Control And Operation Characteristic Analysis Of A MicroGrid

A MicroGrid is a low voltage network with different micro generators (micro sources) and loads operating to supply electric power for the local area. It can operate in grid-connected or islanded, which has the ability to enhance the reliability of electrical energy supply. Most of micro sources in a MicroGrid are interfaced through power electronic converters. The interfaces of converters increase the flexibility of control but reduce the inertia of the MicroGrid. Lack of inertia and two operation modes pose difficulties in maintaining a power balance between generation and consumption and controlling the network frequency of an islanded MicroGrid. The connection of different types of micro sources to form a MicroGrid is an important task. This thesis concentrates on a control strategy and operation characteristics of an inverter-interfaced MicroGrid. The main jobs of the thesis are as the following.Firstly, the various control techniques and different control approaches in a MicroGrid used to date are summarised and comparied.Secondly, the frequency stability of a MicroGrid, in which the frequency is regulated by different micro sources using P-f & Q-V droop control,was analyzed. Considering the decentralization and the capacity difference of micro sources in a MicroGrid, how to choose the right droop gains is investigated when different micro sources use P-f&Q-V droop controllers. Then the frequency stability of a grid-connected and an islanded MicroGrid is investigated based on the right droop gains. The differences between the frequency stability of a grid-connected MicroGrid and the power angle stability of the conventional generator are analyzed. Moreover, a small-signal state-space model is used to explore the frequency stability of an islanded MicroGrid in which the frequency is regulated by different micro sources using P-f&Q-V droop controllers. The eigenvalues are calculated when the load resistance and impedance, the equivalent line impedance and the droop gains change. And the factors that induce the frequency unstable are analysed.Thirdly, the control scheme of inverter-interfaced MicroGrid based on the peer-to-peer and master-slave is designed considering the generation characteristics of micro sources and the characteristics of the load power demand. A multi-loop feedback controller is obtained to control the micro sources that are controlled by the P-f&Q-V droop. The outer loop of the multi-loop feedback controller is P-f&Q-V droop controller. The inner voltage controller and current controller insure the output voltage of the inverter of the micro source equal to the output voltage of the outer P-f&Q-V droop controller by reducing the effect of the disturbances of loads or other micro sources. The closed-loop output impedance of the interfaced inverter can be inductive by designing the parameters of the inner controller that can decouple the transferred active power and reactive power. Difference PQ controllers for the interface of current source inverter and the voltage source inverter, are designed to control the output power of the micro sources equal to the reference power.Finally, the transient characteristics of the MicroGrid are investigated in different operation conditions based on the control scheme. The impact of induction machine loads on the transient stability of the MicroGrid, and the transient response of the intentional islanding, the disconnection of a micro source, the output power change of the micro source using PQ control, the change of load power and the connection with the main grid, are shown in this thesis. The impact of different types of faults and the different locations of faults on the transient stability of the MicroGrid are also investigated.