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| The Direction Protection Based on the Amplitude Ratio of the Backward Traveling Wave for the Symmetrical Monopole Flexible DC System |
| Li Ye1, Li Bin2, Liu Xiaoming1, He Jiawei2, Jia Guanlong1 |
1. State Key Laboratory of Reliability and Intelligence of Electrical Equipment School of Electrical Engineering Hebei University of Technology Tianjin 300132 China;
2. Key Laboratory of Smart Grid of Ministry of Education School of Electrical and Information Engineering Tianjin University Tianjin 300072 China |
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Abstract The flexible DC grid is considered as an important trend of the power system development in the future, with the advantages in large-scale renewable energy access, new urban power supply and so on. However, the DC fault propagation speed of the flexible DC grid is faster, which will damage the whole system in a few milliseconds. Therefore, the fast and reliable direction protection is of great significance for DC line protection. The traditional direction protection based on the amplitude ratio of the forward and backward traveling wave can identify the fault direction fast independent of the line boundary, which has a wide application prospect. However, for the symmetrical monopole flexible DC system, the traditional direction protection cannot identify the pole-to-ground fault direction, because the equivalent impedance of the converter is changing as the result of the dynamic change of the input quantity of the sub-modules. Therefore, a novel direction protection based on the traveling wave is proposed, which is independent of the line boundary and not affected by the converter control influence.
Firstly, the basic principle of the traditional direction protection based on the ratio of the forward traveling wave and the backward traveling wave is introduced. And on this basis, the applicability in the symmetrical monopole flexible DC system is analyzed. For the symmetrical monopole flexible DC system, when the DC pole-to-ground fault occurs, the control response of the converter station will affect the DC fault characteristics, generating the additional fault traveling waves which cannot be accurately calculated. In this case, the traditional traveling wave based direction protection criterion cannot identify the forward and backward faults explicitly in the symmetrical monopole flexible DC system. According to the analysis of fault traveling wave propagation process, there is no backward traveling wave when the backward fault occurs. Therefore, the direction criterion based on the ratio of the backward traveling wave amplitude of each line is proposed. And the proposed direction criterion need not depend on the line boundary elements and is not affected by the nonlinear control of converter. On this basis, the direction protection scheme for the symmetrical monopole flexible DC system is proposed. In the proposed direction protection scheme, the ratio of transient voltage amplitude on both sides of DC reactor is firstly used to identify whether the fault occurs at DC lines, and then the ratio of backward traveling wave amplitude of each line is used to determine the fault line. Finally, a large number of simulation cases are carried out to verify the feasibility and superiority of the proposed direction protection.
The conclusions can be drawn as follows: For the flexible DC system, there is no backward traveling wave at the initial stage of the fault when the backward fault occurs. But when the forward fault occurs, the amplitude of the backward traveling wave is large. Based on this characteristic, a new direction protection principle based on the amplitude ratio of the backward traveling wave of adjacent lines is proposed in this paper. Compared with the traditional traveling wave based direction protection, the performance of the proposed protection is not affected by the dynamic switching process of converter station sub-module after the fault, and the reliability and sensitivity are greatly improved.
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Received: 06 January 2022
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2023, Vol. 38 
