Electric power conversion systems – Current conversion – Having plural converters for single conversion
Utility Patent
2000-01-27
2001-01-02
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C323S906000
Utility Patent
active
06169678
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a photovoltaic power generation apparatus and to a control method thereof. More particularly, the present invention relates to a photovoltaic power generation apparatus having a plurality of solar battery arrays and a plurality of power converters and to a control method thereof.
Recently, the warming of the earth, caused by carbon dioxide emission involved in the use of fossil fuels, and radioactive contamination caused by accidents in nuclear power plants and radioactive waste have become social issues. Such issues regarding the global environment and energy are rapidly gaining the interest of many people. Photovoltaic power generation, which employs solar rays, an inexhaustible clean energy source, is regarded by the world to be the energy source of tomorrow.
There are various sizes of photovoltaic power generation apparatuses employing solar batteries, and the electric power generated by a given apparatus ranges from several watts to several thousand kilowatts. For general residences, a photovoltaic power generation apparatus, comprising solar batteries of 3 to 5 kW and power converters of 3 to 5 kW, is generally employed. For apartment houses or public buildings or the like where solar batteries can be installed in a wider area than general residences, a photovoltaic power generation apparatus comprising solar batteries of about 10 kW and power converters of about 10 kW, or a photovoltaic power generation apparatus comprising solar batteries of about 10 kW and two or three power converters of 3 to 5 kW connected in parallel are employed.
FIG. 2
shows a photovoltaic power generation apparatus constructed by connecting three units, each comprising a 3.3 kW solar battery and a 3.5 kW power converter, in parallel. In
FIG. 2
, reference numeral
31
denotes a 3.3 kW solar battery array;
32
, a 3.5 kW power converter connected to the solar battery array
31
;
33
, a switchboard connected to the power converter
32
;
34
, a transaction power meter box connected to the switchboard
33
; and
35
, a commercial power system connected to the transaction power meter box
34
. Note herein that the 3.5 kW power converter
32
is of the type used for general residences.
The control circuit of the power converter
32
serves to protect the power converter
32
by detecting an abnormal voltage of the commercial power system
35
. The protection operation is described hereinafter. When an abnormal voltage of the commercial power system
35
is detected once or a number of times, alternating-current power output of the power converter
32
is suspended and a stand-by mode begins (hereinafter referred to as “operation suspension”). When an abnormal voltage is no longer detected and a predetermined time period lapses, the alternating-current power output of the power converter
32
is automatically resumed (hereinafter referred to as “operation resume”).
Needless to say, an abnormal voltage of the commercial power system
35
not only imposes small-scale problems, but also can have large effects on the entire society. For this reason, the frequency and voltage of the commercial power system
35
must be controlled to maintain predetermined values.
Connecting the commercial power system
35
with the power converter
32
requires a distribution line. Since the distribution line has an impedance, a voltage drop is unavoidable at the time of power transmission/reception. For instance, in the system shown in
FIG. 9A
, the distribution line between the commercial power system
5
and user
6
has an impedance
7
. Because of this, the voltage VR at the user
6
at the power receiving end (e.g., voltage at the switchboard
33
in
FIG. 2
) falls below the system voltage VS of the commercial power system
5
. Therefore, the system voltage VS of the commercial power system
5
is set slightly high, taking into account the voltage drop caused by the impedance
7
. For instance, the low-voltage output of a pole transformer is set to 105V. Thus, when no load is applied, the voltage VR at the user
6
at the power receiving end is 105V.
On the other hand, when a current flows backward from the photovoltaic power generation apparatus to commercial power system
5
as shown in
FIG. 9B
, the voltage VR at the user
6
at the power receiving end exceeds 105V because of the influence of the impedance
7
. For instance, while the user
6
is receiving power from the commercial power system
5
and consuming
30
A, assume that the voltage drops by 4V and the voltage VR at the power receiving end falls to 101V. In this case, in order to send
30
A back from the user
6
to the commercial power system
5
, the voltage drop needs to be compensated. Thus, the voltage VR at the power receiving end becomes 109V. As described above, because of the influence of the impedance
7
of a distribution line, when the backward current becomes large, the voltage at the user
6
at the power receiving end (voltage at the switchboard
33
or the connection point of the power converter
32
in
FIG. 2
) becomes high.
In order to maintain a certain voltage for all users, the commercial power system
5
turns on/off switches of a power generating station and substation in correspondence with varying electric power demands. Therefore, the voltage of the commercial power system
5
is not constant, but is changing every moment.
FIG. 10
shows an actual measurement result of voltage of the commercial power system
5
, measured for a day by the inventors of the present invention. At the points indicated by the arrows A, the system voltage VS reaches about 107V. This means that the voltage at the connection point of the power converter
32
rises or falls according to the level of backward current flow and also according to changes in the level of voltage of the commercial power system
5
.
Along these lines the apparatus shown in
FIG. 2
is used wherein where three power converters
32
, which suspend operation by detecting an abnormal voltage and resume operation automatically when no abnormal voltage is detected, are connected in parallel. When an abnormal state such as a rise in the system voltage VS is detected, the three power converters
32
detect the abnormal voltage and stop operation at the same time. Even after recovery, operation suspension and operation resume may repeatedly occur. This is disadvantageous not only because an electric stress is imposed on a part of the photovoltaic power generation apparatus, but also because the amount of power generated by the photovoltaic power generation apparatus is reduced.
SUMMARY OF THE INVENTION
The present invention solves the above-described problems, and has as its object to prevent generation of an electrical stress or reduction of the power generation amount in a photovoltaic power generation apparatus, caused by plural power converters which detect an abnormal state and simultaneously suspend operation or repeat operation suspension and operation resume.
Another objet of the present invention is to provide a photovoltaic power generation apparatus capable of suspending operation of only a part of power converters when an abnormal state is detected.
In order to achieve the above objects, the present invention provides a photovoltaic power generation apparatus comprising: plural solar battery arrays, each of which is constructed by plural solar battery modules; and plural power converters, each of which is connected to one of the plural solar battery arrays, for converting direct-current power generated by the plural solar battery arrays to alternating-current power so as to provide the alternating-current power to a commercial power system,
wherein each of the plural power converters comprises a detector for detecting an abnormal state of output of the alternating-current power of each of the power converters to suspend power conversion operation of each of the power converters, and operation suspension timing of at least one of the plural power converters is earlier than the others.
Furthermore, the presen
Kondo Hiroshi
Manabe Naoki
Takehara Nobuyoshi
Berhane Adolf Deneke
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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