Case study on over-current protection action of Gaoling back-to-back HVDC system

Commutation failure is one of the most common faults in the high-voltage direct current (HVDC) transmission system, which can lead to results such as: DC voltage reducing, DC current increasing, DC transmission power reducing, shorter life span of the converter valves, converter transformer DC magnetic bias and over-voltage of weak AC system on inverter-side. And in severe conditions, it may result in DC block fault and other serious problems. A DC block event occurred in the Gaoling back-to-back DC unit I caused by AC system disturbance is described. AC system failure made DC system to produce commutation failure and to result in DC over-current protection action and unit I block fault. Based on detailed event process, the principle of DC over-current protection action is discussed in detail, combining of Gaoling back-to-back DC system previous commutation failures. Finally, the recommendations and improvements about enhancing the security and stability level of AC and DC systems are given from four aspects such as: the DC system protection configuration, AC system stabilization level, DC control and protection system, and quality of power grid operating personnel. © State Grid Electric Power Research Institute Press.