Control apparatus of inverter and power generation system using such control apparatus

To provide a cheap inverter which uses a single digital microprocessor and can execute a current control and a maximum power control without changing a design of a hardware even for any switching device or solar cell, there is provided a control apparatus of an inverter for converting a DC electric power into an AC electric power by using a switching device, wherein the control apparatus is constructed by an inversion discrimination circuit for receiving an output current or an output voltage and a reference signal and for discriminating whether a switching state of the switching device should be inverted or not and a gate pulse signal forming unit for changing the switching state of the switching device in response to an inversion request signal which is outputted from the inversion discrimination circuit. The gate pulse signal forming unit has a CPU for forming a gate pulse signal by an interrupting process by using the inversion request signal as an interruption control signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a control apparatus of an inverter and a power 
generation system using such a control apparatus. More particularly, the 
invention relates to a control apparatus of a system linkage inverter 
which is linked to a commercially available power source system and a 
power generation system using such a control apparatus and capable of 
coping with an unstable power generation such as a solar power generation. 
2. Related Background Art 
A solar power generation system that converts an inexhaustible clean solar 
energy into an electric power is desirable. Particularly, in recent years, 
legal preparation is progressing and a system linkage system for 
converting a DC electric power which is generated by a solar cell into an 
AC power by using an inverter and for supplying the AC power to the 
commercially available power source system can be used on a full scale. 
FIG. 1 shows an example of a general solar power generation system. An 
output of a solar cell 1 is connected to a commercially available power 
source system 3 through a system linkage inverter (hereinafter, simply 
referred to as an inverter) 2. The inverter 2 has therein: an input filter 
(DC side filter) 21 having a coil and a capacitor; a switching unit 22 
having a semiconductor switch which is on/off controlled by a gate control 
signal and the like; an output filter (AC side filter) 23 having a coil 
and a capacitor; and a control apparatus 24 for controlling the operation 
of an inverter. The control apparatus 24 has a maximum power control unit 
241 and an instant value current control unit 242. The inverter 2 can also 
have a protecting apparatus and the like. 
The maximum power control unit 241 changes an operation point of a solar 
cell in accordance with a change in insolation intensity or temperature, 
thereby extracting the maximum electric power from the solar cell. The 
control unit 241 inputs a solar cell voltage Vpv and a solar cell current 
Ipv and arithmetically operates an output current instruction value of the 
inverter so that a solar cell output becomes maximum. More particularly, 
the control unit 241 arithmetically operates and calculates a voltage such 
that the solar cell electric power becomes maximum and controls the 
current instruction value so that the solar cell voltage is equal to such 
a voltage. It is a general way to use a digital microprocessor (what is 
called a micom) as an arithmetic operating apparatus. As an example of 
such a maximum power control unit, there is a unit as disclosed in 
Japanese Patent Application Laid-open No. 62-85312. 
The instant value current control unit 242 inputs the current instruction 
value from the maximum power control unit 241 and controls the switching 
unit 22 by a gate control signal so that an output current of the inverter 
almost coincides with the current instruction value. As an example of such 
an instant value current control unit, there is a unit as disclosed in 
U.S. Pat. No. 4,424,557. 
An example of the operation of the instant value current control unit will 
now be described with reference to FIGS. 1 and 2. The instant value 
current control unit 242 inputs the current instruction value from the 
maximum power control unit 241 and sends a gate control signal (on/off) 
instruction signal to the switching unit 22. FIG. 2 shows relations among 
the gate signal, output current waveform, and current instruction value in 
this instance. As shown in FIG. 2, as for the instant value current 
control signal, a gate control signal is transmitted so that the output 
current almost coincides with the current instruction value. Namely, when 
the gate signal is at the high (H) level, the output current increases. 
When the output current exceeds the instruction value by a predetermined 
amount or more, the gate signal is set to the low (L) level, thereby 
reducing the output current. When the output current is lower than the 
instruction value by a predetermined amount or more, the gate signal is 
again set to the H level, thereby increasing the output current. By such a 
control operation, the output current is almost made coincide with the 
instruction value. In the inverter for solar power generation system, in 
order to reduce noises and an output current distortion, in many cases, 
the highest frequency of the gate signal is set to a fairly high value in 
a range from about 10 kHz to 30 kHz. 
The maximum power control unit and instant value current control unit are 
ordinarily individually constructed. This is because control speeds which 
are required for the maximum power control and the instant value current 
control are quite different. Since it is sufficient that the maximum power 
control unit merely traces a fluctuation of the insolation, a control unit 
having a lower switching speed can be used. According to the study of the 
inventors et al., it is sufficient that the control unit can operate at a 
slow period such as 0.1 second. However, the instant value current control 
system continuously compares the output current with the current 
instruction value and must decide an on/off of the gate from the 
comparison result. Moreover, in order to reduce an output current 
distortion and noises, it is required to turn on/off the switching unit at 
a frequency of 10 kHz or higher. For example, in order to realize a 
switching speed of 10 kHz, it is necessary to suppress a control period to 
a value less than at most 100 microseconds. If both of the control 
operations are executed by one digital microprocessor, all of control 
logics can be assembled as a software, so that a flexibility of the system 
remarkably rises. However, if such a construction is ordinarily embodied, 
both of the maximum power control and the instant value current control 
are executed in the control period (for example, 100 .mu.sec), so that an 
extremely high speed microprocessor and an extremely high speed 
analog/digital converter are necessary. They are very expensive and there 
is a problem such that the inverter cannot be cheaply constructed. 
Therefore, in general, the instant value current control unit constructed 
like a hardware and the maximum power control unit constructed like a 
software are combined as mentioned above. Although it is easy to perform 
the instant value current control by the hardware, since two control units 
of the hardware and the digital microprocessor are provided, the costs 
rise and an installing place is necessary. When a new switching device 
appears, it is necessary to change the design of the hardware of the 
current control unit. 
An inverter control apparatus using an interruption by a timer has been 
disclosed in Japanese Patent Application Laid-open No. 57-25171. However, 
such an apparatus cannot be applied to an inverter having an input such 
that it successively changes in accordance with an insolation like a solar 
cell. 
As mentioned above, a proper control apparatus suitable for a linkage 
inverter to perform the instant value current control (waveform formation 
control) and the maximum power control by using a single CPU is not yet 
realized. 
SUMMARY OF THE INVENTION 
The invention is made in consideration of the above problems and it is an 
object of the invention to provide a cheap inverter which uses a single 
digital microprocessor and which can execute a current control and a 
maximum power control without changing a design of a hardware even for any 
switching device or solar cell. 
Another object of the invention is to provide a control apparatus of an 
inverter for converting a DC electric power into an AC electric power by 
using a switching device and for reversely supplying to a commercially 
available power source system, comprising: inversion discriminating means 
for comparing an output current or an output voltage of the inverter with 
a predetermined reference signal, thereby discriminating whether a 
switching state of the switching device is inverted or not; and gate pulse 
signal forming means for inverting the switching state of the switching 
device in response to an inversion request signal which is outputted from 
the discriminating means and, wherein the gate pulse signal forming means 
is constructed by a digital CPU, and the CPU receives the inversion 
request signal as an interruption control signal and forms a gate pulse 
signal to invert the switching state of the switching device in an 
inversion request interruption processing routine after the interruption 
control signal was inputted. 
Still another object of the invention is to provide a power generation 
system comprising: an inverter having such a control apparatus; a solar 
cell for supplying a DC electric power to the inverter; and a commercially 
available power source system as an additional device of the inverter, 
wherein an output electric power of the solar cell is reversely supplied 
to the commercially available power source system. 
To accomplish the above object, according to the invention, there is 
provided a control apparatus of an inverter for converting a DC electric 
power into an AC electric power by using a switching device, comprising: 
an inversion discrimination circuit for receiving an output current or an 
output voltage and a reference signal, thereby discriminating whether a 
switching state of the switching device should be inverted or not; and 
gate pulse signal forming means for changing the switching state of the 
switching device in response to an inversion request signal which is 
outputted from the inversion discrimination circuit and, wherein the gate 
pulse signal forming means is constructed by a CPU and inputs the 
inversion request signal as an interruption control signal of the CPU. 
When considering the switching operation of the inverter, it will be 
understood that a period of time when some process is functionally 
necessary is nothing but a moment for switching the switch. In the 
invention, by paying attention to such a point, the moment for inverting 
is discriminated by a hardware manner and an interruption request signal 
is generated, thereby allowing the CPU to execute the switching operation 
of the switch only for such an instantaneous time. With this method, the 
CPU ordinarily can devote itself to a process such as protecting process, 
maximum power process, or the like such that it has a relatively enough 
response time and can execute a process to form a switching gate pulse by 
using an interrupting process in an extremely short response time. With 
this method, since the gate pulse forming process is described as a 
software of the CPU, even when the switching device is changed, there is 
no need to change the hardware. Further, an independence of mutual 
processing routines is remarkably raised and a debugging can be easily 
performed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiment 1 
FIG. 3 shows a preferred example of a construction of a solar power 
generation system using an inverter according to the invention. 
In FIG. 3, the same component elements as shown in FIG. 1 are designated by 
the same reference numerals. Reference numeral 4 denotes a CPU (central 
processing unit); 5 an inversion discrimination circuit; and 6 a 
converter. The converter 6 is connected to the commercially available 
power source system 3 and supplies a desired current value as a reference 
current to the inversion discrimination circuit 5. An output current from 
the switching unit 22 is supplied to the inversion discrimination circuit 
5. 
According to the inverter 2 in FIG. 3, an analog comparator and a proper 
main process by the CPU are combined and an all-off time to turn off all 
of the switches of the switching unit 22 is set. In FIG. 3, a gate driving 
circuit (not shown) of the switching unit 22 is directly driven by a 
digital output signal from the digital CPU 4. A switch inversion request 
signal from the inversion discrimination circuit 5 is coupled to an 
interruption signal line of the CPU 4. 
As a solar cell 1, there is a cell such that amorphous silicon or crystal 
silicon is used in a photoelectric converting layer. However, there is no 
limitation when embodying the invention. In the embodiment, 56 (14 
series.times.4 parallel) amorphous solar cell modules (trade name: UPM880) 
made by USSC Co., Ltd. are used as a solar cell 1 and an array having an 
output of 200V and 5.6 A is constructed. 
The output of the solar cell 1 is inputted to the switching unit 22 through 
the DC side filter 21 having a capacitor of a large capacitance. In the 
embodiment, an aluminum electrolytic capacitor of 4700 .mu.F is used as a 
capacitor of the DC side filter 21. As such a capacitor, it is necessary 
to select a capacitor such that it can withstand a ripple current and a 
release voltage of the solar cell 1. 
As a switching unit 22, what is called a full bridge circuit using four 
self arc-extinguishing type switching devices such as MOSFETs, IGBTs, or 
the like is preferably used. As another circuit, a half bridge circuit 
using two devices or the like can be also used. In the embodiment, a full 
bridge circuit shown in FIG. 4 is constructed by using four MOSFETs 
(2SK1405: 600V and 15 A) made by Hitachi Ltd. The output of the switching 
unit 22 is connected to the commercially available power source system 
(100V, 60 Hz) through an output reactor (10 A, 10 mH). 
As shown in FIG. 4, the four MOSFETs have two sets (221 and 222; 223 and 
224) of MOSFETs in each of which a source S and a drain D are 
interconnected. The drains D of the MOSFETs (221 and 223), of which is 
source S is interconnected, are respectively connected to the positive (+) 
terminal. The sources S of the MOSFETs (222 and 224), of which are 
interconnected drains D, are respectively connected to the negative (-) 
terminal. Outputs are taken out from the nodes where the sources S and 
drains D of the MOSFETs are connected. An output from the CPU 4 is 
supplied to a gate G of each MOSFET. 
A current trace control type is used as an instant value current control 
system (inversion discrimination system). FIG. 5 shows a specific example 
of a circuit to execute it. In the inversion discrimination circuit 5 in 
FIG. 5, a voltage obtained by multiplying a predetermined constant to a 
system voltage is applied as a reference current to a reference signal 
input terminal Iref. An inverter output current is supplied to an output 
current input terminal Iinv and is compared by comparators COMP1 and 
COMP2, respectively. Inversion request signals having opposite phases are 
outputted. 
R1 to R9 and R12 denote fixed resistors and VR10, VR11, VR13, and VR14 
indicate variable resistors, respectively. 
Resistance values of those resistors are set as follows: R1 to R8=100 
k.OMEGA.; R9, R12=5.1 k.OMEGA.; VR10, VR13=500 .OMEGA.; and VR11, VR14=200 
.OMEGA.. 
A state of the operation of the inversion discrimination circuit 5 will now 
be explained with reference to FIG. 2. When a difference between the 
reference current Iref and the output current Iinv is out of a 
predetermined current width .DELTA., an output occurs so that an inversion 
request signal R or S is set to the high logic level H. In the embodiment, 
general operational amplifiers TL072 are used as arithmetic operational 
amplifiers U1-1 and U1-2 and MAX909 made by Maxim Co., Ltd. in which an 
output is at the TTL level and an operating speed is high is used as a 
comparator. Particularly, although there is no severe limitation with 
respect to those devices, in order to connect to the digital CPU, it is 
convenient to use devices such that digital levels (CMOS, TTL) are 
outputted to the outputs R and S. It is necessary to set the current width 
.DELTA. to a proper value in consideration of an efficiency and a 
distortion of the inverter. Generally, when .DELTA. is reduced, the number 
of switching times increases and an efficiency deteriorates. However, a 
distortion decreases. When .DELTA. is increased, the opposite phenomena 
occur. Therefore, it is necessary to decide the current width .DELTA. in 
consideration of conditions such as a distortion and the like. 
The outputs R and S of the inversion discrimination circuit 5 are connected 
to the interruption input terminal of the digital CPU. It is desirable to 
use a CPU having a plurality of interruption input terminals. Unless 
otherwise, it is sufficient to enable a plurality of interruption inputs 
to be connected by using a circuit for an interruption control as 
represented by an IC (model name, 8251) made by Intel Co., Ltd. 
In the embodiment, a micomboard for learning (trade name; MTK7702A) made by 
Mitsubishi Electric Corporation is used. A device M7710 (made by 
Mitsubishi Electric Corporation; clock frequency is 25 MHz) is installed 
as a CPU on the board. Such a CPU is what is called a 1-chip type and has 
an A/D converter, a D/A converter, a timer, a parallel I/O, an RAM, and 
three interruption input terminals. The CPU is suitable for embodying the 
invention. In the embodiment, the outputs R and S are connected to an 
interruption 0 and an interruption 1, respectively. Bits 0 to 4 of a 
parallel port No. 6 are used as a gate pulse. 
An output of the parallel port is insulated by a photocoupler and, after 
that, it is sent to a gate driving circuit of the switching device. A 
well-known circuit can be used as a gate driving circuit. In many cases, 
since recommended gate driving circuits are disclosed in brochures of the 
power devices of respective companies, they can be also used. 
An inverter control apparatus according to the invention is constructed as 
mentioned above. A construction of a software will now be described. 
In the control apparatus of the invention, it is necessary to describe two 
independent processing programs of a switch inversion processing routine 
described as an interrupting process and a main control routine for 
executing other processes such as an optimum operation point trace control 
and the like. In case of the invention, since the mutual programs don't 
need to be aware of their existence, the program can be fairly easily 
described and a maintenance performance is raised. 
According to the embodiment, a process for flickering an LED connected to 
the fifth parallel port of the CPU M7710 is executed in the main control 
process. FIG. 6 shows such a main control process. The operation which is 
described by the main control routine must have a delay control (response) 
time slower than a switch inverting operation. Unless otherwise, a time 
for an interrupting process becomes an amount that cannot be relatively 
ignored and obstructs the operation. 
The description contents of the interruption processing routine is as shown 
in FIGS. 7A and 7B. The process for ALLOFF (all of the switching devices 
are turned off. In the embodiment, the ALLOFF process is executed for a 
time of 2 .mu.seconds) is executed for a predetermined time. After that, a 
status inversion to a current increasing mode (the FETs 221 and 224 are 
turned on) and a current decreasing mode (the FETs 222 and 223 are turned 
on) is executed. By the ALLOFF process, it is possible to prevent an 
accident such that the switching devices (the FETs 221 and 222, the FETs 
223 and 224 in FIG. 4) serially connected to the solar cell output are 
simultaneously made conductive due to a delay of the arc extinction or the 
like and the circuit is short-circuited and the switching devices are 
broken or the like. It is a feature of the embodiment that an ALLOFF time 
is decided in a software manner. When the kind of switching device is 
changed, there is a case where the ALLOFF time has to be changed. Even in 
such a case, the control apparatus of the embodiment doesn't need the 
change of hardware which requires a time and costs. Since the switching 
operation can be intelligently executed by the CPU, many various processes 
can be performed for an unexpected situation and a safety as a system can 
be raised. 
According to the control apparatus, while executing the flickering 
operation of the LED after resetting, only when there is a switch 
inversion interruption, the switch inversion is performed and the output 
current waveform is controlled. In the apparatus, since the phase 
synchronization with the system is automatically accomplished by the 
reference current Iref to the inversion discrimination circuit, the 
control processing program is remarkably simple as mentioned above. 
Embodiment 2 
In the embodiment, IGBT (CM50DY-12H) made by Mitsubishi Electric 
Corporation is used as a switching device and the other hardware 
construction is substantially the same as the embodiment 1. The 
description of the interruption processing routine is changed as shown in 
FIGS. 8A and 8B. Specifically speaking, a time from the previous inversion 
is measured by using a timer. When the measured time is too short, a time 
waiting process is executed. In the embodiment, the shortest switch time 
is set to 0.1 msec. By performing a limiting process of the shortest 
switch time as mentioned above, a switching frequency can be limited to 10 
kHz or less. Since the switching operation of the IGBT is slower than that 
of the MOSFET, it is necessary to limit the switching frequency. However, 
in the control apparatus of the invention, there is no need to modify the 
hardware in association with it. A memory for storing the time is commonly 
used for the interruption 0 and the interruption 1. 
Embodiment 3 
In the embodiment, in addition to the foregoing embodiments, the main 
control routine is rewritten as shown in FIG. 9 and the optimum operation 
point trace (MPPT) control operation is executed by the main control 
routine. 
Namely, after the current instruction value increasing mode was set, a 
current amplitude instruction is renewed and the apparatus waits for 0.1 
second. Subsequently, a PV (solar cell) current/voltage is read and a PV 
electric power is calculated. A check is now made to see if the PV (solar 
cell) electric power has increased than the previous calculation result. 
If YES, the current amplitude instruction is renewed. When the PV power is 
not increased than the previous calculation result, the current 
instructing mode is inverted. After that, the current amplitude 
instruction is renewed. 
As mentioned above, the current instruction is renewed in accordance with 
an increase or decrease in PV power. In association with the optimum 
operation point trace, a circuit is constructed as shown in FIG. 10 so 
that the micomboard can also fetch the solar cell voltage and current to 
the A/D input terminal. 
To fetch the solar cell voltage, it is a general way that the voltage of 
the solar cell is dropped by a proper voltage dividing resistor and, after 
that, the dropped voltage is inputted to the A/D input terminal by using 
an insulating amplifier. To fetch the solar cell current, after 
insulating, it is sufficient that the current is converted into the 
voltage by using a DCCT (DC current transformer) using a Hall element and 
the voltage is inputted to the A/D input terminal of the CPU. As other 
methods, there are a method of using a precise resistor for a current 
detection and the like. An output of a D/A converter is multiplied to the 
system voltage by an analog multiplier 243 and the resultant voltage is 
used as a reference current Iref, thereby enabling a magnitude of an 
inverter output current to be controlled by the CPU. The maximum power 
control unit 241 is assembled in the CPU 4. 
There is a mountain climbing method as an MPPT control operation method. In 
the embodiment, the mountain climbing method is used. A portion surrounded 
by a broken line in FIG. 9 mentioned above shows a flowchart for the 
mountain climbing method. Namely, the current instruction value is 
increased, a PV power at that time is calculated, and when the power 
drops, the current instruction value is decreased and, when the power 
rises, the current instruction value is further increased. A waiting time 
of 100 msec (0.1 second) is provided for a control loop in order to allow 
the inverter to trace the operation. Even if there is a waiting time for 
limiting the switching time, a processing time of the interruption 
processing routine is at most 0.2 msec or less. Therefore, there is a 
difference of 500 times between both of the above control times. 
When the apparatus is operated in a day under a blue sky as mentioned 
above, the inverter of the embodiment doesn't abnormally operate but 
execute the maximum power point tracing operation. No problem occurred 
with respect to an output current distortion and the shortest switching 
time. As mentioned above, the MPPT operation and the output waveform 
forming operation can be embodied by the single CPU. 
As a comparison example, FIG. 11 shows a flowchart of an example of a 
program for processing the waveform formation and the main control 
operation by only a software. In this case, relatively complicated 
processes such as system synchronizing process, phase discriminating 
process, and the like are necessary. 
As mentioned above, according to the invention, there is provided the 
control apparatus of the system linkage inverter for converting the DC 
electric power into the AC electric power by using the switching device 
and reversely supplying the AC electric power to the commercially 
available power source system, comprising: the inversion discriminating 
means for inputting an output current or output voltage and a reference 
signal and for discriminating whether a switching state of the switching 
device is inverted or not; and the gate pulse signal forming means for 
changing the switching state of the switching device in response to an 
inversion request signal which is outputted from the inversion 
discriminating means and, wherein the gate pulse signal forming means is 
constructed by the digital CPU and the inversion request signal is 
inputted as an interruption control signal of the CPU. The control 
apparatus has the following effects. 
(1) The parameters such as ALLOFF time and shortest switching time which 
are peculiar to the switching device can be easily changed. 
(2) The gate block function can be easily realized. 
(3) The lowest or highest switching frequency can be easily limited. 
(4) In the solar power generation, the MPPT control unit and the waveform 
control unit can be constructed by a cheap single CPU of a relatively slow 
speed. Thus, the costs of the inverter control apparatus can be reduced. 
(5) Since the interrupting process is used, the maximum power control 
process and the waveform formation control (gate pulse generating) process 
can be described as exactly independent programs. Therefore, the debugging 
of each process can be also independently easily executed and a developing 
efficiency can be raised.