Grid-Connected Control Strategy of VSG under Complex Grid Voltage Conditions

Abstract

Under complex grid conditions, the grid voltage usually has an imbalance, low order harmonics, and a small of DC bias. When the grid voltage contains low order harmonics and a small amount of DC bias component, the inverter's output current cannot meet the grid connection requirements, and there is a three-phase current imbalance in the control strategy of common VSG under unbalanced voltage. A theoretical analysis of non-ideal power grids is carried out, and a VSG control strategy under complex operating conditions is proposed. Firstly, the third-order generalized integrator (TOGI) is used to eliminate the influence of the DC component of grid voltage. An improved delay signal cancellation (DSC) method is proposed to control the balance current and power fluctuation under unbalanced voltage based on the method of common VSG positive and negative sequence separation, It also eliminates the harmonic of command current. Then, the improved quasi proportional resonant controller (QPR) cascaded PI is used to suppress the harmonic current further so that the harmonic content of grid-connected current can meet the grid-connected requirements and achieve the three-phase current balance. Finally, the proposed strategy is verified by simulation under the control objectives of the current balance, active power, and reactive power constant.