Study On Monitoring And Protection System Of DC Micro-Grid

With the global economy development, the demands of energy growth rapidly, especially the demand for electricity, causing severe environmental problems. Under the pressure of energy demands and environmental protection, distributed generation emerged. In order to maximize the value of distributed generation, the concept of micro-grid has become a hot research topic of the moment. DC micro-grid has the advantages of less transform links, simpler control and easier access for distributed power. With the continuous innovation of power electronics technology, the development of DC micro-grid has been receiving spotlights. Basing on DC micro-grid, the monitoring and protection system were discussed, providing technical support for the promotion of DC micro-grid engineering applications.Taking a DC ring system of400V voltage rate and80kW capacity as an example, the article analyzes the networking mode, components and operation characteristics. This system was connected by bidirectional DC/DC converter, which makes the system has clear advantages in flow control, system protection and reliability. The article details the various components of the model selection, meanwhile, the strategy of system control, operation mode, standby mode and fault diagnosis are presented. Basing on the structure and operation characteristics of the DC micro-grid, the researches of monitoring methods and protection strategies are carried out.According to the needs of protection, the voltage, amplitude and direction of current, power, temperature, battery SOC (State of Charge) and other parameters are necessary to be real-time monitored. The structure and equipments of the monitoring system were introduced, meanwhile, the corresponding hardware circuit were designed. A bunk communication system was applied, which adopts RS-485as underlying communication and industrial Ethernet communication as upper communication, not only improving the anti-interference ability, but also addressing the communication problems brought by long distance communication. Battery SOC is an important parameter for battery management and protection. An improved ampere-hour integral method for monitoring the SOC of storage device was presented, increasing the accuracy of SOC estimation effectively.Combining with the operation mode of the DC micro-grid, basing on the characteristics of DC current direction, a protection scheme with directional current protection strategy as main protection, current quick-breaking protection, definite time over-current protection and definite time over-voltage protection as secondary protection, was applied. Besides, thermal overload protection and battery overcharge/overdischarge protection were also taken into account. The protective algorithms of each protection strategy were detailed. The protection scheme with large scope and fast fault diagnosis can effectively protect the full-line of the system selectively.