JP4819375B2 - System connection method and system connection inverter for distributed power supply - Google Patents

Description

  The present invention relates to a grid interconnection method and a grid interconnection inverter that link a distributed power source such as photovoltaic power generation to a commercial power grid.

  An electric power system has been put into practical use in which distributed power sources such as solar power generation and fuel cell power generation are connected to a commercial power system, and surplus power flows backward to the commercial power system. In such a power system, for example, a grid-connected inverter as shown in Patent Document 1 is used, and DC power generated by sunlight or the like is boosted to a necessary voltage and converted to AC power by an inverter circuit and a filter circuit. However, it is connected to the commercial power system and performs a grid operation that outputs power.

In the case of interconnection operation using such a grid interconnection inverter, if the commercial power system is stopped due to an accident or construction during the interconnection operation, a single operation state of the distributed power source occurs. In this state, a problem arises from the viewpoint of system security and supply reliability, and therefore, when a single operation state of the distributed power source occurs, an operation of disconnecting the distributed power source from the system is performed.
When this is done automatically, it is possible to detect changes in frequency and voltage, but after the system is shut down, the frequency of the active and reactive power consumed by the distributed power generation output and the load is in equilibrium. Since no change or voltage change appears, the system stoppage cannot be detected in such a state, and it is difficult to automatically perform the shut-off operation.

Therefore, as a solution to this, there is a method of detecting isolated operation by giving periodic reactive power fluctuations to the power generation output of the distributed power source and detecting periodic frequency fluctuations, etc. appearing at the time of transition to isolated operation by that action. Proposed (Distributed power system interconnection technical guidelines (JEAG 9701-2001)).
In this case, the following method can be considered as a periodic reactive power fluctuation method.
(1) Change the reactive power fluctuation in a sine wave.
(2) Change the reactive power fluctuation in a triangular waveform.
(3) Change the reactive power fluctuation in a rectangular waveform.
In Patent Document 1, the rectangular wave (square wave) of (3) is changed.

Japanese Patent Laid-Open No. 9-23660

However, each of the above fluctuation methods has the following problems. In the case of (1), since the reactive power fluctuates slowly, the frequency fluctuations that appear during isolated operation are small, and false detection is likely to occur. In the case of (2), as in (1), the frequency fluctuation that appears during isolated operation is small and easily misdetected, and the output voltage frequency of the distributed power supply constantly fluctuates, which increases the distortion of the output current waveform. Cheap.
On the other hand, in the case of (3), the frequency fluctuation that appears during isolated operation is easy to detect and easy to detect, but the output reactive power greatly fluctuates in a short time. large. Further, in Patent Document 1 in which the reactive power is changed in the shape of (3), a method for fixing the reactive power to a predetermined value when a frequency change is detected is disclosed, but there is no description about the predetermined value. If the output when the power is fixed is close to the power factor of the load, the frequency change is less likely to appear, so there is a possibility of erroneous detection.

  Therefore, in view of such a problem, the present invention provides a grid interconnection method and a grid interconnection inverter that can reliably detect a single operation of a distributed power source even if the load phase is delayed or advanced. The purpose is to provide.

In order to solve the above-mentioned problem, the invention described in claim 1 is a system connection method of a distributed power source that converts the direct current output of the distributed power source into an alternating current and reversely flows a current to the commercial power system. The reactive power that alternately fluctuates in the forward phase and the delayed phase is superimposed on the current to be flowed, and is superimposed if the reactive power fluctuation causes the frequency at the linkage point to deviate from the preset first frequency range. the reactive power of the phase continues longer predetermined period fixed in phase than the variation cycle of the reactive power at that time, while the reactive power of the phase is fixed, the frequency at the connection point is the first frequency If it deviates from the predetermined second frequency range set wider than the range, it is determined that the operation is independent, and the AC output of the distributed power supply is stopped.
By this method, if frequency fluctuation occurs, the reactive power phase is fixed and determined at that phase, so that the isolated operation can be detected and stopped regardless of the phase state of the load. As a result, the isolated operation state is not continued, and safety during maintenance, maintenance, etc. near the interconnection point can be ensured. In addition, if the reactive power phase is fixed and if it does not deviate from the second frequency range even after a certain period of time, it can be reset to cause periodic fluctuations in the reactive power, and malfunction can be prevented.

The invention of claim 2 is a system connection method for a distributed power source that converts the direct current output of the distributed power source into an alternating current and reversely flows the current to the commercial power system, and advances the current to the reverse power flow in a constant cycle, When the reactive power that alternately varies in the delay phase is superimposed, and the frequency at the linkage point deviates from the preset first frequency range due to this reactive power variation, the alternating variation of the superimposed reactive power is stopped. The phase at that time is gradually increased and the state is continued for a certain period longer than the fluctuation period of the reactive power. While the phase of the reactive power is gradually increased, the frequency at the linkage point is the first frequency range. If it deviates from the predetermined second frequency range set wider, it is determined that the operation is independent, and the AC output of the distributed power supply is stopped .
According to this method, the reactive power component is gradually increased in addition to stopping the phase fluctuation of the reactive power when the frequency fluctuation occurs, so that it is possible to more reliably determine the single operation. In addition, if it does not deviate from the second frequency range even after a lapse of a certain period after the phase fluctuation is stopped, it can be reset and the periodic fluctuation of the reactive power can be carried out, and malfunction can be prevented.

The invention according to claim 3 is a grid-connected inverter of a distributed power source for inputting a DC output of the distributed power source, converting the AC power into an AC power by an inverter circuit, and causing a reverse power flow to a commercial power system, the inverter circuit The control unit for controlling the current is a target current setting unit that creates an output current target value of the active current based on the reference phase of the current synchronized with the voltage phase of the interconnection point voltage, and is constant based on the reference phase of the current A phase fluctuation unit that generates a reactive power component that alternately fluctuates between a lead phase and a delay phase in a cycle, and an alternating current that generates a periodic reactive power fluctuation by adding the reactive power component to the output current target value A modulation unit that modulates the current to be output to the interconnection point, and the phase variation unit includes a first frequency determination unit that determines whether or not the output frequency is out of a predetermined first frequency range. The frequency is the first circumference If it is out of the range, the reactive power is fixed at the phase at that time, the phase locked state is maintained for a certain period longer than the reactive power fluctuation period, and the modulation unit is set wider than the first frequency range. the second has a second frequency determining section having the criteria of frequency ranges, the output frequency, characterized in that the stop et modulation operation deviated from the second frequency range.
With this configuration, when frequency fluctuations occur, the reactive power phase is fixed and determined at that phase, so that independent operation can be detected and stopped regardless of the phase state of the load. As a result, the isolated operation state is not continued, and safety during maintenance, maintenance, etc. near the interconnection point can be ensured. In addition, if the reactive power phase is fixed and if it does not deviate from the second frequency range even after a certain period of time, it can be reset to cause periodic fluctuations in the reactive power, and malfunction can be prevented.

The invention according to claim 4 is a grid-connected inverter of a distributed power source for inputting a DC output of the distributed power source, converting it into AC power by an inverter circuit, and causing a reverse power flow to a commercial power system, wherein the inverter circuit The control unit for controlling the current is a target current setting unit that creates an output current target value of the active current based on the reference phase of the current synchronized with the voltage phase of the interconnection point voltage, and is constant based on the reference phase of the current A phase fluctuation unit including a fluctuation period generation unit that generates a reactive power component that is alternately changed to a lead phase and a delay phase in a cycle, and a reactive power gradual increase unit that gradually increases the output of the reactive power by stopping the phase fluctuation; A modulation unit that adds the reactive power component to the output current target value and modulates the alternating current that causes periodic reactive power fluctuations to be output to a connection point;
The phase varying unit includes a first frequency determining unit that determines whether an output frequency is out of a predetermined first frequency range, a reactive power gradually increasing unit that gradually increases reactive power by multiplying a reactive power component by a coefficient larger than 1. When the output frequency is out of the first frequency range, the alternating fluctuation of the superposed reactive power is stopped, the phase at that time is gradually increased, and the state is changed to the fluctuation period of the reactive power. The modulation unit has a second frequency determination unit having a second frequency range set wider than the first frequency range as a determination criterion, and the output frequency is the second frequency range. The modulation operation is stopped when it is out of the frequency range .
With this configuration, the reactive power component is gradually increased in addition to stopping the reactive power phase fluctuation when the frequency fluctuation occurs, so that it is possible to more reliably determine the single operation. In addition, if it does not deviate from the second frequency range even after a lapse of a certain period after the phase fluctuation is stopped, it can be reset and the periodic fluctuation of the reactive power can be carried out, and malfunction can be prevented.

According to the present invention, when the frequency fluctuation occurs, the reactive power phase is fixed or gradually increased at that phase, so that the independent operation can be detected and stopped regardless of the phase state of the load. . As a result, the isolated operation state is not continued, and safety during maintenance, maintenance, etc. near the interconnection point can be ensured. In addition, if the reactive power phase is fixed and if it does not deviate from the second frequency range even after a certain period of time, it can be reset to cause periodic fluctuations in the reactive power, and malfunction can be prevented.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, embodiments of the invention will be described in detail with reference to the drawings. FIG. 1 is a configuration diagram showing a first embodiment of a grid-connected inverter of a distributed power source according to the present invention, wherein 1 is a DC power source (distributed power source) such as solar power generation distributed, and 2 is a DC power source. AC conversion unit for converting to AC, 3 is a control unit for controlling the AC conversion unit 2, 4 is a commercial power system, 5 is a load, and the control unit 3 detects the output current and returns to the connection point M by feedback control. It is configured to output a predetermined current.

  The AC conversion unit 2 includes an inverter circuit 10 and an LC filter 11, and 12 indicates current detection means. The AC converter 2 boosts the DC power supplied from the DC power source 1 by a chopper circuit or the like (not shown), converts it into a pulsed AC by the inverter circuit 10, smoothes it into a sine wave by the LC filter 11, and commercial power Outputs to system 4.

The control unit 3 includes a phase detection unit 15 for detecting a zero cross point of the output voltage waveform, a PLL 16, a target current setting unit 17 for setting a target current of an alternating current output to the commercial power system, and a phase for giving a phase variation. A fluctuation unit 18, a modulation unit 19 that generates a control signal for the inverter circuit 10, and a frequency detection unit 20 are provided, and the PLL 16 provides a phase to the target current setting unit 17 and the phase fluctuation unit 18 based on the zero cross point detected by the phase detection unit 15. A reference signal is being output.
Then, the target current setting unit 17 sets the target value (target current value) of the active current to be output by multiplying the sine wave signal by the output current target value.

  The phase fluctuation unit 18 includes a reactive power settling unit 22, a fluctuation period creation unit 23, and a first frequency determination unit 24. A reactive power settling value having a predetermined magnitude is output from the reactive power settling unit 22, and an output current The amount of reactive power added to is determined. Then, a periodic waveform having a preset waveform shape is output from the fluctuation period creating unit 23, and periodic phase fluctuation is given to the reactive power.

Specifically explaining this fluctuation waveform, it is a waveform as shown in FIG. 2 (a), and the fluctuation value of the created waveform is periodically fluctuated between −1.0 and 1.0 in a trapezoidal shape, It is updated every time the voltage phase obtained by the phase detector 15 is counted as 0 degree, and is updated so as to change, for example, one cycle at 24 counts, that is, change one cycle at 24 voltage waveforms.
FIG. 2 is a diagram showing the relationship between phase fluctuation and voltage frequency when the operation is switched from the interconnection operation to the independent operation. (A) is the phase fluctuation waveform, and (b) is the interconnection point corresponding to the phase fluctuation of (a). The voltage frequency characteristic diagram of M is shown, and the case where the commercial power system stops at the point P0 and the single operation of the distributed power supply (DC power supply) is shown.

  When the change in the detected frequency exceeds the first frequency range W1 (reference B) that is a fixed range centered on the reference frequency, the first frequency determination unit 24 starts the timer 25 and activates the timer 25. During this time, the switch 26 is turned off, and the supply of the zero cross detection signal is cut off. As a result, the fluctuation period is not updated, and the same phase value is supplied when the reference B is exceeded.

The modulation unit 19 includes a PI calculation unit 27 that performs PI calculation, a PWM modulation unit 28 that performs PWM modulation, and a second frequency determination unit 29 that detects whether the output frequency is within the frequency range set by the reference A. The calculated target current value is compared with the value of the output current detected by the current detection means 12, the deviation is PI-calculated by the PI calculation unit 27, and the result is PWM-modulated by the PWM modulation unit 28, and the control signal of the inverter circuit 10 Have created.
The second frequency determination unit 29 stops the modulation operation of the PWM modulation unit 28 when the change in the detected frequency exceeds the second frequency range W2 (reference A) wider than the first frequency range W1. .

  The frequency detection unit 20 detects the voltage frequency of the interconnection point by counting how many times, for example, a 50 μS counter is generated at the generation interval between the voltage phase 0 degree and the next voltage phase 0 degree. However, the 50 μS counter is not absolute, and if a finer frequency fluctuation is detected, the counter time may be changed.

Hereinafter, the operation of the control unit 3 will be specifically described. However, the control of each circuit of the control unit 3 is performed by arithmetic processing of a microcomputer (not shown).
(1) The output current from the DC power source 1 to the interconnection point M is controlled as follows.
First, the phase detector 15 detects the zero cross point of the voltage at the connection point M, obtains the generation timing of the phase 0 degree therefrom, and obtains 0 degree of the internal phase (ωt) created by the phase 0 degree and the internal clock. Synchronized by the PLL 16. Based on the internal phase (ωt) synchronized with the output voltage phase in this way, the target current setting unit 17 performs a sine calculation to create sin (ωt), and outputs this value by multiplying this value by the output current target value. Set the target value (target current value) of the active current.

Then, the modulation unit 19 compares the target current value with the output current value detected by the current detection means, performs PI calculation on the deviation, and creates a control signal for the inverter circuit 10 by PWM modulation based on the result. To do.
By operating four switches (not shown) of the inverter circuit based on the control signal thus created, the phase is synchronized with the voltage phase of the interconnection point, and the magnitude is equal to the output current target value. Current is output.

(2) The operation of the phase changing unit 18 will be described.
First, cosine (ωt) is generated by cosine calculation from the internal phase (ωt) synchronized with the voltage phase of the interconnection point M. This value is multiplied by the reactive power set value preset by the reactive power settling unit 22 and further multiplied by the value generated by the fluctuation cycle creation unit 23, and then added to the calculated target current value.

  Due to the reactive power added in this way, a slight fluctuation of the voltage frequency occurs at the interconnection point M as shown in the T1 part of FIG. This is because the output phase advances when the reactive power of the phase is superimposed on the target current value, and after synchronizing with the phase of the immediately previous output by the PLL 16, the phase component is advanced again based on that and the phase component is output. When the period of 1 continues, the voltage frequency at the interconnection point M increases as shown in FIG. On the contrary, when the period of the fluctuation period is −1, the phase frequency is lowered again after the PLL 16 synchronizes with the phase of the immediately preceding output, and then the reference phase is used as a reference. However, since the energy from the commercial power system 4 is large relative to the output of the grid interconnection inverter, the fluctuation in the frequency is absorbed and only a slight fluctuation appears, and the waveform distortion should be kept small during the grid operation. Can do.

The fluctuation period will be described. When the fluctuation value output from the fluctuation period generator 23 is “1”, the cos (ωt) value is 90 degrees ahead of the sin (ωt) value. . This is because the lead phase component of 90 degrees in magnitude corresponding to the reactive power set value is superimposed on the target current value. As a result, the current output to the system is also the target current value superimposed with a component whose phase is advanced 90 degrees by the reactive power settling value.
Conversely, when the value given from the fluctuation cycle creation unit 23 is “−1”, it indicates that the reactive power component created based on cos (ωt) is inverted and delayed by −90 degrees. This is because the target current value is superimposed on the target current value by a delay phase component of 90 degrees in magnitude corresponding to the reactive power setting value, and the current output to the system also has a phase corresponding to the reactive power setpoint value of 90 degrees. The delayed component is superimposed.

(3) When the distributed power source is operated independently.
As described above, during the grid connection operation, the energy from the commercial power system 4 is large with respect to the output of the grid connection inverter, so that the frequency variation hardly appears. However, when the commercial power system 4 is stopped and the DC power supply 1 is operated alone, the voltage frequency of the interconnection point M changes greatly according to the reactive power phase to be output, and the T2 portion in FIG. As shown, the frequency change rate becomes large.

Specifically, when the system is stopped when the load power factor is 1, in the phase state, in the phase state, the voltage phase is increased to advance to the voltage phase of the interconnection point synchronized by the PLL. In addition, it decreases when the phase is delayed.
Also, when the load phase is delayed and the system is stopped, when the fluctuation cycle is negative, the load and the output of the distributed power source are balanced, and voltage frequency changes are less likely to appear, but on the positive side. When shifted, the change (increase) in the voltage frequency appears remarkably.
On the other hand, when the system is stopped when the load phase is the leading phase, a change (decrease) in the voltage frequency appears noticeably when the fluctuation cycle is negative for the same reason.

This will be specifically described below with reference to the waveform diagram of FIG. When the single operation occurs at the point P0, the change in the output frequency increases from that point and deviates from the reference B (first frequency range W1). Then, the phase of the reactive power is fixed at that time (P1 point). FIG. 2 shows a state in which the phase is fixed with a delay phase. If the commercial power system is stopped, the frequency will decrease.
Thereafter, when the isolated operation is continued, the frequency change is further increased and deviates from the reference A (second frequency range W2) at the point P2 (below the lower limit of the second frequency range W2). Then, a stop signal is output from the second frequency determination unit 29, and the PWM modulation unit 28 stops its operation.
On the contrary, if the phase is fixed in the state of the leading phase, the frequency increases and exceeds the reference B, and exceeds the reference A when the isolated operation continues.
If the reference A is not exceeded until the timer 25 expires, the fluctuation cycle creation unit 23 resumes operation when the time is up, and the output phase resumes cyclic fluctuations.

  As described above, when the frequency fluctuation occurs, the reactive power phase is fixed and determined based on the phase, so that the isolated operation can be detected and stopped regardless of the phase state of the load. As a result, the isolated operation state is not continued, and safety during maintenance, maintenance, etc. near the interconnection point can be ensured. In addition, if the reactive power phase is fixed and if it does not deviate from the second frequency range even after a certain period of time, it can be reset to cause periodic fluctuations in the reactive power, and malfunction can be prevented.

FIG. 3 shows a second embodiment of the grid interconnection inverter. The difference from the first embodiment is that the phase varying unit 18 has a reactive power gradually increasing unit 31, and the same components as those in FIG. To do.
The reactive power gradual increase unit 31 includes a switch 33 controlled by the slope coefficient generation unit 32 and the first frequency determination unit 24. When the timer 25 is started when the frequency change exceeds the reference B, the fluctuation amount is fixed. The reactive power component is multiplied by a slope coefficient larger than 1. As a result, the reactive power output whose phase fluctuation is stopped gradually increases and the reactive power component of the output increases.

  The operation of the control unit in FIG. 3 will be described with reference to FIG. FIG. 4 is a diagram showing the relationship between the phase fluctuation and the voltage frequency when switching from the grid operation to the independent operation, as in FIG. 2, where (a) shows the phase fluctuation waveform and (b) shows the phase fluctuation of (a). The voltage frequency characteristic diagram of the corresponding interconnection point is shown, and the case where the commercial power system is stopped at the point P0 as in FIG.

When the isolated operation occurs, the change in the output frequency increases and deviates from the reference B, and the phase of the reactive power is fixed at that time (P1 point). At the same time, the reactive power gradually increasing unit 31 operates to gradually increase the fixed phase and gradually increase the reactive power. FIG. 4 shows a state in which the phase is deviated from the reference B due to the delayed phase. If the system is stopped, the frequency is decreased.
Thereafter, when the isolated operation is continued, the frequency change is further increased and deviates from the reference A at the point P2. Then, a stop signal is output from the second frequency determination unit 29, and the PWM modulation unit 28 stops its operation. On the other hand, when the phase is fixed in the lead phase state, the frequency increases and exceeds the reference B. When the isolated operation continues, the frequency exceeds the reference A, and the PWM modulator 28 similarly stops the output.
If the reference A is not exceeded until the timer 25 expires, the fluctuation cycle creation unit 23 resumes operation when the time is up, and the output phase resumes cyclic fluctuations.

As described above, when the frequency fluctuation occurs, the reactive power phase is fixed and determined based on the phase, so that the isolated operation can be detected and stopped regardless of the phase state of the load. As a result, the isolated operation state is not continued, and safety during maintenance, maintenance, etc. near the interconnection point can be ensured. In addition, since the reactive power component is gradually increased in accordance with the stoppage of the phase variation of the reactive power, it is possible to more reliably determine the single operation.
In addition, if it does not deviate from the second frequency range even after a lapse of a certain period after the phase change of reactive power is stopped , it can be reset and periodic fluctuations of reactive power can be performed, and malfunctions can be prevented. Become.

  In the above embodiment, the periodic waveform of the reactive current superimposed on the target current value has a trapezoidal shape, but may have a rectangular wave shape as in the prior art. Further, specific values of the first frequency range W1 and the second frequency range W2 are individually set in accordance with the capacity of the distributed power source and the characteristics of the connection point.

1 is a circuit block diagram illustrating a first embodiment of a grid-connected inverter of a distributed power source according to the present invention. FIG. 2 is a relationship diagram of phase fluctuation and voltage frequency for explaining the operation of FIG. 1, (a) is a phase fluctuation waveform, and (b) is a voltage frequency characteristic diagram of a connection point corresponding to the phase fluctuation of (a). . It is a circuit block diagram which shows 2nd Embodiment of the grid connection inverter of the distributed power source which concerns on this invention. FIG. 4 is a relationship diagram of phase fluctuation and voltage frequency for explaining the operation of FIG. 3, (a) is a phase fluctuation waveform, and (b) is a voltage frequency characteristic diagram of a connection point corresponding to the phase fluctuation of (a). .

  1 .... DC power supply (distributed power supply) 2 .... AC conversion unit 3 .... control unit 4 .... commercial power system 5 .... load 10 .... inverter circuit 17 .... target current setting unit, 18 .. Phase variation unit, 19 .. Modulation unit, 22 .. Reactive power establishment unit, 23 .. Variation period creation unit, 24 .. First frequency determination unit, 29 .. Second frequency determination unit, 31. Reactive power gradually increasing section, M ·· interconnection point, W 1 ··· first frequency range, W 2 ··· second frequency range.

Claims (4)

A system connection method for a distributed power source that converts the direct current output of the distributed power source into an alternating current and reverses the current to the commercial power system.
The reactive power that alternately fluctuates in the forward phase and the delayed phase is superimposed on the current to be reversely flowed. The phase of the reactive power is fixed at the current phase and continues for a certain period longer than the reactive power fluctuation period,
While reactive power of phase are fixed, when out of a predetermined second frequency range whose frequency is set wider than the first frequency range at linkage points, distributed power supply is determined that the isolated operation A system interconnection method for a distributed power source, characterized in that AC output is stopped. A system connection method for a distributed power source that converts the direct current output of the distributed power source into an alternating current and reverses the current to the commercial power system.
The reactive power that alternately fluctuates in the forward phase and lagging phase is superimposed on the current to be reversely flowed. If the frequency at the linkage point deviates from the preset first frequency range due to this reactive power fluctuation, it is superimposed. Stop the alternating fluctuation of the reactive power and gradually increase the phase at that time, and continue the state for a certain period longer than the fluctuation period of the reactive power,
While the phase of the reactive power is gradually increased, if the frequency at the linkage point deviates from the predetermined second frequency range set wider than the first frequency range, it is determined that the operation is independent, and the distributed power source A system interconnection method for a distributed power source, characterized in that AC output is stopped . It is a grid-connected inverter of a distributed power source for inputting the DC output of the distributed power source, converting it to AC power by an inverter circuit, and causing reverse flow to the commercial power system,
A control unit that controls the inverter circuit includes a target current setting unit that creates an output current target value of an active current based on a reference phase of a current synchronized with a voltage phase of a connection point voltage;
A phase fluctuation unit that generates a reactive power component that is alternately changed to a lead phase and a delay phase in a constant cycle based on a reference phase of the current;
A modulation unit that adds the reactive power component to the output current target value and modulates the alternating current that causes periodic reactive power fluctuations to be output to a connection point;
The phase variation unit includes a first frequency determination unit that determines whether the output frequency is out of a predetermined first frequency range. If the output frequency is out of the first frequency range, the phase at that time Reactive power is fixed, and the phase fixed state is maintained for a certain period longer than the reactive power fluctuation period,
The modulation unit includes a second frequency determining section having the first second frequency range that is wider set than the frequency range criteria, et modulation of the output frequency deviates from the second frequency range A grid-connected inverter of a distributed power source characterized by stopping operation. It is a grid-connected inverter of a distributed power source for inputting the DC output of the distributed power source, converting it to AC power by an inverter circuit, and causing reverse flow to the commercial power system,
A control unit that controls the inverter circuit includes a target current setting unit that creates an output current target value of an active current based on a reference phase of a current synchronized with a voltage phase of a connection point voltage;
Based on the reference phase of the current, a fluctuation period generation unit that generates a reactive power component that is alternately changed to a leading phase and a lagging phase, and a reactive power gradual increase that gradually increases the reactive power output by stopping the phase fluctuation A phase variation section with a section,
A modulation unit that adds the reactive power component to the output current target value and modulates the alternating current that causes periodic reactive power fluctuations to be output to a connection point;
The phase varying unit includes a first frequency determining unit that determines whether the output frequency is out of a predetermined first frequency range, and a reactive power gradually increasing unit that gradually increases the reactive power by multiplying the reactive power component by a coefficient larger than 1. When the output frequency is out of the first frequency range, the alternating fluctuation of the superposed reactive power is stopped, the phase at that time is gradually increased, and the state is changed to the fluctuation of the reactive power. Last for a period longer than the cycle,
The modulation unit includes a second frequency determination unit having a second frequency range set wider than the first frequency range as a determination criterion, and performs a modulation operation when an output frequency is out of the second frequency range. A grid-connected inverter of a distributed power source characterized by stopping the operation .

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