JP2796035B2 - Inverter system interconnection protection method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for protecting an inverter connected to an AC power system and operating while converting DC power into AC power and supplying the AC power to a load.

[0002]

2. Description of the Related Art A typical example of such a conventional apparatus is shown in FIG. 7 and will be described below.

[0003] The DC power of a DC power supply 1 composed of a solar cell or a fuel cell is converted into AC power by an inverter bridge 2, and the AC power is converted to a high frequency by PWM (pulse width modulation) control by a filter including a reactor 3 and a capacitor 4. The fraction is removed and supplied to the load 9.

On the other hand, AC power for ordinary households supplied from an AC power system 8 via a circuit breaker 7 and a pole transformer 6 is supplied to a load 9, and the AC power of the inverter bridge 2 is supplied to the AC power system. Connect to 8 and drive. The AC voltage supplied to the load 9 is detected by a voltage detector 10,
A sine wave signal V _S is input to the current reference circuit 12 via the band pass filter 16. The current reference circuit 12 multiplies the control signal V _C output from the amplifier 11 by the sine wave signal V _S to output a current reference I ^* . The current reference I ^* and the output current I of the inverter bridge 2 detected by the current detector 5 are input to the amplifier 13 and input to the PWM control unit 14,
The inverter bridge 2 is PWM-controlled so that the current deviation becomes zero via the drive unit 15.

[0005] The phase of the current reference I ^* substantially coincides with the phase of the AC voltage supplied to the load 9, and AC power with a high power factor is supplied from the inverter bridge 2.

When a solar cell is used as the DC power supply 1, the difference between the voltage reference V ^* and the voltage of the DC power supply 1 is amplified by the amplifier 11 so as to extract the maximum power.
_The constant voltage control of the DC power supply 1 is performed by outputting _C , but the detailed description is omitted because it is not directly related to the present invention.

In such a distribution system, when performing maintenance and inspection of the load side including the pole transformer 6, the circuit breaker 7 is opened and separated from the AC power system 8 to perform the maintenance. In this case, the state on the load side is monitored by the voltage relay 17 and the frequency relay 18, and when it is disconnected from the AC power system 8, this is detected by the abnormality detection circuit 19, the operation of the drive unit 15 is stopped, and the inverter bridge 2 is stopped. To stop.

[0008]

However, when the circuit breaker 7 is opened and disconnected from the AC power system 8, if the power supplied from the inverter bridge 2 and the power of the load 9 are balanced, the abnormality detecting circuit 19 will be described. In this case, the abnormality detection is not performed, and the inverter bridge 2 may keep operating. This state is called islanding.
ing) or reverse charging, which is dangerous when maintenance is performed.

In particular, when an induction motor is connected to the load 9, the AC power system 8
, The voltage and frequency tend to be maintained, making it difficult to detect reverse charging.

As a method for preventing the reverse charging, a band pass filter system, a power fluctuation system, and a harmonic voltage monitoring system described below have been proposed, and these will be described below.

<Band-Pass Filter Method> The magnitude and phase of the current reference I ^{* in} FIG. 7 are as shown in FIG. The reverse charge, if the inverter is not reactive power reactive power and the load is required to match the supply, the frequency of the AC power supply will be displaced from the rated frequency f _0, the balance of active power decreases the current value However, the voltage of the AC power supply decreases, the voltage relay 17 detects an abnormality, and the inverter stops.

Here, the load impedance is shown in FIG.
Let us consider the case of the delayed power factor angle φ between R and L shown in FIG. When the inverter is connected to an AC power system having a large power supply capacity, the inverter is operating at the rated frequency f ₀ , and thus the inverter outputs power having a power factor of 1 due to the band-pass filter characteristics shown in FIG. When the circuit breaker 7 is opened, the load 9
When the frequency rises to balance the reactive power output from the inverter and the reactive power of the load due to the delay power factor angle φ, and when the frequency rises to f ₁ , the phase of the delay power factor angle φ The inverter supplies power that is delayed only by the delay. In this case, the current value only slightly decreases from I ₀ to I ₁ , and the fluctuation of the voltage is not large. This voltage fluctuation depends on the supplied active power and the active power consumed by the load.

However, when the band-pass filter 16 is not provided, the inverter operates so that a current having a power factor of 1 always flows. Therefore, in the case of FIG. Although the frequency does not become infinite due to a delay or the like, the frequency becomes extremely high).

As described above, the band-pass filter 16 acts to suppress the system frequency fluctuation at the time of reverse charging. This also acts to suppress the effect of lowering the frequency in the case of a leading power factor load.
In the case of reverse charging, voltage fluctuation is in the direction of expanding, but frequency fluctuation is in the direction of suppressing, and depending on the conditions, detection of reverse charging may be delayed.

<Power fluctuation method> The fluctuation of the current reference I ^* output from the current reference circuit 12 by the fluctuation circuit 21 of FIG. 7 at a low frequency within a certain range, and the output from the inverter when the circuit breaker 7 is opened. By disturbing the balance between the power to be applied (active power and reactive power) and the power of the load, the voltage and frequency are changed to detect reverse charging. However, even in this method, when a large number of inverters are connected in parallel, the phases of the power fluctuations of the inverters are scattered, and there is a case where no power fluctuation occurs as a whole and the detection cannot be performed.

<Harmonic Voltage Monitoring System> The harmonics of the load-side voltage are monitored by the harmonic detection circuit 20, and the harmonics (third, fifth,
The reverse charging is detected by the increase of the seventh harmonic).
However, in this method, when a large number of loads having a rectifier circuit of the capacitor input type, such as an inverter air conditioner and a television, are used, the third, fifth, and seventh harmonics increase in a steady state, and the reliability of detection is increased. It decreases significantly.

The present invention has been made to solve these problems, and an object of the present invention is to provide an inverter system interconnection protection method and a device capable of detecting reverse charging more reliably. Is to do.

[0018]

Means for Solving the Problems In order to achieve the above object, the following configuration is provided.

According to a first aspect of the present invention, there is provided an inverter system interconnection protection method for converting DC power into AC power and operating in connection with an AC power system, wherein a voltage phase of the AC power system and the inverter output is controlled. Controlling the output current of the inverter so as to match a current reference output at a predetermined phase in synchronization with the inverter, and when the inverter is disconnected from the AC power system, the frequency of the output voltage of the inverter, the frequency change rate The method is such that the phase of the current reference is changed so that the frequency changes to a predetermined value based on the distortion voltage near the zero cross.

According to a second aspect of the present invention, there is provided a system interconnection protection device for an inverter which converts DC power into AC power and operates in connection with an AC power system, wherein the output current of the inverter is controlled in accordance with a current reference. Current control means for controlling, a current reference circuit synchronized with the phase of the output voltage of the AC power system and the inverter, and determining the phase of the current reference according to a separately provided phase command; and an output voltage of the inverter. Control means for changing the phase of the current reference so that the frequency changes in the positive feedback loop based on the frequency and the frequency change rate.

According to a third aspect of the present invention, as the control means of the second aspect, the phase change rate is reversed when the frequency of the output voltage of the inverter deviates from the rated frequency by a predetermined value or more.

According to a fourth aspect of the present invention, there is provided a current reference circuit according to the second aspect, wherein the current reference circuit has a characteristic of changing an amplification factor according to a gain command given separately, and the control means controls a frequency of an output voltage of the inverter. , The characteristic of lowering the gain command based on the frequency change rate.

According to a fifth aspect of the present invention, as the current control means according to the second aspect, the current reference is controlled so that the frequency changes in a positive feedback loop based on a voltage distortion near a zero crossing of the output voltage of the inverter. It has the characteristic of changing the phase.

According to a sixth aspect of the present invention, as the current control means according to the fourth aspect, a change in the output voltage of the inverter with respect to a voltage distortion near the zero crossing of the output voltage of the inverter, a current reference, or a change in the output current of the inverter. On the basis of this, a characteristic for reducing the gain command is provided.

[0025]

According to the first aspect of the present invention, when the inverter is disconnected from the AC power system, the current phase is changed based on the frequency of the inverter output voltage, the frequency change rate, and the distortion voltage near zero crossing. As a result, the frequency changes to a predetermined value, so that a method for surely stopping the inverter can be obtained.

According to the invention corresponding to claim 2, when the inverter is disconnected from the AC power system, the control means changes the frequency in the positive feedback loop based on the frequency of the output voltage of the inverter and the frequency change rate. As a result, the phase of the current reference is changed so that the frequency can be changed rapidly and the inverter can be stopped at high speed.

According to the third aspect of the present invention, when the frequency of the output voltage of the inverter deviates from the rated frequency by a predetermined value or more, the phase change rate of the control means is reversed, so that the leading current and the lagging current are balanced. Frequency can be stabilized, and divergence of the frequency can be prevented.

According to the fourth and sixth aspects of the present invention, when the inverter is disconnected from the AC power system and changes the frequency, the characteristic that the gain command is reduced based on the frequency of the output voltage of the inverter and the frequency change rate is provided. As a result, the current amplitude can be reduced, and the inverter can be safely stopped.

According to the fifth aspect of the present invention, when the inverter is disconnected from the AC power system of the inverter under light load or no load, the frequency is positively feedback based on the voltage distortion near the zero cross of the output voltage of the inverter. By changing the phase of the current reference so as to change in the loop, the inverter can be stopped by detecting the reverse charging state at high speed.

[0030]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of the present invention. In FIG. 1, the structures 1 to 15 are the same as those shown in FIG.

The amplitude of the current reference circuit 12 used in the present invention changes according to the output voltage Vc of the amplifier 11 and the gain G given from the phase gain setting circuit 31, and the amplitude varies according to the phase θ given from the phase gain setting circuit 31. The phase current reference I ^* is output. The phase θ means a phase difference from the phase of the AC voltage.

The AC voltage detected by the voltage detector 10 is
The signal is input to the frequency detection circuit 22 via the band-pass filter 16, where the frequency is detected.
22 is input to the frequency change rate (df / dt) detection circuit 23 and the arithmetic circuit 24. V23 detected by the frequency change rate detection circuit 23 is input to the arithmetic circuit 24, and the arithmetic circuit 24 outputs V24 (f + α (df) from V22 and V23.
/ Dt): α is a coefficient) to output the coefficient setting circuit 30
To enter.

On the other hand, the AC voltage detected by the voltage detector 10 is input to a voltage effective value calculation circuit 26 via a voltage instantaneous value detection circuit 25, and an effective value V26 is obtained by the voltage effective value calculation circuit 26. Is input to the voltage relay 17, where it is compared with the set voltage value to detect an abnormality in the AC voltage, and is input to the abnormality detection circuit 19.

The signal V 22 detected by the frequency detection circuit 22 is input to the frequency relay 18, which detects an abnormality in the system frequency, and inputs this to the abnormality detection circuit 19.

Further, the AC voltage detected by the voltage detector 10 is input to a zero cross detector 28, where it is detected that the AC voltage is near the zero cross, and the detected value is input to a distortion detector 27. Further, the detection value detected by the voltage instantaneous value detection circuit 25 is input to the distortion detector 27. The distortion detector 27 detects the distortion near the zero crossing of the AC voltage, and inputs the detected value V27 to the distortion relay 32, where the case where the detected value V27 is larger than the set value is detected. Is done. The abnormality detection circuit 19 includes the voltage relay 1
7. When the output signals of the frequency relay 18 and the distortion relay 32 are input, a stop command is given to the drive unit 15.

The detected value V27 of the distortion detector 27 is input to a distortion change detector 33, where a change in distortion is detected, and the detected value is input to a coefficient setting circuit 30. The current reference I ^* from the current reference circuit 12 and the effective value V26 calculated by the voltage effective value calculation circuit 26 are input to the change rate detection circuit 29, where dV
/ DI ^* is detected and input to the coefficient setting circuit 30. In the coefficient setting circuit 30, the voltage effective value V and the current reference I
^From the ratio dV / dI ^{* of *,} it is determined whether the voltage has changed due to the change in the inverter output current.

The coefficient setting circuit 30 determines the variables F ₁ and F ₂ for determining the gain G and the phase θ output from the phase / gain setting circuit 31, thereby changing the phase and amplitude of the current reference I ^* . It is configured as follows.

The operation of the apparatus according to the embodiment of the present invention will be described below.

FIG. 2 shows an example of characteristics of the phase / gain setting circuit 31. The gain G and the phase θ are determined according to the variables F ₁ and F ₂ given from the coefficient setting circuit 30. Now the inverter is connected to the AC power system and the circuit breaker 7 in FIG.
Is closed, the system frequency is almost equal to the rated value f ₀ (50H
z or 60 Hz), the inverter outputs a current in the same phase as the AC power supply (power factor = 1), and the current reference I ^* has a gain of 1 (that is, the maximum power point of the solar cell). Output.

Next, the operation at the time of reverse charging (landing) when the circuit breaker 7 is opened will be described.

When the circuit breaker 7 is opened and there is an imbalance between the reactive power of the inverter output and the reactive power of the load, a differential reactive power ΔQ is generated. At this time, the load current Il
In the case of the delay power factor as shown in (e), the power factor of the inverter is normally almost 1 and control is performed so as not to generate reactive power, so that the inverter current I _IN and the load current Il are balanced. The output frequency becomes slightly higher, the leading current of the filter capacitor 4 is increased, the lag current of the load inductance is reduced, and the load power factor is about to become one.
The frequency V22 and the frequency change rate V23 are detected, f + α (df / dt) (α is a coefficient) is obtained by the arithmetic circuit 24, and weighted by the coefficient setting circuit 30 as V24.
Set the variable F _1, F _2, phase, gain setting circuit 31
To determine the gain G and the phase θ to change the phase and magnitude of the inverter current.

Now, the operation will be described for the case where only the current phase is changed. In the case of FIG. 9E, f + α (df / d
Since t)> f ₀ , the variable F ₂ shown in FIG. 2B is moved to the right from the position of f ₀ by this control, and the inverter current phase is advanced. Therefore, the phase of the load current and the phase of the inverter current are separated from each other, so that the frequency of the AC voltage further increases. This effect becomes a positive feedback, the frequency rises further, keep the characteristics as the phase is delayed reversed beyond the maximum value of the phase lead exceeds the certain range F ₂ as shown in FIG. 2 (b). As a result, the leading current flowing through the capacitor 4 becomes larger than the lagging current flowing through the load inductance, and F
_At point a, the load and the power factor of the inverter match and balance.

Incidentally, the straight line φL shown in FIG. 2 (b) is a load current phase, shows a case of power factor = 1 at the frequency f _0. Therefore, in practice it balanced at a position slightly moved to the right from the point of view of F _a.

Next, when the load has a leading power factor as shown in FIG. 9D, at the time of reverse charging, the inverter power factor and the load power factor try to match, the frequency of the AC voltage decreases, and the capacitor current decreases. , The reactor current increases to act to balance the reactive power. The frequency decreases, and f + α (df /
Since dt) <f ₀ , the inverter current is controlled to output a delay in FIG. 2B, so that the frequency decreases due to the positive feedback action in which the frequency of the AC voltage further decreases, and the phase characteristics are reversed. to balance at point F _b.

If the frequency relay 18 is set at, for example, the points Fx and Fy, when ΔQ is not 0, reverse charging can be easily detected and the inverter can be stopped.

In this case, since the frequency is diverged,
Since high voltage is not good for the load,
When the frequency deviation becomes large due to the gain characteristic of FIG. 2A, it is preferable to operate the voltage relay 17 by reducing the inverter current and the AC voltage.

The remaining reverse charge protection is for the case where ΔQ is substantially zero.

Consider the case where the active power of the load and the active power of the inverter are unbalanced under this condition, that is, ΔP is not zero.

In FIG. 9C, since C and L do not consume active power, active power is consumed only by R. If the voltage is V + ΔV (V is the rated voltage), the power consumption of R is
(V + ΔV) ² / R. Here, ΔV is a voltage change during reverse charging.

Therefore, ΔP = [(V + ΔV) ² / R] −
[V ² / R] holds. Therefore, if ΔP is equal to or more than a predetermined value, V + ΔV can be detected by the voltage relay 17 and the inverter can be stopped.

Next, ΔQ which is the most difficult to detect is approximately 0, ΔP
Is substantially zero.

[0052] excitation current i of the AC voltage V _AC and the pole transformer 6
The relation of _ex is shown in FIG. In the vicinity of the zero cross of the AC voltage, the excitation current i _ex becomes a distorted current unlike a sine wave due to the saturation characteristics of the iron core. If breaker 7 is closed, the exciting current i _ex is supplied from the AC power system 8, it holds the small sinusoidal V _AC of voltage distortion.

However, when ΔQ is approximately 0 and ΔP is approximately 0 and the circuit breaker 7 is opened, the exciting current i _ex of the pole transformer 6 is received from the inverter and the load. Since ΔQ is approximately 0, the power is balanced at a power factor of 1;
P is a slight sinusoidal component shown in FIG. 3 (b), can not by sufficiently supplying the peak current in the vicinity of the zero crossing of the voltage, as in the V _AC shown by a broken line in FIG. 3 (b), the voltage , The voltage near the zero cross decreases, and the distortion increases.

[0054] distortion detecting circuit 27 of this strain was detected by the distortion change detection circuit 33 detects a possibility of the reverse charge, shifting the horizontal axis F ₂ on the horizontal axis F ₁ and phase characteristics of the gain characteristic of Fig. 2 Then, the balance of the power is broken, the frequency and the voltage are fluctuated, and the inverter is stopped by the voltage relay 17 and the frequency relay 18.

The horizontal axis F ₂ is better aligned with the direction of f + α (df / dt), but f + α (df / dt).
= F _{0, that} is, when the frequency does not change, shifting the frequency to a lower value as -β can quickly reverse the reverse charging balance with respect to the motor load.

When the output V27 of the distortion detection circuit 27 exceeds a set value, the inverter can be stopped directly by the distortion relay 32.

Next, the distortion change detection circuit 33 detects the distortion increases, so when shifting the horizontal axis F ₁ is the current reference I ^* changes detected by dV / dI ^* circuit 29, dV is the opposite state of charge / When dI ^* exceeds a predetermined value, it is also possible to add a circuit imbalance by positive feedback to the horizontal axis F _1.

When the phase is changed, dV / dI ^* c
It is better to catch the voltage fluctuation with respect to osφ (cosφ = power factor) effective current. Needless to say, the accuracy can be further improved by setting the current reference I ^* to the actually measured current I.

According to the above-described embodiment, when the inverter is disconnected from the AC power system in a state where the inverter and the AC power system are connected and operated, the reactive power of the inverter output and the load of the load are reduced. When the reactive power is slightly unbalanced, the frequency rises or falls slightly. This frequency shift is detected, and if the frequency increases, the inverter current phase is advanced to control the frequency to further increase and diverge.If the frequency decreases, the current phase is sent to reduce the frequency. Let it diverge.

In addition, as the frequency deviates from the rated value, the inverter current value is reduced to reduce the supply of active power, thereby reducing the voltage.

Since the voltage and frequency do not change when the effective and ineffective parts of the power are completely balanced, the increase in the distortion near the zero crossing of the voltage waveform is detected, and the current state and the magnitude of the current are changed to change the balance state. To divergent operation. By doing so, reverse charging can be more reliably prevented.

The present invention is not limited to the embodiment described above, but can be modified as follows.

(1) In the above-described embodiment, in FIG.
The case where the horizontal axis of gain and phase is moved by + α (df / dt) has been described, but this is represented by A (f−f ₀ ) + f ₀ + α (df
/ Dt) (A is a coefficient), the horizontal axis can be moved. A
If = 0, the phase can be moved only when the frequency changes, and in the steady state, the operation with the power factor = 1 can be performed regardless of f.

Further, it is also possible to take measures such as applying a time coefficient to the feedback of the frequency change rate in accordance with the response of the AC power system 8.

[0065] (2) phase characteristic to FIGS. 4 (a) normal operating range of the system frequency as shown in F ₅ ~f ₀ ~F ₄ during the gradual rate of change of theta (or zero), outside the range It is needless to say that various variations are conceivable, such as improving the steady-state power factor by increasing the rate of change of θ and θ.

Further, as shown in FIG. 4 (c), it is conceivable that the change of θ is increased only near the rated frequency, and above that, θ is limited to prevent the power factor from deteriorating.

(3) For the gain characteristic, as shown in FIG. 4B, a curve such as x, y, z is prepared and dV / dI ^* co
Variations such as transition from x to z according to a change in sφ and a time function are also conceivable.

(4) The control circuit of FIG. 1 can be easily realized by using a microprocessor and a memory circuit.
In particular, as shown in FIG. 2, a characteristic in which the relationship between the band pasta filter 16 and the gain is similar but the phase characteristic is completely opposite can be freely stored in the memory. In an analog circuit, this can be achieved by separately using the gain characteristic of the bandpass filter and the phase characteristic of the notch filter.

(5) In FIG. 1, the frequency, frequency change rate, dV / dI ^* , distortion rate change, etc. are input to the coefficient setting circuit 30, but these combinations are free and the distortion rate change is harmonic. Needless to say, it can be caught by the wave change rate.

(6) When the power factor control of the inverter is inserted for stabilizing the voltage, the control may be such that the phase characteristic of FIG. 2 is added to the power factor control angle.

In this case, it is preferable to hold the power factor control when the distortion factor increases or dV / dI ^* exceeds a certain value.

(7) The phase / gain setting circuit 31 shown in FIG. 1 can be substituted by changing the characteristics of the filter 16 to those shown in FIGS. 31 characteristics can also be added.

The characteristics shown in FIGS. 5 and 6 can be easily put into practical use by using an analog filter technique (particularly, a switched capacitor filter). For example, Linear Technology's LT
By using one C1060 IC and connecting the band-pass filter 16a and the notch filter 16b of FIG. 5A in series, it is possible to obtain the gain-phase characteristic of FIG. 5B.

Further, as shown in FIG. 6A, the band-pass filters 1 (f ₁ and f ₂ ) having slightly different center frequencies are used.
By connecting 6c and 16d in series, FIG.
Needless to say, the characteristic shown in FIG. Similar characteristics can be obtained by adding 16c and 16d. In addition, the band-pass filter 16a is directed to the center frequency f _0. The phase characteristic of V16d and the gain characteristic of V16a are used.

(8) It goes without saying that the basic operation does not change even if the gain setting circuit 31 is omitted while the gain setting of the phase / gain setting circuit 31 in FIG. 1 is fixed.

[0076]

According to the present invention described above, it is possible to provide an inverter system interconnection protection method and device thereof that can reliably prevent reverse charging.

[Brief description of the drawings]

FIG. 1 is a block diagram for explaining an embodiment of the present invention.

FIG. 2 is a diagram for explaining characteristics of the phase / gain setting circuit of FIG. 1;

FIG. 3 is a waveform chart for explaining the operation of FIG. 1;

FIG. 4 is a diagram for explaining another embodiment.

FIG. 5 is a diagram for explaining another embodiment.

FIG. 6 is a diagram for explaining another embodiment.

FIG. 7 is a block diagram for explaining an example of a conventional inverter system interconnection protection device.

FIG. 8 is a diagram for explaining the problem of FIG. 7;

FIG. 9 is a diagram for explaining the problem of FIG. 7 and another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... DC power supply, 2 ... Inverter bridge, 3 ... Reactor, 4 ... Capacitor, 5 ... Current detector, 6 ... Pole transformer, 7 ... Breaker, 8 ... AC power system, 9 ... Load, 10
... voltage detector, 11 ... amplifier, 12 ... current reference circuit, 1
3 ... amplifier, 14 ... PWM control circuit, 15 ... drive unit, 1
6 bandpass filter, 17 voltage relay, 18 frequency relay, 19 abnormality detection circuit, 20 harmonic detection circuit, 21 fluctuation circuit, 22 frequency detection circuit, 23
Frequency change rate detection circuit, 24 arithmetic circuit, 25 voltage detection circuit, 26 effective value calculation circuit, 27 distortion detection circuit, 2
8 Zero cross detection circuit 29 Change rate detection circuit 30
... coefficient setting circuit, 31 ... phase / gain setting circuit, 32 ...
Strain relay, 33 ... Strain change detection circuit.

Claims (6)

(57) [Claims] 1. An inverter system interconnection protection method for converting DC power into AC power and operating in conjunction with an AC power system, wherein the inverter is synchronized with a voltage phase of the AC power system and the inverter output at a predetermined phase. Control the output current of the inverter so as to match the current reference to be output, and when the inverter is disconnected from the AC power system, the frequency of the output voltage of the inverter, the rate of change in frequency, and the distortion voltage near zero crossing Changing the phase of the current reference so that the frequency changes to a predetermined value based on the frequency. 2. A system interconnection protection device for an inverter that converts DC power into AC power and operates in connection with an AC power system, wherein current control means controls an output current of the inverter according to a current reference; A current reference circuit that synchronizes with the phase of the output voltage of the AC power system and the inverter and determines the phase of the current reference according to a separately given phase command; and a frequency of the output voltage of the inverter and a rate of frequency change. A system interconnection protection device for an inverter, further comprising control means for changing a phase of the current reference so that the frequency changes in a positive feedback loop based on the frequency. 3. The system interconnection of inverters according to claim 2, wherein the phase change rate is reversed when the frequency of the output voltage of the inverter deviates from the rated frequency by a predetermined value or more. Protective equipment. 4. A current reference circuit according to claim 2, wherein said current reference circuit has a characteristic of changing an amplification factor according to a gain command given separately, and said control means controls said inverter based on a frequency of an output voltage of said inverter and a frequency change rate. An inverter system interconnection protection device having a characteristic of reducing a gain command. 5. The current control means according to claim 2, wherein a characteristic of changing a phase of the current reference so that the frequency changes in a positive feedback loop based on a voltage distortion near a zero crossing of the output voltage of the inverter. A system interconnection protection device for an inverter, comprising: 6. The current control means according to claim 4, wherein the gain command is reduced based on a voltage distortion near a zero crossing of the output voltage of the inverter, a current reference or a change in the output voltage of the inverter with respect to a change in the output current of the inverter. A system interconnection protection device for an inverter, characterized in that the device has a characteristic of causing the system to be connected.