Pilot Protection Scheme of Submarine Cable in Flexible Low-Frequency Transmission System Based on Cross Entropy Algorithm

doi:10.11930/j.issn.1004-9649.202306023

Abstract

Abstract:

There are power electronic converters on both sides of the AC submarine line of flexible low-frequency transmission system, and the fault current characteristics are determined by the converter control strategy, which is different from the traditional synchronous power supply, which will lead to the risk of incorrect operation of the traditional current differential protection. Based on the fault current characteristics of offshore wind power transmission system cable line, this paper analyzes the operational performance of traditional current differential protection and reveals the reasons for the decline of operational performance. In this paper, a novel pilot protection scheme based on cross entropy algorithm is proposed. In this scheme, virtual negative capacitance compensation and negative number processing are carried out on the sampled current of one side, and then the cross entropy between the sampled current and the current of the other side is calculated. According to the change of cross entropy, external and internal faults are judged. The hardware in-loop test results show that the scheme can operate correctly when various types of faults occur in the submarine cable, and has strong resistance to high transition resistance. It can also operate correctly when there are measurement errors in TA. It can be used as a primary protection for flexible low-frequency transmission submarine.

Key words: flexible low-frequency transmission system, pilot protection, virtual negative capacitance, cross entropy algorithm

Weixuan HE, Zhengzhen FAN, Yaotong HUO, Yingyu LIANG. Pilot Protection Scheme of Submarine Cable in Flexible Low-Frequency Transmission System Based on Cross Entropy Algorithm[J]. Electric Power, 2023, 56(11): 38-48.

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URL: https://www.electricpower.com.cn/EN/10.11930/j.issn.1004-9649.202306023

https://www.electricpower.com.cn/EN/Y2023/V56/I11/38

Figures/Tables 18

Fig.1 Topology of offshore wind power by flexible low-frequency AC transmission

Fig.2 Control diagram of M3C low-frequency side

Fig.3 Equivalent circuit diagram of flexible low-frequency AC transmission

Fig.4 Current differential protection performance diagram during normal operation of the system

Fig.5 Current differential protection performance diagram during general fault of the system

Fig.6 Current differential protection performance diagram during serious fault of the system

Fig.7 Diagram of virtual negative capacitance compensation

Fig.8 Comparison of current before and after virtual negative capacitance compensation

Table 1 Probabilistic results of current on two sides of submarine cable during normal operation of the system

区间	1	2	3	4	5
p_i(I_M)	0.342	0.147	0.123	0.085	0.048
q_i(I_N)	0.342	0.147	0.125	0.085	0.048

区间	6	7	8	9	10
p_i(I_M)	0.047	0.046	0.047	0.042	0.063
q_i(I_N)	0.047	0.046	0.047	0.047	0.063

Fig.9 Fault current and cross entropy waveforms at A-phase-to-ground fault under different transition resistance

Fig.10 Current and cross entropy waveforms under normal condition

Fig.11 Current and cross entropy waveforms under three-phase fault of busbar M

Fig.12 Current and cross entropy waveforms under serious fault at F2

Fig.13 Current and cross entropy waveforms under serious fault at F3

Fig.14 Current and cross entropy waveforms under slight fault at F2

Fig.15 Current and cross entropy waveforms under slight fault at F3

Fig.16 External fault current and cross entropy waveforms when different measurement errors exist in TA

Fig.17 Internal fault current and cross entropy waveforms when different measurement errors exist in TA

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