This invention relates to a power generation system utilizing natural energy, a power amount calculation apparatus and a power amount calculation method.
Recent growing awareness of global environmental problems has attracted peoples' attention to a Renewable electricity generation system free of pollution, especially a solar power generation system which uses solar cells. The solar power generation system, however, has its amount of generated power fluctuating depending on the daily insolation, i.e. hours of sunshine. In order to effectively utilize the extra power generated by the solar system and therefore to contribute to the stabilized supply of power, it is preferable to use solar power generation systems installed in individual homes or buildings in combination with a commercial power system.
In other words, the supply of power to loads usually takes place as a result of parallel operation of solar power generation systems and a commercial power source. All or part of power necessary for home consumption is supplied from a solar power generation system, and if the power generated by the solar power generation system becomes more than necessary for the home demand, the extra power is fed to the commercial power source, that is, the “reverse power flow” takes place.
The electric power company purchases reverse flow power by the preset unit price. This is a well-known power service customers' incentive for the installation of a solar power generation system.
The patent documents 1-3 listed below relate to this invention.
Patent Document 1: JP-A-08-70533
Patent Document 2: JP-A-06-332553
Patent Document 3: JP-2002-152976
FIG. 2 diagrammatically shows an example of a distributed power source system using a solar battery as a power source, disclosed in the above patent document 1. In FIG. 2, the distributed power source system comprises a solar cell array 201 and a power conversion apparatus 202 incorporating therein an inverter (inverter circuit) 203 for converting the DC power outputted from the solar cell array 201 into AC power.
The power conversion apparatus 202 incorporates therein a circuit breaker 205 for separating the distributed power source from the commercial power system 204, and a system linkage protection apparatus 212 which includes an islanding detector 207 that detects the dissection of a circuit breaker 206 belonging to the commercial power system 204 on the basis of the fluctuation of frequency or voltage and that dissects the circuit breaker 205 in response to the detection of the dissection of the circuit breaker 206.
The system linkage protection apparatus 212 includes an arithmetic operation unit 208, a variable output unit 209, a control unit 210 and a display 211. The arithmetic operation unit 208 calculates the power generated by the solar cell array 201 on the basis of the output voltage and output current of the solar cell array 201 which are obtained through measurement.
The variable output unit 209 changes the output voltage of the solar cell array 201. The control unit 210, by changing the output voltage of the solar cell array 201 through the control of the variable output unit 209, performs at regular intervals a searching operation for searching the output voltage that makes maximum the generated power calculated by the arithmetic operation unit 208. The display 211 is used to make an indication that generated power amount is abnormal.
The islanding detector 207, the arithmetic operation unit 208, the variable output unit 209 and control unit 210 are integrated into a single entity as a microcomputer 212. By controlling the inverter circuit 203 via the variable output unit 209, the control unit 210 changes the output voltage of the solar cell array 201 and searches the value of voltage that makes maximum the detected value of the output power delivered from the arithmetic operation unit 208.
A solar power generation system is usually provided with a system linkage protection apparatus that monitors the voltage at the point at which the solar power generation system is linked with a commercial power system and that separates the solar power generation system from the commercial power system by actuating a relay when the voltage at the link point assumes a value that does not fall within the rated range, that is, power supply abnormality occurs, and therefore provided with the function of immediately stopping the power generation by the solar power generation system so as to suppress the voltage rise at the link point.
The patent document 2 discloses a technique according to which output suppression control is employed that does not stop the power generation by the solar power generation system but that monitors the voltage at the link point and makes the generated output smaller than the maximum value when the voltage approaches the rated upper limit value.
The patent document 3 discloses a method according to which, in a power system connected with plural distributed power source systems such as solar power generation systems, the plural distributed power source systems are linked with a system administration apparatus via a communication network; each distributed power source system includes a system control unit which transmits the operating information of the distributed power source to the system administration apparatus; and the operating condition is controlled in response to the instruction received from the system administration apparatus.