Analysis And Calculation Methods Of Short Circuit Current In Active Distribution Networks

The fault analysis of distribution network is the basis of studying the safe operation of the distribution network,the relay protection and the fault treatment.By using the radiation structure characteristic of distribution network,the method of port compensation based on phase component is used to calculate short circuit current of distribution network,which can reduce the workload of calculation.Calculation of short circuit current in distribution network by using port compensation method can be used without iterative calculation,however,the calculation error is large due to the influence of the parallel branch in the calculation of the node impedance matrix is ignored.Though the iterative calculation can solve the problem of large error,it needs multiple calculations of port compensation current,which leads to the large computation.In addition,when calculating the fault of an active distribution network,rotating machines are simply treated as PV nodes which are usually used in steady-state power flow calculation in the existing literatures,The model is not accurate enough when the fault is calculated.In view of the shortcomings of the current port compensation method,this paper improves the calculation.Taking the influence of the parallel branch into account when calculating the port node impedance matrix;A more accurate constant electromotive force model is used to replace the commonly used PV model in the calculation of the fault of the active power distribution network.The main contents of this paper are as follows:(1)In the calculation of passive power distribution network fault by using port compensation method,an improved algorithm for port compensation considering the influence of parallel branches is proposed.Through the Y / ? transformation and the parallel branch node admittance matrix correction,the different connection way of the reactive power compensation capacitor and load parallel branch into a unified simple parallel form,in the calculation of the fault port node impedance matrix is relatively simple consider their influence.In this way,the use of a simple way without iteration to calculate more accurate fault current of the passive distribution network.(2)When calculating the fault of the active distribution network,the constant electromotive force model of the rotating machine which is more close to the reality is constructed.The transient electromotive force of the rotating machine is obtained by the steady state power flow calculation,and make full use of the characteristic that the transient electromotive force of the rotating machine is kept unchanged while the fault occurred,the constant electromotive force model is used.Thus,when calculating the fault port node impedance matrix,the rotating machine can be regarded as a parallel branch and the branch admittance matrix is calculated according to the secondary transient impedance.For a distribution network containing only rotating distributed power supplies,it is possible to use the simple way without iteration method to calculate the fault current which is more practical and accurate.(3)When calculating the fault of the power distribution network with inverter,in order to meet the nonlinear characteristics of the inverter,an iterative algorithm for port compensation is proposed.The inverter has the property of current source.When calculating the fault port node impedance matrix,the inverter is treated as an open parallel branch,regardless of its influence.In the process of fault iterative calculation,according to the control strategy of the inverter distributed power supply and the grid voltage,the output current of the inverter is adjusted.For a class of distributed power distribution networks with inverters,a very accurate fault current can be calculated.(4)DIGSILENT simulation software has been used to build a numerical example model,and set up the corresponding fault.At the same time,MATLAB is used to write the fault calculation program,and the algorithm is tested.The results of calculation and the example analysis show that the improved method is feasible and effective.In this paper,the effect of the parallel branch is taken into account when calculating the impedance matrix of the faulty port node,and a more accurate constant electromotive force model is established when the active distribution network fault is calculated.In this way,for the passive and only the rotating distributed power distribution network,using the improved algorithm for non-iterative port compensation considering load impact can obtain more accurate short-circuit current,and the calculation is simple.For the distributed power distribution network with inverter,taking into account the non-linear characteristics of the inverter,using the port compensation iterative algorithm considering the parallel branch,the result is accurate.Therefore,the research results of this paper are of great practical value for the simple and accurate calculation of short circuit current in distribution network.