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Unified Power Flow Controller Impact on the Relays in a Power System
Copyright © IFAC Control of Power Systems and Power Plants, Beijing, China, 1997
UNIFIED POWER FLOW CONTROLLER IMPACT ON THE RELAYS IN A POWER SYSTEM
x. Lombard, P.G. Therond, A. Giard, s. Vitet Direction des Etudes et Recherches Electricite de France. Clamart. France
Abstract-This paper gives an approach on Unified Power Flow Controller (UPFC) impacts on the relays in a Power System. The results are based on relay tests fed by voltages and currents calculated with a complete simulation with EMTP (Electromagnetic Transients Program) code. The series part of the UPFC modifies the line impedance and could disturb the relays measurements. The UPFC design, especially the philosophies of protection for the series transformer, have an impact on line impedance. The type of relays and the configuration of grid have to be taken into account. Copyright © 1998lFAC
Keywords: UPFC, Relays, Protection, EMTP
during abnormal conditions, and the calculation of voltages and currents at the location of the protcctive relays. The second stage was carried out in the EDF protection laboratory: the short-circuit currents calculated by EMTP were applied on real protective relays through digitaVanalog amplifiers. The paper is organized as follows: -Section 2 describes UPFC implementation in the network. -Section 3 describes UPFC impact on relays with short lines -Section 4 describes UPFC impact on relays with long lines
1. IN1RODUCTION
In the context of a network subject to higher and higher loads, utility companies are looking for new equipment's which will enable increased power levels to be transmitted on existing power lines. In recent years, advances in semiconductor technology have made available devices with a gate turn-off capability. These developments offer a new technological base for Flexible Alternating Current Transmission System (FACTS) concepts. One particular new concept called UPFC has been proposed by Gyugyi (1991). With this equipment, a wide range of typical transmission parameters, such as voltage, line impedance and phase angle, can be efficiently controlled.
2. UPFC IMPLEMENT ATION IN THE NETWORK
Having a normal behavior of the protective relays on the power system where a UPFC would be installed is a key point. In this paper, we consider the case where the protection scheme relies on distance protections which measure the impedance between the fault location and the circuit breaker. In this case, the UPFC leakage reactance of the series transformer can potentially disturb the impedance measurement depending if the UPFC is in or out of use. It was therefore decided to check this crucial point. The method used was made of two stages.
In the following paragraphs, we give a description of a UPFC and of main parameters to take into account for its impact on relays. 2.1. Description o/the UPFC
A Unified Power-Flow Controller which is shown in Figure 1, consists of two parts: one is connected in parallel with the network and the other in series with it. Each part is composed of one or more transformers followed by Graetz bridges with GTO thyristors. The two parts are connected through a DC link, smoothed by a capacitor. .
The first stage was the modeling with EMTP of the UPFC including the internal protections, the control
213
ownstream short circuit, the short circuit current ust be derived extremely fast from GTO. As xplained above, the solution is to pro\'ide an lectronic bypass also called a Crowbar. Two options re possible to implement the Crowbar: either on the igh voltage side of the series transformer or on the ow voltage side. the Crowbar is implemented at high voltage side. the mpact on the relays is reduced because the Crowbar hort-cireuits the series transformer, in case of big ontingency. From the network point of view, the FC series part is no more presents and the relav will ot be disturbed during the calculation of the lo~ation !shunt fthe fault. VI f the Crowbar is implemented at low voltage. the mpact on the relays is increased because in case of L -_ _ _ _ _ _ _ _ _..2.-.!..:.!~'___ _ _ _ _ _ _ _.ijown stream short-circuit, the leakage impedance of the series transformer is still present in the line. In the Fig 1: Simplified diagram o/the network and the UPFC UPFC vicinity, the relay calculation could be disturbed by the leakage impedance of the transformer. The UPFC series part is used to modify the active and Therefore, to minimize the impact on the rclays. it is the reactive power flows in the line of the network in interesting to use Crowbar on high voltage. But the which it is inserted. The series part applies a series practical costs for insulation are very high in voltage into the network. The amplitude and the phase comparison with this option. of this voltage can be adjusted in real time. Bus A
!series
Regulation of the shunt part has two main functions: - In continuous operation, the reactive power exchanged depends on the amplitude difference between the AC inverter voJtages and the network. - To regulate the voltage of the capacitor, and to supply the active power drawn by the series part, the shunt part is able to exchange the active power in a transient manner by changing the phase between voltages of the inverter and the network.
Cicuir Breaker
HighVoltage Crowbar
I-I1---+:::J-~I 1
,""-...1
~
I
I
UPFC line
To simulate the behavior of the UPFC, a very complete model has been developed using EMTP (Joncquel, Lombard). Each GTO device, and its diode in antiparallel, is represented. The internal protections as Crowbar and the control during abnormal conditions for the shunt and the series part is included.
Fig 2: Crowbar location on HighlLow side
2.2. Connection between UPFC and the network
2.4. UPFC control during abnormal conditions
Connections between the series part of the UPFC and the network has a significant impact on transients during abnormal conditions on the network or in the UPFC. During severe contingency, the series part of the UPFC can be bypassed by a circuit breaker. This circuit breaker is not quick enough to protect the power electronics of the series part against overvoItage. Therefore in addition an electronic bypass is needed. The mechanical bypass is used for slow action, as for instance to energize and to de-energize the UPFC.
If the UPFC has to restart quickly after a short-circuit, the Crowbar will be sized to withstand the short circuit current. In this first case, the leakage impedance of the UPFC series transformer will be in the line during the whole relay calculation. If the UPFC is not required very quickly after the recovery, the series part could be bypassed in a few periods to reduce the thermal constraint to the Crowbar. In this second case, the leakage impedance of the series part will only be in the line during a couple of periods.
2.3. Protection of the series part
2.5. UPFC internal faults
The series transformer is protected by surge arresters. To limit the overvoltage at the inverter side, during
The most common fault in power electronics is GTO short-circuit. If one full GTO valve fails in short-
Low Voltage Crowbar
214
circuit, when the complementary GTO triggers, the capacity is short-circuited. The series voltage inserted becomes null, but the consequence is an inrush of current by the shunt part, only limited by the leakage impedance of the transformer. In this case the internal system of protection has to block in a few millisecond the trigger of all GTO. An emergency stop of the UPFC is required, including the de-energization of the shunt part. The impact of this fault on relays depends on the philosophy protection, but in general it is negligible because the elimination of this fault by the UPFC itself occurs in a few milliseconds.
Today this type of network has to be operated with a separated busbar, at substations A and C. But the impact on reliability and short-circuit power is negative. The interest of the UPFC is to enforce the power to flow in cables 3 and 4, and to operate this network interconnected. For solving the problem presented, the power specified for the UPFC would be 7 MYA, the maximum AC voltage inserted is 2000 V, (0.5°). 3.2. Network configuration
The objective is to find the most constrained configuration for the network before the fault, in order to test relay in extreme situation. The pow'er flow was adjusted for the maximal current estimated during the winter peak in 1998. The network is operated in N-l configuration, when cable 2 in unavailable.
3. UPFC IMPACT ON NETWORK RELAYS WITH SHORT LINES In this paragraph, we study through a real practical case the UPFC impact on relays with short lines. The first stage was the modeling with EMfP of the UPFC design, and the calculation of voltages and currents at the location of the protective relays.
We choose to exchange the nominal reactive power by the shunt part. A specific law has been established for the series control, which is based on the simple idea that the ratio between the effective load and the thermal capacity of each of the three links must be equal. We have called this the Prorated Thermal Capacity Law (Daniel 96). The simulations performed during the study have shown that the command chosen for the UPFC before the fault has little impact on the relay.
3.1. Description of the network
The network described in Figure 3 has a problem in the division of power flows, which prevents the maximum thermal capacities of each of the cables from being operated. The network shown is a 225 kV network, with underground cables connecting the various substations. Cables 1 and 2 have thermal capacities (500A) which are half those of cables 3 and 4 (lOOOA). In addition, the sum of the lengths of cables 3 and 4 is approximately twice the length of cables 1 or 2, (5km). There are only loads at the substation Band C. The substation A is the power source, the short -circuit current is about 23 kA.
3.3. Networkfaults studied
The network studied is composed by underground cables, with however a small portion of overhead line (see Figure 4). For cables only single phase zero resistance faults were considered. In fact, for cable, three phase and resistive faults are very seldom. For the overhead line portion, three phase faults are considered.
Substation C
Substation C
Substation B cable 1
Substation B
cable 2
#
Single phase short-circuit
Substation A
@Relays
Substation A
Fig 1: 225kV Network configuration
Fig4: Relays and faults modeled
215
receiving node UR. The Current transformer is fully mode led in EMTP, its ratio is 300.
Figure 4 shows the different short-circuit locations, and the relay locations where the currents are measured.
With no UPFC, after the short-circuit, at time T=300ms a transient is immediately followed by a zero current in cable 1, Figure 6.
3.4. Measurement Reductor
With a UPFC, the current in cable 1 is decreased by the UPFC regulation at time T=lOOms, Figure 7.
The Current Transformer, CT, and the Coupling Capacitor Voltage Transformer, ccvr, have been added in the modeling. The transformer saturation curve have been represented as it is important in case of harmonic voltage.
Current lA) I
I
1 I
I 3.5. Simulation results
ft
All short-circuit voltages showed in Figure 4 were systematically computed. The currents in every relays modeled with their reductors are registered. The EMTP simulations follow the scheme presented in Table 1.
¥
~ V
ft ~
ft V
I
WV V ~ ~ WV
5
J 1
10
Sequence 1 2 3 4
5
100
200
JOO li~
Event Time EMTP Initialization Oms~lOOms UPFC Initialization 100ms~200ms Steady state 200ms~300ms Short-circuit 300ms~370ms Short-circuit sustained 370ms~500ms UPFC bypassed
400
500
mS
Fig 6: Current in cable 1 dun'ng phase short eirc/lir in UR (without UPFC)
With UPFC, after the short-circuit, at time T=300ms the current in cable 1 is not zero while the series transformer is in the line. Figures 6 and 7 prove that UPFC has an impact on the current seen by the relays. The series voltage is not negligible and this voltage induced a current in the loop. This non zero current has an impact on the relays, it can disturb the distance calculation of the relays
The following figures give some EMTP results: Figure 5 shows the short-circuit current flowing in the UPFC line. The event of this simulation is a three phase short-circuit in the UPFC receiving node UR. The amplitude of this current is increased after bypassing the series transformer in 70ms at time T=370ms. Current (kAI 40
1
i
Table 1 Event during EMTP simulation
Cu".nt lA)
Current in US
30 20
10
o
- IOL.o---l+0o----20i-o---3-tOo---40~O--~500
-10
lime-
-20
Fie 7: Current in cable 1 durillg tree phase short circuit ill UR (with UPFC)
-30 -40
m5
1 JO
2110
3110
400
5
Time (mS)
o 3.6. Type ofrelays tested
On Figure 4, we show the relays locations, these relays are protection distance relays. Two types of relays are implemented, but the adjustments are different for every relays. In points BA and US a back up relay is implemented in case offailure of the main relay.
Fig 5: Current in cable 3 during three phase short-circuit in UR (with UPFC)
We compare on Figures 6 and 7 the currents flowing in the Current Transformer in CA, without and with the UPFC, during a three phase short circuit at UPFC
216
3. 7. Relays test results
assume that the relay are distance relay with no communication between them. . - The relay BA will see correctly the fault and open the line in substation B during the first stage. - The relay calculation in AB will find that the fault is outside this line 1, the short circuit will be fed by the substation A during the first stage. Finally, during the second stage, the relay AB will open the line I.
The second stage was carried out in the EDF protection laboratory. The short-circuit currents calculated by EMfP were translated in Morgat format, (Bomard 1988) . The software Morgat is associated with a digital/analog converter device, (Monteat 95). Analog current and voltage measurements of CT and CCVT fed real protective relays. For each possible fault shown Figure 4, we tested all the relays with the real adjustments. The tests showed that behaviors of the protective relays were normal despite the presence of the UPFC. In consequence the UPFC implementation in this network could be done with no modifications of the relay adjustments.
~ -----..
Line 1
~
~C::5~O~~~::::::}-~~
~~------------------~ Line 2
4. llv1P ACT ON RELAYS WITH LOOP FLOW IN LONG LINES
Fig 8: 225kV network configuration
It would be interesting to verify if in general, the relay adjustments have to be modified after UPFC implementation in a Power System. The results described above cannot be generalized to all power system configurations, because they depend on the series transformer leakage reactance and on the UPFC bypass protection. So, if another UPFC installation were decided in the future the approach described above would be applied again.
In this case the time for eliminating the fault on line 1 near the substation B will be increased (in comparison with a reference case where no UPFC is on line.) 4.2. Example of solution for the network One solution is to implement a new relay in UR as shown on Figure 9. This relay will have the same adjustments than the relay implemented in AB, the command of relay AB and UR are added to open the circuit breaker. Eventually it is possible to use only one relay by remoting the signals from point UR to the relay AB, but in case of UPFC failure, the line needs an additional protection. If the UPFC is energized during the fault, the relay in UR will detect the fault in first stage, it will open the circuit breaker in substation A for line 1. If the UPFC is not energized during the fault , the relay in AC will detect the fault in first stage, it will open the circuit breaker in substation A for line 1.
The relay adjustments are calculated for a certain network topology, because the line parameter do not change during the time. But the UPFC series part can be inserted or not, because as explained in the previous section, the UPFC can be bypassed during the fault. The distance calculation achieved by the relay can be affected due to the series transformer. This section gives an example of the potential problems which can happen due to UPFC implementation. 4.1. Example ofproblem with distance calculation by relays The leakage impedance of the UPFC series part transformer could be 30% or more in comparison with the line impedance. In this case the distance calculation is affected and the zone calculated by the relay could be modified.
Line 1
For instance in Figure 8, this 225kV system case has the following characteristics: Overhead line 1: Length = 50km, XLI = 17D., 1nl = 800A UPFC : Sn=100MVA, phase shift = 18.4° Leakage impedance of the series transformer is 10%
Line 2 Substation A
2
t
Substation B
Fig9: 225kV network configuration
= O.l 3 x 41.6 = 5.2D.
X
Substation B
Substation A
100
If the UPFC is implemented in substation A, in case of fault near the substation B: In the following, we
217
A second solution consists in using relay with high speed communication. It is expected that this configuration \\-ill delay the line tripping but the relay orders would be more reliable.
development? An Electricite de France answer based on the development of a prototype Unified Power Flow Controller. " , Symposium CIGRE, TOKYO, 1995.
4.3. Conclusion
This section proves that UPFC impact on relay depends on various factors such as UPFC size in comparison with lines in the ·vicinity. Besides, the types and the control of relays in the network have to be taken into account.
D .Daniel, X.Lombard, c.Poumarede, P.G.Th6rond "UPFC implementation in a 225 KV system objectives and technical aspects for operation." Workshop FACTS EPRI, Baltimore, April 1996.
5. CONCLUSION This paper gives an analysis on UPFC impact on the relays in a Power System. The UPFC impact on relay is strongly linked with the UPFC design, especially the rating, the Crowbar location, and the control during abnormal conditions. The results of this study are based on relays tests fed by voltages and currents calculated with a complete EMTP simulation. Through a real particular case, this paper gives the detail parameters to take into account in the relay tests when implementing UPFC. On the particular case studied, the relay adjustments pennit to operate the network with or without the UPFC. This article shows that this result cannot be generalized to every situation. It gives one example which induce potential problem with relays. The main consequences on the Power System seem to be an additional delay to eliminate the fault due to error in location of the fault. To solve this problem, we suggest to add a relay after the UPFC. This new particular field of study will be investigated in the future. 6. ACKNOWLEDGEMENTS The authors gratefully acknowledge E. Joncquel, D. Daniel and A. Dieudonne for their contributions to the study.
7. REFERENCES
L. Gyugyi "A unified power flow control concept for flexible AC Transmission systems. " ACIDC Power Transmission Conference. lEE Conf. Pub. 345 ; 1991 pp 19-26
P. Bornard, P.Erhard, P.Fauquembergue, " MORGAT: a data processing program for testing transmission line protective relays" , IEEEIPES, voL.3 , no4, October 1988, ppI419-1426. A. Montmeat, A. Giard, 1. Roguin, " MORGAT for testing MV and EHV protective relays" , 6th Conf. Power Electronics and European Applications. 19-21 Sept. 1995. Seville Spain
8. BIOGRAPHIES Xavier Lombard graduated from Ecole Nationale Superieure d'Ingenieurs Electriciens de Grenoble in 1991, he has been with EDF since that date. He worked 15 months on hannonic initialization in EMTP at Hydro Quebec. His research activities is mainly involved with the design the control and the modeling of advanced power conversion systems. He is member of IEEE and CIGRE. Andre Giard receiving his engineering degree from ENS d'electricite et de Mecanique, Nancy in 1975. He is with EDF since 1976 and is research engineer in the Power system Department (protection and control). His main fields of interest are Power System stability, protection and planning. He is member of SEE and CIGRE. Pierre Guy Therond (M'93), he received the diploma of engineer from Ecole Superieure d'Electricite (Paris) in 1983 and the Ph.D degree in physics from the University of Grenoble in 1986. He joined the R&D Division of Electricite de France in 1987. From 1990, he is the head of the group "New Technologies for Networks", managing R&D on power electronics and superconductivity. He is member of SEE, IEEE, CIGRE.
L. Gyugyi, et ai, "The unified power flow controller a new approach for Transmission control " IEEE frans. Power System , 94 SM 474-7 PWRD E . Joncquel, X. Lombard "A Unified Power flow Controller for the ElectroMagnetic Transients Program", 6th European Conf. Power Electronics and Applications. 19-21 Sept. 1995. Seville Spain. X. Lombard, P.G. Therond, "Control of Unified Power Flow Controller: comparison of methods on the basis of an EMTP detailed model". IEEE 96 SM 511-6 PWRD.
D.Daniel, A. Le Du, C.Poumarede, P.G.Therond, B.Langlet, 1.P.Taisne, V.Collet-BilIon "Power electronics: an effective tool for network
218
Sylvain Vitet was born in Paris, France. He received the Engineering Diploma from the ENS des mines de St Etienne, France 1985, and the management Diploma of the institut d' Administration des entreprises, Paris, in 1989. In 1986, he joined the EDF Company. Until 1992, he worked on pollution phenomena on power systems. From 1992 to 1996, he worked as a project manager on alternative solution for the 400kV network. From 1996, he is the head of the group "Power System Dynamics, Protections and Control". He is member of SEE, IEEE, CIGRE.
UNIFIED POWER FLOW CONTROLLER IMPACT ON THE RELAYS IN A POWER SYSTEM
x. Lombard, P.G. Therond, A. Giard, s. Vitet Direction des Etudes et Recherches Electricite de France. Clamart. France
Abstract-This paper gives an approach on Unified Power Flow Controller (UPFC) impacts on the relays in a Power System. The results are based on relay tests fed by voltages and currents calculated with a complete simulation with EMTP (Electromagnetic Transients Program) code. The series part of the UPFC modifies the line impedance and could disturb the relays measurements. The UPFC design, especially the philosophies of protection for the series transformer, have an impact on line impedance. The type of relays and the configuration of grid have to be taken into account. Copyright © 1998lFAC
Keywords: UPFC, Relays, Protection, EMTP
during abnormal conditions, and the calculation of voltages and currents at the location of the protcctive relays. The second stage was carried out in the EDF protection laboratory: the short-circuit currents calculated by EMTP were applied on real protective relays through digitaVanalog amplifiers. The paper is organized as follows: -Section 2 describes UPFC implementation in the network. -Section 3 describes UPFC impact on relays with short lines -Section 4 describes UPFC impact on relays with long lines
1. IN1RODUCTION
In the context of a network subject to higher and higher loads, utility companies are looking for new equipment's which will enable increased power levels to be transmitted on existing power lines. In recent years, advances in semiconductor technology have made available devices with a gate turn-off capability. These developments offer a new technological base for Flexible Alternating Current Transmission System (FACTS) concepts. One particular new concept called UPFC has been proposed by Gyugyi (1991). With this equipment, a wide range of typical transmission parameters, such as voltage, line impedance and phase angle, can be efficiently controlled.
2. UPFC IMPLEMENT ATION IN THE NETWORK
Having a normal behavior of the protective relays on the power system where a UPFC would be installed is a key point. In this paper, we consider the case where the protection scheme relies on distance protections which measure the impedance between the fault location and the circuit breaker. In this case, the UPFC leakage reactance of the series transformer can potentially disturb the impedance measurement depending if the UPFC is in or out of use. It was therefore decided to check this crucial point. The method used was made of two stages.
In the following paragraphs, we give a description of a UPFC and of main parameters to take into account for its impact on relays. 2.1. Description o/the UPFC
A Unified Power-Flow Controller which is shown in Figure 1, consists of two parts: one is connected in parallel with the network and the other in series with it. Each part is composed of one or more transformers followed by Graetz bridges with GTO thyristors. The two parts are connected through a DC link, smoothed by a capacitor. .
The first stage was the modeling with EMTP of the UPFC including the internal protections, the control
213
ownstream short circuit, the short circuit current ust be derived extremely fast from GTO. As xplained above, the solution is to pro\'ide an lectronic bypass also called a Crowbar. Two options re possible to implement the Crowbar: either on the igh voltage side of the series transformer or on the ow voltage side. the Crowbar is implemented at high voltage side. the mpact on the relays is reduced because the Crowbar hort-cireuits the series transformer, in case of big ontingency. From the network point of view, the FC series part is no more presents and the relav will ot be disturbed during the calculation of the lo~ation !shunt fthe fault. VI f the Crowbar is implemented at low voltage. the mpact on the relays is increased because in case of L -_ _ _ _ _ _ _ _ _..2.-.!..:.!~'___ _ _ _ _ _ _ _.ijown stream short-circuit, the leakage impedance of the series transformer is still present in the line. In the Fig 1: Simplified diagram o/the network and the UPFC UPFC vicinity, the relay calculation could be disturbed by the leakage impedance of the transformer. The UPFC series part is used to modify the active and Therefore, to minimize the impact on the rclays. it is the reactive power flows in the line of the network in interesting to use Crowbar on high voltage. But the which it is inserted. The series part applies a series practical costs for insulation are very high in voltage into the network. The amplitude and the phase comparison with this option. of this voltage can be adjusted in real time. Bus A
!series
Regulation of the shunt part has two main functions: - In continuous operation, the reactive power exchanged depends on the amplitude difference between the AC inverter voJtages and the network. - To regulate the voltage of the capacitor, and to supply the active power drawn by the series part, the shunt part is able to exchange the active power in a transient manner by changing the phase between voltages of the inverter and the network.
Cicuir Breaker
HighVoltage Crowbar
I-I1---+:::J-~I 1
,""-...1
~
I
I
UPFC line
To simulate the behavior of the UPFC, a very complete model has been developed using EMTP (Joncquel, Lombard). Each GTO device, and its diode in antiparallel, is represented. The internal protections as Crowbar and the control during abnormal conditions for the shunt and the series part is included.
Fig 2: Crowbar location on HighlLow side
2.2. Connection between UPFC and the network
2.4. UPFC control during abnormal conditions
Connections between the series part of the UPFC and the network has a significant impact on transients during abnormal conditions on the network or in the UPFC. During severe contingency, the series part of the UPFC can be bypassed by a circuit breaker. This circuit breaker is not quick enough to protect the power electronics of the series part against overvoItage. Therefore in addition an electronic bypass is needed. The mechanical bypass is used for slow action, as for instance to energize and to de-energize the UPFC.
If the UPFC has to restart quickly after a short-circuit, the Crowbar will be sized to withstand the short circuit current. In this first case, the leakage impedance of the UPFC series transformer will be in the line during the whole relay calculation. If the UPFC is not required very quickly after the recovery, the series part could be bypassed in a few periods to reduce the thermal constraint to the Crowbar. In this second case, the leakage impedance of the series part will only be in the line during a couple of periods.
2.3. Protection of the series part
2.5. UPFC internal faults
The series transformer is protected by surge arresters. To limit the overvoltage at the inverter side, during
The most common fault in power electronics is GTO short-circuit. If one full GTO valve fails in short-
Low Voltage Crowbar
214
circuit, when the complementary GTO triggers, the capacity is short-circuited. The series voltage inserted becomes null, but the consequence is an inrush of current by the shunt part, only limited by the leakage impedance of the transformer. In this case the internal system of protection has to block in a few millisecond the trigger of all GTO. An emergency stop of the UPFC is required, including the de-energization of the shunt part. The impact of this fault on relays depends on the philosophy protection, but in general it is negligible because the elimination of this fault by the UPFC itself occurs in a few milliseconds.
Today this type of network has to be operated with a separated busbar, at substations A and C. But the impact on reliability and short-circuit power is negative. The interest of the UPFC is to enforce the power to flow in cables 3 and 4, and to operate this network interconnected. For solving the problem presented, the power specified for the UPFC would be 7 MYA, the maximum AC voltage inserted is 2000 V, (0.5°). 3.2. Network configuration
The objective is to find the most constrained configuration for the network before the fault, in order to test relay in extreme situation. The pow'er flow was adjusted for the maximal current estimated during the winter peak in 1998. The network is operated in N-l configuration, when cable 2 in unavailable.
3. UPFC IMPACT ON NETWORK RELAYS WITH SHORT LINES In this paragraph, we study through a real practical case the UPFC impact on relays with short lines. The first stage was the modeling with EMfP of the UPFC design, and the calculation of voltages and currents at the location of the protective relays.
We choose to exchange the nominal reactive power by the shunt part. A specific law has been established for the series control, which is based on the simple idea that the ratio between the effective load and the thermal capacity of each of the three links must be equal. We have called this the Prorated Thermal Capacity Law (Daniel 96). The simulations performed during the study have shown that the command chosen for the UPFC before the fault has little impact on the relay.
3.1. Description of the network
The network described in Figure 3 has a problem in the division of power flows, which prevents the maximum thermal capacities of each of the cables from being operated. The network shown is a 225 kV network, with underground cables connecting the various substations. Cables 1 and 2 have thermal capacities (500A) which are half those of cables 3 and 4 (lOOOA). In addition, the sum of the lengths of cables 3 and 4 is approximately twice the length of cables 1 or 2, (5km). There are only loads at the substation Band C. The substation A is the power source, the short -circuit current is about 23 kA.
3.3. Networkfaults studied
The network studied is composed by underground cables, with however a small portion of overhead line (see Figure 4). For cables only single phase zero resistance faults were considered. In fact, for cable, three phase and resistive faults are very seldom. For the overhead line portion, three phase faults are considered.
Substation C
Substation C
Substation B cable 1
Substation B
cable 2
#
Single phase short-circuit
Substation A
@Relays
Substation A
Fig 1: 225kV Network configuration
Fig4: Relays and faults modeled
215
receiving node UR. The Current transformer is fully mode led in EMTP, its ratio is 300.
Figure 4 shows the different short-circuit locations, and the relay locations where the currents are measured.
With no UPFC, after the short-circuit, at time T=300ms a transient is immediately followed by a zero current in cable 1, Figure 6.
3.4. Measurement Reductor
With a UPFC, the current in cable 1 is decreased by the UPFC regulation at time T=lOOms, Figure 7.
The Current Transformer, CT, and the Coupling Capacitor Voltage Transformer, ccvr, have been added in the modeling. The transformer saturation curve have been represented as it is important in case of harmonic voltage.
Current lA) I
I
1 I
I 3.5. Simulation results
ft
All short-circuit voltages showed in Figure 4 were systematically computed. The currents in every relays modeled with their reductors are registered. The EMTP simulations follow the scheme presented in Table 1.
¥
~ V
ft ~
ft V
I
WV V ~ ~ WV
5
J 1
10
Sequence 1 2 3 4
5
100
200
JOO li~
Event Time EMTP Initialization Oms~lOOms UPFC Initialization 100ms~200ms Steady state 200ms~300ms Short-circuit 300ms~370ms Short-circuit sustained 370ms~500ms UPFC bypassed
400
500
mS
Fig 6: Current in cable 1 dun'ng phase short eirc/lir in UR (without UPFC)
With UPFC, after the short-circuit, at time T=300ms the current in cable 1 is not zero while the series transformer is in the line. Figures 6 and 7 prove that UPFC has an impact on the current seen by the relays. The series voltage is not negligible and this voltage induced a current in the loop. This non zero current has an impact on the relays, it can disturb the distance calculation of the relays
The following figures give some EMTP results: Figure 5 shows the short-circuit current flowing in the UPFC line. The event of this simulation is a three phase short-circuit in the UPFC receiving node UR. The amplitude of this current is increased after bypassing the series transformer in 70ms at time T=370ms. Current (kAI 40
1
i
Table 1 Event during EMTP simulation
Cu".nt lA)
Current in US
30 20
10
o
- IOL.o---l+0o----20i-o---3-tOo---40~O--~500
-10
lime-
-20
Fie 7: Current in cable 1 durillg tree phase short circuit ill UR (with UPFC)
-30 -40
m5
1 JO
2110
3110
400
5
Time (mS)
o 3.6. Type ofrelays tested
On Figure 4, we show the relays locations, these relays are protection distance relays. Two types of relays are implemented, but the adjustments are different for every relays. In points BA and US a back up relay is implemented in case offailure of the main relay.
Fig 5: Current in cable 3 during three phase short-circuit in UR (with UPFC)
We compare on Figures 6 and 7 the currents flowing in the Current Transformer in CA, without and with the UPFC, during a three phase short circuit at UPFC
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3. 7. Relays test results
assume that the relay are distance relay with no communication between them. . - The relay BA will see correctly the fault and open the line in substation B during the first stage. - The relay calculation in AB will find that the fault is outside this line 1, the short circuit will be fed by the substation A during the first stage. Finally, during the second stage, the relay AB will open the line I.
The second stage was carried out in the EDF protection laboratory. The short-circuit currents calculated by EMfP were translated in Morgat format, (Bomard 1988) . The software Morgat is associated with a digital/analog converter device, (Monteat 95). Analog current and voltage measurements of CT and CCVT fed real protective relays. For each possible fault shown Figure 4, we tested all the relays with the real adjustments. The tests showed that behaviors of the protective relays were normal despite the presence of the UPFC. In consequence the UPFC implementation in this network could be done with no modifications of the relay adjustments.
~ -----..
Line 1
~
~C::5~O~~~::::::}-~~
~~------------------~ Line 2
4. llv1P ACT ON RELAYS WITH LOOP FLOW IN LONG LINES
Fig 8: 225kV network configuration
It would be interesting to verify if in general, the relay adjustments have to be modified after UPFC implementation in a Power System. The results described above cannot be generalized to all power system configurations, because they depend on the series transformer leakage reactance and on the UPFC bypass protection. So, if another UPFC installation were decided in the future the approach described above would be applied again.
In this case the time for eliminating the fault on line 1 near the substation B will be increased (in comparison with a reference case where no UPFC is on line.) 4.2. Example of solution for the network One solution is to implement a new relay in UR as shown on Figure 9. This relay will have the same adjustments than the relay implemented in AB, the command of relay AB and UR are added to open the circuit breaker. Eventually it is possible to use only one relay by remoting the signals from point UR to the relay AB, but in case of UPFC failure, the line needs an additional protection. If the UPFC is energized during the fault, the relay in UR will detect the fault in first stage, it will open the circuit breaker in substation A for line 1. If the UPFC is not energized during the fault , the relay in AC will detect the fault in first stage, it will open the circuit breaker in substation A for line 1.
The relay adjustments are calculated for a certain network topology, because the line parameter do not change during the time. But the UPFC series part can be inserted or not, because as explained in the previous section, the UPFC can be bypassed during the fault. The distance calculation achieved by the relay can be affected due to the series transformer. This section gives an example of the potential problems which can happen due to UPFC implementation. 4.1. Example ofproblem with distance calculation by relays The leakage impedance of the UPFC series part transformer could be 30% or more in comparison with the line impedance. In this case the distance calculation is affected and the zone calculated by the relay could be modified.
Line 1
For instance in Figure 8, this 225kV system case has the following characteristics: Overhead line 1: Length = 50km, XLI = 17D., 1nl = 800A UPFC : Sn=100MVA, phase shift = 18.4° Leakage impedance of the series transformer is 10%
Line 2 Substation A
2
t
Substation B
Fig9: 225kV network configuration
= O.l 3 x 41.6 = 5.2D.
X
Substation B
Substation A
100
If the UPFC is implemented in substation A, in case of fault near the substation B: In the following, we
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A second solution consists in using relay with high speed communication. It is expected that this configuration \\-ill delay the line tripping but the relay orders would be more reliable.
development? An Electricite de France answer based on the development of a prototype Unified Power Flow Controller. " , Symposium CIGRE, TOKYO, 1995.
4.3. Conclusion
This section proves that UPFC impact on relay depends on various factors such as UPFC size in comparison with lines in the ·vicinity. Besides, the types and the control of relays in the network have to be taken into account.
D .Daniel, X.Lombard, c.Poumarede, P.G.Th6rond "UPFC implementation in a 225 KV system objectives and technical aspects for operation." Workshop FACTS EPRI, Baltimore, April 1996.
5. CONCLUSION This paper gives an analysis on UPFC impact on the relays in a Power System. The UPFC impact on relay is strongly linked with the UPFC design, especially the rating, the Crowbar location, and the control during abnormal conditions. The results of this study are based on relays tests fed by voltages and currents calculated with a complete EMTP simulation. Through a real particular case, this paper gives the detail parameters to take into account in the relay tests when implementing UPFC. On the particular case studied, the relay adjustments pennit to operate the network with or without the UPFC. This article shows that this result cannot be generalized to every situation. It gives one example which induce potential problem with relays. The main consequences on the Power System seem to be an additional delay to eliminate the fault due to error in location of the fault. To solve this problem, we suggest to add a relay after the UPFC. This new particular field of study will be investigated in the future. 6. ACKNOWLEDGEMENTS The authors gratefully acknowledge E. Joncquel, D. Daniel and A. Dieudonne for their contributions to the study.
7. REFERENCES
L. Gyugyi "A unified power flow control concept for flexible AC Transmission systems. " ACIDC Power Transmission Conference. lEE Conf. Pub. 345 ; 1991 pp 19-26
P. Bornard, P.Erhard, P.Fauquembergue, " MORGAT: a data processing program for testing transmission line protective relays" , IEEEIPES, voL.3 , no4, October 1988, ppI419-1426. A. Montmeat, A. Giard, 1. Roguin, " MORGAT for testing MV and EHV protective relays" , 6th Conf. Power Electronics and European Applications. 19-21 Sept. 1995. Seville Spain
8. BIOGRAPHIES Xavier Lombard graduated from Ecole Nationale Superieure d'Ingenieurs Electriciens de Grenoble in 1991, he has been with EDF since that date. He worked 15 months on hannonic initialization in EMTP at Hydro Quebec. His research activities is mainly involved with the design the control and the modeling of advanced power conversion systems. He is member of IEEE and CIGRE. Andre Giard receiving his engineering degree from ENS d'electricite et de Mecanique, Nancy in 1975. He is with EDF since 1976 and is research engineer in the Power system Department (protection and control). His main fields of interest are Power System stability, protection and planning. He is member of SEE and CIGRE. Pierre Guy Therond (M'93), he received the diploma of engineer from Ecole Superieure d'Electricite (Paris) in 1983 and the Ph.D degree in physics from the University of Grenoble in 1986. He joined the R&D Division of Electricite de France in 1987. From 1990, he is the head of the group "New Technologies for Networks", managing R&D on power electronics and superconductivity. He is member of SEE, IEEE, CIGRE.
L. Gyugyi, et ai, "The unified power flow controller a new approach for Transmission control " IEEE frans. Power System , 94 SM 474-7 PWRD E . Joncquel, X. Lombard "A Unified Power flow Controller for the ElectroMagnetic Transients Program", 6th European Conf. Power Electronics and Applications. 19-21 Sept. 1995. Seville Spain. X. Lombard, P.G. Therond, "Control of Unified Power Flow Controller: comparison of methods on the basis of an EMTP detailed model". IEEE 96 SM 511-6 PWRD.
D.Daniel, A. Le Du, C.Poumarede, P.G.Therond, B.Langlet, 1.P.Taisne, V.Collet-BilIon "Power electronics: an effective tool for network
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Sylvain Vitet was born in Paris, France. He received the Engineering Diploma from the ENS des mines de St Etienne, France 1985, and the management Diploma of the institut d' Administration des entreprises, Paris, in 1989. In 1986, he joined the EDF Company. Until 1992, he worked on pollution phenomena on power systems. From 1992 to 1996, he worked as a project manager on alternative solution for the 400kV network. From 1996, he is the head of the group "Power System Dynamics, Protections and Control". He is member of SEE, IEEE, CIGRE.