Research On Comprehensive Protection Strategy Of Low Voltage Microgrid

With the increasing demand for energy and higher requirements for environmental protection,the large-scale application of renewable energy has promoted the development of microgrids.With its advantages of flexible power generation,environmental friendliness and high efficiency,the microgrid has become a research hotspot in countries all over the world.However,the fault characteristics of the microgrid in the island operation mode are quite different from those in the grid-connected operation,which makes the traditional relay protection strategy cannot be applicated in microgrids,and it's easy to cause protection malfunction or refuse to trip.Therefore,it is necessary to propose a new protection strategy to meet the development needs of the future microgrid.In this paper,the master-slave control mode of microgrid based on PQ control and V/f control is introduced.The peer-to-peer control mode of microgrid based on droop control is analyzed.The output characteristics and fault characteristics of distributed generator are researched.The low-voltage microgrid is simulated in different operation modes and different fault types by using MATLAB/Simulink software.The influence of different transition resistances on the bus voltage is verified.The difference of fault currents at both ends of the feeder is studied,which paves the way for the next step of designing microgrid protection strategy.This dissertation also proposed a modified low-voltage acceleration inverse-time overcurrent protection(ITOC-LVA).The voltage at relays are corrected by the anti-fault resistance algorithm,and the voltage correction value is used as an acceleration factor to accelerate the action of the inverse-time overcurrent protection.This method can tackle the low fault impedance and be employed in both grid-connected mode and islanded mode without the requirement of communication system.The effects to normal operation of the microgrid of some DGs' offline under the islanded operation mode are analyzed.The adaptive overcurrent protection under different DG access states(ITOC-VDGS)is proposed.The DG access status is sent to the central control unit by the communication network,and the protection parameters are adaptively adjusted to improve the sensitivity and speed of protection.Integrating the two proposed microgrid protection strategies to form a comprehensive protection method.By detecting the abnormality of the bus voltage,the busbar sending blocking-signal criterion is proposed,which effectively solves the problem of selectivity caused by comprehensive protection.By utilizing the self-starting process identification criterion(SSI),the self-starting coefficient can be neglected from operating current of relays,which can enhance the sensitivity performance of the protection method.