1. Field of the Invention
The present invention relates to a power conditioner for converting a direct current power generated by direct current power sources, such as a thin film solar cell and a fuel cell, into an alternating current power for utility interconnection, and a solar photovoltaic power generation system in which the power conditioner is used.
2. Description of the Related Art
In perspective of global environment protection, there are proactively ongoing projects in recent years for developing a power generation system using a solar cell attracting attention as an inexhaustible clean energy. In an example of the power generation system, a direct current power generated by the solar cell is converted by a power conditioner into an alternating current power having a commercial frequency for utility interconnection so that the converted alternating current power is supplied to a household load connected to a commercial power system, and surplus power, if the alternating current power exceeds power consumption by the household load, is returned to the system as a reverse power flow.
The power conditioner used in the solar photovoltaic power generation system conventionally includes an inverter for converting the direct current power generated by the solar cell into the alternating current, and a protection device for utility interconnection. There are two types of devices as such a power conditioner; an insulator device wherein a direct current unit and an alternating current unit are electrically insulated from each other by an insulation transformer, and a non-insulated type device wherein the insulation transformer is not used. Comparing these two different devices, the latter non-insulated type device, which is more advantageous in its power conversion efficiency, is more often used. An example of the latter device is recited in Japanese Patent Cited Document 1 (Unexamined Japanese Patent Publication No. 2002-10496).
FIG. 20 illustrates an example of a solar photovoltaic power generation system provided with a non-insulated type power conditioner. A power conditioner 36 illustrated in the drawing is operable for utility interconnection with a commercial power source 2. The power conditioner 36 is provided with a smoothing capacitor 33 for smoothing power generated by and outputted from a solar photovoltaic panel 1, an inverter 34 for PWM control, a filter 35 having a reactor and a capacitor, and a control circuit not shown.
In the power conditioner 36, the generated power outputted from the solar photovoltaic panel 1 is smoothed by the smoothing capacitor 33. The inverter 34 has switching elements 37-40, which are, for example, four MOSFET where diodes are connected in reverse parallel to each other. The power conditioner 36 controls a switching operation of the switching elements 37-40 in the inverter 34 (the switching elements are switched on and off) at such a high frequency as around 18 kHz to thereby convert the generated power of the solar photovoltaic panel 1 smoothed by the smoothing capacitor 33 into an alternating current power synchronizing with a commercial power system and outputs the converted power. The power conditioner 36 supplies the alternating current power thus converted by way of the filter 35 to a load not shown or supplies it to the system as a reverse power flow. A solar cell dominantly used in the solar photovoltaic panel 1 today is a crystalline solar cell superior in its power conversion efficiency.
Another example of the solar cell increasingly often used is an inexpensive thin film solar cell advantageous in that a significantly reduced volume of silicon to be used as its material, its simplified production process, enabling the production of a solar photovoltaic panel having a larger area. A known disadvantage of the thin film solar cell made of amorphous silicon is that a potential on a negative-electrode side thereof is possibly lower than a ground potential, which expedites aging degradation.
To avoid the aging degradation of the thin film solar cell, it is necessary to adjust its potential on the negative-electrode side to be equal to the ground potential. An insulator power conditioner, wherein a direct current unit and an alternating current unit are electrically insulated from each other by an insulation transformer, can set the potential on the negative-electrode side of the solar cell, which is the direct-current side, to the ground potential.
As opposed to the insulator power conditioner, a non-insulated type power conditioner 36 illustrated in FIG. 20 excellent in its power conversion efficiency, however, has the problem to be solved, that is failure to keep the potential on the negative-electrode side, which is an input side of the power conditioner 36, at the ground potential level because different reference potentials are used on the direct-current side and the alternating-current side.
In direct current power sources other than the solar cell, such as a fuel cell and a power generator, it is desirable to ground their negative-electrode side in view of better safety for such an event as electrical leak. The conventional non-insulated type power conditioner had difficulty in keeping the potential on the negative-electrode side of the direct current power source at a potential level equal to a ground potential of an alternating-current output.