Abstract: A grid-connected inverter includes first and second power conversion circuits, a contactor and a control circuit. The first conversion circuit converts a first DC voltage to a second DC voltage. The second conversion circuit converts the second DC voltage to an AC voltage. The contactor connects an output side of the second conversion circuit to a power system. The control circuit includes a decision circuit and controls start and stop operations of the conversion circuits, and opening and closing of the contactor. The decision circuit decides whether a condition of the contactor is abnormal by detecting, after the control circuit controls the contactor to be open, whether or not a value of the second DC voltage is less than a threshold value, and if the value of the second DC voltage is detected to be not less than the threshold value, decides that the condition of the contactor is abnormal.

Abstract: In a unit inverter system where multiple unit inverters are connected in parallel, the quantity of operating unit inverters is determined in accordance with an amount of power to be converted. A gate signal of a semiconductor switching element of a unit inverter is turned off after an output current of the inverter is reduced when reducing the quantity of inverter units, thereby improving the partial load efficiency of the system without an adverse effect on the system. A regulator connected to the inverter determines dead time of the inverter according to the output current value and an average output current value of the unit inverters, waits for the determined dead time so as to reduce the output current of the unit inverter, and then turns off the gate signal.

Abstract: During single operation of a distributed power supply system that has been disconnected from a commercial electric power system, a frequency increase monitoring circuit is operated and an instruction to output a larger amount of a constant reactive current is given to a reactive current controlling unit. After the output frequency of the distributed power supply exceeds a frequency increase level, the level of an active current is limited in accordance with the level of the outputted reactive current.