Study On Relay Protection Of Distribution System With Distributed Generations

An approach to apply the new energy to generate electricity, the new energy distributed generation can reconcile the contradictory between the economic development and the environmental protection efficiently, as well as improve the power supply reliability after all. With growing number of the new energy distributed generations connected with the distribution system, some problems come up regarding the overcurrent relay protection. This paper discusses the main issues of the overcurent protection with the connection of the distributed generations, and presents the solutions to the problems, including the discussion on the short circuit current calculation and the relay protection when taking the distributed generation into account.The fault analysis model of the invert-interfaced renewable energy source is different from the traditional generation. Firstly, failure analysis model of the invert-interfaced renewable energy sources is discussed.The controlled current source under different control conditions is presented as the simplified fault analysis model after analyzing the fault current characteristics of the invert-interfaced generations with the common double-loop constant power current-mode control strategy. Moreover, short circuit current simplified model calculation formula is deduced to provide for the setting calcution of the relay protection.With the connection of the distributed generations, the fault current level and direction is changed, affecting the reliability of the corresponding relay protection. The paper theoretically analyzes the special problems regarding to the relay protections of different positions and discusses the main issues of the relay protection system considering different connection manners by the simulation methods, with the corresponding proposal for the improved protection system put forward.The proposal for improved protection system is proposed as follows.(1) For the situation of distributed generation connected to the feeder line, the scheme of restricting the penetration level is proposed, with the calculation method of the maximum penetration level and the cooperation program of the line protection and the DG protection presented afterwards.(2) In the case of a single invert-interfaced generation connected to bus, an improved overcurrent protection is proposed, with the added lowvolage reverse direction protection and the improved setting calculation method considering the invert-interfaced generation.(3) Considering the flexible network topology and operation mode caused by the different types of DGs connected to the system, a novel distance protection scheme is proposed,combining the instantaneous distance protection with inverse time distance relay protection, to ensure the effective fault isolation and the connection of the DGs.In addition, an improved pilot protection is presented to deal with the cooperation problem between the long line and the short line.The two kinds of protection schemes mentioned above is simple, practical and reliable with less information needed, which meet the present requirements of the protection of the distribution system with DGs connected.