For example, power generated as DC current by a photovoltaic panel can be system-interconnected with a commercial AC system via a power conditioner which is a power conversion device. The system interconnection can be performed for not only a single-phase AC system but also a three-phase AC system (for example, see Patent Literature 1 (FIG. 2)).
FIG. 24 is an example of a circuit diagram of a power conversion device used in a case of performing system interconnection from a DC power supply to a three-phase AC system. In FIG. 24, a power conversion device 200 generates AC power based on DC power received from a photovoltaic panel 201 as a DC power supply, and supplies the power to a three-phase AC system 220. The power conversion device 200 includes a capacitor 202, a step-up circuit 203, a smoothing circuit 205 for smoothing the voltage of a DC bus 204, a three-phase inverter circuit 207, and three pairs of AC reactors 208 to 210 and capacitors 211 to 213. The smoothing circuit 205 is formed by connecting two capacitors 206 in series for the purpose of obtaining the withstand voltage property and connecting six sets of such two capacitors 206 in parallel for the purpose of obtaining the capacitance. The capacitance of the smoothing circuit as a whole is several mF, for example.
In this example, the photovoltaic panels 201, the capacitors 202, and the step-up circuits 203 are provided for three systems, and these systems are connected in parallel to the DC bus 204. For example, if input voltage from one photovoltaic panel 201 is DC 200V and the current thereof is 30 A, power of 6 kW per system and power of 18 kW in total can be generated. The line-to-line voltage of the three-phase AC system 220 is 400V.
For the output of the photovoltaic panel 201, the step-up circuit 203 performs maximum power point tracking (MPPT) control to obtain an optimum operating point. The output of the step-up circuit 203 is smoothed by the smoothing circuit 205 having a large capacitance, to become the voltage of the DC bus 204. This voltage is subjected to switching by the three-phase inverter circuit 207, thereby generating three-phase AC voltage including a high-frequency component. The high-frequency component is removed by the AC reactors 208 to 210 and the capacitors 211 to 213, whereby a waveform that allows system interconnection with the three-phase AC system 220 is obtained.
Here, the voltage of the DC bus 204 is required to be equal to or higher than the wave crest value of AC 400V, which is 400×√2, i.e., about 566V, but is set at 600V, considering some margin. In a case where the voltage of the DC bus 204 is 600V, when a switching element in the three-phase inverter circuit 207 is turned off, due to resonance by a floating inductance and the capacitance of the switching element, voltage that greatly exceeds 600V is applied to the switching element. Therefore, in order to reliably prevent insulation breakdown of the switching element, for example, withstand voltage property of 1200V which is twice as high as the voltage of the DC bus is required. In addition, the withstand voltage property of 1200V is also required for the smoothing circuit 205, and in the configuration in FIG. 24, withstand voltage property of 600V is required for each capacitor.