Performance of Grid Connected Photovoltaic

Performance of Grid Connected Photovoltaic
1
Performance of Grid Connected Photovoltaic Inverter with Maximum Power Point
Tracker and Power Factor Control
S. Mekhilef, M.E, Ahmed, and M. A. A. Younis
Dept. of Electrical Engineering, University of Malaya
50603 Kula Lumpur, Malaysia
ABSTRACT
Detailed analysis, simulation and hardware results of grid
connected inverter with maximum power point tracker and
power factor control in Malaysian climate are presented. A
six-switch topology inverter with symmetrical Pulse Width
Modulation (PWM) switching technique is used. A low pass
filter is incorporated in the circuit to filter out unwanted
harmonics and produce a sinusoidal AC current. Low total
harmonic current distortion at the inverter output can be
achieved. The three-phase PWM switching pattern was
developed using Xilinx FPGA. The developed system is also
capable of adjusting the power factor to unity. A 3kVA power
transformer with a 1:2 ratio is constructed to provide galvanic
isolation for better circuit performance and circuit protection.
Hardware results for proposed solar photovoltaic inverter
configuration interconnected to the grid are also presented.
From the experimental results it is confirmed that the
harmonic distortion of the inverter output waveform is within
the limits laid down by the utility companies.
Index Terms—grid connected inverter, maximum power point
tracker, FPGA, and photovoltaic systems
Malaysia is located almost on the equator and is blessed
with an abundance of sunlight almost all year round So
obviously, with the right planning and strategies that are
coupled to the right technology and development in the
market, the potential for photovoltaic system as an alternative
source of power in this country looks promising and is
constantly gaining ground and popularity. To harvest the vast
solar energy especially in tropical country like Malaysia, it
would be desirable if the energy conversion units are simple,
reliable, low cost and high efficiency. High efficiency can be
achieved by the use of all the power generated for the unit and
even contribute to the gird while the energy is not used. The
output power induced in the photovoltaic modules is
influenced by an intensity of solar cell radiation, temperature
of the solar cells and so on. Therefore, to maximize the
efficiency of the renewable energy system, it is necessary to
track the maximum power point of the input source [6-14].
The prototype of the three-phase grid connected inverter
with maximum power point tracker control and power factor
correction was tested with and without the MPPT. The tests
were carried out at a power greater than 3kW. The current
performance proved to be satisfactory and complies with
IEEE recommended practice on utility interface of PV system,
IEEE Std 929-2000 and UL1741-1999.
1. INTRODUCTION
In recent years, the increase of energy demand and the
problems of fossil-fuel sources due to their environmental
pollution and future shortages, have led to the development of
technologies needed to use nonpolluting alternative energy
sources such as solar and wind sources [1-4]
It is beyond doubt that especially the high development of
the power electronics has made the energy produced by the
above alternative sources accessible and at the same time also
at low cost [3-8]. Moreover, it has allowed the spreading of
the Distributed Generation (DG), consisting in the use of a
great number of small and medium generation systems
connected to the distribution grid to feed a dedicated
consumer or to be support of the grid itself [9-10]
Power supply reliability and power quality have become
important issues for all kind of power electronics systems
including
photovoltaic
systems.
Interconnecting
a
photovoltaic system with utility, it is necessary that the PV
system should meet the harmonic standard and the active
power supply requirement. Several utility connected
photovoltaic systems have been proposed [13-15]. Among
these systems, the most common type is the parallel running
PV system with bi-directional power flow to provide unity
power factor on the utility line.[16-17]
2. THE PROPOSED SYSTEM TOPOLOGY
The first significant decision that an inverter designer must
make is the choice of an overall circuit topology. The PV
array voltage and utility grid interconnect voltage drive the
topology selection [9-12]. There can be wide DC input voltage
variations resulting from various combinations of array power,
temperature and module configuration. The primary topology
consideration is whether or not to use a DC-to-DC converter
stage between the PV array and the DC to AC inverter block
to pre-regulate the DC voltage. The inverter with DC-to-DC
converter stage will operate over a wider DC input range but
with a cost premium and lower conversion efficiencies at
some operating points. The topology that provides the best
energy yield under a given set of operating conditions will be
determined by the remaining system components [15-18].
The overall circuit of the proposed three-phase grid
connected inverter system consists of a solar array, DC-DC
boost converter acting as maximum power point tracker,
power conditioning unit, a low pass LC filter, a high power
transformer, a phase-locked loop circuit, Xilinx FPGA as a
main PWM generator that is capable of generating high
frequency PWM as well as controllable displacement factor,
and a personal computer as shown in figure 1.
CCECE/CCGEI May 5-7 2008 Niagara Falls. Canada
978-1-4244-1643-1/08/$25.00 ” 2008 IEEE
001129
2
Solar
Array
Three Phase
Inverter
DC-DC
Converter
Low Pass
Filter
ADC
Magnetic
Switch
Three Phase
Power Transformer
Load
Phase
Lock
Loop
Isolation
&
Driver
Utility
PWM
Generator
(Xilinx)
Isolation
Amplifier
Personal Computer
Fig. 1: Overall block diagram of three-phase grid connected inverter
A six power switching device bridge topology has been
chosen to form a controlled bridge that simplified the circuit
since fewer power switching devices are involved. High
frequency three-phase PWM with less noise interference is
generated using Xilinx FPGA.
The closed loop system is formed by the feedback of the
output voltage to a personal computer via an isolation
amplifier. A program was developed using Genie software is
used as a main feedback controller of the system that
processes the reference input and the feedback signal to
produce the required modulation index to the DC-DC
converter and also the power-conditioning unit. The
modulation index is then fed into the PWM generator circuit
that consists of a FPGA and phase-locked loop. The pulses
produced from the generator unit are used to drive the power
switching devices via isolated driver circuits.
A. PWM Pattern for Three-Phase Inverter
The developed PWM is based on symmetrical sampling.
The triangular carrier signal and the sinusoidal modulating
signal are used. The principle of PWM generation is shown in
figure 2.
(b)
Fig. 2: (a) Principle of PWM generation, (b)Three 600 sine waveforms
portions
Each phase is been divided into six 60 segments; the PWM
is generated for the first 60 only by storing sixty samples of
red phase (A) and sixty samples of blue phase (B) in a look-up
table and the yellow phase (C) segment is derived by the
addition of red and blue phases to form a three-phase
modulating signal as shown in figure 2-b. This technique
reduces the usage of Configurable Logic Blocks (CLB) in the
Xilinx FPGA, and the memory requirements. The decoding of
the look-up table to form the full wave of the three phase
modulating signals is shown in table I.
TABLE I
DECODING OF LOOK-UP TABLE
(a)
Angle ()
Red phase
0-60
60-120
120-180
180-240
240-300
300-360
a
c
b
a
c
b
Blue phase
c
b
a
c
b
a
Yellow phase
b
a
c
b
a
c
B. Inverter Circuit Configuration
The proposed main circuit comprises of a capacitor, DCDC boost converter, three phase inverter with six power
001130
3
IGBTs (S1 to S6) types BUP314, six fast switching diodes
(D1 to D6) type BYP101, a low pass LCR filter, a three phase
power transformer, load, and utility supply as shown in figure
3. The low pass filter absorbs the high order of harmonic
components produced by the PWM switching in the inverter
and produces almost sinusoidal current at the output of the
inverter within the limits of the power supply utility. The
three-phase power transformer is not only meant to step-up
the voltage level but also to provide ohmic isolation for better
protection.
Solar Arrays
ipv
DC/DC Converter
Ld
Power Inverter
is
Dd
S1
D1
S3
D3
S5
determines the angle of current lag in the system i.e. lagging
power factor. When a negative cycle occurs, the counter
remains at the reset state. Similarly, during the negative cycle
another counter operates. However, this condition will
determine the angle of lead of the current in the system i.e.
leading power factor.
Shifting of the PWM patterns is carried out by the delay or
advance of the reset signal. The reset signal is connected to all
the re-settable modules. A positive triggering edge of the
positive and negative cycle is used as a reference by the reset
signal. Advancing or delaying the reset signal by the external
command will force the current in the main circuit to lead or
lag the supply voltage. The principle operation of the reset
unit is shown in figure 5.
D5
IA
Cd
Vpv
Cs
Sd
A
Vs
IB
IC
B
C
S4
Utility
Vr
Vy
Vb
Magnetic
switch
ir
ia1
ib2
iy
ib1
ic2
ib
ic1
i2
S6
Transformer
Load
ia2
i3
D4
i1
D6
S2
D2
Low pass filter
iL1
L1
iL2
L2
iL3
L3
r
r
r
c
c
c
Fig. 3: Schematic diagram of the proposed three-phase grid connected inverter
The operation of the three phase power inverter can be
divided into six modes, termed Mode I, Mode 2, Mode 3,
Mode 4, Mode 5, and Mode 6 as shown in table II. In this
analysis, the inverter is connected to the Y-connected resistive
load and the transformer core is unsaturated.
Mode
TABLE II
MODES OF OPERATION
Phase angle ()
S1
S3
S5
S2
S6
S4
Mode1
Mode2
Mode3
Mode4
Mode5
Mode6
0 /3
/3 2/3
2 /3 4/3
4/3 5 /3
5/3 2
On
On
On
-
On
On
On
On
On
On
On
On
On
-
On
On
On
-
On
On
On
Fig. 5: Principle of operation of the reset unit
D.Maximum Power Point Tracker
1) Energy production from the system
The total energy production by the PV system daily for five
months has been collected; the amount of energy generated by
the PV system is changing from one day to another this is due
to weather changes i.e. during cloudy and raining day. The
average monthly energy production by the PV system is
shown in figure 6. During April the PV system generates the
highest level of energy for the sun.
C.Power Factor Correction Unit
The power factor correction unit consists of two eight-bit
counters, two eight-bit comparators, a VHDL code block, and
a few logic gates are used to form a reset unit. This unit is
clocked by the signal derived from carrier unit. During the
positive cycle, one counter starts counting from 0 to 180.
During the counting process, the comparator compares the
counter value with the external input data. If the values are
equal, it produces a pulse output signal. This signal
Energy (kWh)
120
100
80
60
40
20
0
1
2
3
4
5
M o n th s
Fig. 6: Monthly energy production
001131
The output current of the PV system has been captured for a
4
period of four days from 7:15 in the morning till 19:30 in the
evening it can be seen from the plots that the peaks values
occurs between 13:00 and 16:30 as shown in figure 7.
time. The input (in parallel with PV) and output capacitors
values are 4700f and 470F, respectively. The input inductor
value is 10mH and is wound on ferrite core with 1mm air gap.
The system efficiency is defined as:
K
Po
Pin
Po
Po Pd
Where Pin and Po are the DC-DC converter input and output
power, respectively, while Pd is the power loss. The power
loss consists of the IGBP and diode conduction and switching
losses, the inductor core and copper losses and the control
system power consumption.
The theoretical values were calculated using data given by
the manufacturers of the circuit elements. The theoretical and
measured efficiency for various output power levels is shown
in figure 9. It is seen that the efficiency is quite high
approximately 90% and relatively constant for a wide output
power range.
Fig. 7: PV output current
0.95
2)MPPT Implementation
Theoretical
0.9
Efficiency (%)
The MPP tracking process is shown in figure 8. The starting
points vary, depending on the atmospheric condition, while
the modulation index is changed continuously, resulting in the
system steady-state operation around the maximum power
point. The proposed MPPT was implemented in two different
stages. The first stage senses the output voltage and current,
digitise them using the ADC .The digital values are then used
by the Genie software to generate the modulation index which
is used as input to Xilinx FPGA to generate the required
PWM in the second stage.
0.85
Measured
0.8
0.75
0.7
0.65
0.6
500
PV Output
Power
Steady state
operation
1000
1500
2000
2500
3000
Power (W)
Fig. 9: System efficiency under PV MPPT conditions at 25 0C
Pmax
Possible
starting points
Time
Fig. 8: MPP tracking process
A prototype MPPT system has been developed using the
above-described method and was tested in the laboratory. The
PV array used with this system consists of 42 SP75 Siemens
modules, producing a 3.15kW maximum power at an
irradiation level of 1kW/m2 and temperature of 250C. In order
to test the proposed system under various atmospheric
conditions, the PV array was first simulated with a DC power
supply by adjusting its output voltage and current limit
settings. The power switch consists of one IGBT rated at
600V, 50A, while the diode has a 200ns reverse-recovery
The actual PV output power and corresponding theoretical
maximum output power for various irradiation levels is shown
in figure 10(a). It is seen that the proposed system always
tracks the PV maximum power point. Figure 10(b) shows the
PV output power for various irradiation levels, with the MPPT
control disconnected and with the DC-DC converter
modulation index set such that the PV array produces the
maximum power at each at 1 kW/m2 at 250C. The theoretical
maximum PV power at each irradiation level is also indicated
in the figure. A comparison between figure 10 (a) and (b)
shows that the use of the proposed MPPT control system
increases the PV output power by as much as 18% for the
irradiation in the range of 0.2-0.75kW/m2.
Since both the sun irradiation and the air temperature
change slowly during the daytime, the system is expected to
track effectively the PV maximum power point, under normal
changes of atmospheric conditions (e.g., cloud shadowing).
This has been also verified experimentally by partially
covering the PV modules and noting that the system tracked
successfully.
001132
5
the voltage and has an inductance of 8mH, with 1:2 ratio. A
pure resistive load of 300 ohm is connected in star
configuration. The AC side filter is a passive low pass LCR
filter connected in delta configuration.
The low order unwanted harmonics can be filtered out by
connecting a low pass filter at the output inverter as shown in
figure 12.
PV Output Power (W)
3000
Theoretical Power
2500
Measured Power
2000
1500
1000
!
500
0
0
0.25
0.50
0.75
1
!
2
Irradiation (kW/ m )
(a)
3000
2500
2000
(a)
1500
1
1000
PV Output Power
500
0
x: 5ms/div, y: 5A/div
Maximum Power
0
0.25
0.50
0.75
Current 5A/div
PV Output Power (W)
!
1
Irradiation (kW/m2)
2
(b)
Fig. 11: (a) Theoretical and measured PV output power under MPPT
conditions at various irradiation levels and (b) the actual PV output power and
the corresponding theoretical maximum PV power at various irradiation levels
(PV maximum power at 1 kW/m2 at 250C)
3
0
20
40
60
80
100
Time 10ms/div
3. EXPERIMENTAL RESULTS
(b)
A prototype model of a three-phase grid connected inverter
was constructed and tested to compare the performance of the
inverter with the Pspice® simulated results. The block
diagram of the experimental set-up is shown in figure 1.
A window based graphical user interface software was
developed using the Genie software program where the
information on the output voltage, displacement angle and
modulation index are displayed on the PC screen. The changes
on the output voltage are also displayed on-line. The
displacement angle and modulation index can be easily
change by clicking the scroll icon provided on the screen. The
AC output voltage, and DC input voltage are read on-line by
the PC via a voltage isolation amplifier and display the value
on the screen. A general purpose data acquisition card PCL816 is slotted in the PC to interface with the external signals.
The system is also capable of shifting the PWM signal that
will consequently change the power factor. The isolation
transformer is connected in Delta/Wye connection to step up
Fig.12: Inverter output current (a) before filter (b) after filter
It can be seen clearly from table III that the 3rd to 9th are
lower than 4.0%, 11th to 15th are lower than 2.0% the and
THD is 4.2% which comply with IEEE Std 519-1992.
TABLE III
DISTORTION LEVEL OF THE OUTPUT CURRENT
5th
7th
9th
11th
13th
Harmonics
3rd
15th
Distortion L
1.1%
3.2%
3.1%
3.7%
3.5%
1.8%
1.5%
Power factor adjustment is demonstrated in figure 13(a) for
leading, figure 13(b) for lagging, The inverter could be forced
to operate at unity power at any load condition. The power
factor adjustment data is entered via personal computer using
Genie software.
001133
6
T
Displacement angle
3.
Volt, A (5V, 5A /div)
Voltage
Current
4.
!
5.
6.
7.
time, mS
(a)
8.
T
Displacement angle
Volt, A (5V, 5A /div)
Voltage
Current
!
time, mS
(b)
Fig. 13: Effect of shifting PWM pattern on operating power factor
4. CONCLUSION
The prototype of the three-phase grid connected inverter
with maximum power point tracker control and power factor
correction was tested with and without the MPPT. The tests
were carried out at a power greater than 3kW. The current
performance proved to be satisfactory and complies with
IEEE recommended practice on utility interface of PV system.
The PV array output power delivered to the inverter can be
maximized using MPPT control system, which consist of a
power conditioner to interface the PV output to the inverter,
and a control unit, which drives the power conditioner such
that it extracts the maximum power from the PV array. Low
cost and low power consumption MPPT system has been
developed and tested. Experimental results show that the use
of the proposed MPPT control increases the PV output power
by as much as 18%.
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001134
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