Voltage fluctuations resulting from variable output power of renewable energy sources are strictly challenging power quality in distributed generation systems. This paper presents a control method for Distributed-STATCOM (D-STATCOM) to alleviate variation of both positive-and negative-sequence voltages. The D-STATCOM simultaneously operates as fundamental positive-sequence admittance and fundamental negativesequence conductance to restore the positive-sequence voltage to the nominal value as well as reduce the negative-sequence voltage to an allowable level. Both admittance and conductance are dynamically tuned to improve voltage-regulation performances in response to load changes and power variation of renewable sources. A proportional-resonant current regulator with selectively harmonic compensation is realized to control the fundamental current of the D-STATCOM as well as reduce the harmonic current, which could be an advantage in practical applications due to high voltage distortion in low-voltage Microgrids. Voltage-regulation performances are discussed for different D-STATCOM location as well as different D-STATCOM current. Computer simulations and laboratory tests validate effectiveness.

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This paper presents a seamless transition method for droop-controlled inverters to operate in both islanding and grid-connected modes. A local PLL and a virtual inductance are designed to ride through transient when the inverter switches between two modes with no synchronization. The proposed method can cooperatively work with well-developed droop controls so that the inverters are able to share load among them as well as subsist under transient events of the utility. Theoretical analysis and experimental results validate effectiveness of the proposed method. I.

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