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A novel pilot protection scheme for LCC-HVDC transmission lines based on smoothing-reactor voltage
Electric Power Systems Research 168 (2019) 261–268
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A novel pilot protection scheme for LCC-HVDC transmission lines based on smoothing-reactor voltage☆
T
⁎
Yongli Li, Yunke Zhang , Jinzhao Song, Liang Zeng, Jingqiu Zhang The Key Laboratory of Smart Grid of the Ministry of Education, School of Electrical and Information Engineering, Tianjin University, Tianjin 300072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pilot protection HVDC transmission line Smoothing-reactor voltage HVDC transmission system
This paper proposes a novel pilot protection scheme for LCC-HVDC transmission lines based on smoothingreactor voltage (SRV). The proposed scheme is derived taking into consideration transient capacitance current and fault resistance. The polarity characteristics of SRVs are theoretically analyzed under internal and external faults in the fault transient period. And for internal fault, the polarities of SRVs at both sides are positive; when an external fault occurs, the polarity of SRV is negative at faulted side and positive at the other side. Based on these characteristics, a new back-up protection scheme is proposed. Moreover, a fault pole selection criterion is proposed. A ± 800 kV LCC-UHVDC test system is established to verify the accuracy and validity of the proposed protection scheme under different fault conditions. Comprehensive simulations show that the proposed method can identify internal and external faults reliably, with high sensitivity and selectivity, and its performance is inspiring under different fault locations and fault resistances. Furthermore, the proposed protection needs a lower sampling frequency and does not require data synchronization, making it valuable for applications in HVDC transmission system.
1. Introduction With the remarkable advantages of large capability and long distance power transmission, fast and flexible control, low losses and asynchronous power grid interconnections, line-commutated converter based high voltage direct current (LCC-HVDC) transmission system has been wildly applied in the modern power system [1–3]. HVDC transmission line normally crosses complex terrain, and operates in extreme climate environment, which lead to high fault rate on the line. When the protection of dc line is failure to clear the fault reliably, the pole control protection will take action and block the line, seriously threatening the reliable and security operation of power transmission systems [4]. Currently, traveling-wave protection (TWP) is used as the main protection, and current differential protection is acted as the back-up protection for dc lines. Nevertheless, the main protection, which uses the change rate of voltage or voltage variation to detect faults, is sensitive to fault resistance. When an external fault occurs, transient capacitance current may be causing the mal-operation of the back-up protection. As a result, the protection needs a time delay up to hundreds of milliseconds, and its response ability is slow. During the time delay
period, the faulty line of internal fault will be blocked by the pole control protection, and this phenomenon has occurred commonly in practical HVDC system [5]. Therefore, the used protection in HVDC transmission system has lots of problems, and unable to ensure security and stability of the system. To overcome these problems caused by fault resistance or transient capacitance current, a number of improved protections have been presented in Refs. [6–13]. Several improved traveling wave protections have been reported in Refs. [6,7]. Traveling wave protection can operate quickly in most case, but requires high sampling rate. In Refs. [8–10], the single-ended protections based on boundary feature have been proposed. The methods can identify the fault with short operation time, but the sensitivity problem exists for long-distance fault with high resistance. Presently, back-up protections have been proposed in Refs. [11–13]. Based on R–L model, a distance protection scheme was proposed in Ref. [11]. In Ref. [12], improved differential current protection was presented. A directional protection scheme based on reactive energy was presented in Ref. [13]. However, the performance of backup protections [11–13] is strongly influenced by fault resistance. Therefore, it is important to study a new back-up protection for HVDC transmission lines.
☆
This work was supported in part by National Key Research and Development Program of China (2016YFB0900603) and Technology Projects of State Grid Corporation of China (52094017000W). ⁎ Corresponding author. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.epsr.2018.12.012 Received 28 May 2018; Received in revised form 7 November 2018; Accepted 12 December 2018 0378-7796/ © 2018 Elsevier B.V. All rights reserved.
Electric Power Systems Research 168 (2019) 261–268
Y. Li et al.
In this paper, a pilot protection scheme for HVDC transmission lines is proposed based on SRV. According to the superposition theorem, the superimposed network of HVDC system is used to theoretically analyze that the polarity characteristics of SRVs are notably different for internal and external faults. These characteristics are used to construct a new back-up protection scheme. Moreover, a fault pole selection criterion is proposed. A ± 800 kV LCC-UHVDC system is built in PSCAD/ EMTDC and used to validate the proposed protection under different fault locations, fault resistances and sampling frequencies.
2. Analysis of smoothing-reactor voltage 2.1. Equivalent circuit of HVDC transmission system
Fig. 2. Superimposed network for the pole-pole fault.
According to the Thevenin’s theorem and switch function theory [14], the converter of HVDC system with ac system, can be replaced by a dc voltage source and an equivalent inductance at the dc side. The influences of distributed capacitance and fault resistance are considered. In order to simplified analysis of SRV, HVDC transmission line is represented as a lumped parameter model, and DC filters installed at the both terminals of lines are not considered. The equivalent circuits of HVDC system for positive and negative poles are shown in Fig. 1(a) and (b), respectively. In Fig. 1, uR and uI are the equivalent dc voltage sources at rectifier side and inverter side, respectively. M and N denote buses at the two terminals of HVDC transmission lines, respectively. LM and LN are the equivalent inductances at terminal M and N, respectively. Lsr is the inductance of smoothing reactor. The SRVs of positive and negative poles at rectifier side and inverter side are denoted as uMp, uMn, uNp, uNn. For dc transmission lines, Ri is the resistance, Li is the self-inductance, i = 1, 2, and C is the line-to-ground capacitance. According to the superposition theorem, the equivalent network of HVDC transmission system can be divided into two separate parts, including the stable dc network and superimposed network [13]. As the SRVs uMp, uMn, uNp and uNn can be seen as 0 for the stable dc network, only the superimposed network is taken into consideration to analyze the polarity characteristics of SRVs for internal and external faults.
2.2. Internal fault The superimposed network of bipolar fault is shown in Fig. 2. ΔiM and ΔiN are the fault component of smoothing-reactor current at M side and N side, respectively. uf is the equivalent voltage source at the fault point, and Rf is the fault resistance. According to Fig. 2, the voltage uf can be expressed as: dΔiM dt dΔi L2) dtN
⎧uf = 2(LM + Lsr + L1)
+ 2R1 ΔiM + Rf i f
⎨uf = 2(LN + Lsr + ⎩
+ 2R2 ΔiN + Rf i f
(1)
Then, Eq. (1) can be expressed as:
⎧ dΔiM = dt
uf − (2R1 ΔiM + Rf i f )
⎨ dΔiN = ⎩ dt
uf − (2R2 ΔiN + Rf i f )
>0
2(LM + Lsr + L1)
>0
2(LN + Lsr + L2)
(2)
So there are, dΔiM dt dΔi Lsr dtN
⎧ ΔuMp = ΔuNp = Lsr
>0
⎨ ΔuMn = ΔuNn = ⎩
>0
(3)
where ΔuMp and ΔuMn are the fault component SRVs at rectifier side. ΔuNp and ΔuNn are the fault component SRVs at inverter side. From Eq. (3), when a bipolar fault occurs on the dc transmission lines, it can be obviously obtained that the polarities of ΔuMp, ΔuMn, ΔuNp and ΔuNn are positive. The superimposed network of positive-pole to ground fault is shown in Fig. 3. With the same principle, the relationship between the current fault components and the fault-point voltage uf can be described as:
⎧ dΔiM = dt
uf − (R1 ΔiM + Rf i f )
⎨ dΔiN = ⎩ dt
uf − (R2 ΔiN + Rf i f )
LM + Lsr + L1 LN + Lsr + L2
>0 >0
So there is,
Fig. 1. Equivalent circuit of HVDC system. (a) The positive network. (b) The negative network.
Fig. 3. Superimposed network for the positive-pole to ground fault. 262
(4)
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Fig. 4. Superimposed network for the external faults. (a) The external fault at rectifier side. (b) The external fault at inverter side. dΔiM dt dΔi Lsr dtN
⎧ ΔuMp = Lsr
>0
⎨ ΔuNp = ⎩
>0
(5)
According to Eq. (5), when a positive-pole to ground fault occurs, the polarities of ΔuMp and ΔuNp are positive. In the same principle, when a negative-pole to ground fault occurs, the polarities of ΔuMn and ΔuNn are the same with that of positive-pole to ground fault. Therefore, for internal fault, the polarities of fault component SRVs at both sides are positive. 2.3. External fault The superimposed networks of external faults at rectifier side and inverter side are shown in Fig. 4(a) and (b), respectively. In Fig. 4, iC is the discharging current of equivalent capacitive C. When external faults occur, the fault component currents ΔiM and ΔiN and capacitive current iC can be described as:
ΔiM >
1 1 iC and ΔiN > iC 2 2
Fig. 6. Simulation results when a metallic ground fault f3 occurs at the middle point of positive line. (a) SRVs of positive line and negative line at rectifier side. (b) SRVs of positive line and negative line at inverter side. (c) SRVs at both sides, and set voltage. (d) Polarities of SRVs, and operation status of relay protection.
(6)
From Eq. (6), it is proved that the polarities of ΔiM and ΔiN are not determined by the capacitive current iC.
Fig. 5. Bipolar HVDC transmission system. 263
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Table 1 Simulation results under different fault conditions in bipolar mode. Fault distance from rectifier side (km)
Fault resistance (Ω)
uM (kV)
pM
uN (kV)
pN
λ
Fault identification result
100
0 300 500
143.9 39.0 28.0
1 1 1
132.6 34.8 27.0
1 1 1
2.44 2.47 2.45
Positive pole fault
1000
0 300 500
96.1 36.9 26.1
1 1 1
96.9 31.4 21.9
1 1 1
0.41 0.40 0.42
Negative pole fault
1907
0 300 500
230.1 116.1 81.6
1 1 1
234.3 143 96.1
1 1 1
0.95 0.92 0.90
Positive–negative pole fault
Table 2 Simulation results under different fault conditions in unipolar mode. Fault location
Fault distance from rectifier side (km)
Fault resistance (Ω)
uMp (kV)
pM
uNp (kV)
pN
Fault identification result
f3
100
0 300 500 0 300 500 0 300 500
165.31 44.26 31.50 82.09 31.79 20.73 40.69 18.90 13.27
1 1 1 1 1 1 1 1 1
89.17 22.84 15.41 101.14 32.63 21.47 119.00 46.98 28.57
1 1 1 1 1 1 1 1 1
Internal fault
1000
1907
Internal fault
Internal fault
For external fault at rectifier side shown in Fig. 4(a), the voltage uf can be derived as:
uf = (LN + Lsr + L2)
dΔiN dt
+ R2 ΔiN − (Lsr + L1)
dΔiM dt
− R1 ΔiM + Rf i f (7)
In the fault transient period, the increasing trend of ΔiM at rectifier side is negative, and the increasing trend of ΔiN at inverter side is positive [15]. Therefore, ΔuMp and ΔuNp should be,
⎧ ΔuMp = ⎨ ΔuNp = ⎩
dΔi Lsr dtM dΔi Lsr dtN
Fig. 7. Simulation results when a ground fault f3 with 500 Ω occurs at the middle point of positive line. (a) SRVs of positive line and negative line at rectifier side. (b) SRVs of positive line and negative line at inverter side. (c) SRVs at both sides, and set voltage. (d) Polarities of SRVs, and operation status of relay protection.
<0 >0
(8)
For the external fault at inverter side, with the same principle, ΔuMp and ΔuNp should be,
⎧ ΔuMp > 0 Δu < 0 ⎨ ⎩ Np
⎧ ΔuMp = uMpf − uMpn Δu = uNpf − uNpn ⎨ ⎩ Np
(9)
(10)
where uMPf and uNPf are the post-fault SRVs at rectifier side and inverter side, respectively. uMpn and uNpn are the pre-fault SRVs at rectifier side and inverter side, respectively. From Eq. (10), the polarities of ΔuMp and ΔuNp are decided by uMpf, uNpf, uMpn and uNpn. The SRVs uMpn and uNpn can be seen as 0 during the normal operation of HVDC system. Thus, the polarities of ΔuMp and ΔuNp are similar with that of uMpf and uNpf. In summary, Eq. (10) can be simplified as:
According to Eqs. (6), (8) and (9), when an external fault occurs, the polarities of fault component SRVs at both sides are not influenced by the capacitive current of dc transmission line. When an external fault occurs at rectifier side, the polarity of ΔuMp is negative and the polarity of ΔuNp is positive. And when an external fault occurs at inverter side, an analogous conclusion can be acquired that the polarities of fault component SRVs are reverse. For external fault, therefore, the polarity of fault component SRV is negative at faulted side and positive at the other side.
⎧ ΔuMp = uMpf Δu = uNpf ⎨ ⎩ Np
(11)
2.4. Comparison between SRV and fault component SRV From Eq. (11), it is noted that the post-fault SRVs can be substituted for the fault component SRVs to identify internal and external faults.
Taking the positive pole of HVDC transmission system as an example, the fault component SRVs ΔuMp and ΔuNp can be calculated as, 264
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Fig. 8. Simulation results of external fault f1. (a) SRVs of positive line at rectifier side and inverter side. (b) SRVs at both sides, and set voltages. (c) Polarities of SRVs, and operation status of relay protection.
Fig. 9. Simulation results of external fault f6. (a) SRVs of positive line at rectifier side and inverter side. (b) SRVs at both sides, and set voltages. (c) Polarities of SRVs, and operation status of relay protection.
2.5. Summary
⎧uM (k ) = uMp (k ) + uMn (k ) ⎨ ⎩uN (k ) = uNp (k ) + uNn (k )
According to the analysis above, it can be concluded that the polarities of SRVs at both sides are positive for internal fault, no matter bipolar fault or monopole (positive pole or negative pole) fault; when an external fault occurs, the polarity of SRV is negative at faulted side and positive at the other side. Therefore, based on these different characteristics of SRV in the fault transient period, a novel pilot protection for HVDC transmission lines can be constructed.
As is analyzed in Section 2, internal fault or external fault for HVDC transmission lines can be identified by using the different polarity characteristics of SRV. Therefore, the protection criterion of internal fault is,
⎧ pM = 1 p =1 ⎨ ⎩ N
3. Protection scheme
(14)
When a fault occurs, according to the polarity identification criteria Eq. (12), the polarity of SRV at each side is identified independently. And merely one-side signal which is the polarity of SRV, is needed to send to the other side. Then, an internal or external fault can be distinguished by the protection criterion Eq. (14). For the proposed protection, therefore, data synchronization is not required.
3.1. Protection criterion The polarity identification criteria of SRV is given as: NT
⎧ 1 ⎪1, if NT ∑ ui (k ) > uset ⎪ k=1 pi = NT ⎨ 1 ⎪− 1, if N ∑ ui (k ) < − uset T ⎪ k=1 ⎩
(13)
3.2. Threshold setting In order to avoid the mal-operation caused by disturbance, and considering that the approximate error between SRV and fault component SRV has some adverse effects on Eq. (11), the setting value uset is,
(12)
where i = M and i = N denote the rectifier side and the inverter side, respectively. NT is the total number of sampling point in 5 ms. k is an integer and k = 1, 2, 3, …. ui(k) is the SRV at the i side, and uset is the setting value. For a bipolar HVDC system, the calculated method of SRV is described as follows:
uset = kr kc k set UN
(15)
where kr is reliability coefficient. kc represents the voltage measured point at each side, kc = 2 for unipolar HVDC system and kc = 4 for 265
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where λset1 and λset1 are the threshold values of positive line and negative line fault, respectively. According to the reliability requirement of practical HVDC system, λset1 and λset1 can be adjusted to optimal value. To ensure sufficient margin of Eq. (17), λset1 is set to 1.3, and λset2 is 0.7.
Table 3 Simulation results of different external faults in bipolar mode. Fault location (km)
Fault type
uM (kV)
pM
uN (kV)
pN
Fault identification result
f1
ag
−168.7
−1
183.3
1
External fault at rectifier side
f6
abg ab abc ag
−239.0 −238.8 −236.1 100.7
−1 −1 −1 1
236.4 237.3 241.1 −147.2
1 1 1 −1
f2
abg ab abc pg
165.9 173.1 203.5 −167.3
1 1 1 −1
−231.8 −231.3 −233.9 174.9
−1 −1 −1 1
f5
ng pn pg
−167.9 −343.7 134.7
−1 −1 1
175.0 243.3 −157.9
1 1 −1
ng pn
132.9 240.7
1 1
−157.9 −322.9
−1 −1
4. Simulation results External fault at inverter side
A ± 800 kV LCC-UHVDC system with rated current 4 kA is built for simulation under the PSCAD/EMTDC environment [17]. And the mainly parameters come from practical project. The schematic diagram of the HVDC system is shown in Fig. 5. f1, f2, f5 and f6 are external faults. f3 and f4 are internal faults, located on dc transmission lines. Taking the measured voltage at rectifier side as example, up1 and up2 are the measured voltage of positive pole, and un1and un2 are of negative pole. The frequency-dependent model is used to simulate the dc transmission line with the whole length of 1907 km [18]. And the line parameters are given as follows: resistance Rl = 0.00664 Ω/km, inductance Ll = 2.093 mH/km, conductance Gl = 0.01 nS/km and capacitance Cl = 12.422 nF/km. The 12/24/36 triple-tuned dc filters are installed on each side. The simulation step is 10 μs, and the initial sampling frequency is 10 kHz. The fault time is set as 0.9 s, and the duration time of fault is 0.1 s. According to Eq. (15), the threshold value uset is determined, where uset = 8 kV in unipolar mode and uset = 16 kV in bipolar mode. In order to verify the correctness and effectiveness of the proposed protection scheme, various types of faults are tested, and the influences of fault distance, fault resistance, sampling frequency, power flow and lightning disturbance on the proposed protection are analyzed as follows.
External fault at rectifier side
External fault at inverter side
where AG means single-phase ground fault, ABG means double-phase ground fault, AB means double-phase fault, ABC means three-phase fault, PG means positive-pole ground fault, NG means negative-pole ground fault, PN means positive–negative pole fault.
bipolar HVDC system. And UN is the rated voltage of HVDC system. Considering the error of the voltage sensor, kset is taken as 0.005 [16]. 3.3. Fault pole selection For a bipolar HVDC system, when an internal fault occurs on the positive or negative pole, fault pole selection is necessary to isolate the faulty line, maintain continuous operation of the healthy line and enhance the reliability of power system. Due to the coupling effect of positive and negative lines, the SRV of the non-fault line may changes suddenly, and it is considerably smaller than that of the faulty line. Thus, the fault pole selection coefficient λ, with the weight coefficient of both sides, is defined in Eq. (16).
λ=
∑
uMp (k )
k1 kN=T1
∑
The SRVs at both sides and relay protection operation are depicted in Fig. 6, when a metallic ground fault occurs at the middle point of positive pole (953.5 km away from the rectifier side). In Fig. 6, uMp and uMn are the SRVs of positive line and negative line at rectifier side, respectively. uNp and uNn are the SRVs of positive line and negative line at inverter side, respectively. uM and uN are the SRVs at both sides, respectively. PO represents the operation status of relay protection. As shown in Fig. 6(a), during normal operation, the SRVs at both sides are of very small value. And after fault, the SRVs are quickly increasing and larger than the threshold uset (16 kV). Then, the polarities of SRVs in Fig. 6(b) are positive, pM = 1 and pN = 1. Therefore, an internal fault occurred on the protected line is identified correctly, and the proposed protection operates reliably within 10 ms in Fig. 6(c). The simulation result shows that the proposed method is of high sensitivity and selectivity.
NT
NT
∑
4.1. Simulation result of internal fault
+ uMn (k )
k=1
uNp (k )
k2 kN=T1
∑
uNn (k )
(16)
k=1
where k1 and k2 are the weight coefficients of rectifier side and inverter side, respectively. Considering that HVDC transmission line is symmetrical, k1 and k2 are set equal to 0.5. Therefore, the fault pole selection criterion is defined as:
⎧ λ > λ set1,for positive line fault λ set1 > λ > λ set2 ,for positive-negative line fault ⎨ ⎩ λ set2 > λ,for negative line fault
(17)
Table 4 Simulation results of different external faults in unipolar mode. Fault location
Fault type
uMp (kV)
pM
uNp (kV)
pN
Fault identification result
f1
ag abg ab abc pg pg ag abg ab abc
−79.61 −108.39 −111.87 −111.89 −167.84 70.75 29.66 52.01 51.01 52.94
−1 −1 −1 −1 −1 1 1 1 1 1
46.54 73.00 67.65 85.42 97.32 −44.40 −66.90 −103.25 −103.34 −105.72
1 1 1 1 1 −1 −1 −1 −1 −1
External fault at rectifier side
f2 f5 f6
266
External fault at rectifier side External fault at inverter side External fault at inverter side
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of SRV uN at the inverter side is positive after a time delay in Fig. 8(b). Therefore, an external fault at the rectifier side is identified. For external fault at f6, the SRV uN at the inverter side is quickly less than the threshold value −uset, and the polarity pN is negative. And the polarity of uM is positive after a time delay in Fig. 9(b). As a result, an external fault at the inverter side is identified within 10 ms. In summary, when a fault occurs at the rectifier side or inverter side of HVDC system, the fault can be detected as an external fault reliably. And the proposed protection is maintained. However, it is should be noted that the SRV of non-fault side is larger than the setting value uset after a time delay, and the time delay is mainly caused by the transmission of electrical signals (voltage and current) on the dc line and the smoothing reactor Lsr [19]. For a LCC-HVDC system, the smoothing reactor Lsr is constant. Therefore, the time delay increases with the length of dc line increasing. Tables 3 and 4 show that the simulation results of different external faults. The simulation results prove that the proposed protection can correctly identify external fault, and has high selectivity and reliability.
Table 5 Simulation results under different sampling frequencies. Sampling frequency (kHz)
Fault location
UM (kV)
UN (kV)
Fault identification result
2
f3 f5 f3 f5 f3 f5 f3 f5
85.2 200.6 91.3 203.9 91.1 203.5 91.9 203.1
96.1 −208.1 98.8 −229.3 101.0 −233.9 103.1 −237.0
Internal fault External fault Internal fault External fault Internal fault External fault Internal fault External fault
5 10 20
4.2. Influence of fault distance To verify the performance of the proposed method, various types of internal faults in bipolar mode are simulated, and the simulation results are listed in Table 1. Considering that the LCC-UHVDC system can operate in unipolar mode, the simulation results of unipolar mode are shown in Table 2. Tables 1 and 2 show that internal faults can be identified correctly and reliably. Furthermore, the selection of fault pole is of high discrimination under different fault conditions. Therefore, the proposed protection is immune to fault distance and of high reliability and selectivity.
4.5. Influence of sampling frequency To verify the performance of the proposed protection, internal fault f3 occurred at positive line (1907 km away from the rectifier side), and external fault f5 under different sampling frequencies are simulated. The simulation results are shown in Table 5. As shown in Table 5, it is proved that the proposed protection scheme can correctly identify internal fault and external fault, and has high selectivity and reliability. Therefore, the proposed protection has a certain immunity of sampling frequency, and needs a lower sampling frequency to perform well.
4.3. Influence of fault resistance To verify the sensitivity of the proposed protection, the simulation results of faults with different fault resistances are shown in Fig. 7, Tables 1 and 2. From Fig. 7, it can be observed that the polarities of SRVs are positive, and an internal fault is identified reliably within 10 ms. As shown in Tables 1 and 2, it is obvious that the SRVs at both ends are decrease with fault resistance increasing. However, the polarity of SRV is still positive, as the SRV is prominently larger than the threshold uset. Hence, the proposed protection scheme performs well under different fault conditions, and is of high sensitivity for faults with high resistance.
4.6. Influence of power flow During normal operation, the rated voltage and rated current of HVDC system would be changed in a small range. The influence of power flow on the performance of the proposed protection is studied under different operation modes. As shown in Table 6, it is proved that the proposed protection scheme can correctly identify internal fault and external fault, and its performance is well in the operation modes. As a result, the performance of the proposed method is insensitive to power flow change.
4.4. Simulation result of external fault The simulation results of external fault located at f1 and f6 with three-phase ac fault are shown in Figs. 8 and 9, respectively. For external fault at f1, the SRV uM at the rectifier side is quickly less than the threshold value −uset, and the polarity pM is negative. And the polarity
4.7. Influence of lightning disturbance To study the influence of lightning disturbance on the proposed
Table 6 Simulation results under different power flows. Operation mode
Fault location
uM (kV)
pM
uN (kV)
pN
Fault identification result
UN = 1.0 p.u. IN = 0.9 p.u.
f2 f3 f5 f2 f3 f5 f2 f3 f5 f2 f3 f5 f2 f3 f5 f2 f3 f5
−172.80 94.67 130.75 −172.96 93.79 126.11 −173.48 91.08 126.97 −172.71 59.81 102.02 −156.26 89.00 129.89 −136.23 77.33 111.87
−1 1 1 −1 1 1 −1 1 1 −1 1 1 −1 1 1 −1 1 1
172.44 95.79 −162.56 162.99 93.92 −163.38 151.83 89.77 −165.17 118.98 75.88 −167.52 165.53 93.90 147.27 140.46 79.63 −128.61
1 1 −1 1 1 −1 1 1 −1 1 1 −1 1 1 −1 1 1 −1
External fault Internal fault External fault External fault Internal fault External fault External fault Internal fault External fault External fault Internal fault External fault External fault Internal fault External fault External fault Internal fault External fault
UN = 1.0 p.u. IN = 0.8 p.u.
UN = 1.0 p.u. IN = 0.7 p.u.
UN = 1.0 p.u. IN = 0.5 p.u.
UN = 0.9 p.u. IN = 1.0 p.u.
UN = 0.8 p.u. IN = 1.0 p.u.
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to verify the accuracy and validity of the proposed method. Comprehensive simulations show that the proposed protection can accurately identify internal and external faults, with high sensitivity and selectivity. Moreover, it is insensitive to the sampling frequency and fault resistance. In summary, the proposed protection scheme performs well under different fault conditions, with asynchronous data and a lower sampling frequency, making it valuable and prospective for applications in HVDC transmission system.
Table 7 Simulation results of lightning disturbance at different locations. Lightning location
uM (kV)
pM
uN (kV)
pN
Fault identification result
External fault
Line
Distance (km)
Positive pole
100 1000 1907
−26.27 −28.26 −24.03
−1 −1 −1
25.89 23.87 19.89
1 1 1
Negative pole
100 1000 1907
26.90 25.93 20.71
1 1 1
−30.27 −26.57 −29.49
−1 −1 −1
References [1] L. Willis, N. Stig, HVDC transmission: yesterday and today, IEEE Power Energy Mag. 5 (2007) 22–31. [2] Y. Liang, G. Wang, H. Li, Time-domain fault-location method on HVDC transmission lines under unsynchronized two-end measurement and uncertain line parameters, IEEE Trans. Power Deliv. 30 (2015) 1031–1038. [3] M. Farshad, J. Sadeh, A novel fault-location method for HVDC transmission lines based on similarity measure of voltage signals, IEEE Trans. Power Deliv. 28 (2013) 2483–2490. [4] S. Mirhosseini, M. Akhbari, Wide area backup protection algorithm for transmission lines based on fault component complex power, Int. J. Electr. Power Energy Syst. 83 (2016) 1–6. [5] A. Li, Z. Cai, Q. Sun, X. Li, D. Ren, Z. Yang, Study on the dynamic performance characteristics of HVDC control and protections for the HVDC line fault, IEEE Power and Energy Society General Meeting (2009) 1–5. [6] J. Wu, H. Li, G. Wang, Y. Liang, An improved traveling-wave protection scheme for LCC-HVDC transmission lines, IEEE Trans. Power Deliv. 32 (2017) 106–116. [7] D. Wang, H.L. Gao, S.B. Luo, G.B. Zou, Travelling wave pilot protection for LCCHVDC transmission lines based on electronic transformers differential output characteristic, Int. J. Electr. Power Energy Syst. 28 (2017) 283–290. [8] G. Song, X. Chu, S. Gao, X. Kang, Z. Jiao, A new whole-line quick-action protection principle for HVDC transmission lines using one-end current, IEEE Trans. Power Deliv. 30 (2015) 599–607. [9] J. Liu, N. Tai, C. Fan, W. Huang, Protection scheme for high-voltage direct current transmission lines based on transient AC current, IET Gener. Transm. Distrib. 9 (2015) 2633–2643. [10] S. Gao, X. Chu, Q. Shen, X. Jin, J. Luo, Y. Yun, G. Song, A novel whole-line quickaction protection principle for HVDC transmission lines using one-end voltage, Int. J. Electr. Power Energy Syst. 65 (2015) 262–270. [11] Y. Qin, M. Wen, J. Zheng, Y. Bai, A novel distance protection scheme for HVDC lines based on R–L model, Int. J. Electr. Power Energy Syst. 100 (2018) 167–177. [12] J. Zheng, M. Wen, Y. Chen, X. Shao, A novel differential protection scheme for HVDC transmission lines, Int. J. Electr. Power Energy Syst. 94 (2018) 171–178. [13] S. Luo, X. Dong, S. Shi, B. Wang, A directional protection scheme for HVDC transmission lines based on reactive energy, IEEE Trans. Power Deliv. 31 (2016) 559–567. [14] Y. Jiang, A.K. Ekstrom, General analysis of harmonic transfer through converters, IEEE Trans. Power Electron. 12 (1997) 287–293. [15] S. Gao, G. Song, Z. Ma, X. Jin, Novel pilot protection principle for high-voltage direct current transmission lines based on fault component current characteristics, IET Gener. Transm. Distrib. 9 (2015) 468–474. [16] F. Kong, Z. Hao, S. Zhang, B. Zhang, Development of a novel protection device for bipolar HVDC transmission lines, IEEE Trans. Power Deliv. 29 (2014) 2270–2278. [17] Manitoba HVDC Research Center, PSCAD/EMTDC User’s Manual, Winnipeg, MB, Canada, 2003. [18] Y. Zhang, Y. Li, J. Song, B. Li, X. Chen, A new protection scheme for HVDC transmission lines based on the specific frequency current of DC filter, IEEE Trans. Power Deliv. (2018), https://doi.org/10.1109/TPWRD.2018.2867737 Early Access paper. [19] N.M. Haleem, A.D. Rajapakse, Local measurement based ultra-fast directional ROCOV scheme for protecting Bi-pole HVDC grids with a metallic return conductor, Int. J. Electr. Power Energy Syst. 98 (2018) 323–330. [20] J. Sneath, A.D. Rajapakse, Fault detection and interruption in an earthed HVDC grid using ROCOV and hybrid DC breakers, IEEE Trans. Power Deliv. 31 (2016) 973–981. [21] R. Li, L. Xu, L. Yao, DC fault detection and location in meshed multi-terminal HVDC systems based on DC reactor voltage change rate, IEEE Trans. Power Deliv. 32 (2017) 1516–1526.
method, a lightning current with 1.2/50 μs double exponential waveform is applied on dc lines at different locations [9]. The simulation results are shown in Table 7. As shown in Table 7, when the lighting occurs, the polarities of uM and uN can be identified, then an external fault is recognized. As a result, the performance of the proposed method is not affected by lightning disturbance. 4.8. Comparison with other protection methods For LCC-HVDC transmission lines, current differential protection is used as the backup protection. The backup protection, which needs a delay time of 500 ms, may not act as the backup protection reliably, leading to blocked HVDC pole. To solve this problem, a novel pilot protection scheme based on smoothing-reactor voltage for LCC-HVDC transmission lines is proposed in this paper. The proposed protection scheme is derived taking into consideration transient capacitance current and fault resistance, and can accurately identify internal and external faults in 10 ms, with high sensitivity and selectivity. Currently, none of previous works has been proposed back-up protection based on smoothing-reactor voltage. Although the change rate of DC reactor voltage has been used to constructed primary protection scheme for VSC-HVDC grid in Refs. [19–21], the sampling frequency of these proposed schemes [19–21] is higher than that of most practical TWP device (the sampling frequency of which is 6.7 kHz or 10 kHz) [6]. A novel differential protection scheme for HVDC transmission lines based on the compensation of the distributed capacitive current is proposed in Ref. [12]. In Ref. [12], synchronous data of both ends is required. Besides, Ref. [12] introduces that the protection will trip in 15 ms when an internal fault with high fault resistance occurs. However, the new proposed method in this paper can trip in 10 ms. 5. Conclusion A novel pilot protection scheme for LCC-HVDC transmission lines based on smoothing-reactor voltage has been proposed in this paper. The proposed scheme is derived taking into consideration transient capacitance current and fault resistance. According to the superposition theorem, the superimposed network of HVDC system is used to theoretically analyze that the polarity characteristics of SRVs are notably different for internal fault and external fault. These characteristics are used to construct a new back-up protection scheme. And a fault pole selection criterion is proposed. A ± 800 kV LCC-UHVDC system is used
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A novel pilot protection scheme for LCC-HVDC transmission lines based on smoothing-reactor voltage☆
T
⁎
Yongli Li, Yunke Zhang , Jinzhao Song, Liang Zeng, Jingqiu Zhang The Key Laboratory of Smart Grid of the Ministry of Education, School of Electrical and Information Engineering, Tianjin University, Tianjin 300072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pilot protection HVDC transmission line Smoothing-reactor voltage HVDC transmission system
This paper proposes a novel pilot protection scheme for LCC-HVDC transmission lines based on smoothingreactor voltage (SRV). The proposed scheme is derived taking into consideration transient capacitance current and fault resistance. The polarity characteristics of SRVs are theoretically analyzed under internal and external faults in the fault transient period. And for internal fault, the polarities of SRVs at both sides are positive; when an external fault occurs, the polarity of SRV is negative at faulted side and positive at the other side. Based on these characteristics, a new back-up protection scheme is proposed. Moreover, a fault pole selection criterion is proposed. A ± 800 kV LCC-UHVDC test system is established to verify the accuracy and validity of the proposed protection scheme under different fault conditions. Comprehensive simulations show that the proposed method can identify internal and external faults reliably, with high sensitivity and selectivity, and its performance is inspiring under different fault locations and fault resistances. Furthermore, the proposed protection needs a lower sampling frequency and does not require data synchronization, making it valuable for applications in HVDC transmission system.
1. Introduction With the remarkable advantages of large capability and long distance power transmission, fast and flexible control, low losses and asynchronous power grid interconnections, line-commutated converter based high voltage direct current (LCC-HVDC) transmission system has been wildly applied in the modern power system [1–3]. HVDC transmission line normally crosses complex terrain, and operates in extreme climate environment, which lead to high fault rate on the line. When the protection of dc line is failure to clear the fault reliably, the pole control protection will take action and block the line, seriously threatening the reliable and security operation of power transmission systems [4]. Currently, traveling-wave protection (TWP) is used as the main protection, and current differential protection is acted as the back-up protection for dc lines. Nevertheless, the main protection, which uses the change rate of voltage or voltage variation to detect faults, is sensitive to fault resistance. When an external fault occurs, transient capacitance current may be causing the mal-operation of the back-up protection. As a result, the protection needs a time delay up to hundreds of milliseconds, and its response ability is slow. During the time delay
period, the faulty line of internal fault will be blocked by the pole control protection, and this phenomenon has occurred commonly in practical HVDC system [5]. Therefore, the used protection in HVDC transmission system has lots of problems, and unable to ensure security and stability of the system. To overcome these problems caused by fault resistance or transient capacitance current, a number of improved protections have been presented in Refs. [6–13]. Several improved traveling wave protections have been reported in Refs. [6,7]. Traveling wave protection can operate quickly in most case, but requires high sampling rate. In Refs. [8–10], the single-ended protections based on boundary feature have been proposed. The methods can identify the fault with short operation time, but the sensitivity problem exists for long-distance fault with high resistance. Presently, back-up protections have been proposed in Refs. [11–13]. Based on R–L model, a distance protection scheme was proposed in Ref. [11]. In Ref. [12], improved differential current protection was presented. A directional protection scheme based on reactive energy was presented in Ref. [13]. However, the performance of backup protections [11–13] is strongly influenced by fault resistance. Therefore, it is important to study a new back-up protection for HVDC transmission lines.
☆
This work was supported in part by National Key Research and Development Program of China (2016YFB0900603) and Technology Projects of State Grid Corporation of China (52094017000W). ⁎ Corresponding author. E-mail address: [email protected] (Y. Zhang). https://doi.org/10.1016/j.epsr.2018.12.012 Received 28 May 2018; Received in revised form 7 November 2018; Accepted 12 December 2018 0378-7796/ © 2018 Elsevier B.V. All rights reserved.
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In this paper, a pilot protection scheme for HVDC transmission lines is proposed based on SRV. According to the superposition theorem, the superimposed network of HVDC system is used to theoretically analyze that the polarity characteristics of SRVs are notably different for internal and external faults. These characteristics are used to construct a new back-up protection scheme. Moreover, a fault pole selection criterion is proposed. A ± 800 kV LCC-UHVDC system is built in PSCAD/ EMTDC and used to validate the proposed protection under different fault locations, fault resistances and sampling frequencies.
2. Analysis of smoothing-reactor voltage 2.1. Equivalent circuit of HVDC transmission system
Fig. 2. Superimposed network for the pole-pole fault.
According to the Thevenin’s theorem and switch function theory [14], the converter of HVDC system with ac system, can be replaced by a dc voltage source and an equivalent inductance at the dc side. The influences of distributed capacitance and fault resistance are considered. In order to simplified analysis of SRV, HVDC transmission line is represented as a lumped parameter model, and DC filters installed at the both terminals of lines are not considered. The equivalent circuits of HVDC system for positive and negative poles are shown in Fig. 1(a) and (b), respectively. In Fig. 1, uR and uI are the equivalent dc voltage sources at rectifier side and inverter side, respectively. M and N denote buses at the two terminals of HVDC transmission lines, respectively. LM and LN are the equivalent inductances at terminal M and N, respectively. Lsr is the inductance of smoothing reactor. The SRVs of positive and negative poles at rectifier side and inverter side are denoted as uMp, uMn, uNp, uNn. For dc transmission lines, Ri is the resistance, Li is the self-inductance, i = 1, 2, and C is the line-to-ground capacitance. According to the superposition theorem, the equivalent network of HVDC transmission system can be divided into two separate parts, including the stable dc network and superimposed network [13]. As the SRVs uMp, uMn, uNp and uNn can be seen as 0 for the stable dc network, only the superimposed network is taken into consideration to analyze the polarity characteristics of SRVs for internal and external faults.
2.2. Internal fault The superimposed network of bipolar fault is shown in Fig. 2. ΔiM and ΔiN are the fault component of smoothing-reactor current at M side and N side, respectively. uf is the equivalent voltage source at the fault point, and Rf is the fault resistance. According to Fig. 2, the voltage uf can be expressed as: dΔiM dt dΔi L2) dtN
⎧uf = 2(LM + Lsr + L1)
+ 2R1 ΔiM + Rf i f
⎨uf = 2(LN + Lsr + ⎩
+ 2R2 ΔiN + Rf i f
(1)
Then, Eq. (1) can be expressed as:
⎧ dΔiM = dt
uf − (2R1 ΔiM + Rf i f )
⎨ dΔiN = ⎩ dt
uf − (2R2 ΔiN + Rf i f )
>0
2(LM + Lsr + L1)
>0
2(LN + Lsr + L2)
(2)
So there are, dΔiM dt dΔi Lsr dtN
⎧ ΔuMp = ΔuNp = Lsr
>0
⎨ ΔuMn = ΔuNn = ⎩
>0
(3)
where ΔuMp and ΔuMn are the fault component SRVs at rectifier side. ΔuNp and ΔuNn are the fault component SRVs at inverter side. From Eq. (3), when a bipolar fault occurs on the dc transmission lines, it can be obviously obtained that the polarities of ΔuMp, ΔuMn, ΔuNp and ΔuNn are positive. The superimposed network of positive-pole to ground fault is shown in Fig. 3. With the same principle, the relationship between the current fault components and the fault-point voltage uf can be described as:
⎧ dΔiM = dt
uf − (R1 ΔiM + Rf i f )
⎨ dΔiN = ⎩ dt
uf − (R2 ΔiN + Rf i f )
LM + Lsr + L1 LN + Lsr + L2
>0 >0
So there is,
Fig. 1. Equivalent circuit of HVDC system. (a) The positive network. (b) The negative network.
Fig. 3. Superimposed network for the positive-pole to ground fault. 262
(4)
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Fig. 4. Superimposed network for the external faults. (a) The external fault at rectifier side. (b) The external fault at inverter side. dΔiM dt dΔi Lsr dtN
⎧ ΔuMp = Lsr
>0
⎨ ΔuNp = ⎩
>0
(5)
According to Eq. (5), when a positive-pole to ground fault occurs, the polarities of ΔuMp and ΔuNp are positive. In the same principle, when a negative-pole to ground fault occurs, the polarities of ΔuMn and ΔuNn are the same with that of positive-pole to ground fault. Therefore, for internal fault, the polarities of fault component SRVs at both sides are positive. 2.3. External fault The superimposed networks of external faults at rectifier side and inverter side are shown in Fig. 4(a) and (b), respectively. In Fig. 4, iC is the discharging current of equivalent capacitive C. When external faults occur, the fault component currents ΔiM and ΔiN and capacitive current iC can be described as:
ΔiM >
1 1 iC and ΔiN > iC 2 2
Fig. 6. Simulation results when a metallic ground fault f3 occurs at the middle point of positive line. (a) SRVs of positive line and negative line at rectifier side. (b) SRVs of positive line and negative line at inverter side. (c) SRVs at both sides, and set voltage. (d) Polarities of SRVs, and operation status of relay protection.
(6)
From Eq. (6), it is proved that the polarities of ΔiM and ΔiN are not determined by the capacitive current iC.
Fig. 5. Bipolar HVDC transmission system. 263
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Table 1 Simulation results under different fault conditions in bipolar mode. Fault distance from rectifier side (km)
Fault resistance (Ω)
uM (kV)
pM
uN (kV)
pN
λ
Fault identification result
100
0 300 500
143.9 39.0 28.0
1 1 1
132.6 34.8 27.0
1 1 1
2.44 2.47 2.45
Positive pole fault
1000
0 300 500
96.1 36.9 26.1
1 1 1
96.9 31.4 21.9
1 1 1
0.41 0.40 0.42
Negative pole fault
1907
0 300 500
230.1 116.1 81.6
1 1 1
234.3 143 96.1
1 1 1
0.95 0.92 0.90
Positive–negative pole fault
Table 2 Simulation results under different fault conditions in unipolar mode. Fault location
Fault distance from rectifier side (km)
Fault resistance (Ω)
uMp (kV)
pM
uNp (kV)
pN
Fault identification result
f3
100
0 300 500 0 300 500 0 300 500
165.31 44.26 31.50 82.09 31.79 20.73 40.69 18.90 13.27
1 1 1 1 1 1 1 1 1
89.17 22.84 15.41 101.14 32.63 21.47 119.00 46.98 28.57
1 1 1 1 1 1 1 1 1
Internal fault
1000
1907
Internal fault
Internal fault
For external fault at rectifier side shown in Fig. 4(a), the voltage uf can be derived as:
uf = (LN + Lsr + L2)
dΔiN dt
+ R2 ΔiN − (Lsr + L1)
dΔiM dt
− R1 ΔiM + Rf i f (7)
In the fault transient period, the increasing trend of ΔiM at rectifier side is negative, and the increasing trend of ΔiN at inverter side is positive [15]. Therefore, ΔuMp and ΔuNp should be,
⎧ ΔuMp = ⎨ ΔuNp = ⎩
dΔi Lsr dtM dΔi Lsr dtN
Fig. 7. Simulation results when a ground fault f3 with 500 Ω occurs at the middle point of positive line. (a) SRVs of positive line and negative line at rectifier side. (b) SRVs of positive line and negative line at inverter side. (c) SRVs at both sides, and set voltage. (d) Polarities of SRVs, and operation status of relay protection.
<0 >0
(8)
For the external fault at inverter side, with the same principle, ΔuMp and ΔuNp should be,
⎧ ΔuMp > 0 Δu < 0 ⎨ ⎩ Np
⎧ ΔuMp = uMpf − uMpn Δu = uNpf − uNpn ⎨ ⎩ Np
(9)
(10)
where uMPf and uNPf are the post-fault SRVs at rectifier side and inverter side, respectively. uMpn and uNpn are the pre-fault SRVs at rectifier side and inverter side, respectively. From Eq. (10), the polarities of ΔuMp and ΔuNp are decided by uMpf, uNpf, uMpn and uNpn. The SRVs uMpn and uNpn can be seen as 0 during the normal operation of HVDC system. Thus, the polarities of ΔuMp and ΔuNp are similar with that of uMpf and uNpf. In summary, Eq. (10) can be simplified as:
According to Eqs. (6), (8) and (9), when an external fault occurs, the polarities of fault component SRVs at both sides are not influenced by the capacitive current of dc transmission line. When an external fault occurs at rectifier side, the polarity of ΔuMp is negative and the polarity of ΔuNp is positive. And when an external fault occurs at inverter side, an analogous conclusion can be acquired that the polarities of fault component SRVs are reverse. For external fault, therefore, the polarity of fault component SRV is negative at faulted side and positive at the other side.
⎧ ΔuMp = uMpf Δu = uNpf ⎨ ⎩ Np
(11)
2.4. Comparison between SRV and fault component SRV From Eq. (11), it is noted that the post-fault SRVs can be substituted for the fault component SRVs to identify internal and external faults.
Taking the positive pole of HVDC transmission system as an example, the fault component SRVs ΔuMp and ΔuNp can be calculated as, 264
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Fig. 8. Simulation results of external fault f1. (a) SRVs of positive line at rectifier side and inverter side. (b) SRVs at both sides, and set voltages. (c) Polarities of SRVs, and operation status of relay protection.
Fig. 9. Simulation results of external fault f6. (a) SRVs of positive line at rectifier side and inverter side. (b) SRVs at both sides, and set voltages. (c) Polarities of SRVs, and operation status of relay protection.
2.5. Summary
⎧uM (k ) = uMp (k ) + uMn (k ) ⎨ ⎩uN (k ) = uNp (k ) + uNn (k )
According to the analysis above, it can be concluded that the polarities of SRVs at both sides are positive for internal fault, no matter bipolar fault or monopole (positive pole or negative pole) fault; when an external fault occurs, the polarity of SRV is negative at faulted side and positive at the other side. Therefore, based on these different characteristics of SRV in the fault transient period, a novel pilot protection for HVDC transmission lines can be constructed.
As is analyzed in Section 2, internal fault or external fault for HVDC transmission lines can be identified by using the different polarity characteristics of SRV. Therefore, the protection criterion of internal fault is,
⎧ pM = 1 p =1 ⎨ ⎩ N
3. Protection scheme
(14)
When a fault occurs, according to the polarity identification criteria Eq. (12), the polarity of SRV at each side is identified independently. And merely one-side signal which is the polarity of SRV, is needed to send to the other side. Then, an internal or external fault can be distinguished by the protection criterion Eq. (14). For the proposed protection, therefore, data synchronization is not required.
3.1. Protection criterion The polarity identification criteria of SRV is given as: NT
⎧ 1 ⎪1, if NT ∑ ui (k ) > uset ⎪ k=1 pi = NT ⎨ 1 ⎪− 1, if N ∑ ui (k ) < − uset T ⎪ k=1 ⎩
(13)
3.2. Threshold setting In order to avoid the mal-operation caused by disturbance, and considering that the approximate error between SRV and fault component SRV has some adverse effects on Eq. (11), the setting value uset is,
(12)
where i = M and i = N denote the rectifier side and the inverter side, respectively. NT is the total number of sampling point in 5 ms. k is an integer and k = 1, 2, 3, …. ui(k) is the SRV at the i side, and uset is the setting value. For a bipolar HVDC system, the calculated method of SRV is described as follows:
uset = kr kc k set UN
(15)
where kr is reliability coefficient. kc represents the voltage measured point at each side, kc = 2 for unipolar HVDC system and kc = 4 for 265
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where λset1 and λset1 are the threshold values of positive line and negative line fault, respectively. According to the reliability requirement of practical HVDC system, λset1 and λset1 can be adjusted to optimal value. To ensure sufficient margin of Eq. (17), λset1 is set to 1.3, and λset2 is 0.7.
Table 3 Simulation results of different external faults in bipolar mode. Fault location (km)
Fault type
uM (kV)
pM
uN (kV)
pN
Fault identification result
f1
ag
−168.7
−1
183.3
1
External fault at rectifier side
f6
abg ab abc ag
−239.0 −238.8 −236.1 100.7
−1 −1 −1 1
236.4 237.3 241.1 −147.2
1 1 1 −1
f2
abg ab abc pg
165.9 173.1 203.5 −167.3
1 1 1 −1
−231.8 −231.3 −233.9 174.9
−1 −1 −1 1
f5
ng pn pg
−167.9 −343.7 134.7
−1 −1 1
175.0 243.3 −157.9
1 1 −1
ng pn
132.9 240.7
1 1
−157.9 −322.9
−1 −1
4. Simulation results External fault at inverter side
A ± 800 kV LCC-UHVDC system with rated current 4 kA is built for simulation under the PSCAD/EMTDC environment [17]. And the mainly parameters come from practical project. The schematic diagram of the HVDC system is shown in Fig. 5. f1, f2, f5 and f6 are external faults. f3 and f4 are internal faults, located on dc transmission lines. Taking the measured voltage at rectifier side as example, up1 and up2 are the measured voltage of positive pole, and un1and un2 are of negative pole. The frequency-dependent model is used to simulate the dc transmission line with the whole length of 1907 km [18]. And the line parameters are given as follows: resistance Rl = 0.00664 Ω/km, inductance Ll = 2.093 mH/km, conductance Gl = 0.01 nS/km and capacitance Cl = 12.422 nF/km. The 12/24/36 triple-tuned dc filters are installed on each side. The simulation step is 10 μs, and the initial sampling frequency is 10 kHz. The fault time is set as 0.9 s, and the duration time of fault is 0.1 s. According to Eq. (15), the threshold value uset is determined, where uset = 8 kV in unipolar mode and uset = 16 kV in bipolar mode. In order to verify the correctness and effectiveness of the proposed protection scheme, various types of faults are tested, and the influences of fault distance, fault resistance, sampling frequency, power flow and lightning disturbance on the proposed protection are analyzed as follows.
External fault at rectifier side
External fault at inverter side
where AG means single-phase ground fault, ABG means double-phase ground fault, AB means double-phase fault, ABC means three-phase fault, PG means positive-pole ground fault, NG means negative-pole ground fault, PN means positive–negative pole fault.
bipolar HVDC system. And UN is the rated voltage of HVDC system. Considering the error of the voltage sensor, kset is taken as 0.005 [16]. 3.3. Fault pole selection For a bipolar HVDC system, when an internal fault occurs on the positive or negative pole, fault pole selection is necessary to isolate the faulty line, maintain continuous operation of the healthy line and enhance the reliability of power system. Due to the coupling effect of positive and negative lines, the SRV of the non-fault line may changes suddenly, and it is considerably smaller than that of the faulty line. Thus, the fault pole selection coefficient λ, with the weight coefficient of both sides, is defined in Eq. (16).
λ=
∑
uMp (k )
k1 kN=T1
∑
The SRVs at both sides and relay protection operation are depicted in Fig. 6, when a metallic ground fault occurs at the middle point of positive pole (953.5 km away from the rectifier side). In Fig. 6, uMp and uMn are the SRVs of positive line and negative line at rectifier side, respectively. uNp and uNn are the SRVs of positive line and negative line at inverter side, respectively. uM and uN are the SRVs at both sides, respectively. PO represents the operation status of relay protection. As shown in Fig. 6(a), during normal operation, the SRVs at both sides are of very small value. And after fault, the SRVs are quickly increasing and larger than the threshold uset (16 kV). Then, the polarities of SRVs in Fig. 6(b) are positive, pM = 1 and pN = 1. Therefore, an internal fault occurred on the protected line is identified correctly, and the proposed protection operates reliably within 10 ms in Fig. 6(c). The simulation result shows that the proposed method is of high sensitivity and selectivity.
NT
NT
∑
4.1. Simulation result of internal fault
+ uMn (k )
k=1
uNp (k )
k2 kN=T1
∑
uNn (k )
(16)
k=1
where k1 and k2 are the weight coefficients of rectifier side and inverter side, respectively. Considering that HVDC transmission line is symmetrical, k1 and k2 are set equal to 0.5. Therefore, the fault pole selection criterion is defined as:
⎧ λ > λ set1,for positive line fault λ set1 > λ > λ set2 ,for positive-negative line fault ⎨ ⎩ λ set2 > λ,for negative line fault
(17)
Table 4 Simulation results of different external faults in unipolar mode. Fault location
Fault type
uMp (kV)
pM
uNp (kV)
pN
Fault identification result
f1
ag abg ab abc pg pg ag abg ab abc
−79.61 −108.39 −111.87 −111.89 −167.84 70.75 29.66 52.01 51.01 52.94
−1 −1 −1 −1 −1 1 1 1 1 1
46.54 73.00 67.65 85.42 97.32 −44.40 −66.90 −103.25 −103.34 −105.72
1 1 1 1 1 −1 −1 −1 −1 −1
External fault at rectifier side
f2 f5 f6
266
External fault at rectifier side External fault at inverter side External fault at inverter side
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of SRV uN at the inverter side is positive after a time delay in Fig. 8(b). Therefore, an external fault at the rectifier side is identified. For external fault at f6, the SRV uN at the inverter side is quickly less than the threshold value −uset, and the polarity pN is negative. And the polarity of uM is positive after a time delay in Fig. 9(b). As a result, an external fault at the inverter side is identified within 10 ms. In summary, when a fault occurs at the rectifier side or inverter side of HVDC system, the fault can be detected as an external fault reliably. And the proposed protection is maintained. However, it is should be noted that the SRV of non-fault side is larger than the setting value uset after a time delay, and the time delay is mainly caused by the transmission of electrical signals (voltage and current) on the dc line and the smoothing reactor Lsr [19]. For a LCC-HVDC system, the smoothing reactor Lsr is constant. Therefore, the time delay increases with the length of dc line increasing. Tables 3 and 4 show that the simulation results of different external faults. The simulation results prove that the proposed protection can correctly identify external fault, and has high selectivity and reliability.
Table 5 Simulation results under different sampling frequencies. Sampling frequency (kHz)
Fault location
UM (kV)
UN (kV)
Fault identification result
2
f3 f5 f3 f5 f3 f5 f3 f5
85.2 200.6 91.3 203.9 91.1 203.5 91.9 203.1
96.1 −208.1 98.8 −229.3 101.0 −233.9 103.1 −237.0
Internal fault External fault Internal fault External fault Internal fault External fault Internal fault External fault
5 10 20
4.2. Influence of fault distance To verify the performance of the proposed method, various types of internal faults in bipolar mode are simulated, and the simulation results are listed in Table 1. Considering that the LCC-UHVDC system can operate in unipolar mode, the simulation results of unipolar mode are shown in Table 2. Tables 1 and 2 show that internal faults can be identified correctly and reliably. Furthermore, the selection of fault pole is of high discrimination under different fault conditions. Therefore, the proposed protection is immune to fault distance and of high reliability and selectivity.
4.5. Influence of sampling frequency To verify the performance of the proposed protection, internal fault f3 occurred at positive line (1907 km away from the rectifier side), and external fault f5 under different sampling frequencies are simulated. The simulation results are shown in Table 5. As shown in Table 5, it is proved that the proposed protection scheme can correctly identify internal fault and external fault, and has high selectivity and reliability. Therefore, the proposed protection has a certain immunity of sampling frequency, and needs a lower sampling frequency to perform well.
4.3. Influence of fault resistance To verify the sensitivity of the proposed protection, the simulation results of faults with different fault resistances are shown in Fig. 7, Tables 1 and 2. From Fig. 7, it can be observed that the polarities of SRVs are positive, and an internal fault is identified reliably within 10 ms. As shown in Tables 1 and 2, it is obvious that the SRVs at both ends are decrease with fault resistance increasing. However, the polarity of SRV is still positive, as the SRV is prominently larger than the threshold uset. Hence, the proposed protection scheme performs well under different fault conditions, and is of high sensitivity for faults with high resistance.
4.6. Influence of power flow During normal operation, the rated voltage and rated current of HVDC system would be changed in a small range. The influence of power flow on the performance of the proposed protection is studied under different operation modes. As shown in Table 6, it is proved that the proposed protection scheme can correctly identify internal fault and external fault, and its performance is well in the operation modes. As a result, the performance of the proposed method is insensitive to power flow change.
4.4. Simulation result of external fault The simulation results of external fault located at f1 and f6 with three-phase ac fault are shown in Figs. 8 and 9, respectively. For external fault at f1, the SRV uM at the rectifier side is quickly less than the threshold value −uset, and the polarity pM is negative. And the polarity
4.7. Influence of lightning disturbance To study the influence of lightning disturbance on the proposed
Table 6 Simulation results under different power flows. Operation mode
Fault location
uM (kV)
pM
uN (kV)
pN
Fault identification result
UN = 1.0 p.u. IN = 0.9 p.u.
f2 f3 f5 f2 f3 f5 f2 f3 f5 f2 f3 f5 f2 f3 f5 f2 f3 f5
−172.80 94.67 130.75 −172.96 93.79 126.11 −173.48 91.08 126.97 −172.71 59.81 102.02 −156.26 89.00 129.89 −136.23 77.33 111.87
−1 1 1 −1 1 1 −1 1 1 −1 1 1 −1 1 1 −1 1 1
172.44 95.79 −162.56 162.99 93.92 −163.38 151.83 89.77 −165.17 118.98 75.88 −167.52 165.53 93.90 147.27 140.46 79.63 −128.61
1 1 −1 1 1 −1 1 1 −1 1 1 −1 1 1 −1 1 1 −1
External fault Internal fault External fault External fault Internal fault External fault External fault Internal fault External fault External fault Internal fault External fault External fault Internal fault External fault External fault Internal fault External fault
UN = 1.0 p.u. IN = 0.8 p.u.
UN = 1.0 p.u. IN = 0.7 p.u.
UN = 1.0 p.u. IN = 0.5 p.u.
UN = 0.9 p.u. IN = 1.0 p.u.
UN = 0.8 p.u. IN = 1.0 p.u.
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to verify the accuracy and validity of the proposed method. Comprehensive simulations show that the proposed protection can accurately identify internal and external faults, with high sensitivity and selectivity. Moreover, it is insensitive to the sampling frequency and fault resistance. In summary, the proposed protection scheme performs well under different fault conditions, with asynchronous data and a lower sampling frequency, making it valuable and prospective for applications in HVDC transmission system.
Table 7 Simulation results of lightning disturbance at different locations. Lightning location
uM (kV)
pM
uN (kV)
pN
Fault identification result
External fault
Line
Distance (km)
Positive pole
100 1000 1907
−26.27 −28.26 −24.03
−1 −1 −1
25.89 23.87 19.89
1 1 1
Negative pole
100 1000 1907
26.90 25.93 20.71
1 1 1
−30.27 −26.57 −29.49
−1 −1 −1
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method, a lightning current with 1.2/50 μs double exponential waveform is applied on dc lines at different locations [9]. The simulation results are shown in Table 7. As shown in Table 7, when the lighting occurs, the polarities of uM and uN can be identified, then an external fault is recognized. As a result, the performance of the proposed method is not affected by lightning disturbance. 4.8. Comparison with other protection methods For LCC-HVDC transmission lines, current differential protection is used as the backup protection. The backup protection, which needs a delay time of 500 ms, may not act as the backup protection reliably, leading to blocked HVDC pole. To solve this problem, a novel pilot protection scheme based on smoothing-reactor voltage for LCC-HVDC transmission lines is proposed in this paper. The proposed protection scheme is derived taking into consideration transient capacitance current and fault resistance, and can accurately identify internal and external faults in 10 ms, with high sensitivity and selectivity. Currently, none of previous works has been proposed back-up protection based on smoothing-reactor voltage. Although the change rate of DC reactor voltage has been used to constructed primary protection scheme for VSC-HVDC grid in Refs. [19–21], the sampling frequency of these proposed schemes [19–21] is higher than that of most practical TWP device (the sampling frequency of which is 6.7 kHz or 10 kHz) [6]. A novel differential protection scheme for HVDC transmission lines based on the compensation of the distributed capacitive current is proposed in Ref. [12]. In Ref. [12], synchronous data of both ends is required. Besides, Ref. [12] introduces that the protection will trip in 15 ms when an internal fault with high fault resistance occurs. However, the new proposed method in this paper can trip in 10 ms. 5. Conclusion A novel pilot protection scheme for LCC-HVDC transmission lines based on smoothing-reactor voltage has been proposed in this paper. The proposed scheme is derived taking into consideration transient capacitance current and fault resistance. According to the superposition theorem, the superimposed network of HVDC system is used to theoretically analyze that the polarity characteristics of SRVs are notably different for internal fault and external fault. These characteristics are used to construct a new back-up protection scheme. And a fault pole selection criterion is proposed. A ± 800 kV LCC-UHVDC system is used
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