Research On The Mechanism And Evaluation Methods Of AC/DC Hybrid System Cascading Failure

HVDC technology is one of the important means to transmit large capacity of power from the large-scale energy bases to load centers.It has made substantial progress in our country in recent years and a hybrid AC/DC power grid has been formed.Strong HVDC system with weak AC support is the major risk for the receiving power systems.Meanwhile,the interaction between AC/DC might cause complicated cascading reaction in the case of system faults,and hinder a further development of the AC/DC power system.Therefore,the right sense of potential risks and the estimation of the relevant stability in hybrid AC/DC power system have the theoretical and practical significance on the planning and safe operation of power grid.The mechanism of cascading failure in hybrid AC/DC power system and the relevant stability evaluation methods are analyzed and discussed in this dissertation.The main research contents involve:Firstly,the HVDC commutation process and the affecting factors are analyzed.Secondly,from the viewpoint of power system planning and operation,the influence of converter commutation is analyzed while a new HVDC is integrated.Then,an interpretation,from a new viewpoint,of converter commutation process in multi-infeed HVDC system is built and the affecting factors are illustrated.Based on this model,the conclusion is drawn that the risk in multi-infeed HVDC systems is higher than single-infeed HVDC systems,as the original commutated reactance increases.Therefore,intensive landing points of HVDC system would cause simultaneously or sequentially commutation failure under AC system faults,which will lead to a new type of voltage instability with a narrow time scales,high-impaction and multicomponent cascading failure characteristics.Based on the analysis of reactive power and voltage coupling,the mechanism of cascading failure in hybrid AC/DC system is studied considering the effects of HVDC control and reconstruction simulation in PSCAD.In order to demonstrate the main effecting factors and evaluate hybrid AC/DC power system stability,the differential algebra equation(DAE)model enabling the description of various coupling in hybrid AC/DC power system is established,the peculiarity of this model is that the dynamic characteristic of HVDC systems,interactions between AC/DC systems,and electrical coupling between different DC systems can be described.Then utilizing linearized model,the eigenvalues are computed.The results indicated that AC system strength,coupling impedance and HVDC control parameters have certain influence on the system stability.At last,on the aspects of the evaluation methods and their applications on large-scale hybrid AC/DC power systems,some common indexes are pointed out.To solve the problem of location selection and connection mode,a method based on MISCR index is proposed and applied in actual power grid planning of east China.On this basis,HVDC access capability for the actual grid is validated on simulation.The results of the analysis can provide some reference for power grid planning and scientific decision.