Study On First And Subsequence Commutation Failure Mitigation Strategy Of HVDC System

Commutation failure is one of the most common faults in HVDC systems with grid commutated converters.When AC fault occurs on the inverter side of HVDC,the amplitude and phase jump of commutation voltage leads to the sharp decrease of commutation margin,causing the first commutation failure.Commutation failure gives rise to the interruption of DC transmission power and the rise of DC current,which makes the DC system deviate from the normal operation state and damage the safety of equipment.In general,the first commutation failure after AC fault is unavoidable.After the system recovers from the first commutation failure,if the control system is not adjusted properly,the subsequent commutation failure occurs during the fault duration or clearing period,which may cause repeated high-power impact on the AC system and possible DC blocking,which may endanger the power grid security.The firing control system can adjust the operation state of HVDC system by changing the firing phase of converter valve.Therefore,in-depth analysis of HVDC control system and corresponding improvement of control strategy are of great significance to reduce the probability of the first commutation failure and avoid the subsequent commutation failure.In this paper,the basic principles of the control strategies of the pole control system on the rectifier side and the inverter side are introduced.Then,three basic control strategies of CIGRE,SIEMENS and ABB are introduced,and the transient response and the immunity performance of commutation failure with different control strategies are analyzed in detail.Then,in view of the slow response speed and large control error of the extinction angle controller commonly,a first commutation failure suppression method is proposed to deal with AC fault quickly.In this method,the firing angle order is calculated based on the amplitude of the zero-sequence and αβ component of the three-phase voltage at the commutation bus,and it is used as the upper limit of the output of the original controller.In this way,the firing angle order of the original control system is quickly corrected to increase the commutation margin of each converter valve,and the probability of first commutation failure is effectively reduced.The equidistant pulse control based on phase-locked oscillator is generally adopted in projects,and it generates firing signal sequences of equal intervals according to the firing angle order and the synchronous voltage phase of the phase-locked loop(PLL).During the recovery period after AC faults,PLL fails to lock in time because of the fast change of AC voltage phase may cause,which results in a large deviation between the actual firing angle and its order.When the actual firing angle is larger than its order,subsequent commutation is likely to occur due to insufficient commutation margin.Therefore,a fault recovery strategy based on phase detection of commutation voltage is proposed.The phase error caused by PLL is calculated by detecting the phase of commutation voltage,and the firing angle order is compensated to reduce the deviation of the actual firing angle and its order.This method effectively reduces the probability of commutation failure and makes the system recovery more stable.The predictive calculation-based extinction angle control generates the firing angle order by means of open loop.Under the steady-state condition,the result of this calculation is accurate.But when a fault occurs in the AC system,a serious calculation error exists,which increases the risk of commutation failure.Therefore,an improved control is proposed.In this control,the firing angle order of each converter valve is separately calculated according to its commutation voltage,and considering the influence of the firing pulse control system,a compensation term is added to eliminate the deviation between actual firing angle and its order.To ensure the equidistant pulse control of each converter valve,the minimum firing angle order of all valves is taken as the output of the controller.Finally,the above-mentioned research work is summarized,and the future research work of commutation failure suppression strategy is put forward.