JPH06245388A - Reverse charge protecting device for system linkage inverter - Google Patents

Abstract

PURPOSE:To early detect disconnection of an output side of an inverter from an AC system in a protecting device for the inverter to be operated by converting DC power into AC power and linking it to the AC system. CONSTITUTION:The reverse charge protecting device for a system linkage inverter detects a voltage phase and frequency of an output side of an inverter 2 by detectors 10, 22, 25, obtains a reference phase from the voltage phase, generates a current reference of a phase responsive to this reference phase by generators 12, 26, controls an output current of the inverter by a controller 13, corrects the reference phase in response to the frequency by correctors 23, 24, early detects disconnection of the output side of the inverter from an AC system and protects it.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system interconnection protection device for an inverter that converts direct current power into alternating current power and supplies it to a load, and operates in conjunction with an alternating current power system.

[0002]

2. Description of the Related Art A typical example of a conventional device of this type is shown in FIG. 6 and described below.

DC power from 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 a filter composed of a reactor 3 and a capacitor 4 removes high frequency components by PWM control and supplies it to a load 9. To be done.

On the other hand, AC power for general households supplied from the AC power system 8 via the circuit breaker 7 and the pole transformer 6 is supplied to the load 9, and the inverter bridge 2 is connected to the AC power system 8. Drive in line. The AC voltage supplied to the load 9 is detected by the voltage detector 10, and the sinusoidal signal V _s is input to the current reference circuit 12 via the bandpass 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 obtain the current reference I ^*. Is output. This current reference I ^* And the output current I of the inverter bridge 2 detected by the current detector 5 and the amplifier 13
Is input to the inverter bridge 2 through the PWM controller 14 and the driver 15 so that the current deviation becomes zero. Current reference I ^* The phase of is almost the same as the phase of the AC voltage supplied to the load 9, and AC power having a high power factor is supplied from the inverter bridge 2.

When a solar cell is used as the DC power source 1, the voltage reference V ^{* is} set so as to extract the maximum power ^. However, since it is not directly related to the present invention, detailed description thereof will be omitted.

[0006] In such a power distribution system, when performing maintenance inspection on the load side including the pole transformer 6, the breaker 7 is opened and disconnected from the AC power system 8. In this case, the state of the load side is monitored by the voltage relay 17 and the frequency relay 18, and when disconnected from the AC power system, the abnormality detection circuit 19 detects this and stops the operation of the drive unit 15 to stop the inverter. ing.

[0007]

However, when the circuit breaker 7 is opened and disconnected from the AC power system 8, the abnormality detection circuit 19 indicates that the power supplied from the inverter bridge 2 and the power of the load 9 are in balance. No detection is done,
The inverter may continue to operate. This condition is called islanding or reverse charging and is dangerous when performing maintenance. Especially when an induction motor is connected to the load, the back electromotive voltage of the motor tends to maintain the voltage and frequency of the system, making it difficult to detect reverse charging. The following proposals have been made as methods for preventing this reverse charging.

Frequency fluctuation system: The phase of the system reference voltage input to the current reference circuit 12 is phase-shifted within a fixed amount by the fluctuation circuit 21, and when the circuit breaker 7 is opened, it is reversed from the fluctuation of the inverter output frequency. Detect charging. However, this method uses inverter power (including reactive power)
When the power of the load is completely balanced, the frequency and voltage may not change and detection may not be possible.

Power fluctuation method: The fluctuation circuit 21 causes a current
Current reference I output from the quasi circuit 12^* Within a certain amount
When the circuit breaker 7 is opened at the low frequency,
The balance between the power output from the data
Reverse voltage is detected by changing the voltage and frequency with. Shi
However, even with this method, many inverters are connected in parallel.
If this happens, the phase of the power fluctuation of each inverter will vary.
If there is no power fluctuation as a whole and it cannot be detected.
There is a match.

Harmonic voltage monitoring system: The harmonic detection circuit 20 monitors the harmonics of the load side voltage, and when the circuit breaker 7 is opened, the harmonics (third, fifth and seventh harmonics) increase. By doing so, reverse charging is detected. However, in this method, when a large number of loads having a capacitor input type rectifier circuit, such as an inverter air conditioner and a television, are used, the third, fifth and seventh harmonics increase in a steady state and the detection reliability becomes high. Markedly reduced.

The present invention has been made in order to solve these problems, and an object thereof is to provide a system interconnection protection device capable of more reliably detecting reverse charge even in the above-mentioned state. To provide.

[0012]

In order to achieve the above object, the present invention relates to an inverter protection device that converts DC power into AC power and operates in an AC system and operates in a voltage phase on the output side of the inverter. The frequency is detected, a reference phase is obtained from the voltage phase, an AC current reference having a phase corresponding to the reference phase is generated to control the output current of the inverter, and the reference phase is corrected according to the frequency. As a result, it is detected that the output side of the inverter is disconnected from the AC system, and the protection operation is performed. Further, the reference phase is corrected to a lead phase or a lag phase according to the increase or decrease of the frequency. Further, when the frequency is within a predetermined range before and after the rated frequency, the correction amount of the reference phase is reduced or the correction is not performed.

[0013]

When the AC side of the inverter is disconnected from the AC power system, the frequency of the AC power source slightly increases or decreases depending on the load condition. If this is detected and the frequency increases, the phase of the inverter current is advanced and shifted to the direction where the frequency further increases, and if the frequency decreases, the phase of the inverter current is delayed to shift to the direction where the frequency further decreases. The load, the load is rapidly lost, and the voltage, frequency, and frequency change rate of the AC power supply are detected to detect reverse charging at an early stage, and the operation of the inverter is stopped by this detection output. Further, in a predetermined range before and after the rated frequency, the phase change of the inverter current is reduced or the phase change is not performed so that the steady-state operation is stably performed.

[0014]

FIG. 1 shows an embodiment of a protection device for a grid interconnection inverter according to the present invention. In FIG. 1, the same parts as those in FIG. 6 are designated by the same reference numerals and the description thereof will be omitted. The AC voltage V ₁ supplied to the load 9 is detected via the voltage detection circuit 10 and the PLL
A (phase locked loop) circuit 22 obtains a signal SY synchronized with V ₁ .

From this signal SY, a sine wave circuit 26 obtains a sine wave synchronized with V ₁ through a phase shift circuit 23, the current reference circuit 12 synchronizes with V _1, and the output V _c of the amplifier 11 is adjusted to the magnitude. Sine wave current reference I ^* with proportional amplitude Is output. AC signal V from signal SY by frequency detection circuit 25
The frequency f of ₁ is detected, and the function circuit 24 generates a signal θ corresponding to the frequency f. The phase shift circuit 23 outputs the synchronization signal SYX whose phase has changed according to the signal θ. in this case,
When the AC voltage V ₁ is at the rated frequency, the synchronizing signal SYX having a phase in which the power factor is approximately 1 is output.

On the other hand, from the output of the voltage detector 10 (generally a transformer is used), the voltage relay 17, the frequency relay 18,
The frequency change rate relay 27 detects the amount of change, the abnormality detection circuit 19 detects an abnormality, and the inverter drive unit 15 is turned off to stop the inverter. In the above configuration, the load 9
Consider the case where the circuit breaker 7 is opened in a state where the inverter output currents are almost balanced. The output current of the inverter is basically controlled so that the power factor is approximately 1, that is, the voltage V and the inverter current I _INV are in the same phase as shown in FIG.

When the load has an inductance L as shown in (a) and has a delay power factor, the frequency rises to operate the inverter output current and the load current in balance.
It operates in a direction in which the frequency of the AC voltage increases in an attempt to reduce the current flowing through the inductance L. At this time,
The frequency detector 25 detects that the frequency has risen and acts so as to advance the inverter current phase slightly. Then, in order to balance with this inverter current, the frequency of the AC power supply acts positively to further increase, and the frequency of the AC power supply rises rapidly, so this is detected by the frequency relay 18 and the frequency change rate relay 27, and an abnormality is detected. The circuit 19 stops the inverter. That is, in the case of a lag power factor load, a forward current is supplied to positively act to break the balance.

Next, when the load has a capacitance C as shown in (b) and a leading power factor, in order to operate the inverter output current and the load current in a balanced manner, the frequency is lowered and the current flowing in the capacitor C is reduced. The load is reduced and the load shifts to approach the load of unity.

When the frequency detector 25 detects a decrease in frequency and the phase shift circuit 23 operates to delay the inverter current phase, the frequency is further decreased and the voltage of the capacitor C is delayed in phase, that is, the frequency is decreased. It acts positively and the frequency of the AC power supply drops rapidly. That is, the lagging current is supplied to the leading power factor load to quickly lose the balance. The frequency relay 18 and the frequency change rate relay 27 detect this and the abnormality detection circuit 19 stops the inverter.

In the case of a load having a power factor of 1, if the output current of the inverter and the current of the load are not balanced, the voltage becomes abnormal. Therefore, the voltage relay 17 detects this and the abnormality detection circuit 19 stops the inverter.

Next, the induction motor (IM), which has been conventionally considered to be the most difficult to detect reverse charging, is connected to the load as shown in FIG.
Consider the case of (d). The equivalent circuit in this case is approximated by a parallel circuit of the exciting inductance l ₀ and the resistance obtained by dividing the secondary resistance γ ₂ of IM by “slip” S. The vector of voltage and current at this time is shown in FIG.
₂ is the torque component current, and the current i ₀ is the magnetic flux component current. Frequency of the counter electromotive voltage of the induction motor is f-f _s is proportional to the rotational speed of (f _s is "slip" frequency) i.e. the induction motor.
(F is the power frequency) Therefore, the frequency of the AC power source to cut off the circuit breaker 7 becomes f-f _s.

When this frequency is detected, it is lower than the power supply frequency, so the delay current is made to flow out from the inverter by the phase shift circuit. Therefore, a current i which is delayed from that in FIG. 2E flows through the induction motor. As a result, that is, the torque current l
₂ decreases or becomes negative, and the motor slows down. Since this acts on the forward and return loops, the AC power supply frequency drops more rapidly than the free run, and the abnormality detection 19 is activated by the frequency relay to stop the inverter. An example of the function generator 24 is shown in FIG. In (a), the phase is advanced with respect to the rated frequency f ₀ in proportion to the increase in frequency, and is limited to a certain limit. When the power supply frequency deviates from the rated value, the power factor deviates from 1, but when the system is connected, the frequency change is extremely small and the power factor also becomes approximately 1. In (b), the power factor is 1 because the phase does not change near the rated frequency, and the phase shift is performed when the frequency deviates from a constant value. (C) shows a gradual change in phase near the rated frequency,
A method of increasing the phase shift as the frequency deviates from the rated value. (D) is a method of outputting a current with a power factor of 1 from the inverter without changing the phase near the rated frequency, and performing a fixed phase shift when the frequency deviates from a constant value. And so on.

As described above, according to the present invention, the frequency change is increased by shifting the current phase of the inverter in a direction that promotes the frequency change at the time of reverse charging, and the frequency change or the frequency change rate relay is used to increase the frequency change. It is possible to obtain the reverse charge protection device for the grid-connected inverter that detects the occurrence of the above and stops the inverter to prevent the reverse charge promptly. Another embodiment of the present invention is shown in FIG.

When the filter circuit 16 is inserted before the PLL circuit 22 in FIG. 1, the frequency is detected by the frequency detection circuit 25, and the phase shift amount by the filter is corrected by the filter phase correction circuit 28 within the required frequency range. By adding the output of the circuit and the function generator 24 by the adder circuit 29 and phase-shifting by the phase shift circuit 23, the same operation as in FIG. 1 can be performed.

It is needless to say that the method of detecting by the zero-cross method without using the PLL circuit 22 in FIG. 4 can be similarly applied. It is also clear that the filter phase correction circuit can be omitted if the phase delay of the filter is small in the range of use. Further, the accuracy of reverse charge detection can be improved by adding a distortion rate detection circuit or the like as the abnormality detection relay.

A circuit for advancing the phase with respect to an increase in frequency can be configured by the notch filter shown in FIG. 5, and can be realized by inserting this circuit into the voltage phase detection circuit or the current reference circuit. . In the actual use, since a frequency relay that determines reverse charging at a rated frequency of about ± 1 Hz is used, the phase may be advanced in this range.

[0027]

According to the present invention, when the frequency of the AC power supply does not rise above the rated value, the current phase of the inverter is advanced so as to further raise the frequency, and when the frequency falls below the rated value, The reverse of the grid-connected inverter that delays the current phase and further lowers the frequency to break the power balance and stop the inverter due to frequency or voltage abnormalities to detect reverse charge reliably and at high speed. A charge protection device can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing a load state for explaining the operation of the present invention.

FIG. 3 is a diagram showing an example of a function of a function generator 24 shown in FIG.

FIG. 4 is a diagram showing another embodiment of the present invention.

5 is a diagram showing an example of a filter phase correction circuit 28 of FIG.

FIG. 6 is a block diagram of a conventional protection device for a grid interconnection inverter.

[Explanation of symbols]

1 ... DC power supply, 2 ... Inverter bridge, 3 ... Reactor, 4 ... Capacitor, 5 ... Current detector,
6 ... Transformer, 7 ... Breaker, 8 ... AC power system, 9 ... Load, 10 ... Voltage detector, 11, 13 ... Amplifier, 12 ... Current reference circuit, 14 ... PWM control circuit, 15 ... Drive section, 16 ... Filter, 17 ... Voltage relay, 18 ... Frequency relay, 19 ... Abnormality detection circuit, 20 ... Harmonic detection circuit, 21 ... Loosening circuit, 22
... PLL circuit, 23 ... Phase shift circuit, 24 ... Function generating circuit, 25 ... Frequency detection circuit, 26 ... Sine wave circuit,
27 ... Frequency change rate relay, 28 ... Filter phase correction circuit,
29 ... Adder circuit.

Claims (3)

[Claims] 1. A protection device for an inverter, which converts direct-current power into alternating-current power and operates in connection with an alternating-current system, detects a voltage phase and a frequency on an output side of the inverter, and obtains a reference phase from the voltage phase. While controlling the output current of the inverter by generating an AC current reference having a phase corresponding to this reference phase, and correcting the reference phase according to the frequency, the output side of the inverter is disconnected from the AC system. Reverse charge protection device for grid-connected inverters, which detects and performs protection operation. 2. The reverse charge protection device for a grid interconnection inverter according to claim 1, wherein the reference phase is corrected to a lead phase or a lag phase according to an increase or decrease of the frequency. 3. The system according to claim 1, wherein the correction amount of the reference phase is reduced or not corrected in a predetermined range of the frequency before and after the rated frequency. Reverse charge protection device for inverters.