Travelling wave directional pilot protection for hybrid LCC-MMC-HVDC transmission line

Electrical Power and Energy Systems 115 (2020) 105431

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Travelling wave directional pilot protection for hybrid LCC-MMC-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 266061, PR China

ARTICLE INFO

ABSTRACT

Keywords: Travelling wave Directional pilot protection Hybrid LCC-MMC-HVDC Transmission line PSCAD/EMTDC

Currently, line commutated converter (LCC) and modular multilevel converter (MMC) based hybrid high voltage direct current (HVDC) is applied in actual power transmission system. Generally, LCC-MMC-HVDC transmission line adopts travelling wave (TW) protection as primary protection which includes voltage change criterion, voltage change rate criterion and current change rate criterion. In fact, it has threshold issue and no fault direction discrimination ability which could lead to wrong operation to external fault. Besides, it has not considered different structures of LCC rectifier station and MMC inverter station. In order to overcome these disadvantages, the paper proposes a novel TW directional pilot protection for hybrid LCC-MMC-HVDC transmission line. Firstly, special TW propagation process characteristic in hybrid LCC-MMC-HVDC transmission system is analyzed. Secondly, a novel fault direction discrimination algorithm and complete implementation scheme are proposed. Thirdly, using PSCAD/EMTDC, a ± 400 kV hybrid LCC-MMC-HVDC transmission system simulation model is established. Eventually, a series of simulations prove that the proposed TW protection actions correctly with different fault locations, different fault resistances and different fault types.

1. Introduction LCC-HVDC transmission technology has several advantages: large transmission capacity, high transmission efficiency and long transmission distance. However, its inverter station suffers from commutation failure issue for long time [1–4]. Because of the characteristic of IGBT and high frequency PWM strategy, MMC-HVDC transmission technology has no commutation failure issue and could supply power to passive system [5,6]. But the construction cost of MMC-HVDC station is much larger than LCC-HVDC converter station. Hence, to obtain advantages of LCC-HVDC and MMC-HVDC transmission technologies, hybrid LCC-MMC-HVDC transmission system is applied in actual power grid. Generally, rectifier station and inverter station adopt LCC-HVDC converter and MMC-HVDC converter, respectively. Currently, hybrid LCC-MMC-HVDC transmission line adopts TW protection as primary protection, and it includes voltage change criterion, voltage change rate criterion and current change rate criterion [7,8]. Essentially, it is transient signal based protection method rather than TW based protection method. And there are several shortcomings: (1) no direction discrimination ability. As a kind of single-ended signal based protection principle, it cannot identify internal fault or external fault correctly; (2) fault resistance issue. The amplitudes of VTW and CTW decrease significantly with large fault resistance; (3) threshold ⁎

issue. The value of threshold is directly related to protection’s reliability and sensibility. In order to overcome these shortcomings, a number of scholars have proposed novel protection principles. Wang et al. [9] proposes a travelling wave directional pilot protection for hybrid HVDC transmission line. It has high reliability but is not suitable for hybrid LCC-MMCHVDC transmission line. Wang et al. [10] proposes a novel hybrid directional comparison pilot protection scheme for the hybrid LCC-VSC transmission lines. But the adaptability of proposed method at hybrid LCC-MMC-HVDC transmission line requires further research. Tang et al. [11] proposes a LCC and MMC hybrid HVDC topology with DC line fault clearance capability. Zheng et al. [12] proposes a transient energy based pilot protection principle which is more reliable than conventional protection principle. Gijare et al. [13] pays attention to permanent faults on DC overhead lines in hybrid converter based HVDC system. Wang et al. [14] analyzes transmission line boundary characteristics of the hybrid HVDC system. However, boundary protection principle requires more study. Deng et al. [15] proposes a single-ended travelling wave protection algorithm based on full waveform in the time and frequency domains. Zou et al. [16] proposed a novel transientenergy-based directional pilot protection method for HVDC line. But, their adaptability at multi-terminal transmission system requires further research.

Corresponding author. E-mail address: [email protected] (D. Wang).

https://doi.org/10.1016/j.ijepes.2019.105431 Received 9 January 2019; Received in revised form 28 June 2019; Accepted 22 July 2019 Available online 03 August 2019 0142-0615/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature LCC HVDC IGBT VTW PG

PN MMC TW PWM CTW NG M-CTW

line commutated converter high voltage direct current insulated gate bipolar transistor voltage travelling wave positive pole to ground

To overcome these shortcomings, the paper proposes a novel travelling wave directional pilot protection principle for hybrid LCC-MMCHVDC transmission line. In Chapter 2, the TW propagation characteristic in LCC-MMC-HVDC system is analyzed. In Chapter 3, a novel directional pilot protection principle is proposed including fault direction discrimination algorithm and complete protection implementation scheme. In Chapter 4, a number of simulations prove that the novel protection principle actions correctly with different fault types, different fault resistances and different fault locations. In Chapter 5, a concise summary is described.

2.2. Reflection and refraction of TW Fig. 2 provides the TW propagation process in hybrid LCC-MMCHVDC transmission system. As can be seen, the incident TWs (VTW is u1 and CTW is i1) propagate along line 1, and refraction and reflection occur at impedance discontinuity point (generally busbar or fault). Besides, the TW 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 definition of forward direction of VTW is from transmission line to earth. The definition of forward direction of CTW is its actual propagation direction (i1 and i3 from left to right, i2 from right to left). Hence, reflection and refraction coefficients are shown as:

2. TW Propagation characteristic 2.1. Relationship between initial VTW and CTW

= (Z2 Z1)/(Z2 + Z1) = (Z2 Z1)/(Z2 + Z1) u = 2Z2/(Z2 + Z1) i = 2Z1/(Z2 + Z1) u i

Fig. 1 is lossless transmission line’s equivalent model [17], and it is assumed that fault happens at F when t = 0 . Assume the initial VTW is uF . Considering C is distributed capacitance per unit length of transmission line, it will be charged to:

dQ = CuF dx

where u and

(1)

dt

0

dQ CuF dx = lim = CuF v dt 0 dt dt

= LiF dx = LCuF v dx

(2)

(3)

where d is magnetic flux; L is the inductance per unit length of transmission line. According to Eq. (3), the electrical filed around this short transmission line can be described as:

E = lim dt

0

d LCuF v dx = lim = LCuF v 2 dt 0 dt dt

and i are VTW and CTW reflection coefficients, respectively. are VTW and CTW refraction coefficients, respectively.

(a) Refraction and reflection at rectifier station At rectifier side, there is large smoothing reactor installed on transmission line. In fact, smoothing reactor is a TW impedance discontinuous point, and the TW impedance of smoothing reactor is

(4)

where E is electrical field strength. Assume dx is small enough, then:

E

i

Fig. 3 is a typical ±400 kV hybrid LCC-MMC-HVDC transmission system, and it consists of three parts: LCC-HVDC rectifier station, HVDC transmission line and MMC-HVDC inverter station. At rectifier station, it adopts constant DC current control strategy as Appendix A shows. At inverter station, it adopts constant DC voltage control and constant reactive power control strategies as Appendix B shows. The length of transmission line is 300 km and the power transmitted on transmission line is 800 MW. RP, RN, IP and IN are relays installed at different positions. And F1, F2, F3, F4 and F5 are PG internal fault, NG internal fault, PN internal fault, PG external fault at rectifier side and PG external fault at inverter side, respectively. Every upper bridge arm and lower bridge arms of MMC contain N (N = 10) submodules (SMs), and the structure of SM is also provide. The following will analyze TW refraction and reflection at two ends of transmission line.

where iF is the initial CTW; dt is propagation time of TW of dx length; v is the TW propagation speed along transmission line. Meanwhile, the magnetic flux of inductance of transmission line can be described as:

d

u

(5)

uF

Taking Eq. (5) into Eq. (4):

v=

1 LC

(6)

Taking Eq. (6) into Eq. (2):

uF = iF

L C

(8)

2.3. Typical hybrid LCC-MMC-HVDC transmission system

where dQ is the amount of electric charge; dx is the length of transmission line. Assume dx is small enough, according to Eq. (1):

iF = lim

positive pole to negative pole modular multilevel converter travelling wave pulse width modulation current travelling wave negative pole to ground modulation current travelling wave

(7)

Apparently, initial VTW and CTW have same polarities, and the ratio of initial VTW to CTW is L /C (constant value).

Fig. 1. Transmission line’s equivalent model. 2

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refraction occur at inverter station. The reflection TW returns to fault location, and the refraction TW passes through C and L directly into ground. Hence, the TW impedance at inverter station is:

Zinv =

much larger than transmission line. Hence, based on Eq. (8):

= (Zrec Zline)/(Zrec + Zline ) = (Zrec Zline)/(Zrec + Zline ) i, rec = 2Zrec /(Zrec + Zline ) u, rec u, rec

= 2Zline/(Zrec + Zline )

k1 C

j

k2 + 2j L = 4 fL C

k1 + k2 2 fC

(10)

where Zinv is TW impedance at inverter station; and f are angular frequency and time frequency, respectively; k1 and k2 are the numbers of inserted submodule capacitances at upper bridge arm and lower bridge arm at the TW arrival time of inverter station (1 k1, k2 N ), respectively. Hence, based on Eq. (8), Eq. (10) and Fig. 4, the reflection and refraction coefficients at relay can be described as:

Fig. 2. TW reflection and refranction.

i, rec

j

= (Zinv Zline)/(Zinv + Zline) = (Zinv Zline)/(Zinv + Zline) u, inv = 2Zinv /(Zinv + Zline ) u, inv i, inv

(9)

i, inv

where u, rec and i, rec are VTW and CTW reflection coefficients at rectifier station, respectively; u, rec and i, rec are VTW and CTW refraction coefficients at rectifier station, respectively. Zrec and Zline are TW impedances of smoothing reactor and transmission line, 1 < u, rec = i, rec < 1, u, rec respectively. Apparently, >0, i, rec > 0 . (b) Refraction and reflection at inverter station

= 2Zline/(Zinv + Zline )

(11)

where u, inv and i, inv are VTW and CTW reflection coefficients at inverter station, respectively; u, inv and i, inv are VTW and CTW refraction coefficients at inverter station, respectively. Zinv and Zline are TW impedances of inverter station and transmission line, respectively. Apparently, 1 < u, inv = i, inv < 1, u, inv > 0, i, inv > 0 . 3. Protection principle

Ignoring AC power system impedance, Fig. 4 provides typical grounding fault on hybrid LCC-MMC-HVDC transmission line. After fault occurs, the TW generated by grounding fault propagates along transmission line. At inverter station, there exists TW impedance discontinuity point at TW arrival moment because of submodule capacitance C and bridge arm reactance L in Fig. 3, and reflection and

3.1. TW propagation characteristic in hybrid LCC-MMC-HVDC transmission system Fig. 5(a) and (b) describe TW propagation process with internal fault and external fault, respectively. L and R represents rectifier station

Fig. 3. Typical LCC-MMC-HVDC transmission system. 3

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Fig. 4. Equivalent structure of inverter station under typical grounding fault.

and inverter station, respectively. L1, L 2, L 3 and L 4 are different TW mutation points at rectifier station. R1, R2 and R3 are different TW mutation points at inverter station. Table 1 and 2 provide two ends’ TW mutations with internal fault and external fault, respectively. Lu and Li are VTW and CTW reflection coefficients at rectifier side, respectively. Fu and Fi are VTW and CTW reflection coefficients at fault location, respectively. Ru and Ri are VTW and CTW reflection coefficients at inverter side, respectively. Fu and Fi are VTW and CTW refraction coefficients at fault location, respectively. Lu and Li are VTW and CTW refraction coefficients at rectifier side, respectively. uF and iF are initial VTW and CTW, respectively.

where k (t ) is CTW modulation coefficient; i (t ) is CTW value. Define fault direction discrimination parameter:

E=

(14)

(i) Rectifier side under internal fault. In Fig. 5(a), it is a forward fault 1 < Lu = Li < 1 and for rectifier side. Considering 1 < Fu = Fi < 1, based on Table 1:

Define CTW modulation coefficient: (12)

E=

(1 +

×

ts + td ts

=

where t is sampling point; u (t ) and i (t ) are VTW and CTW values, respectively. Then, define M-CTW:

i k (t ) = k ( t ) i ( t )

u (t ) ik (t ) dt

where t is sampling point; ts and td are start time and time length of data window, respectively; u (t ) and ik (t ) are VTW and M-CTW values, respectively. The following will analyze fault direction discrimination parameters of two ends under different fault types:

3.2. Fault direction discrimination algorithm

k (t ) = |u (t )|/|i (t )|

ts + td ts

(1 +

Lu )(1

|1 + Lu | Li )[ |1 Li |

+

| Lu Fu (1 + Lu )| | Li Fi (1 Li )|

+

2 | Lu 2 | Li

2 Fu (1 + Lu )| 2 Li )| Fi (1

+

]

uF (t ) iF (t ) dt Lu )

2 (1

+1+1+

)×

ts + td ts

uF (t ) iF (t ) dt < 0

(15) (ii) Inverter side under internal fault. In Fig. 5(a), it is a forward fault 1 < Lu = for inverter side. Considering

(13)

Fig. 5. TW propagation process. 4

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Table 1 Two ends’ TW mutations with internal fault. No.

Rectifier side

No.

VTW

L1 L2

(1 + Lu Fu (1 2 2 Lu Fu (1

L3

Li Fi

CTW

Lu ) uF

(1

+

Lu ) uF

+

Lu) uF

(1 +

×

ts + td ts

=

(1 +

|1 + Ru | Ri )[ |1 Ri |

Ru )(1

VTW

1<

Ru

=

Ri

< 1,

Fu

Li ) iF

R1 R2

Lu Fu (1

Li ) iF

R3

2 Lu Fu Fu (1

Li ) iF

Li Fi (1 2 2 Li Fi (1

< 1, 1 < Fu = Fi < 1, > 0 , based on Table 1:

E=

and

>0

2 (1

+

Fu / Fi

+

|u (t + 1)

+

+

2 | Lu Fu Fu (1 + Ru)| 2 | Li Fi Fi (1 Ri )|

ts + td ts

uF (t ) iF (t ) dt < 0

| Lu Fu (1 + Ru)| | Li Fi (1 Ri )|

=

2 Lu (1

Fu / Fi

+

)×

| 2 2 Lu | | Lu Fu Lu | + | Lu2 Fu + 2 | Li Fi Li | Li Fi Li | ts + td + )× t uF (t ) iF (t ) dt s

+

+1+1

+

]

)×

ts + td ts

M1 = 1 1 M0

uF (t ) iF (t ) dt

+

2 2 | Lu Fu Lu (1 + Ru)| 2 2 | Li Ri )| Fi Li (1

2 Lu (1

=

+

Ru

| Lu (1 + Ru)| Ri )[ | (1 Ri )| Li

Ru )(1

)2 (1

+

]×

ts + td ts

+1+1+

+

)×

ts

E<0 E>0

(20)

1) + 0.9524x (n)

0.9524x (n

(21)

1)

1

1

P+ P

(22)

4.1. Typical fault simulation

uF (t ) iF (t ) dt < 0

Based on Fig. 3, a typical hybrid LCC-MMC-HVDC simulation system is established in PSCAD/EMTDC [9]. And:

(18)

Based on (i), (ii), (iii) and (iv), fault direction discrimination algorithm can be described as:

{

Ri ) iF

4. Simulation analysis

| Lu Fu Lu (1 + Ru)| | Li Fi Li (1 Ri )|

uF (t ) iF (t ) dt ts + td

Ri ) iF

where M1 and M0 are positive sequence signal and zero sequence signal, respectively; P+ and P are positive pole signal and negative pole signal, respectively; (d) Based on positive sequence VTW and CTW, fault direction discrimination parameters of two ends (ER for rectifier side, EI for inverter side) are calculated; (e) Two ends of transmission line exchange fault direction discrimination parameters via optical fiber; (f) Check if ER < 0 & EI < 0 is met; (g) If condition in (f) could be met, internal fault is determined immediately. Otherwise, external fault is determined immediately.

>0

Ri < 1, Lu > 0 based on Table 2:

+

2 Li Fi Fi (1

Ru ) uF

where y (n) and y (n 1) are outputs at sampling point n and n 1, respectively; x (n) and x (n 1) are inputs at sampling point n and n 1, respectively. (c) Phase-mode transformation. In this paper, Karrenbauer transformation is adopted as following shows:

(iv) Inverter side under external fault. In Fig. 5(b), it is a forward fault for inverter side. Considering 1 < Lu = Li < 1, 1 < Fu = Fi < 1, 1 < Ru = and Li > 0 ,

Lu Li (1

+

Ri ) iF

Li Fi (1

u (t )| >

y (n) = 0.9048y (n

(17)

E=

(1

Ru ) uF

where t is sampling point; n is sampling start point; is data window ( = 10 in this paper); u is DC voltage; is threshold ( = 10 kV in this paper). (b) High pass filter can filter out low frequency signal and obtain high frequency TW signal by differential equation:

(iii) Rectifier side under external fault. In Fig. 5(b), it is a backward 1 < Lu = fault for rectifier side. Considering 1 < Fu = Fi < 1, Lu > 0 and Li > 0 , based on Table 2: Li < 1, | Lu | Lu Li ( | | Li

Ru ) uF

+

t=n

(16)

E=

(1 +

CTW

n+

uF (t ) iF (t ) dt Ru )

Inverter side

forward fault backward fault

(i) For LCC-HVDC rectifier side, it adopts 12-pulse structure. The value of smoothing reactor is 0.2H. The air core reactance of transformer is 0.2 p.u.. The value of reactor of MMC inverter station’s grounding pole is 0.5H. (ii) For MMC-HVDC inverter side, to accelerate simulation speed, the number of half bridge’s SMs is set to N = 10 . The submodule capacitance C and bridge arm reactance L are 1000 µF and 0.07H, respectively. The air core reactance of transformer is 0.2 p.u.. (iii) The detailed structure and parameters of transmission line is provided in Appendix C.

(19)

3.3. Protection implementation scheme Fig. 6 is the schematic of protection implementation, and it can be separated to several steps: (a) Protection starts. The protection start time identification algorithm is: Table 2 Two ends’ TW mutations with external fault. No.

Rectifier side VTW

No. CTW

Inverter side VTW

L1

Lu uF

Li iF

R1

Lu (1

L2

Lu Fu Lu uF 2 2 Lu Fu Lu uF

Li Fi Li iF 2 2 Li Fi Li iF

R2

Lu Fu Lu (1 2 2 Lu Fu Lu (1

L3

R3

5

+

CTW

Ru) uF

+

Ru ) uF

+

Ru) uF

Li (1 Li Fi Li (1 2 2 Li Fi Li (1

Ri ) iF Ri ) iF

Ri ) iF

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Fig. 6. Schematic of protection implementation.

Fig. 7. DC voltages and DC currents of two ends with typical internal fault (F1).

Fig. 8. VTW and M-CTW of two ends with typical internal fault (F1).

6

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Fig. 9. VTW and M-CTW of two ends with typical external fault at rectifier side (F4).

Fig. 10. VTW and M-CTW of two ends with typical external fault at inverter side (F5).

Fig. 11. The performance of proposed protection with different fault locations.

(iv) The sampling rate is set as 1 MHz to obtain whole bandwidth of TW. (v) The data window td in Eq. (14) is set to 1 ms.

resistance). uRP , uRN , uIP and uIN are DC voltages of rectifier side’s positive pole, rectifier side’s negative pole, inverter side’s positive pole and inverter side’s negative pole, respectively. iRP , iRN , iIP and iIN are DC currents of rectifier side’s positive pole, rectifier side’s negative pole, inverter side’s positive pole and inverter side’s negative pole, respectively. Based on Fig. 7, Fig. 8 provides VTW and M-CTW of two ends

Fig. 7 provides DC voltages and DC currents of two ends with typical internal fault (F1, 30 km away from rectifier station, 0 fault 7

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Fig. 12. The performance of proposed protection with different fault resistances.

Fig. 13. The performance of proposed protection with different transmission line lengths.

4.2. Different fault locations To verify the performance of proposed protection with different fault locations, a series of simulations are carried out as Fig. 11 shows. The fault resistance is 0 . In Fig. 11, ER and EI are fault direction discrimination parameters of rectifier side and inverter side, respectively. Eventually, it could be concluded that the proposed protection principle could identify fault direction correctly with different fault locations. 4.3. Different fault resistances To verify the performance of proposed protection with different fault resistances, a series of simulations are carried out as Fig. 12 shows. The faults are set in the middle of transmission line in simulation model. In Fig. 12, ER and EI are fault direction discrimination parameters of rectifier side and inverter side, respectively. Eventually, it could be concluded that the proposed protection principle could identify fault direction correctly with different fault resistances.

Fig. 14. Operation delay analysis.

with typical internal fault (F1, 30 km away from rectifier station). uR1 and iR1k are positive sequence VTW and M-CTW of rectifier side, respectively. uI1 and iI 1k are positive sequence VTW and M-CTW of inverter side, respectively. Eventually, the fault direction discrimination parameters of rectifier side and inverter side are 4.827 × 105 V 2s and 4.854 × 105 V 2s , respectively. Afterwards, based on calculation results of fault direction discrimination parameters, according to Eq. (19) and Fig. 6, the proposed protection principle can discriminate internal fault correctly. Fig. 9 provides typical external fault (F4, PG fault, fault resistance is 0 ) at rectifier side. Fig. 10 provides typical external fault (F5, PG fault, fault resistance is 0 ) at inverter side. uR1 and iR1k are positive sequence VTW and M-CTW of rectifier side, respectively. uI1 and iI 1k are positive sequence VTW and M-CTW of inverter side, respectively. In Fig. 10 and Fig. 10(b), VTW and M-CTW overlap approximately. In Fig. 9, the fault direction discrimination parameters of rectifier side and inverter side are 2.001 × 105V 2s and 3.018 × 105 V 2s , respectively. In Fig. 10, the fault direction discrimination parameters of rectifier side and inverter side are 1.499 × 105 V 2s and 2.134 × 105 V 2s , respectively. Afterwards, based on calculation results of fault direction discrimination parameters in Fig. 9 and 10, according to Eq. (19) and Fig. 6, the proposed protection principle can discriminate external fault correctly.

4.4. Different transmission line lengths To verify the performance of proposed protection with different transmission line lengths, a series of simulations are carried out as Fig. 13 shows. The fault locates at middle of transmission line. In Fig. 13, ER and EI are fault direction discrimination parameters of rectifier side and inverter side, respectively. Eventually, it could be concluded that the proposed protection principle could identify fault direction correctly with different transmission line lengths. 5. Operation delay analysis Fig. 14 provides diagram of operation delay analysis. If fault happens at F when t = 0 , the relays of two ends detect the arrival of TW at tR1 and tI1, respectively. Afterwards, the data process delay (including data window) of two ends are tR and tI , respectively. Finally, the transmission delay of fault direction discrimination results of two ends are tR3 and tI3 , respectively. lR is distance between F and LCC-HVDC rectifier station. lI is distance between F and MMC-HVDC inverter station. L is the whole length of transmission line. Taking LCC-HVDC 8

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rectifier station as an example, its operation time is:

l l L l L T = max(tR2, tR3) = max( R + tR, I + tI + ) = I + tI + v v v v v L tI + 2 v

brid LCC-MMC-HVDC transmission line. It has several advantages:

• High reliability. For dependability aspect, the proposed protection (23)

where v is TW propagation speed. Based on Eq. 23, the operation delay includes two parts:

• •

(i) The data process delay ( tI ) includes data window (1 ms in this paper) and calculation time. Considering the computing ability of relay, the calculation time is assumed 1 ms . (ii) Assume the TW propagation speed v 300 km/ms , the propagation delay 2L /v is related to transmission line length. For example, if transmission line length varies from 300 km to 1000 km, the propagation delay varies from 2 ms to 6.67 ms.

principle has ability to distinguish fault section (internal fault or external fault) and action correctly with different fault locations, different fault resistances and different fault types. For security aspect, the proposed protection principle will not action to external faults such as converter bus fault, DC reactor fault, et al. Light data transmission burden. Only fault direction discrimination parameters need to be exchanged through optical fiber. No strict data synchronization system.

Declaration of Competing Interest The authors declare that there is no conflict of interests regarding the republication of this paper.

6. Summary The paper proposes a novel TW directional pilot protection for hyAppendix A. Control strategy of LCC rectifier station Fig. 15.

Fig. 15. Control strategy of LCC rectifier station.

Appendix B. Control strategy of MMC inverter station Fig. 16.

Fig. 16. Control strategy of MMC inverter station.

9

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Appendix C. HVDC transmission line structure and parameter Fig. 17.

Fig. 17. HVDC transmission line structure and parameter.

Appendix D. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.ijepes.2019.105431.

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