| With the world's energy shortage and increasing environmental pollution,the use of renewable energy as a new generation technology will become the power system development trend.Grid-connected operation of distributed generation(DG)changes the traditional structure of single radiation power supply network.At the same time,bidirectional flow of power flow and load current affects the safe and stable operation of power system.Therefore,after studying the distributed generation control strategy,as well as deeply analyzing the distributed generation fault characteristics and the impact on the traditional protection,this paper proposes a protection scheme adapted to the distributed power distribution network.First of all,the structure and working principle of inverter-interfaced distributed generation are briefly analyzed and the dual-loop control strategy of three-phase grid-connected inverter is studied in depth.Focusing on asymmetrical fault conditions,the dual-dq current control technology with positive and negative sequence decoupling is adopted,as well as suppressing the negative sequence component is taken as the control objective to improve the symmetry of inverter-interfaced distributed generation output current.At the same time,considering the grid-connected requirements of low-voltage crossing,the fault output characteristics of inverter-interfaced distributed generation are deeply analyzed.Secondly,this paper expounds the influence of grid-connected DG on traditional protection from the perspective of mathematics.It focuses on analyzing the impact on the original protection of lines and automatic reclosing devices when distributed generation accesses to distribution network by different types,locations and capacity.A typical lOkV distribution network with distributed generations is built in MATLAB/Simulink,verifying the impact of distributed generation on the traditional protection of distribution network.Thirdly,a new protection scheme of distribution network with DGs based on phase comparison of positive sequence component of fault current is proposed.According to the fault output characteristics of inverter-interfaced distributed generation,the relationship between the phase of positive sequence fault component in current and the voltage drop at point of common coupling is deduced.By analyzing the phase relationship between the currents at both ends of the line when an internal or external fault occurs,the identification criterion of line protection is constructed.At the same time,the rate of phase change is taken as the start-up criterion of protection to reduce the invalid communication of the protection device.The protection scheme requires only current information,reducing the installation of voltage transformers;at the same time,it has good applicability for different transition resistance and fault types.Finally,an improved differential protection principle based on amplitude ratio of node branch current is proposed.By considering the influence of CT transient and steady factors,the threshold value of protection is quantified.At the same time,the influence of the unmeasurable branch of the distribution network on the protection of internal or external is taken into account.Focusing on the problem of protection malfunction caused by untestable branch at upstream of distributed generation,the auxiliary criterion of node branch is used to eliminate the blind spot of main criterion and improve the protection reliability of power system.The simulation results show that the proposed scheme is simple and feasible,with good ability to resist synchronization and CT transmission error as well as good applicability to the complex situation of distribution network. |
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