Abstract: A power conversion system including: a self-excited power converter that converts power between an alternating-current (AC) power system and a direct-current (DC) line; an AC circuit breaker disposed between the AC power system and the self-excited power converter; an arrester connected between the DC line and a ground or between an AC line and the ground, the AC line connecting the AC circuit breaker and the self-excited power converter; and a control device. The control device causes the self-excited power converter to stop a switching operation when the control device detects a ground fault accident on the DC line, and opens the AC circuit breaker after the self-excited power converter stops the switching operation.

Abstract: A multilevel converter includes a first arm connected between a positive voltage terminal and an alternating-current terminal and a second arm connected between the alternating-current terminal and a negative voltage terminal. Each of the first and second arms includes a plurality of cascaded unit cells. Each unit cell has a capacitor charged to a direct-current voltage and outputs a voltage across terminals of the capacitor or 0 V. The plurality of unit cells as being helically cascaded implement a reactor.

Abstract: Each phase arm of a power conversion device includes at least one converter cell connected in series. For each converter cell, an element driving unit is provided which turns on one switching element in the converter cell as a startup element. The element driving unit is supplied with power from a DC capacitor, and when voltage of the DC capacitor exceeds startup voltage Vsh, turns on the startup element. Thus, at the time of startup of the power conversion device, the DC capacitor in each converter cell is initially charged to desired voltage.

Abstract: A power generation system is provided in which, when a static frequency converter (SFC) is connected to synchronous generator's armature windings, an AC exciter performs AC excitation by allowing a d-axis winding and a q-axis winding of the AC exciter to configure d-q orthogonal axes; and, at the time of steady-state operation of the synchronous generator, an alternating current(s) supplied from an electric power source is rectified by a thyristor excitation device, and also the AC exciter thereby performs DC excitation by connecting the d-axis winding and the q-axis winding in series with each other.

Abstract: An air conditioner includes a compressor; an outdoor heat exchanger; an indoor heat exchanger; an outdoor fan configured to supply outdoor air to the outdoor heat exchanger; and an indoor fan configured to supply indoor air to the indoor heat exchanger. The air conditioner performs a heating operation by driving both the indoor fan and the outdoor fan and by causing refrigerant to flow in one direction through both the indoor heat exchanger and the outdoor heat exchanger and so that a defrost operation in which both the indoor fan and the outdoor fan are stopped is performed by causing the refrigerant to flow in the opposite direction to that in the heating operation. When a defrost failure is caused by the defrost operation the outdoor fan is driven, and the indoor fan is stopped for a predetermined period of time, after which the defrost operation is resumed.

Abstract: A multilevel converter includes a first arm connected between a positive voltage terminal and an alternating-current terminal and a second arm connected between the alternating-current terminal and a negative voltage terminal. Each of the first and second arms includes a plurality of cascaded unit cells. Each unit cell has a capacitor charged to a direct-current voltage and outputs a voltage across terminals of the capacitor or 0 V. The plurality of unit cells as being helically cascaded implement a reactor.

Abstract: A power generation system is provided in which, when a static frequency converter (SFC) is connected to synchronous generator's armature windings, an AC exciter performs AC excitation by allowing a d-axis winding and a q-axis winding of the AC exciter to configure d-q orthogonal axes; and, at the time of steady-state operation of the synchronous generator, an alternating current(s) supplied from an electric power source is rectified by a thyristor excitation device, and also the AC exciter thereby performs DC excitation by connecting the d-axis winding and the q-axis winding in series with each other.

Abstract: Each phase arm of a power conversion device includes at least one converter cell connected in series. For each converter cell, an element driving unit is provided which turns on one switching element in the converter cell as a startup element. The element driving unit is supplied with power from a DC capacitor, and when voltage of the DC capacitor exceeds startup voltage Vsh, turns on the startup element. Thus, at the time of startup of the power conversion device, the DC capacitor in each converter cell is initially charged to desired voltage.

Abstract: An air conditioner (1) includes: a compressor (21); an outdoor heat exchanger (23); an indoor heat exchanger (13); an outdoor fan (25); and an indoor fan (15). In a case where the outdoor heat exchanger (23) has frost, the indoor fan (15) and the outdoor fan (25) are stopped, and a refrigerant is flowed in a direction opposite to a warming operation so as to perform a defrosting operation. In a case where a temperature of the refrigerant output from the compressor (21) declines below a predetermined temperature during the defrosting operation, it is determined to be defective defrosting. In a case of the defective defrosting by the defrosting operation, the outdoor fan (25) is driven, the indoor fan (15) is stopped and the refrigerant is flowed in a same direction as the warming operation so as to perform a defrosting preparation operation. Thereafter, the defrosting operation is resumed.