Research On Frequency Control Strategy Under Islanding Mode Of Microgrid

In recent years,the permeability of distributed generation technology based on renewable energy is increasing gradually in power system.Although wind and solar energy resources are clean,and they have unlimited reserves,because of the potential problems such as undulatory property,intermittence and non-dispatching,their access to the power grid will lead to security,stability and power quality problems.The birth of microgrid has effectively solved the contradiction between renewable energy generation technology and power grid.Microgrid is a small power generation and distribution system consisting of various distributed generators,energy storage devices,energy conversion devices,loads,monitors and protectors.It can control,protect and manage itself.It can operate both in the power grid and independently.When the microgrid is connected to the power grid,the voltage and frequency of the microgrid are supported by the power grid,and the relationship between the microgrid and the external power grid is coordinated by reasonably dispatching of micro power sources in the microgrid.When the microgrid operates independently,the voltage deviation and frequency deviation caused by the intermittent energy fluctuation and load fluctuation are regulated by the micro power sources with the capability of voltage regulation and frequency regulation in the microgrid.Operation control is the key to the safety and stability of microgrid operation,and the control of inverters is the key to the control of microgrid.When there are multiple main power sources in the microgrid,coordinated control among them is usually required.The first level control in the independent operation mode of microgrid is mainly researched in this paper,and an improved droop control strategy based on the method of low-pass filtering is proposed.In this paper,a microgrid consisting of wind turbine,photovoltaic cell,diesel generator and battery energy storage system is taken as the research object,and several models of micro power sources are built by MATLAB/Simulink.The influences of temperature and irradiance on the output characteristics of photovoltaic cell and the influences of pitch angle and wind speed on the output characteristics of wind turbine are discussed.A maximum power point tracking circuit based on perturbation observation method is built.Then the starting characteristics of diesel generator are simulated.Then the battery energy storage system and bidirectional DC-DC circuit are built to ensure that the voltage of DC bus remains constant during charging and discharging of battery.For the current-controlled inverters of voltage source type,three kinds of control technologies of inverters are mainly introduced,including V/f control,droop control and voltage of DC bus control,and the corresponding controllers are designed.The effectiveness of the three controllers is verified by simulation examples.Finally,the performances of the master-slave control based on V/f control and the peerto-peer control based on droop control are simulated and compared in the case of independent operation of microgrid.In traditional microgrid with two droop controllers,the high frequency components in system frequency obtained by the method of low-pass filtering are used to feedback control.By moving the droop curve in parallel,the operating points of the two main power sources move.This method has a certain reference value for the follow-up research of frequency control in independent operation mode of microgrid.