Methods to Prevent Commutation Failures in HVDC Systems

Abstract Commutation failure (CF) is the main fault type in HVDC (High voltage direct current) system with line commuted converter. So, Commutation faults are the main disadvantages of line commutated converter HVDC systems. There are some situations that cause the fault in these types of faults that occur in thyristor valve compartments. These situations can be seen in the magnitude of the commutation voltage, phase angle reversal and sudden changes, a sudden change in DC currents or failure of ignition controls. To mitigate CFs, considerable researches have been done either in modelling or control methods. However, with the operation of more and more large-capacity HVDC projects, the coupling characteristics between HVDC and power grid are more complex. The existing reviews tend to focus only on the CFs of a single HVDC itself. Also, in the event of any commutation fault, the system can be put back into operation after the fault is completely eliminated. However, if the valve ignition circuits are designed to be blocked at a pre-set low voltage and the blockage is lifted under normal voltage conditions, they can be partially protected in the event of a commutation fault. Otherwise, the fault process can be prolonged. This paper provides more systematic overview of CFs also the influences of the occurring moment on the commutation process are observe quantitatively to predict and prevent commutation failures effectively. Then, according to the critical fault voltage and extinction angle CFs reactance is calculation and presented based by formulations. The concept of the commutation failure domain and security domain is determined to estimate the probability of commutation failures. Furthermore, the direct current and RMS voltage calculation methods under asymmetrical faults are presented and direct current is predicted rather than real time measured and the RMS voltage under asymmetrical faults can be calculated instantaneously. In particular, the advantages and limits of various CFs prevention methods are analyzed and compared in order to reveal the latest research result.