Advanced control techniques for parallel inverter operation without control interconnections

In order to ensure true redundancy, expandability and good reliability of a power supply system, a distributed Uninterruptible Power Supply (UPS) system is required. A distributed UPS system can be realized by connecting many inverters in parallel. A technically challenging aspect of such a system is to develop a flexible control technique for proper load sharing among the parallel connected inverters without control interconnection and in the presence of unit to unit variations and the line impedance imbalances. The primary goal of this research is to develop such a new control technique to achieve a proper load sharing in a distributed fashion.; Existing control techniques to operate inverters in parallel either require some form of control interconnections or do not guarantee the proper sharing of the reactive and distortion power. The interconnecting lines restrict the location of the inverters, and they are sources of noise and failure. In order to obtain a truly distributed power supply system with reliability, redundancy and expandability, these control interconnections will have to be removed.; In this research, a new concept of sharing the reactive and distortion power has been developed. Two small signals, at a frequency other than the power frequency and its harmonics, are injected for the reactive and distortion power sharing in the power supply system. This technique allows proper sharing of both linear and nonlinear load among the inverters without any control interconnections. The technique can also automatically compensate for the differences between the units and the effect of line impedance. Techniques are also developed to enable the units to self-synchronize with the ac bus before connection. A unique method to handle the overload conditions and the constraints on the voltage adjustment band has been developed. In addition, efficient ways of calculating the reactive and distortion power have been developed. The proposed techniques were implemented in a fixed point DSP and were found to work well in a prototype system consisting of two commercial single-phase inverters under various operating conditions.