Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
1997-05-29
2001-01-09
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S055000
Reexamination Certificate
active
06172889
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inverter apparatus for supplying a power to a load the inverter apparatus being in interconnection with an AC power system. The present invention also relates to an inverter apparatus islanding operation detecting method for detecting the event that the inverter apparatus has started an islanding operation as dissociated from the AC power system due to a power failure or the like of the AC power system.
2. Description of the Background Art
Conventionally, there has been an inverter apparatus as shown in FIG.
14
. This inverter apparatus
100
subjects a DC power inputted from a DC power source
101
to PWM (Pulse Width Modulation) control in an inverter main circuit
102
, thereby converting the DC power into an AC power. Then, after a high-frequency component is removed from the converted AC power by a filter comprised of a reactor
103
and a capacitor
104
, the resulting AC power is supplied to a load
105
. The load
105
is further supplied with an AC power from an AC power system
108
via a breaker
106
and a pole transformer
107
, so that the inverter apparatus
100
is operated in interconnection with the AC power system
108
.
The inverter main circuit
102
is subjected to the PWM control as follows. That is, a load supply voltage V
o
supplied to the load
105
is detected by a voltage detector
109
. Then, the detected load supply voltage V
o
is inputted to a band-pass filter
110
to pass through it and made to be a sine wave signal Va which is inputted to a current reference generator circuit
111
. The current reference generator circuit
111
multiplies the inputted sine wave signal Va by an output command signal Vc so as to generate a current reference signal Ic and input it to an error amplifier circuit
112
.
On the other hand, an output current I
o
of the inverter main circuit
102
is detected by a current detector
113
and inputted to the error amplifier circuit
112
. The error amplifier circuit
112
generates an error amplification signal E based on the inputted current reference signal Ic and the output current I
o
and inputs it to a PWM control circuit
114
. The PWM control circuit
114
generates a gate pulse signal Gp by executing a pulse width modulation process based on the inputted error amplification signal E and inputs it to a gate drive circuit
115
. The gate drive circuit
115
switchingly controls switching elements (not shown) constituting the inverter main circuit
102
by the inputted gate pulse signal Gp, thereby converting the DC power of the DC power source
101
into an AC power.
The inverter apparatus
100
detects the event that the inverter apparatus has entered into an islanding operation state as follows. It is to be noted that the islanding operation means a state in which the DC power source
101
(e.g., a solarlight power generation system) continues supplying a DC power when the AC power system
108
fails.
When the inverter apparatus
100
enters into the islanding operation state, a voltage abnormality and a frequency abnormality occur in the load supply voltage V
o
supplied to the load
105
. This is for the reasons as follows. That is, the amplitude and phase of the current reference signal Ic relative to the load supply voltage V
o
are as shown in
FIGS. 15A and 15B
due to the transfer characteristics of the band-pass filter
110
. Therefore, when the breaker
106
is opened thereby making the inverter apparatus
100
enter into the islanding operation state and when a reactive power supplied from the inverter apparatus
100
and a reactive power required by the load
105
do not coincide with each other, the output of the inverter apparatus
100
(referred to as an inverter output hereinafter) exhibits variations such as (i) deviation of its frequency from the rated frequency (e.g., 50/60 Hz when the AC power system
108
is the commercial power source) f
o
, (ii) reduction of its current value, and (iii) reduction of its voltage value. For example, in the case where the load impedance of the inverter apparatus
100
when it enters into the islanding operation state has a delayed phase &phgr; as shown in
FIG. 15B
, the frequency of the inverter output balances when it is increased up to f
1
. In this stage, the current value of the inverter output reduces from I
0
to I
1
, and the voltage value reduces in accordance with it.
Therefore, this inverter apparatus
100
is provided with a voltage abnormality detector circuit
116
and a frequency abnormality detector circuit
117
for the detection of the voltage abnormality and the frequency abnormality of the load supply voltage V
o
and decides whether or not the inverter apparatus
100
has entered into the islanding operation according to the result of detection. Based on the result of decision, the operation of the gate drive circuit
115
is stopped, so that the output of the inverter main circuit
102
is stopped and the DC power source
101
is disconnected from the AC power system
108
by a system linking relay (not shown) of the inverter main circuit
102
, thereby preventing the islanding operation of the inverter apparatus
100
.
However, the background art inverter apparatus
100
that prevents the islanding operation in the aforementioned manner has had the problem that the frequency variation and the voltage variation of the inverter output reduce when the power factor of the load impedance of the inverter output is proximate to one, causing the islanding operation state to be hardly detected by the voltage abnormality detector circuit
116
or the frequency abnormality detector circuit
117
.
For the above reasons, there has been conventionally proposed a method for detecting the islanding operation state by detecting a distortion generated in the output waveform of the inverter output in the islanding operation state. This is as follows. That is, when the inverter apparatus
100
enters into the islanding operation state, an excitation current is supplied from the inverter apparatus
100
to the pole transformer
107
, and this excitation current causes a distortion at or around the zero-crossing point of the output waveform of the inverter output supplied to the load
105
. Therefore, by measuring the distortion and a rate of change of the distortion, the islanding operation state is detected.
However, in order to detect the islanding operation in a manner as described above, it is required to newly provide a circuit for detecting the distortion and the rate of change of the distortion. This has caused an increase in number of the components and a complication of the construction of the inverter apparatus, which has been a factor for increasing the cost of the inverter apparatus.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to allow the islanding operation to be surely detected simply and at low cost even when the power factor of the inverter output with respect to the load impedance is proximate to one.
In order to achieve the aforementioned object, the present invention provides an inverter apparatus islanding operation detecting method for detecting an event when an inverter apparatus, which converts an output of a direct current power source into an alternating current through waveform control synchronized with a zero-crossing point and supplies the alternating current to a load in interconnection with an alternating current power system, has been dissociated from the alternating current power system and has started an islanding operation, comprising:
imparting to an inverter output waveform a distortion for causing a variation in an output frequency in an islanding operation stage of the inverter apparatus, and detecting the islanding operation of the inverter apparatus by detecting a frequency variation of an inverter output caused by the distortion or a variation attributed to the frequency variation.
Also, the present invention provides an inverter apparatus having: an inverter main circuit for converting an outp
Eguchi Masaki
Kodama Hirokazu
Nakata Hirofumi
Takebayashi Tsukasa
Han Y. J.
Riley Shawn
Sharp Kabushiki Kaisha
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