Probability Analysis Of Short-circuit Current And Research On Critical Proportion Of Inverter-interfaced Generators Connected To Power System

Along the rapid development of clean energy,the grid-connected installed capacity of inverter-interfaced generators has increased significantly.The low voltage ride through process of the inverter-interfaced generators is uncertain,since the output characteristics of the inverter-interfaced generators are related to the control strategy of inverter,which makes the short-circuit current output by the inverter-interfaced generators random.Grid-connected inverter-interfaced generators will increase the short-circuit current level of the grid.As its proportion of grid-connected access increases,the short-circuit current of the grid may affect the safe operation of system equipment and carrying conductors.Analyzing the probability distribution of short-circuit current of the power system after the inverter-interfaced generators is connected to the grid can provide a basis for the planning and design of the power grid and guarantee its safe and stable operation.Therefore,this paper focuses on the related content of the short-circuit current probability analysis and the calculation of critical proportion of this power system with inverter-interfaced generators.The main contents of this paper are as follows:(1)The disconnection probability analysis model of inverter-interfaced generators based on maximum entropy principle is established.Starting with the control strategy of the low voltage ride through(LVRT)of inverter,a fault equivalent model determined by the degree of voltage drop at the grid connection point is derived according to the output characteristics of the inverter-interfaced generators.Considering the uncertainty of the inverter-interfaced generators disconnecting from the grid during the LVRT process,the probability density function of the terminal voltage and the fault duration is determined based on the principle of maximum entropy,then the off-grid probability evaluation model can be obtained.On this basis,a probabilistic analysis method for the output short-circuit current of the inverter-interfaced generators is proposed.Finally,through a fault simulation of a photovoltaic power station,the accuracy of probability density function of random variable is verified,and the influence of the uncertainty of the low voltage ride through(LVRT)process of inverter-interfaced generators on its short-circuit current is analyzed.(2)The probability distribution of short-circuit current in power system after grid-connection of inverter-interfaced generators is analyzed.On the basis of the fault equivalent model of inverter-interfaced generator,the grid-connected inverter-interfaced generator station is equivalent to a single inverter-interfaced generator through the method of power loss equivalence and capacity weighted equivalence.The node voltage equation can be written by combining the fault components of the short-circuit current injected by the inverter-interfaced generators with the fault component network of the traditional power grid,the three-phase short-circuit current of power system is calculated by superposition theorem.Finally,considering the off-grid uncertainty when the grid-side fault occurs,the probability distribution of short-circuit current in the power system is obtained through fault simulation.(3)The calculation method for the critical proportion of the grid-connected access of high-proportion inverter-interfaced generators connected to the grid is proposed.According to the verification content about short-circuit current during the selection of the switching equipment and conductors in the power system,the short-circuit current exceeding probability evaluation index is proposed and the evaluation system is established.Based on the calculation method of the proportion of grid-connected access of inverter-interfaced generator,a process for evaluating the probability of grid short-circuit current exceeding the standard is proposed,and a calculation method for proportion of the grid-connected access of high-proportion inverter-interfaced generators is obtained.Finally,through simulation analysis,calculating the critical proportion of the grid-connected access of the inverter-interfaced generators in an Anhui power system.