Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
2000-02-28
2001-09-04
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C363S079000
Reexamination Certificate
active
06285572
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power supply system in which a DC output of a DC power supply whose electric energy increases and decreases, such as solar cells, wind power generators, and fuel cells, is converted to an AC output by a plurality of inverters and is supplied to a system, and concerns a technique for controlling the inverters with high efficiency. In addition, the present invention relates to a parallel-connected system in which electric power generated by a power generating means such as solar cells is converted by inverters to electric power corresponding to a commercial power supply, and is outputted to the commercial power supply.
2. Description of the Related Art
As such a power supply system, a photovoltaic power generation system using solar cells is generally known.
FIG. 6
is a system diagram of a conventional photovoltaic power generation system. This photovoltaic power generation system is configured such that a plurality of solar cells (DC power supply)
101
are arranged on the roof of a house, DC outputs generated by these solar cells
101
are collected into one output by a junction box
102
, and this DC output is then converted to an AC output through an inverter
103
. Subsequently, the power is supplied to the branch circuit inside the house and a commercial-use power system
106
through a distribution board
104
. Incidentally, reference numeral
105
denotes an in-house load connected to the branch circuit.
Generally, the inverter has the characteristic that its efficiency declines extremely during a low output. There has been a problem in that if DC/AC conversion is effected by a single inverter in correspondence with the estimated maximum energy generated by the photovoltaic power generation system, the DC/AC conversion efficiency declines during a low output. To solve such a problem, Japanese Patent Application Laid-Open (JP-A) No. 6-165513, for example, discloses a system in which a plurality of inverters with small outputs are connected in parallel, and the number of inverters which are run is increased or decreased in correspondence with the energy generated by the solar cells so as to suppress the decline in the conversion efficiency during a low output.
In addition, in a parallel-connected system, the DC power generated by a generating apparatus such as a photovoltaic power generator is converted to AC power corresponding to a commercial power supply by the inverters, and is then supplied to the commercial power supply.
With the inverters used in such a parallel-connected system, independent operation due to service interruption of the commercial power supply is prevented, and the system interconnection is protected against an overvoltage, an undervoltage, a frequency rise, and a frequency drop in the commercial power supply.
With the inverters used in the parallel-connected system, the most efficient operation is possible during the output of rated power. However, with the power generator using solar cells, since the generated power increases and decreases due to the quantity of solar radiation and the like, the inverters are subjected to maximum power point tracking control (MPPT control) so that the output efficiency becomes highest in correspondence with the increase or decrease in the generated power when the input power is less than the rated power.
As described above, with the inverters whose output power is large, if the input power is excessively low with respect to the rated power, the output efficiency drops extremely. For this reason, a proposal has been made that, with the parallel-connected system, a plurality of inverters be connected in parallel, and the number of driven inverters be set in correspondence with the input power, so that even when the generated power is low, the inverters can be driven efficiently.
With the conventional method, the number of inverters which are driven is determined merely in correspondence with the output power, and no consideration is given to the selection of the inverters which are driven. For this reason, only particular inverters are driven during a low output, and the other inverters are driven only when the output has increased, with the result that the running time of the particular inverters becomes longer than that of the other inverters. Hence, there has been a problem in that the service life of the particular inverters with a long running time expires earlier than the other inverters.
Furthermore, there has been a problem in that if particular inverters among the plurality of inverters are not effective, the overall system fails to work.
In addition, there is a problem in that if the respective output powers of the plurality of inverters are individually controlled, conversely, the conversion efficiency drops depending on the generated power, the number of driven inverters, and so on. Further, if the individual inverters are separately provided with system integration protection when the plurality of inverters are run in parallel, there are cases where their mutual outputs and protective operations interfere with each other, rendering appropriate protection impossible.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a method of operating a power supply system having a plurality of inverters, such that the inverters are driven with high efficiency, thereby overcoming the above-described drawbacks of the conventional art.
To this end, in accordance with a first aspect of the invention, there is provided a method of operation for a power supply system having a plurality of inverters connected in parallel with a DC power supply whose generated electric energy increases or decreases, in which the inverters convert an electric output from the DC power supply to frequency- and voltage-controlled AC power and output the AC power to a system, the method comprising the steps of: (a) setting one of the inverters to serve as a master unit and the other inverters to serve as slave units, wherein the master unit controls the slave units; and (b) allowing the master unit to control the slave units on the basis of at least one of an increase or decrease in the electric energy from the DC power supply and an increase or decrease in the AC power outputted from the inverters.
In accordance with the above-described first aspect of the invention, of the plurality of inverters, one inverter which is set as the master unit controls the operation of the remaining inverters, whose order has been set in accordance with a predetermined rule, on the basis of the increase or decrease of electric energy of the DC power supply or an increase or decrease of the amount of AC power output from the inverter.
Further, in this aspect of the invention, when running of a generator is suspended, the master unit sets a master unit which is to be used during the start of the next running of the generator. Such a setting can be effected on the basis of integrated values of the running times of the inverters or their amounts of output power.
As a result, the integrated values of the running times or output powers can be substantially equalized among the plurality of inverters, and it is possible to prevent the running times of particular inverters from becoming long.
Further, the inverters are respectively connected to remote controllers for remote controlling, and the remote controllers are connected to each other in such a manner as to be capable of transmitting and receiving signals to and from one another. The operation of the inverters is effected through the remote controllers.
Further, the ordering of slave units to be run next may be randomly set by using random numbers.
Further, the ordering of slave units to be run next may be set in the ascending order of the running times thereof.
Further, the ordering of slave units to be run next may be set in the ascending order of the amounts of output power thereof.
Another object of the invention is to provide an efficient parallel-connected system in which a plurality of inv
Madenokouji Masaki
Makino Yasuhiro
Morita Isao
Onizuka Keigo
Knobbe Martens Olson & Bear LLP
Patel Rajnikant B.
Sanyo Electric Co,. Ltd.
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