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Travelling wave directional pilot protection for hybrid HVDC transmission line
Electrical Power and Energy Systems 107 (2019) 615–627
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Travelling wave directional pilot protection for hybrid HVDC transmission line
T
⁎
Dong Wang , Mengqian Hou, Mengyou Gao, Feng Qiao School of Automation and Electronic Engineering, Qingdao University of Science & Technology, No.99, Songling Road, Qingdao 266042, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Travelling wave Directional pilot protection Hybrid HVDC Transmission line PSCAD/EMTDC
As known, hybrid high-voltage direct current (HVDC) is wide applied in power system. Because two converter stations are line commutated converter based HVDC (LCC-HVDC) type and voltage source converter based HVDC (VSC-HVDC) type respectively, the HVDC transmission line (HVDC-TL) has special travelling wave (TW) propagation characteristic. Especially, due to the capacitance installed at VSC-HVDC station, the relay at VSC-HVDC side cannot detect voltage TW (VTW) signal. Meanwhile, TW protection is widely applied in HVDC transmission system, but it has no fault direction recognition ability. Hence, the paper proposes a novel TW directional pilot protection. Firstly, the paper analyzes the special propagation characteristics of TW along HVDC-TL. Secondly, the paper proposes novel fault direction discrimination criterions based on the integration of instantaneous power and current at LCC-HVDC side and VSC-HVDC side, respectively. Meanwhile, faulty pole selection criterion is also proposed based on the ratio of positive pole current to negative pole current. Thirdly, based on PSCAD/EMTDC, a simulation model of ± 400 kV LCC-VSC-HVDC transmission system is established. And the simulation results prove that the proposed protection method is immune to fault location, fault resistance and fault type.
1. Introduction HVDC transmission system has several advantages over alternating current (AC) transmission system like long transmission distance, high transmission efficiency and large transmission capacity [1–5]. Especially, LCC-HVDC converter station has larger transmission capacity and lower project construction costs than other converter stations. But, due to the control strategy of line commutated converter, LCC-HVDC inverter station is highly dependent on DC/AC voltage level and suffers from commutation failure issue for a long time [6–9]. Compared with LCC-HVDC converter station, VSC-HVDC converter station has no commutation failure but higher construction cost to obtain the same transmission capacity as LCC-HVDC converter station. In order to acquire the common advantages of LCC-HVDC and VSC-HVDC converter stations, LCC-VSC-HVDC hybrid transmission systems are applied in current power system for two main purpose: (1) In order to power weak AC system which has heavy loads, considering acquiring large transmission capacity, no commutation failure issue and lower construction cost, it is generally designed as LCC-VSC-HVDC hybrid transmission system. (2) In order to overcome commutation failure issue of inverter station of existing LCC-HVDC project, taking reconstruction cost into account, conventional LCC-HVDC transmission system could be ⁎
upgraded to LCC-VSC-HVDC hybrid system. As main protection principle of HVDC transmission line, TW protection is widely used including voltage change criterion, voltage change rate criterion and current change rate criterion. However, it has a number of disadvantages: (1) No direction discrimination ability. As a single-ended protection principle, external fault may lead to wrong operation; (2) Threshold issue. Considering only voltage and current amplitude signals are adopted in protection principle, the threshold value directly affects the reliability and sensibility of protection. (3) Fault resistance. The VTW and current TW (CTW) amplitudes will significantly decrease if fault resistance is very large. In order to overcome these disadvantages, a number of scholars have proposed novel protection principles. Through comparing the special frequency band voltage or current integration value with threshold, the protection principle proposed by Song et al. [10] and Gao et al. [11] can identify internal or external fault. It has fast operation speed, but sensibility issue may occur with large fault resistance. Liu et al. [12] proposes a novel special frequency band current differential pilot protection principle, but it requires strict time synchronization system. Ha et al. [13] proposes a TW based current differential pilot protection. Just same with [12], strict time synchronization system also affects its application in actual power system. Wang et al. [14] proposes a novel protection which is suitable for hybrid HVDC-TL, but it may
Corresponding author. E-mail address: [email protected] (D. Wang).
https://doi.org/10.1016/j.ijepes.2018.12.028 Received 28 September 2018; Received in revised form 27 November 2018; Accepted 16 December 2018 0142-0615/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 2. Reflection and refraction of TW.
Fig. 1. HVDC-TL’s equivalent model.
uF = iF
have operation speed issue. A faulty component of current characteristic based protection is proposed by Gao et al. [15], but it may have slow operation speed and threshold issue. Liu et al. [16] proposes a hybrid protection based on TW protection and boundary protection, but it may be too complex to be applied in actual power system. Other novel protection principles can be found in [17–21]. To solve these issues, the paper proposes a novel protection principle. In Section 2, the general propagation characteristic of TW is described. In Section 3, novel direction pilot protection principle is proposed including TW propagation characteristic along hybrid HVDC-TL, fault direction discrimination principle of two ends of HVDC-TL, faulty pole selection method and protection implementation method. In Section 4, a series of simulations are carried out including the influences of different fault locations and different fault resistances. In Section 5, a concise summary is given.
2.2. Reflection and refraction of TW Fig. 2 shows the reflection and refraction of TW. As can be seen, the incident wave (VTW u1 and CTW i1) propagates along line 1, then reflection and refraction occur at impedance discontinuity point (fault or busbar). And the wave impedance at line 1 and line 2 are Z1 and Z2 , respectively. The reflection VTW and CTW are u2 and i2 , respectively. The refraction VTW and CTW are u3 and i3 , respectively. The forward directions of VTW and CTW in this paper are from HVDC-TL to ground and its actual propagation direction (i1 and i3 from left to right, i2 from right to left), respectively. Hence:
u + u2 = u3 ⎧ 1 i − i2 = i3 ⎪ ⎪1 i1 = u1/ Z1 ⎨ ⎪i2 = u2/ Z2 ⎪i3 = u3/ Z3 ⎩
2.1. Relationship between initial VTW and CTW Fig. 1 is lossless HVDC-TL’s equivalent model [22], and fault happens at F when t = 0 . The initial VTW is uF . Considering C is distributed capacitance per unit length of HVDC-TL, it will be charged to:
dt → 0
dQ CuF dx = lim = CuF v dt → 0 dt dt
(1)
⎧ λ u = (Z2 − Z1)/(Z2 + Z1) ⎪ λi = (Z2 − Z1)/(Z2 + Z1) ⎨ βu = 2Z2/(Z2 + Z1) ⎪ β = 2Z /(Z + Z ) 1 2 1 ⎩ i
(2)
3. Protection method 3.1. Typical hybrid HVDC transmission system
(3)
where dΦ is magnetic flux; L is the inductance per unit length of HVDCTL. Based on Eq. (3), the electrical filed around this short HVDC-TL can be described as:
E = lim
dt → 0
dΦ LCuF v dx = lim = LCuF v 2 dt → 0 dt dt
Fig. 3 is typical ±400 kV LCC-VSC-HVDC transmission system, and it consists of three parts: LCC-HVDC rectifier station, HVDC-TL and VSCHVDC inverter station. At rectifier station, it adopts constant current control strategy. At inverter station, it adopts constant DC and AC voltages control strategy. The length of HVDC-TL is 300 km and the power transmitted on HVDC-TL is 800 MW. RP, RN, IP and IN are relays installed at different positions. Besides, the transmission system simulation model is established in PSCAD/EMTDC. And F1, F2, F3, F4 and F5 are positive pole to ground (PG) internal fault, negative pole to ground (NG) internal fault, positive pole to negative pole (PN) internal fault, PG external fault at rectifier side and PG external fault at inverter side, respectively.
(4)
where E is electrical field strength. If dx is infinite small, then:
E ≈ uF
(5)
Taking Eq. (5) into Eq. (4):
v=
1 LC
(9)
where λ u and λi are VTW and CTW reflection coefficients, respectively. βu and βi are VTW and CTW refraction coefficients, respectively.
where iF is the initial CTW; dt is propagation time of TW of dx length; v is the TW propagation speed along HVDC-TL. Considering the magnetic flux of inductance of HVDC-TL, it can be described as:
dΦ = LiF dx = LCuF v dx
(8)
Based on Eq. (8), reflection and refraction coefficients could be described as:
where dQ is amount of electric charge; dx is the length of HVDC-TL. If dx is infinite small, based on Eq. (1):
iF = lim
(7)
Apparently, initial VTW and CTW have same polarities, and the ratio of initial VTW to CTW is a constant value L/ C .
2. TW propagation
dQ = CuF dx
L C
3.2. TW propagation characteristics
(6)
Figs. 4 and 5 describe TW propagation characteristics under internal
Taking Eq. (6) into Eq. (2): 616
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Fig. 3. Typical ± 400 kV LCC-VSC-HVDC transmission system.
be seen in Fig. 3, there are large smoothing reactors installed in LCCHVDC rectifier station, and the paper choose smoothing reactor to be equivalent to rectifier station. Besides, there are large capacitances installed between HVDC-TL and ground at VSC-HVDC inverter station, and its impedance in TW frequency band (several kHz to hundreds of kHz) is approximately equal to 0Ω. In other words, in Fig. 3, IP and IN are approximately grounded directly (only for TW frequency bandwidth). Hence, in Figs. 4 and 5, R is grounded directly (only for TW frequency bandwidth). These assumptions are proved in Appendix A. Table 1 is two ends’ TW at different mutation points under internal fault. Tables 2 and 3 are two ends’ TW at different mutation points under external faults at rectifier and inverter sides, respectively. λLu and λLi are VTW and CTW reflection coefficients at rectifier side, respectively. λFu and λFi are VTW and CTW reflection coefficients at fault point, respectively. λRu and λRi are VTW and CTW reflection coefficients at inverter side, respectively. βLu and βLi are VTW and CTW refraction coefficients at rectifier side, respectively. βFu and βFi are VTW and CTW refraction coefficients at fault point, respectively. βRu and βRi are VTW and CTW refraction coefficients at inverter side, respectively. Generally, − 1 < λFu = λFi < 1, βFu > 0, βFi > 0 . And:
Fig. 4. TW propagation characteristics under internal fault.
fault and external fault, respectively. And Fig. 5(a) and (b) are external faults occur at rectifier and inverter sides, respectively. L and R represents LCC-HVDC and VSC-HVDC stations, respectively. L1, L 2, L3 and L 4 are different TW mutation points at LCC-HVDC station. R1, R2, R3 and R 4 are different TW mutation points at VSC-HVDC station. As can
Fig. 5. TW propagation characteristics under external faults. 617
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Table 1 Two ends’ TW at different mutation points under internal fault. No.
Relay R1
No.
VTW
CTW
L1 L2
(1 + λLu) uF λLu λFu (1 + λLu) uF
− (1 − λLi ) iF − λLi λFi (1 − λLi ) iF
L3 ⋯
2 2 λLu λFu (1 + λLu) uF ⋯
2 2 − λLi λFi (1 − λLi ) iF ⋯
Relay R1
No.
VTW
CTW
R1 R2
0 0
− (1 − λRi) iF − λLi βFi (1 − λRi) iF
R3 ⋯
0 ⋯
2 − λLi λFi βFi (1 − λRi ) iF ⋯
In Fig. 5(b), it is forward fault for LCC-HVDC side, based on Table 3:
Table 2 Two ends’ TW at different mutation points under external fault at rectifier side. No.
Relay R2
t + t2
E = − ∫t 1 1
Relay R2
[βRu βRi (1 + λLu )(1 − λLi ) uF (t ) iF (t )
+ λRu λFu λRi λFi βRu βRi (1 + λLu )(1 − λLi ) uF (t ) iF (t ) VTW
CTW
L1
βLu uF
βLi iF
L2
λLu λFu βLu uF
λLi λFi βLi iF
L3 ⋯
2 2 λLu λFu βLu uF ⋯
2 2 λLi λFi βLi iF ⋯
VTW
CTW
R1 R2
0
− βLi (1 − λRi) iF
0
− λLi λFi βLi (1 − λRi) iF
R3 ⋯
0 ⋯
2 2 − λLi λFi βLi (1 − λRi ) iF ⋯
2 2 2 2 + λRu λFu λRi λFi βRu βRi (1 + λLu )(1 − λLi ) uF (t ) iF (t )
+ ⋯] dt < 0
Based on Eqs. (13)–(15), fault direction discrimination criterion at LCC-HVDC side is:
{
⎧− 1 < λLu = λLi = (ZR − ZTL)/(ZR + ZTL) < 1 ⎪ λRu = λRi = (ZI − ZTL)/(ZI + ZTL) ≈ −1 ⎪ ⎪ βLu = 2ZR/(ZR + ZTL) > 0 ⎨ βLi = 2ZTL/(ZR + ZTL) > 0 ⎪ β = 2Z /(Z + Z ) ≈ 0 I I TL ⎪ Ru ⎪ βRi = 2ZTL/(ZI + ZTL) ≈ 2 ⎩
E>0 E<0
backward fault forward fault
Considering there are no VTW signals with internal fault and rectifier side’s external fault, define fault direction discrimination parameter at VSC-HVDC side:
(10)
Q= (11)
uR = (1 + λRu ) uF ≈ 0kV
∫t
t3 + t 4
i (t ) dt
where t3 and t4 are start time and time length of data window, respectively. In Fig. 4, it is forward fault for VSC-HVDC side, based on Table 1:
3.3. Fault direction discrimination principle at LCC-HVDC side
t + t4
Q = − ∫t 3
Define fault direction discrimination parameter at LCC-HVDC side: t 1+ t2
3
t + t4
≈ − ∫t 3 3
where t1 and t2 are start time and time length of data window, respectively. In Fig. 4, it is forward fault for LCC-HVDC side, based on Table 1: t + t2
Q=−
∫t
t3 + t 4
3
[(1 + λLu )(1 − λLi ) uF (t ) iF (t ) + λLu λFu λLi λFi (1 + λLu )(1
+
(13)
Q=
In Fig. 5(a), it is backward fault for LCC-HVDC side, based on Table 2:
[βLi (1 − λRi ) iF (t ) + λLi λFi βLi (1 − λRi ) iF (t )
2 λLi2 λFi βLi (1
∫t
t3 + t 4
3
− λRi ) iF (t )] dt
(19)
2 2 [βRi iF (t ) + λRi λFi βRi iF (t ) + λRi λFi βRi iF (t )] dt
(20)
Based on Eqs. (18)–(20):
[βLu βLi uF (t ) iF (t ) + λLu λFu λLi λFi βLu βLi uF (t ) iF (t ) 2 2 2 2 + λLu λFu λLi λFi βLu βLi uF (t ) iF (t ) + ⋯] dt > 0
(18)
In Fig. 5(b), it is backward fault for VSC-HVDC side, based on Table 3:
2 2 2 2 + λLu λFu λLi λFi (1 + λLu )(1 − λLi ) uF (t ) iF (t ) + ⋯] dt < 0
t + t2
(1 − λRi ) iF (t ) dt
In Fig. 5(a), it is forward fault for VSC-HVDC side, based on Table 2:
− λLi ) uF (t ) iF (t )
E = ∫t 1 1
− λRi ) iF (t )] dt
(12)
1
E = − ∫t 1 1
[(1 − λRi ) iF (t ) + λLi βFi (1 − λRi ) iF (t )
λLi2 λFi βFi (1
+
u (t ) i (t ) dt
(17)
3
Hence, this is the reason why there is no VTW at inverter station.
∫t
(16)
3.4. Fault direction discrimination principle at VSC-HVDC side
Based on Eq. (10), the VTW at inverter side is:
E=
(15)
− sgn(iF ) sgn(Q) = ⎧ + ⎨ ⎩ sgn(iF)
(14)
forward fault backward fault
(21)
Table 3 Two ends’ TW at different mutation points under external fault at inverter side. No.
Relay R1
No.
VTW
CTW
L1
βRu (1 + λLu) uF
− βRi (1 − λLi ) iF
L2
λRu λFu βRu (1 + λLu) uF
− λRi λFi βRi (1 − λLi ) iF
L3 ⋯
2 2 λRu λFu βRu (1 + λLu) uF ⋯
2 2 − λRi λFi βRi (1 − λLi ) iF ⋯
618
Relay R2 VTW
CTW
R1 R2
βRu uF
βRi iF
λRu λFu βRu uF
λRi λFi βRi iF
R3 ⋯
2 2 λRu λFu βRu uF ⋯
2 2 λRi λFi βRi iF ⋯
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• #5: Based on E and η, rectifier side can identify fault direction and
Considering uF and iF have same polarities, if PG fault occurs, the polarities of uF and iF of faulty line are both negative. If NG fault occurs, the polarities of uF and iF of faulty line are both positive. Hence, the polarity of faulty line is:
sgn(Q) =
⎧+ 1 ⎪− 1 ⎨− 1 ⎪+ 1 ⎩
> < < >
0 0 0 0
PG inernal fault NG inernal fault PG exernal fault NG exernal fault
• •
(22)
Based on Eq. (22), if faulty pole can be selected correctly, the fault direction can be identified.
4. Simulation analysis 4.1. Typical fault simulation
3.5. Faulty pole selection method
Based on Fig. 3 and Appendix B, a typical DC-AC hybrid simulation system is established in PSCAD/EMTDC. Besides, the HVDC-TL adopts frequency depended model as appendix 1 shows. The sampling rate is set as 1 MHz. Fig. 7 is the DC voltages and currents with typical PG internal fault (F1, 30 km from rectifier station). Due to the special structure and control strategy, the voltages at inverter side have no mutations. Based on high pass filter, after low frequency band components are filtered out, Fig. 8 provides VTWs and CTWs, and the propagation characteristics are exactly as Section 3.2 shows. The fault direction discrimination parameters and faulty pole selection parameters of rectifier side (EP , EN and η) and inverter side (QP , QN and η ) are provided in Table 4. Eventually, the fault direction discrimination results of two ends are both forward, and internal fault is identified correctly. Figs. 9 and 10 are the VTWs and CTWs with typical PG external faults at rectifier side (F4) and inverter side (F5), respectively. And the TW propagation characteristics are exactly as Section 3.2 shows. The fault direction discrimination parameters and faulty pole selection parameters of rectifier side (EP , EN and η) and inverter side (QP , QN and η) are provided in Table 4. Eventually, the fault direction discrimination result of the relay close to fault is backward, and external fault is identified correctly.
As known, the TW amplitude of faulty HVDC-TL is much larger than non-faulty HVDC-TL [22]. And if PN fault occurs, the TW amplitudes of positive and negative HVDC-TLs are approximately equal to each other. Hence, define faulty pole selection parameter:
η = lg
|IP, max | |IN , max |
(23)
where |IP, max | and |IN , max | are the maximum values of positive pole and negative pole CTWs in data window, respectively. Define faulty pole selection criterion:
⎧ η > 0.1 − 0.1 < η < 0.1 ⎨ ⎩ η < − 0.1
PG fault PN fault NG fault
faulty pole. Based on Q and η, inverter side can identify fault direction and faulty pole, too; #6: Two ends exchange fault direction discrimination results; #7 & #8: If the fault direction discrimination results of two ends are both forward faults, internal fault can be identified and relay actions to fault. Otherwise, external fault can be identified and relay returns.
(24)
3.6. Protection implementation method Fig. 6 is the schematic of protection implementation, and it can be separated to several steps:
• #1: Protection starts; • #2: High pass filter. It can filter out low frequency band signal and retain TW signal; #3: • Fault direction discrimination parameters of rectifier side (E) and inverter side (Q) are calculated, respectively; • #4: Faulty pole selection parameters of both ends are calculated;
4.2. Simulation results with different fault locations To verify the performance of proposed protection with different fault locations, a series simulation is carried out as Figs. 11 and 12
Fig. 6. Schematic of protection implementation. 619
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Fig. 7. DC voltages and currents with typical internal fault (F1).
Fig. 8. VTWs and CTWs with typical internal fault (F1).
Table 4 Simulation results of typical internal and external faults. No.
F1 F4 F5
Rectifier side
Inverter side
EP (J)
EN (J)
η
Results
QP (mC)
QN (mC)
η
Results
−511.159 2755.506 −82.398
−2.736 −3.483 −20.158
0.878 1.414 0.116
Forward Backward Forward
24.582 23.188 −12.799
−6.962 −4.636 3.500
0.166 0.057 0.738
Forward Forward Backward
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Fig. 9. VTWs and CTWs with typical external fault at rectifier side (F4).
4.3. Simulation results with different fault resistances
shows. Figs. 11 and 12 are the fault direction discrimination parameters and faulty pole selection parameters of two ends, respectively. It simulates different fault locations of HVDC-TL from rectifier side to inverter side (the fault resistance is 0Ω), and considers three different fault types (F1, F2 and F3). Eventually, based on Figs. 11 and 12, it is concluded that the proposed protection can identify fault direction directly with different fault locations.
To verify the performance of proposed protection with different fault resistances, a series simulation is carried out as Figs. 13 and 14 shows. Figs. 13 and 14 are the fault direction discrimination parameters and faulty pole selection parameters of two ends, respectively. It simulates different fault resistances from 0Ω to 300Ω (the fault locates at middle of the HVDC-TL), and considers three different fault types (F1, F2 and F3). Eventually, based on Figs. 13 and 14, it is concluded that the proposed protection can identify fault direction directly with
Fig. 10. VTWs and CTWs with typical external fault at inverter side (F5). 621
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Fig. 11. Fault direction discrimination parameters of two ends with different fault locations.
Fig. 12. Faulty pole selection parameters of two ends with different fault locations. 622
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Fig. 13. Fault direction discrimination parameters of two ends with different fault resistances.
Fig. 14. Faulty pole selection parameters of two ends with different fault resistances.
side and VSC-HVDC side, respectively. Meanwhile, faulty pole selection criterion is also proposed based on the ratio of positive pole current to negative pole current. Thirdly, based on PSCAD/EMTDC, a simulation model of ±400 kV LCC-VSC-HVDC transmission system is established. And the simulation results prove that the proposed protection method has several advantages:
different fault resistances. 5. Summary Because two converter stations are LCC-HVDC type and VSC-HVDC type respectively, the HVDC-TL has special TW propagation characteristic. Especially, due to the capacitance installed at VSC-HVDC station, the relay at VSC-HVDC side cannot detect VTW signal. Considering the TW protection principles applied in HVDC transmission system has no fault direction recognition ability, the paper proposes a novel TW directional pilot protection. Firstly, the paper analyzes the special propagation characteristics of TW along HVDC-TL. Secondly, the paper proposes novel fault direction discrimination criterions based on the integration of instantaneous power and current at LCC-HVDC
• Direction discrimination ability. No matter LCC-HVDC side or VSCHVDC side, the fault direction can be identified correctly; • Faulty pole selection ability. No matter PG fault, NG fault or PN fault, the faulty pole can be distinguished correctly; • High reliability. The proposed protection method can distinguish internal fault and external fault reliably and is immune to fault location, fault resistance and fault type;
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Fig. 15. Structure of HVDC-TL.
Fig. 16. TW impedance analysis of HVDC-TL and converter stations.
Fig. 17. Detailed structure of ± 400 kV LCC-VSC-HVDC transmission system.
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Fig. 18. Control strategy of LCC rectifier station.
Fig. 19. Control strategy of VSC inverter station.
• No time synchronization system.
prospect to improve protection performance in actual LCC-VSC-HVDC transmission system.
Hence, the proposed protection method has a good application
Appendix A. TW impedance analysis of HVDC-TL and converter stations Fig. 15 is the structure of HVDC-TL. To acquire frequency dependent characteristic of HVDC-TL, based on Carson formula, Dommel proposed depth of complex penetration [23]:
P=
ρ 2πfμ
(25)
where ρ is the resistivity of earth ( ρ = 100Ωm ); f is frequency; μ is the permeability of vacuum ( μ = 4π × Meanwhile, the equivalent radius of four bundled conductors is:
GMR =
4
10−7H/m ).
4rd3
(26)
where r is radius of single bundled wire (r = 0.0134 m ). d is the distance between bundled wires(d = 0.450 m ). Ignoring mutual impedance, the inductance per length of HVDC-TL is:
LTL =
μ 2(h + P ) ln 2π GMR
(27)
Ignoring mutual conductance, the capacitance per length of HVDC-TL is:
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CTL =
ε 2h ln 2π GMR
(28)
10−12
H/m ). where ε is vacuum dielectric constant (ε = 8.854 × Based on Eq. (7), the TW impedance of HVDC-TL is: ZTL =
LTL = CTL
μ ln[2(h + P )] − lnGMR ε ln(2h) − lnGMR
(29)
For LCC-HVDC converter station, considering the value of smoothing reactor is large enough, the paper choose smoothing reactor to be equivalent to rectifier station as following shows:
ZR ≈ ZSR = 2πfLSR i
(30)
where ZR and ZSR are impedance values of rectifier station and smoothing reactor, respectively; L is inductance of smoothing reactor (L = 0.2H). For VSC-HVDC converter station, considering the parallel connection of large capacitance and converter, the paper choose capacitance to be equivalent rectifier station as following shows:
ZI ≈ ZC = −
1 i 2πfC
(31)
where ZI and ZC are impedance values of inverter station and smoothing reactor, respectively; C is capacitance installed at VSC-HVDC inverter station (C = 0.001F ). Fig. 16 is the TW impedance analysis of HVDC-TL and converter stations. Apparently, in the range of 10 kHz to 1 MHz, ∣ZTL ∣, ∣ZR ∣ ≫ 0, ∣ZI ∣ ≈ 0 . Appendix B. Simulation model details Fig. 17 is detailed structure of ±400 kV LCC-VSC-HVDC transmission system including LCC-HVDC rectifier station, VSC-HVDC inverter station, 300 km length of over-head T-line, and other parameters. Fig. 18 is the control strategy of LCC rectifier station. It adopts constant current control strategy. By comparing measured DC current and setting DC current, based on the algorithm in Fig. 18, the control order (firing angle) is given to LCC converter in Fig. 17. Fig. 19 is the control strategy of VSC inverter station. It adopts constant DC voltage control and constant AC voltage control strategies. Fig. 19(a) is outer-loop control. By comparing measured DC voltage with setting DC voltage, the reference D-axis current is obtained. Similarly, the reference Qaxis current is obtained, too. Fig. 19(b) is inner-loop control. Based on reference D-axis and Q-axis currents obtained in outer-loop control, reference D-axis and Q-axis voltages are calculated. Afterwards, using sinusoidal pulse width modulation (SPWM) algorithm, control order is obtained and given to VSC converter in Fig. 17. Appendix C. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.ijepes.2018.12.028.
[11] Gao SP, Chu X, Shen QY, Jin XF, Yun YY, et al. A novel whole-line quick-action protection principle for HVDC transmission lines using one-end voltage. Int J Electr Power Energy Syst 2015;65:262–70. https://doi.org/10.1016/j.ijepes.2014.10.013. J.L. [12] Liu J, Fan CJ, Tai NL. A novel pilot directional protection scheme for HVDC transmission line based on specific frequency current. In: 2014 International conference on power system technology; 2014. p. 976–82. https://doi.org/10.1109/ POWERCON.2014.6993931. [13] Ha HX, Yu Y, Yi RP, Bo ZQ, Chen B. Novel scheme of travelling wave based differential protection for bipolar HVDC transmission lines. In: 2010 International conference on power system technology; 2010. p. 1–6. https://doi.org/10.1109/ POWERCON.2010.5666376. [14] Wang Y, Zhang BH. A novel hybrid directional comparison pilot protection scheme for the LCC-VSC hybrid hvdc transmission lines. In: 13th International conference on development in power system protection 2016 (DPSP); 2016. p. 1–6. https://doi. org/10.1049/cp.2016.0112. [15] Gao SP, Song GB, Ma ZB, Jin XF. Novel pilot protection principle for high-voltage direct current transmission lines based on fault component current characteristics. IET Gener, Transmiss Distrib 2015;9:468–74. https://doi.org/10.1049/iet-gtd. 2014.0313. 6. [16] Liu XL, Osman AH, Malik OP. Hybrid traveling wave/boundary protection for monopolar HVDC line. IEEE Trans Power Deliv 2009;24(2):569–78. https://doi. org/10.1109/TPWRD.2008.2002687. [17] Abu-Elanien AE, Abdel-Khalik AS, Massoud AM, Ahmed S. A non-communication based protection algorithm for multi-terminal HVDC grids. Electr Power Syst Res 2017;144:41–51. https://doi.org/10.1016/j.epsr.2016.11.010. [18] Zhang Y, Tai NL, Xu B. Travelling wave-based pilot direction comparison protection for HVDC line. Int Trans Electr Energy Syst 2012;23(8):1304–16. https://doi.org/ 10.1002/etep.1657. [19] Song GB, Chu X, Cai XL, Gao SP. A novel pilot protection principle for VSC-HVDC cable lines based on fault component current. Int J Electr Power Energy Syst 2013;53:426–33. https://doi.org/10.1016/j.ijepes.2013.05.001. [20] Parker M, Finney S, Holliday D. Dc protection of a multi-terminal HVDC network featuring offshore wind farms. Energy Proc 2017;142:2195–201. https://doi.org/ 10.1016/j.egypro.2017.12.588. proceedings of the 9th International Conference on
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Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Travelling wave directional pilot protection for hybrid HVDC transmission line
T
⁎
Dong Wang , Mengqian Hou, Mengyou Gao, Feng Qiao School of Automation and Electronic Engineering, Qingdao University of Science & Technology, No.99, Songling Road, Qingdao 266042, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Travelling wave Directional pilot protection Hybrid HVDC Transmission line PSCAD/EMTDC
As known, hybrid high-voltage direct current (HVDC) is wide applied in power system. Because two converter stations are line commutated converter based HVDC (LCC-HVDC) type and voltage source converter based HVDC (VSC-HVDC) type respectively, the HVDC transmission line (HVDC-TL) has special travelling wave (TW) propagation characteristic. Especially, due to the capacitance installed at VSC-HVDC station, the relay at VSC-HVDC side cannot detect voltage TW (VTW) signal. Meanwhile, TW protection is widely applied in HVDC transmission system, but it has no fault direction recognition ability. Hence, the paper proposes a novel TW directional pilot protection. Firstly, the paper analyzes the special propagation characteristics of TW along HVDC-TL. Secondly, the paper proposes novel fault direction discrimination criterions based on the integration of instantaneous power and current at LCC-HVDC side and VSC-HVDC side, respectively. Meanwhile, faulty pole selection criterion is also proposed based on the ratio of positive pole current to negative pole current. Thirdly, based on PSCAD/EMTDC, a simulation model of ± 400 kV LCC-VSC-HVDC transmission system is established. And the simulation results prove that the proposed protection method is immune to fault location, fault resistance and fault type.
1. Introduction HVDC transmission system has several advantages over alternating current (AC) transmission system like long transmission distance, high transmission efficiency and large transmission capacity [1–5]. Especially, LCC-HVDC converter station has larger transmission capacity and lower project construction costs than other converter stations. But, due to the control strategy of line commutated converter, LCC-HVDC inverter station is highly dependent on DC/AC voltage level and suffers from commutation failure issue for a long time [6–9]. Compared with LCC-HVDC converter station, VSC-HVDC converter station has no commutation failure but higher construction cost to obtain the same transmission capacity as LCC-HVDC converter station. In order to acquire the common advantages of LCC-HVDC and VSC-HVDC converter stations, LCC-VSC-HVDC hybrid transmission systems are applied in current power system for two main purpose: (1) In order to power weak AC system which has heavy loads, considering acquiring large transmission capacity, no commutation failure issue and lower construction cost, it is generally designed as LCC-VSC-HVDC hybrid transmission system. (2) In order to overcome commutation failure issue of inverter station of existing LCC-HVDC project, taking reconstruction cost into account, conventional LCC-HVDC transmission system could be ⁎
upgraded to LCC-VSC-HVDC hybrid system. As main protection principle of HVDC transmission line, TW protection is widely used including voltage change criterion, voltage change rate criterion and current change rate criterion. However, it has a number of disadvantages: (1) No direction discrimination ability. As a single-ended protection principle, external fault may lead to wrong operation; (2) Threshold issue. Considering only voltage and current amplitude signals are adopted in protection principle, the threshold value directly affects the reliability and sensibility of protection. (3) Fault resistance. The VTW and current TW (CTW) amplitudes will significantly decrease if fault resistance is very large. In order to overcome these disadvantages, a number of scholars have proposed novel protection principles. Through comparing the special frequency band voltage or current integration value with threshold, the protection principle proposed by Song et al. [10] and Gao et al. [11] can identify internal or external fault. It has fast operation speed, but sensibility issue may occur with large fault resistance. Liu et al. [12] proposes a novel special frequency band current differential pilot protection principle, but it requires strict time synchronization system. Ha et al. [13] proposes a TW based current differential pilot protection. Just same with [12], strict time synchronization system also affects its application in actual power system. Wang et al. [14] proposes a novel protection which is suitable for hybrid HVDC-TL, but it may
Corresponding author. E-mail address: [email protected] (D. Wang).
https://doi.org/10.1016/j.ijepes.2018.12.028 Received 28 September 2018; Received in revised form 27 November 2018; Accepted 16 December 2018 0142-0615/ © 2018 Elsevier Ltd. All rights reserved.
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Fig. 2. Reflection and refraction of TW.
Fig. 1. HVDC-TL’s equivalent model.
uF = iF
have operation speed issue. A faulty component of current characteristic based protection is proposed by Gao et al. [15], but it may have slow operation speed and threshold issue. Liu et al. [16] proposes a hybrid protection based on TW protection and boundary protection, but it may be too complex to be applied in actual power system. Other novel protection principles can be found in [17–21]. To solve these issues, the paper proposes a novel protection principle. In Section 2, the general propagation characteristic of TW is described. In Section 3, novel direction pilot protection principle is proposed including TW propagation characteristic along hybrid HVDC-TL, fault direction discrimination principle of two ends of HVDC-TL, faulty pole selection method and protection implementation method. In Section 4, a series of simulations are carried out including the influences of different fault locations and different fault resistances. In Section 5, a concise summary is given.
2.2. Reflection and refraction of TW Fig. 2 shows the reflection and refraction of TW. As can be seen, the incident wave (VTW u1 and CTW i1) propagates along line 1, then reflection and refraction occur at impedance discontinuity point (fault or busbar). And the wave impedance at line 1 and line 2 are Z1 and Z2 , respectively. The reflection VTW and CTW are u2 and i2 , respectively. The refraction VTW and CTW are u3 and i3 , respectively. The forward directions of VTW and CTW in this paper are from HVDC-TL to ground and its actual propagation direction (i1 and i3 from left to right, i2 from right to left), respectively. Hence:
u + u2 = u3 ⎧ 1 i − i2 = i3 ⎪ ⎪1 i1 = u1/ Z1 ⎨ ⎪i2 = u2/ Z2 ⎪i3 = u3/ Z3 ⎩
2.1. Relationship between initial VTW and CTW Fig. 1 is lossless HVDC-TL’s equivalent model [22], and fault happens at F when t = 0 . The initial VTW is uF . Considering C is distributed capacitance per unit length of HVDC-TL, it will be charged to:
dt → 0
dQ CuF dx = lim = CuF v dt → 0 dt dt
(1)
⎧ λ u = (Z2 − Z1)/(Z2 + Z1) ⎪ λi = (Z2 − Z1)/(Z2 + Z1) ⎨ βu = 2Z2/(Z2 + Z1) ⎪ β = 2Z /(Z + Z ) 1 2 1 ⎩ i
(2)
3. Protection method 3.1. Typical hybrid HVDC transmission system
(3)
where dΦ is magnetic flux; L is the inductance per unit length of HVDCTL. Based on Eq. (3), the electrical filed around this short HVDC-TL can be described as:
E = lim
dt → 0
dΦ LCuF v dx = lim = LCuF v 2 dt → 0 dt dt
Fig. 3 is typical ±400 kV LCC-VSC-HVDC transmission system, and it consists of three parts: LCC-HVDC rectifier station, HVDC-TL and VSCHVDC inverter station. At rectifier station, it adopts constant current control strategy. At inverter station, it adopts constant DC and AC voltages control strategy. The length of HVDC-TL is 300 km and the power transmitted on HVDC-TL is 800 MW. RP, RN, IP and IN are relays installed at different positions. Besides, the transmission system simulation model is established in PSCAD/EMTDC. And F1, F2, F3, F4 and F5 are positive pole to ground (PG) internal fault, negative pole to ground (NG) internal fault, positive pole to negative pole (PN) internal fault, PG external fault at rectifier side and PG external fault at inverter side, respectively.
(4)
where E is electrical field strength. If dx is infinite small, then:
E ≈ uF
(5)
Taking Eq. (5) into Eq. (4):
v=
1 LC
(9)
where λ u and λi are VTW and CTW reflection coefficients, respectively. βu and βi are VTW and CTW refraction coefficients, respectively.
where iF is the initial CTW; dt is propagation time of TW of dx length; v is the TW propagation speed along HVDC-TL. Considering the magnetic flux of inductance of HVDC-TL, it can be described as:
dΦ = LiF dx = LCuF v dx
(8)
Based on Eq. (8), reflection and refraction coefficients could be described as:
where dQ is amount of electric charge; dx is the length of HVDC-TL. If dx is infinite small, based on Eq. (1):
iF = lim
(7)
Apparently, initial VTW and CTW have same polarities, and the ratio of initial VTW to CTW is a constant value L/ C .
2. TW propagation
dQ = CuF dx
L C
3.2. TW propagation characteristics
(6)
Figs. 4 and 5 describe TW propagation characteristics under internal
Taking Eq. (6) into Eq. (2): 616
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Fig. 3. Typical ± 400 kV LCC-VSC-HVDC transmission system.
be seen in Fig. 3, there are large smoothing reactors installed in LCCHVDC rectifier station, and the paper choose smoothing reactor to be equivalent to rectifier station. Besides, there are large capacitances installed between HVDC-TL and ground at VSC-HVDC inverter station, and its impedance in TW frequency band (several kHz to hundreds of kHz) is approximately equal to 0Ω. In other words, in Fig. 3, IP and IN are approximately grounded directly (only for TW frequency bandwidth). Hence, in Figs. 4 and 5, R is grounded directly (only for TW frequency bandwidth). These assumptions are proved in Appendix A. Table 1 is two ends’ TW at different mutation points under internal fault. Tables 2 and 3 are two ends’ TW at different mutation points under external faults at rectifier and inverter sides, respectively. λLu and λLi are VTW and CTW reflection coefficients at rectifier side, respectively. λFu and λFi are VTW and CTW reflection coefficients at fault point, respectively. λRu and λRi are VTW and CTW reflection coefficients at inverter side, respectively. βLu and βLi are VTW and CTW refraction coefficients at rectifier side, respectively. βFu and βFi are VTW and CTW refraction coefficients at fault point, respectively. βRu and βRi are VTW and CTW refraction coefficients at inverter side, respectively. Generally, − 1 < λFu = λFi < 1, βFu > 0, βFi > 0 . And:
Fig. 4. TW propagation characteristics under internal fault.
fault and external fault, respectively. And Fig. 5(a) and (b) are external faults occur at rectifier and inverter sides, respectively. L and R represents LCC-HVDC and VSC-HVDC stations, respectively. L1, L 2, L3 and L 4 are different TW mutation points at LCC-HVDC station. R1, R2, R3 and R 4 are different TW mutation points at VSC-HVDC station. As can
Fig. 5. TW propagation characteristics under external faults. 617
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Table 1 Two ends’ TW at different mutation points under internal fault. No.
Relay R1
No.
VTW
CTW
L1 L2
(1 + λLu) uF λLu λFu (1 + λLu) uF
− (1 − λLi ) iF − λLi λFi (1 − λLi ) iF
L3 ⋯
2 2 λLu λFu (1 + λLu) uF ⋯
2 2 − λLi λFi (1 − λLi ) iF ⋯
Relay R1
No.
VTW
CTW
R1 R2
0 0
− (1 − λRi) iF − λLi βFi (1 − λRi) iF
R3 ⋯
0 ⋯
2 − λLi λFi βFi (1 − λRi ) iF ⋯
In Fig. 5(b), it is forward fault for LCC-HVDC side, based on Table 3:
Table 2 Two ends’ TW at different mutation points under external fault at rectifier side. No.
Relay R2
t + t2
E = − ∫t 1 1
Relay R2
[βRu βRi (1 + λLu )(1 − λLi ) uF (t ) iF (t )
+ λRu λFu λRi λFi βRu βRi (1 + λLu )(1 − λLi ) uF (t ) iF (t ) VTW
CTW
L1
βLu uF
βLi iF
L2
λLu λFu βLu uF
λLi λFi βLi iF
L3 ⋯
2 2 λLu λFu βLu uF ⋯
2 2 λLi λFi βLi iF ⋯
VTW
CTW
R1 R2
0
− βLi (1 − λRi) iF
0
− λLi λFi βLi (1 − λRi) iF
R3 ⋯
0 ⋯
2 2 − λLi λFi βLi (1 − λRi ) iF ⋯
2 2 2 2 + λRu λFu λRi λFi βRu βRi (1 + λLu )(1 − λLi ) uF (t ) iF (t )
+ ⋯] dt < 0
Based on Eqs. (13)–(15), fault direction discrimination criterion at LCC-HVDC side is:
{
⎧− 1 < λLu = λLi = (ZR − ZTL)/(ZR + ZTL) < 1 ⎪ λRu = λRi = (ZI − ZTL)/(ZI + ZTL) ≈ −1 ⎪ ⎪ βLu = 2ZR/(ZR + ZTL) > 0 ⎨ βLi = 2ZTL/(ZR + ZTL) > 0 ⎪ β = 2Z /(Z + Z ) ≈ 0 I I TL ⎪ Ru ⎪ βRi = 2ZTL/(ZI + ZTL) ≈ 2 ⎩
E>0 E<0
backward fault forward fault
Considering there are no VTW signals with internal fault and rectifier side’s external fault, define fault direction discrimination parameter at VSC-HVDC side:
(10)
Q= (11)
uR = (1 + λRu ) uF ≈ 0kV
∫t
t3 + t 4
i (t ) dt
where t3 and t4 are start time and time length of data window, respectively. In Fig. 4, it is forward fault for VSC-HVDC side, based on Table 1:
3.3. Fault direction discrimination principle at LCC-HVDC side
t + t4
Q = − ∫t 3
Define fault direction discrimination parameter at LCC-HVDC side: t 1+ t2
3
t + t4
≈ − ∫t 3 3
where t1 and t2 are start time and time length of data window, respectively. In Fig. 4, it is forward fault for LCC-HVDC side, based on Table 1: t + t2
Q=−
∫t
t3 + t 4
3
[(1 + λLu )(1 − λLi ) uF (t ) iF (t ) + λLu λFu λLi λFi (1 + λLu )(1
+
(13)
Q=
In Fig. 5(a), it is backward fault for LCC-HVDC side, based on Table 2:
[βLi (1 − λRi ) iF (t ) + λLi λFi βLi (1 − λRi ) iF (t )
2 λLi2 λFi βLi (1
∫t
t3 + t 4
3
− λRi ) iF (t )] dt
(19)
2 2 [βRi iF (t ) + λRi λFi βRi iF (t ) + λRi λFi βRi iF (t )] dt
(20)
Based on Eqs. (18)–(20):
[βLu βLi uF (t ) iF (t ) + λLu λFu λLi λFi βLu βLi uF (t ) iF (t ) 2 2 2 2 + λLu λFu λLi λFi βLu βLi uF (t ) iF (t ) + ⋯] dt > 0
(18)
In Fig. 5(b), it is backward fault for VSC-HVDC side, based on Table 3:
2 2 2 2 + λLu λFu λLi λFi (1 + λLu )(1 − λLi ) uF (t ) iF (t ) + ⋯] dt < 0
t + t2
(1 − λRi ) iF (t ) dt
In Fig. 5(a), it is forward fault for VSC-HVDC side, based on Table 2:
− λLi ) uF (t ) iF (t )
E = ∫t 1 1
− λRi ) iF (t )] dt
(12)
1
E = − ∫t 1 1
[(1 − λRi ) iF (t ) + λLi βFi (1 − λRi ) iF (t )
λLi2 λFi βFi (1
+
u (t ) i (t ) dt
(17)
3
Hence, this is the reason why there is no VTW at inverter station.
∫t
(16)
3.4. Fault direction discrimination principle at VSC-HVDC side
Based on Eq. (10), the VTW at inverter side is:
E=
(15)
− sgn(iF ) sgn(Q) = ⎧ + ⎨ ⎩ sgn(iF)
(14)
forward fault backward fault
(21)
Table 3 Two ends’ TW at different mutation points under external fault at inverter side. No.
Relay R1
No.
VTW
CTW
L1
βRu (1 + λLu) uF
− βRi (1 − λLi ) iF
L2
λRu λFu βRu (1 + λLu) uF
− λRi λFi βRi (1 − λLi ) iF
L3 ⋯
2 2 λRu λFu βRu (1 + λLu) uF ⋯
2 2 − λRi λFi βRi (1 − λLi ) iF ⋯
618
Relay R2 VTW
CTW
R1 R2
βRu uF
βRi iF
λRu λFu βRu uF
λRi λFi βRi iF
R3 ⋯
2 2 λRu λFu βRu uF ⋯
2 2 λRi λFi βRi iF ⋯
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• #5: Based on E and η, rectifier side can identify fault direction and
Considering uF and iF have same polarities, if PG fault occurs, the polarities of uF and iF of faulty line are both negative. If NG fault occurs, the polarities of uF and iF of faulty line are both positive. Hence, the polarity of faulty line is:
sgn(Q) =
⎧+ 1 ⎪− 1 ⎨− 1 ⎪+ 1 ⎩
> < < >
0 0 0 0
PG inernal fault NG inernal fault PG exernal fault NG exernal fault
• •
(22)
Based on Eq. (22), if faulty pole can be selected correctly, the fault direction can be identified.
4. Simulation analysis 4.1. Typical fault simulation
3.5. Faulty pole selection method
Based on Fig. 3 and Appendix B, a typical DC-AC hybrid simulation system is established in PSCAD/EMTDC. Besides, the HVDC-TL adopts frequency depended model as appendix 1 shows. The sampling rate is set as 1 MHz. Fig. 7 is the DC voltages and currents with typical PG internal fault (F1, 30 km from rectifier station). Due to the special structure and control strategy, the voltages at inverter side have no mutations. Based on high pass filter, after low frequency band components are filtered out, Fig. 8 provides VTWs and CTWs, and the propagation characteristics are exactly as Section 3.2 shows. The fault direction discrimination parameters and faulty pole selection parameters of rectifier side (EP , EN and η) and inverter side (QP , QN and η ) are provided in Table 4. Eventually, the fault direction discrimination results of two ends are both forward, and internal fault is identified correctly. Figs. 9 and 10 are the VTWs and CTWs with typical PG external faults at rectifier side (F4) and inverter side (F5), respectively. And the TW propagation characteristics are exactly as Section 3.2 shows. The fault direction discrimination parameters and faulty pole selection parameters of rectifier side (EP , EN and η) and inverter side (QP , QN and η) are provided in Table 4. Eventually, the fault direction discrimination result of the relay close to fault is backward, and external fault is identified correctly.
As known, the TW amplitude of faulty HVDC-TL is much larger than non-faulty HVDC-TL [22]. And if PN fault occurs, the TW amplitudes of positive and negative HVDC-TLs are approximately equal to each other. Hence, define faulty pole selection parameter:
η = lg
|IP, max | |IN , max |
(23)
where |IP, max | and |IN , max | are the maximum values of positive pole and negative pole CTWs in data window, respectively. Define faulty pole selection criterion:
⎧ η > 0.1 − 0.1 < η < 0.1 ⎨ ⎩ η < − 0.1
PG fault PN fault NG fault
faulty pole. Based on Q and η, inverter side can identify fault direction and faulty pole, too; #6: Two ends exchange fault direction discrimination results; #7 & #8: If the fault direction discrimination results of two ends are both forward faults, internal fault can be identified and relay actions to fault. Otherwise, external fault can be identified and relay returns.
(24)
3.6. Protection implementation method Fig. 6 is the schematic of protection implementation, and it can be separated to several steps:
• #1: Protection starts; • #2: High pass filter. It can filter out low frequency band signal and retain TW signal; #3: • Fault direction discrimination parameters of rectifier side (E) and inverter side (Q) are calculated, respectively; • #4: Faulty pole selection parameters of both ends are calculated;
4.2. Simulation results with different fault locations To verify the performance of proposed protection with different fault locations, a series simulation is carried out as Figs. 11 and 12
Fig. 6. Schematic of protection implementation. 619
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Fig. 7. DC voltages and currents with typical internal fault (F1).
Fig. 8. VTWs and CTWs with typical internal fault (F1).
Table 4 Simulation results of typical internal and external faults. No.
F1 F4 F5
Rectifier side
Inverter side
EP (J)
EN (J)
η
Results
QP (mC)
QN (mC)
η
Results
−511.159 2755.506 −82.398
−2.736 −3.483 −20.158
0.878 1.414 0.116
Forward Backward Forward
24.582 23.188 −12.799
−6.962 −4.636 3.500
0.166 0.057 0.738
Forward Forward Backward
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Fig. 9. VTWs and CTWs with typical external fault at rectifier side (F4).
4.3. Simulation results with different fault resistances
shows. Figs. 11 and 12 are the fault direction discrimination parameters and faulty pole selection parameters of two ends, respectively. It simulates different fault locations of HVDC-TL from rectifier side to inverter side (the fault resistance is 0Ω), and considers three different fault types (F1, F2 and F3). Eventually, based on Figs. 11 and 12, it is concluded that the proposed protection can identify fault direction directly with different fault locations.
To verify the performance of proposed protection with different fault resistances, a series simulation is carried out as Figs. 13 and 14 shows. Figs. 13 and 14 are the fault direction discrimination parameters and faulty pole selection parameters of two ends, respectively. It simulates different fault resistances from 0Ω to 300Ω (the fault locates at middle of the HVDC-TL), and considers three different fault types (F1, F2 and F3). Eventually, based on Figs. 13 and 14, it is concluded that the proposed protection can identify fault direction directly with
Fig. 10. VTWs and CTWs with typical external fault at inverter side (F5). 621
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Fig. 11. Fault direction discrimination parameters of two ends with different fault locations.
Fig. 12. Faulty pole selection parameters of two ends with different fault locations. 622
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Fig. 13. Fault direction discrimination parameters of two ends with different fault resistances.
Fig. 14. Faulty pole selection parameters of two ends with different fault resistances.
side and VSC-HVDC side, respectively. Meanwhile, faulty pole selection criterion is also proposed based on the ratio of positive pole current to negative pole current. Thirdly, based on PSCAD/EMTDC, a simulation model of ±400 kV LCC-VSC-HVDC transmission system is established. And the simulation results prove that the proposed protection method has several advantages:
different fault resistances. 5. Summary Because two converter stations are LCC-HVDC type and VSC-HVDC type respectively, the HVDC-TL has special TW propagation characteristic. Especially, due to the capacitance installed at VSC-HVDC station, the relay at VSC-HVDC side cannot detect VTW signal. Considering the TW protection principles applied in HVDC transmission system has no fault direction recognition ability, the paper proposes a novel TW directional pilot protection. Firstly, the paper analyzes the special propagation characteristics of TW along HVDC-TL. Secondly, the paper proposes novel fault direction discrimination criterions based on the integration of instantaneous power and current at LCC-HVDC
• Direction discrimination ability. No matter LCC-HVDC side or VSCHVDC side, the fault direction can be identified correctly; • Faulty pole selection ability. No matter PG fault, NG fault or PN fault, the faulty pole can be distinguished correctly; • High reliability. The proposed protection method can distinguish internal fault and external fault reliably and is immune to fault location, fault resistance and fault type;
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Fig. 15. Structure of HVDC-TL.
Fig. 16. TW impedance analysis of HVDC-TL and converter stations.
Fig. 17. Detailed structure of ± 400 kV LCC-VSC-HVDC transmission system.
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Fig. 18. Control strategy of LCC rectifier station.
Fig. 19. Control strategy of VSC inverter station.
• No time synchronization system.
prospect to improve protection performance in actual LCC-VSC-HVDC transmission system.
Hence, the proposed protection method has a good application
Appendix A. TW impedance analysis of HVDC-TL and converter stations Fig. 15 is the structure of HVDC-TL. To acquire frequency dependent characteristic of HVDC-TL, based on Carson formula, Dommel proposed depth of complex penetration [23]:
P=
ρ 2πfμ
(25)
where ρ is the resistivity of earth ( ρ = 100Ωm ); f is frequency; μ is the permeability of vacuum ( μ = 4π × Meanwhile, the equivalent radius of four bundled conductors is:
GMR =
4
10−7H/m ).
4rd3
(26)
where r is radius of single bundled wire (r = 0.0134 m ). d is the distance between bundled wires(d = 0.450 m ). Ignoring mutual impedance, the inductance per length of HVDC-TL is:
LTL =
μ 2(h + P ) ln 2π GMR
(27)
Ignoring mutual conductance, the capacitance per length of HVDC-TL is:
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CTL =
ε 2h ln 2π GMR
(28)
10−12
H/m ). where ε is vacuum dielectric constant (ε = 8.854 × Based on Eq. (7), the TW impedance of HVDC-TL is: ZTL =
LTL = CTL
μ ln[2(h + P )] − lnGMR ε ln(2h) − lnGMR
(29)
For LCC-HVDC converter station, considering the value of smoothing reactor is large enough, the paper choose smoothing reactor to be equivalent to rectifier station as following shows:
ZR ≈ ZSR = 2πfLSR i
(30)
where ZR and ZSR are impedance values of rectifier station and smoothing reactor, respectively; L is inductance of smoothing reactor (L = 0.2H). For VSC-HVDC converter station, considering the parallel connection of large capacitance and converter, the paper choose capacitance to be equivalent rectifier station as following shows:
ZI ≈ ZC = −
1 i 2πfC
(31)
where ZI and ZC are impedance values of inverter station and smoothing reactor, respectively; C is capacitance installed at VSC-HVDC inverter station (C = 0.001F ). Fig. 16 is the TW impedance analysis of HVDC-TL and converter stations. Apparently, in the range of 10 kHz to 1 MHz, ∣ZTL ∣, ∣ZR ∣ ≫ 0, ∣ZI ∣ ≈ 0 . Appendix B. Simulation model details Fig. 17 is detailed structure of ±400 kV LCC-VSC-HVDC transmission system including LCC-HVDC rectifier station, VSC-HVDC inverter station, 300 km length of over-head T-line, and other parameters. Fig. 18 is the control strategy of LCC rectifier station. It adopts constant current control strategy. By comparing measured DC current and setting DC current, based on the algorithm in Fig. 18, the control order (firing angle) is given to LCC converter in Fig. 17. Fig. 19 is the control strategy of VSC inverter station. It adopts constant DC voltage control and constant AC voltage control strategies. Fig. 19(a) is outer-loop control. By comparing measured DC voltage with setting DC voltage, the reference D-axis current is obtained. Similarly, the reference Qaxis current is obtained, too. Fig. 19(b) is inner-loop control. Based on reference D-axis and Q-axis currents obtained in outer-loop control, reference D-axis and Q-axis voltages are calculated. Afterwards, using sinusoidal pulse width modulation (SPWM) algorithm, control order is obtained and given to VSC converter in Fig. 17. Appendix C. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.ijepes.2018.12.028.
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