JPH07101972B2 - Active filter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a harmonic component generated by a harmonic generating load, which adversely affects other loads connected to the power system when power is supplied from the power system to the harmonic generating load. The present invention relates to an active filter which is installed at a power source input terminal of a harmonic generating load to inject a compensating current to prevent the harmonic from flowing out into a power system.

2. Description of the Related Art An active filter which is the basis of the present invention will be described with reference to FIGS.

This active filter is configured by combining an injection circuit and a voltage source inverter, and controls the switching element of the voltage source inverter on / off by the level and polarity of the sum of the harmonic current of the harmonic generation load and the harmonic current of the injection circuit. To do.

That is, as shown in FIG. 3, the active filter is installed between the electric power system 1 and the harmonic generating load 2 fed from the electric power system 1, and the harmonic generating load 2 is installed.
An active filter for injecting a harmonic current for canceling a harmonic current flowing from the power system 1 into the power system 1, the first and second impedance elements 3A being connected to the power system 1. , 3B in series circuit, and a harmonic for detecting the sum of the harmonic current flowing from the harmonic generating load 2 to the power system 1 and the harmonic current flowing from the injection circuit 3 to the power system 1. A wave current detection circuit 4, a hysteresis comparator 5 having a negative predetermined value as a lower threshold value and a positive predetermined value as an upper threshold value, and an output Δi of the harmonic current detection circuit 4 as an input; An output voltage is not applied to the connection point of the first and second impedance elements 3A and 3B of the injection circuit 3, and the sum of the both harmonic currents is calculated according to the output of the hysteresis comparator 5. The sum of both harmonic current respectively a structure and a voltage source inverter 6 for turning on and off the switching element in a direction approaching to zero when it exceeds a predetermined value polarity.

In this case, the first impedance element 3A of the injection circuit 3 is composed of the capacitor (−jX _C1 ) C ₁ , and the second impedance element 3B is the series circuit of the capacitor C ₂ and the reactor L ₂ (jX _L2 −jX _C2 ). The impedance Z ₂ of the second impedance element 3B is selected such that Z ₂ = jX _L2 -jX _C2 = 0 or Z ₂ ≈0 with respect to the fundamental wave of the power system 1. With this injection circuit 3,
It is possible to significantly reduce the inverter rating.

The harmonic current detection circuit 4 is composed of current transformers 7 and 8, an adder 9, and a fundamental wave removal filter (for example, a notch filter) 10. The load current i _L is taken out by the current transformer 7 and
i _C1 is taken out by the current transformer 8, both are added by the adder 9, and then passed through the fundamental wave elimination filter 10 to obtain the harmonic current i _Lh in the load current i _L and the harmonic current i _ch in the current i _C1. Is output as the sum Δi.

The output Δi of the harmonic current detection circuit 4 is input to the hysteresis comparator 5.

The hysteresis comparator 5 is changed to the high level output level from the low level when the harmonic current detection circuit 4 outputs Δi as shown in FIG. 4 has exceeded the upper threshold I _0/2, the lower output level when the threshold -I _0/2 falls below is adapted to change from the high level to the low level.

The output of this hysteresis comparator 5
Each switching element Q ₁ ~ of the voltage source inverter 6 shown in the figure
By turning Q _{6 on} and off, a harmonic current i _ch (having a polarity opposite to i _Lh ) for compensating the harmonic current i _Lh flowing in the harmonic generating load 2 is supplied to the injection circuit 3.

FIG. 5 shows a circuit diagram for explaining the operation of the three-phase voltage source inverter 6. In the figure, Q ₁ and Q ₂ are A-phase switching elements, Q ₃ and Q ₄ are B-phase switching elements, and Q ₅
And Q ₆ are C phase switching elements, E is a DC power supply, and D ₁
~ D ₆ is a diode connected in antiparallel to the switching elements Q _{1 to} Q ₆ , Tr is a harmonic transformer, and L _{ia to} L _ic are reactors.

FIG. 6 shows an enlarged version of the harmonic current i _ch , and the operation will be described in more detail based on this figure.

In the case of i _ch <−i _Lh , when Δi = − (1/2) I ₀ is reached like the point, the output of the hysteresis comparator 5 becomes low level, and for example, the switching element of the lower arm of phase A is used.
Turn on Q ₂ .

As a result, the harmonic current i _ch increases and i _ch > −i _Lh . After this, when Δi = (1/2) I ₀ is reached as shown by the point, the output of the hysteresis comparator 5 becomes high level, for example, the switching element of the upper arm of the A phase.
Turn on Q ₁ . As a result, the harmonic current i _ch decreases, i _ch <−i _Lh , and so on. Therefore, the harmonic current
i _ch is the harmonic current −i while changing zigzag in the width of I _0.
It will change along _Lh .

FIG. 7 shows a waveform diagram of each part of FIG. The same figure (A) shows the system current i _S , the same figure (B) shows the load current i _L , the same figure (C) shows the current −i _C1 flowing through the capacitor C ₁ , and the same figure (D) shows the capacitor C ₂ The current i _C2 flowing in
Indicates the output current i _INV of the voltage source inverter 6.

The same applies to the B phase and the C phase.

Problems to be Solved by the Invention The active filter described above is based on the harmonic current detection circuit 4
When the fundamental wave removal in is insufficient, the voltage source inverter 6 also outputs the fundamental wave voltage, and this fundamental wave voltage is applied to the injection circuit 3, but the reactor L ₂ of the injection circuit 3 and the capacitor C ₂ are connected in series. Since the circuit is set to resonate at the frequency near the fundamental wave of the power system 1, the impedance element 3B short-circuits the output end of the voltage source inverter 6 with respect to the frequency near the fundamental wave, A large current having a frequency close to the fundamental wave flows to the impedance element 3B, that is, the output current i _INV of the voltage source inverter 6 contains a large amount of the fundamental wave component as shown in FIG. 7 (E). As a result, the capacity of the voltage source inverter 6 has to be increased.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an active filter capable of reducing the current at a frequency near the fundamental wave flowing from the voltage source inverter to the injection circuit.

Means for Solving the Problems The active filter according to the present invention is installed between a power system and a harmonic generation load fed from the power system, and harmonics flowing out from the harmonic generation load to the power system. An active filter for injecting a harmonic current for canceling a current into the power system, comprising: a first impedance element composed of a first capacitor and a second capacitor and a reactor connected to the power system. An injection circuit including a series circuit of two impedance elements, and a harmonic current detection for detecting a sum of a harmonic current flowing from the harmonic generation load to the power system and a harmonic current flowing from the injection circuit to the power system. Circuit and the negative predetermined value as the lower threshold and the positive predetermined value as the upper threshold, and the output of the harmonic current detection circuit The output voltage is not applied to the connection point between the hysteresis comparator that is an input and the first and second impedance elements of the injection circuit, and the harmonic current of the harmonic generation load and the harmonic current according to the output of the hysteresis comparator. A voltage source inverter that turns on and off a switching element in a direction in which the sum of the harmonic currents approaches zero when the sum of the harmonic currents of the injection circuit exceeds a predetermined value of both polarities; and the first and second X is the reactance of the capacitor
_C1 and X _C2 and the voltages across the first and second capacitors V _C1 and V _C2 , respectively, the voltage across the second capacitor V _C2 to the voltage across the first capacitor V _C1
To the reactance ratio X of the first and second capacitors
_A first subtractor for subtracting the value obtained by multiplying _C2 / X _C1 , and a second subtractor for subtracting the output of the first subtractor when the output of the harmonic current detection circuit is input to the hysteresis comparator
And a subtractor.

Action As described above, the active filter of the present invention detects the sum of the harmonic current of the harmonic generation load and the harmonic current of the injection circuit by the harmonic current detection circuit, and outputs the output of the harmonic current detection circuit. The difference from the output of the first subtractor is input to the hysteresis comparator, and the voltage across the second capacitor V _C2 to the voltage across the first capacitor V _C1
The reactance ratio of the first and second capacitors X _C2 / X
_{Since the} correction signal is obtained by subtracting the value multiplied by _C1 ,
A harmonic current for compensating the harmonic current of the harmonic generation load is caused to flow through the injection circuit, and a current having a large same wave number near the fundamental wave flows from the voltage source inverter to the second impedance element due to the function of the correction signal. Can be suppressed, and the inverter capacity can be reduced.

Embodiment An embodiment of the present invention will be described with reference to FIGS. 1 and 2.

The circuit shown in FIG. 1 is similar to the circuit shown in FIG.
Is added, and the others are the same as in FIG. 3, so
Description of the same part will be omitted.

Specifically, the compensating arithmetic circuit 12 calculates the voltage of the capacitor C ₂ .
V from _C2 to the voltage V _C1 of the capacitor C ₁ by subtracting the voltage multiplied by the reactance ratio X _C2 / X _C1 for capacitor C ₁ of the capacitor C ₂ The result of the subtraction by K times, harmonic current detection circuit 4
Output Δi is input to the hysteresis comparator 5 and is subtracted. The constant circuits 13 and 14 and the subtracters 15 and 16 are subtracted.
Composed of and. Next, it will be described in detail that a large current near the fundamental wave included in the inverter current can be suppressed by providing the compensation arithmetic circuit 12.

In order to prevent the current of the frequency close to the fundamental wave from flowing from the voltage source inverter 6 to the impedance element 3B of the injection circuit 3, a signal that detects the fundamental wave component in the output of the voltage source inverter 6 and cancels it is applied. It suffices to feed back to the input of the hysteresis comparator 5.

Therefore, it is the above-mentioned compensation arithmetic circuit 12 that tries to detect only the fundamental wave component in the output voltage of the voltage source inverter 6 without using a bandpass filter with a delay.

For the voltage V _C2 across the capacitor C ₂ , the voltage source inverter 6
And by output, so appear superimposed by a voltage of the power system 1, it is necessary to remove the effects of the power system 1 side voltage, to the voltage across the capacitor C ₁
The V _C1 and minus multiplied by Riatansu ratio X _C2 / X _C1 of the capacitor C _1, C _2, the output of the subtracter 15 becomes to correspond only to the fundamental wave in the output of the voltage source inverter 6.

Of course, the voltages V _C1 and V _C2 include not only the fundamental wave component but also the harmonic wave component, and the harmonic wave compensating function is slightly lowered by inputting this harmonic wave component to the hysteresis comparator 5. However, this is negligible. Hereinafter, this point will be described. C ₁ , C ₂
For the nth harmonic current components i _C1n and i _C2n (n includes non-integer orders) flowing in the Therefore, this correction term acts on the low-frequency component and acts on the harmonic component in a small amount. Therefore, if the fundamental wave component is corrected by 100, the fifth harmonic component is 20
Therefore, the performance as an active filter is slightly lowered, but this is not a problematic amount.

FIG. 2 shows the waveform chart of each part of FIG. 1, where (A) shows the system current i _S , (B) shows the load current i _L , and (C) shows the current of the capacitor C ₁ . i _C1 , (D) shows the output Δi ₁ of the constant circuit 14, that is, the above-mentioned correction term, and (E) shows the output current i _INV of the voltage source inverter 6.

As is clear from this figure, the fundamental wave component input to the hysteresis comparator 5 is compensated by the correction term of FIG. 2 (D), so that the output current i _INV of the voltage source inverter 6 has almost no fundamental wave component. It will not be included. Therefore, almost no current having a frequency near the fundamental wave flows from the voltage source inverter 6 into the injection circuit 3, and as a result, the capacity of the voltage source inverter 6 can be reduced.

Advantageous Effects of Invention The active filter of the present invention detects the sum of the harmonic current of the harmonic generation load and the harmonic current of the injection circuit by the harmonic current detection circuit, and outputs the harmonic current detection circuit and the output of the harmonic current detection circuit. The difference from the output of the subtractor of 1 is input to the hysteresis comparator, and when the output of the harmonic current detection circuit exceeds a predetermined value of both polarities according to the output of the hysteresis comparator, the harmonic generation load A harmonic current injection circuit for compensating for the harmonic current of the harmonic generation load by turning on and off the switching element of the voltage source inverter in the direction in which the sum of the harmonic current and the harmonic current of the injection circuit approaches zero. In addition to the method of flowing into
The value obtained by subtracting the value obtained by multiplying the voltage V _C1 across the first capacitor by the reactance ratio X _C2 / X _C1 of the first and second capacitors from the voltage V _C2 across the second capacitor as a correction signal to the hysteresis comparator. Since the input is made, the frequency component near the fundamental wave contained in the output of the harmonic current detection circuit can be canceled, and the current at the frequency near the fundamental wave from the voltage source inverter can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing the structure of an embodiment of the present invention, and FIG.
1 is a waveform diagram of each part of FIG. 1, FIG. 3 is a circuit diagram of an active filter which is the basis of the present invention, FIG. 4 is an operation characteristic diagram of a hysteresis comparator, and FIG. 5 is an operation explanation circuit of a voltage type inverter. FIG. 6 and FIG. 6 are enlarged waveform diagrams for explaining the operation, and FIG. 7 is a waveform diagram of each part of FIG. 1 ... Power system, 2 ... Harmonic generation load, 3 ... Injection circuit, 3A, 3B ... Impedance element, 4 ... Harmonic current detection circuit, 5 ... Hysteresis comparator, 6 ... Voltage source inverter, 15,16 …… Subtractor

Claims (1)

[Claims] 1. A harmonic current for canceling a harmonic current flowing from the harmonic generating load to the power system, the harmonic current being installed between the power system and a harmonic generating load fed from the power system. An active filter for injecting into the power system, the first impedance element comprising a first capacitor connected to the power system and a second
Of a second impedance element formed of a series circuit of a capacitor and a reactor, a sum of a harmonic current flowing from the harmonic generating load to the power system and a harmonic current flowing from the injection circuit to the power system. A harmonic current detection circuit for detecting, a negative predetermined value as a lower threshold value and a positive predetermined value as an upper threshold value, and a hysteresis comparator that inputs the output of the harmonic current detection circuit, and The output voltage is not applied to the connection point of the first and second impedance elements of the injection circuit, and the sum of the harmonic current of the harmonic generation load and the harmonic current of the injection circuit according to the output of the hysteresis comparator. Is a voltage-type inverter that turns on and off the switching element in the direction in which the sum of both harmonic currents approaches zero when the current exceeds a predetermined value for both polarities. Data and, both ends of the second capacitor when the voltage across the first and the reactance of the second capacitor and X _C1, X _C2 respectively the first and second capacitors and V _C1, V _C2, respectively A first subtractor for subtracting a value obtained by multiplying the voltage V _C1 across the first capacitor by the reactance ratio X _C2 / X _C1 of the first and second capacitors from the voltage V _C2; and the first subtractor. Second subtractor that subtracts the output of the above when the output of the harmonic current detection circuit is input to the hysteresis comparator.