Application of Voltage Source Converter in Interphase Power Controller for Power Flow Control and Fault Limitation M. Farmad*, S. Farhangi*, S. Afsharnia* and G. B. Gharehpetian+ of Electrical and Computer Engineering, University of Tehran, Iran + Department of Electrical Engineering, Amirkabir University of Technology, Tehran, Iran * Department
e-mail:
[email protected] Abstract— In this paper, the analysis of Interphase Power Controller (IPC) equipped with the conventional phase shifting transformer (PST) is reviewed. Then, the Static Series Synchronous Compensator (SSSC) which incorporate the voltage source converter (VSC) is utilized for substitution of the PST in the IPC. The simplified model of the VSC-based IPC has been presented. The capabilities of this new device for power flow control and the short circuit current limiting have been investigated in Iran electric network. Index Terms— Interphase power controller, Phase shifting transformer, Static series synchronous compensator, Load flow control, Short circuit currents.
I
I. INTRODUCTION
nterconnected power systems are subjected to overrating of the circuit breakers and related substation equipment due to short circuit level. Splitting the existing bus into more sections, utilizing of series reactors in transmission lines and using transformers with high impedance or replacing overduty substation circuit breakers and related equipments are normal traditional alternatives to reduce the short circuit levels. Splitting an existing bus into more than one section mitigates the substation fault problem in a relatively cost effective manner, but operating flexibility and reliability will be decreased. In practice, it may be difficult to obtain permission to change the existing bus configuration. Series reactors can neither completely eliminate the fault current contributions nor efficiently reduce the transmission constraints. At normal conditions, series reactors absorb reactive power. Under heavy loading conditions, this solution can make more problems for voltage regulation. Replacing the under-rated circuit breakers and associated substation equipments with higher interrupting devices, is another method to overcome the fault duty problem. Depending on voltage levels, the number of circuit breakers involved and desired new rating for the breakers, the replacement of breakers can be expensive. In addition, scheduling large number of circuit breaker replacements imposes planning and engineering challenges. However, none of the above methods provide additional transmission capability or ability to control and redirect the power flow.
k,(((
A comprehensive review of interphase power controller (IPC) concepts and applications can be found in [1]. In [2], it has been shown that an IPC equipped with the conventional phase shifting transformer can be used as not only a power flow controller but also as a fault limiter. In this paper, after reviewing the properties and equations of IPC, the model and a typical control circuit for SSSC and its main part which is a VSC, are investigated and its ability for power flow control is presented. Then, with substitution of the PST in IPC with a VSC, a VSC-based IPC has been introduced and analyzed. Finally, the upcoming problem of Iran electric network in relation with short circuit level of some the 230 kV substations has been explained and the capability of VSC-based IPC to control power flow and overcome this problem has been described. II.
SYSTEM MODELING
A. IPC An IPC is a series-connected controller of active and reactive power consisting, in each phase, of inductive and capacitive branches subjected to separately phase-shifted voltages provided by phase shifting transformers (PST) [3]. For short circuit mitigation applications, the reactances of the reactor and the capacitor are selected to be equal and tuned at the fundamental frequency, so as to impose infinite impedance to the short circuit current. Each terminal of the IPC behaves as a voltage dependent current source and provides the IPC with the unique decoupling effect property and short circuits on one side of the IPC will not be transferred to the other side while controlling power flow under normal and postcontingency conditions. The IPC is therefore well suited for system interconnections that require no additional synchronizing power and new power transmission corridors [4]. Furthermore, additional interconnections can be integrated even in strongly meshed power networks with high short circuit levels without exceeding given breaking capacities in the surrounding substations. This offers the chance to interconnect systems even when the short circuit levels are already in a range which would not normally allow new interconnections to be built, without having to replace some circuit breakers and/or other system components. It can also absorb or submit reactive power to the interconnected systems
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without transmitting MVAR. The positive or negative amount of reactive power injected into or taken from the system is therefore the same at both terminals of the IPC. Un-tuned IPC can be used for power flow control, especially for increasing transfer capability with the existing transmission facilities. The controllable parameters are the phase shift angles and/or the branch impedances. The phase shifting devices can be conventional or electronically switched phase shifting transformers. One of the important characteristics of an IPC is that the active power through the device is held nearly constant for a large range of angles across the device. An IPC with electronically switched phase shifting devices add dynamic regulation capabilities to the power system. As a consequence, not only the steady state stability properties but also the transient stability performance of the power system is improved, remarkably. After disturbances, such as line outages, electronically based IPC can quickly redistribute the power flow to a predefined state and therefore increase the system security. Overloaded transmission corridors, which could occur in subsequent operational problems and further outages, are thus avoided and therefore it can control power flow with fast response. Another advantage of electronically switched phase shifting components is their maintenance free operation. Fig. 1 shows a tuned IPC in series with a transmission line connected two power systems.
VS ∠δ N1
IS
αA
αB
IPC jX A
jX
VS ∠(δ + α A )
− jX A
V R ∠0 IR
Vm ∠δ m
N2
VSVR cos(δ + α IPC ) X IPC V V sin(δ + α IPC ) QR = − S R X IPC α + αB α IPC = A 2 XA X IPC = α − αB 2 sin( A ) 2 PR =
PR =
(3)
(4)
2V S V R sin α cos δ XA
(5)
Since δ is a relatively small angle in power system, according to the (5), the active power is almost constant and independent to characteristics of the networks connected to the above mentioned line. Meanwhile the operator of the system can control the active power of the transmission line by adjusting α in IPC. If only one PST is in the upper or the lower branch of IPC, then α A or α B will be equal to zero. The templates PR in relation to VSVR / X A
δ
for different
amounts of α will be as demonstrated in figures 2 and 3 respectively showing the active power firmness.
Fig. 1. A tuned IPC in series with a transmission line connecting two power systems
0.4
α B = −20 o
The variables and parameters in Fig. 1 are defined as follows: Magnitude of sending bus voltage V
0.3
S
α B = −10 o
0.2
VR
Magnitude of receiving bus voltage
IS
Angle of sending bus voltage Sending end current
IR
Receiving end current
-0.1
αA αB
Phase shift of upper PST
-0.2
XA Vm
Reactance of reactor and capacitor in IPC
δm X
(2)
In a special case, if α A = −α B = α , then the active transmitted power in a transmission line equipped with such IPC is given by (5):
for variation of
VS ∠(δ + α B )
0.1 P/(VsVr/Xa )
δ
(1)
α B = 0o 0
α B = 10 o α B = 20 o
Phase shift of lower PST
-0.3
-0.4 -30
-20
-10
0
10
20
30
δ
Magnitude of the voltage in transmission line after IPC Angle of the voltage in transmission line after IPC
Fig. 2. Active power variation in a transmission line equipped with an IPC (α A = 0)
Reactance of the transmission line
According to Fig. 1, the equations governing the behavior of the IPC are obtained and given as follows: k,(((
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amplitude of the current and its relative angle (| I | and
0.4
α A = 20 o
θ ir ) . Vqref
0.3
multiplying the converter gain (kinv ) to Vqref , Vdcref will be
0.1 P/(VsVr/Xa)
( X qref ) .Then, by
the desired compensating reactance
α A = 10 o
0.2
will be equal to the current amplitude multiplied by
αA = 0
0
calculated. A PI controller regulates Vdc to Vdcref by adding a
o
small phase angle ( β ) . By adding
-0.1
α A = −10 o
capacitive compensation) or
-0.2
compensation),
α A = −20 o
-0.3
calculating
-0.4 -30
-20
-10
0
10
20
30
θ vr
θv
π /2
− π / 2 to θ ir (in to
θ ir (in
inductive
will be determined and used for
by summing with or subtracting from
β .This
angle will be used for gate pattern command.
δ
Fig. 3. Active power variation in a transmission line equipped with an IPC (α B = 0)
B. SSSC The SSSC injects a nearly sinusoidal and controllable voltage in series with the transmission line. The main part of the SSSC is a Voltage Source Converter supplied by a dc-link capacitor. The injected voltage is perpendicular to the line current, and therefore emulates an inductive or a capacitive reactance in series with the transmission line, and influences the electric power flow in the line as illustrated in Fig. 4.
VS ∠δ N1
jVq ∠(δ / 2) +
−
jX
I∠(δ / 2)
VR ∠0 Fig. 5. A typical control block diagram for SSSC
N2
C. VSC-based IPC In steady state operation of network, SSSC has the same function as PST [7]. Therefore, it can be used instead of PST in the structure of IPC, which is analyzed in this sub-section. This new combination can be nominated as VSC-based IPC. The single line diagram of this component in series with a transmission line is illustrated in Fig. 6.
Vq
VS
VS = VR = V
I
δ /2 VR
VS ∠δ
Fig. 4. A SSSC in series with a transmission line connecting two power systems
According to Fig. 5, the transmitted active power of the line is defined as follows:
N1
V [V sin δ − Vq cos(δ / 2)] (6) X So the injected voltage (Vq ) can control the active power and
k,(((
IL
Vm ∠δ m jX A
IS
jX
VR ∠0 IR
N2
VSC1
C1
P=
is almost perpendicular to the line current. There are many control methods for SSSC in the literature, and in this paper, the control approaches used in [5] and [6] are utilized. The control block diagram is given in Fig. 5. A phase locked loop (PLL) is used to measure the left side voltage angle of the connection point of SSSC. With aiming this angle (θ ) , the direct and quadrature components of the three phase line currents ( I d and I q ) are calculated and used to determine the
Vq 1
C2
VSC2
IC
Vq2
− jX A
Fig. 6. A single line diagram of VSC-based IPC
In Fig. 6, Vq1 and Vq 2 are adjusted to be perpendicular to I L and
I C respectively. By considering the Norton
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equivalent of each branch of VSC-based IPC and performing necessary simplifications including elimination of parallel jX A and − jX A , the final diagram will be as Fig. 7.
VS ∠δ
IQ
Vm ∠δ m
IS
N1
jX
VR ∠0 IR
TABLE I SOME OF 230 KV SUBSTATIONS ENCOUNTERED WITH SHORT CIRCUIT LEVEL PROBLEM
No
Fig. 7. Simplified equivalent circuit of VSC-based IPC
2
The line current, I Q , is dependent only to Vq1 and Vq 2 as described in (7). This current can be controlled by the injected voltages. It should be noted that, if one VSC is omitted, the remaining VSC control the line current. The point "m" voltage and the transmitted active power of the line equipped with VSC-based IPC can be determined by (8) and (11).
I Q ==
jX A
V m ∠δ m = V R +
= −I S
X (Vq1 − Vq 2 ) XA
Vq1 = Vq1∠α 1
(7)
3
4
5
6
7
(8) 8
(9) 9
Vq 2 = Vq 2 ∠α 2
(10) 10
PS =
VS [Vq1 sin(δ S − α 1 ) − Vq 2 sin(δ S − α 2 )] XA III.
the faulted bus
N2
1
Vq1 − Vq 2
No & name of
(11)
11
12
CASE STUDY AND SIMULATION
4780 [N-PRN2] 3120 [AZADG2] 3360 [MONTG2] 3450 [PARKJ2] 3300 [KAN1 2] 3580 [SHUSH2] 3400 [NAMAS2] 3540 [SADAT2] 3430 [PARDI2] 3380 [MOSAL2] 3150 [ESLSH2] 3630 [TEHPS2]
Total fault
The most
current
contribution from
(A)
(bus no & name)
54746.8
53499.7
52292.8
51166.7
45854
44405.2
44073.7
43154.3
39191
37711.9
37212.5
35148.8
3160 [FIBAH2] 4780 [N-PRN2] 3450 [PARKJ2] 3300 [KAN1] 3450 [PARKJ2] 3230 [GHORK2] 3450 [PARKJ2] 3320 [KAN2] 3400 [NAMAS2] 3230 [GHORK2] 3160 [FIBAH2] 3510 [REYN2]
Contribution
(A)
(%)
23430
43
12259
23
10054
19
10412
20
12462
27
24437
55
30545
69
34549
80
21929
56
27443
73
16820
45
7026
20
A. Problem description Short circuit analysis of Iran electric network shows that in some of 230 kV substations, short circuit currents will exceed the capability of the existing circuit breakers. Some of these substations are listed in Table I. Figure 8 shows the short circuit current contributions for the seventh row of Table I. In Reference [2], the conventional IPC was investigated to mitigate the short circuit level for all of the above mentioned cases, while maintaining the acceptable load flow conditions. In the present work, VSC-based IPC has been utilized to reduce the short circuit level. The results have been presented for the case demonstrated in Fig. 8.
Fig. 8. Short circuit current in bus 3400 and its contribution from neighboring buses
k,(((
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transmitted active power through the transmission line becomes 0 ( as a consequence of being equal reactances of the capacitor and the reactor in IPC), 9, 21, 43 and 98 MW respectively and the remaining power, P2 for the bus 3400 is supplied by the other bus (3430). In other words, the active power of the line is completely under control of the system operator. Figures 10 and 11 show the amount of active and reactive powers in lines connected to bus 3400 and the DC bus voltage variations during the simulation.
BRK1 VSC
jX A 3400
3450
1
Z th1
BRK 2
− jX A BRK 2
1 o PI equivalent for line 1
3430
2 load
y
B. Simulations Some of the advantages of using VSC-based IPC are as follows: 1) Substitution of conventional phase shifting transformer in interphase power control by VSC decreases the capital investment and operational costs. 2) Power flow control is more flexible and faster because of utilization of power electronics elements. To present the ability of VSC-based IPC for power flow controlling and also mitigation of short circuit level, simulations for the seventh case of Table I are carried out in PSS/E and PSCAD/EMTDC environments. Load flow studies shows that for supplying 320 MW load connected to the bus 3400, 220 MW from the bus 3450 and 100 MW from the bus 3430 flow toward it. Three phase short circuit current at the bus 3400, is 44 kA and approximately 69% of this current is supplied from the bus 3450 (Fig. 8). According to Fig. 9, VSC-based IPC including only one VSC in the reactor branch is added to the line 3400-3450. To study the ability of this component for power flow control, the following scenarios are carried out. The parameters of the power system in Fig. 9 are given in the appendix. During the first second of the simulation, by closing BRK1 and opening BRK2, the component is out of service and the network is in its initial condition. Then, by changing over the switches, VSC-based IPC is activated. By adjusting X qref to 0, -10, -20, -30 and -40 ohm, the
y
Advanced Graph Frame 350 300 250 200 150 100 50 0 -50 140 120 100 80 60 40 20 0 -20 -40 0.0
Fig. 10.
P1
P2
Q1
Q2
1.0
2.0
3.0
4.0
5.0
Power flow variations in the lines connected to bus 3400
influenced by the amount of
X qref
in VSC-based IPC
Fig. 11. DC bus voltage variations influenced by the amount of
X qref
in
VSC-based IPC
The other consequence of using VSC-based IPC in the above network is short circuit level mitigation. Adding this component to the network decreases the short circuit current at the bus 3400 effectively. As indicated in Figure 12, the first three phase short circuit is applied at t=1 sec. Since the VSCbased IPC is out of service, the peak of short circuit current is approximately 70 kA. After adding this component to the network and applying another three phase short circuit at t=3 sec, the current decreases to 25 kA. So it is not necessary to substitute over-duty substation circuit breakers and associated equipments with the new and higher capacity components.
Z th 2
2 o PI equivalent for line 2 Fig. 9. VSC-based IPC added to the network
k,(((
. . .
6.0
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REFERENCES
Fig. 12. Three phase short circuit current at bus 3400 with and without VSC-based IPC
IV. CONCLUSION In this paper, the conventional IPC and VSC in SSSC structure have been investigated. By combining these two elements, voltage source converter-based interphase power controller (VSC-based IPC) has been introduced. It can be used in the interconnected power systems to control active power and decrease the short circuit current. As a case study, the application of VSC-based IPC in Iran electric network has been investigated. The simulation results show the effectiveness of this IPC for power control and short circuit level mitigation.
[1]
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[2]
M. Farmad, S. Farhangi, S. Afsharnia, G.B. Gharehpetian, “An efficient algorithm for determining the values of elements of interphase power controller as a fault limiter”, Power Systems Conference and Exposition, PSCE '06. 2006 IEEE PES, pp. 1493 – 1497
[3]
N.G. Hingorani, L. Gyugui, 2000, “ Understanding FACTS: Concepts and technology of flexible AC transmission systems”, Institute of Electrical and Electronic Engineers, New York
[4]
J. Brochu, P. Pelletier, F. Beauregard, G. Morin, “The interphase power controller, a new concept for managing power flow within AC networks”, PWRD IEEE Transactions, vol. 9, No. 2, pp. 833-841, Apr. 1994
[5]
C. Schauder and H. Mehta, “Vector analysis and control of advanced static VAR compensators” IEE Proceedings-C, vol. 140, No. 4, pp. 299306, July 1993
[6]
Anil C. Pradhan and P.W. Lehn, “Frequency domain analysis of the static synchronous series compensator", IEEE Transactions on Power Delivery, Sep. 28, 2004
[7]
E. Acha, C. R. Fuerte-Eaquivel, H. Ambriz-Perez, C. AngelesCamacho, ,2004, “FACTS, Modeling and simulation in power networks”, John Wiley and Sons, England
APPENDIX The parameters of the system in Fig. 9 are:
Network voltage = 230 kV Rated frequency = 50 Hz Base MVA = 100 Z th1 = 0.62422 + j * 3.14755Ω Z th 2 = 0.87285 + j * 6.95635Ω X A = 81.68Ω Load = 320MW + 110MVAR trasmission line 1( pu ) : R = 0.00012, X = 0.00173, B = 0.05932 trasmission line 2( pu ) : R = 0.00019, X = 0.00280, B = 0.09614
k,(((
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