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. 𝑐𝑜𝑠𝑡 𝐶 = √2/3 −𝑆𝑖𝑛𝑡 [ 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). 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