352 / 2021-11-10 14:51:45

PSCAD-Based Analysis of Operating Overvoltage at the Neutral Point of a 500kV High-Voltage DC Converter Transformer

HVDC transmission,Converter transformers,Neutral point grounding,Switching overvoltage,Fault current

Power system and automation > Power system fault simulation and risk assessment

In order to solve overvoltage problems at the neutral point of high-voltage DC (HVDC) converter transformers and ensure the safe and stable operation of HVDC transmission systems, this paper analyses the effect of the neutral point grounding via a small reactance on the zero-sequence impedance of the system. Grounding the neutral point via a small reactance increases the zero-sequence impedance of the system and thus effectively suppresses the single-phase short-circuit current. The 500kV unipolar high-voltage DC transmission system is modeled using PSCAD/EMTDC simulation software and simulated for different types of faults. Comparative analyses are carried out considering different types of faults and different neutral point grounding configurations such as direct grounding and grounding via 4 Ω, 8 Ω, 13 Ω, 15 Ω and 17 Ω small reactances. The simulation results show that when the neutral point of the converter transformer is earthed by a small reactance, the single-phase short-circuit current can be suppressed effectively. However, with increasing the resistance value of the small reactance, the suppression effect will gradually weaken and causes the neutral point voltage to rise significantly. Compared to the direct earthing, the single-phase short-circuit fault current can be reduced by 14.7% when the neutral point is earthed by a small reactance of 17 Ω. However, the resultant neutral point overvoltage is most serious in this case, with a peak value of 81.20 kV.