JP2940777B2 - Grid connection system and its disconnection control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system interconnection system for supplying power to a load by a parallel operation of a power supply device equipped with an inverter and a commercial power system, and relates to an independent operation of the inverter when a commercial power system fails. The feature is in the prevention means.

[0002]

2. Description of the Related Art In recent years, a relatively small-capacity distributed power supply of several kW provided with a DC power supply such as a solar cell or a fuel cell has been interconnected (connected) to a commercial power system via an inverter circuit.
Various interconnection systems for supplying power to loads such as home appliances have been proposed.

In a system interconnection system, in order to ensure the safety of maintenance work of a commercial power system, at the time of an unexpected power outage or a work power outage of a commercial power system, a switch is operated to release the inverter, thereby disconnecting the inverter. The function of disconnecting the circuit from the commercial power system, that is, the function of detecting the isolated operation of the inverter circuit is indispensable.

A conventional system interconnection system is configured to detect a system power failure based on output voltage fluctuations and output frequency fluctuations of an inverter circuit, and to disconnect the inverter circuit from a commercial power system. However, in such a grid-connected system, when the output power of the inverter circuit and the power consumption of the load are almost equal, the output voltage and output frequency of the inverter circuit hardly fluctuate. There was a problem that it was not possible.

Therefore, in order to reliably detect a system power failure due to frequency fluctuation, a waveform extracting means (filter) is used in feedback control of the inverter circuit, in which the center frequency of the frequency component extracted from the output voltage is slightly different from the commercial frequency. A detection method has been proposed in which the output frequency of the inverter circuit is positively changed from the commercial frequency to the center frequency for waveform extraction at the time of a power failure.

[0006]

In the above detection method,
At the time of interconnection, the frequency of the output voltage of the inverter circuit is regulated to the commercial frequency by the commercial power system, so that the phase of the output current of the inverter deviates from the phase of the output voltage, and the operating power factor of the inverter circuit decreases. It will be smaller. There is also a problem that a dead zone is generated depending on the load.

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a system interconnection system capable of reliably preventing the inverter circuit from operating alone without lowering the operating power factor of the inverter circuit and without generating a dead zone. , And a disconnection control method.

[0008]

According to a first aspect of the present invention, there is provided a system comprising: a power supply for outputting DC power; an inverter circuit for converting the DC power to AC power; A commercial power system that supplies power to a load in connection with a circuit, a waveform extracting unit that extracts a fundamental frequency component of a voltage at a connection point between the inverter circuit and the commercial power system, and an output of the waveform extracting unit. An inverter control unit that controls the inverter circuit so as to output a current synchronized with a signal, and a system protection unit that disconnects the inverter circuit from a commercial power system when a frequency abnormality of the voltage at the interconnection point occurs. , A first and a second waveform extracting circuit connected in parallel as the waveform extracting means, wherein the first and second waveform extracting circuits are connected to each other. The center frequency of the is set to a value smaller by a predetermined amount than the commercial frequency, and constituted the center frequency of the extracted components of the second waveform extracting circuit is set to a value larger by a predetermined amount than the commercial frequency.

In a system according to a second aspect of the present invention, the center frequency of the extracted component of the first waveform extracting circuit is set to a value smaller by about 2 Hertz than the commercial frequency, and
The center frequency of the extracted component of the waveform extraction circuit is set to a value approximately 2 Hertz higher than the commercial frequency, and the selectivity of the first and second waveform extraction circuits is set to 15 to 30. .

According to a third aspect of the present invention, in the method, the frequency of the voltage at the interconnection point is detected at a constant cycle, and a predetermined time is measured from the time when the frequency abnormality is first detected. And when the number of times of frequency abnormality detection is larger than a specific number of times, the inverter circuit is disconnected from the commercial power system at a point in time after the specified time has elapsed.

According to a fourth aspect of the present invention, in the method, the frequency of the voltage at the interconnection point is detected at a constant cycle, and a specified time is measured from the time when the frequency abnormality is first detected. A method in which the inverter circuit is disconnected from the commercial power system when a plurality of frequency abnormalities are detected after a lapse of time.

[0012]

During parallel operation of the inverter circuit and the commercial power system, the output signal of the waveform extracting means has an output waveform synchronized with the system voltage of the commercial frequency, and an output current of the inverter at the commercial frequency synchronized with the system voltage is output.

[0013] Further, when the commercial power system fails, the operating frequency of the waveform extracting means shifts from the commercial frequency to approach the center frequency of the first waveform extracting circuit or the second waveform extracting circuit, and the frequency of the inverter output current is reduced. Will vary from frequency. As a result, the frequency of the interconnection point voltage changes similarly, and a frequency abnormality occurs.

On the other hand, the frequency of the interconnection point voltage is monitored by the system protection means, and when the abnormality occurrence period within a prescribed time is longer than a prescribed time, or when the prescribed time has elapsed since the first occurrence of the failure. If an abnormality frequently occurs later, the inverter circuit is disconnected.

[0015]

FIG. 1 is a block diagram showing a schematic configuration of a system interconnection system SY according to the present invention.

The system interconnection system SY includes a solar cell 1 having a predetermined rated output (for example, an optimum operating voltage of 200 V and an optimum operating power of 3 kW) for converting solar energy into DC power.
It is mainly configured with an inverter circuit 2 that converts DC power into AC power and a commercial power system 3 and supplies power to a load 5 such as various home appliances connected to the distribution line L.

The inverter circuit 2 comprises a plurality of bridge-connected switching elements. The inverter circuit 2 is supplied with a pulse width modulated switching control signal from an inverter control unit 14 having a microprocessor for performing various processes described below.

A switch 4 for system protection is provided between the inverter circuit 2 and the commercial power system 3. The switch 4 brings the inverter circuit 2 and the commercial power system 3 into an interconnected state or a disconnected state according to a signal from the system abnormality detection circuit 6.

The system abnormality detection circuit 6 determines whether the connection point voltage V3 and the frequency thereof are within a proper range in the operation regulation of the commercial power system 3 together with the switching state signal SK indicating the state of the switch 4. A connection point state detection signal SJ indicating the above is sent to the inverter control unit 14. Then, in response to the input of the disconnection instruction signal SA from the inverter control unit 14, the system abnormality detection circuit 6 opens the switch 4 to disconnect the inverter circuit 2 from the commercial power system 3, and the interconnection instruction signal S
In response to the input of B, the switch 4 is closed and the inverter circuit 2 is connected to the commercial power system 3.

That is, the switch 4, the system abnormality detection circuit 6,
The inverter control section 14 constitutes a system protection unit that disconnects the inverter circuit 2 from the commercial power system 3 when a system abnormality including a power failure occurs.

The proper range of the voltage is 95 to 107 V when the reference value is 101 V, and the proper range of the frequency is ± 1% of the fundamental frequency (49.5 to 50.5 Hz or 59.4). ６60.6 Hz). The interconnection point voltage V3, which is a voltage on the distribution line L, is detected by the first voltage detection means 7 including a transformer (PT).

In the system interconnection system SY, the inverter circuit 2 and its feedback control system perform power conversion of the voltage-type current control system. That is, the output voltage V1 of the solar cell 1 that fluctuates according to the amount of solar radiation is detected by the second voltage detecting means 8 including an isolation amplifier, and the output voltage V1 of the solar cell 1 is detected by the differential amplifier 9.
Error signal S obtained by amplifying the difference between voltage command value Vref and voltage command value Vref
a is generated. The input error signal Sa becomes one input signal of the multiplier 10. In addition, the value of the optimal operating point of the solar cell 1 is set as the voltage command value Vref.

The multiplier 10 receives the other input signal as
The fundamental frequency component S11 of the interconnection point voltage V3 extracted by the waveform extraction unit 11 is input. Then, a current command value signal Si indicating a control target value is generated by multiplying the input error signal Sa by the fundamental frequency component S11. That is, the magnitude (amplitude) of the output current of the inverter circuit 2 is set by the input error signal Sa, and the phase of the output current is set by the system voltage at the interconnection point.

The current command value signal Si and the actual output current value detected by the current detecting means 13 comprising a current transformer are input to an operational amplifier (error amplifier) 12 to generate a current error signal SΔi. This current error signal SΔi is input to the inverter control unit 14.

The inverter controller 14 generates a switching signal having a pulse width adjusted by comparing the current error signal SΔi with a reference triangular wave signal of about 20 kHz, and outputs the switching signal to the inverter circuit 2.

With such feedback control, as a normal operation, the AC power having a power factor of 1 whose current value is set so as to extract the maximum power from the solar cell 1 and whose current phase is the same as the system voltage is supplied to the inverter circuit. 2 to load 5
Output to the side.

As is clear from the above description, in the control of the inverter circuit 2, the output frequency is specified by the waveform extracting unit 11. FIG. 2 is a circuit diagram of the waveform extracting unit 11, and FIG. 3 is a Bode diagram showing frequency response characteristics of the waveform extracting unit 11.

The waveform extracting section 11 includes a variable resistor 113 for adjusting the output balance, and two band-pass filters (BPF) 11 connected in parallel via the variable resistor 113.
0, 111 and an amplifying unit 112, and is composed of a total of three operational amplifiers and predetermined electronic components.

The bandpass filters 110 and 111 have different center frequencies of the passbands (extracted components). That is, in the first bandpass filter 110, the circuit constant is set so that the center frequency is 48 Hz, which is smaller by 2 Hz than the commercial frequency (here, 50 Hz).
In the second bandpass filter 111, circuit constants are set such that the center frequency is 52 Hz, which is 2 Hz higher than the commercial frequency.

In this embodiment, the selectivity Q of each of the band-pass filters 110 and 111 is set to 22. The reciprocal of the selectivity Q indicates a pass bandwidth, and the gain characteristic curve becomes steeper as the selectivity Q increases.

As shown in FIG. 3, the waveform extracting unit 11
There are three points (48 Hz, 50 Hz, 52 Hz) at which the differential value becomes zero in the frequency-gain characteristics, and the gain becomes the highest value at two points (48 Hz, 52H) on both sides of them. That is, when the operating frequency is 48 Hz, 50 Hz, or 52 Hz, the waveform extracting means 11
Operation becomes stable, especially when the operating frequency is 48 Hz or 5 Hz.
It is most stable at 2 Hz.

Focusing on the frequency-phase characteristics, when the operating frequencies are 48 Hz, 50 Hz, and 52 Hz, the phase difference between the input waveform and the output waveform is almost 0 degrees.

The waveform extracting section 11 having such characteristics
Tends to operate at a frequency (48 or 52 Hz) where the gain is the highest and the phase difference is 0 degree in terms of the circuit configuration. That is, an attempt is made to extract a signal component of 48 or 52 Hz.

However, during the interconnection operation between the inverter circuit 2 and the commercial power system 3, the power capacity of the commercial power system 3 is orders of magnitude larger than the power capacity of the solar cell 1 and the frequency is stable. The system point voltage V3 is defined by the commercial power system 3, and the frequency of the output current of the inverter circuit 2 is maintained at the commercial frequency (50 Hz). As a result, the operating frequency of the waveform extracting unit 11 is almost fixed to the commercial frequency at the metastable point, and the commercial power system 3
A voltage waveform having the same phase as that of the system voltage and having a frequency of 50 Hz is output.

Next, the system interconnection system SY having the above configuration
The control of interconnection and disconnection in will be described. When the output voltage V1 of the solar cell 1 is lower than the minimum value required for power conversion (for example, 170 V), such as at night,
The inverter circuit 2 is in an operation stop state, and the switch 4 is in an open state for safety and power saving.

The inverter control section 14 is operated by the control power supplied from the solar cell 1, and monitors the output voltage V1 of the solar cell 1 by taking in the output of the second voltage detecting means 8 during operation.

When the output voltage V1 of the solar cell 1 reaches a predetermined value at which interconnection is possible, the inverter control unit 14 sends an interconnection instruction signal SB to the system abnormality detection circuit 6, and the switch 4 is closed to close the inverter circuit. After it is confirmed by the open / close state signal SK that the system 2 and the commercial power system 3 are interconnected, the inverter circuit 2 is started with a slow start to shift the system to the interconnected operation state.

During the interconnection operation, as described above,
Inverter circuit 2 with output frequency 50 Hz and operating power factor 1
Operates, and the load 5 is output from the inverter circuit 2.
, Only the active power is supplied, and all the reactive power required by the load 5 is supplied from the commercial power system 5. In addition,
If the output voltage V1 of the solar cell 1 falls below a predetermined value during the interconnection operation, the inverter control unit 14 outputs
After the disconnection instruction signal SA is sent to the switch 4, the switch 4 is opened, and after the disconnection of the inverter circuit 2 from the commercial power system 3 is confirmed by the open / close state signal SK, the inverter circuit 2 is returned to the stop state. It is.

On the other hand, as a process for a system abnormality except for a power failure, the inverter control unit 14 performs a connection point voltage V3 based on a state detection signal SJ output from the system abnormality detection circuit 6.
When at least one of them is out of the proper range and the state continues for a specified time or more, a disconnection instruction is given to the system abnormality detection circuit 6.

Then, as a process for a system power failure, the inverter control unit 14 performs an islanding prevention process. FIG.
FIG. 5 is a flowchart showing an example of the islanding prevention process, and FIG.
5 is a timing chart of the islanding prevention process of FIG.

The inverter control unit 14 fetches the connection point state detection signal SJ from the system abnormality detection circuit 6 at a constant period (for example, 10 ms) St, and monitors the frequency of the connection point voltage V3 (# 1). The period St is measured by a software timer of the microprocessor.

Here, when a power failure occurs in the commercial power system 3, the inverter circuit 2 enters the isolated operation state, so that the waveform extracting unit 11 has the highest gain, that is, the operation frequency of 48 Hz or 52 Hz. Attempt to transition to state. Since the operating frequency can be shifted to both the high side and the low side with respect to the commercial frequency, the operating frequency always changes regardless of whether the load 5 is inductive or capacitive. That is, there is no dead zone that occurs when the shift of the operating frequency with respect to the commercial frequency is limited in one direction as in the related art.

As the operating frequency of the waveform extracting section 11 changes, the frequency of the interconnection point voltage V3 (load voltage) changes from the commercial frequency, and the change depends on the characteristics of the inverter control system. Is caused to return, for example, a periodic change that changes around the commercial frequency.

When the inverter control unit 14 first detects the frequency abnormality, the specified time T0 (for example, 600 m
Set a timed timer for measuring s), start counting the number of times of abnormality detection, and thereafter increment the abnormality detection counter every time frequency abnormality is detected until the specified time T0 elapses (# 2-4). . Note that the specified time T0 is a time setting condition (for example, 0.5 to 1
S) in consideration of the response time of each unit.

When the time counting by the timed timer ends, the count value of the abnormality detection counter, that is, the number of abnormality detections within the specified time T0, is compared with a specific value (for example, 30) determined as a reference value (#). 5).

When the count value is larger than the specific value, the inverter control unit 14 determines that the system is in the isolated operation state, turns off the switch 4 and disconnects the inverter circuit 2 from the commercial power system 3. After that, the display of the occurrence of the system blackout is performed (# 6, 7). Also, when the frequency abnormality is instantaneous or the abnormality detection is erroneous detection due to noise and the count value is smaller than a specific value, it is determined that it is not a system power failure, and the abnormality detection counter is reset. The monitoring of the frequency is started again (# 8). In the example of FIG. 5, after the specified time T0 has elapsed, the disconnection instruction is issued with a delay of the required time for state determination, and the inverter circuit 2 is disconnected with a delay of the response time of the switch 4 (about 10 ms). .

FIG. 6 is a timing chart showing another example of the islanding prevention process. In FIG. 6, the inverter control unit 14 takes in the interconnection point state detection signal SJ at a constant cycle, monitors the frequency of the interconnection point voltage V3, and when a frequency abnormality is first detected, a specified time T1 (for example, 500 m
Set a first timed timer to time s). When the next frequency abnormality is detected for a certain period (for example, 500 ms) after the end of the time measurement, a predetermined time T2 (for example, 100
ms), a second timed timer is set, and when the frequency abnormality is detected a plurality of times (for example, two times) during the counting of the predetermined time T2, an instruction to disconnect the inverter circuit 2 is issued.
If the number of times of abnormality detection within the predetermined time T2 is one or less, the second timed timer is set when the frequency abnormality is detected again after the time measurement of the predetermined time T2 ends, and the number of times of abnormality detection is counted. In other words, as long as the number of abnormality detections within the predetermined time T2 does not become plural, the second timed timer is repeatedly set each time a frequency abnormality is detected, and the number of abnormality detections is monitored. The period during which the setting of the second timed timer is repeated corresponds to the reaction time (0.5 to 1 second) defined as the timed setting condition.

500 ms after the end of the first timed timer
If the frequency abnormality is not detected a plurality of times during the predetermined time T2 during the period of, after 500 ms has elapsed, the state returns to the state before the first frequency abnormality was detected, and the monitoring of the frequency is restarted.

Next, the results of an experiment for confirming the operation of the system interconnection system SY will be described. As a confirmation experiment, a 3 kW pure resistance load was used as the load 5, an active power of 3 kW was supplied from the inverter circuit 2, and a load balanced state where power was not supplied from the commercial power system 3 to the load 5 was created. And the commercial power system 3 was disconnected to simulate a system power failure state, the first voltage detecting means 7 detected the interconnection point voltage V3, and the operation of the system protection means was confirmed.

As a result of this experiment, after the occurrence of the simulated power failure,
The frequency of the interconnection point voltage V3 changes from 50 Hz to 51 H within seconds.
z or more, and it was confirmed that the inverter circuit 2 was disconnected from the commercial power system 3 within the range of 0.5 to 1 second.

Each of the above-described band pass filters 11
The same experiment was performed by changing the selectivity Q between 0 and 111.
As a result, when the selectivity Q was set to 15 to 30, it was confirmed that the inverter circuit 2 was disconnected in a short time and reliably.

According to the above-described embodiment, at the time of system power failure,
In addition to opening the switch 4, the inverter circuit 2
Therefore, double protection of the commercial power system 3 can be achieved.

In the interconnected operation state, when the power generated by the solar cell 1 falls below a predetermined value due to a decrease in the amount of solar radiation, not only the operation of the inverter circuit 2 is stopped but also the switch 4 is opened. Therefore, the commercial power system 3
No power is supplied from the inverter circuit 2 to the inverter circuit 2 side, and wasteful power consumption can be suppressed.

According to the above-described embodiment, it is possible to satisfy the time setting condition to be strictly adhered to at the time of interconnection with the commercial power system 3, and to reliably prevent the islanding operation without malfunction due to noise. In particular, according to the disconnection control method of FIG. 5, when the frequency of the interconnection point voltage V3 periodically changes and the frequency abnormality occurs intermittently, that is, when the frequency is within a proper range within the specified time T0. Even in such a case, the isolated operation can be prevented, and the frequency abnormality during the non-power failure and the frequency abnormality during the power failure can be distinguished. Further, according to the disconnection control method of FIG. 6, periodic frequency abnormalities can be detected during a prescribed period of power failure detection, and software can be simplified.

In the above-described embodiment, the operating frequency of the waveform extracting unit 11 is changed from 50 Hz to 48 or 52 at the time of system power failure.
When the frequency changes in the direction of Hz, the gain of the waveform extracting unit 11 increases, so that the current command value Si increases and the output current of the inverter circuit 2 increases. As a result, the load voltage drops and a voltage abnormality occurs at the interconnection point. Therefore, the voltage abnormality can be detected and the inverter circuit 2 can be disconnected from the commercial power system 3.

[0056]

According to the first and second aspects of the present invention,
During the interconnection operation between the inverter circuit and the commercial power system, the output signal of the waveform extracting means has an output waveform synchronized with the system voltage of the commercial frequency, and the inverter output current of the commercial frequency synchronized with the system voltage is output. Since only the active power is supplied to the load, the generated power of the DC power supply can be effectively used without lowering the operating power factor of the inverter circuit.

When the commercial power system is out of service, the operating frequency of the waveform extracting means shifts from the commercial frequency toward the center frequency of the first or second waveform extracting circuit, and the frequency of the inverter output current changes from the commercial frequency. As a result, the frequency of the interconnection point voltage also changes in the same manner, so that even in a load equilibrium state in which the output power of the inverter circuit and the power consumption of the load are substantially equal, a system blackout is reliably detected without causing a dead zone. And the inverter circuit can be disconnected from the commercial power system.

According to the third and fourth aspects of the present invention, it is possible to satisfy a time setting condition to be strictly adhered to when connecting to a commercial power system, and to realize reliable prevention of islanding without malfunction caused by noise. Can be.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a grid interconnection system according to the present invention.

FIG. 2 is a circuit diagram of a waveform extracting unit.

FIG. 3 is a Bode diagram showing a frequency response characteristic of a waveform extracting unit.

FIG. 4 is a flowchart illustrating an example of an islanding prevention process;

FIG. 5 is a timing chart of the islanding prevention process of FIG. 4;

FIG. 6 is a timing chart showing another example of the islanding prevention process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Solar cell (power supply) 2 Inverter circuit 3 Commercial power system 11 Waveform extraction part (waveform extraction means) 14 Inverter control part 110,111 Bandpass filter (waveform extraction circuit) S11 Output signal SY System interconnection system St period T0, T1 Specified time V3 Interconnection point voltage

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Yasuhiro Makino 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (56) References JP 2-7832 (JP, A) JP 1987-689429 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H02J 3/00-5/00 G01R 19/165

Claims (4)

(57) [Claims] A power supply for outputting DC power; an inverter circuit for converting the DC power to AC power; a commercial power system interconnecting the inverter circuit to supply power to a load; A waveform extraction unit that extracts a fundamental frequency component of a voltage at a point of connection with a power system, and an inverter control unit that controls the inverter circuit so as to output a current synchronized with an output signal of the waveform extraction unit.
System protection means for disconnecting the inverter circuit from the commercial power system when a frequency abnormality of the voltage at the interconnection point occurs, wherein the system is connected in parallel as the waveform extraction means. First and second
The center frequency of the extracted component of the first waveform extracting circuit is set to a value smaller than the commercial frequency by a fixed amount, and the center frequency of the extracted component of the second waveform extracting circuit is A system interconnection system characterized by being set to a value that is higher than a commercial frequency by a certain amount, and 2. A center frequency of an extracted component of the first waveform extracting circuit is set to a value smaller by about 2 Hertz than a commercial frequency, and a central frequency of an extracted component of the second waveform extracting circuit is a commercial frequency. 2. The system interconnection system according to claim 1, wherein the first and second waveform extraction circuits are set to a value that is larger by about 2 Hertz, and the selectivity of the first and second waveform extraction circuits is set to 15 to 30. 3. 3. The interconnection system according to claim 1, wherein the frequency of the voltage at the interconnection point is detected at a constant cycle, and a specified time is counted from the time when the frequency abnormality is first detected. And
When the number of times of detection of the frequency abnormality within the specified time is greater than a specified number, the inverter circuit is disconnected from the commercial power system at a time after the specified time has elapsed. Method. 4. The grid interconnection system according to claim 1, wherein the frequency of the voltage at the interconnection point is detected at a constant cycle, and a specified time is counted from the time when the frequency abnormality is first detected. And
A disconnection control method, wherein the inverter circuit is disconnected from the commercial power system when a plurality of frequency abnormalities are detected after the specified time has elapsed.