Utility interactive power conditioning systems islanding problem solutions

This dissertation deals with the islanding problem of single-phase, self-commutated utility interactive inverters with the direct feedback loops. The objectives of the study and research are to get a complete understanding of the islanding problem and to find a solution for the problem. That is to give the utility interactive inverters the ability of preventing the is landing operation mode.; In this dissertation, the modeling and simulation work for the utility interactive inverter with the direct feedback loops has been considered. The inverter's closed-loop transfer function and frequency responses in different situations have been established. The dynamic characteristics of inverters in the islanding mode have been discussed in some detail.; Two solutions to the islanding problem are introduced in this dissertation. The first solution consists of modifying the feedback and control system of the inverters with direct feedback loops. The inverters after modification have a stable, unique output frequency in the normal operation mode. After the utility power sources are separated from the distribution line and the islanding occurs, the inverter output frequency will be removed from the nominal frequency and be out of the limit rapidly, then the built-in under/over frequency protection system will shut off the inverter.; The second solution is called differential impedance protection. Before and after the continuous islanding occurs, the values of inverter output voltage, current and frequency only change a little, but the ratio dV/dI, called differential impedance, will change significantly. The detailed analytical verification is given. A circuit to detect the differential impedance change has been designed. The principle of circuitry is introduced, too.