Research On Control And Calculation Method Of Short Circuit Current Based On Line Switching And UPFC

With the continuous increase of power supply capacity and the continuous strengthening of the power grid structure,the problem of short-circuit currents has become a problem.More and more short-circuit currents of substations have approached or even exceeded the rated interrupting capacity of the circuit breakers,which has become a major safety hazard.Short-circuit current exceeding has become a common problem in China's power grid and is an important factor restricting the development of the power grid.Therefore,it is very urgent and necessary to study the short-circuit-current-limiting scheme.In addition,with the application of the unified power flow controller in the actual power grid,research on the calculation method of the system short-circuit current considering UPFC is of great significance.The main work in this paper is as following:(1)This paper focuses on the measures of line breaking,which is widely applied to limiting short-circuit current,and proposes an algorithm to search all feasible scheme to limiting current.First,the expression of nodal self-impedance to line switching is derived based on the branch addition method,which proves that there is a monotonic relationship between the node short-circuit current and the line switching.Then,the current-limiting scheme is classified,and the properties is derived.Based on that,the searching problem is transformed into a search of a special set of "boundary feasible schemes",which avoids exhausting all line-breaking schemes.Second,using the approximate linearized expression of node self-impedance,the search efficiency of the algorithm is further improved.Finally,the validity of the proposed search algorithm is verified by cases studies.(2)Flexibility is a major feature of smart grid operation.With the development of equipment level and communication technology,dynamic optimization and adjustment of transmission line operation status in grid operation has broad application prospects.Short-circuit current safety is a necessary condition for transmission switching to be used in practice.This paper first establishes an optimal transmission switching model that considers short-circuit current constraints,and then uses the full current-limiting schemes to replace the non-convex and nonlinear nodal self-impedance constraints.The original problem is decomposed into the optimal transmission switching main problem and the short-circuit current safety sub-problem,which are solved iteratively by adding cut planes.Finally,case study shows that the full current-limiting schemes has certain application prospects in the optimal transmission switching.(3)In view of current unified power flow controller(UPFC)device and the fault handling strategy adopted,a calculation method for short-circuit current of UPFCnearby bus was derived based on the electric network theory.UPFC-nearby bus was described by transfer admittance.Since the UPFC series side is bypass protected immediately after the nearby bus fault,a series voltage source was added equivalently to the fault component in the superposition method,thereby affecting the SCC calculation result of the fault bus.The relative change of the calculation result is related to the UPFC operating voltage and the current distribution coefficient.The effects of operating conditions and UPFC parameters on the SCC calculation result were studied in IEEE9 system,and was verified by PSCAD.Using IEEE118 system,the influence of UPFC on SCC in large-scale power grid was analyzed.The results showed that UPFC has the opposite effect on the SCC on different sides,and the influence on the SCC of the nearby bus decreases with the increase of the electrical distance.