Study On Over-Current Protection Scheme Of Distribution System With Microgrid

Because of the interconnection of microgrid, the topology of distribution network has changed from the traditional simple single-source radial-network to complicated multi-source loop-network. Meanwhile, the features such as power How distribution and the level and direction of fault current have also changed, which cause the current protection of distribution network refusing operation or misoperation, making the original current protection can not meet the requirements of relay protection after microgrid access. So it is necessary to study the relay protection scheme of the distribution network including microgrid to ensure the sale and stable operation and the reliability of power supply of the distribution network.Firstly, the fault characteristics of inverter-based distributed generation using PQ control are analyzed in this article. The distributed generations in microgrid are mainly inverter-based distributed generation, which rely on the device of power electronics transformation such as DC-AC or AC-DC-AC transformation to complete energy conversion. When the microgrid is under parallel operation, the distributed generations usually adopt PQ control and output constant active power and reactive power. The fault features of distributed generation are greatly relate to the types of distributed generation and control mode. Therefore, this article first analyzes the fault characteristics of inverter-based distributed generation using PQ control by simulation experiment, providing the reference for studying the impact on the current protection of distribution network. Simulation results show that the inverter-based distributed generation with PQ control is able to maintain constant output power according to the power reference value for remote fault. The output voltage of the DG is reduced and the output current is increased. The output fault current of the DG may exceed the endurance capacity of the inverter electronic components for close-in fault. Thus the inverter-based distributed generation could not work normally and be out of service.Secondly, in this article, the influences of microgrid access on the power flow distribution, current protection and automatic reclosing of distribution network are analyzed in the simulation.The simulation model of distribution network containing microgrid is established in PSCAD/RMTDC, the influence of microgrid access on the power flow distribution and the influence of different fault location and the different microgrid capacities on the current protection and automatic reclosing of distribution network are analyzed by the simulation experiment. Simulation results indicate that when the microgrid access to the distribution network, the topology of the distribution network is changed, the power flow distribution is different, even bi-directional power flow may occur. In the fault condition the access of mierogrid will induce that the level and direction of the fault current changes, leading to the current protection of distribution network refusing operation or misoperation and automatic reelosing failure. The original current protection of distribution network can not meet the requirements of relay protection. and the greater the mierogrid network capacity is, the more serious the impact degree is. In addition, the influence on the power flow distribution. current protection and automatic reclosing can be reduced by improving the matching degree of the output power and the internal load of the mierogrid.Finally, an improved pilot overeurrent protection scheme of closed-loop operation distribution network with mierogrid is designed. According to the shortcomings and the insufficiency of the current pilot overeurrent protection of distribution network containing mierogrid proposed in the literature, an improved pilot overeurrent protection scheme is designed to overcome the shortcomings. The principle and configuration of the protection scheme are illustrated and the logic diagram of the protection scheme is designed. The simulation model of elosed-loop operation distribution network with mierogrid is established in PSCAD/EMTDC to validate the effectiveness of the protection scheme. Simulation results show that the improved pilot overeurrent protection scheme can rapidly and reliably clear the fault to make up for the shortcomings of the protection scheme in the literature. The designed protection scheme is of good adaptability for the connection of mierogrid and suitable for the distribution network operate under the open-loop mode. The proposed protection scheme has a good application prospect.