| The development of electric power system is seriously limited by energy crisis and environment pollution.Therefore,the distributed generation technologies,which use renewable energy,such as wind and solar,as the primary energy have made great progress.With the increasing capacity of distributed generation(DG),the relay protection of traditional distribution network face huge challenges,and the conventional methods can't meet the requirements any more.Exploring the protection strategy of the distribution network with DG plays a significant role in the security and stability operation of power systems.This article takes distribution network of three-phase inverter as the research object,and studies the relay protection scheme to adapt to the distribution network from many aspects,including electrical model?theoretical analysis and simulation experiments and so on.At the beginning,the DG type and its working principle is discusses which is widely used at present,and the electrical model of three-phase inverter distributed generation is built by the characteristics of control strategy of inverter,which provides the theoretical basis and simulation conditions for later research.Next,the structure and protection configuration of the traditional distribution network are introduced.At the same time,the short-circuit current calculation formula of the distribution network with DG is deduced,and the factors affecting the short-circuit current are qualitatively analyzed.The simulation shows that the existence of the DG has the effect of shunting and boosting the short-circuit current,and the short-circuit current is affected by factors such as the fault point and the power supply capacity,so that the protection method is invalid,the cooperation of the protections is lost,the protection maybe refuse to move or malfunction.Then,the conventional power directional elements can not meet the relay protection requirements of DG distribution networks is discussed,and a direction pilot protection based on power-frequency polarized current method using current traveling wave comparison is presented.By analyzing the output current characteristics of DG at different fault locations and comparing the amplitude of fault current component on both sides of line and DG under different grid-connected conditions,the fault component extraction algorithm in traveling wave protection is proved Network applicability.The proposed scheme takes the pre-fault current as the polarized current and obtains the polarity relation by calculating the waveform correlation of the pre-fault current and the initial traveling wave reconstructed by the S-transform filter.Combining the power,the fault direction can be judged.The simulation results show that the proposed scheme is independent of the initial fault angle,transition resistance,location of grid point and noise,and it can judge the fault direction without extracting high-frequency transient signals,so as to realize directional pilot protection.At last,a wide-area backup protection algorithm based on nodes classifying searching is proposed.In order to cope with the flexible distribution network,the different information matrix is defined and updated in real time,calculated and amended,from which can determine the fault location and get backup protection node set,and then according to the search order allocate the time to achieve the cooperation between the protection.It is shown that the scheme can give a quick and effective tripping scheme,which not only can reduce the requirements of the communication system,but also does not depend on the setting current.And this scheme is very suitable for the protection of the distribution network with DG.The application of DG is the main direction of the future development of intelligent electric grid and power system.The proposed scheme provides a valuable theoretical reference for the protection of distribution network with DG,and creates a great circumstance for the further development of distributed generation technology. |
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