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Research on regional centralized protection scheme for distribution network integrated with electric vehicles
Electrical Power and Energy Systems 119 (2020) 105903
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Research on regional centralized protection scheme for distribution network integrated with electric vehicles
T
Jing Maa,b,⁎, Jing Liua, Gengyu Yanga, A.G. Phadkeb a b
State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
ARTICLE INFO
ABSTRACT
Keywords: Electric vehicle Distribution network Short circuit current calculation Regional protection
In view of the difficulty in balancing between selectivity and sensitivity of current protection in distribution networks integrated with electric vehicles, a regional current protection method applicable to distribution networks with large-scale electric vehicle penetration is proposed. First, considering the effect of the converter control strategy of electric vehicle chargers, a theoretical analysis is made to determine the expression of the short circuit current that the electric vehicle charger provides when faults occur on feeders. On this basis, according to the minimum short circuit current, the current protection criterion that meets the requirement of sensitivity alone is established. And then, combining multiple start-up signals of the criterion on the fault line and multiple-level neighboring lines after fault occurs, the regional current protection scheme for distribution network is constructed. Finally, simulation tests conducted in Real-Time Digital Simulator (RTDS) of IEEE 33-bus system verify that the proposed method can effectively avoid mal-cooperation between different protection zones which is common in traditional current protection. Besides, the proposed scheme is not affected by factors such as the charging capacity of electric vehicles, the number and the location of charging facilities, etc., and can accurately identify the fault location even when the information is partly missing or in error.
1. Introduction Massive integration of electric vehicles to the distribution network not only greatly affects the variation of operation mode and network structure of distribution network, but also determines the distortion characteristics of electrical variables when a fault occurs on feeders. As the penetration rate of electric vehicles in the distribution network ever increases, the transient characteristic of fault current has become ever more complicated, thus protection setting and the coordination between different protection zones has become more difficult, and the contradiction between four requirements of relay protection (i.e. reliability, selectivity, speed and sensitivity) ever more prominent. Therefore, there is need for a strategy to harmonize the four requirements and improve the performance of relay protection [1–6]. Current protection schemes mainly include limiting penetration capacity schemes [7,8], adaptive protection schemes [9–12] and fault positioning schemes based on communication technology [13–16], etc. Limiting penetration capacity schemes take into account factors such as harmonic constraint, voltage constraint, protection adaptability, etc. and obtain the allowed penetration capacity of electric vehicle in the distribution network passively. Adaptive protection schemes are based
⁎
on the original protection configuration, and by calculating parameters such as system emf (electromotive force), equivalent impedance, branch coefficient, etc. can adjust protection setting values online, thus identifying and clearing the fault. As for fault positioning schemes based on communications, by using the acquisition, transmission, and processing of information such as the amplitude, phase, and protection operation status, etc. of local or global electrical quantities in the distribution network, the fault position can be located. However, when electric vehicles are integrated on a large scale, the structure and operation mode of distribution network will vary frequently [17]. Existing protection method cannot fundamentally solve the problems in protection setting and coordination between different protection zones, as well as the contradiction between the requirements of selectivity and sensitivity. Therefore, it is urgent to study new protection schemes adaptable to distribution network with large-scale integration of electric vehicles [18]. In view of the above problems, a regional current protection method applicable to a distribution network with large-scale electric vehicle penetration is proposed in this paper. Considering the effect of converter control strategy, the analytical expression of short circuit current the electric vehicle charger injects into the distribution network when
Corresponding author at: No. 52 Mailbox, North China Electric Power University, No. 2 Beinong Road, Changping District, Beijing 102206, China. E-mail address: [email protected] (J. Ma).
https://doi.org/10.1016/j.ijepes.2020.105903 Received 27 August 2019; Received in revised form 29 January 2020; Accepted 3 February 2020 Available online 12 February 2020 0142-0615/ © 2020 Elsevier Ltd. All rights reserved.
Electrical Power and Energy Systems 119 (2020) 105903
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fault occurs on feeders is obtained. On this basis, by analyzing the effect of electric vehicle charger on the parameters of protection backside system in the distribution network, the minimum short circuit current feed-in from electric vehicle charger to the distribution network is calculated. And then, the current protection criterion that only meets the requirement of sensitivity is established according to the value of the minimum short circuit current. After fault occurs, combining multiple start-up signals of the criterion on multiple-level neighboring lines, the regional current protection scheme for distribution network is constructed. Finally, simulation tests conducted in Real-Time Digital Simulator (RTDS) verify the correctness and feasibility of the proposed scheme.
of short circuit current output from the converter of electric vehicle charger is:
id =
CUdc
dUdc dt
1.5Ud
iq* =
Q 1.5Eg
2.1. Analytical expression of short circuit current of electric vehicle charger considering converter control
iq =
2 Imax
( i = min (
3
P = 2 (ed id + eq iq) = 2 Eg id Q=
ed iq) =
3 E i 2 g q
id = min (1)
PC = udc idc
id2
PBSS 1.5Eg
(5)
(7)
P BSS ,I 1.5Eg max 2 Imax
)
id2 ,
Q 1.5Eg
)
(8)
(9)
IBE = f (Eg )
When an asymmetrical inter-phase fault occurs in the distribution network, the current that electric vehicle charger injects into the distribution network will contain negative-sequence component and various harmonics. In order to improve the characteristics of output current, scholars put forward the positive-sequence component control strategy, which takes the positive-sequence component of voltage at grid-connecting point Eg+ as reference, at the same time filtering out the harmonic components of power generated by the negative-sequence component. Thus, when asymmetrical fault occurs in the distribution network, the short circuit current will not contain negative-sequence component and can be expressed as:
(2)
id = min iq = min
P BSS 1.5Eg+ 2 Imax
, Imax id2 ,
Q 1.5Eg+
(10)
In (10), the electric vehicle charger can be equalized to a non-linear current source controlled by the positive-sequence component of voltage at PCC (Point of Common Coupling), i.e.
(3)
IBE = f (E g + )
(1) When d-axis current component id exceeds the limited value Imax, the reference value of d-axis component of short circuit current output from the converter of electric vehicle charger is shown in (4). In this case, q-axis current component iq = 0.
id = Imax
+ PBSS
1.5Eg
According to (8), the amplitude and the phase of the short circuit current of electric vehicle charger are determined by the voltage at grid-connecting point Eg . Therefore, when fault occurs, the electric vehicle charger can be equalized to a current source model controlled by the voltage at grid-connecting point Eg :
In (2), P is the active power the distribution network provides for the electric vehicle charger, PBSS is the active power absorbed by the electric vehicle battery, and PC is the active power absorbed by the DC capacitance. In (2), the active power absorbed by the DC capacitance PC can be expressed as:
dU = CUdc dc dt
dUdc dt
(6)
q
where P is the active power, Q is the reactive power, and Eg is the amplitude of the voltage at the grid-connecting point. id and iq are the daxis and q-axis components of VSR output current. ed and eq are the daxis and q-axis components of voltage at the grid-connecting point. The electric vehicle charger usually applies the current inner-loop plus power outer-loop control strategy based on feedforward compensation and PI control mode, thus the electric vehicle charger can be equalized to a current source model related to the control command value [20,21]. Limited by the physical property of converter itself, when fault occurs in the distribution network, the short circuit current allowed through the converter of electric vehicle charger I cannot exceed the maximum value Imax [22,23]. When the amplitude of controller output short circuit current exceeds the maximum short circuit current allowed through the converter, the amplitude of short circuit current will be limited to be Imax and the control mode will be changed to constant power control with the output of active power as priority. The exchange of active power between the distribution network and electric vehicle charger can be expressed as:
P = PC + PBSS
CUdc
where Q is the controller command value of converter output reactive power. According to the reference values of d-axis and q-axis current components shown in (4)–(7), consider that the current inner loop is capable of fast and accurate tracking, the d-axis and q-axis components of electric vehicle charger output current can be expressed as:
The electric vehicle charger charges the battery of electric vehicles through three-phase voltage PWM rectifier, i.e. Voltage Source Rectifier (VSR). In dq rotating coordinate system, according to the grid voltage directional vector control technique, the charging power on AC side of VSR [19] can be expressed as:
3 (e i 2 q d
=
(3) When d-axis current component id does not exceed the limited value Imax, if the amplitude of short circuit current output I does not exceed Imax, according to (1) the reference value of q-axis component is shown in (6); if the amplitude of short circuit current output I exceeds Imax, according to the amplitude-limiting strategy of controller, the reference value of q-axis component is shown in (7).
2. Analytical expressions of short circuit currents of electric vehicle charger and distribution network
3
+ PBSS
(11)
The proposed method considers the charger type of existing EVs and is applicable to both DC fast charging and AC slow charging. This paper analyzes the transient characteristics of short circuit current EVs inject into the distribution network via grid-side converter in detail. For chargers using DC fast charging and AC slow charging, the grid-side converter and control strategy are the same, as well as the transient characteristics of output short circuit currents, thus the proposed method is applicable to both charger types.
(4)
(2) When d-axis current component id does not exceed the limited value Imax, according to (1)-(3), the reference value of d-axis component 2
Electrical Power and Energy Systems 119 (2020) 105903
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Fig. 3. Voltage-current phasor diagram when fault occurs.
charging facility is integrated, Eg = Um + Im Zg1. Es the equivalent emf of protection backside system when the electric vehicle charger is integrated, Es = Um + Is Zg1. After fault occurs in the distribution network, since the electric vehicle charger applies the control strategy with active power as a priority, the charging current of electric vehicle charger IBE and the measured voltage at grid-connecting bus Um are approximately reversed in phase. The measured current at the relaying point Im lags behind Um in phase by L . According to Kirchhoff’s current law (KCL), the current in protection backside system Is can be expressed as:
Is = IBE + Im
According to (12) and (13), the relationship between different voltage and current phasors can be obtained, as shown in Fig. 3. It can be seen that, Es < Eg , i.e. the amplitude of system equivalent emf drops after electric vehicle is integrated. Meanwhile, since the equivalent impedance of the protection backside system remains unchanged, the amplitude of the short circuit current at the relaying point will drop after the electric vehicle charger is integrated. In this case, if current protection is still set in the traditional way, the sensitivity of protection will go lower and the protection may refuse to operate. When three-phase fault occurs in the distribution network, there is only positive-sequence network, and similar analyzing process can be conducted, which is not repeated here. Therefore, when an inter-phase fault occurs at ZL on the feeders, the short circuit current at the relaying point Imf can be expressed with protection backside equivalent emf Es and equivalent impedance Zg as:
Fig. 1. Phase-to-phase metallic fault analysis.
2.2. Analytical expression of short circuit current of the distribution network When a fault occurs in the distribution network, the equivalent emf and system impedance of protection backside system is affected by the injection current from electric vehicle charger. As shown in Fig. 1, take the protection device at bus A as an example, the electric vehicle charger can be equalized to a non-linear current source, and the other parts can be substituted with the equivalent Thevenin circuit. ZL1 and ZLoad1 are the equivalent positive-sequence impedances of feeders and load. ZL2 and ZLoad2 are the equivalent negative-sequence impedances of feeders and load. Suppose phase B-to-C fault occurs at bus B, consider that the injection current from electric vehicle charger only contains positive-sequence component, the equivalent model of electric vehicle charger only exists in the positive-sequence network, as shown in Fig. 1(b). The negative-sequence network is shown in Fig. 1(c), which is the same as traditional negative-sequence network of distribution system, without the equivalent model of electric vehicle charger. Therefore, the following analysis is focused on the effect of electric vehicle chargers on the positive-sequence network. The electric vehicle charger can be equalized to a constant current source at any moment, thus the Thevenin equivalent circuit of the positive-sequence network can be obtained, shown in Fig. 2. It can be seen from Fig. 2 that, the injection current from electric vehicle charger does not change the positive-sequence equivalent impedance of protection backside system Zg1, but only affects the equivalent emf Es :
Es=(Eg Zg1 + IBE ) Zg1 = Eg + Zg1 IBE
(13)
Imf = K d
Es Zg + ZL
(14)
where K d is a fault type coefficient. For phase-to-phase fault, K d = 3 2 ; for three-phase short circuit fault, K d = 1. 3. Regional current protection scheme based on protection startup information 3.1. Regional protection operation criterion Considering that traditional current protection may refuse to operate after an electric vehicle is integrated due to lowered sensitivity, a new protection setting method that gives priority to sensitivity is put forward. (1) Current protection Zone-II guarantees adequate sensitivity for faults occurring at the end of the protected line; and (2) current protection Zone-III guarantee adequate sensitivity for faults occurring at the end of the subordinate line. The protection range of current protection set according to the requirement of sensitivity is shown in Fig. 4. Take protection 2 for example, the setting formula of Zone-II and Zone-III are shown in (15) and (16) respectively.
(12)
where Eg is the equivalent emf of protection backside system when no
I2II = I2III = Fig. 2. Thevenin equivalent circuit of positive-sequence network.
Ik . C . min II Ksen Ik . D . min III Ksen
(15) (16)
where Ik.C.min and Ik.D.min are minimum short circuit currents that go 3
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Fig. 5. A 10 kV distribution network in a certain area.
another feeder line. After fault occurs, the start-up information of current protection zone-II and zone-III on different feeder lines is sent to the regional host computer, which then returns corresponding tripping information to circuit breakers 1–6 according to the calculation results of protection criteria. Suppose fault occurs at F1 in Fig. 4, in this case, Zone-II and Zone-III of protection 1 and 2 will all start, while Zone-II and Zone-III of protection 3, 4, 5 will not start. The host computer will calculate according to the protection criteria concerning protection 1 and 2 respectively. For protection 2, E1 = 1 means the fault occurs on the line where protection 2 is; E2 = 0 means the fault occurs on the subordinate line of protection 2; (E1 + E2) = 1, thus the ‘Operate’ command is issued. Similarly, for protection 1, it can be calculated that (E1 + E2) = −1, thus the host computer determines the fault is not on this line and issues blocking signal to protection 1. Therefore, mal-cooperation between protection 1 and protection 2 can be effectively avoided.
Fig. 4. Protection range of setting method that gives priority to the requirement of sensitivity.
through the protection device when a phase-to-phase fault occurs at the end of feeders, when the charging facility of electric vehicle is in fullII III load operation. The sensitivity coefficient Ksen and Ksen are set to 1.2. This setting method can guarantee adequate sensitivity for current protection, but may fail to meet the requirement of selectivity, as shown in Fig. 4. (1) When fault occurs at F1, since the operation delay of zoneII of protection 1 is the same as that of protection 2, they will both operate at the same time, i.e. selectivity is not satisfied. (2) When fault occurs at F2, if zone-II of protection 2 fails to operate, since the operation delay of zone-III of protection 1 is shorter than that of protection 2, zone-III of protection 1 will operate before zone-III of protection 2, i.e. selectivity is not satisfied. Concerning the above problems, a regional current protection scheme based on multiple fault information is proposed in this paper. The regional host computer is installed at the outlet of the terminal substation feeder line. By collecting the start-up information of current protection Zone-II and Zone-III on different feeder lines, the host computer can make a centralized decision according to protection criteria shown in (17), and then send corresponding commands (‘operate’, ‘not operate’ or ‘block’) to circuit breakers on different feeder lines.
1 ((ISII = 1) E1 =
( 0 1 ((ISII = 1)
E2 =
i,
As for problems such as loss of protection start-up information and transmission error which may exist in a regional protection system, the following countermeasure is taken. (1) Loss of protection start-up information When problems such as protection device failure, measuring device disconnection and communication module fault, etc. occur in the distribution network protection system, the host computer will not be able to receive the start-up information of certain protection device, i.e. certain start-up information will be missing. In this case, the start-up information of Zone-II and Zone-III of this protection device are both set to be ‘1′ for protection criteria calculation. If the fault still exists after current protection operates, the protection start-up information is changed from ‘1′ to ‘0′ and the protection criteria are re-calculated. Commonly seen cases of protection start-up information loss include three types—loss of start-up information of protection on the fault line, on the upstream line of the fault point and on the downstream line of the fault point. When the start-up information of protection on the fault line is missing, the default start-up information is ‘1′, thus the fault can still be accurately cleared. When the start-up information of protection on healthy line in the upstream of fault point is missing, the missing information can be compensated by the start-up information of protection on the downstream line, thus the proposed method is fault-tolerant. For example, suppose fault occurs at F2 in Fig. 4 and the start-up information of protection 2 is missing, regional protection can still make the correct identification. For protection 2, E1 = 0, E2 = −1, E1 + E2 = −1 < 0, thus protection 2 is blocked; for protection 3, E1 = 1, E2 = 0, E1 + E2 = 1, thus protection 3 should operate. The regional host computer will issue a tripping command and clear the fault correctly. When the start-up information of protection on non-fault line in the downstream of fault point is missing, the host computer will set the missing information as ‘1′, thus it is identified that the fault is on the
i, ILII = 0)
((
ILIII
= 0)) others
(ISIII = 1)) (
0
(ISIII = 1))
3.2. Fault tolerance analysis
((
i = 1, ILII(i) = 1)
i = 1, ILIII(i) = 1)) others
Operate (E1 + E2 ) > 0 Not operate (E1 + E2 ) = 0 Block (E1 + E2 ) < 0
(17)
where the I with superscripts ‘II’ and ‘III’ represent the start-up information of current protection Zone-II and Zone-III respectively. Subscript ‘S’ represents protection device on the protected line; subscript ‘L’ represents protection device on the subordinate line. i is the number of the protection device. ‘1′ means current protection device starts; ‘0′ means current protection device does not start. ‘Operate’ means the host computer issues tripping command to circuit breakers on this feeder line; ‘Not operate’ means the fault is not on this line, and the host computer does not issue tripping command to circuit breakers on this feeder line; ‘Block’ means the fault is within the operation range of this protection device, but not on the feeder line where the protection device is, thus this protection device need not operate, and in order to avoid override tripping and guarantee selectivity of current protection, the host computer issues blocking signal to circuit breakers on this feeder line. Take the 10 kV distribution network in Fig. 5 for example. Protection 1, 2, 3 and 4 are on the same feeder line, protection 5 and 6 are on 4
Electrical Power and Energy Systems 119 (2020) 105903
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line where the protection start-up information is missing. After certain time delay (the length of time delay is set according to engineering practice of real distribution network, usually shorter than the time delay of traditional current protection zone-II), if the host computer detects that the fault still exists, the missing information will be reset from ‘1′ to ‘0′ and protection criteria will be re-calculated. And then the host computer will send ‘Operate’ command to the right protection device. Therefore, in this case, the fault line can still be cleared accurately, without losing selectivity. For example, in Fig. 4 suppose fault occurs at F1 and the start-up information of protection 3 is missing, first it will be determined that the fault is on line CD. After certain time delay, the host computer detects that the fault is not cleared, thus the start-up information of protection 3 is reset to be ‘0′ for criteria calculation. The criteria calculation result of protection 2 is: E1 = 1, E2 = 0, E1 + E2 = 1, thus the host computer will issue tripping command to protection 2 and clear the fault. It can be seen that, when a single signal is missing, the proposed regional protection scheme can still correctly clear the fault line.
Fig. 6. Diagram of IEEE 33-bus distribution system.
1.8 MW. The regional host computer is installed at power source node 1. The start-up information of Zone-II and Zone-III of all current protection devices in the distribution system are sent to the host computer. 4.2. Verification of the calculated short circuit current of the distribution network
(2) Transmission error in protection start-up information
Set phase-to-phase fault and three-phase fault at the terminal point of distribution line L8-9, i.e. F1 respectively. When the charging capacity applies different values, the short circuit current that goes through protection R6-7 can be calculated according to (14), and the waveforms of calculated value and simulated value are shown in Fig. 7, where the solid line represents the calculated value, and the dashed line represents the simulated value. The simulation curves represent the amplitudes of short circuit current phasors in the transient process, not the instantaneous values of short circuit currents. As for the calculated values, first, the phasor values of voltage and current measured at the relaying point (i.e. Um and Im ) are processed according to certain algorithm, and then the equivalent emf Es and equivalent impedance Zg of protection backside system are calculated, thus according to (14), the amplitude of short circuit current phasor can be calculated. It can be seen that, in the fault duration, the calculation result and simulation result of short circuit current are all very close in different cases, with
Transmission error in protection start-up information refers to false transmitting of protection start-up information due to communication problems, which may render ‘0′ as ‘1′ or ‘1′ as ‘0′. Commonly seen transmission errors in start-up information include three types—transmission error in start-up information of protection on the fault line, on the upstream line of the fault point and on the downstream line of the fault point. When the start-up information of Zone-II or Zone-III of protection on the fault line or the upstream line of the fault point is falsely transmitted as ‘0′ (which should be ‘1′), the start-up information of two protection devices can complement each other. The transmission error in one signal does not affect the global identification result of protection criteria. When the start-up information of protection on the downstream line of the fault point is falsely transmitted as ‘1′ (which should be ‘0′), the host computer will first identify that the fault is on this line and will issue tripping command. After certain time delay, the host computer will detect that the fault still exists, thus the start-up information will be changed to ‘0′ and protection criteria will be re-calculated, so that the fault can be cleared correctly and protection selectivity is preserved. Suppose fault occurs at F1 and there is transmission error in the start-up information of protection 3, i.e. the start-up information is falsely transmitted as ‘1′, in this case the criteria calculation result of protection 3 is: E1 = 1, E2 = 0, E1 + E2 = 1, thus protection 3 will operate. After protection 3 operates, if the fault still exists, the start-up information of protection 3 will be changed to ‘0′, and the criteria calculation result of protection 2 becomes: E1 = 1, E2 = 0, E1 + E2 = 1, thus the fault will be cleared reliably. Therefore, the proposed scheme is tolerant of transmission errors in protection start-up information.
(a) Phase-to-phase fault.
4. Simulation verification 4.1. Test system To verify the correctness and effectiveness of the short circuit current calculation method and regional current protection scheme proposed in this paper for distribution network with large-scale electric vehicle integration, the IEEE 33-bus standard distribution system is built in the RTDS, as shown in Fig. 6. The IEEE 33-bus system is a complex distribution network with crisscross long and short lines. Line parameters of the region used for research in this paper are shown in Table A1 in Appendix A. It can be seen that, lines between node 4 and node 10 and lines between node 6 and node 29 are crisscross long and short lines. Five charging stations are integrated to the system at different locations, the maximum charging capacity of each station being
(b) Three-phase short circuit fault Fig. 7. Calculated and simulated values of short circuit current in the case of inter-phase faults. 5
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the maximum error being only 0.17%. Besides, as the charging capacity increases, the short circuit current decreases. When the charging capacity increases to 9 MW, compared with when it is 0, the short circuit current of phase-to-phase fault decreases by approximately 16%, while the short circuit current of three-phase fault decreases by approximately 18%. The proposed method can accurately calculate the short circuit current at the relaying point when the charging capacity applies different values and different types of fault occur in the distribution network, laying a foundation for the setting of Zone-II and Zone-III of regional current protection.
Table 2 Setting results of current zone-II and III of proposed method. Relevant to F1
Relevant to F2
Relay number
IsetII/kA
IsetIII/kA
Relay number
IsetII/kA
IsetIII/kA
R4-5 R5-6 R6-7 R7-8 R8-9 R9-10 R10-11
0.965 0.793 0.707 0.644 0.545 0.472 0.463
0.794 0.719 0.643 0.546 0.473 0.462 0.415
R4-5 R5-6 R6-26 R26-27 R27-28 R28-29 R29-30
0.965 0.793 0.629 0.604 0.585 0.513 0.483
0.794 0.719 0.605 0.586 0.513 0.483 0.421
4.3. Verification of regional current protection scheme (1) Calculation of current protection setting value according to the requirement of sensitivity According to Section 4.2, as the charging capacity increases, the short circuit current of the distribution network decreases. For traditional protection set according to selectivity, the sensitivity of protection will decrease, even to inadequacy. Take current protection zone-II for example. The setting values of Zone-II of line protection R4-5, R5-6, R6-7 and R10-11 according to the requirement of selectivity (traditional method) and sensitivity (proposed method) are shown in Tables 1 and 2. It can be seen that the setting value of Zone-II calculated according to traditional method is inadequate for the sensitivity (the sensitivity coefficient KsenII < 1) and cannot cover the whole line length of the protection line. As shown in Fig. 8, when three-phase short circuit fault occurs at the endpoint of line L8-9 (F1), the amplitude of measured current at protection R8-9 Imf is smaller than the setting value of traditional method IsetII_tra, thus traditional protection will refuse to operate. However, Imf is bigger than the setting value of the proposed method IsetII_new, thus regional centralized protection will operate correctly. The proposed method not only guarantees adequate sensitivity for protection, but also is not affected by the branch coefficient and need not cooperate with neighboring lines, thus protection setting is greatly simplified. However, the proposed method does not consider the cooperation between different protection devices, thus selectivity of protection may not be satisfied. For example, when a three-phase fault occurs at 70% line length from the relaying point (i.e. F1) on line L8-9, Zone-II of protection R8-9 and R7-8 and Zone-III of protection R6-7 will all start. If Zone-II of protection R8-9 fails to operate since the operation time delay of Zone-II of protection R7-8 is shorter than that of Zone-III of protection R8-9, protection R7-8 will operate instead of protection R8-9, thus the requirement of selectivity is not satisfied. When a phase-to-phase fault occurs at 70% line length from the relaying point (i.e. F2) on line L27-28, Zone-II and Zone-III of protection R26-27 and R27-28 and Zone-III of protection R6-26 will all start. Since the operation time delays of Zone-II of protection R26-27 and R27-28 are the same, protection R26-27 will operate instead of protection R27-28, thus the requirement of selectivity is not satisfied. In order to guarantee the selectivity of protection, the regional current protection scheme is used
Fig. 8. Comparison between the setting results of traditional method and the proposed method.
to identify the fault location. (2) Verification of regional current protection scheme When a three-phase fault occurs at 70% line length from the relaying point (i.e. F1) on line L8-9, according to (17), the start-up status of different protection devices and the calculation result of regional current protection criteria can be obtained, shown in Table 3. It can be seen that the regional host computer issues tripping command to protection R8-9, and sends blocking command to protection R6-7 and R7-8, effectively prevent protection R6-7 and R7-8 from override operating. The other protection devices do not start, thus the host computer does not issue any command to them. Therefore, the proposed regional protection scheme can accurately locate the fault. When a phase-to-phase fault occurs at 70% line length from the relaying point (i.e. F2) on line L27-28, according to (17), the start-up status of different protection devices and the calculation result of
Table 1 Current segment-II setting value of traditional method and proposed method. Relay number
R4-5 R5-6 R6-7 R7-8 R8-9 R9-10 R10-11
Traditional method
Table 3 Criterion calculation results of the protection when fault occurs in F1.
Proposed method
Relay number
IsetII/kA
KsenII
IsetII/kA
KsenII
1.398 1.252 1.139 0.964 0.837 0.820 0.790
0.79 0.73 0.72 0.78 0.76 0.67 0.68
0.965 0.793 0.707 0.644 0.545 0.472 0.463
1.2 1.2 1.2 1.2 1.2 1.2 1.2
R4-5 R5-6 R6-7 R7-8 R8-9 R9-10 R10-11
6
Start-up information
Criterion result
II
III
E1
E2
E1 + E2
0 0 0 1 1 0 0
0 0 1 1 1 0 0
0 0 0 0 1 0 0
0 0 −1 −1 0 0 0
0 0 −1 −1 1 0 0
Action signal
Not Operate Not Operate Block Block Operate Not Operate Not Operate
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protection R28-29 on the downstream line, its Zone-II or Zone-III start-up information is set as ‘1′. According to the regional protection criteria, for protection R27-28, E1 + E2 = −1, thus protection R27-28 is blocked; for protection R28-29, E1 + E2 = 1, thus protection R28-29 will operate to cut line L28-29,. Since after line L28-29 is cut, the fault still exists, the start-up information of protection R28-29 is reset as ‘0′ and protection criteria are re-calculated. Finally, the calculation result of regional protection criteria is the same as when there is no transmission error, as can be seen in Table 4, and the fault is correctly cleared by protection R27-28.
Table 4 Criterion calculation results of the protection when fault occurs in F2. Relay number
R4-5 R5-6 R6-26 R26-27 R27-28 R28-29 R29-30
Start-up information
Criterion result
II
III
E1
E2
E1 + E2
0 0 0 1 1 0 0
0 0 1 1 1 0 0
0 0 0 0 1 0 0
0 0 −1 −1 0 0 0
0 0 −1 −1 1 0 0
Action signal
Not Operate Not Operate Block Block Operate Not Operate Not Operate
5. Conclusion Concerning the contradiction between the requirements of current protection on selectivity and sensitivity caused by large-scale penetration of electric vehicles in the distribution network, a regional centralized protection scheme for distribution network integrated with electric vehicles is proposed based on current protection set according to the requirement of sensitivity. The proposed scheme has the following advantages:
regional current protection criteria can be obtained, shown in Table 4. It can be seen that protection R27-28 receives tripping command from the host computer. Protection R6-26 and R26-27 receive blocking command from the host computer, thus mal-operation of protection R6-26 and R26-27 is avoided. The other protection devices do not start, thus the host computer does not issue any command to them. Therefore, the proposed regional protection scheme can effectively solve the contradiction between sensitivity and selectivity and identify the fault reliably.
(1) Only logical signals of protection operation information need to be transmitted, thus the requirement on the bandwidth and synchronicity of communication is low. Besides, without the laddering time delay, the operation speed of backup protection is effectively improved. (2) The fault characteristic of electric vehicle charger controller is considered comprehensively, thus the proposed method is highly sensitive and adaptable, unaffected by the fault type and the capacity and location of charging facilities. (3) The proposed scheme is applicable to complex distribution network with crisscross long and short lines. Problems in traditional current protection are solved, such as the difficulty in protection setting and coordination between different protection zones, as well as the malcooperation between backup protection zones caused by large-scale penetration of electric vehicles to the distribution network. Besides, the proposed method is fault tolerant with relatively large redundancy.
4.4. Fault tolerance Verification of regional protection scheme (1) Loss of protection start-up information When a three-phase fault occurs at F2 on line R27-28, if the start-up information of protection R26-27 on the upstream line or protection R2728 on the fault line is missing, the missing information will be taken as ‘1′. According to Section 3.2, the calculation result of regional protection criteria is consistent with when there is no information missing, as shown in Table 4. If the start-up information of protection R28-29 on the downstream line is missing, the host computer will set both the zone-II and Zone-III start-up information of protection R28-29 as ‘1′. According to Table 5, the host computer identifies that the fault is on line L28-29, but the fault is not cleared yet. Therefore, the start-up information of protection R28-29 is reset as ‘0′ and protection criteria are re-calculated, which yields the same result as when there is no information missing, as shown in Table 4, and the fault is correctly cleared by protection R27-28.
This paper conducts study on the problems of relay protection in distribution network integrated with electric vehicles. Currently, distributed generation and electric vehicles are integrated to the distribution network simultaneously on an ever larger scale, thus the transient characteristics of fault current will become more complicated, and the network structure more flexible and varying. The joint effect of two integration types on relay protection and adapting strategies requires further and in-depth research.
(2) Transmission error in protection start-up information If there is transmission error in the start-up information of protection R26-27 on the upstream line or protection R27-28 on the fault line, according to Section 3.2, the calculation result of regional protection criteria is consistent with when there is no transmission error, as shown in Table 4. If there is transmission error in the start-up information of
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Table 5 Calculation results of protection criterion when fault occurs in F2 and loss of start-up information. Relay number
R4-5 R5-6 R6-26 R26-27 R27-28 R28-29 R29-30
Start-up information
Criterion result
II
III
E1
E2
E1 + E2
0 0 0 1 1 1 0
0 0 1 1 1 1 0
0 0 0 0 0 1 0
0 0 −1 −1 −1 0 0
0 0 −1 −1 −1 1 0
Acknowledgment
Action signal
This work is supported by National Key Research and Development Program of China (2018YFB0904003), National Natural Science Foundation of China (51822703), and Chinese Universities Scientific Fund (2018JQ01), Chinese Universities Scientific Fund (2018ZD01).
Not Operate Not Operate Block Block Block Operate Not Operate
7
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Appendix A (See Table A1)
Table A1 Line parameters of IEEE 33-node system.
8
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Power Deliv 2017;32(1):411–9. [13] Shuai Z, Chao S, Xin Y, et al. Fault analysis of inverter-interfaced distributed generators with different control schemes. IEEE Trans Power Deliv 2018;33(3):1223–35. [14] Alwash SF, Ramachandaramurthy VK, Mithulananthan N. Fault-location scheme for power distribution system with distributed generation. IEEE Trans Power Deliv 2015;30(3):1187–95. [15] Cavalcante PAH, Almeida MCD. Fault location approach for distribution systems based on modern monitoring infrastructure. IET Gener Transm Distrib 2018;12(1):94–103. [16] Khond SV, Dhomane GA. Optimum coordination of directional overcurrent relays for combined overhead/cable distribution system with linear programming technique. Protect Contr Modern Power Syst 2019;4(1):9. [17] Biswal M, Biswal S. A positive-sequence current based directional relaying approach for CCVT subsidence transient condition. Protect Contr Modern Power Syst 2017;2:18. [18] Lin H, Sun K, Tan ZH, et al. Adaptive protection combined with machine learning for microgrids. IET Gener Transm Distrib 2019;13(6):770–9. [19] Yilmaz M, Krein PT. Review of the impact of vehicle-to-grid technologies on distribution systems and utility interfaces. IEEE Trans Power Electron 2013;28(12):5673–89. [20] Kim YJ, Wang J. Power hardware-in-the-loop simulation study on frequency regulation through direct load control of thermal and electrical energy storage resources. IEEE Trans Smart Grid 2018;9(4):2786–96. [21] Hajahmed MA, Illindala MS. The Influence of Inverter-based DGs and Their Controllers on Distribution Network Protection. IEEE Trans Ind Appl 2014;50(4):2928–37. [22] Kong X, Zhang Z, Yin X, et al. Study of fault current characteristics of the DFIG considering dynamic response of the RSC. IEEE Trans Energy Convers 2014;29(2):278–87. [23] Coffele F, Booth C, Dysko A. An Adaptive Overcurrent Protection Scheme for Distribution Networks. IEEE Trans Power Deliv 2015;30(2):561–8.
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Contents lists available at ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Research on regional centralized protection scheme for distribution network integrated with electric vehicles
T
Jing Maa,b,⁎, Jing Liua, Gengyu Yanga, A.G. Phadkeb a b
State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
ARTICLE INFO
ABSTRACT
Keywords: Electric vehicle Distribution network Short circuit current calculation Regional protection
In view of the difficulty in balancing between selectivity and sensitivity of current protection in distribution networks integrated with electric vehicles, a regional current protection method applicable to distribution networks with large-scale electric vehicle penetration is proposed. First, considering the effect of the converter control strategy of electric vehicle chargers, a theoretical analysis is made to determine the expression of the short circuit current that the electric vehicle charger provides when faults occur on feeders. On this basis, according to the minimum short circuit current, the current protection criterion that meets the requirement of sensitivity alone is established. And then, combining multiple start-up signals of the criterion on the fault line and multiple-level neighboring lines after fault occurs, the regional current protection scheme for distribution network is constructed. Finally, simulation tests conducted in Real-Time Digital Simulator (RTDS) of IEEE 33-bus system verify that the proposed method can effectively avoid mal-cooperation between different protection zones which is common in traditional current protection. Besides, the proposed scheme is not affected by factors such as the charging capacity of electric vehicles, the number and the location of charging facilities, etc., and can accurately identify the fault location even when the information is partly missing or in error.
1. Introduction Massive integration of electric vehicles to the distribution network not only greatly affects the variation of operation mode and network structure of distribution network, but also determines the distortion characteristics of electrical variables when a fault occurs on feeders. As the penetration rate of electric vehicles in the distribution network ever increases, the transient characteristic of fault current has become ever more complicated, thus protection setting and the coordination between different protection zones has become more difficult, and the contradiction between four requirements of relay protection (i.e. reliability, selectivity, speed and sensitivity) ever more prominent. Therefore, there is need for a strategy to harmonize the four requirements and improve the performance of relay protection [1–6]. Current protection schemes mainly include limiting penetration capacity schemes [7,8], adaptive protection schemes [9–12] and fault positioning schemes based on communication technology [13–16], etc. Limiting penetration capacity schemes take into account factors such as harmonic constraint, voltage constraint, protection adaptability, etc. and obtain the allowed penetration capacity of electric vehicle in the distribution network passively. Adaptive protection schemes are based
⁎
on the original protection configuration, and by calculating parameters such as system emf (electromotive force), equivalent impedance, branch coefficient, etc. can adjust protection setting values online, thus identifying and clearing the fault. As for fault positioning schemes based on communications, by using the acquisition, transmission, and processing of information such as the amplitude, phase, and protection operation status, etc. of local or global electrical quantities in the distribution network, the fault position can be located. However, when electric vehicles are integrated on a large scale, the structure and operation mode of distribution network will vary frequently [17]. Existing protection method cannot fundamentally solve the problems in protection setting and coordination between different protection zones, as well as the contradiction between the requirements of selectivity and sensitivity. Therefore, it is urgent to study new protection schemes adaptable to distribution network with large-scale integration of electric vehicles [18]. In view of the above problems, a regional current protection method applicable to a distribution network with large-scale electric vehicle penetration is proposed in this paper. Considering the effect of converter control strategy, the analytical expression of short circuit current the electric vehicle charger injects into the distribution network when
Corresponding author at: No. 52 Mailbox, North China Electric Power University, No. 2 Beinong Road, Changping District, Beijing 102206, China. E-mail address: [email protected] (J. Ma).
https://doi.org/10.1016/j.ijepes.2020.105903 Received 27 August 2019; Received in revised form 29 January 2020; Accepted 3 February 2020 Available online 12 February 2020 0142-0615/ © 2020 Elsevier Ltd. All rights reserved.
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fault occurs on feeders is obtained. On this basis, by analyzing the effect of electric vehicle charger on the parameters of protection backside system in the distribution network, the minimum short circuit current feed-in from electric vehicle charger to the distribution network is calculated. And then, the current protection criterion that only meets the requirement of sensitivity is established according to the value of the minimum short circuit current. After fault occurs, combining multiple start-up signals of the criterion on multiple-level neighboring lines, the regional current protection scheme for distribution network is constructed. Finally, simulation tests conducted in Real-Time Digital Simulator (RTDS) verify the correctness and feasibility of the proposed scheme.
of short circuit current output from the converter of electric vehicle charger is:
id =
CUdc
dUdc dt
1.5Ud
iq* =
Q 1.5Eg
2.1. Analytical expression of short circuit current of electric vehicle charger considering converter control
iq =
2 Imax
( i = min (
3
P = 2 (ed id + eq iq) = 2 Eg id Q=
ed iq) =
3 E i 2 g q
id = min (1)
PC = udc idc
id2
PBSS 1.5Eg
(5)
(7)
P BSS ,I 1.5Eg max 2 Imax
)
id2 ,
Q 1.5Eg
)
(8)
(9)
IBE = f (Eg )
When an asymmetrical inter-phase fault occurs in the distribution network, the current that electric vehicle charger injects into the distribution network will contain negative-sequence component and various harmonics. In order to improve the characteristics of output current, scholars put forward the positive-sequence component control strategy, which takes the positive-sequence component of voltage at grid-connecting point Eg+ as reference, at the same time filtering out the harmonic components of power generated by the negative-sequence component. Thus, when asymmetrical fault occurs in the distribution network, the short circuit current will not contain negative-sequence component and can be expressed as:
(2)
id = min iq = min
P BSS 1.5Eg+ 2 Imax
, Imax id2 ,
Q 1.5Eg+
(10)
In (10), the electric vehicle charger can be equalized to a non-linear current source controlled by the positive-sequence component of voltage at PCC (Point of Common Coupling), i.e.
(3)
IBE = f (E g + )
(1) When d-axis current component id exceeds the limited value Imax, the reference value of d-axis component of short circuit current output from the converter of electric vehicle charger is shown in (4). In this case, q-axis current component iq = 0.
id = Imax
+ PBSS
1.5Eg
According to (8), the amplitude and the phase of the short circuit current of electric vehicle charger are determined by the voltage at grid-connecting point Eg . Therefore, when fault occurs, the electric vehicle charger can be equalized to a current source model controlled by the voltage at grid-connecting point Eg :
In (2), P is the active power the distribution network provides for the electric vehicle charger, PBSS is the active power absorbed by the electric vehicle battery, and PC is the active power absorbed by the DC capacitance. In (2), the active power absorbed by the DC capacitance PC can be expressed as:
dU = CUdc dc dt
dUdc dt
(6)
q
where P is the active power, Q is the reactive power, and Eg is the amplitude of the voltage at the grid-connecting point. id and iq are the daxis and q-axis components of VSR output current. ed and eq are the daxis and q-axis components of voltage at the grid-connecting point. The electric vehicle charger usually applies the current inner-loop plus power outer-loop control strategy based on feedforward compensation and PI control mode, thus the electric vehicle charger can be equalized to a current source model related to the control command value [20,21]. Limited by the physical property of converter itself, when fault occurs in the distribution network, the short circuit current allowed through the converter of electric vehicle charger I cannot exceed the maximum value Imax [22,23]. When the amplitude of controller output short circuit current exceeds the maximum short circuit current allowed through the converter, the amplitude of short circuit current will be limited to be Imax and the control mode will be changed to constant power control with the output of active power as priority. The exchange of active power between the distribution network and electric vehicle charger can be expressed as:
P = PC + PBSS
CUdc
where Q is the controller command value of converter output reactive power. According to the reference values of d-axis and q-axis current components shown in (4)–(7), consider that the current inner loop is capable of fast and accurate tracking, the d-axis and q-axis components of electric vehicle charger output current can be expressed as:
The electric vehicle charger charges the battery of electric vehicles through three-phase voltage PWM rectifier, i.e. Voltage Source Rectifier (VSR). In dq rotating coordinate system, according to the grid voltage directional vector control technique, the charging power on AC side of VSR [19] can be expressed as:
3 (e i 2 q d
=
(3) When d-axis current component id does not exceed the limited value Imax, if the amplitude of short circuit current output I does not exceed Imax, according to (1) the reference value of q-axis component is shown in (6); if the amplitude of short circuit current output I exceeds Imax, according to the amplitude-limiting strategy of controller, the reference value of q-axis component is shown in (7).
2. Analytical expressions of short circuit currents of electric vehicle charger and distribution network
3
+ PBSS
(11)
The proposed method considers the charger type of existing EVs and is applicable to both DC fast charging and AC slow charging. This paper analyzes the transient characteristics of short circuit current EVs inject into the distribution network via grid-side converter in detail. For chargers using DC fast charging and AC slow charging, the grid-side converter and control strategy are the same, as well as the transient characteristics of output short circuit currents, thus the proposed method is applicable to both charger types.
(4)
(2) When d-axis current component id does not exceed the limited value Imax, according to (1)-(3), the reference value of d-axis component 2
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Fig. 3. Voltage-current phasor diagram when fault occurs.
charging facility is integrated, Eg = Um + Im Zg1. Es the equivalent emf of protection backside system when the electric vehicle charger is integrated, Es = Um + Is Zg1. After fault occurs in the distribution network, since the electric vehicle charger applies the control strategy with active power as a priority, the charging current of electric vehicle charger IBE and the measured voltage at grid-connecting bus Um are approximately reversed in phase. The measured current at the relaying point Im lags behind Um in phase by L . According to Kirchhoff’s current law (KCL), the current in protection backside system Is can be expressed as:
Is = IBE + Im
According to (12) and (13), the relationship between different voltage and current phasors can be obtained, as shown in Fig. 3. It can be seen that, Es < Eg , i.e. the amplitude of system equivalent emf drops after electric vehicle is integrated. Meanwhile, since the equivalent impedance of the protection backside system remains unchanged, the amplitude of the short circuit current at the relaying point will drop after the electric vehicle charger is integrated. In this case, if current protection is still set in the traditional way, the sensitivity of protection will go lower and the protection may refuse to operate. When three-phase fault occurs in the distribution network, there is only positive-sequence network, and similar analyzing process can be conducted, which is not repeated here. Therefore, when an inter-phase fault occurs at ZL on the feeders, the short circuit current at the relaying point Imf can be expressed with protection backside equivalent emf Es and equivalent impedance Zg as:
Fig. 1. Phase-to-phase metallic fault analysis.
2.2. Analytical expression of short circuit current of the distribution network When a fault occurs in the distribution network, the equivalent emf and system impedance of protection backside system is affected by the injection current from electric vehicle charger. As shown in Fig. 1, take the protection device at bus A as an example, the electric vehicle charger can be equalized to a non-linear current source, and the other parts can be substituted with the equivalent Thevenin circuit. ZL1 and ZLoad1 are the equivalent positive-sequence impedances of feeders and load. ZL2 and ZLoad2 are the equivalent negative-sequence impedances of feeders and load. Suppose phase B-to-C fault occurs at bus B, consider that the injection current from electric vehicle charger only contains positive-sequence component, the equivalent model of electric vehicle charger only exists in the positive-sequence network, as shown in Fig. 1(b). The negative-sequence network is shown in Fig. 1(c), which is the same as traditional negative-sequence network of distribution system, without the equivalent model of electric vehicle charger. Therefore, the following analysis is focused on the effect of electric vehicle chargers on the positive-sequence network. The electric vehicle charger can be equalized to a constant current source at any moment, thus the Thevenin equivalent circuit of the positive-sequence network can be obtained, shown in Fig. 2. It can be seen from Fig. 2 that, the injection current from electric vehicle charger does not change the positive-sequence equivalent impedance of protection backside system Zg1, but only affects the equivalent emf Es :
Es=(Eg Zg1 + IBE ) Zg1 = Eg + Zg1 IBE
(13)
Imf = K d
Es Zg + ZL
(14)
where K d is a fault type coefficient. For phase-to-phase fault, K d = 3 2 ; for three-phase short circuit fault, K d = 1. 3. Regional current protection scheme based on protection startup information 3.1. Regional protection operation criterion Considering that traditional current protection may refuse to operate after an electric vehicle is integrated due to lowered sensitivity, a new protection setting method that gives priority to sensitivity is put forward. (1) Current protection Zone-II guarantees adequate sensitivity for faults occurring at the end of the protected line; and (2) current protection Zone-III guarantee adequate sensitivity for faults occurring at the end of the subordinate line. The protection range of current protection set according to the requirement of sensitivity is shown in Fig. 4. Take protection 2 for example, the setting formula of Zone-II and Zone-III are shown in (15) and (16) respectively.
(12)
where Eg is the equivalent emf of protection backside system when no
I2II = I2III = Fig. 2. Thevenin equivalent circuit of positive-sequence network.
Ik . C . min II Ksen Ik . D . min III Ksen
(15) (16)
where Ik.C.min and Ik.D.min are minimum short circuit currents that go 3
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Fig. 5. A 10 kV distribution network in a certain area.
another feeder line. After fault occurs, the start-up information of current protection zone-II and zone-III on different feeder lines is sent to the regional host computer, which then returns corresponding tripping information to circuit breakers 1–6 according to the calculation results of protection criteria. Suppose fault occurs at F1 in Fig. 4, in this case, Zone-II and Zone-III of protection 1 and 2 will all start, while Zone-II and Zone-III of protection 3, 4, 5 will not start. The host computer will calculate according to the protection criteria concerning protection 1 and 2 respectively. For protection 2, E1 = 1 means the fault occurs on the line where protection 2 is; E2 = 0 means the fault occurs on the subordinate line of protection 2; (E1 + E2) = 1, thus the ‘Operate’ command is issued. Similarly, for protection 1, it can be calculated that (E1 + E2) = −1, thus the host computer determines the fault is not on this line and issues blocking signal to protection 1. Therefore, mal-cooperation between protection 1 and protection 2 can be effectively avoided.
Fig. 4. Protection range of setting method that gives priority to the requirement of sensitivity.
through the protection device when a phase-to-phase fault occurs at the end of feeders, when the charging facility of electric vehicle is in fullII III load operation. The sensitivity coefficient Ksen and Ksen are set to 1.2. This setting method can guarantee adequate sensitivity for current protection, but may fail to meet the requirement of selectivity, as shown in Fig. 4. (1) When fault occurs at F1, since the operation delay of zoneII of protection 1 is the same as that of protection 2, they will both operate at the same time, i.e. selectivity is not satisfied. (2) When fault occurs at F2, if zone-II of protection 2 fails to operate, since the operation delay of zone-III of protection 1 is shorter than that of protection 2, zone-III of protection 1 will operate before zone-III of protection 2, i.e. selectivity is not satisfied. Concerning the above problems, a regional current protection scheme based on multiple fault information is proposed in this paper. The regional host computer is installed at the outlet of the terminal substation feeder line. By collecting the start-up information of current protection Zone-II and Zone-III on different feeder lines, the host computer can make a centralized decision according to protection criteria shown in (17), and then send corresponding commands (‘operate’, ‘not operate’ or ‘block’) to circuit breakers on different feeder lines.
1 ((ISII = 1) E1 =
( 0 1 ((ISII = 1)
E2 =
i,
As for problems such as loss of protection start-up information and transmission error which may exist in a regional protection system, the following countermeasure is taken. (1) Loss of protection start-up information When problems such as protection device failure, measuring device disconnection and communication module fault, etc. occur in the distribution network protection system, the host computer will not be able to receive the start-up information of certain protection device, i.e. certain start-up information will be missing. In this case, the start-up information of Zone-II and Zone-III of this protection device are both set to be ‘1′ for protection criteria calculation. If the fault still exists after current protection operates, the protection start-up information is changed from ‘1′ to ‘0′ and the protection criteria are re-calculated. Commonly seen cases of protection start-up information loss include three types—loss of start-up information of protection on the fault line, on the upstream line of the fault point and on the downstream line of the fault point. When the start-up information of protection on the fault line is missing, the default start-up information is ‘1′, thus the fault can still be accurately cleared. When the start-up information of protection on healthy line in the upstream of fault point is missing, the missing information can be compensated by the start-up information of protection on the downstream line, thus the proposed method is fault-tolerant. For example, suppose fault occurs at F2 in Fig. 4 and the start-up information of protection 2 is missing, regional protection can still make the correct identification. For protection 2, E1 = 0, E2 = −1, E1 + E2 = −1 < 0, thus protection 2 is blocked; for protection 3, E1 = 1, E2 = 0, E1 + E2 = 1, thus protection 3 should operate. The regional host computer will issue a tripping command and clear the fault correctly. When the start-up information of protection on non-fault line in the downstream of fault point is missing, the host computer will set the missing information as ‘1′, thus it is identified that the fault is on the
i, ILII = 0)
((
ILIII
= 0)) others
(ISIII = 1)) (
0
(ISIII = 1))
3.2. Fault tolerance analysis
((
i = 1, ILII(i) = 1)
i = 1, ILIII(i) = 1)) others
Operate (E1 + E2 ) > 0 Not operate (E1 + E2 ) = 0 Block (E1 + E2 ) < 0
(17)
where the I with superscripts ‘II’ and ‘III’ represent the start-up information of current protection Zone-II and Zone-III respectively. Subscript ‘S’ represents protection device on the protected line; subscript ‘L’ represents protection device on the subordinate line. i is the number of the protection device. ‘1′ means current protection device starts; ‘0′ means current protection device does not start. ‘Operate’ means the host computer issues tripping command to circuit breakers on this feeder line; ‘Not operate’ means the fault is not on this line, and the host computer does not issue tripping command to circuit breakers on this feeder line; ‘Block’ means the fault is within the operation range of this protection device, but not on the feeder line where the protection device is, thus this protection device need not operate, and in order to avoid override tripping and guarantee selectivity of current protection, the host computer issues blocking signal to circuit breakers on this feeder line. Take the 10 kV distribution network in Fig. 5 for example. Protection 1, 2, 3 and 4 are on the same feeder line, protection 5 and 6 are on 4
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line where the protection start-up information is missing. After certain time delay (the length of time delay is set according to engineering practice of real distribution network, usually shorter than the time delay of traditional current protection zone-II), if the host computer detects that the fault still exists, the missing information will be reset from ‘1′ to ‘0′ and protection criteria will be re-calculated. And then the host computer will send ‘Operate’ command to the right protection device. Therefore, in this case, the fault line can still be cleared accurately, without losing selectivity. For example, in Fig. 4 suppose fault occurs at F1 and the start-up information of protection 3 is missing, first it will be determined that the fault is on line CD. After certain time delay, the host computer detects that the fault is not cleared, thus the start-up information of protection 3 is reset to be ‘0′ for criteria calculation. The criteria calculation result of protection 2 is: E1 = 1, E2 = 0, E1 + E2 = 1, thus the host computer will issue tripping command to protection 2 and clear the fault. It can be seen that, when a single signal is missing, the proposed regional protection scheme can still correctly clear the fault line.
Fig. 6. Diagram of IEEE 33-bus distribution system.
1.8 MW. The regional host computer is installed at power source node 1. The start-up information of Zone-II and Zone-III of all current protection devices in the distribution system are sent to the host computer. 4.2. Verification of the calculated short circuit current of the distribution network
(2) Transmission error in protection start-up information
Set phase-to-phase fault and three-phase fault at the terminal point of distribution line L8-9, i.e. F1 respectively. When the charging capacity applies different values, the short circuit current that goes through protection R6-7 can be calculated according to (14), and the waveforms of calculated value and simulated value are shown in Fig. 7, where the solid line represents the calculated value, and the dashed line represents the simulated value. The simulation curves represent the amplitudes of short circuit current phasors in the transient process, not the instantaneous values of short circuit currents. As for the calculated values, first, the phasor values of voltage and current measured at the relaying point (i.e. Um and Im ) are processed according to certain algorithm, and then the equivalent emf Es and equivalent impedance Zg of protection backside system are calculated, thus according to (14), the amplitude of short circuit current phasor can be calculated. It can be seen that, in the fault duration, the calculation result and simulation result of short circuit current are all very close in different cases, with
Transmission error in protection start-up information refers to false transmitting of protection start-up information due to communication problems, which may render ‘0′ as ‘1′ or ‘1′ as ‘0′. Commonly seen transmission errors in start-up information include three types—transmission error in start-up information of protection on the fault line, on the upstream line of the fault point and on the downstream line of the fault point. When the start-up information of Zone-II or Zone-III of protection on the fault line or the upstream line of the fault point is falsely transmitted as ‘0′ (which should be ‘1′), the start-up information of two protection devices can complement each other. The transmission error in one signal does not affect the global identification result of protection criteria. When the start-up information of protection on the downstream line of the fault point is falsely transmitted as ‘1′ (which should be ‘0′), the host computer will first identify that the fault is on this line and will issue tripping command. After certain time delay, the host computer will detect that the fault still exists, thus the start-up information will be changed to ‘0′ and protection criteria will be re-calculated, so that the fault can be cleared correctly and protection selectivity is preserved. Suppose fault occurs at F1 and there is transmission error in the start-up information of protection 3, i.e. the start-up information is falsely transmitted as ‘1′, in this case the criteria calculation result of protection 3 is: E1 = 1, E2 = 0, E1 + E2 = 1, thus protection 3 will operate. After protection 3 operates, if the fault still exists, the start-up information of protection 3 will be changed to ‘0′, and the criteria calculation result of protection 2 becomes: E1 = 1, E2 = 0, E1 + E2 = 1, thus the fault will be cleared reliably. Therefore, the proposed scheme is tolerant of transmission errors in protection start-up information.
(a) Phase-to-phase fault.
4. Simulation verification 4.1. Test system To verify the correctness and effectiveness of the short circuit current calculation method and regional current protection scheme proposed in this paper for distribution network with large-scale electric vehicle integration, the IEEE 33-bus standard distribution system is built in the RTDS, as shown in Fig. 6. The IEEE 33-bus system is a complex distribution network with crisscross long and short lines. Line parameters of the region used for research in this paper are shown in Table A1 in Appendix A. It can be seen that, lines between node 4 and node 10 and lines between node 6 and node 29 are crisscross long and short lines. Five charging stations are integrated to the system at different locations, the maximum charging capacity of each station being
(b) Three-phase short circuit fault Fig. 7. Calculated and simulated values of short circuit current in the case of inter-phase faults. 5
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the maximum error being only 0.17%. Besides, as the charging capacity increases, the short circuit current decreases. When the charging capacity increases to 9 MW, compared with when it is 0, the short circuit current of phase-to-phase fault decreases by approximately 16%, while the short circuit current of three-phase fault decreases by approximately 18%. The proposed method can accurately calculate the short circuit current at the relaying point when the charging capacity applies different values and different types of fault occur in the distribution network, laying a foundation for the setting of Zone-II and Zone-III of regional current protection.
Table 2 Setting results of current zone-II and III of proposed method. Relevant to F1
Relevant to F2
Relay number
IsetII/kA
IsetIII/kA
Relay number
IsetII/kA
IsetIII/kA
R4-5 R5-6 R6-7 R7-8 R8-9 R9-10 R10-11
0.965 0.793 0.707 0.644 0.545 0.472 0.463
0.794 0.719 0.643 0.546 0.473 0.462 0.415
R4-5 R5-6 R6-26 R26-27 R27-28 R28-29 R29-30
0.965 0.793 0.629 0.604 0.585 0.513 0.483
0.794 0.719 0.605 0.586 0.513 0.483 0.421
4.3. Verification of regional current protection scheme (1) Calculation of current protection setting value according to the requirement of sensitivity According to Section 4.2, as the charging capacity increases, the short circuit current of the distribution network decreases. For traditional protection set according to selectivity, the sensitivity of protection will decrease, even to inadequacy. Take current protection zone-II for example. The setting values of Zone-II of line protection R4-5, R5-6, R6-7 and R10-11 according to the requirement of selectivity (traditional method) and sensitivity (proposed method) are shown in Tables 1 and 2. It can be seen that the setting value of Zone-II calculated according to traditional method is inadequate for the sensitivity (the sensitivity coefficient KsenII < 1) and cannot cover the whole line length of the protection line. As shown in Fig. 8, when three-phase short circuit fault occurs at the endpoint of line L8-9 (F1), the amplitude of measured current at protection R8-9 Imf is smaller than the setting value of traditional method IsetII_tra, thus traditional protection will refuse to operate. However, Imf is bigger than the setting value of the proposed method IsetII_new, thus regional centralized protection will operate correctly. The proposed method not only guarantees adequate sensitivity for protection, but also is not affected by the branch coefficient and need not cooperate with neighboring lines, thus protection setting is greatly simplified. However, the proposed method does not consider the cooperation between different protection devices, thus selectivity of protection may not be satisfied. For example, when a three-phase fault occurs at 70% line length from the relaying point (i.e. F1) on line L8-9, Zone-II of protection R8-9 and R7-8 and Zone-III of protection R6-7 will all start. If Zone-II of protection R8-9 fails to operate since the operation time delay of Zone-II of protection R7-8 is shorter than that of Zone-III of protection R8-9, protection R7-8 will operate instead of protection R8-9, thus the requirement of selectivity is not satisfied. When a phase-to-phase fault occurs at 70% line length from the relaying point (i.e. F2) on line L27-28, Zone-II and Zone-III of protection R26-27 and R27-28 and Zone-III of protection R6-26 will all start. Since the operation time delays of Zone-II of protection R26-27 and R27-28 are the same, protection R26-27 will operate instead of protection R27-28, thus the requirement of selectivity is not satisfied. In order to guarantee the selectivity of protection, the regional current protection scheme is used
Fig. 8. Comparison between the setting results of traditional method and the proposed method.
to identify the fault location. (2) Verification of regional current protection scheme When a three-phase fault occurs at 70% line length from the relaying point (i.e. F1) on line L8-9, according to (17), the start-up status of different protection devices and the calculation result of regional current protection criteria can be obtained, shown in Table 3. It can be seen that the regional host computer issues tripping command to protection R8-9, and sends blocking command to protection R6-7 and R7-8, effectively prevent protection R6-7 and R7-8 from override operating. The other protection devices do not start, thus the host computer does not issue any command to them. Therefore, the proposed regional protection scheme can accurately locate the fault. When a phase-to-phase fault occurs at 70% line length from the relaying point (i.e. F2) on line L27-28, according to (17), the start-up status of different protection devices and the calculation result of
Table 1 Current segment-II setting value of traditional method and proposed method. Relay number
R4-5 R5-6 R6-7 R7-8 R8-9 R9-10 R10-11
Traditional method
Table 3 Criterion calculation results of the protection when fault occurs in F1.
Proposed method
Relay number
IsetII/kA
KsenII
IsetII/kA
KsenII
1.398 1.252 1.139 0.964 0.837 0.820 0.790
0.79 0.73 0.72 0.78 0.76 0.67 0.68
0.965 0.793 0.707 0.644 0.545 0.472 0.463
1.2 1.2 1.2 1.2 1.2 1.2 1.2
R4-5 R5-6 R6-7 R7-8 R8-9 R9-10 R10-11
6
Start-up information
Criterion result
II
III
E1
E2
E1 + E2
0 0 0 1 1 0 0
0 0 1 1 1 0 0
0 0 0 0 1 0 0
0 0 −1 −1 0 0 0
0 0 −1 −1 1 0 0
Action signal
Not Operate Not Operate Block Block Operate Not Operate Not Operate
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protection R28-29 on the downstream line, its Zone-II or Zone-III start-up information is set as ‘1′. According to the regional protection criteria, for protection R27-28, E1 + E2 = −1, thus protection R27-28 is blocked; for protection R28-29, E1 + E2 = 1, thus protection R28-29 will operate to cut line L28-29,. Since after line L28-29 is cut, the fault still exists, the start-up information of protection R28-29 is reset as ‘0′ and protection criteria are re-calculated. Finally, the calculation result of regional protection criteria is the same as when there is no transmission error, as can be seen in Table 4, and the fault is correctly cleared by protection R27-28.
Table 4 Criterion calculation results of the protection when fault occurs in F2. Relay number
R4-5 R5-6 R6-26 R26-27 R27-28 R28-29 R29-30
Start-up information
Criterion result
II
III
E1
E2
E1 + E2
0 0 0 1 1 0 0
0 0 1 1 1 0 0
0 0 0 0 1 0 0
0 0 −1 −1 0 0 0
0 0 −1 −1 1 0 0
Action signal
Not Operate Not Operate Block Block Operate Not Operate Not Operate
5. Conclusion Concerning the contradiction between the requirements of current protection on selectivity and sensitivity caused by large-scale penetration of electric vehicles in the distribution network, a regional centralized protection scheme for distribution network integrated with electric vehicles is proposed based on current protection set according to the requirement of sensitivity. The proposed scheme has the following advantages:
regional current protection criteria can be obtained, shown in Table 4. It can be seen that protection R27-28 receives tripping command from the host computer. Protection R6-26 and R26-27 receive blocking command from the host computer, thus mal-operation of protection R6-26 and R26-27 is avoided. The other protection devices do not start, thus the host computer does not issue any command to them. Therefore, the proposed regional protection scheme can effectively solve the contradiction between sensitivity and selectivity and identify the fault reliably.
(1) Only logical signals of protection operation information need to be transmitted, thus the requirement on the bandwidth and synchronicity of communication is low. Besides, without the laddering time delay, the operation speed of backup protection is effectively improved. (2) The fault characteristic of electric vehicle charger controller is considered comprehensively, thus the proposed method is highly sensitive and adaptable, unaffected by the fault type and the capacity and location of charging facilities. (3) The proposed scheme is applicable to complex distribution network with crisscross long and short lines. Problems in traditional current protection are solved, such as the difficulty in protection setting and coordination between different protection zones, as well as the malcooperation between backup protection zones caused by large-scale penetration of electric vehicles to the distribution network. Besides, the proposed method is fault tolerant with relatively large redundancy.
4.4. Fault tolerance Verification of regional protection scheme (1) Loss of protection start-up information When a three-phase fault occurs at F2 on line R27-28, if the start-up information of protection R26-27 on the upstream line or protection R2728 on the fault line is missing, the missing information will be taken as ‘1′. According to Section 3.2, the calculation result of regional protection criteria is consistent with when there is no information missing, as shown in Table 4. If the start-up information of protection R28-29 on the downstream line is missing, the host computer will set both the zone-II and Zone-III start-up information of protection R28-29 as ‘1′. According to Table 5, the host computer identifies that the fault is on line L28-29, but the fault is not cleared yet. Therefore, the start-up information of protection R28-29 is reset as ‘0′ and protection criteria are re-calculated, which yields the same result as when there is no information missing, as shown in Table 4, and the fault is correctly cleared by protection R27-28.
This paper conducts study on the problems of relay protection in distribution network integrated with electric vehicles. Currently, distributed generation and electric vehicles are integrated to the distribution network simultaneously on an ever larger scale, thus the transient characteristics of fault current will become more complicated, and the network structure more flexible and varying. The joint effect of two integration types on relay protection and adapting strategies requires further and in-depth research.
(2) Transmission error in protection start-up information If there is transmission error in the start-up information of protection R26-27 on the upstream line or protection R27-28 on the fault line, according to Section 3.2, the calculation result of regional protection criteria is consistent with when there is no transmission error, as shown in Table 4. If there is transmission error in the start-up information of
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Table 5 Calculation results of protection criterion when fault occurs in F2 and loss of start-up information. Relay number
R4-5 R5-6 R6-26 R26-27 R27-28 R28-29 R29-30
Start-up information
Criterion result
II
III
E1
E2
E1 + E2
0 0 0 1 1 1 0
0 0 1 1 1 1 0
0 0 0 0 0 1 0
0 0 −1 −1 −1 0 0
0 0 −1 −1 −1 1 0
Acknowledgment
Action signal
This work is supported by National Key Research and Development Program of China (2018YFB0904003), National Natural Science Foundation of China (51822703), and Chinese Universities Scientific Fund (2018JQ01), Chinese Universities Scientific Fund (2018ZD01).
Not Operate Not Operate Block Block Block Operate Not Operate
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Appendix A (See Table A1)
Table A1 Line parameters of IEEE 33-node system.
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