Analysis and improvement of the ‘5.26’ bipolar trip of Yun-Guang HVDC islanded operation

Islanded operation of HVDC sending system can reduce the influence of HVDC fault on the parallel AC system, enhance the security of the system and increase the transmission capacity. Due to relatively small system inertia and ESCR, there are difficulties of the frequency and voltage control in the islanded sending system. The conventional analysis are focuses on the process after HVDC bipolar trip. An unexpected bipolar trip occurred after the monopolar trip test in the onsite commissioning of Yun-Guang HVDC islanded operation on May 26th, 2013. Process analysis shows that direct causes of the bipolar trip are the reverse-action of PSS, the low ESCR, and frequency adaptability of the HVDC overvoltage protection. An supplymentary test, concentrating on frequency adaptability of the HVDC overvoltage protection, was conducted on July 14th, 2013. But the temporary overvoltage risk still existed in the supplymentary test. Wide-area PSS and high-voltage-side voltage control are proposed to solve temporary overvoltage problem after HVDC trip. In the EMTDC/PSCAD simulation, overvoltage decreases about 60 kV to 80 kV by using these two measures simultaneously.