Analysis And Research Of Microgrid Protection

Protection of microgrid is one of the most challenging and crucial problems that should to be solved.Distributed generations,loads and control systems are introduced in this paper.Based on the research of characters and problems of microgrid, A microgrid is simulated and protections are classified properly while the protection of the line and the load is the focus of the study.The main jobs of the thesis are as following.The existing protections cannot function properly for the characteristics of microgrid.These characteristics are:power electronics interfaced distributed generation cannot supply large fault current;plug-and-play distributed generation may reduce the sensitivity of protection,lead to the malfunction of protection and expand or narrow the scope of protection;protection should function properly whether the microgrid is grid-connected or islanded;control approaches of a distributed generation affects the transients after faults;ground system of a microgrid affects the level of the fault current;a fault should cleared timely for the power electronic devices in a microgrid are lack of inertia and response fastly.Improved distance protection which will not be affected by the current-limit characteristic is used to protect the line of a microgrid. Traditional distance protection principle was used in zone Ⅰ, Setting value is calculated in accordance with the escape of measured impedance when a line teminal short-circuit fault.The affection of the plug-and-play generation is considered when calculate settings in zone Ⅱ.Actual impedance can be obtained by calculating the output current of the distributed generation and the measured current.For the output current and the location of the distributed generation are unknown,measured impedance can not accurately reflect the actual impedance.The maximum output current can be obtained by make provisions for the parameters of the generation in relevant microgrid criteria. The calculating of the seting value in zone Ⅱ followed the principle of the most unfavorable when the generation is located at the point of protection installation.In order to avoid malfuntion,protection zone Ⅱ is compatible with the following line in time setting and operate after some time delay.Simulation in PSCAD environment proved that when protection zone Ⅰ failed to operate for the connection of the plug-and-play generation,protection zone Ⅱ operate after several seconds delay.The sum of each phase current is adopted as the fault criterion in the protection of the load and the essence is the detection of the leakage current.Lines in microgrid can be adjusted reasonably to facilitate the extraction of the current.The method is able to reflect the ground faults the amount of which account for a large percent.The method response fastly and will not be affected by control systems. Peer-to-peer microgrid is simulated and the generations are based on droop control principle.Voltage loop controller and current loop controller are adopted to improve the quality of the output power. The protection of the load was proved to be valid in an islanded microgrid for faults were simulated.