JP3341047B2 - Multi-level power converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has at least two power converters for converting a DC voltage into multi-level AC voltage pulses by PWM control, and each power converter has a plurality of DC sides connected in series. The present invention relates to a multilevel power converter (converter / inverter system) connected via a DC stage circuit including a filter capacitor.

[0002]

2. Description of the Related Art In recent years, multi-level power converters that enable the use of high-frequency switching elements and reduce harmonics by increasing the number of output levels have begun to spread. In a multilevel power converter, it is important to control the intermediate voltage of the series capacitor of the DC stage to be constant in order to output a voltage as instructed.

For example, Japanese Unexamined Patent Publication No. 7-75345 describes a method of controlling a neutral point voltage of a three-level inverter.

[0004]

However, the neutral point voltage control of the inverter has a problem that the number of pulses in one cycle generally decreases in a region where the output frequency is high, and the effect of the neutral point voltage control becomes weak.

An object of the present invention is to effectively realize neutral point voltage control in a whole operation region in a converter / inverter system.

[0006]

Means for performing neutral point voltage control on the converter side, means for performing neutral point voltage control on the inverter side, and means for selecting this neutral point voltage control are provided on the converter side. The means for performing the neutral point voltage control and the means for performing the neutral point voltage control on the inverter side are provided by a pulse mode signal of the inverter, or a deviation signal of the midpoint voltage of the DC stage comprising the filter capacitor, or the output frequency of the inverter. Is selected using a signal corresponding to the magnitude of

As a result, even in the high-speed region where the number of pulses of the inverter decreases, the number of pulses on the converter side is almost always constant, so that the neutral point voltage control can be effectively performed.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. A multi-level power converter 1 (hereinafter, referred to as a converter) for converting a single-phase AC to a DC indicates a three-level converter including switching elements 11a to 11h and rectifying elements 12a to 12h as shown in FIG. ing. An AC power supply 3 is connected to the AC side of the converter 1 via a transformer 2. In addition, filter capacitors 41 and 4 are provided on the DC side.
A power converter 5 (hereinafter, referred to as an inverter) for converting a direct current to an alternating current is connected via a capacitor 2 (hereinafter, abbreviated as a capacitor), and an induction motor 6 for driving an electric vehicle is also connected. A multi-level power converter 5 (hereinafter, referred to as an inverter) that converts DC from a converter into AC is
As shown in FIG. 2B, the switching elements 51a to 51
1 and a three-level inverter composed of rectifying elements 52a to 52l and 53a to 53f. The detailed description of the main circuit configuration of the converter / inverter of FIG. 2 is omitted since it is described in the above-mentioned patent publication.

A positive DC voltage edp and a negative DC voltage edn detected by voltage detectors 71 and 72, respectively.
From the DC voltage ed, which is the sum of these, and the DC voltage difference Δe, which is the difference, by the adder 81 and the subtractor 82.
Calculate d. AC voltage e detected by voltage detector 73
Using s, the AC current is detected by the current detector 74, and the DC voltage ed, the converter control circuit 83 generates a modulated wave ymc.

A converter neutral point voltage controller 84 converts the DC voltage difference Δed into a modulated wave correction signal Δym
Obtain c. Modulated wave ymc and modulated wave correction signal Δymc
, A pulse signal is generated by the PWM controller 85, and the switching elements 11a to 11h of the converter 1 are turned on / off.

The inverter current i detected by the current detector
Using the mm, the speed signal fr detected by the speed detector 76, and the DC voltage ed, the inverter controller 86 generates a modulated wave ymi and a pulse mode signal mode.
Further, a modulated wave correction signal Δymi is obtained from the DC voltage difference Δed by the inverter neutral point voltage controller 87. Using the modulated wave ymi, the pulse mode signal mode, and the modulated wave correction signal Δymi, a pulse signal is generated by the PWM controller 88, and the switching elements 51a to 111 of the inverter 5 are turned on / off.

The gate start signal generator generates a gate start signal Gsc for the converter and a gate start signal Gsi for the inverter. Converter PWM
The controller 85 and the PWM controller 88 of the inverter generate pulse signals only when the respective gate start signals are input, and operate the converter 1 and the inverter 5 respectively. Neutral point voltage control selector 90 generates neutral point voltage control operation signals Npc and Npi of converter and inverter from gate start signals Gsc and Gsi of the converter and inverter and pulse mode signal mode. Converter neutral point voltage controller 8
The neutral point voltage controller 87 of the inverter 4 and the inverter operates only when the respective neutral point voltage control operation signals Npc and Npi are input.

Next, the operation of the embodiment of FIG. 1 will be described with reference to FIG.

In a converter / inverter system in which the neutral point of the DC stage circuit is common, the neutral point voltage control may be basically performed by either the converter or the inverter. Has already been put to practical use in DC electric vehicles, and it was considered that this should be used.

By the way, in the converter / inverter system, the inverter is started after the DC voltage ed exceeds a predetermined value due to the operation of the converter. Therefore, as shown in FIG. 3, shortly after the gate start signal Gsc of the converter goes high, the gate start signal Gsi of the inverter goes high. Therefore, during this period, only the converter performs the charging operation of the capacitors 41 and 42, and the neutral point voltage control by the inverter cannot be performed. After the inverter 5 is started, the neutral point voltage control operation signal Npi of the inverter is set high, and the neutral point voltage control is performed on the inverter side. However, when the rotation speed of the induction motor 6 increases, the number of pulses in one cycle of the inverter output decreases. In the neutral point voltage control, the rise or fall of the pulse finally given to the inverter 5 is a certain amount (time).
Therefore, if the amount of operation is the same, the effect of the neutral point voltage control will decrease as the number of pulses decreases. Unnecessarily increasing the operation amount causes problems such as current distortion. Therefore, a sufficient compensation effect may not be obtained only by the neutral point voltage control of the inverter.

In order to solve such a problem, a converter neutral point voltage controller 84 is provided, and when the converter operates as described above and the inverter is not started, the converter neutral point voltage control is performed. The operation signal Npc is set high, and the neutral point voltage control is performed on the converter side. Further, the neutral point voltage control operation signal Npc of the converter is set high even in a region where the speed is high and the number of pulses of the inverter is small.
In FIG. 3, for generality, the pulse mode is described by a number,
.. Are represented as pulse mode 1, pulse mode 2,. Here, it is assumed that the region after the pulse mode n is a region where the number of pulses is small and a sufficient compensation effect cannot be obtained only by the neutral point voltage control of the inverter.

Here, in the high-speed range where the neutral point voltage control operation signal Npc of the converter is high, the neutral point voltage control operation signal Npi of the inverter is set to low so that the neutral point voltage control of the inverter is not performed. But this is PW
This is because, depending on the method and constant of M, neutral point voltage control of the inverter not only has no effect, but also may distort the waveform or promote fluctuation of the neutral point voltage. When there is no such fear, as shown by the broken line in FIG.
pi high and the inverter neutral point voltage controller 87
There is no problem even if you keep on operating.

Further, when the neutral point voltage control operation signal Npi of the inverter is high, the neutral point voltage control operation signal Npc of the converter is set to low. This is for minimizing the chance of occurrence of harmonics and DC deviation that may occur on the power supply side by doing so. Again, if such a phenomenon is not a problem, there is no problem even if the neutral point voltage control operation signal Npc of the converter is kept high.

The pulse mode, which is a region where the neutral point voltage control is performed by the converter, may be a small pulse mode such as one pulse, two pulses or three pulses, an overmodulation mode, or the like. It is obvious that these vary depending on the main circuit specifications, the PWM modulation method, the carrier frequency, and the like.

FIG. 4 shows a second embodiment of the present invention. Basically the same as the embodiment of FIG. 1, but the neutral point voltage control selector 90
c, and outputs the neutral point voltage controller 87 of the inverter.
Is always operating during the inverter operation. This is an embodiment in which it is not necessary to stop the neutral point voltage control of the inverter when the neutral point voltage control is performed by the converter.
Although the effect is the same as that of the embodiment of FIG. 1, the device can be slightly simplified.

FIG. 5 shows a third embodiment of the present invention. In the neutral point voltage control selector 91, the inverter gate start signal Gsi is inverted by the NOT circuit 911, and the
AND at 912. The relay circuit 913 outputs high when the DC voltage difference Δed exceeds a certain threshold value. OR
The circuit 914 calculates the logical sum of these and outputs the result as a converter neutral point voltage control operation signal Npc. This is inverted by a NOT circuit 915 and output as an inverter neutral point voltage control operation signal Npi.

In this embodiment, before starting the inverter,
When the DC voltage difference Δed after the start of the inverter is large, the neutral point voltage controller 84 of the converter is operated. Since the operation is performed regardless of the speed, it is possible to obtain a good compensation effect even when the parameter of the device fluctuates, the operation is performed in a transient state, or an accident occurs. Also, as in the embodiment of FIG. 4, the output of the inverter neutral point voltage control operation signal Npi may be omitted as occasion demands.

FIG. 6 shows a fourth embodiment of the present invention. In the neutral point voltage control selector 92, the speed signal fr is compared with a specific speed Fr1 by a comparison circuit 921, and an OR circuit 91
4 is output. Others are the same as the embodiment of FIG.

In this embodiment, when the speed becomes equal to or higher than Fr1, the neutral point voltage controller 84 of the converter operates. Therefore, basically the same effects as those of the embodiment of FIG. 1 can be obtained. As in the embodiment of FIG. 5, the output of the inverter neutral point voltage control operation signal Npi may be omitted as occasion demands.

FIG. 7 shows a fifth embodiment of the present invention. In the neutral point voltage control selector 93, the modulation factor K is calculated by the amplitude calculator 931 from the modulation wave signal ymi of the inverter, and this is compared with a specific modulation factor K1 by the comparison circuit 921.
Output to the OR circuit 914. Others are the same as the embodiment of FIG.

In the present embodiment, the neutral point voltage controller 84 of the converter operates when the modulation factor is equal to or higher than K1. The modulation rate is generally controlled so as to be proportional to the speed fr until the one-pulse mode is entered, and becomes a constant value in the one-pulse region, so that basically the same effect as in the embodiment of FIG. 1 can be obtained. it can. Further, as in the embodiment of FIG. 6, the output of the inverter neutral point voltage control operation signal Npi may be omitted as occasion demands.

In the above description, a three-level converter / inverter system has been described as an example. However, similar effects can be expected in other multi-level converters with the same configuration.

[0028]

According to the present invention, the neutral point voltage control can be effectively performed in the entire operation range by combining or switching the neutral point voltage control of the converter and the inverter according to the operation state of the converter / inverter system. .

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a main circuit configuration of the power converter of the embodiment of FIG.

FIG. 3 is a diagram showing the operation of the embodiment of FIG. 1;

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

FIG. 5 is a diagram showing a third embodiment of the present invention.

FIG. 6 is a diagram showing a fourth embodiment of the present invention.

FIG. 7 is a diagram showing a fifth embodiment of the present invention.

[Explanation of symbols]

1 ... Converter, 11a to 11h ... Switching element,
12a to 12h, 13a to 13d: rectifier, 2: transformer, 3: AC power supply, 41, 42: filter capacitor,
5. Inverter, 51a to 51l ... Switching element,
52a-521, 53a-53f rectifier, 6 induction motor, 71-73 voltage detector, 74, 75 current detector, 76 speed detector, 81 adder, 82 subtractor, 83 Converter controller, 84: Converter neutral point voltage controller, 85, 88: PWM controller, 86: Inverter controller, 87: Inverter neutral point voltage controller, 89
... Gate start signal generator, 90-93 ... Neutral point voltage control selector.

──────────────────────────────────────────────────の Continued on the front page (72) Inventor Satoshi Horie 1070 Ma, Hitachinaka-shi, Ibaraki Co., Ltd. Inside the Mito Plant of Hitachi, Ltd. (56) References JP-A-7-75345 (JP, A) JP-A-7-250478 (JP, A) JP-A-8-98548 (JP, A) JP-A-6-319263 (JP, A) JP-A 5-26871 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H02M 7/48 H02M 7/5387 H02P 5/41 302

Claims (3)

(57) [Claims] 1. A multi-level PWM control power converter for performing PWM control of a plurality of switching elements to convert a DC voltage having positive, negative and intermediate voltages into a multi-level AC voltage pulse, and comprising at least two power converters. The DC converters are connected on the DC side via a DC stage composed of a filter capacitor having positive, negative and intermediate voltages. The AC side of the first power converter is connected to an AC power source having a constant frequency. In the multi-level power converter in which a variable frequency variable voltage is output on the AC side of the second power converter and a load is connected to the output end, the control of the midpoint voltage of the DC stage circuit is performed by the first side. Means for controlling the midpoint voltage of the DC stage circuit with a second multilevel power converter; and means for controlling the midpoint voltage of the DC stage circuit with the multilevel power converter. A multi-level power conversion device comprising: selection means for selecting a control means and a midpoint voltage control means using the second power converter by using a pulse mode signal of the second power converter. . 2. A multi-level PWM control power converter for converting a DC voltage having positive, negative and intermediate voltages into a multi-level AC voltage pulse by PWM-controlling a plurality of switching elements. The DC converters are connected on the DC side via a DC stage composed of a filter capacitor having positive, negative and intermediate voltages. The AC side of the first power converter is connected to an AC power source having a constant frequency. In the multi-level power converter in which a variable frequency variable voltage is output on the AC side of the second power converter and a load is connected to the output end, the control of the midpoint voltage of the DC stage circuit is performed by the first side. Means for controlling the midpoint voltage of the DC stage circuit with a second multilevel power converter; and means for controlling the midpoint voltage of the DC stage circuit with the multilevel power converter. A multi-level power conversion device, comprising: selection means for selecting a control means and a midpoint voltage control means by the second power converter using a deviation signal of the midpoint voltage of the DC stage circuit. . 3. A multi-level PWM control power converter for performing PWM control of a plurality of switching elements to convert a DC voltage having positive, negative and intermediate voltages into a multi-level AC voltage pulse, and comprising at least two power converters. The DC converters are connected on the DC side via a DC stage composed of a filter capacitor having positive, negative and intermediate voltages. The AC side of the first power converter is connected to an AC power source having a constant frequency. In the multi-level power converter in which a variable frequency variable voltage is output on the AC side of the second power converter and a load is connected to the output end, the control of the midpoint voltage of the DC stage circuit is performed by the first side. Means for controlling the midpoint voltage of the DC stage circuit with a second multilevel power converter; and means for controlling the midpoint voltage of the DC stage circuit with the multilevel power converter. Selecting means for selecting the control means and the midpoint voltage control means by the second power converter using a signal corresponding to the magnitude of the output frequency of the second power converter. Multi-level power converter.