Research On Current Protection Scheme Of Distributed Generation Accessing Distribution Network

With the promotion and application of distributed power sources,it has brought serious problems to the power grid while generating high efficiency.Because many distributed generators are connected to the grid by grid-connected inverters,the output characteristics make the short circuit current level without distributed generators change obviously when short-circuit faults occur,which brings great impact on the normal operation of protection devices.And the difference in distributed power access location and rated power output also has an impact on the distribution network.In this paper,in order to solve the problem of malfunction of the current protection device,when the distribution network with distributed power supply fails at different locations,an improved scheme for the current protection calculation value of the distribution network with distributed power supply is proposed.Aiming at the situation of DG connected doubly-fed distribution network at the beginning,middle and end buses,the fault equivalent circuit is established to calculate the short-circuit current.In view of the fluctuation of DG output,the regularity of short-circuit current changing with grid-connected voltage is proposed.And according to the output characteristics of the inverter-type distributed power supply and the gridconnected mode,the equivalent model is established according to the control strategy of the inverter-type distributed power source.And based on the theoretical analysis of the Power Systems Computer Aided Design platform,the simulation model is built.It lays the foundation for the study of short-circuit current when three-phase short circuit occurs in the distribution network with distributed power connection.According to the variation law of short-circuit current with the fluctuation of gridconnected point voltage,the relationship between grid-connected point voltage and DG output power is analyzed,and the variation law of short-circuit current with the fluctuation of DG output power is further obtained.According to the variation of short-circuit current with DG output power fluctuation,it is found that if the short-circuit current level of DG at rated power output is still adjusted,the current protection will be misoperated.The accuracy of the theoretical analysis is verified by simulating the fluctuation of DG power with different power output values.According to the variation rule of short-circuit current when the output power o f DG fluctuates,the power value of maximum short-circuit current is determined when the output power of DG is fluctuated at different positions of feeder in distribution network.Because the current protection setting of distribution network with DG is based on short-circuit current level when it operates under rated conditions.Therefore,if the power value is greater than the rated power value of DG output,the current protection of distribution network with DG will still be adjusted according to the original setting principle.If the power value is less than the rated power value of DG output,the current protection of distribution network will be re-adjusted according to the short-circuit current level at this power.Ensure the normal operation of current protection.The new scheme is verified and analyzed by a specific example.In this paper,a new current protection setting scheme is proposed to solve the problem of incorrect operation of current protection devices.And taking the simulation results as reference value,the setting values of instantaneous current quick-break protection and time-limited current quick-break protection for each line of distribution network after the distributed power supp ly is connected are re-adjusted.The new setting value can ensure the normal operation of line protection when the rated output power of the distributed power supply is different after the distributed power supply is connected to the distribution network.