Research Of The Key Technologies Of Control And Protection For Microgrid Connected To Distributed Network

As an environment-friendly, efficient and economical power generation technology, the distributed generation has been more and more attention. However, the steady impact to the traditional distribution network becomes serious go along with permeability of grid-connected distributed generation (DG) increases. With the development of power industry technology, the microgrid which is an intelligent and efficient distributed energy management has been proposed and becomes an important developing direction in power technology. Based on advanced information control technology and the power technology, the microgrid is composed of integrates energy storage, control and protection operates. It encourages users to take part in the electricity market. Besides, it plays an important role in promoting the efficient of new energy and the sustainable development. But, being a new energy management control technology, the microgrid has to overcome complexity problems of control and protection. Its impact to the traditional power system is also yet to be studied and solved. In this paper, the unit model of DG was built in PSCAD/EMTDC simulation environment. And the microgrid management and control model was build further. Several key issues based on this model were studied and explored.Firstly, the DG model based on three-phase voltage inverter was modelled. Its mathematical model, control theories and methods were researched in detail. To satisfy the needs of the power control, the amplitude and frequency control method for the power decoupling control of the DG was used. Further, the LCL filter was employed to suppress high harmonic. The mathematical model and design principles of the LCL filter were studied. Simulation results showed the LCL filter could effectively eliminate the DG's current harmonics and improved power quality。The DG based on the amplitude and frequency control worked reliable with good performance in the power decoupling control. Its dynamic response speed met the engineering requirements.Secondly, the "peer to peer" and "play and plug" property of microgrid were researched. The wired and wireless control strateg were compared to each other. Then, the master-slave parallel method belonged in to the wired control scheme was employed. An energy management was used in the islanding to keep power balance and maintain the voltage and frequency. The master DG used V/f control method to maintain voltage and frequency and to assume power load switching. The slave DG used the constant active and reactive power control. The response of load switching and power control changing were simulated. Simulation results corroborated the effectiveness of the master-slave control strategy. In addition, the model switch strategy of the microgrid was analysed. And an effective seamless switching control scheme was provided. Simulation results showed the proposed scheme is able to achieve switching seamless.Then, to ensure the microgrid could disconnect quickly from the grid when the fault occured, the anti-islanding protection strategy was researched. The simulation results showed the DG based on amplitude and frequency control performed well in anti-islanding. To further reduce the non-detection zone and detection failure problems caused by the power matching, a novel and easy-to-implement islanding detection method using the positive feedback disturbance of the active power was introduced. The synchronization perturbation could be realized without communication for multiple DGs to achieve rapid islanding detection with negligible NDZ. So, the proposed method could improve the anti-islanding ability of microgrid.Finally, considering the microgrid of different capacities and different position, the impact to selectivity, sensitivity and reliability of over-current protection scheme was studied. Different faults in the distribution network was analysed and simulated. It pointed out that the impact to the over-current protection could be reduced if the power matched in the microgrid. In addition, the DG model based on PQ control, PV control and constant current control method of different fault feature were established. Simulation results pointed out that the control scheme of DG needed to be considered while to transform the over-current protection. In the end, the transformation scheme for the distribution network protection considering the connected microgrid was studied.