112
CHAPTER 5
INTERLINE POWER FLOW CONTROLLER SYSTEM
5.1
GENERAL
In the IPFC structure, a number of inverters are linked together at
their DC terminals. Each inverter can provide a series reactive compensation,
as an SSSC, for its own line. However, the inverters can transfer real power
between them via their common DC terminal. This capability allows the IPFC
to provide both reactive and real power compensation for some of the lines
and thereby optimize the utilization of the overall transmission systems. The
main objective of an IPFC is to optimize both real and reactive power flow
among multilines, and transfer power from overloaded to under loaded lines.
The model of the closed loop controlled IPFC is developed using MATLAB
and the simulation results are presented. The closed loop control of the IPFC
is obtained using the PI controller.
5.2
MODEL OF THE IPFC USING MATLAB
An elementary IPFC, composed of voltage source converters 1 and
2 with a common DC link, provides the series compensation for lines 1 and 2.
Controlling the real power flow is achieved by approximately adjusting the
angle of the IPFC in lines 1 and 2. The reactive power flow is achieved by
adjusting the input voltage of lines 1 and 2. The model of the IPFC is
developed and simulated using MATLAB simulink.
113
5.2.1
Model of a Basic Transmission Line using MATLAB
The transmission line is designed with the resistance of the line and
the reactance. The transmission line is simulated for AC sources of 110kV
and 120 kV. The circuits of the primary and secondary lines are shown in
Figures 5.1 and 5.2 respectively. The voltage at the sending and receiving
ends is observed. The power across the load resistor and load inductor is also
taken for an uncompensated line. The primary transmission line consists of
the voltage source V1 and the source impedance is represented as Z1. The
impedance of the transmission lines is represented as Z2 and Z3. The load in
the primary transmission line is represented using ZL1.
Figure 5.1 Model of the primary transmission line
The secondary transmission line consists of the voltage source V2
and the source impedance is represented as Z1. The impedance of the
transmission lines is represented as Z2 and Z3. The load in the secondary
transmission line is represented using ZL2.
114
Figure 5.2 Model of the secondary transmission line
From the values of the Power Engineer’s hand book, appropriate
values are calculated for the real time application, and such values are used as
impedance of primary and secondary transmission lines. From the impedance
values, the loads of the primary and secondary transmission lines are
calculated.
Source voltage V=110kV
ZT = (118 + j 217) ohm
ZL = (432 + j 324) ohm
I
= (V/ZT) = 165.5,
-45.77o A.
VR = I*ZL= 714161.245,
P
= VR*I*cos
-8.94o V
= (714161.245,
-8.94o) * (165.5,
-45.77o) * (0.8)
P = 9.46146 MW
Q
= VR*I*sin
= (714161.245, -8.94o) * (165.5,
-45.77o) * (0.6)
Q = 7.096 MVAR
The Schematic of the transmission line models for 120kV and
110kV are shown in Figures 5.3 and 5.4 respectively.
115
Figure 5.3 Model of the basic transmission line for 120kV
Figure 5.4 Model of the basic transmission line for 110kV
The reactive power in the load for 120kV and 110kV basic
transmission lines is shown in Figures 5.5 and 5.6 respectively. This power
was obtained without using any interlink. By introducing the IPFC controller
the power is improved.
116
Figure 5.5 Reactive power in the 120kV transmission line
Figure 5.6 Reactive power in the 110kV transmission line
The reactive power in the 120kV and 110kV transmission lines is
7.825 MVAR and 7.086 MVAR respectively. This transmission line values
coincide with the theoretical value of 7.096 MVAR.
5.2.2
Model of the Closed Loop Controlled IPFC
The circuit model of the closed loop IPFC system with primary and
secondary loads is shown in Figure 5.7. The load voltage is sensed and it is
rectified. It is then compared with a reference signal. The multiple PWM
technique is used. The output of the pulse generator is used to drive the
switches of the inverter. Subsystem 1 represents the pulse generation module,
which is shown in Figure 5.8. Subsystem 2 senses the voltage across the
117
primary load, and subsystem 3 senses the voltage across the secondary load.
Subsystems 2 and 3 are shown in Figures 5.9 and 5.10 respectively.
Subsystem 4 represents the circuit of the PI controller.
Figure 5.7 Circuit of the closed loop controlled IPFC
118
Terminator7
1
2
m
a
g
2
Conn2
T erminator3
Terminator6
XL7
g
1
Ideal Switch2
k
m
m
Terminator1
Thyristor1
g
Pulse
Generator2
a
Thyristor2
a
g
2
Ideal Swi tch1
Conn3
k
m
Terminator
1
3
g
Conn1
2
1
g
Ideal Switch
a
g
Pulse
Generator3
Conn4
2
Ideal Switch3
Thyristor3
m
4
m
Thyristor
Pulse
Generator
1
g
k
m
k
m
Terminator2
Terminator4
Terminator5
Pulse
Generator1
AND
Pulse
Generator6
1
Pulse
Generator4
In1
>=
Repeating
Sequence
Logical
Operator3
AND
Logical
Operator
Relational
Operator
Pul se
Generator5
Pulse
Generator7
AND
AND
Logical
Operator2
Logi cal
Operator1
Figure 5.8 Subsystem 1 of the closed loop IPFC
Diode4
1
Conn1
Diode5
Diode7
2 RL10
Conn2
RL11
+
v
-
V5
1
Out1
Diode6
Figure 5.9 Subsystem 2 of the closed loop IPFC
119
Diode
1
2
Diode3
Conn2
RL8
RL9
Conn1
+
v
-
V4
Diode1
1
Out1
Diode2
Figure 5.10 Subsystem 3 of the closed loop IPFC
Simulation is done with the primary line input as 120kV and the
secondary line input as 110kV. The reactive power output of the secondary
load is 7.108MVAR. The reactive power output of the closed loop IPFC
system with unequal voltages is shown in Figure 5.11.
Figure 5.11 Reactive power of the secondary load with unequal voltages
in the closed loop IPFC
The model of the closed loop IPFC system is shown in Figure 5.7.
Simulation is done with equal voltages on the input sides [primary and
secondary line inputs as 110kV]. The reactive power output of the secondary
load is 7.109MVAR. The reactive power output of the closed loop IPFC
system with equal voltages is shown in Figure 5.12.
120
Figure 5.12 Reactive power of the secondary load with equal voltages in
the closed loop IPFC
The model of the closed loop IPFC system is shown in Figure 5.7.
Simulation is done with equal voltages on the input sides and different phase
angles [primary line input source voltage at 110kV with phase angle 15o and
secondary line source voltage at 110kV with 30o phase angle]. Due to the
difference in the phase angle, there is an increase in the real power of the
secondary load when compared to the primary load.
The real power of the primary load is 8.8 MW and that of the
secondary load is 9.51 MW with the primary line operating at 110
and secondary line operating at 110
15 kV
30 kV as shown in Figure 5.13. By
controlling the phase angle, the flow of real power between lines having equal
input voltages can be controlled.
121
a
b
Figure 5.13 Real power of the primary load and the secondary load
a. Real power of the primary load
b. Real power of the secondary load
5.2.3
Model of the IPFC in a Four-Bus Test System
In this section, we illustrate the proposed IPFC dispatch strategy for
maximizing the voltage stability limited power transfer in a four-bus test
system. Perhaps the most common approach in the voltage stability analysis is
to increase the system loading Pload and observe the resulting voltage variation
V on the critical buses. The analysis is frequently presented in the form of
Power Voltage (PV) curves, which are now being used in many power control
centers.
To generate consistent PV curves, we modify the IPFC control
strategy slightly by enforcing the desired circulating power Pc at all operating
conditions, regardless of whether the VSCs are at their rated capacities or not.
That is, if both VSCs are below their rated capacities, then, besides requesting
122
a specific power circulation level, the series VSCs will regulate the line active
power flows. The reactive power flow is no longer enforced.
The four-bus test system shown in Figure 5.14 has two equivalent
generators and two equal amounts of loads at bus2 and bus4.
Figure 5.14 The four-bus test system
From the reference Xuan Wei et al (2004), there are two
transmission paths with line 3-4 weaker than line 1-2 (stronger line 1-2 and
weaker line 3-4). An IPFC is sited at bus 1 with each VSC on one of the
parallel lines on the two transmission paths. It is noted that by closing the
switches A and B, the IPFC is bypassed, which is referred to as the
uncompensated system. The IPFC is in service if switches A and B are
opened.
By increasing the loads Pload1 at bus 2, Pload2 at bus 4 and the
necessary amount of generation at bus 1and bus 3, we can investigate the
variation of voltage V2 at bus 2 and V4 at bus 4, with and without the IPFC.
Note that a positive Pc denoted that power is circulating from VSC2 on line 34 to VSC1 on line 1-2.
123
5.2.3.1
Model of the closed loop IPFC with variations in the firing
angle of VSC1
The single-phase four bus closed loop IPFC system has been
simulated in MATLAB Simulink, using the power system toolbox as shown
in Figure 5.15. Switches A and B are opened for the time period of 0.3 sec to
0.6 sec. Then the IPFC is in service during the above mentioned time period,
which is referred to as the compensated system. Switches A and B are closed
for the remaining time period; the IPFC is bypassed, which is referred to as
the uncompensated system. By increasing the loads at bus 2 and bus 4, we can
investigate the variation of the output voltage and real power at bus 4, with
and without the IPFC.
The electrical data of the system used in the present work is as below:
Generator 1 & 2
:
15.7kV
Transformer
:
15.7/ 400 kV, 1000MVA, r=0.0059p.u.,
l = 0.127 p.u.
Series transformer
:
3/ 45 kV, 160 MVA, r = 0.005p.u., l = 0.06 p.u.
Transmission line
:
r = 3.2
/ 100km, l = 103 mH/100km,
c = 1.1 F/ 100km
Continuous
i
-
+
pow ergui
T2
T1
1
2
Z1
i
1
Z2
1
2
V
ZL1
ZL3
2
T3
V1
PQ
+
v
-
I
Vo3
p,q
Scope1
T1
Conn1
Out2
Conn2
Subsystem3
Conn2
Conn1
Conn4
Conn3
In1Out1
Subsystem1
1
Subsystem
2
T4
+
i
-
z2
V
i1
z1
PQ
I
1
1
2
ZL2
2
V2
+
v
-
p1,q1
Scope2
Vo
T6
T5
Conn1
Out2
Conn2
Subsystem2
124
Figure 5.15 Circuit model of the closed loop IPFC system with changes in the firing angle of VSC1
125
Without the IPFC, the output of the stronger line is 2.1*(108) W
and 2.8*(108) VAR and the output of the weaker line is 2.0289*(108) W and
2.7098*(108) VAR, which are shown in Figures 5.16 and 5.17 respectively.
8
3
x 10
P(w)
2
a
1
0
0
0.1
0.2
0.3
0.4
0.5
Time(s)
0.6
0.7
0.8
0.9
1
8
3
Q(var)
b
x 10
2
1
0
0
0.1
0.2
0.3
0.4
Time(s)
0.5
0.6
0.7
0.8
Figure 5.16 Real power and reactive power of the stronger line
a. Real power of the stronger line.
b. Reactive power of the stronger line.
8
3
x 10
P(w)
2
a
1
0
0
0.1
0.2
0.3
0.4
0.5
Time(s)
0.6
0.7
0.8
0.9
1
0.1
0.2
0.3
0.4
0.5
Time(s)
0.6
0.7
0.8
0.9
1
8
b
Q(var)
3
x 10
2
1
0
0
Figure 5.17 Real power and reactive power of the weaker line
a. Real power of the weaker line.
b. Reactive power of the weaker line
126
The subsystem of the closed loop controlled IPFC system with
firing angle changes in VSC1 is shown in Figure 5.18. A reference signal is
compared with a ramp signal, and its output is given as pulse to switches of
the converter. The firing angle of the pulse generators in VSC1 is varied, and
the corresponding variations in real power and output voltage are observed for
different loads. As referred from the reference [43], Xuan Wei (2004), the
power was exchanged from VSC1 to VSC2 or vice versa.
By increasing the power circulation from VSC1 to VSC2 for the
load considered in the range of 600 MW to 800 MW, reactive power is taken
from the stronger line 1-2 and injected into the weaker line 3-4, allowing a
higher voltage at bus 4. Conversely, the load is considered in the range of 800
MW to 950 MW, by increasing the power circulation from VSC2 to VSC1,
bus 4 voltage is decreased and bus 2 voltage is raised.
127
Timer
g
m
1
2
Timer1
g
m
1
2
Ideal Switch
Ideal Switch1
T5 T7
P5
P6
S1
1e-06
+
-
Conn3
4
R2 Conn4
m
2
m
2
V1
v
3
S2
g
a
g
a
S5 S7
g
1
m
k
m
k
g
1
1
Conn1
T3
T6
+
v
-
T4
P8
g
1
V8
g
1
P7
S6
S3
S4
m
2
g
a
m
2
g
a
S8
m
k
m
k
T8
T2
2
R1
T1
Conn2
+
v
-
V4
P3
P1
AND
AND
1
LO2
Scope2
LO1
>=
P11
P2
AND
AND
LO3
LO
4
Figure 5.18 Subsystem of the closed loop IPFC with changes in the firing
angle of VSC1
For the load considered in the range of 600 MW to 800 MW, the
real power flows from VSC1 to VSC2 and the output power variations in the
weaker line 3-4 are as shown in Figure 5.19. For the load considered in the
range of 800 MW to 950 MW, the real power flows from VSC2 to VSC1 is as
shown in Figure 5.20.
128
Figure 5.19 Real power in the weaker line for 650 MW load
Figure 5.20 Real power in the weaker line for 900 MW load
The change in real power for various loads from 600MW to
950MW with the change in power (Pc) is shown in Figure 5.21. The change in
the output voltage for various loads from 600MW to 950MW is shown as
power voltage curve in Figure 5.22.
129
Figure 5.21 Change in the real power of the weaker line
Figure 5.22 Power voltage curve of the weaker line
5.2.3.2
Model of the closed loop IPFC with variations in the firing
angle of VSC2 with reference to the sending end voltage
The model of the closed loop controlled IPFC system with firing
angle variations in VSC2 taking sending end voltage as reference is shown in
Figure 5.17, and the subsystem is shown in Figure 5.23. A reference signal is
compared with a ramp signal, and its output is given as pulses to the switches
of the converter. The firing angle of the pulse generators in VSC2 is varied
130
and the corresponding variations in the reactive power & output voltage of the
weaker line are observed for different loads.
Timer
g
m
1
2
Timer1
g
m
1
2
Ideal Switch
Ideal Switch1
P2 T10
T9
P1
4
S1
g
1
3
S2
m
2
g
a
g
a
S5 S7
Conn3
R2
m
2
m
k
m
k
g
1
Conn4
1
C
2
T3
V8
T11
g
1
m
T12
+
v
-
T4
Conn2
S3
S4
m
2
g
a
g
g
1
m
k
S6
T8
m
2
+
-
k
v
Conn1
a
V1
T2
P4
R1
T1
+
v
-
P3
V4
P7
P5
AND
AND
1
Scope3
LO2
LO1
>=
P8
Scope2
P6
AND
AND
LO3
LO4
Figure 5.23 Subsystem of the closed loop IPFC with changes in the firing
angle of VSC2 taking the sending end voltage as reference
The reactive power variation in the weaker line 3-4 with a load in
the range of 600 MW to 750 MW is shown in Figure 5.24. It is observed that
131
the reactive power increases from 1.6858*(10^8) VAR to 1.8934*(10^8)
VAR, and voltage increases from 1.631*(10^4) V to 1.73*(10^4) V. Thus,
there is a reactive power flow from VSC1 to VSC2.
Figure 5.24 Reactive power in the weaker line for 650 MW load
For the load considered in the range of 800MW to 950 MW, the
reactive power variation in the weaker line is shown in Figure 5.25. It is
observed that the power decreases from 1.92*(10^8) VAR to 1.89*(10^8)
VAR and voltage decreases from 1.87*(10^4) V to 1.73*(10^4) V. Thus,
there is a reactive power flow from VSC2 to VSC1.
Figure 5.25 Reactive power in the weaker line for 900 MW load
132
The change in the output voltage for the loads considered in the
range of 600MW to 950MW, with the change in reactive power (Qc) for bus 4
is shown in Figure 5.26.
Figure 5.26 Change in the reactive power of the weaker line
5.2.3.3
Model of the closed loop IPFC with variations in the firing
angle of VSC2 with reference to the receiving end voltage
The model of the closed loop controlled IPFC system with firing
angle changes in VSC2, taking the receiving end voltage as reference is shown
in Figure 5.17, and the subsystem is shown in Figure 5.27. A reference signal
is compared with a ramp signal and its output is given as pulse to the switches
of the converter. The firing angle of the pulse generators in VSC2 is varied,
and the corresponding variations in the reactive power and output voltage of
the weaker line are observed for different loads.
133
Timer
g
m
1
2
T imer1
Ideal Switch
g
m
1
2
Ideal Switch1
P4
P3
T10
1
g
3
4
Conn3
Conn4
2
2
m
S2
m
a
S1
g
a
g
S5 S7
1
g
k
m
k
m
T9
1
C
2
T3
1
g
R2
S3
m
m
S4
a
g
1
g
k
S6
S8
2
k
m
V8
T 11
T12
g
+
v
-
T4
Conn2
2
+
-
m
v
Conn1
a
V1
T2
T1
P5
R1
P13
+
v
-
V4
0
0
>=
1 Constant
>=
Constant1
2
-1
In1In3
Product1
In2
3
>=
Product
>=
P9
P1
AND
AND
ramp
ramp
LO2
LO1
Figure 5.27 Subsystem of the IPFC with changes in the firing angle of
VSC2 taking the receiving end voltage as reference
For a load of 650 MW, the reactive power output in the weaker line
is shown in Figure 5.28. It is observed that the power increases from
1.97*(10^8) VAR to 1.99*(10^8) VAR, and the voltage increases from
1.7*(10^4)V to 1.73*(10^4) V. Thus, there is a flow of reactive power from
VSC1 to VSC2.
Figure 5.28 Reactive power in the weaker line for 650 MW load
134
5.3
CONCLUSION
The IPFC is simulated for the compensation and power flow
management of the multiline transmission system. In the IPFC structure, the
converters are linked together at their DC terminals. Each inverter can provide
a series reactive compensation, as an SSSC, for its own line. However, the
converters can transfer real power between them through their common DC
terminal. This capability allows the IPFC to provide both real and reactive
compensation for some of the lines and thereby optimize the utilization of the
overall transmission system. In particular, the IPFC can equalize both real and
reactive power flow in the lines, relieve the overloaded lines from the burden
of the reactive power flow, compensate against resistive as well as reactive
voltage drops, and provide a concerted multiline counter measure during
dynamic disturbances.
A new dispatch strategy for an IPFC operating at the rated capacity
is proposed. When the IPFC operates at its rated capacity, it can no longer
regulate the line active power flow set point or the reactive power flow set
point or both. In such cases, the dispatch strategy switches to a power
circulation set point control to co-optimize both series VSCs, without
exceeding one or both rated capacities. The dispatch results show that the
IPFC can improve the power transfer in the system. The power circulation
between the two VSCs can be used to adjust bus voltages to improve the
voltage stability limit transfer. The simulation results are in line with the
predictions. The IPFC is capable of balancing the power through the lines.
The power quality is improved since the IPFC permits additional power. The
circuit models for the IPFC system are developed using MATLAB. These
models are used for simulating a four bus system. The simulation results
using MATLAB are presented. The IPFC increases the real power transfer
and improves the voltage profile.