Research On Wide-Area Protection Strategy Of Multi-Infeed AC/DC Interconnect System

With the gradual establishment of the AC/DC interconnection system with the scale and voltage level at the forefront of the world in China,the regional power grids in China are interconnected by AC/DC transmission.But at the same time,the traditional relay protection technology based on lumped parameter model uses Kirchhoff s law and power frequency electrical quantity information to construct the protection principle,while the nonlinear characteristics of AC/DC interconnected system fault lead to the traditional fault analysis method is no longer applicable,and the traditional relay protection may fail to operate or misoperate in the DC access scenario.Therefore,focusing on the scene of commutation failure caused by AC line fault in AC/DC interconnected system,this paper studies the methods of DC line protection,AC line protection and AC line reclosing in AC/DC interconnected system.For DC line protection in AC/DC hybrid system,concerning its poor immunity to fault resistance in the case of in-zone fault and mal-operation problem in the case of out-of-zone fault,a new pilot directional protection scheme based on the voltage difference between positive and negative poles is proposed.First,by analyzing the operating conditions of converter in different time intervals,the differential expressions of feed-in current from AC system to DC system are derived.On this basis,combining AC-side fault component network,the calculation models of voltage difference between positive and negative poles in different operating conditions are built.And then,according to the consistency between the calculated voltage difference between positive and negative poles and the actual voltage difference,the protection criterion to identify in-zone and out-of-zone faults on DC line is constructed.Concerning the mal-operation of distance protection in AC/DC hybrid system due to commutation failure in DC system caused by AC-side fault,a novel AC line distance protection scheme based on fault likeness factor is proposed.First,by analyzing the conducting state of converter in different time intervals,the expression of feed-in current from DC system to AC system is derived.On this basis,combining AC-side fault component network,the virtual fault location equation considering the impact of feed-in current from DC system is established.Thus,by solving the equation consisting of information at multiple time sections,the virtual fault location is calculated.And then,according to the consistency between virtual fault location and actual fault location,the fault likeness factor is constructed to distinguish between in-zone and out-of-zone faults.Concerning the difficulty in distinguishing between permanent and instantaneous faults on AC line in AC/DC hybrid system due to relatively low amplitudes of fault recovery voltage and fault arc voltage,a method to identify single-phase reclosing fault based on virtual instantaneous power is proposed.First,by analyzing AC-side networks in the case of permanent fault and instantaneous fault,the calculation models of instantaneous power difference between two ends of AC line are built.And then,the instantaneous power difference is decomposed into two parts:virtual instantaneous power which is affected by the currents on two line ends and virtual instantaneous power which is affected by the current on one line end alone.On this basis,according to the feature of virtual instantaneous power affected by the currents on two line ends,i.e.it is 0 in the case of instantaneous fault,but not 0 in the case of permanent fault,a fault nature identification criterion to distinguish between permanent and instantaneous faults is constructed.In view of the fact that there are few related researches on wide-area protection of AC/DC interconnected systems,a new principle of wide-area backup protection for AC/DC hybrid systems based on vector similarity is proposed.First,according to the action signals of DC line protection and AC line protection proposed in this article,combined with the action of other components,an action matrix is formed.Then,by extracting the actual protection signal and measuring the cosine similarity between the actual action signal and the action matrix,the reliable and fast identification of the faulty component is realized.