Electrical transmission or interconnection systems – Plural supply circuits or sources – Plural converters
Reexamination Certificate
2001-07-11
2002-09-17
Riley, Shawn (Department: 2838)
Electrical transmission or interconnection systems
Plural supply circuits or sources
Plural converters
Reexamination Certificate
active
06452290
ABSTRACT:
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
The present invention relates to an inverter parallel operation system in which a balance is maintained among effective powers output by inverters under parallel operation, and load distribution among the inverters under parallel operation is accomplished by controlling in such a manner that the output frequencies of the inverters under parallel operation is caused to agree with frequencies predetermined in accordance with the output effective power from each or any one of the inverter.
2. DESCRIPTION OF THE PRIOR ART
Inverters that are operated in parallel are publicly known, and Japanese Patent Publication No. 2678991 and Japanese Published Unexamined Patent Application No. 2000-32764, for example, disclosed the art of balancing effective powers among inverters under parallel operation.
These prior-arts are based on the fact that output effective powers can be controlled by advancing or lagging the frequency (phase) thereof.
These prior arts, however, take no account of balancing output reactive powers among inverters under parallel operation.
The aforementioned Japanese patent Publication No. 2678991 and Japanese Published Unexamined Patent Application No. 2000-32764 disclosed that effective powers are balanced among the inverters so as to eliminate unwanted cross-current powers among the inverters under parallel operation.
To eliminate the aforementioned cross-current power, it is necessary to accurately grasp the state of generation of cross-current power. That is, how to grasp the generation of cross-current power is of critical importance.
FIG. 26
is a circuit diagram of a system where two inverters are in parallel operation.
Numerals
122
A and
122
B in the figure refer to a-c generators,
133
A and
133
B to rectifier circuits with input voltages V
A
and V
B
,
134
A and
134
B to smoothing capacitors,
135
A and
135
B, and
136
A and
136
B to output terminals, respectively,
137
A and
137
B to inverters or inverter circuits with input voltages V
A
and V
B
,
138
A and
138
B to filter circuits, and F to a common load, respectively.
FIG. 27
is an equivalent circuit diagram in which only cross-currents in the circuit configuration of
FIG. 26
are taken into account. Assuming that an effective power output by an inverter (
137
A) is P
A
,
P
A
≅
E
1
2
4
(
r
1
2
+
x
1
2
)
{
2
x
1
(
θ
1
-
θ
2
)
+
2
r
1
(
E
1
-
E
2
)
}
where r
1
=r
2
, and x
1
=x
2
. When the phase difference between internal voltages is assumed to be &thgr;
1
−&thgr;
2
=0,
P
A
≅
2
r
1
(
E
1
-
E
2
)
·
E
1
2
4
(
r
1
2
+
x
1
2
)
=
r
1
·
E
1
2
2
(
r
1
2
+
x
1
2
)
(
E
1
-
E
2
)
,
and when E
2
>E
1
, PA takes a negative value. That is, a cross-current power is allowed to flow in the inverter
137
A.
The cross-current power flowing in the inverter
137
A is returned to the input side of an H bridge comprising the inverter, charging the smoothing capacitor
134
A connected to the input side of the H bridge to raise the terminal voltage of the smoothing capacitor
134
A. The cross-current power flowing in the inverter
137
A is not returned to the generator side. For this reason, the terminal voltage of the smoothing capacitor
134
A continues to increase. This terminal voltage V becomes a voltage V determined by
E
N
=½ CV
2
=J+½CV
0
2
J=∫(−P
A
)dt
where E
N
: Energy
J: Amount of inflow power
C: Capacitance of the smoothing capacitor
V: Terminal voltage across the smoothing capacitor
V
0
: Initial value.
As the terminal voltage V of the smoothing capacitor
134
A rises, the input voltage of the H bridge of the inverter
137
A rises. As will be described later, however, the internal voltage E
1
of the inverter
137
A is fixed at a value determined by a sine-wave standard signal V
sin
. in the PWM circuit which will be described later. Furthermore, raising the output voltage (which can be considered the internal voltage E
1
) means that control is performed to increase the amplitude of the sine-wave standard signal V
sin
.
Since the presence of a cross-current power causes the terminal voltages of the smoothing capacitors
134
A and
134
B to unwantedly increase, as described above, it is necessary to inhibit the increase.
In the configuration shown in
FIG. 26
, the inverter circuits
137
A and
137
B on each side output high-frequency alternating voltages of a so-called square waveform as switching control is accomplished by on-off signals from the PWM circuit (not shown). The filter circuit
138
A (the same applies to the filter circuit
138
B) comprises two choke coils across the terminals
153
A and
135
A, and across the terminals
154
A and
136
A, and a capacitor across the terminal
135
A and
136
A, for example. Needless to say, the filter circuit
138
A filters the high-frequency alternating voltage of a square waveform output by the inverter circuit
137
A, and works in such a manner as to direct a desired sine-wave voltage of 50 Hz, for example, to the load.
In each inverter device as shown in
FIG. 26
, an output voltage (internal voltage) or output current of its own is extracted due to the need for detecting the power generated by itself.
In such a case, particularly when extracting the output voltage, various contrivances have to be worked out to cope with the effects of the high-frequency alternating voltage of a square waveform generated by the inverter device or the effects of the noise voltage introduced from the load.
FIG. 28
shows an example of conventional methods of drawing potentials.
FIG. 29
shows an example of the method of drawing potentials disclosed in Japanese Patent Publication No. 2688660. Like numerals in each figure correspond to those used in FIG.
26
.
In
FIGS. 28 and 29
, two choke coils are provided because there are two types of high frequencies and noises passing through the filter circuits (hereinafter referred to collectively as noises); noises a and b recirculating into the filter circuit
138
A in an opposite phase, and noises c and d flowing in the filter circuit
138
A in the same phase. The two choke coils are designed to inhibit both types of noises.
In the configuration shown in
FIG. 28
, a potential on the terminal
135
A of a choke coil connected between the terminals
153
A and
135
A and a potential on the terminal
136
A of the choke coil connected between the terminal
154
A and
136
A are introduced to a differential amplifier A where the output voltage of the inverter circuit is extracted as a difference between the two potentials.
In the configuration shown in
FIG. 29
, a potential on the terminal
153
A of a choke coil connected between the terminals
153
A and
135
A and a potential on the terminal
154
A of the choke coil connected between the terminal
154
A and
136
A are introduced to a differential amplifier A where the output voltage of the inverter circuit is extracted as a difference between the two potentials.
The configurations shown in
FIGS. 28 and 29
have their advantages and disadvantages. In the configuration shown in
FIG. 2
, noises caused by high-frequency components from the inverter circuit can be easily suppressed, while noises from the load side are led to the input side of the differential amplifier. In some cases, therefore, an additional filter circuit may have to be added to the load side of the terminals
135
A and
136
A in
FIG. 28
, or on the input side of the differential amplifier.
In the configuration shown in
FIG. 29
, noises from the load side can be easily suppressed, but those from the inverter circuit side tend to be left intact. Furthermore, the presence of choke coils may cause a phase shift in the original output voltage (original sine-wave voltage) in the inverter circuit.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an i
Hayashi Hidetake
Nakamura Yuji
Namai Masao
Takimoto Hitoshi
Yoshioka Toru
McGlew and Tuttle , P.C.
Riley Shawn
Sawafuji Electric Co. Ltd.
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