thesis_report__jagannath_4313658.
Delft University of Technology
Efficient and
Economical integration
of EV and PV
J. A. Obulampalli
ii
Efficient and Economical integration
of EV and PV
By
J. A. Obulampalli
in partial fulfilment of the requirements for the degree of
Master of Science
in Electrical Power Engineering
at the Delft University of Technology,
to be defended publicly on 17th July 2015
Supervisor:
Prof. dr. P. Bauer ,
Dr. ir. Jos van der Burgt,
Thesis committee:
Prof. dr. P. Bauer,
Dr. L. Ramirez,
Dr. J. Rueda Torres,
Dr. Jos van der Burgt,
TU Delft
DNV GL
TU Delft
TU Delft
TU Delft
DNV GL
iii
iv
Preface
This work was carried out in association with the new-energy technologies group of DNV GL, a
consultancy company with its energy division headquartered in Arnhem, Netherlands and TU
Delft.
DNV GL – Energy, which is the Energy vertical of DNV GL (i.e. one of its four Business Areas), has
2300 energy experts and provides services and solutions in the form of Energy Advisory,
Laboratories for type testing, Renewables certifications and Research and innovation. DNV GL
possesses specialized knowledge in the area of both existing and new energy infrastructures,
design and engineering, substation automation systems, connections, stations and of managing
and maintaining complex grids. The vision of DNV GL is to make the world safer, smarter and
greener for the future.
The thesis is a part of "Novel E-MObility Model - NEMO" project, which is funded by the
governments of The Netherlands, Germany and Denmark. The idea behind the project is to
create integrated software to assess the impact of electrical vehicles on the grid from a
technical and economic point of view. The thesis aims at investigating the extent of the asset
loading and voltage problems due to integration electric vehicles and photovoltaic into the
distribution system and finding the most efficient and economical solution.
The thesis mainly focusses to answer the following questions:
 What is the extent of over-loading and voltage problems due to EV/PV integration in
distribution systems?
 What is the most economical and effective solution for integration of EV/PV among the
solutions mentioned in literature?
J. A. Obulampalli
Delft, July 2015
v
Acknowledgement
I would like to thank DNV GL and TU Delft for the opportunity they have given me to realize this M.Sc.
Thesis at the New Energy Technologies section of DNV GL.
During the thesis period I received immense technical guidance and support from Dr. ir. Jos van der
Brugt, Thank you for your support and patience during this process. I wish to express my sincere thanks
to Dr. Martijn Huibers and Santiago Penate Vera for their guidance and suggestions during the process.
I would like to thank Prof. Pavol Bauer for his guidance and for the opportunity to learn and work in his
association. I would like to whole heartedly thank, Ir. Gautham Ram for his constant support and
suggestions during master thesis.
At last I would like to thank and express my gratitude to my parents, my brother and friends for their
support and inspiration.
-Jagannath Obulampalli
vi
Contents
1
2
3
Introduction ............................................................................................................................. 1
1.1 Literature review on Integration of EV/PV into the distribution grid .............................. 2
1.1.1
EV Integration........................................................................................................... 2
1.1.2
PV Integration ........................................................................................................... 2
1.1.3
EV/PV Integration..................................................................................................... 3
1.1.4
Research gap ............................................................................................................. 4
1.2
Solutions for integration of EV/PV into the distribution grids ........................................ 5
1.3
Research Questions: ......................................................................................................... 5
1.4
Research Methodology..................................................................................................... 5
1.5
Structure of the report ...................................................................................................... 6
Voltage Variation and Mitigation Solutions ........................................................................... 7
2.1 Network Reinforcement ................................................................................................... 7
2.2
On load tap changer.......................................................................................................... 8
2.3
Static VAr control ............................................................................................................ 8
2.4
Storage.............................................................................................................................. 9
2.5
Curtailment of power at point of common coupling (PCC)............................................. 9
2.6
Reactive power control by PV inverter Q(U), Q(P)....................................................... 10
2.7
Demand response by local price signals ........................................................................ 10
2.8
SCADA + direct load control......................................................................................... 11
2.9
Wide area control ........................................................................................................... 11
2.10
Prioritisation of Solutions for the integration of PV/EV into the Distribution system.
12
Existing grid and Network Reinforcement ........................................................................... 13
3.1 Description of Case study .............................................................................................. 14
3.2
EV penetration Limits .................................................................................................... 14
3.2.1
Bogfinkevej:............................................................................................................. 14
3.2.2
Rindum Mølleby: .................................................................................................... 15
3.2.3
Conclusion .............................................................................................................. 16
3.3
PV Integration limits ...................................................................................................... 16
3.3.1
Bogfinkevej:............................................................................................................ 16
3.3.2
Rindum Mølleby : ................................................................................................... 16
3.4
Grid Reinforcement ........................................................................................................ 17
vii
3.4.1
Cable upgrade: ........................................................................................................ 17
3.4.2
Transformer upgrade:.............................................................................................. 22
3.5
4
Conclusion...................................................................................................................... 22
Reactive power control ......................................................................................................... 24
4.1 Reactive Power and its effects ....................................................................................... 24
4.1.1
4.2
PV Inverter Topology ............................................................................................. 26
4.2.2
Control of Active and reactive power of Inverter ................................................... 27
Impact of reactive power on EV charging ..................................................................... 28
4.3.1
Bogfinkevej distribution grid .................................................................................. 28
4.3.2
Rindum Mølleby Distribution grid ......................................................................... 35
4.3.3
EV integration limits with reactive power flow...................................................... 35
4.3.4
Rindum Mølleby distribution grid with upgraded Cables ...................................... 36
4.4
6
PV inverters operating in VAr mode.............................................................................. 26
4.2.1
4.3
5
Impact of Cable impedance on reactive power control........................................... 25
Yearly simulation of Distribution system with high PV and high EV penetration........ 41
4.4.1
Bogfinkevej Distribution grid ................................................................................. 41
4.4.2
Rindum Mølleby Distribution grid ......................................................................... 47
4.5
Disscussion ..................................................................................................................... 50
4.6
Conclusion...................................................................................................................... 50
Storage .................................................................................................................................. 51
5.1 PLATOS ......................................................................................................................... 51
5.1.1
Profile simulation and results analysis.................................................................... 52
5.1.2
Optimal Storage location ........................................................................................ 52
5.1.3
Optimum power and Energy sizing ........................................................................ 52
5.1.4
Optimal storage dispatch......................................................................................... 52
5.1.5
Simulation Logic..................................................................................................... 53
5.2
Bogfinkevej Distribution system.................................................................................... 54
5.3
Rindum Mølleby distribution grid.................................................................................. 56
5.4
Conclusion...................................................................................................................... 57
Conclusion ............................................................................................................................ 58
Annex ............................................................................................................................................ 60
Annex A : Details of network reinforcement for 100% EV penetration in bogfinkevej
distribution system. ................................................................................................................... 60
Bibliography.................................................................................................................................. 65
viii
List of tables
Table 1:Details of Distribution grids ............................................................................................ 13
Table 2:EV intergration limit for Bogfinkevej feeder 1 ............................................................... 14
Table 3: EV integration limits for Bogfinkevej ............................................................................ 15
Table 4: EV integration limits: Rindum Mølleby ......................................................................... 15
Table 5: PV integration limits of Bogfinkevej.............................................................................. 16
Table 6: PV Integration Limits Rindum Mølleby ......................................................................... 17
Table 7:network upgrade details: bogfinkevej distribution system .............................................. 18
Table 8: Network reinforcement details Rindum Mølleby distribution system ........................... 21
Table 9: Cable types with their R/X ratios.................................................................................... 25
Table 10: Bogfinkevj grid data ...................................................... Error! Bookmark not defined.
Table 11: EV integration limits with reactve power control......................................................... 29
Table 12: Comparison of reactive power control and traditional grid upgrade ............................ 30
Table 13: rindum grid summary ................................................................................................... 35
Table 14: EV integration limits with reactive power flow ........................................................... 35
Table 15: EV integration limits with cable upgraded ................................................................... 36
Table 16: Comparison of reactive power control with traditional grid upgrade........................... 36
Table 17: Rindum EV integration limits with reconfigured transformer position........................ 37
Table 18: Energy Exchange between the DSO and the consumer................................................ 41
Table 19: Losses comparision with different Solutions................................................................ 42
Table 20:Energy Exchange between the DSO and the consumer................................................. 47
Table 21: Losses comparision with different Solutions................................................................ 47
Table 22: Storage details for bogfinkevej diatribution system ..................................................... 55
Table 23: Comparision of solutions for Bogfinkevej Distribution system ................................... 55
Table 24: Storage details Rindum Mølleby distribution system................................................... 56
Table 25: Comparision of solutions for Rindum Mølleby distribution system ............................ 57
ix
List of Figures
Figure 1:Cable details of the bogfinkevej distribution network before upgrade .......................... 18
Figure 2:Cable details of Bogfinkevej distribution system after upgrade for 60% EV penetration
....................................................................................................................................................... 19
Figure 3:Cable details of Bogfinkevej distribution system after upgrade for 100% EV
penetration:.................................................................................................................................... 20
Figure 4:Cable details of Rindum Mølleby without cable upgrade .............................................. 21
Figure 5:Cable Details of Rindum Mølleby with cable upgrade for 40% EV penetration. .......... 22
Figure 6: EV penetration forecast ................................................................................................. 23
Figure 7:Circuit with a resistance and reactance to compare the sending in voltage and recieving
end voltage .................................................................................................................................... 24
Figure 8: Voltage variation with change in power factor ............................................................. 25
Figure 9: Circuit for checkingvoltage variation of different cable types with p.f. ....................... 25
Figure 10: Graph showing the variation of Voltage with different cable types............................ 25
Figure 11: Schematic of inverter operating in VAr mode ............................................................ 26
Figure 12: Grid connected inverter circuit .................................................................................... 27
Figure 13: Equivalent circuit and phasor diagram of single phase inverter ................................. 28
Figure 14: Flow chart of PLATOS ............................................................................................... 53
Figure 15: location of Storage in Bogfinkevej grid ...................................................................... 54
Figure 16: location of storage in Rindum Mølleby grid ............................................................... 56
Figure 17:Voltage profile before cable upgrade for feeder 1 with 32 EV’s ................................. 62
Figure 18:Voltage profile after cable upgrade for feeder 1 with 32 EV’s .................................... 62
Figure 19: Voltage profile before cable upgrade for feeder 2 with 51 EV’s ................................ 63
Figure 20:Voltage profile after cable upgrade for feeder 2 with 51 EV’s .................................... 63
Figure 21: Voltage profile before cable upgrade for feeder 4 with 26 EV’s ................................ 64
Figure 22:Voltage profile after cable upgrade for feeder 4 with 26 EV’s .................................... 64
x
0
1
1 Introduction
Electric vehicle (EV) is an element of the future transition towards a clean and sustainable energy
system. The sustainability and the impact of electrical vehicles on the environment depends upon the
source of electrical energy. Electrical energy produced from fossil fuels has the largest carbon footprint
and the energy from renewable sources like hydro, wind and photovoltaic have near zero Green House
gases emission during operation. Thus to maximize the positive impact of electrical vehicles on the
environment, electrical energy should be sourced efficiently from renewable sources. The most efficient
way to deliver energy is to generate energy at the load center to avoid transmission losses , and
Distribution generation does exactly the same.
The existing electrical grid is designed for Centralized generation and distributed loads and with the
integration of renewables, the power flow is altered. When a significant magnitude of distributed p ower
flows from the LV system to the MV system, it can lead to issues like over-voltage, overloading of assets
in the transmission system, harmonics, and intermittency in available power. EV’s with their ability of
decentralized storage of electrical energy, can contribute towards solving the challenge of integrating
distributed generation (DG) into existing grids. But their initial deployment into existing grids can cause
local problems like under-voltage and peak loading[1] due to tendency of an EV’s user to charge during
the evening when the household demand is also high
The study aims at investigating the extent of the above said problems due to integration Electric
Vehicles and Photovoltaics into the distribution system and finding the most efficient and economical
solution. Part of the simulation is done using the NEMO tool suite which generates profile, simulates the
load flow and optimizes the storage and grid parameters for a complete year. The load flow and grid
analysis is performed through DIgSILENT PowerFactory via the PLATOS interface.
The Thesis assigned was a part of "Novel E-MObility Model - NEMO" project. The idea behind the project
is to create integrated software to simulate and assess the impact of Electrical vehicles on the grid from
a Technical and Economic point of view.
The software developed is through integration of existing simulation software from the project partners
DNV GL - Energy, Fraunhofer ISE and EMD. EnergyPRO from EMD generates the profiles for households,
distributed generators and SimTOOL from Fraunhofer ISE performs load flow studies and Demand side
management. PLATOS from DNV GL is an optimization tool for determining location, type and size of
storage systems in order to avoid voltage or grid overload problems which result from variable
distributed generators or electrical loads.
1
1.1 Literature review on Integration of EV/PV into the distribution grid
Photovoltaic systems convert solar power into DC power and is then converted into grid compatible AC
power through the use of an inverter. A PV system can be modelled as a PV node or PQ node for load
flow analysis based on the control system of the inverter[2]. If the converter controls Active power (P)
and Voltage (V) independently then it’s modelled as a PV node and If Active power (P) and Reactive
power (Q) are controlled independently then it is represented as a PQ node. Electrical vehicle charger is
modelled as a PQ node as it is considered a load. Thus both PV and EV can be considered as a load with
different sign convention, EV can be considered a positi ve load and PV can be considered as a negative
load.
The voltage across two terminals of a cable is governed by the equation:
∆V = (𝐼𝑎𝑐𝑡𝑖𝑣𝑒 + 𝑗𝐼𝑟𝑒𝑎𝑐𝑡𝑖𝑣𝑒 )(𝑅 + 𝑗𝑋)
It can be seen that when EV and PV are modelled as PQ nodes the voltage change depends on the
magnitude of active and reactive current flowing through the feeder. Therefore there is a possibility of
over voltage (>1.05p.u.) in case of PV generation and under voltage (<0.95p.u.) in case of EV charging.
Due to the increased flow of active and reactive currents, the cable and transformer may be overloaded
(>100% rated kVA).The IEC 60038 defines that the supply voltage of 220/400V can be between 0.9p.u. to
1.1p.u. In the simulation below, the voltage limits are considered to be +/-0.05p.u.
1.1.1 EV Integration
The impact of Electrical vehicles on the transmission system is studied by S.W.Hadley [3] based on
vehicle characteristics, charging characteristics, time of plug in. The reports concludes that changes to
the load profiles due to EV charging and broadly states that the distribution system should be planned
and reinforced to accommodate the non-intermittent nature of Electrical vehicle charging. Study of a LV
distribution grid in suburban Dublin, Ireland by Richardson[4], compares the impact of controlled
charging and uncontrolled charging on the distribution network. In controlled charging the voltage drop
across the feeder is evident and same is curtailed by using controlled charging. The controlled charging
is optimized based on the voltage limits and thermal limits of the system. In the simulation presented in
the paper, voltage limitation is predominant over transformer/cable thermal loading and concludes that
the same may not be true for distribution systems with lower thermal capacities and higher charging
powers. Evaluation done by Taylor[5], in his paper includes the impact of EV’s on the life of the
transformer. The life of the transformer is reduced due to the increased loading and increased stress on
the dielectric.
Voltage unbalance in the distribution system due to single phase charging of electrical vehicles is studied
by Farhad[6] , which suggests that EV’s will have a larger impact on the voltage unbalance for the
consumers at the end of the feeder than the consumers near the transformer. In conclusion it can be
said that the impact of Electric vehicles on the distribution network is well studied.
1.1.2 PV Integration
The impact of photovoltaic systems on the Distribution network is almost the same as EV due to th e fact
that they can be considered as PQ nodes with opposite flow of power. One of the differentiating
2
characteristics of PV system is its output intermittency due to cloud shading which leads to sudden
voltage dip/rise[7]. Baran et al.[8] considers a high penetration of residential PV systems and
investigates the consequences of PV on Protection and Voltage regulation. His study concludes that the
impact on protection is limited due to fact that inverters can limit their currents during faults and can
quickly disconnect from the system. Thomsom[9] through a study of a LV network in Leicester, UK
concludes that PV integration limits are constrained by the existing network parameters and that the
impact of self-consumption on the transformer loading is limited as the residential peak doesn’t coincide
with the PV peak. A comparison of urban and rural distribution network by Tonkoski [10], suggests that
the voltage rise due to PV would be greater in rural networks due to longer distance between two
adjacent houses.
PV inverters and EV charging stations are electronic non-linear loads/generators which generate
harmonics. Schalabbach[11]’s study on PV inverters and Lu Yanxia[12]’s study on Electric vehicle charger
reiterates the same. Harmonics generated can be removed through active and passive filters and are not
major hindrances to EV and PV integration
1.1.3 EV/PV Integration
It can be observed that problems associated with PV and EV integration into the distribution system are
well studied and can be broadly classified into voltage, asset loading, phase voltage unbalance and
harmonics. These problems can be addressed individually or through smart operation of EV in
combination with PV.
The issue of voltage and loading can be addressed by traditional method like network reinforcement in
which the assets leading to voltage drop and overloading can either be replaced or reinforced by adding
parallel transformer or cables. Another solution studied by Ram[13][14] for voltage support is the use of
power electronic assisted On-Load tap changing transformer. OLTC can provide voltage support both in
the case of EV charging and PV generation as the tap change can either be positive or negative and its
integration with power electronics can provide fast response to PV intermittency. The major drawback is
that it doesn’t solve the problem of the loading of the assets.
Network reconfiguration in distribution systems studied by Baran[15], can be implemented by the use
of sectionalizing switches to alter the power flow in the distribution system to decrease the losses and
loading of the distribution system . This is usually implemented in MV networks where switching
alternatives are present and usually not feasible due to the radial nature of distribution networks.
Active power curtailment for PV inverters to avoid over voltage is studied by Tonkoski [16], in which the
active power is curtailed as a function of power generated. The study concludes that the inverters
downstream on the feeder will have to curtail larger magnitudes of power which may affect their
revenue. This solution can be compared to limiting the charging power of Electric vehicles studies by
Quian[17]
Reactive power control to support voltage in a distribution system can be done through PV inverters and
or through the use of shunt capacitors, shunt reactors, synchronous condensers, Static VAr
compensators and dynamic voltage restorers. The switching of shunt capacitors and reactors need to be
controlled as we need to consume reactive power during PV generation and generate reactive power
during EV charging. To avoid the complex control system and installation cost of th e capacitors and
reactors, we consider PV inverter to be the source/sink of continuous variable reactive power. The
inverter reactive power control technologies for voltage support are reviewed by Demirok [18] and a
3
new reactive power control method is proposed which is a combination of cosᵠ(P) and Q(U). The
proposed reactive control methods gives a higher PV integration limits. Based on the above study, the
reactive power control method can be replicated for EV charging as well, to get a higher EV integrati on
limit like the study by Huang[16].
Reactive power compensation has its limitations for voltage support in the distribution systems of EU
countries as the R/X ratios of under-ground cables in LV system are between 5 to 2[19] and needs
significant amount of reactive power with the risk of overloading the cables.
Hashemi[20] projects integration of Storage as a solution for voltage quality issues and peak shaving in
systems with high penetration of PV and EV. The storage absorbs and stores PV energy and discharges
during EV charging. By this charging and discharging process, the voltage problems and loading
problems are solved. The simulations show that the energy storage capacity needed for over voltage
prevention is dependent on the location of the customer on the feeder. The storage capacity is less for
consumers near the transformer. Traube[21] in his paper mitigates PV intermittency through the use of
EV charging and discharging, in this process EV is being used as a storage system. This is can be an
interesting option for work place charging where the EV’s can be charged during peak PV output in the
afternoon. The main limitation for using EV’s for energy storage in residential distribution systems is the
low simultaneity factor as most of the EV are not connected to the residential grid in the afternoon
when the PV output is at its maximum.
1.1.4 Research gap
The above mentioned studies and solutions either mitigate the problems of voltage and loading by
treating them as individual problems or try to mitigate them by smart operation of EV and PV. There is a
lack of research work in terms of using the PV inverters to control reactive power during EV charging. As
cited above reactive power compensation has been studied individually either from EV point of view, or
from PV point of view.
A study of the reactive power capabilities of PV inverters by Ellis[22] states that inverters in principle can
provide reactive power capability at zero active power but, the functionality is not standard in the
industry due to regulations and standardisation. Maknouninjad[23] in his paper discusses steps involved
for operating the inverter in VAr mode during the night. The inverter consumes negligible amount of
active power from the grid to pre charge the DC bus capacitor and regulate the DC bus voltage within
limits. This reactive power capability can provide voltage support during EV charging which is usually
during the evening/night when the PV active power generation is not present. This thesis tries to fill this
gap by using PV inverters for EV charging and comparing it with other methods to get the most effective
and economic solution in terms of impact on voltage and congestion/loading for integration of electric
vehicles and PV systems into LV distribution system.
4
1.2 Solutions for integration of EV/PV into the distribution grids
The solutions for voltage support and loading mentioned in the above literature study can be broadly
classified into :
 Network reinforcement
 On-load tap changer
 Network reconfiguration
 Active power/ charging power curtailment
 Reactive power control
 Storage
The solutions can be further classified based on the stakeholder implementing it. The report:
Prioritization of technical solutions available for the integration of PV into the distribution grid [24]
classifies the solution on the basis of implementation by the stake holders. The following solution are
studied in brief in the following chapter and Network reinforcement, Distributed storage and reactive
power solutions are studied for their effectiveness in later chapters.
 DSO Solutions
o Network reinforcement
o On-load Tap Changer (MV/LV transformer)
o DSO Storage
o Network Reconfiguration
 Prosumer Solutions
o Prosumer Storage
o Self-consumption/generation by tariff incentives
o Curtailment of power at point of common coupling (PCC)
o Active power control
o Reactive power control
 Interactive solutions ( interaction between DSO and Prosumer)
o Demand response price signals
o SCADA + Direct load Control
o Wide area voltage control
1.3 Research Questions:
The thesis mainly focusses to answer the following questions:
 What is the extent of over-loading and voltage problems due to EV/PV integration in distribution
systems?
 What is the most economical and effective solution for integration of EV/PV among the
solutions mentioned in literature?
1.4 Research Methodology
Based on the following study each of the solution will be studied in brief and the solutions with high
priority namely reactive power control, grid reinforcement, storage will be studied to get the best
solution in technical terms and economical terms. To get the efficient and economical solution we follow
the below methodology:
5





Find the EV integration and PV integration limits for voltage and load violations.
Increase the EV integration limits with the use of grid reinforcement.
Increase the EV/PV integration limits by use of reactive power control.
Increase the EV/PV integration limits by use of Storage.
Compare the cost associated with the each method with the traditional grid reinforcement
technologies.
1.5 Structure of the report
The report is structured in the following way:
 Introduction of the thesis along with the literature study and key solutions
 Review of the different solutions and identification of the priority solutions
 Description of the grid studied and the extent to which EV/PV can be integrated into the existing
grid along with study of network reinforcement as a solution for EV/PV integration
 Study of reactive power control as a solution and its impact in the LV distribution system.
 Storage as a solution and description of PLATOS tool.
 Conclusion which compares the network reinforcement, reactive power control and storage as
solutions for EV/PV integration.
6
2
2
Voltage Variation and Mitigation Solutions
As mentioned in the section 1.1, Voltage drop across two terminals of a transmission system is governed
by the equation:
∆V = (𝐼𝑎𝑐𝑡𝑖𝑣𝑒 + 𝑗𝐼𝑟𝑒𝑎𝑐𝑡𝑖𝑣𝑒 )(𝑅 + 𝑗𝑋)
Therefore it can be said that voltage variation mitigation can be done by varying the magnitude of
resistance (R), reactance (X), active current( Iactive )and reactive ccurrent (Ireactive). Active and reactive
current are related to each other through power factor.
Let us see how the following parameters change with the different solutions mentioned in the previous
chapter
2.1 Network Reinforcement
This solution is a traditional method of increasing the network potential to accommodate PV and EV
charging. In this method, assets operating close to their thermal rating are reinforced by replacing them
with higher power capacity cables and transformers. Cables causing the highest magnitude of voltage
variation are replaced with cables of lower resistance and R/X ratio. This solution can also include
building of new feeder or substation instead of reinforcing the existing infrastructure
Reconfiguring the network for altering the power flow in the system to maintain voltage under limits
and avoid loading of assets can be done through the operation of sectionalizing switches. This solution is
usually used in MV systems where the topology is usually meshed but operated radially. Reconfiguration
of the system is usually done to avoid the outages and needs a communication network to be able to
intelligently configure the network.
Another type of network reconfiguration is closed-loop operation in which each load/generated is fed
from two different sources/sinks. This causes the equivalent impedance of the circuit to decrease, thus
leading to a better voltage profile. But, in case of a technical fault the impact on the system will be larger
due to larger area of impact and fault impedance and will lead to reliability issues.
Advantage:
 The solution and can be used for mitigating both voltage and loading.
 It is simple from design and execution point of view
 Can be readily used where there is a possibility for reconfiguration
Disadvantage:
 High cost of Investment, along with disruption of power supply during reinforcement.
7



In rural areas where the distance between two households in comparatively larger,
reinforcements of larger magnitude than in urban areas is needed to keep the voltage within
limits.
Lack of future knowledge about possible loads variation and generators may lead to recurring
investments.
Need intelligent control and communication between the feeders and the Distribution sys tem
for control of sectionalizing switches.
2.2 On load tap changer
The use of a series transformer or an on-load tap changer to boost the voltage to compensate for
voltage sag is not new and the same can be applied for integration of distributed generation by bucking
the voltage across a distribution line. The essence of the method is that the supply voltage V s plus the
additional series voltage gives the load bus voltage.
𝑉𝑙 = 𝑉𝑠 ± 𝑉𝑠𝑒𝑟𝑖𝑒𝑠
The series transformer can be combined with power electronics control for sub-cycle response to
voltage variation, power factor control[ 25], thus giving the capability to control reactive power in the
system.
Advantages:
 Can be effectively used in cases where the voltage problems are predominant in comparison to
line loading
Disadvantage:
 Installation of Series transformer is not simpler in terms of down time or work needed when
compared to traditional grid reinforcement.
 Most of the transformers presently in operation on the distribution network are not fitted with
OLTC
 Impact on the loading of assets is non-existent.
2.3 Static VAr control
Static VAr compensators are fast acting reactive power control equipment which control the reactive
component of current in the system. In cases of over-voltage the reactive power is absorbed and during
under-voltage the reactive power is generated leading to change in voltage. They are usually used in the
MV/HV segments of the transmission systems where the reactance is higher than resistance.
Advantage:
 Effective in voltage support
 Loading of the assets can be minimized when the reactive power compensators are located
outside the substations and point of demand.
Disadvantage:
 Expensive when placed only to provide voltage support
 The line loading is increased due to reactive power flow, and not usually used in distribution
systems with under-ground cables as they have high R/X ratios.
 Switching of capacitors/reactors provides voltage support in steps
8

Communication infrastructure is needed between the feeder far end and transformer for
effective control.
2.4 Storage
Energy Storage to improve power quality in terms of Voltage Depressions and Power interruptions have
been in practice for a long time[26] for industrial plants with critical processes. Its usage in the electrical
utilities and distribution systems were limited by the cost, centralized generation, maturity of
technology and lack of tools to assess the benefits of Storage technology during planning[3]. In recent
times due to growing Distributed energy generation and their presence at the end consumer of the –
distribution network gives Energy Storage a renewed need and push.
Storage technologies integrated into the grid can be used for applications like:
 Instantaneous Applications (Seconds): Mainly rapid spinning reserve, Primary frequency control,
ride-through capability. For these applications, the battery should be able to deliver high power
in short periods.
 Short Term applications (minutes): Secondary and tertiary frequency regulation, Smoothing of
power output from renewable energy sources, Demand side management by active and reactive
power control. For these applications, the battery should be able to deliver high power and
medium energy in short periods.
 Mid-term (minutes to few hours): load balancing, peak shaving, generation in micro grids. For
these applications, the battery should be able to deliver high power and high energy in short
periods
 Long term( few days) : Energy Supply in cases where the demand is for few days in a year and is
not economic to build transmission infrastructure. Eg. Festival areas with high demand for a few
days.
In our study we consider using storage for mitigating the problem of loading and voltage through
discharge of battery during peak demand and low local generation and charging the battery during peak
generation and low consumption. For storage to be effective for voltage, the storage has to be
distributed to avoid overloading of lines and voltage drop.
Advantages:
 Can be effectively used for decreasing the loading of assets
 Voltage support can be realized by either distributing the storage or interfacing them with an
inverter to either absorb or generate reactive power
Disadvantage:
 Expensive when compared to other solutions
 Lack of regulations/standardization regarding the ownership of Storage
 Needs energy residential energy management systems to optimally operate the battery
2.5 Curtailment of power at point of common coupling (PCC)
In this solution, the prosumer is responsible for limiting his consumption or generation based upon the
strength of the network (i.e. in case of congestion or voltage problems). This can be done through use of
home-based energy storage or by demand response, e.g. charging an electric vehicle at the time of PV
9
peak output. Controllable curtailment needs installation of smart meters and residential energy
management systems which implies additional investment to the prosumer.
Advantages:
 Effective for both voltage support and loading
Disadvantages:
 Needs regulatory changes as present regulations like the RES Directi ve 1 in Germany gives
unlimited priority to renewable energy
 Suitable for PV systems as the peaks occur only for a few instances in a year, but not for EV
charging, because the charging level is consistent throughout th year and may need additional
reinforcement.
2.6 Reactive power control by PV inverter Q(U), Q(P)
PV Inverters are able to control active and reactive powers being generated or consumed by them.
Therefore it is possible to consume reactive power from the system during PV generation and gene rate
reactive power during EV charging. This creates an opposite flow of reactive power which can control
the voltage in the system. But this additional flow of power in the system can create over loading of the
system as the amount of reactive power required to effect the voltage like active power is almost 2 to 5
times the active power. In this method, the reactive power can be a function of the active power
generated, Q(P), or as a function of the voltage of PCC, Q(U).
Advantages:
 Can be an effective solution where Voltage problems are predominant over asset loading like in
rural distribution systems.
 Solution is technically available and doesn’t need any additional grid investment
Disadvantages:
 Not Suitable for networks operating near rated thermal loading
 Present Regulations only recommend reactive power control of systems above 30kWp
 Inverter reactive control is not a standard in industry
2.7 Demand response by local price signals
The effective impact on voltage and loading of assets in the network can be reduced by demand side
management (DSM) in which the peak generation or consumption can be controlled by spreading them
out in time. The DSM is effected through the use of price signals sent by the DSO or indirectly by TSO
based on the network limitations, demand and generation forecasts. In the paper by Shengrong Bu[27]
different electricity prices are defined within the DSO network according to grid loading.
1
The RES directive stipulates that :
1. To the extent required by the objectives set out in the Directive, the connection of new RE installations should be
allowed as soon as possible. In order to accelerate grid connection procedures, M ember states may provide for priority
connection capacities for new installations producing electricity from renewable energy sources. (recital (61)).
2. Priority access and guaranteed access for electricity from RES are important for integrating RES into the internal
market in electricity. Priority access to the grid provides an assurance given to the connected RES-E generators that
they will be able to sell and transmit the RES-E in accordance with connection rules all the times, whenever the sources
becomes available. In the event that RES-E is integrated into the spot market, guaranteed access ensures that all the
electricity sold and supported abtains access to the grid (rectial(60)).
10
This solution requires the installation of smart energy systems which can receive the price information
from the system operator, control the residential loads of the consumer based on his settings and give
feedback about the system to the DSO.
Advantages:
 Effective in controlling both Voltage problem and loading
 Its “first step” towards an intelligent grid
Disadvantages:
 Needs a strong communication network
 Parameterization of prices for different distribution areas is difficult
 Prosumers should be protected from being discriminated on the basis of network strength.
2.8 SCADA + direct load control
An alternative to demand side management through price signals is direct load control by the DSO which
can include controlling the PV generation or by controlling the charging magnitude and time of an
electric vehicle. An additional payment can be introduced for customers to get flexibility in load control.
This solution needs the installation of smart meters and energy management systems that can be
managed by SCADA (supervisory control and data acquisition) which can provide remote access of the
loads to the DSO.
Advantages:
 Removes the complexity of assigning prices based on network capacity
 Can be used for effectively for peak load mitigation
Disadvantages:
 As mentioned in other solutions, a strong communication network is needed which increases
the investment on the network
 Not feasible for fast voltage variations
 User loses control of their loads and this might not be favorable to them
 DSO interferes with market parties (customer and supplier), which is not allowed according to
the rules of unbundling
2.9 Wide area control
This solution is a combination of the above mentioned voltage and reactive power control like
Transformer OLTC, distribution voltage regulators and reactive power control through PV inverters. The
solutions are coordinated to optimize voltage and power factor for the complete distribution system
through voltage and power factor measurements at several points in the distribution system. The
components required for this control are available but their integration needs effort both in technical
and economical terms. The effect on voltage is evident but its impact on the loading is minimal and in
some cases cause overloading due to flow of reactive power.
Advantages:
 Coordinated approach between the different voltage control equipments can lead to better
results
Disadvantages:
 Proper coordination between the different devices makes it difficult to implement
11
2.10 Prioritisation of Solutions for the integration of PV/EV into the
Distribution system.
The discussed solutions are evaluated based on the following criteria:
 Impact on voltage
 Impact on congestion
 Technology readiness
Solution
Impact on
Voltage
Impact on
asset loading
Network
Reinforcement
On load tap
changer/Series
Transformer
Static VAr
control
Storage
High impact
(3)
High impact
(3)
High impact
(3)
Negligible
Impact (1)
Medium
impact (2)
High impact
(3)
High impact
(3)
High impact
(3)
Curtailment of
power at PCC
Reactive power
control by PV
inverter
Demand
response by
local price
signals
SCADA + Direct
load control
Wide area
Voltage control
Technical
feasibility in
Implementation
Immediate
(3)
Immediate
(3)
Priority
Immediate
(3)
Immediate
(3)
Near future
(2)
Immediate
(3)
Low
(6)
High
(9)
Medium
(8)
Medium
(8)
High impact
(3)
Negligible
Impact (1)
High impact
(3)
High impact
(3)
Medium
Impact
(2)
High impact
(3)
Future
(1)
Low
(7)
High impact
(3)
High impact
(3)
High impact
(3)
Negligible
Impact(1)
Future
(1)
Future
(1)
Low
(7)
Low
(5)
High
(9)
Medium
(7)
It can be seen that Network reinforcement and storage have the highest priority based on the impact on
voltage, loading and ease of implementation. From the medium priority solutions, we select reactive
power control through inverter as the next feasible solution as implementation of curtailment of power
and Active power control needs changes to the existing directives and is a deterrent to the installation of
renewable energy systems. Therefore the solutions that will be discussed in detailed and compared are:
 Network reinforcement
 Reactive power control through PV inverters
 Storage
12
3
3
Existing grid and Network Reinforcement
The analysis and simulations to find the existing possible integration levels of EV and PV were performed
on two LV grids from the Danish Stakeholder of the NEMO project. The grids were modeled in Power
factory and simulated for a complete year in hour time steps using varying levels of EV and PV
penetration to assess the impact on the power quality.
Normal (or slow) domestic charging of Electric Vehicles (EV) is assumed to consume a power of 3.7kW
and takes 6 hours to charge 22.2 kWh battery. This is a constant demand unlike other household
appliances, and potentially causes under-voltages (<0.95 p.u) due to increased voltage drop in the cables
and overloading of transformers due to increased demand. Distributed generation (DG) on the other
hand may lead to over-voltages (>1.05p.u.) and transformer overloading due to flow of power from the
distribution grid to the transmission grid.
The following constraints were considered while simulating the grids:
 Power Loading limit : 100% of rated value
 Under-voltage <0.95p.u.
 Over-voltage >1.05p.u.
The IEC 60038 defines that the supply voltage of 220/400V can be between 0.9p.u. to 1.01p.u.. For our
simulation we consider 0.95 and 1.05 as the limits to account for the voltage variations from the HV and
MV side.
Table 1:Details of Distribution grids
Construction
Transformer
Cables (3-phase)
Distance between the
substation and last house
connected to the feeder
Consumption per year
Nr. of connected houses
Bogfinkevej
1970’s
250kVA
150mm2 : 0.21km
95mm2 : 0.51km
50mm2 : 1.94km
Feeder 1 : 0.41km
Feeder 2 : 0.38km
Feeder3 : 0.38km
Feeder4 : 0.26km
458MWh
141
Rindum Mølleby
2000’s
400kVA
150mm2 : 2.55km
95mm2 : 0.55km
Feeder1 : 1.04km
Feeder2 : 0.546km
Feeder3 : 0.687km
399 MWh
93
The two distribution networks are ideal for studying the interaction of EV and DG as they offer different
limitations for EV/DG penetration in terms of transformer loading and feeder cable length. Bogfinkevej
13
which supplies to 141 households has a relatively less transformer capacity of 250kVA and Rindum
Mølleby which supplies to 93 households has a long feeders leading to higher voltage drop.
3.1 Description of Case study
It is expected that intelligent operation of EV charging points and Distributed Generation (DG) can solve
the above mentioned problems of voltage level and transformer loading while keeping the grid
reinforcement investment to a minimum. To cross check the above mentioned solution we follow the
following procedure:
 Check the EV integration limits and DG integration limits of the existing grids
 Reinforce the grid to accommodate 40%, 60% and 100% EV penetration, to get an idea about
grid reinforcement investment.
The household load, EV charging load and PV generation profiles for the si mulation are taken from
EnergyPRO by EMD International A/S.
3.2 EV penetration Limits
We need to get the maximum number of vehicles that can charge from an existing distribution grid to
analyse the capacity of the present existing grid. To get the maximum worst case number of EV’s that
can be charged, we take into account time instance on a winter day (02.01.2012 1800hrs) when the
house hold load is at its peak and EV charging imposes an additional load.
3.2.1 Bogfinkevej:
For feeder 1 which has 32 houses, let us suppose each house may have an EV, there can be 2^32
possibilities in which the EV’s can be distributed. To avoid huge computation, we increase the number of
EV’s from the substation to the end of the feeder till cases with under-voltages arise. The same is done
from the feeder far end.
Table 2:EV intergration limit for Bogfinkevej feeder 1
Charging Scheme
Charging without delay
(charging at 6pm)
Charging with delay
(Charging at 1am)
EV placement
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Max. No. of vehicles charging
16
8
19
10
By taking the average of the maximum number of vehicles charging from the transformer end and the
feeder far end we can get the limiting number of electrical vehicles that can charge from a particular
feeder. We can see that charging without delay can accommodate 12 EV’s and charging with time delay
can accommodate 14 EV’s which correspond to 31% and 46% of EV penetration. The increase in EV
charging limit from 6pm to 1am is not large as an EV charging power is high as compared to the general
house hold demand.
14
For other feeders the same method of determining the limit was applied. The EV penetration of feeder 2
is better due to its topology in which many houses are connected near the transformer through
branches. The numbers of houses connected to the feede r 2, feeder 3 and feeder 4 are 51, 32 and 26
respectively.
Table 3: EV integration limits for Bogfinkevej
Feeder Details
Feeder 1: 32 Houses
Feeder 2 : 51 Houses
Feeder 3: 32 houses
Feeder 4 : 26 Houses
All the feeders combined
EV placement
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Max. No. of vehicles charging
16
8
41
13
19
15
17
24
88
60
When transformer overloading is not considered, the number of vehicles that can be charged from the
complete distribution system without voltage violations under worst case scenario of EV positioning (ie.
Charging from the feeder far end) is 60, which accounts to 42% of EV penetration.
When transformer overloading is considered the maximum number of vehicles that can be charged from
the complete distribution system is restricted to 37 EV’s or 26.2% of EV penetration.
3.2.2 Rindum Mølleby:
The distribution system of Rindum Mølleby looks to be strong when compared to Bonfinkeveg as its
transformer and cables have a higher rating and the number of houses it supplies to is less.
This is not actually the case, Bogfinkevej has an ability of accommodate a higher penetration of EV than
Rindum Mølleby due to the fact that the length of all the feeders in Bogfinkevej are almost half that of
Rindum Mølleby
Table 4: EV integration limits: Rindum Mølleby
Feeder Details
Feeder 1
34 houses
Feeder 2
19 houses
Feeder 3
40 houses
All feeders combined
EV placement
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Max. No. of vehicles charging
1
0
14
12
10
12
25
24
15
Rindum Mølleby has a combined EV penetration limit of 18.2% limited by the voltage drop across the
feeder. If the under-voltage due to voltage drop in cables is not considered then the EV penetration limit
is limited by the transformer loading to 82 EV’s or a penetration level of 89%.
3.2.3 Conclusion
It can be said that in case of Bogfinkevej distribution grid, the main limiting factor for EV Penetration is
the transformer loading and in case of Rindum Mølleby the limiting factor is the cable voltage drop due
to long length of the feeders.
3.3 PV Integration limits
PV/DG integration limit is defined as the maximum power that the DG can generate without violating
the voltage levels, line thermal capacity and substation transformer capacity. The PV integration limit is
defined by considering the worst case scenario when the DG generation is at its maximum and the load
is at its minimum.
It is assumed that the aim of a domestic PV system owner is to be able to produce enough energy for
charging EV. The worst case EV demand is 6000 kWh/year. Denmark experiences yearly average of 3.15
sun hours per day (The Nordic Folkecenter for Renewable Energy, 2014) 4, So the average PV sizing
should be 5kWp.
𝑃𝑉 𝑆𝑦𝑠𝑡𝑒𝑚 𝑝𝑒𝑎𝑘 𝑜𝑢𝑡𝑝𝑢𝑡 =
𝑌𝑒𝑎𝑟𝑙𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 (𝑘𝑊ℎ)
[365 ∗ 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑠𝑢𝑛 ℎ𝑜𝑢𝑟𝑠]
3.3.1 Bogfinkevej:
To get the PV integration limits we start placing 5kW PV Systems from the transformer side to the
feeder end and vice versa. The time instant simulated is on a summer day (20-06-2012) at 12pm when
the PV system produces maximum power and the household load is at its minimum. The PV systems are
placed until voltage violations are observed.
Table 5: PV integration limits of Bogfinkevej
Feeder Details
Feeder 1
Feeder 2
Feeder 3
Feeder 4
PV positioning
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
PV penetration in kW
95
75
185
85
95
80
105
70
The worst case PV integration limits for the complete distribution system system is 310kW but the
transformer capacity restricts the capacity to 250kW.
3.3.2 Rindum Mølleby :
16
The same method of estimating the PV integration limits used in bogfinkevej is implemented in for Rindum
Mølleby and the results are presented in the table below.
Table 6: PV Integration Limits Rindum Mølleby
Feeder Details
Feeder 1
Feeder 2
Feeder 3
PV positioning
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
Near the transformer
Far end of the feeder
PV penetration in kW
60
40
80
75
100
60
In worst case scenario the maximum PV generation that can be integrated without any over-voltages is
175kW and is restricted by the cable voltage drop.
3.4 Grid Reinforcement
The idea behind upgrading the grid was to analyze the magnitude of grid reinforcement required to
integrate Electric Vehicles and DG without dispatchable loads, storage, reactive power control etc. The
grid was upgraded for EV penetration levels of 40%, 60% and 100%.
3.4.1 Cable upgrade:
The aim of the cable upgrade is to get an estimate on the cable length that needs to be upgraded to be
able to accommodate EV charging at 40%, 60% and 100% . Transformer over-loading is not considered
as the limiting factor while running the simulations. The EV charging load is superimposed on the peak
house hold demand.
To simulate the distribution grid response, we select a time instant when the household demand is at its
peak on a winter day (02.01.2012 18:00). The EV charging load is imposed on the household demand
and the voltage profile is plotted.
Based on the voltage drop observed, the cables with relatively larger voltage drop is upgraded and
again simulated until the voltage drop is above 0.95 p.u.
3.4.1.1 Bogfinkevej
For 40% EV penetration level, no cable upgrade was needed. For 60 % penetration, 340m from the total
cable length of 2.72km has to be upgraded which is 12% of the total cable under use.
950m of cable was upgraded from a total cable length of 2.72km to keep the voltage within limits for a
penetration level of 100%. This can be a possible solution where the feeder is strong as in case of feeder
4 where only 26.4m of cable had to be changed to accommodate 100% EV penetration. The details of
the cable upgrade for Bogfinkevej distribution system with 100% EV integration are in annexure A.
17
Table 7:network upgrade details: bogfinkevej distribution system
40% EV penetration
60% EV penetration
90% Ev penetration
Traditional grid upgrade
Transformer upgrade : 400kVA
Transformer upgrade: 500kVA
Cable upgrade: 340m from 2.72km
Transformer upgrade:600kVA
Cable upgrade: 950m from 2.72km
Figure 1:Cable details of the bogfinkevej distribution network before upgrade
18
Figure 2:Cable details of Bogfinkevej distribution system after upgrade for 60% EV penetration
19
Figure 3:Cable details of Bogfinkevej distribution system after upgrade for 100% EV penetration:
3.4.1.2 Rindum Mølleby:
For 40% EV penetration, a cable upgrade of 1.7km is needed from a total cable length of 2.81km. For
60% EV penetration, all the cables must be upgraded to 400mm2 from their existing cable ratings and
the grid needs to be reconfigured due to the large lengths of the feeder. Simulation for 100% EV
penetration was not considered due to the length of the feeders and need for complete reconfiguration
of the grid.
20
Table 8: Network reinforcement details Rindum Mølleby distribution system
40% EV Penetration
60% EV Penetration
90% EV Penetration
Traditional grid upgrade
Cable upgrade : 1.7km from 2.9km
Cable upgrade : Complete replacement of cable with
400mm2
Need complete reconfiguration of the grid
Figure 4:Cable details of Rindum Mølleby without cable upgrade
21
Figure 5:Cable Details of Rindum Mølleby with cable upgrade for 40% EV penetration.
3.4.2 Transformer upgrade:
The results from the EV integration limits for Bogfinkevej conclude that the cables are strong enough
to accommodate 42% of EV penetration but the transformer loading restricts it to 26.2%. Therefore an
transformer upgradation by replacement of 250kVA with 400kVA can accommodate 42% EV penetration
till the grid needs complete upgrade.
Rindum Mølleby transformer of 400kVA can accommodate upto 89% of Ev penetration but the cables
restrict it. So, transformer upgrade as a single solution is not feasible.
3.5 Conclusion
The EV penetration forecast is taken from the Green eMotion project Deliverable D4.3 – B2[28] Grid
Impact studies of electric vehicles.
22
The three different penetration profiles are shown
in Figure 6. The high profile corresponds to the high
forecast for Denmark. The medium profile
corresponds to the medium penetration forecast
for Denmark and the high penetration forecast for
EU. Both are representative of the expected
evolution of EVs, based on an assumption of steady
technological progress. The low profile corresponds
to the low penetration forecast for the EU.
From the EV integration limits of the Bogfinkevej
suggests that grid upgradation is not required until
EV penetration reaches 26.2%. The EV penetration
Figure 6: EV penetration forecast
levels in Denmark will reach this limit in 2021 if high
forecast is considered and in 2028 if medium forecast is considered. For Rindum the grid upgradation
has to take place in 2018.
It can also be said that network reinforcement cannot be a solution for 100% EV integration in cases like
Rindum Mølleby where a complete reconfiguration of the feeders is needed. Based upon the feeder
lengths, it can also be generalised that distribution network in urban areas have a larger capacity to
accommodate EV/PV installations in comparison to rural networks, where the distance between the
houses in the feeder is larger.
23
4
4
Reactive power control
4.1 Reactive Power and its effects
The voltage drop across two terminals is governed by the equation
∆V = (𝐼𝑎𝑐𝑡𝑖𝑣𝑒 + 𝑗𝐼𝑟𝑒𝑎𝑐𝑡𝑖𝑣𝑒 )(𝑅 + 𝑗𝑋))
It can be seen that both active and reactive current are involved in
voltage drop across two terminals. The voltage drop can be
mitigated by either delivering the reactive power required at the
point of load or by reversing the flow of reactive power against the
direction of active power. Voltage control by adjusting reactive
power flow is common is high-voltage transmission systems and also
in medium voltage distribution systems due to the fact that HV
transmission lines have higher reactance than compared to
resistance per unit length.
Let us consider a circuit as shown in the figure.7 , Ea be the sending
end voltage, V a be the receiving end voltage, Ra, Xs be the resistance
and the reactance of the line respectively.
Figure 7:Circuit with a resistance and
reactance to compare the sending end voltage
and recieving end voltage
To study the impact of reactive power on the voltage drop, let’s consider three cases where the load
consumes reactive power (lagging power factor), neither absorbs nor produces reactive power (unity
power factor) and last case in which the load produces reactive power (leading power factor).
When the load operates at lagging power factor and consumes reactive power, the additional flow of
reactive power causes higher drop in voltage compared to unity power factor where the reactive power
doesn’t flow. When the load operates in leading power factor, the receiving end voltage can be higher
than the sending end voltage due to overcompensation of voltage drop due to the reverse flow of
reactive power.
This compensation of reactive power at the point of load or reverse flow of reactive power can be used
to improve the voltage in a transmission system.
24
Figure 8: voltage variation with change in power factor
4.1.1 Impact of Cable impedance on reactive power control
Low-Voltage distribution systems in the EU have underground cables that are predominantly ohmic
when compared to their per unit reactance. Thus the impact reactive power has on the voltage drop
across a LV cable is less compared to HV system.
The cables in the Bogfinkevej and Rindum Mølleby distribution system have a cross section area of
50mm2 , 95mm2 and 150mm2. These have a R/X ratio ranging from 2.5 to 7, so one has to take care that
the reactive power is used efficiently without overloading the cables. The R/X ratios of cables used in the
distribution system are listed.
Table 9: Cable types with their R/X ratios
Figure 9: Circuit for checkingvoltage variation of
different cable types with p.f.
Cable type
400 mm2
240 mm 2
150 mm 2
95 mm2
50 mm2
R/X ratio
0.68
1
2.5
4.25
7
To check the impact of the cable ratios on reactive power
control for voltage support, we consider a simple circuit with
different cable types of length 500m from the source to the
load. A load of sufficient magnitude is selected to get a voltage
drop of 5% and the variation in voltage due to change in power
factor is plotted for different cable types.
It can be seen form the voltage vs power factor graph that
cables with lower R/X ratios have a larger impact with reactive
power control. With a high R/X ratio of 4.25 (95mm2 cable), the
voltage improves by 0.01p.u. and even this small improvement
could be useful in some cases.
Figure 10: Graph showing the variation of Voltage with
different cable types
25
4.2 PV inverters operating in VAr mode
PV inverters have the ability to control reactive power but this is not a standard feature for residential
PV inverters, as at present regulations, like German grid code VDE 4105, suggest reactive power control
only for PV systems with a peak output greater than 30kWp and systems below 30kWp are operated at
unity power factor.
On an average PV inverters interact with the grid only for 6-10 hours a day during PV generation, thus
limiting its utilization and return on investment of the inverter. The utilization of PV inverters can be
increased by operating them in reactive power compensation mode when the PV power i s small. During
PV generation the inverter consumes some active PV power as losses and for powering the control
circuitry. In the absence of PV power, the inverter has to draw some active power from the grid to be
able to regulate the DC bus voltage, compensate for switching losses and inject the desired level of
reactive power into the grid.
This reactive power control capability of the PV inverter during the evening coincides with the tendency
of the EV user to charge his vehicle in the evening after returning from work. By using PV inverter for
reactive power control, we eliminate the need for oversizing the EV charger to accommodate variable
power factor and it can also be considered as a possible solution that can be replicated for large
domestic loads such as heat pumps.
4.2.1 PV Inverter Topology
A normal grid connected PV system consists of
a PV modules, DC-DC converter, inverter with a
DC coupling capacitor and grid connection as
represented in the figure. In the absence of PV
power, the inverter can be operated in reactive
power mode by decoupling the DC-DC
converter and the PV module as shown in the
figure.
Operating the inverter in VAr mode involves
the following steps 5:
 Pre-charging the DC bus capacitance
 Regulating the DC bus voltage within limits
while regulating the injected reactive power.
Figure 11: schematic of inverter operating in VAr mode
Pre-charging of the capacitor can be done through either a separate DC connection from an external
source or by operating the inverter as a line rectifier. The paper by Maknouninejad [29] studies and
explains the use of inverter switches/anti parallel diodes to charge the capacitor and provides design
equations for maximum current flowing during capacitor charging.
26
4.2.2 Control of Active and reactive power of Inverter
The inverter circuit is depicted in the figure12 The inverter measures the DC link Voltage Voltage V dc ,
the grid voltage va ,vb, vc, the inverter current i a, i b, i c, and the load current i La, i Lb, i Lc. The DC-DC converter
is used to boost/buck the operating voltage to the maximum power point tracking for the PV array
output.
Figure 12: Grid connected inverter circuit
The a,b,c voltage and current of the three phase system measured is converted into dq coordinate axis
according to the theory of instantaneous reactive power and the theory of the Park transform 6. The
relationship of the currents in the two different coordinates is given by the following equation:
idq0 =Ciabc , iabc=C-1 idq0 .
where idq0 =[id, iq, i0 ]T , iabc=[ia, ib, ic]T , and C is the park transform matrix.
𝑐𝑜𝑠𝑡
𝐶 = √2/3
−𝑆𝑖𝑛𝑡
[ 1/√2
2
2
)
𝐶𝑜𝑠(𝑡 + )
3
3
2
2
−𝑆𝑖𝑛(𝑡 − ) −𝑆𝑖𝑛(𝑡 + )
3
3
1/√2
1/√2
]
𝐶𝑜𝑠(𝑡 −
Where,  is the angle frequency of the grid voltage and C -1=CT because C is an orthogonal matrix.
By adjusting the amplitude and the phase of the output voltage of the inverter, the value of the active
and reactive power and their direction can be controlled. When the output voltage of the inverter is
higher than that of the grid, the inverter outputs inductive reactive power and when the output voltage
of the inverter is lower than that of the grid, the inverter outputs the capacitive reactive power 7 .
27
The equivalent circuit and the and the vector diagram of the single phase inverter is shown in figure 13.
In the equivalent circuit, L is the filtering inductor between the grid and the inverter, V 0 is the output
voltage, I is the output current of the inverter, V g is the grid voltage and V l is the inductor voltage.
In the vector diagram, I d and Iq are the active and reactive current output of the inverter, I load is the
current of the load, I q= -Iload and is the angle between V o and V g.
Figure 13: equivalent circuit and phasor diagram of single phase inverter
The vectors x’ represent I, I d, Iq, V l, V g and V 0 when active power output increases and reactive power
remains constant. It can be seen that the amplitude and the lead forward phase angle of the output
voltage increases when active power output is increased and reactive power is kept constant. When the
reactive power is increases and the active power output is kept constant, the amplitude increases but
the lead forward phase angle decreases.
4.3 Impact of reactive power on EV charging
The impact of Reactive power flow in increasing the EV integration limi ts is studied in the same manner
that was used for getting the EV integration limits of the existing grid and reinforced grid. EV’s are
placed progressively at each house from the transformer end of the feeder to get the best possible case
and from the feeder far end to get the worst case scenario.
The EV integration limits are calculated by assuming that the EV charger operates at unity power factor
and consumes only active power. The reactive power is generated by the PV inverter that should have
been idle due to lack of PV generation during the night. The reactive power generated flows opposite to
the direction of active power and reduces the voltage drop in comparison to unidirectional flow of active
power.
4.3.1 Bogfinkevej distribution grid
The Bogfinkevej distribution system as mentioned earlier was built in 1970’s and supplies to 141 houses.
The feeder details are mentioned again for quick reference.
28
It can be seen that the distribution system has a majority of cable sized 50mm 2 which provides the least
voltage variation with change in power factor. Therefore it is a good example to see if reactive power
control can be good solution for integration of electric vehicles .
Table 10: Bogfinkevj grid data
Construction
Transformer
Cables (3-phase)
Distance between the
substation and last house
connected to the feeder
Consumption per year
Nr. of connected houses
Bogfinkevej
1970’s
250kVA
150mm2 : 0.21km
95mm2 : 0.51km
50mm2 : 1.94km
Feeder 1 : 0.41km
Feeder 2 : 0.38km
Feeder3 : 0.38km
Feeder4 : 0.26km
458MWh
141
The existing grid can accommodate 61 (43% of 141 houses) Electric vehicles during peak winter
household demand. This is considering only the feeder capacity to maintain the voltage levels above
.95p.u. and when the transformer loading is considered, the EV charging is limited to 37EV’s or 26% of
141 houses.
Therefore, transformer is the first limiting factor for the bogfinkevej distribution system and the voltage
drop limits the EV penetration.
4.3.1.1 EV integration limits with Reactive power flow
With reactive power control, the EV integration limits are significantly improved for the distribution
system with 3 out of the 4 feeders being able to accommodate 100% of EV charging and the EV
integration limits increases from 31% to 65%.
The PV inverters are limited to operate at 0.8 leading p.f. equivalent of the EV charging active power to
avoid increased loading of the transformer/cables and losses. Generating 0.8 p.f. equivalent of reactive
power would increase the apparent power flowing in the system by 25% (because P=S*cosfi).
Table 11: EV integration limits with reactve power control
Feeder1 : 32 houses
From the transformer
Feeder far end
Feeder 2 : 51 houses
Bogfinkevej distribution system
Unity p.f.
0.95 p.f.
0.9 p.f.
leading
leading
20
24
26
10
12
14
0.85 p.f.
Leading
27
16
0.8 p.f.
Leading
29
21
29
From the transformer
Feeder far end
Feeder3 : 32 houses
From the transformer
Feeder far end
Feeder 4 : 26 houses
From the transformer
Feeder far end
42
15
48
21
50
35
51
51
51
51
20
16
23
18
26
22
29
28
32
32
20
24
26
26
26
26
26
26
26
26
It can be seen that reactive power control for EV charging through PV inverters can be a viable solution
for Bogfinkevej in comparison to traditional grid upgrade. In traditional grid upgrade both the
transformer and the cables need to be replaced to achieve 90% EV penetration whereas through
reactive power control only the transformer needs to be replaced.
The comparison between traditional grid upgrade and reactive power control to increase the EV
penetration level from the present 26% to 90% presented in the table below:
Table 12: Comparison of reactive power control and traditional grid upgrade
40% EV penetration
60% EV penetration
90% Ev penetration
Reactive Power Control
Transformer upgrade: 400kVA
Transformer upgrade: 500kVA
Traditional grid upgrade
Transformer upgrade : 400kVA
Transformer upgrade: 500kVA
Cable upgrade: 340m out of 2.72km
Transformer upgrade: 750kVA
Transformer upgrade:600kVA
Cable upgrade 54m out of 2.72km Cable upgrade: 950m out of 2.72km
4.3.1.2 Cost implication
The use of reactive power for mitigating voltage problems effectively reduces the need for reinforcing
the cables in the distribution system. It can be noticed from the above table that in case of 60 % EV
penetration, 340m of cable need not be reinforced if reactive power is used to mitigate the voltage
problems. The same is true for 90% EV penetration where 900m of cable need not be reinforced.
30
4.3.1.3 Bogfinkevej feeder 1
Bogfinkevej Feeeder 1
Feeder 1 Bogfinkevej
near the transformer
Far end of the transformer
0.99
0.99
0.98
0.98
0.97
0.97
0.96
Voltage
Voltage
0.96
0.95
0.95
0.94
0.94
0.93
0.93
0.92
0.92
0.91
0.91
0
5
10
15
20
25
30
35
0
5
10
Number of Ev's
voltage @ 1
Voltage @ 0.95
Voltage @ .85
Voltage @ .8
15
20
25
30
35
Number of EV's
[email protected] .9
voltage @ 1
Voltage @ 0.95
Voltage @ .85
Voltage @ .8
[email protected] .9
The graphs represent the lowest voltage recorded in the feeder 1 when EV are placed progressively from the transformer end of the
feeder and from the feeder far end. It can be seen that the voltage improves
It can be seen that in both the best case scenario and the worst case scenario that the feeder 1 experiences voltage problems. The
profile can be improved by replacing the 150mm2 cable connecting the transformer and the first house by a cable with a lower R/X
ratio and thicker cable as done in the case of Rindum Mølleby which will be discussed later.
31
4.3.1.4 Bogfinkevej feeder 2
It can be observed from the graphs that 100% EV penetration is possible with PV inverters generating 0.9p.f. equivalent reactive
power for 3.7kW active power. Feeder2 consists of 150mm2 , 95mm2 and 50mm2 cables with a majority of them being 50mm2 cables.
It can also be seen that the lowest voltage with 21 EV’s is lower than the lowest voltage with 40 EV’s due to over-compensation of
reactive power. So, it can be said that in some cases the voltage profile improves with more households charging their EV’s and taking
part in reactive power control.
32
4.3.1.5 Bogfinkevej feeder 3
Feeder 3 can accommodate 100% EV charging without any voltage violations when PV inverters are producing 0.8.pf. equivalent
reactive power for active charging power. Unlike feeder 1 and 2 of Bogfinkevej distribution system, feeder 3 doesn’t have any cable
with a rating of 150mm2 and consists of cable with sizes of 95 mm2 and 50 mm2 . Thus it can be said that for feeders with sufficient
capacity, reactive power control can be an effective solution even when the cables have a higher resistance than reactance.
33
4.3.1.6 Bogfinkevej feeder 4
Feeder 4 is a relatively short and is a high EV charging capacity feeder with the last house connected to the transformer being 0.26km
away and having an initial EV penetration limit of 76%.
All the cables in feeder 4 are of 50mm2 type and can accommodate 100% Ev charging with just 0.95 leading p.f. . It reiterates the
observation from feeder 3 that, for feeders with sufficient capacity, reactive power control can be an effective solution even when the
cables have a higher resistance than reactance
34
4.3.2 Rindum Mølleby Distribution grid
The relatively new distribution grid of Rindum Mølleby was constructed around the year 2000 and its
details are mentioned in the table.
Table 13: rindum grid summary
The Rindum Mølleby distribution grid is
mostly made up of cable of 150mm2 size
and should be better suitable for
reactive power control compared to
Bogfinkevej, but the length of the
feeders in a major hinderance.
Construction
Transformer
Cables (3-phase)
Distance between the
substation and last house
connected to the feeder
Consumption per year
Nr. of connected houses
Rindum Mølleby
2000’s
400kVA
150mm2: 2.55km
95mm2: 0.55km
Feeder1 : 1.04km
Feeder2 : 0.546km
Feeder3 : 0.687km
399 MWh
93
The existing grid can accommodate 22
EV’s before experiencing voltage
problems. When only the transformer
capacity is considered, the distribution system can accommodate an EV penetration limit of 89%.
Thus it can be said that voltage drop due to the long distance of the feeders is the first limitation for the
feeder and then the transformer loading acts as a limitation.
4.3.3 EV integration limits with reactive power flow
The EV integration limits with reactive power flow is not significantly different when compared to the EV
integration limits without reactive power. Feeder 1 and feeder3 show no improvement or ne gligible
improvement due to their long distance between the transformer and the first house.
Feeder 2 which is shortest and has the highest capacity with an existing EV integration limit of 19 EV’s
(73%) can attain 100% EV integration limit with EV’s being charged at an equivalent leading power factor
of 0.9.
Table 14: EV integration limits with reactive power flow
Feeder1 : 34 houses
From the transformer
Feeder far end
Feeder 2 :19 houses
From the transformer
Feeder far end
Feeder3 : 40 houses
From the transformer
Rindum Mølleby distribution system
Unity p.f.
0.95 p.f.
0.9 p.f.
leading
leading
4
4
5
2
2
2
0.85 p.f.
Leading
6
2
0.8 p.f.
Leading
6
2
16
14
18
18
19
19
19
19
19
19
15
16
18
19
20
35
Feeder far end
6
6
7
7
8
4.3.4 Rindum Mølleby distribution grid with upgraded Cables
The existing Rindum Mølleby Distribution grid cannot integrate EV’s in their feeders1 and 3 with reactive
power flow. To improve its response to reactive power, we replace the existing cables between the
transformer and the first house with 400mm 2 cable which not only decrease the voltage drop due to
active power but also has a lower R/X ratio of 0.68 compared to R/X ratio of the existing 150mm 2 cable
which is 2.5.
Table 15: EV integration limits with cable upgraded
Feeder1 : 34 houses
From the transformer
Feeder far end
Feeder 2 :19 houses
From the transformer
Feeder far end
Feeder3 : 40 houses
From the transformer
Feeder far end
Rindum Mølleby modified distribution system
Unity p.f.
0.95 p.f.
0.9 p.f.
0.85 p.f.
leading
leading
Leading
21
26
27
30
8
11
14
17
0.8 p.f.
Leading
34
34
16
14
18
18
19
19
19
19
19
19
35
18
37
27
40
40
40
40
40
40
It can be seen that by replacing the existing cables of length 0.64km in feeder 1 and 0.27km in feeder 3,
we can achieve 100 % EV in both the feeders.
The comparison between traditional grid upgrade and reactive power control to increase the EV
penetration level from the present 23% to 90% presented in the table below:
Table 16: Comparison of reactive power control with traditional grid upgrade
60% EV Penetration
Reactive power Control
Cable upgrade: 0.91 km from
2.9km
No upgrade needed
90% EV Penetration
Transformer upgrade to 600KVA
40% EV Penetration
Traditional grid upgrade
Cable upgrade : 1.7km from 2.9km
Cable upgrade : Complete
replacement of cable with 400mm2
Need complete reconfiguration of
the grid
36
For distribution systems like Rindum Mølleby, where either reactive power or traditional grid upgrade
cannot be a complete solution, a combination of both is a viable solution.
4.3.4.1 Rindum Mølleby distribution grid with reconfigured transformer position
The Rindum Mølleby distribution grid like mentioned before has long cable from the transformer to the
first house. The distance between the transformer and the house is reduced to 1m and simulated using
reactive power flow like it was done in the previous chapter.
Table 17: Rindum EV integration limits with reconfigured transformer position
Feeder1 : 34 houses
From the transformer
Feeder far end
Feeder 2 :19 houses
From the transformer
Feeder far end
Feeder3 : 40 houses
From the transformer
Feeder far end
Rindum Mølleby modified distribution system
Unity p.f.
0.95 p.f.
0.9 p.f.
0.85 p.f.
leading
leading
Leading
24
27
28
28
10
12
14
15
0.8 p.f.
Leading
30
17
19
19
19
19
19
19
19
19
19
19
40
40
40
40
40
40
40
40
40
40
Like observed from the results in the previous chapter, at unity power factor feeder2 & 3 can
accommodate 100% EV penetration but feeder1 can charge 10 EV’s in the worst case. This performance
of the feeder can be improved by using reactive power. When the EV is being charged at an effective
power factor of 0.8 leading, the feeder can accommodate 17 EV’s ie.50% EV penetration.
37
4.3.4.2 Rindum Mølleby feeder 1
This graph compares the combination of solution used for improving the EV penetration limits of the Rindum Mølleby feeder 1. It can
be observed that the feeder lowest voltage when 1 EV is placed is around 0.955 due to the long cable between the transformer and the
first house. This voltage drop is reduced when the cable is either replaced or the transformer is placed 1m away for the first house.
It can also be observed that the voltage profile in case of the cable being upgraded or the transformer being shifted is almost the same.
But when reactive power flows, the voltage improvement in case of cable upgrade is better due to lower R/X ratio of 400mm2 cable.
38
4.3.4.3 Rindum Mølleby feeder 2
It can be seen that feeder 2 of Rindum Mølleby can accommodate 100% EV integration when the EV charger is operating at an
equivalent leading power factor of 0.9.
Unlike feeder 1 and feeder 3, combination of solutions are not required to get 100% EV integration and it again reiterates the
observations made in Bogfinkevej feeders that reactive power control can be viable option on its own when the feeders are short and
have sufficient initial capacity to accommodate EV charging.
39
4.3.4.4 Rindum Mølleby feeder 3
This graph like the Rindum Mølleby feeder 1 graph compares the different combination of solutions for integration of EV’s. Reactive
power control alone cannot support voltage, So additional support in the form of cable upgrade and transformer relocation is
considered.
It is observed that transformer shifting alone can accommodate 100% EV charging unlike in the case of feeder1. With the upgraded
cable the voltage profile improves but needs the help of reactive power to get be able to charge 40 EV simultaneously.
40
4.4 Yearly simulation of Distribution system with high PV and high EV
penetration
4.4.1 Bogfinkevej Distribution grid
The Bogfinkevej distribution system is simulated for a complete year with a 60% penetration of EV’s and
photovoltaic systems. The EV’s charging from the residential connection consumes 3.7kW and the peak
output from the PV system is considered to be 5kW. The distribution system is first simulated with the
existing grid and without reactive power, and the results are then compared with the grid with an
upgraded transformer and with reactive power control.
Voltage support through reactive power control is done by operating the EV charger at an equivalent
power factor of 0.8 leading for feeders 1,2,3 and at 0.95leading for feeder 4 based on their existing
capacity to integrate EV’s.
The distribution system has a cumulative peak PV output of 420kW and it experiences over-voltage’s
when the cumulative PV production from the system exceeds 250kW. In the complete year the net PV
production exceeds 250kW in only 665 hours in a year. This is due to self-consumption and the
simultaneity factor of the systems generating power. During instances of PV production exceeding
250kW, the PV systems are made to operate at 0.8p.f. lagging.
The results show that EV’s and PV systems can be integrated into the system effectively by using
reactive power but, at the cost of higher losses due to the increased flow of reactive power as seen in
table 19. If the system operates at a power factor of 0.8, then the apparent power increases by 25%.
Due to the increase in apparent power, the I 2 R losses increase by 56%.
𝐴𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑝𝑜𝑤𝑒𝑟(𝑆) = √𝐴𝑐𝑡𝑖𝑣𝑒 𝑝𝑜𝑤𝑒𝑟 (𝑃) 2 + 𝑅𝑒𝑎𝑐𝑡𝑖𝑣𝑒 𝑝𝑜𝑤𝑒𝑟(𝑄) 2
𝐴𝑐𝑡𝑖𝑣𝑒𝑝𝑜𝑤𝑒𝑟 (𝑃)
𝐴𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑝𝑜𝑤𝑒𝑟 (𝑆) =
𝐶𝑜𝑠(𝑝𝑜𝑤𝑒𝑟𝑓𝑎𝑐𝑡𝑜𝑟(𝜑))
The table below shows the energy consumed and exchanged through the distribution system, for
calculating the change in losses and efficiency due to reactive power .
Table 18: Energy Exchange between the distribution grid and the consumer
Household demand
Electric Vehicle charging demand
Total PV generation
Self-consumption by houses having PV
Energy taken from the grid
Energy fed into the grid
Energy exchange with the grid
(a)
(b)
(c)
(d)
(e)=(a + b - d)
(f) =(c - d)
(e + f)
MWhr for year 2012
444
473
486
129
788
357
1145
41
Table 19: Losses comparision with different S olutions
Original grid
Losses
Diff. in losses
Efficiency
23.41MWhr
97.9%
Grid with reactive
power flow
35.27MWhr
+50.6%
96.91%
Grid with cable
upgrade
19.60MWhr
-16.2%
98.2%
42
43
44
45
46
4.4.2 Rindum Mølleby Distribution grid
The Rindum Mølleby distribution grid is simulated for an entire year like the Bogfinkevej distribution
system to calculate the losses and compare the solutions. One of the solution is by reconfiguring the
transformer near the feeder and the other solution is by using reactive power control in combination
with upgrading the cable with 400mm 2 cable.
By replacing the cable with a 400mm 2 cable, the voltage profile along the long cable improves and its
response to reactive power increases when compared to 150mm2 cable, as seen in the section 4.1.1.
From the graphs below it can be seen that with the existing cables and transformer position, the lowest
voltage during EV charging is 0.935 p.u. and is improved by using reactive power and transformer
repositioning.
The losses in case of both the solutions are compared in the table below. It can be noticed that the
increase in losses due to reactive power flow is not as significant as in the case of Bogfinkevej
distribution system due to the decreased losses in the upgraded cable. When we compare the losses
with and without reactive power in the grid with upgraded cables, we get 35% increase in losses due to
reactive power, which is comparable to Bogfinkevej distribution system case.
Table 20:Energy Exchange between the distribution grid and the consumer
House hold demand
Electric Vehicle charging demand
Total PV generation
Self-consumption by houses having PV
Energy taken from the grid
Energy fed from the grid
Energy exchanged with the grid
(a)
(b)
(c)
(d)
(e)=(a + b - d)
(f) =(c - d)
(e + f)
MWh for year 2012
399
312
316
110
601
206
807
Table 21: Losses comparision with different S olutions
Original grid
Losses
Diff. in losses
Efficiency
15.004 MWhr
98.14%
Grid with reactive
power flow and
upgraded cables
15.47 MWhr
+ 3.13 %
98.08%
Grid with upgraded
cables
9.78 MWhr
-34.8%
98.7%
47
Original grid
Grid with upgraded cables and reactive power
48
49
4.5 Disscussion
It can be inferred from the above simulations that reactive power can be used to effectively integrate EV/PV’s into
the distribution system and using PV inverters for either generating or absorbing reactive power gives additional use
for the existing capacity in the PV inverters. Reactive power can be effective in supporting the voltage even in LV
systems having Underground cables (with an unfavourable X/R ratio).
The Bogfinkevej distribution system can accommodate 100% EV charging in feeder 4 with .95p.f. leading,and feeder 2
with .85 p.f. leading. Each feeder needs to be assessed for its response to reactive power, but it can be said
that .8p.f. leading for the complete distribution system can be used for effective EV integration irrespective to which
feeder the EV is connected. Reactive power saves on the investment cost for asset upgrade but with increased losses
of 50.6%. At the rate of 8cents per kWh, 15MWh which is the losses for the complete year in the distribution system
with reactive power, would cost 1200 Euros which is less compared to the cable upgrade cost of around
50Euros/meter. Upgrading 400m of cables would cost 20,000 Euros; the cost of upgrading the cables can be
considered to be equal to 16 years of losses. Therefore by using reactive power, the DSO can reduce its immediate
capital costs for grid reinforcement.
The Rindum Mølleby distribution grid like said before has low capacity to accommodate EV charging because of the
long length of its feeders and reactive power alone cannot accommodate 100% EV charging. As an alternative, when
reactive power is used in combination with upgraded cables or repositioned transformer, the distribution grid can
accommodate 100% EV charging. So, in cases when either grid reinforcement or reactive power alone cannot
support 100% EV charging, a combination of both the solution can be used. In this case, we save the costs of
completely reconfiguring the feeder as partial reinforcement with reactive power can solve the voltage issues.
Simulating the Rindum distribution network for the complete year shows reinforcing the grid for 60% EV penetration
would lead to a 34% decrease in losses but at the cost of repositioning the transformer at a new locations along with
extension of the MV cables.
It can be said that by using PV inverters for solving voltage problems due to EV charging, We can save on network
reinforcement cost.
4.6 Conclusion
In conclusion to the above discussion, it can be said that reactive power can be viable option for control of voltage drop/rise due to penetration of EV/PV. Reactive power as a solution for mitigating voltage problems during EV
charging can be an interim solution before the complete distribution syste m needs to be upgraded. It is observed
Bogfinkevej distribution system that network reinforcement is not needed with use of reactive power and in case of
Rindum Mølleby a combination of network reinforcement and reactive power can be an effective solution where
either network reinforcement or reactive power alone cannot support voltage during EV charging. The inverter from
a 5KW PV system which generates energy equivalent to the energy consumption of a EV for a complete year can
effectively provide the reactive energy needed for EV charging.
5
5 Storage
Storage as an effective solution mitigating problems like overloading of assets and maintaining voltage levels in the
distribution system. Its effectiveness is dependent on location, maximum power, capacity and dispatch.
The impact of the storage on the voltage is dependent on the location of storage. Storage can be either distributed or
centralised. Centralised storage can be located at the Sub-station, mid-point of the feeder or at the end of the
feeder. Due to the various possible configurations of branches in a feeder, centralised storage may not be an
optimum solution. Decentralised storage on the other hand directly affects the voltage at the source.
Storage as a solution for mitigating voltage problems in the distribution network is evaluated using PLATOS. PLATOS
(Planning tool for optimised storage) is an in-house software development by DNV GL. The main objective of PLATOS
is to achieve the lowest number of voltage violations and lowest number of asset overload using decentralised
storage.
5.1 PLATOS
PLATOS interacts with Digsilent Power factory for performing the load flow simulation and the results from the
steady state load flow are used for defining the objective function.
The formulation of the objective function is as follows:
fobj = W1*Dtransformers +W2 * Dlines +W3 * Dnodes
Where:
• Wi: Weight of the specific factor. This is specified by the user and its default value is 1.
Allows to give some factors a bigger influence in the optimization process.
• Dtransformers: Transformers load deviation.
Dtransformers =Σ transformers (Load - Limit) ; for all Load > Limit
• Dlines: Lines load deviation.
Dlines =Σ lines(Load - Limit) ; for all Load > Limit
• Dnodes: Nodes voltage deviation.
Dnodes =Σ nodes(Voltage - VoltageUpperLimit) ; for all Voltage > VoltageUpperLimit +
Σ nodes (VoltageLowerLimit - Voltage) ; for all Voltage < VoltageLowerLimit
In the simulation done, the voltage limits are set to +
−0.05𝑝. 𝑢. and the loading limits are set to 100% of the rated
value.
The lowest value of the objective function is achieved by injection of power in the designated locations over time
with restrictions imposed by state of charge of the storage device. The following paragraphs describe the important
stages of optimisation and the simulation logic.
5.1.1
Profile simulation and results analysis
The electrical grid is simulated for power flow with an initial profile for a whole year to get the details of node
voltages, loading of transformers and cables. The results are then compared with the voltage and loading limits to
get the difference in comparison to the limits. The storage is then placed on the nodes with either voltage or loading
problems.
5.1.2 Optimal Storage location
The location of storage is defined by the following statement[30] :
“Optimal storage locations are those nodes where there are voltage problems and those nodes receiving the flow of
current in case of overloads for the time series simulation”
This rule doesn’t consider the various possible locations of centralised storage (i.e. one storage system in the grid
under study), but provides a simple logic for placing dispersed storage.
5.1.3 Optimum power and Energy sizing
This stage gives an estimate of the power required to solve the violation and the capacity o f the battery. This is
performed either injecting or absorbing power from a certain bus, the optimisation is done using a gradient method
in order to get the lowest voltage and load violations. In this stage, no power or energy constrains are imposed on
the storage device.
As a result of this stage, we get the maximum hypothetical energy needed and the maximum power that either
needs to be injected or absorbed.
5.1.4 Optimal storage dispatch
In this stage, we have the location, maximum power and the hypothetical maximum energy required in order to
solve the violations. These results are without any constraints on charging, SOC and losses. To get the final values of
the power and energy requirements and to impose the fore mentioned constraints, this stage tries to keep the
devices charged without causing grid problems.
The storage device are modelled with a control module that calculates the storage losses and the state of charge for
every time step. The program tries to keep the dispatch of power from the battery real -time to resemble an actual
dispatch.
5.1.5 Simulation Logic
The above mentioned stages can be represented in the following flowchart:
A. load flow is performed on the grid with the given loads and generators. The results are stored for
analysis
B. The stored results are analysed to check for violations and the objective function is defined
C. The problematic busses are identified and storage is placed at the particular busses
D. Power is dispatched from the batteries without any constraints to get the lowest possible number of
violations. The optimisation is done using convex optimisation
E. Like in stage A, the load flow is performed while taking into account state of charge and standby
losses
F. If in stage E, the power and state of charge are not satisfied, then the power, energy and state of
charge in the previous step is corrected .
Figure 14: Flow chart of PLATOS
5.2
Bogfinkevej Distribution system.
The bogfinkevej distribution system is simulated with the same profile of 60% EV penetration as used in case of
reactive power and grid reinforcement in order to compare the different solutions. As noticed in the previous
chapters, the bogfinkevej distribution system comparatively has a higher capacity to integrate EV from the voltage
point of view and transformer loading is the weakest link in the distribution system .
After an initial load flow, the problematic busses are identified and storage is placed on them. The figure below
represents the problematic busses and the location of storage.
Figure 15: location of S torage in Bogfinkevej grid
It can be noticed that voltage problems exist only in one feeder and the rest don’t require any support. Transformer
loading is not considered for storage and is solved by upgrading the transformer.
After the location of storage, the dispatch of power with and without constraints are performed and the details of
the storage are presented in the below table :
Table 22: S torage details for bogfinkevej diatribution system
Location
Battery energy
capacity (kWh)
10311
213.5188
Battery
power (kW)
No. of houses
Size per house
(kWh)
2
106.7
56.2
13918
285.9348
28.1
3
95.3
56.2
13919
225.2755
18.73333
3
75.0
56.2
13920
237.2626
18.73333
4
59.3
56.2
14321
195.0208
14322
265.535
Battery power
per house
(kW)
14.05
4
48.7
3
88.5
56.2
14.05
56.2
18.73333
It can be seen that a total of 1422 kWh battery is required to mitigate any voltage problems due to EV and
PV charging. This when compared with other solutions is an expensive option but gives the ease of multiple
functions to support the grid when needed.
In the comparison below, the price of 1kWh battery is taken to be 250 Euro and cable upgrade cost is
assumed to be 50€/m. It can be seen that network reinforcement and reactive power have the same costs over
a period of 17 years and network reinforcement and reactive power are most economical solutions in
comparison to storage.
Table 23: Comparision of solutions for Bogfinkevej Distribution system
Grid: Bogfinkevej
Network Reinforcement
Reactive power flow
Storage
Solution
Cable upgrade : 340 m
out of 2720 m
Transformer upgrade :
500kVA
Transformer Upgrade +
20000€
EV integration with 0.8
p.f. leading
Transformer upgrade:
500kVA
Transformer Upgrade +
yearly 1200€ = 20000€
/17years
Total battery capacity :
1422kWh
Transformer upgrade:
500kVA
Transformer Upgrade +
355500 €
Cost
5.3 Rindum Mølleby distribution grid
Like the previous simulation, The Rindum Mølleby distribution grid is also simulated in PLATOS with a profile with
60% EV penetration for comparison with the network reinforcement and reactive power solutions.
The problematic busses are identified and storage is placed on them. The figure below represents the problematic
busses where storage is placed.
Figure 16: location of storage in Rindum Mølleby grid
It can be noticed that unlike in the bogfinkevej grid, voltage violations exists on multiple feeders and different
branches.
After the next two stages, the battery sizing and location is as follows :
Table 24: S torage details Rindum Mølleby distribution system
Location
15526
Battery Size
(kWh)
80.91232
Power (kW)
27.44
No of
houses
4
Size per House
(kWh)
20.2
Power Per
house (kW)
6.86
15527
73.07047
15528
99.75673
16768
78.24177
17393
79.00622
17757
94.55933
17758
83.30845
17759
52.7395
7556
79.68232
7556
112.5668
27.44
27.44
27.44
27.38
27.44
27.44
22.66
27.44
24.95
4
18.2
4
24.9
3
26.0
2
39.5
3
31.5
3
27.7
7
7.53
2
39.84
1
112.5
6.86
6.86
9.14
13.69
9.14
9.14
3.23
13.72
24.95
For the rindum mølleby distribution system, a storage equivalent of 833kWh is required. The comparison of
the different solution is as follows:
Table 25: Comparision of solutions for Rindum Mølleby distribution system
Grid: Rindum Mølleby
Network Reinforcement
Reactive power flow
Storage
Solution
Cable upgrade : 1.7km
from 2.9km
Cable upgrade: 0.91 km
from 2.9km
Total battery capacity :
833kWh
Cost
85000€
45500€
208250 €
5.4 Conclusion
Storage as a solution for mitigating violations due to EV and PV penetration is effective but expensive. The
voltage problems due to EV charging can be mitigated by using a combined storage of 1433kWh for
Bogfinkevej distribution system and 833kWh for Rindum Mølleby distribution system.It can become a
viable option if the price of batteries decreases to a value of around 100Euros/kWh. Like in the case of the
Rindum Mølleby distribution system where, if the cost of battery is around 100Euros/kWh, the cost of
installation of storage for mitigation of voltage problem is 83,300€ which is comparable to traditional grid
upgrade.
In case of Bogfinkevej distribution grid, Storage is the most expensive option among the solutions for
mitigating voltage problems. However, in combination with other additional functions, like frequency
response or energy trading with corresponding income, installation of storage may become a viable option.
6 Conclusion
The study intends to gauge the impact of EV/PV penetration into the distribution system based upon asset over loading and voltage problems and find the most effective and economical solution to mitigate them. The two
distribution grids studied provide insights into the problems and the possible solutions , but it cannot be generalized
completely to all the distribution systems due to various possible configurations of the distribution grids.
Among the various solutions for asset loading and voltage problems mentioned in the literature, network
reinforcement, reactive power control and storage are the solution of interest when prioritized based on impact on
voltage, impact on asset loading and technology readiness.
Impact of EV/PV on the distribution system
Charging of Electrical vehicles during peak household demand can cause voltage problems in the distribution grid
with even a low EV penetration of 20%, but as stated above it cannot be generalized for all distribution systems. It
points to the fact that EV integration is a significant point of concern in the distribution system.
In case of Bogfinkevej distribution grid, the main limiting factor for EV Penetration is the transformer loading and in
case of Rindum Mølleby the limiting factor is the cable voltage drop due to long length of the feeders.
Network Reinforcement
Network Reinforcement is the most commonly applied solution for mitigating voltage problems due its simplicity and
cost. The study of the two distribution grids shows that reinforcing the network may not be viable solution in some
distribution grids like in the case of Rindum Mølleby distribution grid where complete reconfiguration of the grid is
required for 100% EV charging during the peak household demand.
The results from the EV integration limits for Bogfinkevej conclude that the cables are strong enough to
accommodate 42% of EV penetration but the transformer loading restricts it to 26.2%. Therefore an transformer
upgradation by replacement of 250kVA with 400kVA can accommodate 42% EV penetration till t he grid needs
upgrade. For 40% EV penetration level, no cable upgrade was needed. For 60 % penetration, 340m from the total
cable length of 2.72km has to be upgraded which is 12% of the total cable under use.
950m of cable was upgraded from a total cable length of 2.72km to keep the voltage within limits for a penetration
level of 100%. This can be a possible solution where the feeder is strong as in case of feeder 4 where only 26.4m of
cable had to be changed to accommodate 100 % EV penetration.
Rindum Mølleby transformer of 400kVA can accommodate upto 89% of Ev penetration but the cables restrict it. For
40% EV penetration, a cable upgrade of 1.7km is needed from a total cable length of 2.81km. For 60% EV
penetration, all the cables must be upgraded to 400mm2 from their existing cable ratings and the grid needs to be
reconfigured due to the large lengths of the feeder. Simulation for 100% EV penetration was not considered due to
the length of the feeders and need for complete reconfiguration of the grid.
Reactive power control
The simulations show that reactive power control can effectively control voltage drop in the feeders even with
underground cables of high resistance in comparison to reactance. In cases, where reactive power alone cannot
support EV integration, a combination of network reinforcement and reactive power control can be used like in the
case of Rindum Mølleby distribution grid.
Reactive power control for EV charging through PV inverters can be a viable solution for Bogfinkevej in comparison to
traditional grid upgrade. In traditional grid upgrade both the transformer and the cables need to be replaced to
achieve 90% EV penetration whereas through reactive power control only the transformer needs to be replaced.
Reactive power control comes at the cost of additional losses which (over the course of the years) are comparable to
the cost of network reinforcement like in the case of Bogfinkevej distribution system.
Network reinforcement or reactive power control alone cannot integrate 100% of EV penetration in case of Rindum
Mølleby distribution system. A combination of both the solutions is needed for effective integration of electric
vehicles.
Storage
Storage as a solution for mitigating voltage problems in the grid is effective as the power is delivered at the node
facing the voltage problems and doesn’t overload the cables as in the case of reactive power. The voltage problems
due to EV charging can be mitigated by using a combined storage of 1433kWh for Bogfinkevej distribution system
and 833kWh for Rindum Mølleby distribution system. It can become a viable option if the price of batteries
decreases to a value of around 100Euro/kWh, like in the case of the Rindum Mølleby distribution system where if the
cost of battery is around 100Euro/kWh, the cost of installation of storage for mitigation of voltage problem is
83,300€ which is comparable to traditional grid upgrade.
In case of Bogfinkevej distribution grid, Storage is the most expensive option among the solutions for mitigating
voltage problems but in combination with other additional functions, like frequency response or energy trading with
corresponding income, installation of storage may become a viable option.
In conclusion it can be said that each distribution grid needs to be studied separately to get the best suited solution
and no solution can be termed as the panacea for integrating EV/PV into the distribution system.
Annex
Annex A : Details of network reinforcement for 100% EV penetration in Bogfinkevej
distribution system.
The aim of the cable upgrade is to get an estimate on the cable length that need to be upgraded to be able to
accommodate EV charging at every house simultaneously.
To simulate the distribution grid (Bogfinekev,1970) response, We select the a time instant when the house
hold demand is at its peak on a winter day (02.01.2012 18:00). The EV charging load is imposed on the
household demand and the voltage profile is plotted.
Based on the voltage drop observed, the cables with relatively larger voltage drop is upgraded and again
simulated until the voltage drop is above 0.95 p.u.
0.959km of cable was upgraded from a total cable length of 2.72km to keep the voltage within limits. This
can be a possible solution where the feeder is strong as in case of feeder 4 where only 0.02644km of cable
had to changed.
The details of the cable changed are :
Cable Details
Line Details
line_8694_LV_732_LV_61
line_8693_LV_732_LV_60
line_8692_LV_13917_LV_59
line_8691_LV_8690_LV_58
line_8690_LV_8689_LV_57
line_8690_LV_14323_LV_56
line_8689_LV_14324_LV_55
line_8688_LV_8691_LV_54
line_8687_LV_8691_LV_53
line_8687_LV_14330_LV_52
line_8686_LV_14331_LV_51
line_8686_LV_14329_LV_50
line_8685_LV_14332_LV_49
line_8685_LV_13148_LV_48
line_8684_LV_14340_LV_47
line_8683_LV_8682_LV_46
line_8683_LV_14384_LV_45
line_8682_LV_8684_LV_44
line_8682_LV_13141_LV_43
line_8681_LV_8683_LV_42
line_8681_LV_14385_LV_41
Cable type
NAYY4x050
NAYY4x095
NAYY4x095
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x150
NAYY4x095
NAYY4x050
NAYY4x050
NAYY4x095
NAYY4x095
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
Upgrade
NAYY4x150
NAYY4x240
NAYY4x150
NAYY4x400
NAYY4x185
NAYY4x185
NAYY4x150
NAYY4x150
Cable length
(km)
0.02644
0.14461
0.04475
0.03895
0.02663
0.04652
0.034
0.03698
0.01969
0.046
0.04971
0.03738
0.02691
0.03012
0.00958
0.0642
0.0392
0.06178
0.04462
0.06053
0.04456
feeder
4
3
3
2
2
2
2
1
line_732_LV_8688_LV_40
line_732_LV_14383_LV_39
line_14387_LV_8681_LV_38
line_14386_LV_8681_LV_37
line_14383_LV_8682_LV_36
line_14381_LV_8684_LV_35
line_14340_LV_14382_LV_34
line_14338_LV_14339_LV_33
line_14337_LV_14338_LV_32
line_14337_LV_14336_LV_31
line_14336_LV_14334_LV_30
line_14334_LV_8685_LV_29
line_14334_LV_14335_LV_28
line_14332_LV_8686_LV_27
line_14329_LV_8687_LV_26
line_14328_LV_14327_LV_25
line_14327_LV_8688_LV_24
line_14325_LV_8689_LV_23
line_14324_LV_14326_LV_22
line_14322_LV_14321_LV_21
line_14321_LV_10311_LV_20
line_13920_LV_13919_LV_19
line_13919_LV_13918_LV_18
line_13918_LV_8692_LV_17
line_13917_LV_8693_LV_16
line_13916_LV_13915_LV_15
line_13915_LV_8693_LV_14
line_13914_LV_13913_LV_13
line_13913_LV_13912_LV_12
line_13912_LV_8693_LV_11
line_13911_LV_13910_LV_10
line_13910_LV_13909_LV_9
line_13909_LV_8694_LV_8
line_13908_LV_13907_LV_7
line_13907_LV_13906_LV_6
line_13906_LV_13904_LV_5
line_13904_LV_8694_LV_4
line_13904_LV_13905_LV_3
line_13148_LV_14333_LV_2
line_12681_LV_8683_LV_1
line_10311_LV_8692_LV_0
NAYY4x150
NAYY4x150
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x095
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x095
NAYY4x095
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x095
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x095
NAYY4x050
NAYY4x050
NAYY4x095
NAYY4x095
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x050
NAYY4x400
NAYY4x240
NAYY4x150
NAYY4x95
NAYY4x95
NAYY4x95
NAYY4x150
NAYY4x185
NAYY4x150
0.0287
0.21232
0.03001
0.03613
0.03445
0.03711
0.04076
0.01731
0.03835
0.02722
0.04055
0.04989
0.02871
0.02992
0.02974
0.03557
0.02947
0.03691
0.04261
0.0177
0.03081
0.02596
0.03413
0.01772
0.07686
0.03402
0.07138
0.03058
0.02901
0.03283
0.0804
0.05472
0.10152
0.05435
0.03387
0.05763
0.02252
0.03825
0.02218
0.03728
0.06707
2
1
1
2
2
2
2
2
3
The voltage profile before cable upgrade for feeder 1 with 32 EV’s ( 100 % penetration of EV).
Figure 17:voltage profile before cable upgrade for feeder 1 with 32 EV’s
The voltage profile after cable upgrade for feeder 1 with 32 EV’s ( 100% penetration of EV).
Figure 18:voltage profile after cable upgrade for feeder 1 with 32 EV’s
The voltage profile before cable upgrade for feeder 2 with 51 EV’s ( 100 % penetration of EV).
Figure 19: voltage profile before cable upgrade for feeder 2 with 51 EV’s
The voltage profile after cable upgrade for feeder 2 with 51 EV’s ( 100 % penetration of EV).
Figure 20:voltage profile after cable upgrade for feeder 2 with 51 EV’s
The voltage profile before cable upgrade for feeder 4 with 26 EV’s ( 100 % penetration of EV).
Figure 21: voltage profile before cable upgrade for feeder 4 with 26 EV’s
The voltage profile after cable upgrade for feeder 4 with 26 EV’s ( 100 % penetration of EV).
Figure 22:voltage profile after cable upgrade for feeder 4 with 26 EV’s
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