CONTROL OF PARALLEL CONNECTED INVERTERS IN STANDALONE AC SUPPLY-SYSTEMS

A scheme for controlling parallel-connected inverters in a standalone ac supply system is presented in this paper. This scheme is suitable for control of inverters in distributed source environments such as in isolated ac systems, large and distributed uninterruptible power supply (UPS) systems, photovoltaic systems connected to ac grids, and low-voltage dc power transmission meshes. A key feature of the control scheme is that it uses feedback of only those variables that can be. measured locally at the inverter and does not need communication of control signals between the inverters. This is essential for the operation of large ac systems, where distances between inverters make communication impractical. It is also important in high-reliability UPS systems where system operation can be maintained in the face of a communication breakdown. Real and reactive power sharing between inverters can be achieved by controlling two independent quantities-the power angle, and the fundamental inverter voltage magnitude. Simulation results obtained with the control scheme are also presented.