Three-Phase Short-Circuit Current Calculation And Probability Assessment Of Power Systems With Photovoltaic Power Generations

Photovoltaic power generations connected to grid with high penetration will become an important trend of future electric power system.During the fault,the photovoltaic power generations will inject short-circuit current into the system,which may increase the level of short-circuit,resulting in serious problems such as excessive short-circuit current.In the meantime,the fault characteristics of photovoltaic power generations are completely different from conventional power supply,thus the traditional equivalent voltage source method cannot accurately characterize the transient current characteristics of photovoltaic power generations.In addition,the low voltage ride-through ability of them is fuzzy,and the random factors of fault will lead to the randomness of voltage sag on grid-connected point of photovoltaic power generations,making the short-circuit current shows uncertainty during a fault.The accurate assessment of short-circuit current,and the change interval and probability distribution of short-circuit level can provide an effective basis for the electric power system planning and design,protection configuration,and equipment selection.In this paper,the calculation method and the probability evaluation of short-circuit current are studied.The main contents of this paper are as follows:(1)For the problem that the traditional voltage source method cannot characterize the transient characteristics of photovoltaic power generations,thus a full current expression based on the transient characteristics of photovoltaic short-circuit current is proposed,and its short-circuit equivalent model is established.From the perspective of short-circuit current features of photovoltaic power generations,a general expression of full current,including decaying AC periodic component,decaying DC component and steady-state short-circuit current,is derived,and verified by the measured short-circuit current waveform.Then,according to the low voltage ride-through control strategy of photovoltaic power generation and its closed-loop transfer function model,the transfer function of the output voltage and command current is derived.Finally,a transient equivalent model of short-circuit calculation,considering virtual impedance control,is established.(2)Based on superposition theorem,the short-circuit current calculation method of power system with photovoltaic power generations is proposed.Based on the equivalent model of photovoltaic power generations,applying superposition theorem,the fault network can be equivalent to the superposition of normal component network and fault component network,and then the equivalent circuit of photovoltaic power generation is added in the fault component network.According to the different equivalent models of photovoltaic power generations during fault transient and steady-state stages,the network distribution of AC component and DC component of short-circuit current are calculated respectively.Then the peak value of short-circuit current is evaluated effectively.(3)Based on Monte Carlo simulation,a probabilistic evaluation method of short-circuit current,considering low voltage ride-through uncertainty,is proposed.During the low voltage ride-through period of photovoltaic power generations,short-circuit current characteristics and internal potential change process,under different terminal voltage sags,are analyzed.Considering uncertainty of low voltage ride-through ability,the normal distribution probability density function is used to characterize the random characteristics of photovoltaic power generations,then a stochastic evaluation model of low voltage off-grid is established.Based on the calculation method of short-circuit current,considering the randomness of fault information,probability distribution of system short-circuit current is finally determined by short-circuit current characteristic of photovoltaic power generations,low voltage off-grid evaluation model and fault conditions.