Abstract
Considering the advantages and limitations of traditional identification method, combined with the strategy of active detection, the principle of DC grid pilot protection based on active detection is proposed to improve the sensitivity and reliability of hybrid MMC DC grid protection, and to ensure the accurate identification of fault areas in DC grid. By using the DC fault ride-through control strategy of the hybrid sub-module MMC, the fault current at the converter station DC terminal is limited. Based on the high controllability of hybrid MMC, sinusoidal fault detection signals with the same frequency are injected into the line at each converter station. Based on model recognition, the capacitance model condition is satisfied by the detected signals at both ends during external faults whereas not satisfied during internal faults. The Spearman correlation coefficients is then introduced, and the correlation discriminant of capacitance model is constructed to realize fault area discrimination of DC grid. The simulation results show that the active detection protection scheme proposed in this paper can accurately identify the fault area of DC grid, and is not affected by fault impedance and has low sampling rate requirement.
Highlights
Nowadays most of flexible DC transmission projects use half-bridge submodules (HBSMs) based modular multilevel converter (MMC) which lacks DC fault clearance capability, and is suitable for cable schemes with low DC failure rates
7 Conclusion Combing the high controllability of hybrid MMC system, fault ride-through strategy and characteristic signal injection strategy, a pilot protection principle based on active detection is proposed in this paper using model identification
1) For the hybrid MMC converter system, its high controllability can be fully utilized, and the characteristic frequency signal can be injected by controlling the DC voltage through acting on the switching process of sub-modules to realize the control of DC current and voltage
Summary
Nowadays most of flexible DC transmission projects use half-bridge submodules (HBSMs) based modular multilevel converter (MMC) which lacks DC fault clearance capability, and is suitable for cable schemes with low DC failure rates. The Zhangbei 500 kV flexible DC transmission project uses half-bridge submodules based MMC with overhead transmission lines. The recovery and restart process of the DC power system is complicated, which makes power recovery time long. This may lead to the loss of stability of the connected AC system. When the internal fault occurs, the capacitance model is not satisfied and the differential current icd has no direct relationship with ducd/dt. Spearman correlation coefficient is selected as the measurement method of similarity for model identification based on active detection
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