Procedure analysis of UHVDC commutation failure

There may be a commutation failure during a fault or start-up of the UHVDC inverter side. The law of commutation failure process in UHVDC converter is studied here. First, the cause of inverter commutation failures is briefly introduced. Then, the general law for the bridge arm conduction of the inverter in the process of the commutation failure is revealed. Finally, the system debugging recorded data of some commutation failures of Taizhou UHVDC inverter in China is used to verify the law. Research shows that the commutation failure will go through three stages which has obvious features: (i) the commutation failure stage, (ii) the bypass circuit formation stage, and (iii) the commutation failure recovery stage. In the last stage, the AC currents of two conducted phases increase instantly in opposite directions, which are easy to recognise. In the second stage, only the DC current increases, and the AC currents equal 0 approximately. Because of the three stages, the commutation failure valve number is easily recognised. The study on the law of commutation failure process lay the foundation for commutation failure process analysis and identification for relay protection.