The station grounding (NBGS) overcurrent protection is a typical protection for bipole protection of HVDC. Whether its action is correct or not will affect the normal operation of HVDC transmission system directly and the operation safety of equipments. In this paper, a test case of station grounding overcurrent protection malfunction is analyzed theoretically. The condition of NBGS overcurrent test is that two poles of HVDC is in unbalanced operation state and the expected test result is that the control system will execute the order of pole balance and the balanced HVDC system will continue to operate stably.
However, the actual test result is that the station grounding overcurrent protection acted and blocked the HVDC system. From the test waveform, it can be seen that the station grounding current (IDGND) does not disappear rapidly, but decays slowly. At the same time, it is also found that the magnitude of the IDGND is equal to the grounding electrode current (IDEL), but which direction is opposite to the IDEL. It is concluded that a circulating current is formed between the two grounding points, and which leads to the station grounding overcurrent protection malfunction in the test.
By setting the conductivity of grounding electrode in the model reasonably and repeating the above test, the grounding electrode circulating current is not generated, the actual current waveform of grounding electrode is RL fast attenuation, and the NBGS overcurrent protection test achieves the expected results. Therefore, the setting of conductivity of grounding electrode line needs to be handled carefully in the commissioning process for HVDC transmission based on RTDS.
F. Guangxu, L. Lin, Cigre Symposium Aalborg 2019, Session 24, paper 20, June 4 - 7, 2019
KEYWORDS: HVDC, NBGS, RTDS, Conductivity, Station grounding, Overcurrent, Circulating current, Grounding electrode