JPH11308773A - Active filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate for distortions due to the excutation current of a transformer by obtaining the excitation current waveform of the transformer, including a core material in a magnetic flux density waveform, that is obtained from the primary voltage waveform of the transformer and adding the compensation current of an active filter to it. SOLUTION: When harmonics and reactive current generated by a load 2 are detected by a detection circuit 91 of a control circuit 9, the gate signal of a PWM waveform is generated by a PWM circuit 93 with it as a compensation current instruction, the gate control of an inverter main circuit 5 is made by a gate circuit 94 , and a compensation current from the inverter main circuit 4 is supplied to a power line via the transformer 3, an excitation current detection circuit 10 detects the primary voltage waveform of the transformer 3 as the excitation current waveform of the transformer 3 and adds it to the compensation current, thus compensating for the current corresponding to excitation of the transformer 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active filter for suppressing harmonic current and reactive current in a power system.

[0002]

2. Description of the Related Art An active filter of this type detects a harmonic current, a negative-phase current, and a reactive current generated in a load of a consumer, and supplies a current having the opposite polarity to the power supply line to thereby provide a power supply line. To suppress the generation of harmonics, etc.
Eliminates line voltage distortion and the like.

FIG. 5 shows a circuit configuration of an active filter, which shows a three-phase three-wire system.
An output terminal of an inverter main circuit 5 is connected to a phase line via a transformer 3 having a Δ-Δ (triangle-triangle) connection and a harmonic filter 4, and the inverter main circuit 5 uses a capacitor 6 as a DC power supply to supply a compensation current. It is generated and supplied to the line through the filter 4 and the transformer 3. The harmonic current generated in the line is detected by the current detector 7, the line voltage phase is detected by the voltage detector 8, and the control circuit 9 obtains a necessary compensation current waveform and a phase signal from the detected current and voltage to obtain an inverter. A gate control signal for the main circuit 5 is used.

FIG. 6A shows a three-phase four-wire system, and FIG.
The difference is that the transformer 3 has a star-star connection and the filter 4 is provided on the line side. This main circuit configuration is
This is shown in FIG.

[0005]

The conventional active filter can remove a harmonic current, a negative-phase current, a reactive current and the like included in the load current according to the purpose. Does not compensate for the distortion current associated with

Here, in the transformer 3 of the three-phase four-wire active filter, if a Δ winding is provided in the tertiary winding, the in-phase current from the inverter flows through the Δ winding as a circulating current. (A zero-phase current or a third harmonic current of the fundamental wave) cannot be suppressed. For this reason, the transformer 3 is not provided with a tertiary winding, but in this case, the third, fifth, and fifth orders determined by the characteristics of the iron core material and the maximum magnetic flux density during operation are used as the exciting current of the transformer. This includes the seventh harmonic current. This amount is as large as 15 to 60% of the exciting current of the transformer.

As described above, the harmonic current contained in the excitation current is determined by the material of the iron core and the maximum magnetic flux density during operation (that is,
(Power supply voltage), the output is the same even when the output of the active filter is low. This means that the lower the load, the higher the harmonic current contained in the exciting current becomes, which cannot be ignored.

In order to reduce the amount of harmonic current included in the exciting current of the transformer, it is conceivable to lower the magnetic flux density of the iron core. However, the amount of the iron core increases, which leads to an increase in the cost and size of the transformer. .

In particular, a transformer having a gap in an iron core for preventing the transformer from being demagnetized has a large exciting current and a large voltage distortion due to the exciting current.

An object of the present invention is to provide an active filter capable of compensating for a distortion accompanying an exciting current of a transformer.

[0011]

The present invention utilizes the fact that the exciting current of a transformer is determined by its iron core material and its magnetic flux density, and the magnetic flux density is determined by the primary voltage of the transformer. By calculating the transformer excitation current waveform including the iron core material in the density waveform and adding this to the active filter compensation current,
It is designed to compensate for distortion accompanying the exciting current of a transformer, and is characterized by the following configuration.

An active filter for detecting a harmonic current, a reverse-phase current, and a reactive current generated in a load and supplying a compensation current according to a current command having a polarity opposite to the detected current to a power supply line via a transformer. An exciting current detecting circuit is provided for obtaining an exciting current waveform of the transformer from a voltage waveform of a power supply line, and using the exciting current waveform as an addition signal to the current command.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an apparatus configuration diagram showing an embodiment of the present invention, and portions equivalent to those in FIG. 5 or FIG.

The control circuit 9 which conventional become equivalent, the detection current of the harmonic current and a reactive current detecting circuit 9 ₁ a compensation current command, and the output current of the inverter main circuit 5 to obtain this instruction and the current detector 9 ₂ the butt, resulting pulse width and polarity of the PWM waveform according to the deviation to the PWM circuit 9 _3, this PW
The inverter main circuit 5 amplifies the M waveform at the gate circuit 9 ₄
To obtain a gate control signal.

Here, in the present embodiment, an exciting current detection circuit 10 is provided as a circuit added to the control circuit 9. The exciting current detecting circuit 10 detects the exciting current of the transformer 3 from the detection voltage of the voltage detector 8 and outputs
To the current command by adding the detected current from the reactive current detector 9 _1. As a result, an output that compensates for the exciting current of the transformer 3 is obtained in the inverter main circuit 5.

The exciting current waveform of the transformer 3 is determined by the material of the iron core and the magnetic flux density at that time, and the magnetic flux density is determined by the primary voltage of the transformer 3. Therefore, the exciting current detection circuit 10 can obtain the exciting current waveform of the transformer 3 from the voltage detection waveform of the power supply line and the constant determined by the core material of the transformer.

FIG. 2 shows a specific example of the exciting current detecting circuit 10. Average value calculating circuit 10 ₁ calculates the average value by integrating calculating a primary voltage detection waveform of the transformer 3. That is, the average value of the magnetic flux density of the transformer 3 is obtained. A / D converter 10 ₂ converts the average value the average value calculating circuit 10 ₁ is determined to a digital value. On the other hand, the A / D converter 10 ₃
The primary voltage detection waveform of the transformer 3 is converted into a digital value.

[0018] ROM 10 ₄ is written pattern data D ₀ to D _n of the exciting current of the transformer 3, and the lower address A ₀ ~A _K-1 and the digital value from the A / D converter 10 ₃ , The upper addresses A _{K to An} are converted to A / D converters 10 ₂
From the digital value.

[0019] ROM10 ₄ of pattern data D ₀ ~D
_n is divided into pattern regions that are distinguished according to the difference in the magnetic flux density (average value) of the transformer 3, and data corresponding to the phase change of the magnetic flux density is written for each region. Therefore, the upper address is determined according to the average value obtained by the average value calculation circuit 10 ₁ , and the lower address is determined according to the phase obtained by the A / D converter 10 _3. Are sequentially generated.

[0020] The data stream will become equivalent to the sampling data which is proportional to the exciting current waveform of the transformer 3, by conversion to an analog signal by a D / A converter ₁₀₅ is obtained as an excitation current waveform of the transformer 3 be able to. The difference of the exciting current waveform due to the core material of the transformer 3, either leave incorporated in advance in the ROM 10 ₄ pattern data, is set as a conversion constant of the D / A converter _105.

By using this configuration as the excitation current detection circuit 10 and adding it as an excitation compensation signal for the transformer, it is possible to compensate for the reactive current and harmonic current of the excitation current for the transformer.

FIG. 3 shows another circuit configuration of the exciting current detecting circuit 10. Portions figure differs from FIG. 2, instead of the A / D converter _103, lies in that a counter circuit.

The counter circuit, a waveform shaping circuit 10 which converts the primary voltage waveform of the transformer 3 into a rectangular wave of the same frequency ₆
When configured by the counter ₁₀₈ to the multiplying circuit ₁₀₇ and counting inputs the oscillation pulse in synchronism with the square wave signal to pulse oscillation at a frequency N times.

FIG. 4 shows input / output waveforms of each part in this configuration. As shown in FIG. 3A, the detection waveform of the voltage detector 8 is a sine wave. As shown contrast (b), the obtained square wave 1f the waveform shaping circuit _106. Then, as shown in FIG.
Obtain N-fold pulse to 0 _7. A pulse of each digit output and 1f of the counter ₁₀₈ for counting the pulses ROM 10 ₄
As in the case of FIG. 2, the reactive current and the harmonic current of the exciting current of the transformer can be compensated for by using the lower address of.

The exciting current detecting circuit 10 is not limited to the hardware configuration described above, and may have an additional software configuration when the control circuit 9 has a processor configuration.

[0026]

As described above, according to the present invention, the excitation current waveform of the transformer is obtained by including the core material in the magnetic flux density waveform obtained from the primary voltage waveform of the transformer, and this is added to the compensation current of the active filter. Therefore, it is possible to suppress the harmonic current and the reactive current generated in the load while compensating for the distortion accompanying the exciting current of the transformer.

[Brief description of the drawings]

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

FIG. 2 is an excitation current detection circuit (part 1) according to the embodiment;

FIG. 3 is an excitation current detection circuit (part 2) according to the embodiment;

FIG. 4 is a waveform chart of the counter circuit in FIG. 3;

FIG. 5 is a configuration diagram of a conventional three-phase three-wire active filter.

FIG. 6 is a configuration diagram of a conventional three-phase four-wire active filter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Power supply 2 ... Load 3 ... Transformer 4 ... Filter 5 ... Inverter main circuit 9 ... Control circuit 9 ₁ ... Harmonic current / reactive current detection circuit 9 ₃ ... PWM circuit 10 ... Excitation current detection circuit

Claims (1)

[Claims] 1. A harmonic current or a negative-phase current generated in a load,
In an active filter that detects a reactive current and supplies a compensation current according to a current command having a polarity opposite to that of the detected current to a power supply line via a transformer, an excitation current waveform of the transformer is obtained from a voltage waveform of the power supply line. An active filter comprising an exciting current detecting circuit for converting the exciting current waveform into an addition signal to the current command.