Study On Control Methods For The Operation Of AC Micro-grid

As energy and environmental issues become increasingly prominent, the development of clean energy has become an important strategic of the world and social sustainable development. Micro-grid can weaken the impact of distributed generation on the grid, making full use of the benefits and value of distributed generation. The control of operational is one of the key technologies of micro-grid, and the flexible operation modes of micro-grid achieve a variety of economic and technical advantages.There are two operation modes in micro-grid: grid-connected mode and islanded mode. The control strategy for micro-grid operation mode will be divided from two levels in this paper to expand the research. For micro-source control strategy, firstly, micro-sources are divided into intermittent source and controlled source, accounting for the stable power output or not. The DC side of the inverter is simplified, focusing on the inverter control strategy, and a mathematical model of the inverter is established. Secondly, the inner current loop controller is designed and three control modes of micro-source inverter are made depth analysis, namely PQ control, droop control, and V-f control. In grid-connected operation mode, by changing the reference power of PQ control, the PCC flow control can be achieved. It is also pointed out that in islanded operation mode, through coordination control of droop control and V-f control, the the stability of voltage and frequency can be achieved, and for the control of droop coefficient is improved so that the droop coefficient of dynamic changes with micro-grid voltage and frequency, improved droop control allows the system to power shortage occurs, the voltage and frequency of the micro-grid smaller decline. Finally, the pre-synchronization control unit is designed and then acted on the droop control and V-f control to achieve the switch between grid-connected operation mode and islanded mode. For the micro-grid multiple power coordinated control strategy, the master-slave control and peer control is analyzed, showing that peer control is simple, reliable, and easy to implement, and when the operation mode is changed, the control of micro-source can remain unchanged. But the peer control can not guarantee the stability of voltage and frequency. Master-slave control can support voltage and frequency of micro-grid, but the control of micro-source will be changed with the variation of operation modes. Therefore, a integrated control method is adopted in this paper by combining the master-slave control and peer control to not only achieve voltage and frequency stability, but also reduce the possibility of switch failure caused by frequently switch of control mode, improving the reliability of the micro-grid operation.By mutual cooperation between the micro-source control strategy and the micro-grid multiple power coordinated control strategy, voltage and frequency stability in the switch between grid-connected mode and islanded mode can be ensured. The results show mutual cooperation control strategy effectively reduces the switching frequency of master micro-source when the operation mode is changed and improves the reliability of the micro-grid operation.