Distributed generators such as photovoltaic, wind turbine and gas cogeneration system have been paid more attention. Distributed generators are often connected to a grid by inverters. Inverters used in distributed generators are generally controlled by a PLL (Phase Locked Loop) in order to be synchronized with power system frequency and phase. The power system will become more unstable if the capacity of inverter-connected type distributed generators becomes larger and larger, because inverter frequency is controlled just to follow the frequency determined by other synchronous generators. The concept of “Virtual Synchronous Generator” (VSG) which is to control inverters to behave like a real synchronous generator has been proposed. VSG has virtual inertia which is realized by a small energy storage device to mimic rotor''s inertia and obtain synchronizing power. So far, an active power control scheme of VSG which is based on the swing equation of a synchronous generator has been employed in previous studies. However, a reactive power control method for VSG has not been mentioned. In this paper, a droop control for keeping sending end voltage of inverters constant is proposed. A method for sharing reactive power proportional to their capacity is also presented. Applying these methods to the VSG control inverters, equitable sharing of active and reactive power is realized. These methods are verified through circuit simulation. Moreover, experiments are conducted using 10 kW level inverters and simulated transmission lines.
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