Interline photovoltaic (I-PV) power plants for voltage
unbalance compensation
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Citation
Moawwad, Ahmed, Vinod Khadkikar, and James L. Kirtley.
Interline Photovoltaic (I-PV) Power Plants for Voltage Unbalance
Compensation. In IECON 2012 - 38th Annual Conference on
IEEE Industrial Electronics Society, 5330-5334. Institute of
Electrical and Electronics Engineers, 2012.
As Published
http://dx.doi.org/10.1109/IECON.2012.6388961
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Institute of Electrical and Electronics Engineers (IEEE)
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Author's final manuscript
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Wed May 25 22:05:27 EDT 2016
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http://hdl.handle.net/1721.1/79674
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1
Interline Photovoltaic (I-PV) Power Plants For
Voltage Unbalance Compensation
Ahmed Moawwad, Student Member, IEEE, Vinod Khadkikar, Member, IEEE and James L. Kirtley,
Jr., Fellow, IEEE
Abstract------ This paper proposes a stationary-frame control
method for voltage unbalance compensation using Interline
Photovoltaic (I-PV) power system. I-PV power systems are
controlled to compensate voltage unbalance autonomously. The
control system of I-PV plants mainly consists of active and
reactive power droop controllers, voltage and current controllers
and unbalance compensator. This approach is based on injecting
negative sequence current from I-PV power plants to compensate
for the unbalanced loads. Consequently, the Point of Common
Coupling (PCC) voltage remains balanced and regulated. The
results obtained from Matlab/Simulink based simulation
demonstrate the effectiveness of the proposed method in
compensation of voltage unbalance at PCC.
Index Terms—Flexible AC transmission system (FACTS),
interline power system, active and reactive power control, voltage
regulation, photovoltaic power generation and control,
Distributed Generation (DG).
I. INTRODUCTION
Unbalanced voltages can result in adverse effects on
equipment and power system. Under unbalanced conditions,
the power system will incur more losses and be less stable.
Also, voltage unbalance has some negative impacts on
equipment such as induction motors, power electronic
converters and adjustable speed drives (ASDs). Thus, the
International
Electro
technical
Commission
(IEC)
recommends the limit of 2% for voltage unbalance in
electrical systems [1]. A major cause of voltage unbalance is
the connection of unbalanced loads (mainly, single-phase
loads connection between two phases or between one phase
and the neutral). Compensation of voltage unbalance is usually
done using series active power filter through injection of
negative sequence voltage in series with the distribution line
[2-4]. Some other works depend on shunt compensations [59].
On the other hand, it is well-known that the Distributed
Generators (DGs) often consist of a prime mover connected
through an interface converter (e.g. an inverter in the case of
A. Moawwad and V. Khadkikar are with the Electric Power Engineering
Program, Masdar Institute, Abu Dhabi, UAE (E-mails: azidan@masdar.ac.ae
and vkhadkikar@masdar.ac.ae).
J. L. Kirtley, Jr. is with the Massachusetts Institute of Technology,
Cambridge, MA 02139 USA (E-mail: kirtley@MIT.EDU).
This work is supported by Masdar Institute under MIT-Masdar Institute
joint research project grant.
dc/ac conversion) to the ac power distribution system. The
distribution system may be the utility grid or the local grid
formed by a cluster of DGs. The main role of DG inverter is to
adjust output voltage phase angle and amplitude in order to
control the active and reactive power injection. In addition,
compensation of power quality problems can be achieved
through proper control strategies [10], [11]. In [12]-[15], some
approaches are presented to use the DG for voltage unbalance
compensation.
The control method presented in [12] and [13] is based on
using a two-inverter structure one connected in shunt and the
other in series with the grid, like a series-parallel active power
filter. The main role of the shunt inverter is to control active
and reactive power flow, while the series inverter balances the
line currents and the voltages at sensitive load terminals, in
spite of unbalanced grid voltage. This is done by injecting
negative sequence voltage.
A new system configuration for large-scale Photovoltaic
(PV) power system with multi-line transmission/distribution
networks is presented in [16]. In this approach, the PV power
plant is reconfigured in a way that two adjacent power system
networks/ feeders can be interconnected. The inverter modules
in a PV power plant are configured such that the system is
represented as a back to back inverter connected multi-line
system, called as Interline-PV (I-PV) system. The I-PV system
then can be controlled adequately allowing the PV solar plant
to function as a flexible AC transmission system (FACTS)
device, such as, interline power flow controller (IPFC). With
the I-PV system both active and reactive power flow control
and energy management in a multi-line system can be
achieved. Additionally the system can have various
applications, for example, to regulate the feeder voltages, load
reactive power support, real power transfer from over power
generation line to under loaded line, improve the overall
system performance against dynamic disturbances (such as,
power system damping) and so on.
This paper deals with a three-phase balancing method
utilizing the capabilities of the I-PV power system. By the
proposed method, PV converters output additional negative
sequence component of current to reduce unbalanced load
currents injected by the grid. As a result, the voltage
unbalance that is caused by the unbalanced load can be
suppressed. The proposed method is confirmed by some
simulation studies carried out by Matlab/Simulink software.
2
II. I-PV POWER SYSTEM CONFIGURATION
Large scale PV power plants are usually connected at one
feeder. When two distribution feeders are located close to each
other, the PV plant inverters can be reconfigured in such a
way that the two feeders could be interconnected with each
other. This configuration is addressed as Interline-PV (I-PV)
system [16] and is shown in Fig. 1. The two inverters are
connected back to back through switch SD3. To operate the PV
plant as I-PV system, switches SA, SB1, SA1, SD1, SA2, SB, SD2
and SD3 are closed, while switch SB2 is kept open. During
night-time when the PV solar plant does not produce real
power, switches SD1 and SD2 can be opened.
SC
SB
Zeq1
Fig. 2 Positive and negative sequences extraction.
SD2
SB2
iS2
−
T-2
Inv-2
VS2
SA
L11
SD3
SB1
SA1
L21
+
Vpcc1
SA2
+
iS1
VS1
PCC
PCC
−
T-1
Inv-1
SD1
L12
L22
Photovoltaic (PV) Solar Power Station
Large Scale
Load 1
Feeder-1
Vpcc 2
Fig. 1 Interline-PV (I-PV) power plant system configuration.
III. CONTROL SYSTEM FOR VOLTAGE UNBALANCE
MITIGATION FUNCTION
In this section, the control algorithm for PCC voltage
unbalance compensation is discussed. The idea of this
controller is to split the main controller of the PV converters
into positive sequence and negative sequence controllers.
A. Positive and negative sequence extractions:
Fig. 2 shows the structure used to extract the positive and
negative sequence components. The positive sequence
extraction is established on the dq frame theory. It converts the
unbalanced 3-ph quantities (voltage and current) into dc
positive sequence components according to the following
equation:
sin
sin
cos
cos
cos
sin
sin
sin
cos
cos
cos
(2)
Load 2
Feeder-2
sin
The negative sequence is a balanced set of three phases
quantities that rotates in the opposite direction of the positive
sequence component. Thus, when the positive sequence
component rotates in abc direction, the negative sequence
component rotates in acb direction. Hence, the negative
sequence component of the voltage or current is extracted
according to the following equation:
B. Positive and negative sequence controllers
Fig. 3 shows the positive and negative sequence controllers
for I-PV power plants. The positive sequence controller is
*
+
used to inject the reference active power P inv1,dq0 from solar
*
+
cells into the grid and to regulate the PCC voltage V pcc1,dq0
simultaneously as shown in Fig. 3.
The negative sequence controller is used to inject the
negative sequence current Iload1,dq0 required by the unbalanced
load using PV converters. Thus, cancelling the negative
sequence current absorbed from the grid and maintaining the
PCC voltage balanced.
(1)
Where x donates either voltage or current quantities, for example:
Vpcc1,abc , Iinv1,abc or Iload1,abc as shown in Fig. 2.
Fig. 3 Positive and negative sequence controllers.
3
cos
1
sin
cos
1
sin
cos
1
sin
cos
(3)
1
sin
cos
1
sin
cos
1
(4)
IV. SIMULATION STUDY
In this section, a simulation study based on the given I-PV
power system and its control to compensate the unbalanced
PCC voltage is discussed.
+
1
0
-1
0.46
0.48
0.5
0.52
time (sec)
0.54
0.56
0.54
0.56
0.54
0.56
0.54
0.56
0.54
0.56
(a)
1
0
-1
0.46
0.48
0.5
0.52
time (sec)
(b)
Supply 3-ph current (pu)
A. System under consideration
Fig. 4 shows the power distribution network that is used for
the simulation study. The system consists of two feeders,
while the study is performed on just one feeder (i.e. feeder-1).
The voltages of the two feeders are considered as 11 kV. The
loads on the feeders are normalized as PQ loads, located at the
ends of each feeder. The loads have different values on each
feeder and are programmed to emulate balanced and
unbalanced conditions. The simulation results are expressed in
per unit (pu), with base voltage of 11 kV and base MVA of 1.
Appendix-I contains the detailed data for the system under
simulation for balanced and unbalanced conditions.
PCC 3-ph voltage (pu)
sin
The PCC three phase voltage waveforms are shown in Fig.
5 (a). It is noticed that the waveforms of the voltages are
unbalanced in magnitude (whithout cmpensation). When the IPV system Inv-1 is controlled to compensate, this unbalance
in the PCC voltages is mittigated ahcieveing a balance set of
PCC voltages. The load voltage waveforms (Fig. 5 (b)) are
similar to the PCC voltage, due to the small voltage drop on
the impedance between the two buses.
When unbalnced loads are connected without any
compensation applied, the unbalanced currents of the loads are
supplied by the grid as shown in Fig. 5 (c). At time = 0.5 sec,
it is noticed that grid currents are balanced due to
compensation. Fig. 5 (d) shows the unbalnced currents
injected by the PV power plant to compensate for the
unbalanced load current shown in Fig. 5 (e).
Load 3-ph voltage (pu)
The following equations are used to transform the positive
and negative sequence dq components into the corresponding
abc components as shown in Fig. 3.
10
5
0
-5
-10
0.46
0.48
0.5
−
0.52
time (sec)
I-PV 3-ph current (pu)
(c)
Fig. 4 System under consideration for simulation.
4
2
0
-2
-4
0.46
0.48
0.5
0.52
time (sec)
Load 3-ph current (pu)
(d)
B. Simulation Results
Fig 5 shows the voltage and current waveforms of feeder-1
with the proposed controller to compensate for unbalanced
PCC voltage. Following are the important simulation
timelines:
• Time-1 = 0.45 sec: unbalanced loads are connected to
the feeder without any compensation.
• Time-2 = 0.50 sec: Inv-1 starts to compensate for the
unbalanced loads.
10
5
0
-5
-10
0.46
0.48
0.5
0.52
time (sec)
(e)
Fig. 5 Voltage and current waveforms for feeder-1.
Fig. 6 shows the PCC voltage profile of feeder-1. The rms
values of the three phases are shown in Fig. 6 (a). Before
4
PCC 3-ph voltage rms.(pu)
compensation, it is noticed that the rms values are not equal
(0.93 pu for phase-a, 1.00 pu for phase-b and 1.02 pu for
phase-c). These values are noticed to be the same at 0.965 pu
after compensation.
1.05
1
0.95
0.9
Ph-a
0.85
0.46
Ph-b
0.48
Ph-c
0.5
0.52
time (sec)
0.54
0.56
0.54
0.56
PCC voltage positive seq.
Vdq,p (pu)
(a)
1.5
1
0.5
0
-0.5
Vd,p
-1
0.46
Vq,p
0.48
0.5
0.52
time (sec)
shown in Fig. 6 (C). It is noticed that after applying the
suitable compensation for the unbalanced currents, the
negative sequence components of the PCC voltage become
zero (i.e. the PCC voltage is balanced).
The current dq negative sequence components of feeder-1
are shown in Fig. 7. Fig. 7 (a) shows that the load negative
sequence components are supplied by the grid while no
compensation is applied (Fig. 7 (b)). It is also shown that
when compensation is provided by Inv-1, the load negative
sequence currents shown in Fig. 7 (a) are supplied by Inv-1
(Fig. 7 (c)).
Active and reactive powers of the three phases (a, b and c)
supplied by the grid and I-PV power plant are shown in Figs 8
and 9. Fig. 8 (a) shows the active and reactive power supplied
from the grid through phase-a. Figs 8 (b) and (c) show the
powers of phases b and c respectively. It is noticed that before
compensation the powers supplied by the three phases (a, b
and c) of the grid are not balanced. After compensation, the
active power is balanced at 2.5 pu and reactive power at 1.5 pu
for the three phases as shown in Figs 8 (a), (b) and (c).
0.05
0
Vd,n
-0.05
0.46
0.48
0.5
0.52
time (sec)
0.54
Vq,n
0.56
Grid active & reactive
power P & Q (pu)
PCC voltage ngative seq.
Vdq,n (pu)
(b)
4
2
0
P
-2
(c)
0.46
Q-------Phase-a
0.48
0.5
Grid active & reactive
power P & Q (pu)
4
2
0
-2
-4
Id,n
0.46
0.5
0.52
time (sec)
0.54
Grid negative seq. current
Idq,n (pu)
(a)
2
0
-4
Id,n
0.46
Iq,n
0.48
0.54
0.56
0.54
0.56
3
2
1
P
0
0.5
0.52
time (sec)
0.54
0.46
Q------ Phase-b
0.48
0.5
0.52
time (sec)
(b)
0.56
4
-2
0.56
4
Iq,n
0.48
0.54
(a)
Grid active & reactive
power P & Q (pu)
Load negative seq. current
Idq,n (pu)
Fig. 6 PCC voltage profile.
0.52
time (sec)
3
2
1
0
P
-1
0.46
Q------ Phase-c
0.48
0.5
0.52
time (sec)
0.56
(c)
Fig.8 Three phase active and reactive power of the grid side.
0.56
Fig. 9 (a) shows the active and reactive power of Inv-1
(Phase-a). The phase b and c powers are shown in Figs. 9 (b)
and (c) respectively. It is noticed that Inv-1 does not inject any
power into the grid during the compensation process. Instead
the active and reactive power generated by the Inv-1 is
circulated between its phases. Phase-a supplies 0.65 pu active
power as shown in Fig. 9 (a), while phases b and c absorb this
amount of active power (Figs. 9 (b) and (c)). The reactive
power consumed by phase-c (Fig. 9 (c)) is being generated by
phases a and b as shown in Figs 9 (a) and (b).
I-PV negative seq. current
Idq,n (pu)
(b)
2
0
Id,n
-2
0.46
Iq,n
0.48
0.5
0.52
time (sec)
0.54
(c)
Fig. 7 Negative sequence components of feeder-1 currents.
Fig. 6 (b) shows the positive sequence dq components of the
PCC voltage. The negative sequence dq components are
I-PV active & reactive
power P & Q (pu)
5
0.8
0.6
4)
0.4
0.2
0
-0.2
-0.4
P
0.46
Q -------- Phase-a
0.48
0.5
0.52
time (sec)
0.54
0.56
5)
I-PV active & reactive
power P & Q (pu)
(a)
6)
0.5
0
-0.5
P
0.46
Q --------Phase-b
0.48
0.5
0.52
time (sec)
0.54
0.56
7)
I-PV active & reactive
powerP & Q (pu)
(b)
8)
0.2
0
-0.2
-0.4
-0.6
-0.8
P
0.46
9)
Q ---------Phase-c
0.48
0.5
0.52
time (sec)
0.54
0.56
(c)
Fig.9 Three phase active and reactive power of I-PV power plants.
10)
V. CONCLUSION
This paper represents the utilization of an I-PV power plant to
compensate for unbalanced PCC voltages. The basic idea is to
inject the negative sequence currents caused by unbalanced
load using I-PV power plants. The proposed controller
consists of positive and negative sequence loops. The positive
sequence controller is used to regulate balanced PCC voltage,
while the negative sequence is to mitigate the unbalanced
voltage. Though both active and reactive powers are
exchanged and circulated through I-PV inverter, there is no
net consumption of active power by the I-PV system. A
MATLAB/Simulink based simulation study has been
performed to evaluate the effectiveness of proposed negative
sequence controller for the PCC voltage unbalance
compensation.
VI. APPENDIX I
Feeders-1 and -2 system voltage level, Vs1 = Vs2 = 11 kV.
Line parameters: 0.08 + j0.04 ohm/km.
Line lengths: L11 = 20 km, L12 = 4 km.
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