Research On Adaptive Strategy For Overcurrent Protection Of Distribution Grid Including Photovoltaic Power Stations

With the increasingly serious environmental pollution and energy crisis,solar photovoltaic power generation has developed rapidly in recent years, because it does not pollute the environment and has short construction period,has already became very important part of power grid. However,large-capacity PV system has changed the structure of distribution grid from single power supply to Multi-power supplies.It's outputs are limited by environmental factors,will change the amplitude and direction of short circuit current,and then influences the performance of the overcurrent protection of distribution grid,even lead to refusal/incorrect tripping, affect the security and stability of grid. Therefore, this article based on the analysis of short circuit characteristics and the influences to overcurrent protection of PV system,from which point of view that PV system's output will change short circuit current, proposed a adaptive strategy for overcurrent protection of distribution grid by identifying the equivalent impedance of PV system. Specific contents and features are as follows:First,the PV system dynamic model which combines with different control strategy is built on the platform of PSCAD/EMTDC based on the physical model andworking principle and characteristics of PV system.It's maximum output power is 15 MW.Verifing the correctness of this model by simulating the temporary steady state characteristics of PV array and inverter.Secondly, researching the for example PV grid-connected system in different output active power, different transition resistance, failure characteristics of the different positions in case of failure and the impact on the overcurrent protection of distribution grid by theoretical analysis and simulation analysis.Simulation results show that: PV system compared to conventional grid power failure which different characteristics,the maximum fault current rating of 1.2 times;the PV output is proportional to the active power,and inversely proportional to the transition resistance.The short circuit current will change when the fault location,PV system active power output,short-circuit current and transition resistance has changed.The overcurrent protection of distribution grid will be affected:an upstream overcurrent protection ? segment will refusal trip when the upstream line failure;a downstream overcurrent protection I segment will incorrect trip when the downstream line failure.Further analysis revealed the active power output of PV systems is the key factor of the short-circuit current from PV system,and it has close relationship with the equivalent impedance of PV system.Next,proposed a method for identifying the equivalent impedance of PV system by using pattern recognition method and analytical method.The pattern recognition method are based on artificial neural network and support vector machine,the analytical method is based on nonlinear regression theory.Building the model of pattern recognition method and analytical method on the platform of MATLAB, the regression function obtained during the previous relationship between the active power output before fault P_PV and the equivalent impedance during fault P_pvf.Realized the feature that online identifying the P_pvf by P_PV before the fault.For the PV system example, determined the optimum relationship between the P_pvf and P_PV by comparing the analysis results of pattern recognition method and analytical method.Finally,proposed a adaptive strategy for overcurrent protection of distribution grid based on the method that online identifying the P_pvf by P_PV before the fault. Established the formula of adaptive setting value and adaptive criterion,taking into account the impact of the current transition resistance overcurrent protection ? segment.Using a common method for recognizing the instantaneous and permanent fault,utilized a correction factor which equal with changing proportion of short circuit current when the transition resistance changing correct the adaptive setting value of overcurrent protection ? segment based on the correspondence between the transition resistance changing characteristics and the operation characteristics of overcurrent protection.Simulation results show that:the adaptive strategy after correcting can solve the problem that upstream overcurrent protection ? segment refusal trip when the upstream line failure and downstream overcurrent protection I segment incorrect trip when the downstream line failure.This adaptive strategy is correct.