Study of control strategy of DG based on nonlinear droop characteristic

Big droop coefficients tend to reduce stability of micro-grid, while small droop coefficients result in slow responding of inverter. In order to overcome the disadvantage, here a kind of droop control strategy based on nonlinear droop characteristic similar to that of magnetic hysteresis is proposed, through real-time changing droop gain appropriately to improve load power distribution accuracy, to effectively suppress the wide fluctuations of frequency and voltage, and to improve stability of the system. At the same time, a feed forward path is added on the power droop block to further improve the stability. Then the small-signal mathematical model of micro-grid with inverter is developed. Only the outer power loop of the inverter is considered. Each DG units is modeled in state-space form on own reference frame and all are combined together on a common reference frame by network equations. The complete model is linearised around an operating point, resulting in the system matrix. The effect of the droop coefficient and the feed forward path on the small disturbance stability is analysed. The time domain simulation is conducted, in the PSCAD/EMTDC environment. The results verify the effectiveness of the control strategy.