Method and apparatus for controlling a DC to AC inverter system by a plurality of pulse-width modulated pulse trains

A pulse train pulse-width modulated by PWM control using a sine-wave signal and a carrier signal is inverted alternately in positive-negative polarity. With a high-frequency AC signal obtained by the inversion, a primary side of a transformer for insulating input and output from each other is excited. This arrangement makes it possible to employ a high-frequency transformer which has a capacity ratio of about 1/30 and a weight ratio of about 1/20 relative to a commercial-frequency transformer, instead of using it. Thus, an inverter apparatus can be reduced in size and weight as compared with the system using the power-frequency transformer. A sine-wave AC waveform with substantially less distortion similar to the waveform output by the conventional PWM control can be obtained with a simple construction.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an inverter apparatus for converting DC 
power generated by an independent DC power supply such as a solar cell 
into AC power and supplying the power to home- and business-use general AC 
loads or to existing commercial power systems and the like. 
2. Description of the Prior Art 
A conventional inverter apparatus is composed of an inverter bridge made up 
of several switching devices, a transformer for electrically insulating 
the DC power source from commercial power systems or loads, a low-pass 
filter, and a control circuit for performing ON/OFF control on the 
plurality of switching devices constituting the inverter bridge. As the 
above transformer, there have been used power-frequency transformers or 
high-frequency transformers intended for the miniaturization of the 
apparatus. 
First, a conventional example of the inverter apparatus using a 
power-frequency transformer is described with reference to FIG. 14. DC 
power outputted from a solar cell 2 is inputted to an inverter apparatus 
1. The input DC power is converted into AC power by an inverter bridge 32, 
and supplied to a commercial power system 3 via a power-frequency 
transformer 33 which is provided at an output end of the inverter 
apparatus 1 in order to insulate the solar cell 2 from the commercial 
power system 3. A DC capacitor 12 for suppressing the input power 
fluctuating of the inverter apparatus 1 and a DC input current detector 13 
are connected in the preceding stage of the inverter bridge 32. An AC 
filter 16 for removing harmonic components of AC current and an inverter 
output current detector 14 are connected in the succeeding stage of the 
inverter bridge 32. Further, an interconnection relay 15 is provided in 
the succeeding stage of the AC filter 16, whereby interconnection and 
disconnection with the commercial power system 3 is implemented. 
A control circuit 34 of the inverter apparatus 1 is composed of a gate 
drive circuit 35, a PWM (Pulse Width Modulation) control unit 36, an error 
amplifier 37, a carrier signal generator 38, a signal processing unit 39, 
a sine-wave signal storage unit 40, an A/D converter 41, and a D/A 
converter 42. 
The PWM control unit 36 generates a first pulse train signal obtained by 
comparing a sine-wave signal equal in frequency (50/60-several hundreds 
Hz) to the voltage waveform of the aforementioned commercial power system 
3 with a high-frequency (about 10/kHz, but not lower than 10 kHz) carrier 
signal synchronized with the sine-wave signal, a second pulse train signal 
obtained by inverting the first pulse train signal, a third pulse train 
signal obtained by comparing an inverted carrier signal, which is the 
inversion of the aforementioned carrier signal, with the sine-wave signal, 
and a fourth pulse train signal obtained by inverting the third pulse 
train signal. FIGS. 15B, 15C, 15D and 15E show respective waveforms of 
these pulse train signals. It is noted that in FIGS. 15A, 15B, . . . , 
15F, high-frequency waves of about 10 kHz, but not lower than 10 kHz are 
schematically illustrated. 
These first to fourth pulse train signals are inputted to the gate drive 
circuit 35. Based on these signals, gate drive signals equal in frequency 
to the carrier signal are generated for four switching devices Q1 to Q4 
constituting the inverter bridge 32. With the gate drive signals, the 
switching devices Q1 to Q4 are controlled to be turned on and off at the 
same frequency as that of the carrier signal. As a result, an output pulse 
train waveform Ei as shown in FIG. 15F is produced by the inverter bridge 
32. Further, the output waveform Ei is subjected to a harmonic-component 
removing process and a smoothing process by the succeeding-stage AC filter 
16, resulting in a 50/60-several hundreds Hz sine-wave AC output. The 
sine-wave AC output undergoes the input-output insulation by the 
power-frequency transformer 33 and thereafter is inputted to the 
commercial power system 3. In this case, the power-frequency transformer 
33 is excited at a frequency of the 50/60-several hundreds Hz sine-wave AC 
output. 
The A/D converter 41 converts a DC voltage signal V.sub.in and a DC current 
signal I.sub.in, which are analog signals derived from the solar cell 2, 
and a utility line voltage signal V.sub.out into digital quantities, and 
then transmits the resulting signals to the signal processing unit 39. In 
order to maximize the output power from the solar cell 2, the signal 
processing unit 39 performs a maximum power point tracking operation, by 
which the solar cell operating point is made coincident with a maximum 
point on the solar cell output characteristic curve. The signal processing 
unit 39 also reads out a sine-wave signal (50/60-several hundreds Hz) 
which serves as a current command value for controlling the inverter 
apparatus 1, from the sine-wave signal memory unit 40 where a plurality of 
sine-wave signals having different amplitudes are previously stored. The 
sine-wave signal storage unit 40 is normally storing the aforementioned 
sine-wave signals having a plurality of different amplitudes proportional 
to the amplitude of rated output current waveform of the inverter 
apparatus 1, as digital quantities quantized in the unit of half the 
period or one period and every certain time intervals. The D/A converter 
42 converts a read sine-wave signal into an analog signal and then 
transmits it to the error amplifier 37. The error amplifier 37 receives as 
inputs an inverter output current signal Iout derived from the inverter 
output current detector 14, and the aforementioned sine-wave signal. The 
error amplifier 37 compares the two signals with each other to determine 
an error, and outputs a reference wave signal obtained by amplifying the 
error, to the PWM control unit 36. The carrier signal generator 38 outputs 
similarly to the PWM control unit 36 a carrier signal (higher than ten 
kHz) synchronized with the sine-wave signal. As a result, the output 
current of the inverter apparatus 1 undergoes a change in response to the 
sine-wave signal that serves as a current command value. In this case, 
when the output current of the inverter apparatus 1 is controlled with the 
above sine-wave signal, AC power with a power factor of 1 can be supplied 
to the existing commercial power system 3, by providing a sine-wave signal 
of the same phase and the same frequency (50/60-several hundreds Hz) as 
the voltage of the commercial power system 3. 
Next described is a case where a high-frequency transformer is used. An 
apparatus employing a power-frequency transformer is disadvantageous in 
reducing the size and weight of the inverter apparatus because of the 
large weight and capacity of the power-frequency transformer. With the use 
of a high-frequency transformer, on the other hand, such problems can be 
solved. When a high-frequency transformer is used, the transformer needs 
to be excited at high-frequency voltage. An example using the current 
instantaneous value control method developed for this purpose is described 
below with reference to FIG. 16. 
An inverter apparatus 100 is inserted between a solar cell 2 and an 
existing commercial power system 3. The inverter apparatus 100 converts DC 
power generated by the solar cell 2 into AC power of 50/60 Hz, and 
supplies the power to loads in grid-connection with the commercial power 
system 3. In the inverter apparatus 100, the input DC power is converted 
into high-frequency alternating voltage under the current instantaneous 
value control by a high-frequency inverter bridge 4 made up of switching 
devices Q1 to Q4, and fed to the primary side of a high-frequency 
transformer 5. The high-frequency alternating current is rectified by a 
diode bridge 6 on the secondary side of the high-frequency transformer 5, 
and subjected to a harmonic-component removing process and a smoothing 
process by a filter circuit made up of a DC reactor 7 and a capacitor 47 
connected in parallel therewith. Further, the processed current is 
converted into AC power of commercial frequency under polarity reversing 
control by a low-frequency inverter bridge 8 made up of switching devices 
S1 to S4. Then, the power is supplied to the commercial power system 3 via 
an interconnection relay 55 and an AC filter 16. 
A signal processing unit 43 receives as inputs a voltage signal V.sub.in of 
the solar cell 2, a current signal I.sub.in detected by a DC input current 
detector 13, a current (inverter output current) signal I.sub.t on the 
primary side of the high-frequency transformer 5 detected by an inverter 
output current detector 14, and a voltage signal V.sub.out of the 
commercial power system 3. The signal processing unit 43 produces as 
outputs a current command signal and a polarity decision signal. A 
hysteresis comparator 44 receives as inputs a primary-side current I.sub.t 
of the high-frequency transformer 5 detected by the inverter output 
current detector 14 and the aforementioned current command signal. The 
hysteresis comparator 44 performs via a NOT circuit 45 the control of 
alternately turning on and off the switching devices Q1, Q4 and Q2, Q3 
that constitute the high-frequency inverter bridge 4, so that the 
primary-side current of the high-frequency transformer 5 is repeatedly 
reciprocated within a range of a constant width having upper and lower 
limit values around the current command signal. More specifically, with 
respect to the current command signal (I.sub.REF) as shown in FIG. 17, an 
upper limit value I.sup.+ and a lower limit value I.sup.- with a 
specified width .DELTA.I are previously given to the hysteresis comparator 
44 as set values. Then, the primary-side current signal I.sub.t of the 
high-frequency transformer 5 in FIG. 16, which is the actual value of the 
control quantity, is detected by the inverter output current detector 14, 
and fed to the hysteresis comparator 44 together with the current command 
signal. When the current signal I.sub.t, which is the actual value of 
control quantity, exceeds the upper limit set value I.sup.+ of FIG. 17 
(I.sup.+ =I.sub.REF +.DELTA.I), the switching devices Q1, Q4 of the 
high-frequency inverter bridge 4 of FIG. 16 are turned off while the 
switching devices Q2, Q3 are turned on via the NOT circuit 45, so that the 
current gradient is turned into a decrease. On the other hand, when the 
current signal I.sub.t of FIG. 17 decreases below the lower limit set 
value I.sup.- (I.sup.- =I.sub.REF -.DELTA.I), the switching devices Q1, 
Q4 are turned on while the switching devices Q2, Q3 are turned off, so 
that the current signal I.sub.t increases. By performing such switching 
control, the actual value of the current signal I.sub.t transits 
reciprocatingly between I.sup.+ and I.sup.- each time the switching 
operation is effected. In this operation, if a sine-wave signal having the 
same frequency as the commercial power system 3 and having an arbitrary 
amplitude is used as the current command signal (I.sub.REF), the current 
signal I.sub.t changes repeatedly and reciprocatingly responsive to even 
very fast switching operation within a range of .+-..DELTA.I around the 
current command signal. Thus, a sine-wave current waveform having a 
commercial frequency and having an amplitude proportional to that of the 
current command signal can be obtained. As described above, the 
primary-side current of the high-frequency transformer 5 in the inverter 
apparatus 100, i.e., the magnitude of the inverter output current can be 
controlled by the amplitude of the current command signal (I.sub.REF). 
The fold-back control circuit 46 receives as an input the aforementioned 
polarity decision signal and alternately switches the turn-on and -off of 
the switching devices S1, S4 and S2, S3 that constitute the low-frequency 
inverter bridge 8, according to the polarity of the voltage signal 
V.sub.out of the commercial power system 3. By this control, DC power 
rectified into a full-wave rectified current by the diode bridge 6 is 
formed into a sine-wave AC output at the succeeding stage of the 
low-frequency inverter bridge 8. 
For reduction in size and weight of the inverter apparatus, a 
high-frequency transformer is preferably used. The reason is that the 
high-frequency transformer results in about 1/30 the capacity and about 
1/20 the weight of the power-frequency transformer. 
However, the above current instantaneous value control method used as a 
method for exciting the high-frequency transformer with high-frequency 
voltage, superior as it is, has difficulties in optimizing the setting of 
the upper and lower limit values of the hysteresis width for the control 
method. Too large set values result in an increased distortion while too 
small set values result in a decreased width of the pulse train signal 
obtained through a comparison between the current command value and the 
inverter output current. This causes the control system to be more 
unstable than in the PWM control using the low-frequency transformer. A 
further problem is that seeking a more stable control system would lead to 
a more complex control circuit. 
SUMMARY OF THE INVENTION 
The present invention has been developed with a view to substantially 
solving the above described disadvantages and has for its essential object 
to provide a method for controlling an inverter apparatus which allows 
even waveform distortions of inverter output current to be controlled to 
substantially lower level without involving troublesome control 
operations, for the purpose of achieving reduction in size and weight of 
the inverter apparatus, and to provide an inverter apparatus using the 
same method. 
Another object of the present invention is to provide an inverter apparatus 
which can be reduced in size and weight by the use of a high-frequency 
transformer, and which can be easily interconnected with the single-phase 
three-wire system distribution line by the same electrical system. 
A still further object of the present invention is to provide an inverter 
apparatus which can continue the interconnected operation without halting 
the inverter apparatus even when any load unbalance has taken place to the 
single-phase three-wire system distribution line. 
In order to achieve the aforementioned object, there is provided a method 
for controlling an inverter apparatus which converts DC power generated by 
a DC power supply into AC power and then supplies the AC power to loads or 
existing commercial power supply, the method comprising steps of: 
alternately inverting positive/negative polarity of a pulse train 
pulse-width modulated by PWM (Pulse Width Modulation) control using a 
sine-wave signal and a carrier signal; and 
exciting a primary side of a transformer, where input and output are 
insulated from each other, with a high-frequency AC signal obtained 
through the inverting step. 
According to the control method of the present invention, a necessary 
high-frequency AC signal can be obtained only by alternately inverting a 
pulse-width modulated pulse train. This inversion can be achieved, for 
example, by driving the gates of switching devices constituting an 
inverter bridge, with four kinds of pulse train signals. 
The method for alternately inverting the positive/negative polarity of the 
pulse-width modulated pulse train by the above PWM control preferably 
comprises steps of: obtaining a first pulse train signal by comparing the 
sine-wave signal and the carrier signal with each other; obtaining a 
second pulse train signal by inverting the first pulse train signal; 
obtaining a third pulse train signal by generating an inverted carrier 
signal, which is an inversion of the carrier signal, and by comparing the 
inverted carrier signal and the sine-wave signal with each other; 
obtaining a fourth pulse train signal by inverting the third pulse train 
signal; generating a rectangular-wave signal which is equal in frequency 
to the carrier signal and which is shifted in phase by a 1/4 period; 
gating the rectangular-wave signal with the individual pulse train signals 
of the first to fourth pulse train signals; and performing on-off control 
on four switching devices constituting an inverter bridge with four pulse 
train signals obtained by the gating process. The gating process is 
preferably an exclusive-OR operation, whereby four kinds of pulse train 
signals are generated. Then, preferably, the four kinds of pulse train 
signals are fed respectively to the four switching devices constituting 
the inverter bridge and the switching devices are controlled to be turned 
on or off, whereby the aforementioned high-frequency AC signal having the 
same frequency as the carrier signal is obtained. In this case, a 50/60 to 
several hundreds Hz sine-wave signal and a more than several tens kHz 
carrier signal are preferably used and the inverter bridge composed of the 
four switching devices is provided in the inverter apparatus. 
Further, the method for obtaining the aforementioned four pulse train 
signals preferably comprises steps of: generating a first pulse train 
signal by comparing a sine-wave signal and a carrier signal with each 
other and by outputting a first level when the sine-wave signal is greater 
than the carrier signal or outputting a second level when it is smaller; 
generating a second pulse train signal by inverting the first pulse train 
signal; generating a third pulse train signal by comparing an inverted 
carrier signal, which is an inversion of the foregoing carrier signal, 
with the sine-wave signal and by outputting the first level when the 
inverted carrier signal is greater than the sine-wave signal or outputting 
the second level when it is smaller; and generating a fourth pulse train 
signal by inverting the third pulse train signal. In addition, the first 
level and the second level mean the so-called "High" level or "Low" level. 
With such an arrangement, the high-frequency alternating current having a 
pulse-width modulated pulse train inverted alternately of the positive and 
negative polarity becomes equal in frequency to the carrier signal. It is 
noted that an inverted signal of the sine-wave signal may also be used 
instead of the inverted signal of the carrier signal. Similar results can 
be obtained also in this case. 
When the primary side of the transformer is controlled so as to be excited 
with an alternating current having a pulse-width modulated pulse train 
inverted alternately of the positive and negative polarity as described 
above, a similar high frequency alternating current having a pulse-width 
modulated pulse train inverted alternately of the positive and negative 
polarity, which results from transforming the primary-side voltage, is 
outputted on the secondary side of the transformer. Thus, this 
high-frequency alternating current is rectified by a rectifier so as to be 
converted into a pulse-width modulated pulse train output continuous on 
one side. 
In the above-described method of the present invention, the high-frequency 
alternating current excited on the secondary side of the transformer is 
preferably rectified by a rectifier (AC-to-DC converter) so as to be 
converted into a pulse-width modulated pulse train output continuous in 
one polarity. 
The inverter apparatus that makes it feasible to implement the 
above-described inverter control method of the present invention 
comprises: 
a first power conversion unit for converting DC voltage into AC voltage; 
a transformer for obtaining a transformed secondary voltage with the 
resulting AC voltage taken as a primary voltage; 
a second power conversion unit to which secondary-side two lines of the 
transformer are connected and which serves for converting AC voltage into 
DC voltage; 
reactors connected in series to each of two output lines of the second 
power conversion unit; 
a third power conversion unit connected to outputs of the reactors and 
converting DC voltage into AC voltage; and 
a control circuit for controlling turn-on and off of switching devices 
constituting the first power conversion unit and the third power 
conversion unit, the control circuit comprising: 
means for generating a sine-wave signal which is an output target value of 
the inverter apparatus; 
means for generating a carrier signal for performing PWM control using the 
sine-wave signal; 
means for generating a rectangular-wave signal equal in frequency to the 
carrier signal and shifted in phase by a 1/4 period; 
pulse train signal generating means for generating a first pulse train 
signal by comparing the sine-wave signal and the carrier signal with each 
other and by outputting a first level when the sine-wave signal is greater 
than the carrier signal, or outputting a second level when the sine-wave 
signal is smaller than the carrier signal; generating a second pulse train 
signal by inverting the first pulse train signal; generating a third pulse 
train signal by comparing an inverted carrier signal, which is an 
inversion of the carrier signal, and the sine-wave signal with each other 
and by outputting the first level when the inverted carrier signal is 
greater than the sine-wave signal, or outputting the second level when the 
inverted carrier signal is smaller than the sine-wave signal; and 
generating a fourth pulse train signal by inverting the third pulse train 
signal; 
means for gating the first to fourth pulse train signals with the 
rectangular-wave signal; and 
means for performing ON-OFF control on the switching devices constituting 
the first power conversion unit with the gated pulse train signals. 
The first power conversion unit is preferably a high-frequency inverter 
bridge that converts a DC input into a high-frequency alternating current 
(ten kHz or more). The transformer is preferably a high-frequency 
transformer that insulates the input side from the output side of the 
high-frequency inverter bridge. The second power conversion unit is 
preferably a diode bridge for rectifying the high-frequency alternating 
current at the output end of the high-frequency transformer. 
Also, preferably, the reactor smooths a rectified waveform to remove 
high-frequency components, and the third power conversion unit is a 
low-frequency inverter bridge that performs fold-back control with low 
frequency (e.g., 50/60 to several hundreds Hz). In addition, the reactor 
may be provided on the preceding stage of the third power conversion unit. 
According to the above-described inverter apparatus, the apparatus adopts 
an inverter control method in which the primary side of the high-frequency 
transformer is excited by a high-frequency alternating current which has a 
pulse-width modulated pulse train inverted alternately of the positive and 
negative polarity and which is equal in frequency to the carrier signal. 
As a result, the secondary-side output waveform of the high-frequency 
transformer also has a high-frequency AC waveform having a pulse-width 
modulated pulse train inverted alternately of the positive and negative 
polarity. Accordingly, the diode bridge provided at the succeeding stage 
of the high-frequency transformer rectifies the pulse-width modulated, 
alternately positive-negative inverted pulse train signal, whereby a PWM 
pulse train waveform continuous on the positive side is obtained. Then, 
the waveform is smoothed by the DC reactor provided at the succeeding 
stage of the diode bridge, whereby high-frequency components are removed. 
Thus, a DC waveform similar to one which results from full-wave rectifying 
a sine-wave AC waveform of the same frequency as the sine-wave signal can 
be obtained. Further, in the commercial-frequency inverter bridge at the 
succeeding stage, fold-back control is performed in which a DC waveform 
similar to the result of full-wave rectifying the sine-wave AC waveform is 
inverted alternately of the positive and negative polarity, whereby a 
sine-wave AC waveform can be obtained. 
More preferably, the above-described inverter apparatus is provided with a 
center tap on the secondary side of the transformer. In this case, the 
line from the center tap is connected to the neutral line of the 
low-voltage single-phase three-wire system distribution line, the reactor 
is connected to the two output lines of the second power conversion unit, 
capacitors are connected up-and-down symmetrically between the respective 
two lines and the line from the center tap, and the two output lines of 
the third power conversion unit are connected to the respective lines of 
the low-voltage single-phase three-wire system distribution line other 
than the neutral line. 
With such an arrangement, as a center tap is provided on the secondary side 
of the high-frequency transformer, three output lines can be obtained by 
the center tap and other two output lines on the secondary side of the 
high-frequency transformer. Further, two line voltages of the same voltage 
are generated between the center tap and the other two output lines, while 
a line voltage two times as much as the foregoing line voltage is 
generated between the two lines other than the center tap. That is, three 
line voltages in total can be obtained. Then, these three line voltages 
are interconnected with the low-voltage single-phase three-wire system 
distribution line of the commercial power system. However, the three line 
voltages on the secondary side of the high-frequency transformer are not 
of the sine-wave waveform of the commercial power system, but each of a 
high-frequency AC waveform having a pulse-width modulated pulse train 
inverted alternately of the positive and negative polarity as described 
above. For this reason, the three line voltages are once rectified by the 
diode bridge of the above-described construction, and smoothed by a filter 
circuit composed of a DC reactor and a capacitor, so as to be shaped into 
a DC voltage waveform with high-frequency components removed (a waveform 
that results from full-wave rectifying the sine wave of the commercial 
frequency). Further, by the fold-back control of the low-frequency 
inverter bridge, a sine-wave AC waveform of commercial frequency is 
obtained. 
By the above action, the present inverter apparatus allows a high-frequency 
transformer to be used instead of a power-frequency transformer, so that 
the apparatus can be reduced in size and weight. Also, a stable sine-wave 
AC waveform can be obtained by quite a simple control method that involves 
only adding gate processing, typically exclusive-OR operation, to the 
conventional PWM control. Moreover, an interconnected operation with the 
low-voltage single-phase three-wire system distribution line of the 
commercial power system can be performed with three output lines having 
three line voltages (e.g., 100 VAC, 100 VAC, 200 VAC). 
In one embodiment of the present invention, there is further provided a 
current detector provided between the secondary-side center tap of the 
transformer and the neutral line of the single-phase three-wire system 
distribution line and detecting a transient current; and a first control 
unit for performing ON-OFF control on switching devices provided in the 
third power conversion unit for converting DC power into AC power so that 
the transient current will not flow, based on a direction of the transient 
current detected by the current detector. 
According to the above embodiment, a transient current flowing through the 
neutral line is detected by the current detector inserted between the 
neutral line of the single-phase three-wire system distribution line and 
the center tap of the transformer. Then, based on the direction of the 
transient current detected by the current detector, the first control unit 
controls the turn-on and -off of the switching devices of the third power 
conversion unit so that the transient current will not flow. For example, 
if there is a shift of the turning-on and -off time due to variations in 
characteristics of the switching devices, the shift can be corrected by 
adjusting the turning-on and -off time of the switching devices, whereby 
the transient current can be prevented from flowing through the neutral 
line of the single-phase three-wire system distribution line. 
Accordingly, this inverter apparatus can produce a stable AC voltage with 
low distortion. 
In one embodiment of the present invention, there is further provided a 
voltage detector for detecting line voltages between the neutral line and 
the two voltage lines of the single-phase three-wire system distribution 
line; a circuit breaker provided between the center tap of the transformer 
and the neutral line of the single-phase three-wire system distribution 
line; and a second control unit for performing control so as to open the 
circuit breaker when the transient current detected by the current 
detector is equal to or greater than a specified value, and to close the 
circuit breaker when a voltage difference between the line voltages 
detected by the voltage detector is smaller than the specified value. 
According to the above embodiment, the voltage detector detects line 
voltages between the neutral line and the two voltage lines of the 
single-phase three-wire system distribution line. The second control unit 
opens the circuit breaker provided between the center tap of the 
transformer and the neutral line of the single-phase three-wire system 
distribution line when the transient current detected by the current 
detector is greater than a specified value, whereas it closes the circuit 
breaker when the voltage difference between the line voltages detected by 
the voltage detector is lower than a specified value. Accordingly, when 
loads connected between the neutral line and the two voltage lines of the 
single-phase three-wire system distribution line are balanced, 
interconnected operation is performed with the single-phase three-wire 
system distribution line of the commercial power system. On the other 
hand, when the loads are unbalanced, i.e., when the transient current 
detected by the current detector becomes greater than the specified value, 
the control unit opens the circuit breaker to thereby disconnect only the 
neutral line of the single-phase three-wire system distribution line so 
that the interconnected operation is continued with the single-phase two 
wire system utility line in which only the voltage lines of the 
distribution line are connected. Then, when the voltage difference between 
the line voltages detected by the voltage detector becomes lower than the 
specified value, it is decided that the loads have restored their balanced 
state, where the control unit closes the circuit breaker to connect the 
neutral line of the single-phase three-wire system distribution line with 
the center tap of the transformer, so that the interconnected operation is 
continued with the single-phase three-wire system distribution line. 
Consequently, in normal state, interconnection is achieved between the 
single-phase three-wire system distribution line and three output lines, 
so that an interconnection matched to the distribution method of the 
commercial power system can be achieved. On the other hand, when the loads 
become unbalanced, only the neutral line of the single-phase three-wire 
system distribution line is disconnected, whereby the switching devices 
and the like are prevented from damage due to load unbalance. Moreover, 
the interconnected operation can be continued with the single-phase two 
wires without halting the system operation, so that the commercial power 
system can be supplied with power from the solar cell efficiently. 
In one embodiment of the present invention, there is further provided a 
voltage detector for detecting line voltages between the neutral line and 
the two voltage lines of the single-phase three-wire system distribution 
line; a circuit breaker provided between the center tap of the transformer 
and the neutral line of the single-phase three-wire system distribution 
line; and a second control unit for performing control so as to open the 
circuit breaker when a voltage difference between the line voltages 
detected by the voltage detector is equal to or greater than a specified 
value, and to close the circuit breaker when the voltage difference is 
smaller than the specified value. 
According to the above embodiment, the line voltages between the neutral 
line and the two voltage lines of the single-phase three-wire system 
distribution line are detected. Then, the second control unit opens the 
circuit breaker provided between the center tap of the transformer and the 
neutral line of the single-phase three-wire system distribution line when 
the voltage difference between the line voltages detected by the voltage 
detector is greater than a specified value, whereas it closes the circuit 
breaker when the voltage difference is lower than the specified value. 
Accordingly, when the loads connected between the neutral line and the 
voltage lines of the single-phase three-wire system distribution line are 
balanced, interconnected operation is performed with the single-phase 
three-wire system distribution line of the commercial power system. On the 
other hand, when the loads become unbalanced, i.e., when the voltage 
difference between the line voltages detected by the voltage detector is 
not less than the specified value, the second control unit opens the 
circuit breaker to disconnect only the neutral line of the single-phase 
three-wire system distribution line, so that the interconnected operation 
is continued with the single-phase two wires in which only the voltage 
lines of the distribution line are connected. Then, when the voltage 
difference between the line voltages detected by the voltage detector 
becomes lower than the specified value, it is decided that the loads have 
restored their balanced state, where the second control unit closes the 
circuit breaker so that the neutral line of the single-phase three-wire 
system distribution line is connected to the center tap of the 
transformer. Thus, the interconnected operation is continued with the 
single-phase three-wire system distribution line. 
Consequently, in normal state, the interconnection is achieved between the 
single-phase three-wire system distribution line and three output lines, 
so that an interconnection matched to the commercial power system method 
can be achieved. On the other hand, when the loads become unbalanced, only 
the neutral line of the single-phase three-wire system distribution line 
is disconnected, whereby the switching devices and the like are prevented 
from damage due to load unbalance. Moreover, the interconnected operation 
can be continued with the single-phase two wires without halting the 
system operation, so that the commercial power system can be supplied with 
power from the solar cell efficiently. 
In one embodiment of the present invention, there is provide an 
interconnection type inverter apparatus for converting DC power fed from a 
DC power supply into AC power and supplying the AC power to a single-phase 
three-wire system distribution line having two voltage lines and a neutral 
line of a commercial power system, the inverter apparatus comprising: 
a first power conversion unit for converting the DC power derived from the 
DC power supply into AC power; 
a transformer for transforming the resulting AC voltage derived from the 
first power conversion unit and outputting the transformed AC voltage from 
secondary-side output terminals of the transformer, in which a center tap 
provided at a generally midpoint of a winding of the secondary-side output 
terminals is connected to the neutral line of the single-phase three-wire 
system distribution line; 
a second power conversion unit for converting AC power derived from the 
secondary-side output terminals of the transformer into DC power; 
filter circuits for removing high-frequency components superimposed on the 
DC voltage derived from the second power conversion unit; and 
a third power conversion unit for converting DC power derived from the 
filter circuits into AC power, the third power conversion unit having 
output terminals connected to the two voltage lines of the single-phase 
three-wire system distribution line, respectively. 
According to the above inverter apparatus, the first power conversion unit 
converts DC power fed from the DC power supply into AC power. Then, the 
transformer transforms the AC voltage derived from the first power 
conversion unit, and outputs the transformed AC voltage from the 
secondary-side output terminal. Next, the second power conversion unit 
converts AC power derived from the secondary-side output terminal of the 
transformer into DC power. Thereafter, high-frequency components 
superimposed on the DC voltage outputted from the second power conversion 
unit are removed by the filter circuit. The third power conversion unit 
converts the DC power derived from the filter circuit into AC power, and 
outputs a line voltage between the two voltage lines of the single-phase 
three-wire system distribution line connected to the output terminal of 
the third power conversion unit. Further, since the center tap provided at 
a generally midpoint of the secondary-side winding of the transformer is 
connected to the neutral line of the single-phase three-wire system 
distribution line, the third power conversion unit outputs line voltages 
between the respective voltage lines and the neutral line. That is, 
generally equal line voltages are generated between the respective voltage 
lines and the neutral line of the single-phase three-wire system 
distribution line, and a voltage approximately two times as much as the 
line voltages is generated between the two voltage lines. 
Accordingly, the inverter apparatus of the above embodiment can be reduced 
in size and weight by virtue of using a high-frequency transformer in 
place of the power-frequency transformer having two input lines and three 
output lines. Moreover, interconnected operation with the single-phase 
three-wire system distribution line of the commercial power system can be 
implemented with three output lines having three line voltages. 
In one embodiment of the present invention, the filter circuits are 
respectively composed of a reactor whose one end is connected to one of 
the DC voltage output terminals of the second power conversion unit, and a 
capacitor connected between the other end of the reactor and the center 
tap of the transformer. 
According to the above embodiment, the filter circuits respectively 
composed of the reactor and the capacitor remove high-frequency components 
superimposed on the DC voltages between the two DC voltage output 
terminals of the second power conversion unit and the center tap of the 
transformer. 
Accordingly, waveform shaping can be accomplished by removing 
high-frequency components of the DC voltage derived from the second power 
conversion unit by the above filter circuits of simple construction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment 
An embodiment of the inverter apparatus of the present invention is 
described below in detail with reference to FIG. 1. FIG. 1 is a view 
showing a utility interactive photovoltaic system including the inverter 
apparatus of the present invention. 
DC power generated from a solar cell 2 (with a 3.5 kW output and a 270 V 
open voltage) is converted by an inverter apparatus 200 into AC power of 
the same phase and the same frequency 50/60 Hz as a commercial power 
system 3, and supplied to the commercial power system 3. 
The DC power fed to the inverter apparatus 200 from the solar cell 2 is 
converted into a high-frequency alternating current by a high-frequency 
inverter bridge 4, and supplied to the primary side of a high-frequency 
transformer 5. Whereas a 16 to 19 kHz high-frequency transformer is 
employed in the present embodiment, a case where a 19 kHz high-frequency 
transformer is used is described here for convenience. The high-frequency 
transformer 5 has a function of insulating the solar cell 2 side (primary 
side) and the commercial power system 3 side (secondary side) from each 
other. The insulated high-frequency alternating current is rectified by a 
diode bridge 6 composed of diodes D1, D2, D3, and D4 and provided on the 
secondary side of the high-frequency transformer 5. Then, the rectified 
current is subjected to a process of removing high-frequency components 
contained in the rectified waveform and a smoothing process by a filter 
circuit composed of a DC reactor 7 and a capacitor 47, resulting in a 
direct current of full-wave rectified waveform. A low-frequency inverter 
bridge 8 performs fold-back control on the direct current of full-wave 
rectified waveform at low frequency (50/60 Hz), whereby a sine-wave 
alternating current of low frequency is obtained. Switching devices Q1, 
Q2, Q3, Q4 and S1, S2, S3, S4 that constitute the high-frequency inverter 
bridge 4 and the low-frequency inverter bridge 8, respectively, are 
controlled for their turn-on and -off by a control circuit 9 and gate 
drive circuits 10, 11. Also, a DC capacitor 12 for suppressing variations 
in the input power to the inverter apparatus 200, and a DC input current 
detector 13 are provided at the preceding stage of the high-frequency 
inverter bridge 4. An inverter output current detector 14 is connected to 
the primary side of the high-frequency transformer 5. An interconnection 
relay 55 for switching the interconnection and disconnection with the 
commercial power system 3, and an AC filter 16 are provided at the 
succeeding stage of the low-frequency inverter bridge 8. In addition, the 
inverter output current detector 14, which is connected to the primary 
side of the high-frequency transformer 5 in FIG. 1, may instead be 
connected to the secondary side. 
The control circuit 9, as shown in FIG. 2, comprises an A/D converter 17, a 
signal processing unit 18 for generating a sine-wave signal (50/60 Hz) of 
an output target value of the inverter apparatus 200, a carrier signal (19 
kHz) generator 19 for generating a carrier signal for performing PWM 
control together with the sine-wave signal, a rectangular-wave signal 
generator 20 for generating a rectangular-wave signal which is equal in 
frequency to the carrier signal and which is shifted in phase by a 1/4 
period, an inversion circuit 21, a comparison circuit 22 for comparing the 
sine-wave signal and the carrier signal with each other, a NOT circuit 23, 
and an XOR (exclusive-OR) circuit 24. The control circuit 9 outputs pulse 
train signals for turning on and off the four switching devices Q1 to Q4 
of the high-frequency inverter bridge 4 to the gate drive circuit 10 of 
FIG. 1. 
With the above arrangement, the high-frequency inverter bridge 4 converts 
DC power derived from the solar cell 2 into a high-frequency alternating 
current (19 kHz) and the high-frequency transformer 5 insulates input and 
output from each other. The ON-OFF control in the high-frequency inverter 
bridge 4 is carried out in the following way by the control circuit 9 of 
FIG. 2. First prepared, as shown in FIGS. 3B, 3C, 3D, and 3E, are a first 
pulse train signal A1 obtained by comparing the sine-wave signal (50/60 
Hz), which is an output target value of the inverter apparatus 200 and 
which is generated by the signal processing unit 18, with the carrier 
signal (19 kHz), a second pulse train signal A2 obtained by inverting the 
first pulse train signal A1, a third pulse train signal A3 derived by 
comparing an inverted carrier signal obtained by inverting the foregoing 
carrier signal, with the sine-wave signal, and a fourth pulse train signal 
A4 obtained by inverting the third pulse train signal. Next obtained are a 
rectangular-wave signal as shown in FIG. 3F which is equal in frequency to 
the carrier signal and shifted in phase by a 1/4 period and which is 
generated by the rectangular-wave signal generator 20, and fifth to eighth 
pulse train signals B1, B2, B3 and B4 as shown in FIGS. 3G, 3H, 3I, and 3J 
by processing the first to fourth pulse train signals A1, A2, A3, and A4 
for exclusive-OR operation by the XOR circuits 24. Then, these fifth to 
eighth pulse train signals are outputted to the gate drive circuit 10 as 
shown in FIG. 1 to perform the ON-OFF control on the switching devices Q1, 
Q2, Q3, and Q4 constituting the high-frequency inverter bridge 4. Although 
the carrier signal and the following signals in FIGS. 3A, 3B . . . , 3L 
should be shown by high frequencies of several tens to several hundreds 
kHz in contrast to the sine-wave signal, those are schematically 
simplified. As a result of this ON-OFF control, the high-frequency 
transformer 5 is excited at a high frequency (19 kHz) Ei which is equal to 
the frequency of the carrier signal and which has a pulse-width modulated 
pulse train inverted alternately of the positive and negative polarity. 
The output timing of the pulse train signals to the gate drive circuit 10 
in the above process is provided by synchronization with the voltage 
signal V.sub.out of the commercial power system 3. As a result, the 
inverter output current is controlled into the same phase as the utility 
line voltage. 
As described above, since the output waveform of the high-frequency 
transformer S is a high-frequency AC waveform having a pulse-width 
modulated pulse train inverted alternately of the positive and negative 
polarity, the diode bridge 6 provided at the succeeding stage of the 
high-frequency transformer 5 rectifies the pulse train signal inverted 
alternately of the positive and negative polarity, obtaining a pulse-width 
modulated pulse train waveform continuous on the positive side as shown by 
E0 of FIG. 3L. Then, a filter circuit composed of the DC reactor 7 and the 
capacitor 47 provided at the succeeding stage of the diode bridge 6 as 
shown in FIG. 1 performs a high-frequency component removing process and a 
smoothing process, whereby a DC waveform equivalent to a waveform which is 
obtained by full-wave rectifying the sine-wave AC waveform having the same 
frequency as the sine-wave signal is obtained. Further, the 
succeeding-stage low-frequency inverter bridge 8 performs the fold-back 
control of alternately inverting the full-wave rectified sine-wave AC 
waveform from positive to negative, whereby a sine-wave AC waveform is 
obtained. 
Second Embodiment 
In an embodiment as shown in FIG. 4, a control circuit 59 does not use the 
carrier signal generator 19, the rectangular-wave signal generator 20, the 
inversion circuit 21, the comparison circuit 22, the NOT circuit 23, or 
the XOR circuit 24 as shown in FIG. 2, but uses a signal processing unit 
58 serving for all the processes that would be done by the above means. 
That is, the control circuit 9 as shown in FIG. 4 is composed of an A/D 
converter 17 and the signal processing unit 58. 
As for the processes performed in the signal processing unit 58, since the 
unit is made up from digital circuits for PWM operation, the processes are 
not carried out sequentially but in batch processing with inputs of 
necessary signals to the signal processing unit 58. As a result, obtained 
are the fifth to eighth pulse train signals B1, B2, B3, and B4 as shown in 
FIG. 3 for turning on and off the four switching devices of the 
high-frequency inverter bridge 4 of FIG. 1. In this embodiment, the signal 
processing unit 58 is composed of a CPU 25, a ROM 26, a RAM 27, a timer 
28, and an I/O interface 29 as shown in FIG. 4. Based on this arrangement, 
the operation within the signal processing unit 58 is described with 
reference to FIG. 3. In addition, the carrier signal of FIG. 3, which 
should actually be shown by high frequency, is simplified here 
schematically. 
The signal processing unit 58 operates to generate a first pulse train 
signal A1, which becomes a High level if the sine-wave signal is greater 
than the carrier signal and which becomes a Low level if it is less as a 
result of a comparison between the sine-wave signal (50/60 Hz) and the 
carrier signal (19 kHz) (two-dots chain line), a second pulse train signal 
A2 obtained by inverting the first pulse train signal A1, a third pulse 
train A3, which becomes a High level if an inverted carrier signal is 
higher than the sine-wave signal and which becomes a Low level if it is 
lower as a result of a comparison between the inverted carrier signal (19 
kHz) (solid line) obtained by inverting the carrier signal and the 
sine-wave signal, and a fourth pulse train signal A4 obtained by inverting 
the third pulse train signal A3. In addition, it is also possible that the 
"Low" level and the "High" level may be replaced with each other in the 
above arrangement. 
With respect to the method of arithmetic operation, the method of producing 
the first pulse train signal A1 is described. First, intersecting points 
between the sine-wave signal and the carrier signal (two-dots chain line) 
are determined. Values between the individual intersecting points 
correspond to pulse widths, which are the ON time or OFF time of a pulse 
train signal. Next, pulse widths are calculated with respect to one period 
of the sine-wave signal (it also may be calculated with respect to half a 
period). Within the CPU 25 of FIG. 4, the pulse widths are replaced with 
timer counts of the timer 28. Then, a sequence of a timer count train 
equivalent to the pulse widths calculated with respect to one period of 
the sine-wave signal is temporarily stored in the RAM 27. When a 
synchronization signal generated from the voltage signal V.sub.out of the 
commercial power system 3 is inputted to the CPU 25 via the I/O interface 
29, the temporarily stored timer count train is read from the RAM 27 one 
by one and set to the timer 28. When a timer interrupt has taken place, 
the timer is decremented by an extent of the timer count set to the timer 
28. At the same time, bit 1 corresponding to the "High" level of the pulse 
signal or bit 0 corresponding to the "Low" level is outputted to the I/O 
interface 29. By such an operation, until the counter reaches the count 0, 
i.e., until an extent of decrement that corresponds to an event of the 
next interrupt is reached, the "High" level or the "Low" level is 
maintained. The "High" or "Low" level is outputted to the gate drive 
circuit 10 of FIG. 1 via the I/O interface 29, whereby a pulse signal 
having one pulse width is generated. 
The above operation is executed in the CPU 25 by reading operational 
instructions within the ROM 26. When these operations are done for all the 
timer counts temporarily stored in the RAM 27, then the first pulse train 
signal A1 with respect to one period of the sine-wave signal is generated 
and outputted to the gate drive circuit 10 of FIG. 1. The second to fourth 
pulse train signals A2, A3, and A4 are also generated and outputted in the 
same way. The gate drive circuit 10 performs the ON-OFF control on the 
four switching devices Q1, Q2, Q3, and Q4 constituting the high-frequency 
inverter bridge However, the high-frequency transfer 5 is not exited at 
the same high frequency (several tens to hundreds kHz) as that of the 
carrier signal with the pulse train signals A1, A2, A3 and A4. Therefore, 
in actual, exclusive-OR is performed on the rectangular-wave signal which 
is equal in frequency to the carrier signal prepared previously in the RAM 
27 and shown in FIG. 3F, and which is shifted in phase by a 1/4 period, 
and the pulse train signals A1, A2, A3 and A4 respectively. The fifth to 
eighth pulse train signals B1, B2, B3 and B4 are obtained as shown in 
FIGS. 3G, 3H, 3I and 3J. The new fifth to eighth pulse train signals B1, 
B2, B3 and B4 are outputted to the gate drive circuit 10, ON-OFF control 
is performed on the four switching devices Q1, Q2, Q3 and Q4 which 
constitute the high-frequency inverter bridge 4. As a result, the 
high-frequency transformer is excited with a high frequency Ei having the 
same high frequency (several tens to several hundreds kHz) as the carrier 
signal and having a pulse-width modulated pulse train inverted alternately 
of the positive and negative polarity. The control circuit 59, if formed 
from digital circuits in this way, can be simplified in construction. 
Third Embodiment 
An inverter apparatus which is a third embodiment of the present invention 
is described with reference to FIG. 5. An inverter apparatus 300 of the 
present invention is inserted between a solar cell 2 and an existing 
commercial power system 3. DC power generated by the solar cell 2 is 
converted into AC power of 50/60 Hz, and supplied to loads in 
interconnection with the commercial power system 3 while an inverse power 
flow to the commercial power system 3 is also performed. The respective 
lines of the single-phase three-wire system distribution line of the 
commercial power system 3 are hereinafter designated as n line, which is 
the neutral line, u line, and v line as shown in FIG. 5, respectively. 
Referring to the arrangement of the inverter apparatus 300 of the 
embodiment, an input capacitor 12 is provided for suppressing the input 
voltage of the inverter apparatus 300 from any rapid change due to 
variation in the output of the solar cell caused by sunlight variation. 
The DC power inputted to the inverter apparatus 300 is led to the 
high-frequency inverter bridge 4 composed of switching devices Q1 to Q4, 
and converted from DC into AC current. Further, the output of the 
high-frequency inverter bridge 4 is supplied to the primary side of a 
high-frequency transformer 65, where it is electrically insulated. A 
center tap is provided on the secondary side of the high-frequency 
transformer 65 and the line from the center tap is connected to the 
neutral line, n line, of the single-phase three-wire system distribution 
line of the commercial power system via an interconnection relay 75. Also, 
the other two lines on the secondary side of the high-frequency 
transformer 65 are inputted to AC input terminals of the diode bridge 6 
and converted from AC into DC current. DC reactors 7a and 7b are connected 
to the two lines derived from DC output terminals of the diode bridge 6 
up-and-down symmetrically, and further capacitors 30a and 30b inserted 
between the two lines and the neutral line n are connected. Thereafter, 
the DC current is inputted to the low-frequency inverter bridge 8 composed 
of switching devices S1 to S4, where the input is converted again from DC 
to AC current. The two output lines of the low-frequency inverter bridge 8 
are connected to two lines other than the neutral line, n line, of the 
single-phase three-wire system distribution line of the commercial power 
system 3, i.e., u line and v line, via filter circuits 31a and 31b 
arranged up-and-down symmetrically between the interconnection relay 75 
and the neutral line n. The filter circuits 31a and 31b have reactors 
L.sub.3 and L.sub.4, capacitors C.sub.3 and C.sub.4, respectively. 
Next the action of the inverter apparatus of the present embodiment is 
described. First, in the high-frequency inverter bridge 4, gate drive 
signals for IGBT (Insulated Gate Bipolar Transistor) devices Q1 to Q4, 
which are four switching devices constituting the high-frequency inverter 
bridge 4, are generated by a comparison between the sine-wave signal 
(50/60 Hz) and the high-frequency carrier signal (19 kHz), whereby the 
primary side of the high-frequency transformer 65 is excited by a pulse 
train signal that has been subjected to a sine-wave pulse-width 
modulation. In this process, the excitation of the primary side of the 
high-frequency transformer 65 with high-frequency alternating current (19 
kHz) is performed by a pulse train as shown by "a" of FIG. 6A, which 
results from inverting the pulse-width modulated pulse train signal 
alternately of the positive and negative polarity. Although the pulse 
train is illustrated schematically for a better understanding, it has the 
same frequency as the high-frequency carrier signal. As the control method 
for exciting the high-frequency transformer 65 with high-frequency 
alternating current, as described above, the gate drive signals for the 
IGBT devices Q1 to Q4 constituting the high-frequency inverter bridge 4 
are generated in the same way as in the first or second embodiment. That 
is, the control circuit 9 of FIG. 5 has the same structure as the control 
circuit 9 of FIG. 2, and acts as shown in FIG. 3. 
In the manner as described above, the high-frequency alternating current 
(19 kHz) is supplied to the primary side of the high-frequency transformer 
65, and a high-frequency alternating current transformed to a voltage 
corresponding to the turn ratio of the transformer is outputted to the 
secondary side of the high-frequency transformer 65. In this case, the 
high-frequency transformer 65 has both a function of electrically 
insulating the commercial power system 3 and the solar cell 2 from each 
other and another function of transforming the output voltage relative to 
the input voltage at a ratio of transformation corresponding to the turn 
ratio. Further, between the center tap and the two output lines of the 
transformer, the center tap provided on the secondary side of the 
high-frequency transformer 65 causes three line voltages to be generated: 
one between the center tap and the upper line of the transformer output, 
another between the center tap and the lower line of the transformer 
output, and the other between the upper line of the transformer output and 
the lower line of the transformer output. The waveform of these three line 
voltages on the secondary side of the high-frequency transformer 65 is 
also a high-frequency alternating current having a pulse-width modulated 
pulse train inverted alternately of the positive and negative polarity, 
similar to that of the primary side as shown in FIG. 6A. 
The above three line voltages are rectified by the succeeding-stage diode 
bridge 6, resulting in a first line voltage A, a second line voltage B, 
and a third line voltage C, which are DC voltages of pulse-width modulated 
pulse trains continuous on the positive side as shown in FIGS. 6A, 6B, and 
6C, respectively. Further, via the filter circuit composed of the DC 
reactors 7a, 7b provided in series on two output lines of the diode bridge 
6 and the capacitors 30a, 30b provided up-and-down symmetrically between 
the two output lines and the output line derived from the center tap of 
the high-frequency transformer 65, the first line voltage A, the second 
line voltage B, and the third line voltage C have their high-frequency 
ripple components removed from the DC voltage waveforms A, B, and C so as 
to be smoothed as shown by "A'", "B'", and "C'" of FIGS. 7A, 7B, and 7C, 
thus resulting in DC voltage waveforms similar to a waveform resulting 
from full-wave rectifying a low-frequency sine wave. 
Between the first line voltage A' and the second line voltage B' of the 
filter circuit output as shown in FIGS. 7A and 7B, and the third line 
voltage C' as shown in FIG. 7C, which is the line voltage between two 
lines other than the line connected to the center tap of the 
high-frequency transformer 65, there is a relationship that first line 
voltage A'+second line voltage B'=third line voltage C'. If the center tap 
of the transformer 65 is assumed to be the midpoint of the secondary side 
winding, then the first line voltage equals the second line voltage so 
that the third line voltage has a voltage value two times larger than that 
of the first or second line voltage. Also, in the present embodiment, the 
waveform of the current flowing through the two output lines of the filter 
circuit other than one line connected to the center tap is the same 
waveform in phase as the voltage waveform as shown in FIGS. 7A and 7B. 
Further, the two lines constituting the three line voltages (other than the 
one line connected to the center tap) are inputted to the succeeding-stage 
low-frequency inverter bridge 8. Gate terminals of the four IGBT devices 
(S1 to S4) constituting the low-frequency inverter bridge 8 are turned on 
and off alternately between S1, S4 and S2, S3 with commercial frequency 
i.e. power frequency. That is, in synchronization with the valleys of the 
voltage values (0 V points) of the individual line voltages as shown in 
FIGS. 7A and 7B, the switching devices S2 and S3 are turned on while S1 
and S4 are off, and so on. As a result, the waveforms of voltage and 
current shown in FIGS. 7A and 7B have the mountains of full-wave rectified 
sine waves inverted alternately and up-and-down symmetrically, so that the 
voltage waveforms as shown in FIGS. 7A and 7B are converted into sine-wave 
AC waveforms of commercial frequency. Further, the waveforms are smoothed 
by the filter circuits 31a, 31b arranged up-and-down symmetrically between 
the neutral line and the two output lines of the low-frequency inverter 
bridge 8 via the interconnection relay 75, so that voltage waveforms and 
current waveforms of commercial frequency shaped by removal of the 
harmonic-component as shown in FIGS. 7D and 7E can be obtained. 
If the voltage between the output line derived from the AC filter circuit 
31a and the one line connected to the center tap is a first line voltage 
A", the voltage between the output line derived from the AC filter circuit 
31b and the one line connected to the center tap is a second line voltage 
B", and if the voltage between the two output lines from the AC filter 
circuit 31a, 31b is a third line voltage C", then the first line voltage 
waveform A", the second line voltage waveform B", and the third line 
voltage waveform C" are as shown in FIGS. 7D, 7E, and 7F, respectively. 
As in the above-described case, the third line voltage C" has a voltage 
value two times larger than that of the first line voltage A" or the 
second line voltage B". By designing the turn ratio of the high-frequency 
transformer 65 so that the first line voltage and the second line voltage 
are 100 V and the third line voltage is 200 V (in the present embodiment, 
the turn ratio of the transformer is 1:2.2 to 2.7 and the center tap of 
the transformer is provided at the midpoint of the secondary winding since 
the rated input voltage is 200 VDC), the inverter apparatus 300 can 
realize the reduction in size and weight of the apparatus with the use of 
the high-frequency transformer 65. Moreover, the inverter apparatus 300 
has the three lines of u line, n line, and v line including the neutral 
line as the outputs of the inverter apparatus 300 and the individual line 
voltages therebetween have line voltages that can be interconnected to the 
single-phase three-wire system distribution line of the commercial power 
system 3. Thus, system interconnection with the single-phase three-wire 
system distribution line of the commercial power system 3 becomes also 
possible. 
Instead of providing the center tap on the secondary side, the 
high-frequency transformer 65 may also have two secondary side windings, 
where the winding start of one winding and the winding end of the other 
may be connected to each other to replace the aforementioned center tap. 
By combining the above-described main circuit construction of the inverter 
apparatus 300 that the high-frequency transformer 65 is provided with a 
center tap and the center tap is connected to the neutral line of the 
commercial power system 3, with the inverter control method that the 
high-frequency transformer 65 is excited with a high-frequency alternating 
current having a pulse-width modulated pulse train inverted alternately of 
the positive and negative polarity, it becomes possible to use a 
high-frequency transformer which is about 1/30 in capacity ratio and about 
1/20 in weight ratio, instead of the power-frequency transformer. Thus, 
the inverter apparatus can be reduced in size and weight as compared with 
the method using a power-frequency transformer. 
Further, it becomes possible to obtain a sine-wave AC waveform reduced in 
distortion similar to the waveform output by the conventional PWM control 
with quite a simple construction in which, for example, only gate 
processing of exclusive OR operation is added to the conventional PWM 
control. 
It is also possible to realize an inverter apparatus of the high-frequency 
insulation type capable of interconnection with the single-phase 
three-wire system distribution line of the commercial power system by the 
same electrical method. 
Fourth Embodiment 
FIG. 8 shows the main-part arrangement of an inverter apparatus 400 of a 
fourth embodiment. Reference numeral 101 denotes a solar cell; 102 denotes 
a high-frequency inverter bridge serving as a first power conversion unit 
whose input terminal is connected to the DC voltage output terminal of the 
solar cell 101 and which is composed of switching devices Q1 to Q4; 103 
denotes a high-frequency transformer whose primary side input terminals 
are connected to the output terminals of the high-frequency inverter 
bridge 102 and which has a center tap at generally the center of the 
secondary winding; 104 denotes a diode bridge serving as a second power 
conversion unit whose input terminals are connected to the secondary side 
output terminals of the high-frequency transformer 103 and which is 
composed of diodes D1, D2, D3, and D4; 105a denotes a filter circuit which 
is composed of a reactor L.sub.1 whose one end is connected to one of the 
DC output terminals of the diode bridge 104 and a capacitor C.sub.1 
connected between the other end of the reactor L.sub.1 and the center tap 
of the high-frequency transformer 103; 105b denotes a filter circuit which 
is composed of a reactor L.sub.2 whose one end is connected to the other 
of the DC output terminals of the diode bridge 104 and a capacitor C.sub.2 
connected between the other end of the reactor L.sub.2 and the center tap 
of the high-frequency transformer 103; and 106 denotes a 
commercial-frequency inverter bridge serving as a third power conversion 
unit whose input terminals are connected to the DC voltage output 
terminals of the filters 105a, 105b and which is composed of switching 
devices S.sub.1 to S.sub.4. Reference numeral 107 denotes an 
interconnection relay for three circuits, where the output terminals of 
the commercial-frequency inverter bridge 106 and the center tap of the 
high-frequency transformer 103 are connected to their corresponding input 
terminals of the relay; 108a denotes a filter circuit composed of a 
reactor L.sub.3 to one end of which one of the output terminals of the 
commercial-frequency inverter bridge 106 is connected via an 
interconnection relay 107 and a capacitor C.sub.3 which is connected 
between the other end of the reactor L.sub.3 and the center tap of the 
high-frequency transformer 103 via an interconnection relay 107; and 108b 
denotes a filter circuit composed of a reactor L.sub.4 whose one end is 
connected to the other of the output terminals of the commercial-frequency 
inverter bridge 106 via an interconnection relay 107 and a capacitor 
C.sub.4 which is connected between the other end of the reactor L.sub.4 
and the center tap of the high-frequency transformer 103 via an 
interconnection relay 107. Further, a current detector 109 is provided 
between the connection point of the capacitors C.sub.1, C.sub.2 of the 
filter circuits 105a, 105b and the interconnection relays 107. Also 
provided is a control circuit 110 as a first control unit for outputting 
gate signals to the gate terminals of the switching devices S.sub.1 to 
S.sub.4 of the commercial-frequency inverter bridge 106 upon receiving a 
signal representing overcurrent derived from the current detector 109, and 
for outputting an open/close signal to the interconnection relays 107. 
Further, the control circuit 110 is similar to the control circuit 9 of 
FIG. 2 and functions as shown in FIG. 3. In addition, an input capacitor 
C.sub.5 is connected between the input terminals of the high-frequency 
inverter bridge 102, so that the high-frequency inverter bridge 102 is 
suppressed by the input capacitor C.sub.5 from rapid change in the DC 
voltage input against variation in the DC voltage output of the solar cell 
101 due to variation in the quantity of sunlight. 
One end of a commercial power supply 111 is connected to the other end of 
the reactor L.sub.3 of the filter circuit 108a via a voltage line u of the 
single-phase three-wire system distribution line of a commercial power 
system 130. One end of a commercial power supply 112 is connected to the 
other end of the commercial power supply 111. Meanwhile, the other end of 
the commercial power supply 112 is connected to the other end of the 
reactor L.sub.4 of filter circuit 108b via the voltage line v of the 
single-phase three-wire system distribution line of the commercial power 
system 130. A neutral point between the commercial power supplies 111, 112 
of the single-phase three-wire system distribution line is connected to 
ground GND. Also, the neutral point of the commercial power supplies 111, 
112 is connected to the connection point between the capacitors C.sub.3, 
C.sub.4 of the filter circuits 108a, 108b via the neutral line n of the 
single-phase three-wire system distribution line, and the neutral line n 
is connected to the center tap of the high-frequency transformer 103 via 
the interconnection relay 107 and the current detector 109. 
In the inverter apparatus 400 with the above arrangement, the control 
circuit 110 outputs gate signals pulse-width modulated by using a 
high-frequency carrier (20 kHz) similarly to FIG. 3. Receiving the gate 
signals at the gate terminals of the switching devices S.sub.1 to S.sub.4 
of the high-frequency inverter bridge 102, the high-frequency inverter 
bridge 102 converts DC power fed from the solar cell 101 into AC power. 
That is, the control circuit 110 generates a sine-wave signal of the same 
phase and the same frequency (50/60 Hz) as the voltage waveform of the 
commercial power system 130 by an unshown sine-wave generation circuit, 
and outputs gate signals pulse-width modulated based on the sine wave and 
the high-frequency carrier, thereby controlling the inverter output 
current. Then, the high-frequency transformer 103 transforms the AC 
voltage derived from the output terminals of the high-frequency inverter 
bridge 102 at a ratio of transformation according to a specified turn 
ratio. By this high-frequency transformer 103, the DC side of the solar 
cell 101 and the AC side of the commercial power system 130 are 
electrically insulated from each other. Further, by providing a center tap 
to the secondary side winding of the high-frequency transformer 103, there 
can be obtained three pulse-width modulated high-frequency AC voltages 
between the center tap and two output terminals of the high-frequency 
transformer 103, and between the two output terminals of the 
high-frequency transformer 103. 
The pulse-width modulated high-frequency AC voltages from the 
high-frequency transformer 103 are rectified by the succeeding-stage diode 
bridge 104, resulting in a first line voltage V.sub.1, a second line 
voltage V.sub.2, and a third line voltage V.sub.3, which are all DC 
voltages on which a high-frequency ripple voltage has been superimposed, 
as shown in FIGS. 9A, 9B, and 9C. Further, by the filter circuits 105a, 
105b which are composed of the reactor L.sub.1 and capacitor C.sub.1, and 
the reactor L.sub.2 and capacitor C.sub.2, respectively, a first line 
voltage V.sub.11, a second line voltage V.sub.12, and a third line voltage 
V.sub.13 have their high-frequency ripple components smoothed from the DC 
voltage waveforms as shown in FIGS. 10A, 10B, and 10C, resulting in DC 
voltage waveforms resulting from full-wave rectifying the sine wave of the 
commercial frequency. In this case, the peak voltage of the first line 
voltage V.sub.11 and the second line voltage V.sub.12 is 141 VDC, and that 
of the third line voltage V.sub.13 is 282 VDC. 
Among the first line voltage V.sub.11, the second line voltage V.sub.12, 
and the third line voltage V.sub.13 of the output of the filter circuits 
105a, 105b, there is a relationship that 
EQU V.sub.13 =V.sub.11 +V.sub.12. 
That is, since the center tap of the high-frequency transformer 103 is 
provided at generally the midpoint of the secondary side winding, the 
first line voltage V.sub.11 and the second line voltage V.sub.12 become 
generally equal to each other, while the third line voltage V.sub.13 
becomes a voltage value approximately two times larger than that of the 
first line voltage V.sub.11 or the second line voltage V.sub.12. Also, 
since the inverter output current is pulse-width modulated by a sine-wave 
signal of the same phase and the same frequency as the voltage waveform of 
the commercial power system 103, the waveforms of the currents flowing 
through the output terminals of the filter circuits 105a, 105b is a 
waveform equal in phase with the voltage waveforms, as shown in FIGS. 10A 
and 10B. 
Further, the third line voltage V.sub.13 is inputted to the input terminals 
of the succeeding-stage commercial-frequency inverter bridge 106. At this 
point, the control circuit 110 turns on and off alternately the switching 
devices S1, S4 and the switching devices S2, S3 by controlling the gate 
terminals of the switching devices S1 to S4 based on the commercial 
frequency, the switching devices S1 to S4 being four IGBTs constituting 
the commercial-frequency inverter bridge 106. That is, in synchronization 
with the valley bottoms (0 V points) of the voltage value of the line 
voltages V.sub.11, V.sub.12 as shown in FIGS. 10A, 10B, the control 
circuit 110 turns off the switching devices S2, S3 while the switching 
devices S1, S4 are on, and so forth. As a result, voltage waveform and 
current waveform of the line voltage V.sub.11, V.sub.12 are inverted 
alternately of the crests of the individual sine waves of full-wave 
rectified waveforms so as to be converted into sine-wave AC waveforms of 
commercial frequency. Further, the sine-wave AC voltage derived from the 
commercial-frequency inverter bridge 106 has harmonic components removed 
by the filter circuits 108a, 108b so that commercial-frequency AC voltage 
waveforms that have been shaped in waveform can be obtained as shown in 
FIGS. 11A, 11B, and 11C. FIG. 11A shows a first line voltage V.sub.21 
across the capacitor C.sub.3 of the filter circuit 108a, FIG. 11B shows a 
second line voltage V.sub.22 across the capacitor C.sub.3 of the filter 
circuit 108b, and FIG. 11C shows a third line voltage V.sub.23 between the 
two output terminals of the filter circuits 108a, 108b. 
In this case, the ratio of transformation of the high-frequency transformer 
103 is designed so that the third line voltage V.sub.23 has a voltage 
value approximately two times larger than that of the first line voltage 
V.sub.21 or the second line voltage V.sub.22 and that the first line 
voltage V.sub.21 and the second line voltage V.sub.22 are 100 VAC and the 
third line voltage V.sub.23 is 200 VAC with respect to the DC voltage of 
the output of the solar cell 101 (in the inverter apparatus 400 of the 
fourth embodiment, since the rated input voltage is 200 VDC, it is assumed 
that the turn ratio of the high-frequency transformer 103 is 1:2.2 to 2.7 
and the center tap of the high-frequency transformer 103 is at generally 
the midpoint of the secondary winding). 
In this case also, since the gate terminals of the switching devices S1 to 
S4, which are the four IGBTs constituting the commercial-frequency 
inverter bridge 106, are controlled in synchronization with the valley 
bottoms (0 V points) of the voltage value of the line voltages as shown in 
FIGS. 10A and 10B, the switching devices S1, S4 and the switching devices 
S2, S3 are turned on and off alternately, whereby the full-wave rectified 
sine waves are alternately inverted at their crests and converted into 
commercial-frequency sine-wave AC waveforms as shown in FIGS. 11A and 11B. 
In order to accomplish this control, the switching devices S1, S4 or the 
switching devices S2, S3, which are the four IGBTs of the 
commercial-frequency inverter bridge 106, need to be each turned on and 
off simultaneously. However, in stricter sense, there are some cases where 
even if utterly the same gate signals are given, the timing with which 
switching devices S1 to S4 turn on and off may shift due to variations in 
the characteristics of the individual IGBTs. 
For example, referring to FIG. 8, in the case where the same gate signals 
are given from the control circuit 110 to the gate terminals of the 
switching devices S1, S4 so that the switching devices S1, S4 are turned 
on simultaneously, if the switching device S1 is turned on earlier than 
the switching device S4 by a slight time interval due to variation in the 
characteristics of the switching devices S1, S4, then a current flows 
through a path that leads from the upper output terminal of the 
high-frequency transformer 103 via the reactor L.sub.1, the switching 
device S1, the filter circuit 108a, the voltage line u, the neutral line 
n, and the current detector 109, to the center tap of the high-frequency 
transformer 103. Thereafter, in the aforementioned slight time interval, 
when the switching device S4 is turned on, a current flows through a path 
that leads from the center tap of the high-frequency transformer 103 via 
the current detector 109, the neutral line n, the voltage line v, the 
filter circuit 108b, the switching device S4, and the reactor L.sub.2, to 
the lower output terminal of the high-frequency transformer 103. 
Accordingly, when the loads connected to the single-phase three-wire 
system distribution line are balanced, a transient current matching the 
loads flows in a direction from the neutral line n of the commercial power 
system 130 toward the center tap of the high-frequency transformer 103 
during the slight time interval from when the switching device S1 is 
turned on until when the switching device S4 is turned on. Then, when the 
switching device S4 is turned on, a current equivalent to the above 
current flows in a direction reverse to the above direction, so that the 
apparent current becomes zero. 
As seen above, with balanced loads of the single-phase three-wire system 
distribution line of the commercial power system 130, when the switching 
device S1 and the switching device S4 are turned on simultaneously, the 
current flowing through the current detector 109 is zero. However, if the 
timing with which the individual switching devices are turned on is 
shifted, a current flows through the neutral line for a slight time 
interval to which the timing is shifted. That is, when the switching 
device S1 is turned on earlier than the switching device S4 by a slight 
time interval, a transient current flows for the slight time interval from 
the neutral line n of the commercial power system 130 toward the center 
tap of the high-frequency transformer 103 (negative direction). On the 
other hand, when the switching device S4 is turned on earlier than the 
switching device S1 by a slight time interval, a transient current flows 
for the slight time interval from the center tap toward the neutral line n 
(positive direction). 
In addition, even with unbalanced loads of the single-phase three-wire 
system distribution line of the commercial power system 130, there will 
arise a phenomenon similar to the above as shown in FIG. 12B. In this 
case, although some current has previously been flowing through the 
neutral line n, a transient current flows through the current detector 109 
at zero-cross points every half periods of the commercial-frequency sine 
wave with shifted timing with which the switching devices are turned on. 
Likewise, when the switching device S1 and the switching device S4 are off, 
and when the switching device S2 and the switching device S3 are off, the 
same thing will take place. Regardless of whether the loads are balanced 
or unbalanced, the waveform of the current flowing through the neutral 
line n is such that a transient current flows at zero-cross points every 
half periods of the commercial-frequency sine wave either in either the 
positive or negative direction, as shown in FIGS. 12A and 12B. 
Thus, receiving the signal representing an overcurrent derived from the 
current detector 109, the control circuit 110 discriminates the direction 
in which the transient current flows (whether it is the positive or 
negative direction). If it is decided that the transient current has 
flowed in the negative direction, the control circuit 110 controls the 
gate signals so as to delay by a slight time interval the turn-on of the 
switching device S1 and the turn-off of the switching device S3, and to 
expedite by a slight time interval the turn-off of the switching device S2 
and the turn-on of the switching device S4. Meanwhile, if it is decided 
that the transient current has flowed in the positive direction, the 
control circuit 110 controls the gate signals so as to delay by the slight 
time interval the turn-off of the switching device S2 and the turn-on of 
the switching device S4, and to expedite by the slight time interval the 
turn-on of the switching device S1 and the turn-off of the switching 
device S3. In addition, the control circuit 110 controls the gate signals 
in such a way that upper and lower limits of the turn-on and -off time of 
the switching devices S1 to S4 will not exceed predetermined upper and 
lower limits. In this way, the transient current that flows through the 
current detector 109 inserted between the neutral line n of the 
single-phase three-wire system distribution line and the center tap of the 
high-frequency transformer 103 (the current is caused by a shift of the 
ON-OFF timing due to variation in the characteristics of the switching 
devices S1 to S4) can be offset. 
As seen above, the inverter apparatus 400 is inserted between the solar 
cell 101 and the single-phase three-wire system distribution line of the 
existing commercial power system 130, and converts DC power generated by 
the solar cell 101 into AC power of 60/50 Hz. Thus, the inverter apparatus 
400 supplies the power to loads in interconnection with the commercial 
power system 130 and supplies inverse power flow also to the commercial 
power system 130. Accordingly, the inverter apparatus 400 can be reduced 
in size and weight (with a capacity ratio of about 1/4 and a weight ratio 
of about 1/6 relative to the conventional) by using the above 
high-frequency transformer 103. Moreover, since the inverter apparatus 400 
has three lines of the neutral line n, the voltage line u, and the voltage 
line v as its outputs, the interconnected operation with the single-phase 
three-wire system distribution line of the commercial power system 130 can 
be implemented. 
Also, the control circuit 110 controls the timing of the turn-on and -off 
of the switching devices S1 to S4 of the commercial-frequency inverter 
bridge 106 based on the direction of the transient current detected by the 
current detector 109 by a slight time interval. Thus, it is possible to 
prevent any transient current from flowing through the neutral line n with 
the ON-OFF timing of the switching devices shifted due to variations in 
the characteristics of the switching devices. 
Fifth Embodiment 
FIG. 13 shows the main-part arrangement of an inverter apparatus 500 of a 
fifth embodiment of the present invention. In addition to the arrangement 
of the inverter apparatus 400 of the fourth embodiment, a circuit breaker 
218 is provided instead of the interconnection relay between the center 
tap of a high-frequency transformer 203 and a neutral line n of the 
single-phase three-wire system distribution line of a commercial power 
system 230. Also, a load 221 is connected between a voltage line u and the 
neutral line n of the single-phase three-wire system distribution line of 
the commercial power system 230, while a load 222 is connected between a 
voltage line v and the neutral line n. As a means for detecting that the 
loads 221, 222 have become unbalanced, a current detector 209 detects that 
an overcurrent has flowed through the neutral line n of the single-phase 
three-wire system distribution line connected to the center tap of the 
high-frequency transformer 203. Also, as a means for detecting that the 
above unbalanced state has been resolved, there is provided a voltage 
detector 213 for detecting both a line voltage V.sub.un between the 
voltage line u and the neutral line n of the single-phase three-wire 
system distribution line and a line voltage V.sub.vn between the voltage 
line v and the neutral line n. A control circuit 220 is further provided 
as a second control unit for deciding whether the loads are balanced or 
unbalanced, based on the line voltages V.sub.un, V.sub.vn derived from the 
voltage detector 213 to control the opening and closing of the circuit 
breaker 218. It is noted that like components are designated by like 
reference numerals in connection with the inverter apparatus 400 and their 
description is omitted. 
In the inverter apparatus 500 with the above arrangement, in interconnected 
operation with its output terminal connected to the single-phase 
three-wire system distribution line of the commercial power system 230, 
when the loads 221, 222 come into an increasingly unbalanced state, the 
control circuit 220 decides whether or not the signal representing a 
transient current derived from the current detector 209 is greater than a 
predetermined set value. If it is decided that the signal representing the 
transient current is greater than the set value, the control circuit 220 
outputs a control signal representing the opening to the circuit breaker 
218. Then, the circuit breaker 218 is opened so that the connection 
between the inverter apparatus 500 and the neutral line n is shut off, 
with the result that the inverter apparatus 500 is connected only to the 
voltage lines u and v, where the interconnected operation with the 
commercial power system 230 is performed with single-phase two wires, 200 
V. 
On the other hand, while the inverter apparatus 500 is running an 
interconnected operation with single-phase two wires, 200 V of the voltage 
lines u and v of the commercial power system 230 with the circuit breaker 
218 opened as described above, the voltage detector 213 detects the line 
voltage V.sub.un between the voltage line u and the neutral line n, and 
the line voltage V.sub.vn between the voltage line v and the neutral line 
n, which are the line voltages across the loads 221, 222. Receiving 
signals representing the line voltages V.sub.un and V.sub.vn derived from 
the voltage detector 213, the control circuit 220 decides that the voltage 
difference between the line voltages V.sub.un and V.sub.vn is not more 
than a predetermined set value, and then outputs a control signal 
representing the closing to the circuit breaker 218. Then, the circuit 
breaker 218 is closed so that the inverter apparatus 500, which has been 
running an interconnected operation with single-phase two wires, 200 V, 
can return again to the interconnected operation with the commercial power 
system 230 and the single-phase three wires. In addition, the circuit 
breaker 218 is closed by detecting the voltage difference of the line 
voltage V.sub.un between the voltage line u and neutral line n from the 
line voltage V.sub.vn between the voltage line v and neutral line n, based 
on the fact that a reduced voltage difference makes it possible to decide 
that the load unbalanced state has been resolved. In addition, the control 
circuit 220 has a constitution similar to the control circuit 9 of FIG. 2, 
and functions as shown in FIG. 3. 
From the above description, when interconnected operation with the 
commercial power system 230 is done by the single-phase three-wire system 
distribution line in normal operation, the inverter apparatus 500 can 
continue the interconnected operation with the single-phase two wires, 200 
V by disconnecting only the neutral line n of the single-phase three wires 
from the inverter apparatus 500 when the loads 211, 222 connected between 
the neutral line n of the commercial power system 230 and the individual 
voltage lines become unbalanced. Accordingly, even if the loads 221, 222 
have become unbalanced, the interconnected operation is never halted, so 
that efficient power supply can be ensured for the commercial power system 
230. 
Although the solar cells 101, 201 have been employed in the fourth and 
fifth embodiments, the DC power supply is not limited to those but may of 
course be a DC power supply such as a fuel cell. 
Also, although the high-frequency transformers 103, 203 have been provided 
with the center tap at generally the center of the secondary side winding 
in the fourth and fifth embodiments, yet the transformer may be provided 
with two secondary windings, where the winding start of one winding and 
the winding end of the other may be connected to each other and the 
resulting connecting point may be the center tap. 
Although the filter circuits 205a, 205b composed of the reactors L.sub.1, 
L.sub.2 and the capacitors C.sub.1, C.sub.2 have been used in the fourth 
and fifth embodiments, the low-pass filters are not limited to these, but 
may be another if it can remove high-frequency components superimposed on 
the DC voltage derived from the second power conversion unit. That is, the 
filter circuits may be those composed of only reactors whose one end is 
connected to the output terminal of the second power conversion unit. 
In the above fifth embodiment, a current detector 209 inserted between the 
center tap of the high-frequency transformer 203 and the neutral line n 
has been used as a means for detecting any load unbalanced state to detect 
an overcurrent flowing through the neutral line n. However, without being 
limited to this arrangement, any unbalanced state of loads may also be 
detected by detecting a voltage difference of line voltages between the 
neutral line and the individual voltage lines by using a voltage detector 
and by deciding whether or not the voltage difference is greater than a 
specified value. 
As will be clear from the foregoing description, the inverter apparatus of 
the present invention adopts an inverter control method in which the 
primary side of the high-frequency transformer is excited by a 
high-frequency alternating current which has a pulse-width modulated pulse 
train inverted alternately of the positive and negative polarity and which 
is equal in frequency to the carrier signal. As a result, the 
secondary-side output waveform of the high-frequency transformer also has 
a high-frequency AC waveform having a pulse-width modulated pulse train 
inverted alternately of the positive and negative polarity. Accordingly, 
the diode bridge provided at the succeeding stage of the high-frequency 
transformer rectifies the pulse-width modulated, alternately 
positive-negative inverted pulse train signal, whereby a PWM pulse train 
waveform continuous on the positive side is obtained. Then, the waveform 
is smoothed by the DC reactor provided at the succeeding stage of the 
diode bridge, whereby high-frequency components are removed. Thus, a DC 
waveform similar to one which results from full-wave rectifying a 
sine-wave AC waveform of the same frequency as the sine-wave signal can be 
obtained. Further, in the commercial-frequency inverter bridge at the 
succeeding stage, fold-back control is performed in which a DC waveform 
similar to the result of full-wave rectifying the sine-wave AC waveform is 
inverted alternately of the positive and negative polarity, whereby a 
sine-wave AC waveform can be obtained. 
According to the present invention, it becomes possible to use a 
high-frequency transformer which is about 1/30 in capacity ratio and about 
1/20 in weight ratio, instead of the power-frequency transformer. Thus, 
the inverter apparatus can be reduced in size and weight as compared with 
the method using a power-frequency transformer. 
Further, it becomes possible to obtain a sine-wave AC waveform reduced in 
distortion similar to the waveform output by the conventional PWM control 
with quite a simple construction in which, for example, only gate 
processing of exclusive OR operation is added to the conventional PWM 
control. 
It is also possible to realize an inverter apparatus of the high-frequency 
insulation type capable of interconnection with the single-phase 
three-wire system distribution line of the commercial power system by the 
same electrical method. 
Also, the inverter apparatus of the above embodiment is an interconnection 
type inverter for converting DC power fed from a DC power supply into AC 
power and supplying the power to the single-phase three-wire system 
distribution line having two voltage lines and a neutral line of the 
commercial power system, wherein the DC power derived from the DC power 
supply is converted into AC power by a first power conversion unit, an AC 
voltage derived from the first power conversion unit is transformed by a 
transformer, the transformed AC voltage is outputted from the 
secondary-side output terminal, and a center tap provided at generally the 
midpoint of the winding of the secondary-side output terminal of the 
transformer is connected to the neutral line of the single-phase 
three-wire system distribution line, and wherein AC power derived from the 
secondary-side output terminal of the transformer is converted into DC 
power by a second power conversion unit, high-frequency components 
superimposed on the DC voltage derived from the second power conversion 
unit are removed by filter circuits, a third power conversion unit 
converts DC power derived from the filter circuits into AC power, and the 
inverter apparatus is connected to the commercial power system with the 
three lines of two voltage lines and a neutral line of the single-phase 
three-wire system distribution line. 
Therefore, according to the inverter apparatus of the above embodiment, the 
inverter apparatus can be reduced in size and weight and besides 
interconnected operation can be implemented with the single-phase 
three-wire system distribution line, which is adopted in most newly built 
residential houses. 
Also, according to the inverter apparatus of one embodiment, the filter 
circuits are respectively composed of a reactor whose one end is connected 
to one of two DC voltage output terminals of the second power conversion 
unit, and a capacitor connected between the other end of the reactor and 
the center tap of the transformer. 
Therefore, according to the inverter apparatus of the above embodiment, the 
filter circuits remove high-frequency components superimposed on the 
individual DC voltages between the two DC voltage output terminals of the 
second power conversion unit and the center tap of the transformer. As a 
result, waveform shaping can be implemented by removing high-frequency 
components of the DC voltages derived from the second power conversion 
unit by filter circuits of simple construction. 
Also, in the inverter apparatus of one embodiment, a transient current is 
detected by a current detector provided between the secondary-side center 
tap of the transformer and the neutral line of the single-phase three-wire 
system distribution line, and the first control unit performs ON-OFF 
control on the switching devices provided in the third power conversion 
unit for converting DC power into AC power based on the direction of the 
transient current detected by the current detector, so that the above 
transient current will not flow. 
According to the inverter apparatus of the above embodiment, any transient 
current can be prevented from flowing through the neutral line of the 
single-phase three-wire system distribution line, for example, by 
correcting any shift of the turn-on and -off time due to variation in the 
characteristics of the switching devices. Therefore, this inverter 
apparatus can be interconnected with the commercial power system by stable 
AC voltage output with less distortion. 
In the inverter apparatus of one embodiment, the voltage detector detects 
line voltages between the neutral line and two voltage lines of the 
single-phase three-wire system distribution line, and the second control 
unit performs control in such a way that the circuit breaker provided 
between the center tap of the transformer and the neutral line of the 
single-phase three-wire system distribution line is opened when the 
transient current detected by the current detector is not less than a 
specified value, and that the circuit breaker is closed when the voltage 
difference between the individual line voltages detected by the voltage 
detector is smaller than the specified value. 
According to the inverter apparatus of the above embodiment, even when an 
unbalance has taken place to the loads connected to the single-phase 
three-wire system distribution line, the circuit breaker is opened so that 
the interconnected operation with single-phase two wires (200 V) can be 
continued without halting the inverter apparatus, whereas the circuit 
breaker is closed when the loads have been restored to the balanced state, 
so that the interconnected operation with the single-phase three wires is 
restored. Thus, the output of the solar cell can be connected in inverse 
power flow to the commercial power system efficiently. 
In the inverter apparatus of one embodiment, the voltage detector detects 
line voltages between the neutral line and two voltage lines of the 
single-phase three-wire system distribution line, and the second control 
unit opens the circuit breaker provided between the center tap of the 
transformer and the neutral line of the single-phase three-wire system 
distribution line when the voltage difference between the individual line 
voltages detected by the voltage detector is not less than a specified 
value, whereas it closes the circuit breaker when the voltage difference 
is smaller than the specified value. 
According to the inverter apparatus of the above embodiment, even when an 
unbalanced state has taken place to the loads connected to the 
single-phase three-wire system distribution line, the circuit breaker is 
opened without halting the inverter apparatus, so that the interconnected 
operation with single-phase two wires (200 V) is continued. Besides, when 
the loads have been restored to the balanced state, the circuit breaker is 
closed so that the interconnected operation with the single-phase three 
wires is restored. Thus, the output of the solar cell can be connected in 
inverse power flow to the commercial power system efficiently. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.