Virtual Oscillator Control (VOC) is a type of nonlinear grid-forming controller for DC/AC inverters. Compared with the droop control method, the original VOC has a faster transient response in islanded mode. However, its output voltage always contains a third-order harmonic, resulting in a significant third-harmonic current, especially in the grid-connected mode. In order to eliminate this harmonic, a notch-type filter is typically employed, but it affects the synchronization speed of multiple DC/AC inverters connected in parallel in islanded mode. In this paper, by analyzing the nonlinear dynamical equations of the oscillator and simplifying its nonlinear current source, a novel VOC for three-phase DC/AC inverters is proposed, which can successfully eliminate the third-order harmonic in the output voltage of the oscillator. Further, compared with the traditional VOC with notch filter, the grid-synchronization response of the proposed VOC-based DC/AC inverters can be significantly improved. A 3 kW/3phase/120 V experimental prototype system designed on the DSPACE DS1202 platform has been developed to verify the feasibility of the proposed control strategy.
In islanded microgrids, distributed generators (DGs) are employed as distributed compensators to improve the power quality on the load side. Due to the access of unbalanced loads, the low-voltage microgrid will naturally exhibit three-phase unbalance, which may cause additional power losses and deteriorate power quality. Therefore, this paper proposes a novel unbalanced-voltage compensation control method based on the Hopf oscillator, which introduces the negative-sequence voltage and current into the traditional Hopf oscillator to achieve the negative sequence voltage droop characteristics. It can not only realize the negative sequence droop control of a single inverter, but it is also capable to support the reactive power automatic distribution between multi-parallel grid-forming inverters. Compared with the traditional unbalanced voltage compensation method based on filtered negative sequence reactive power, the proposed unbalanced voltage compensation method can slightly improve the dynamic response. Experimental results verify the effectiveness of the proposed control strategy.
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