JP3186962B2 - NPC inverter device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an NPC inverter device for converting a DC voltage having a neutral point into an AC voltage, and more particularly to a DC power supply for outputting a DC voltage, which suppresses the oscillation state of the DC voltage due to LC resonance. , NP with improved control characteristics
It relates to a C inverter device.

[0002]

2. Description of the Related Art Neutral point clamp type inverters (Neutral
Point Clamped Inverter) is called an NPC inverter, and is known to convert a DC voltage having a neutral point into high-quality AC power having less harmonic components. This type of conventional NPC inverter device is shown in FIG. Shown in

In FIG. 4, a DC power supply 1 is an AC power supply 1.
A converter 1c that obtains a positive DC voltage Vdp from a and a converter 1 that obtains a negative DC voltage Vdn from an AC power supply 1b
d is connected in series, and is smoothed between PN through an LC filter including reactors 1e and 1f and capacitors 2 and 3, and has a DC voltage Vdpn having a neutral point C (= Vdp + Vdn).
Is output. The DC voltage Vdpn having this neutral point C is
It is converted into a desired AC voltage by the NPC inverter 5 and supplied to the load 7 as three-phase AC currents Iu, Iv, Iw. The three-phase AC currents Iu, Iv, Iw are detected by the current transformer 6 as feedback currents Iuf, Ivf, Iwf, and are converted by the converter (3
Phase two-phase conversion) 9, the active current component Iqf and the reactive current component Idf
Is converted to a rectangular coordinate signal of The active current component Iqf and the reactive current component Idf are converted by the current controller 10 into an active current reference I
Compared with qr and the reactive current reference Idr, orthogonal coordinate signals of the effective voltage reference Eqr and the invalid voltage reference Edr are output so as to reduce the respective current deviations. These orthogonal coordinate signals Eqr and Edr are converted into three-phase AC voltage references Vu *, Vv * and Vw * by a conversion unit (two-phase three-phase conversion) 11. The three-phase AC voltage references Vu *, Vv *, Vw * are compared with the positive and negative carriers output from the carrier generation unit 12 by the comparator 13 and output as a gate pulse GP. Pulse width modulation (PWM) control is performed by the pulse GP, and the DC voltage Vdpn between PN is changed to the AC voltage reference Vu *, V
v *, Vw *, and is converted into an AC voltage corresponding to the active current component Iqf and the reactive current component Idf of the three-phase AC currents Iu, Iv, Iw.
Is controlled to a value corresponding to the active current reference Iqr and the reactive current reference Idr.

The voltage detectors 20 and 21 detect the positive and negative DC voltages Vdp and Vdn of the DC power supply 1 as voltage feedbacks Vdpf and Vdnf, respectively.
d is added to each AC voltage reference Vu *, Vv *, Vw *, and the difference voltage Δ is calculated based on the corrected AC voltage reference Vu2 *, Vv2 *, Vw2 *.
The AC output voltage of the inverter 5 is controlled so that Vd becomes zero. Thus, when a difference occurs between the DC voltages Vdp and Vdn on the positive side and the negative side of the DC power supply 1, the neutral point potential fluctuates to generate a difference voltage ΔVd, and the AC voltage is set so that the difference voltage ΔVd becomes zero. The AC voltage references Vu2 *, Vv2 *, Vw2 * obtained by correcting the references Vu *, Vv *, Vw * are supplied to the comparator 13. As a result, the NPC inverter 5 is controlled so that a difference occurs between the output power on the positive side and the output power on the negative side, and is controlled so as to suppress the fluctuation of the neutral point potential.

For example, when the positive side DC voltage Vdp becomes larger than the negative side DC voltage Vdn, the AC voltage references Vu *, V
The difference voltage ΔVd is applied so as to bias v * and Vw * in the positive direction, so that the power supplied from the positive DC voltage Vdp increases and the power supplied from the negative DC voltage Vdn decreases. And the fluctuation of the neutral point potential is suppressed as a result.

[0006]

However, in the above-mentioned conventional device, the L composed of the reactors 1e and 1f and the capacitors 2 and 3 due to load fluctuations and output frequency fluctuations.
The DC voltage may oscillate at the resonance frequency of the C filter. Each of converters 1c and 1d includes a voltage control system (not shown) for controlling a DC voltage, and the DC voltage may vibrate depending on the relationship between the voltage control response and the resonance frequency. Further, if the positive DC voltage Vdp and the negative DC voltage Vdn fluctuate while matching, the difference voltage ΔVd is not detected, so that
The above-described correction control cannot suppress the fluctuation of the DC voltage. Further, when the positive DC voltage Vdp and the negative DC voltage Vdn fluctuate in the same phase due to the fluctuation of the effective current component of the AC current output from the inverter 5, the difference voltage ΔVd becomes zero. Can not.

[0007] Therefore, in the conventional device, the voltage control response of the converter must be slowed to cope with it, and the fluctuation of the DC voltage cannot be sufficiently suppressed. Therefore, when an electric motor that drives a rolling mill is applied as a load, the control response to a load change becomes slow, the control output torque fluctuates, the rolling action is affected, and the product quality may be adversely affected.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem. It is an object of the present invention to detect a fluctuation of a DC voltage to correct and control an effective current component of an AC output current, thereby achieving an LC resonance of a DC power supply. It is an object of the present invention to provide an NPC inverter device which suppresses a DC voltage oscillation phenomenon caused by the above and improves control characteristics.

[0009]

In order to achieve the above object, an NPC inverter device according to the present invention is configured as follows. A DC power supply that outputs a DC voltage having a neutral point divided into a positive voltage and a negative voltage through an LC filter, and an NPC that converts the DC voltage into an AC voltage having a desired frequency and supplies an AC current to a load Inverter means, and current control means for controlling the NPC inverter means so as to reduce the current deviation by comparing the active current reference and the reactive current reference with the detected values of the active current component and the reactive current component of the AC current, respectively. Correction means for outputting, as a correction signal, a fluctuation component of the DC voltage in a region of a predetermined frequency or higher, and the effective current reference is corrected by the correction signal so as to suppress the fluctuation of the DC voltage. (Claim 1) A DC power supply that outputs a DC voltage having a neutral point divided into a positive voltage and a negative voltage through an LC filter, and converts the DC voltage into an AC voltage having a desired frequency. A plurality of NPC inverter means for supplying an alternating current to the load; and a means for comparing the active current reference and the reactive current reference with the detection values of the active current component and the reactive current component of the alternating current, respectively, so as to reduce the current deviation. Said N
A plurality of current control means for controlling the PC inverter means;
A correction unit that outputs a fluctuation component of a region of a predetermined frequency or more of the DC voltage as a correction signal, and the respective effective current references given to each current control unit so as to suppress the fluctuation of the DC voltage by the correction signal. to correct. (Claim 2) A calculation unit is provided for obtaining a second correction signal that decreases in accordance with an increase in the active current reference based on the correction signal and the active current reference, and calculates a change in the DC voltage by the second correction signal. The respective effective current references provided to the respective current control means are corrected so as to suppress them. (Claim 3) The calculation section outputs a second correction signal obtained by multiplying the correction signal by a constant gain until the effective current reference exceeds a predetermined value, and outputs a second correction signal when the effective current reference exceeds a predetermined value. 2 Set the correction signal to zero. (Claim 4) The correction means determines the predetermined frequency based on a resonance frequency of the LC filter. (Claim 5)

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS NP according to claim 1 of the present invention
FIG. 1 shows an embodiment of the C inverter device. In the configuration of FIG. 1, reference numeral 30 denotes an effective current correction unit that obtains a correction signal Iqc from the positive DC voltage detection value Vdpf and the negative DC voltage detection value Vdnf, and includes a positive DC voltage detection value Vdpf and a negative DC voltage. An adder 31 for obtaining an addition value (Vdpf + Vdnf) of the voltage detection value Vdnf, that is, a DC voltage Vdpn between PN of the DC power supply 1.
When the DC voltage Vdpn between the PNs fluctuates, a high-pass filter 32 that removes a low-frequency region including a DC component of a fluctuation component equal to or lower than a predetermined frequency and outputs only a high-frequency region as a correction signal Iqc. An adder 33 adds the correction signal Iqc to the effective current reference Iqr. Otherwise, the configuration is the same as that of the related art.

In the above configuration, the NPC inverter 5
Outputs the three-phase AC currents Iu, Iv, Iw based on the active current reference Iqr and the reactive current reference Idr in the same manner as before, and the difference between the positive and negative DC voltages Vdp, Vdn of the DC power supply 1 is different. When the resulting difference voltage ΔVd is detected, the AC voltage references Vu *, Vv *, Vw * are corrected and controlled in the same manner as in the prior art, and the correction control is performed so as to suppress the fluctuation of the neutral point potential.

In this embodiment, when the DC voltage Vdpn between PN of the DC power supply 1 fluctuates, the effective current correction unit 30
Then, a correction signal Iqc in a high frequency region from which a low frequency region including a direct current component below a predetermined frequency of the fluctuation component is removed is added to an effective current reference Iqr via an adder 33. As a result, when the DC voltage Vdpn between the PNs fluctuates, the NPC inverter 5 is controlled so as to suppress the voltage fluctuation.
The effective current components of the alternating currents Iu, Iv, Iw output from the CPU are corrected and controlled. For example, when the DC voltage Vdpn between PN changes in the increasing direction, the effective current component is corrected to increase, and when the Vdpn changes in the decreasing direction, the effective current component is corrected to decrease. As a result, the DC voltage between PN is corrected. V
Acts to suppress fluctuations in dpn.

FIG. 2A is a transfer function showing the characteristics of the high-pass filter 32. Reference numeral 32a denotes a transfer block that passes only a low-frequency region including a DC component whose frequency is equal to or lower than a frequency determined by a predetermined time constant T of the input signal. 32b is a transmission block 32
A subtractor 32c for subtracting the low frequency region of the input signal having passed through the input signal from the input signal, and an amplifier 32c for converting the output signal of the subtractor 32b to a predetermined level and outputting it as a correction signal Iqc. With the above configuration, the input signal (DC voltage Vdp between PN
Only the high frequency component from which the low frequency region including the direct current component below the frequency determined by the predetermined time constant T has been removed from the detected value of Vdpn) is output as the correction signal Iqc.

According to this embodiment, when the DC voltage fluctuates due to the resonance frequency of the LC filter of the DC power supply 1 due to the load fluctuation or the output frequency fluctuation, the time constant T of the high-pass filter 32a is appropriately adjusted. By setting, a damping effect that effectively suppresses the fluctuation of the DC voltage is obtained,
The voltage control response of the converter operating as a DC power supply can be improved without causing an oscillation phenomenon. Therefore, when an electric motor that drives a rolling mill is applied as a load, the problem that the fluctuation of the control output torque of the electric motor is suppressed, the effect on the rolling action is suppressed, and the product quality is adversely affected is also solved.

FIG. 3 shows an embodiment of an NPC inverter device according to claim 2 of the present invention. In this embodiment, a DC voltage having a neutral point C output from one DC power supply 1 is supplied to a plurality of NPC inverters 5 _{1 to} 5 _n , and each of the motors (loads) 7 _{1 to} 7 _n has an independent AC. It is a common power supply system that supplies current.

[0016] In FIG. 3, 15 ₁ to 15 _n are respectively valid (torque) current reference Iqr and reactive (excitation) on the basis of the current reference Idr NPC inverter 5 ₁ to 5 controls the _n and motor (load) 7 ₁ - in the control unit for controlling the 7 _n, an adder 33 ₁ ~ 33 _n for adding the correction signal Iqc to each of the active current reference Iqr ₁ ~Iqr _n it is provided.

In the above configuration, the common DC power supply PN
When the voltage fluctuates, the correction signal Iqc in the high frequency region obtained by removing the low frequency region including the direct current component below the predetermined frequency of the fluctuation component from the effective current correction unit 30 is applied to each torque current reference I.
It is added to qr ₁ ~Iqr _n, each NPC inverter 5 ₁ to 5
The effective (torque) current component of the alternating current output from _n is corrected and controlled so as to suppress the voltage fluctuation between PN.

In this embodiment, each NPC inverter 5 ₁
To 5 _n it can be operated independently, different respective output frequency, but the DC voltage in relation to each of the frequency difference and the LC resonant frequency of the DC power source 1 may vibrate, active current correction High-pass filter 3 of section 30
By setting the time constant T of 2 to an appropriate value, the oscillation of the DC voltage can be suppressed.

FIG. 2B shows an embodiment of an NPC inverter device according to claims 3 and 4 of the present invention. The examples are further added the arithmetic unit 34 ₁ to 34C _n to the embodiment of FIG.

[0020] The arithmetic unit 34 ₁ to 34C _n becomes the gain varying amplifier according to active current reference Iqr ₁ ~Iqr _n respectively, with respect to the correction signal Iqc output from the active current correction unit 30, the effective the second correction signal Iqc ₁ ~I multiplied by the reduced gain with an increase of the current reference Iqr ₁ ~Iqr _n
Outputs qc _n . Each effective current reference given to each current control means is corrected by the second correction signal so as to suppress the fluctuation of the DC voltage.

[0021] FIG. 2 (c) in the gain characteristic of the arithmetic unit 34 ₁ ~34 _n, shows the relationship between the gain change with respect to the active current reference Iqr ₁ ~Iqr _n size (absolute value). Characteristic c1 is an example of the gain to zero in the effective current reference Iqr ₁ size of ～Iqr _n decreases the gain of the arithmetic unit 34 ₁ to 34C _n in accordance with the increase of (absolute value), a predetermined active current reference Iqrx Which corresponds to claim 3.

The characteristic c2 has an effective current reference Iqr _{1 to} Iqr _n
This is an example in which the gain is constant when the magnitude (absolute value) is equal to or less than a predetermined value Iqrx, and the gain is zero when the magnitude exceeds Iqrx.

According to an embodiment of the claims 3 and 4, when supplying a plurality of NPC inverters 5 ₁ to 5 _n, respectively of the electric motor (load) 7 ₁ to _7-n to the alternating current of the common power system, a predetermined The second correction signal Iqc is supplied only to the NPC inverter that supplies the AC current of the active current component equal to or less than the active current reference Iqrx of the NPC, and the NPC that supplies the AC current of the active current component exceeding the predetermined active current reference Iqrx is supplied. By controlling the inverter so as not to apply the second correction signal Iqc, the control for suppressing the fluctuation component of the DC voltage can be performed by a light-load (including no-load) NPC inverter. Therefore, when the DC voltage oscillates due to the resonance of the LC filter of the DC power supply and suppresses the voltage fluctuation, the operation status of each NPC inverter is checked, and the NP having the output current with a margin is provided.
The correction control is performed by the C inverter, and the NPC inverter that supplies the AC current to the electric motor (load) with the rolling load is not supplied with the second correction signal Iqc, so that the rolling operation is not affected.

In the above description, the DC voltage between PN of the DC power supply 1 is obtained by adding the positive DC voltage detection value Vdpf and the negative DC voltage detection value Vdnf. The present invention can be similarly implemented by using a voltage detector that directly detects a DC voltage.

[0025]

According to the present invention, the fluctuation of the DC voltage can be detected and the effective current component of the AC output current can be corrected and controlled so as to suppress the fluctuation. A stable DC voltage can be obtained by suppressing the voltage oscillation phenomenon, and an NPC inverter device with improved control characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment of an NPC inverter device according to claim 1 of the present invention.

FIG. 2 (a) is a block diagram showing the characteristics of a high-pass filter 32, and FIG. 2 (b) is a block diagram of N according to claims 3 and 4 of the present invention.
Main configuration diagram of an embodiment of a PC inverter device, (c) gain characteristic diagram of the arithmetic unit 34 ₁ to 34C _n

FIG. 3 is a configuration diagram of an embodiment of an NPC inverter device according to claim 2 of the present invention.

FIG. 4 is a configuration diagram of a conventional NPC inverter device.

[Explanation of symbols]

1 ... DC power source 1a, 1b ... AC power source 1c, 1d ... converter 1e, 1f ... reactor 2 ... capacitor 5,5 ₁ to 5 _n ... NPC inverter 6 ... current transformer 7, 7 ₁ to _7-n ... load ( 9) Converter (three-phase two-phase conversion) 10 ... Current controller 11 ... Converter (two-phase three-phase conversion) 12 ... Carrier generation unit 13 ... Comparator 15 ₁ to 15 _n ... Current control unit 20, 21 ... voltage detector 30 ... active current correction unit 31 ... adder 32 ... high-pass filter 33, 33 ₁ ~ 33 _n ... adder 34 _1-34
n … Calculation unit

Claims (5)

(57) [Claims] 1. A DC power supply for outputting a DC voltage having a neutral point divided into a positive voltage and a negative voltage via an LC filter, converting the DC voltage into an AC voltage having a desired frequency, and applying the AC voltage to a load. NPC inverter means for supplying a current, and controlling the NPC inverter means to reduce a current deviation by comparing the active current reference and the reactive current reference with the detection values of the active current component and the reactive current component of the AC current, respectively. And a correction unit that outputs a fluctuation component of the DC voltage in a region of a predetermined frequency or more as a correction signal, and corrects the effective current reference so as to suppress the fluctuation of the DC voltage by the correction signal. An NPC inverter device, characterized in that: 2. A DC power supply for outputting a DC voltage having a neutral point divided into a positive voltage and a negative voltage via an LC filter, and converting the DC voltage into an AC voltage having a desired frequency. Multiple NPs that supply alternating current to the load
C inverter means for controlling each of the NPC inverter means so as to reduce the current deviation by comparing the active current reference and the reactive current reference with the detected values of the active current component and the reactive current component of the AC current, respectively. Current control means, and correction means for outputting a fluctuation component of the DC voltage in a region of a predetermined frequency or higher as a correction signal, and the correction signal is given to each current control means so as to suppress the fluctuation of the DC voltage. An NPC inverter device for correcting each effective current reference. 3. The NPC inverter device according to claim 2, further comprising an arithmetic unit for obtaining a second correction signal that decreases in accordance with an increase in the effective current reference, based on the correction signal and the effective current reference. An NPC inverter device wherein each effective current reference provided to each current control means is corrected so as to suppress the fluctuation of the DC voltage by a second correction signal. 4. The NPC inverter device according to claim 3, wherein the arithmetic unit outputs a second correction signal obtained by multiplying the correction signal by a constant gain until the effective current reference exceeds a predetermined value. When the current reference exceeds a predetermined value, the second
An NPC inverter device for setting a correction signal to zero. 5. The NPC inverter device according to claim 1, wherein said correction means determines said predetermined frequency based on a resonance frequency of said LC filter. Inverter device.