Research On The Mechanism And Control Strategy Of Successive Commutation Failure In Multi Infeed Dc System

Line-commutated converter based high voltage direct current(LCC-HVDC)technology has provided promising solutions to overcome the challenge of the inverse distribution of energy resources and load,which has been wildly utilized in long-distance bulk power transmission.With multiple DC circuits connected to the same AC power grid,a complex multi infeed DC(MIDC)system is formed.Commutation failure(CF)is one of the most common failures in HVDC system.Single and continuous CF have adverse effects on DC power transmission,reactive voltage stability,transient stability,etc.Due to the complex coupling among the DC systems,the CF of a certain DC may cause the successive CF of the adjacent DC,which will lead to a more serious regional power loss.Therefore,the research on the mechanism and control strategy of successive CF is helpful to the safety and stability of the power grid.The work of this thesis mainly includes the following three aspects:Firstly,this thesis studies the mechanism of CF,and analyzes the influence of voltage amplitude drop,distortion,drop rate and DC current variation on CF.Furthermore,on the basis of summarizing the reactive power characteristics of AC / DC system,combined with the characteristics of MIDC,this thesis studies the typical scenario of successive CF,and proposes a tracing method for successive CF,which can evaluate the influence of fault DC by other DC.Secondly,based on the mechanism of CF,this thesis proposes a successive CF prediction method considering the commutation voltage distortion and DC current variation.By converting the time-domain to the frequency-domain and combining with the inherent response form of the power system,the commutation voltage and DC current are predicted,and substitute them into the essential mathematical description of commutation failure to achieve the prediction of extinction angle.Furthermore,considering the shortcomings of pure mechanism driven method in the establishment of precise physical model,this thesis proposes a physical model based and data driven successive CF prediction method.In this method,the amplitude and phase of each harmonic of the commutation voltage are taken as the input characteristics,and the extinction angle predicted by the pure mechanism driven method can be modified.Finally,by summarizing the typical control methods of DC system,a suppression strategy of successive CF of multi infeed system based on predicted extinction angle and a progressive recovery strategy of based on additional current instructions are proposed in thi thesis.The former takes the predicted extinction angle as the starting condition,and calculates the rectifier side current order so as to realize the advance control of the control system and suppress the occurrence of successive CF.The latter fully considers the system reactive margin and DC strength to calculate the additional current order,which can realize the fast recovery of power transmission under the condition of ensuring the safe operation of the system.The research of mechanism,influencing factors,prediction methods and control strategies of successive commutation have significant importance to practical engineering.