Abstract-Due to the increasing penetration level of microgrids (MGs), it becomes a critical issue for MGs to help sustaining power system stability. Therefore, ancillary services, such as the low-voltage ride-through (LVRT) capability should be incorporated in MGs in order to guarantee stable operation of the utility grid during grid faults. In this paper, a LVRT control strategy based on positive/negative sequence droop control is proposed for grid-interactive MGs to ride-through voltage sags with not only inductive/resistive, but also complex line impedance. By using the proposed control strategy, MGs can support the grid voltage, make profits, and also ride-through the voltage dip during the whole fault period. A two layer hierarchical control strategy is proposed in this paper. The primary controller consists of voltage and current inner loops, a conventional droop control and a virtual impedance loop, while the secondary controller is based on a positive/negative sequence droop scheme which is able to coordinate the power injection during voltage sags. Experimental results are obtained to verify the effectiveness of the proposed control strategy.Index Terms-Grid-interactive microgrid, hierarchical control, low-voltage ride-through, negative sequence droop control.

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The silicon carbide (SiC) MOSFET is characterized by high operating voltage, temperature, switching frequency and efficiency which enables a converter to achieve high power density. However, at high switching frequency, the crosstalk phenomenon occurs when the gate voltage spike introduced by high dv/dt and voltage ringing forces false turn-on of SiC MOSFET which causes a crow-bar current thereby increasing switching losses. In order to increase the immunity against the crosstalk phenomenon in a half-bridge configuration, this paper presents a gate driver for SiC MOSFET capable of generating the negative turn-off voltage without using a negative power supply. In addition, the effect of parasitic inductances on the switching response is analyzed and an RC snubber is designed using high-frequency based circuit reduction technique to dampen the switching ringing. The performance of the proposed gate driver and the designed RC snubber is validated using simulation and experiment at the 1 MHz switching frequency. The results show that the proposed gate driver with RC snubber eliminates crosstalk by maintaining any spurious gate spike below the gate threshold voltage.

In this paper, a control strategy with low bandwidth communications for paralleled three-phase inverters is proposed to achieve satisfactory voltage unbalance compensation. The proposed control algorithm mainly consists of voltage/current inner loop controllers, a droop controller, a selective virtual impedance loop, and an unbalance compensator. The inner loop controllers are based on the stationary reference frame to better mitigate the voltage distortion under nonlinear loads. Droop control and selective virtual impedance loop achieve accurate current-sharing when supplying both linear and nonlinear loads. Moreover, by adjusting voltage references according to the amplitude of the negative sequence voltage, the unbalance factor, which is mainly caused by single phase generators/loads, can be mitigated to an extremely low value. Finally, an AC microgrid which includes three three-phase three-leg inverters was tested in order to validate the proposed control strategy.