JP4279640B2 - Power converter - Google Patents

Description

  The present invention relates to a power conversion device having a neutral point in a DC voltage, and more particularly to a power conversion device including means for suppressing fluctuations in neutral point potential.

  In a power converter having a neutral point in a DC voltage like a conventionally used three-level inverter, for example, when an AC motor is driven by a three-level inverter, a neutral point divided by a smoothing capacitor is used. Is known to fluctuate at a frequency three times the AC motor drive frequency. This occurs because the power supplied from the positive DC section to the three-level inverter is not instantaneously equal to the power supplied from the negative DC section to the three-level inverter. In the same way, when a three-level converter is applied to the converter side, the potential at the neutral point divided by the smoothing capacitor similarly fluctuates at a frequency three times the power supply frequency.

  When the neutral point potential fluctuates in this way, the drive waveform of the AC motor is disturbed, causing various problems such as increased loss, increased torque pulsation, and unstable control.

  In order to solve the above problems, measures have been adopted in which voltage compensation control is performed in response to potential fluctuations at the neutral point, and countermeasures for suppressing fluctuations in the neutral point potential by increasing the capacity of the smoothing capacitor. However, the countermeasure by the former control is limited in its application such as being effective only when the apparatus has a voltage margin, and the latter countermeasure has a problem that the apparatus is enlarged.

Further, in order to avoid such restrictions, a proposal has been made to add a neutral point suppressing circuit (see, for example, Patent Document 1).
JP 9-65658 A (pages 16-18, FIG. 1)

  The technique disclosed in Patent Document 1 provides a circuit connected from a neutral point to a positive power source and a negative power source via a reactor and a semiconductor switch, and performs conduction control of the semiconductor switch according to the fluctuation of the neutral point potential. Although it was made to perform, not only was the control complicated, but also a reactor was required, so it was not necessarily a simple solution.

  Accordingly, an object of the present invention is to provide a power converter that can suppress neutral point potential fluctuations by simple control without any restriction on control.

  In order to achieve the above object, a power converter according to the present invention divides a DC voltage input from an AC power source through a transformer into a DC voltage, and a DC voltage output from the two-level converter in two. A positive-side and negative-side smoothing capacitor connected to obtain a three-level DC voltage; a three-level inverter that converts the three-level DC voltage into an alternating current; and a secondary-side neutral point of the transformer; A semiconductor switch provided between neutral points of the positive and negative smoothing capacitors and capable of bidirectional energization, a detector for detecting the amount of electricity of the positive and negative smoothing capacitors, and this detection Control means for controlling the energization direction of the semiconductor switch so as to keep the voltage at the neutral point of the positive and negative smoothing capacitors constant according to the amount of electricity detected by the detector. As a feature That.

  According to the present invention, the direction of energization of the conductor switch provided between the secondary neutral point of the transformer and the three-level DC neutral point is controlled according to the fluctuation of the neutral point potential. Therefore, it is possible to provide a power conversion apparatus that can suppress neutral point potential fluctuations by simple control without any restriction on control.

  Embodiments of the present invention will be described below with reference to the drawings.

  1 is a circuit configuration diagram of a power conversion device according to a first embodiment of the present invention.

  As shown in FIG. 1, AC power is supplied from a three-phase AC power source to a two-level converter 2 configured by bridge-connecting power devices via a transformer 1. A series circuit composed of a positive-side smoothing capacitor 3A and a negative-side smoothing capacitor 3B is connected to the DC output portion of the two-level converter 2 to divide the output voltage of the two-level converter 2 into two. A three-level DC input composed of the output voltage of the two-level converter 2 and the neutral point voltage of the smoothing capacitors 3A and 3B is converted into a three-level AC output by the three-level inverter 4, and the AC motor 5 is driven. The three-level inverter 4 is configured to bridge-connect power devices connected in series, and clamp the midpoint of each power device connected in series to a neutral point potential by a diode.

  The amount of electricity input to the two-level converter 2 and the amount of electricity output from the three-level inverter 4 are controlled by the respective control units, but this control unit is not shown in FIG.

  Between the neutral point of the secondary winding of the transformer 1 and the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B, there is provided a semiconductor switch 6 in which power devices α and β are connected in antiparallel. Yes. The power device α of the semiconductor switch 6 is a switch in a direction in which current flows from the neutral point of the secondary winding of the transformer 1 to the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B, and the power device β is vice versa. It is. The voltage across the smoothing capacitors 3A and 3B is detected by the voltage detectors 7A and 7B, respectively, and becomes an input to the neutral point control circuit 8. The neutral point control circuit 8 is configured to control the semiconductor switch 6 in accordance with the voltages detected by the voltage detectors 7A and 7B.

  The operation of the power conversion device of the present invention having the above configuration will be described below.

  As described above, the potential at the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B fluctuates at a frequency three times the output frequency of the three-level inverter 4. Since the amplitude of the fluctuation of the neutral point potential corresponds to the difference between the voltages at both ends of the smoothing capacitors 3A and 3B, the voltages detected by the voltage detectors 7A and 7B are EA and EB, respectively. The circuit 8 calculates the differential voltage EA−EB. When this differential voltage EA-EB is larger than the first predetermined value, the neutral point control circuit 8 turns on the power device α of the semiconductor switch 6 and from the neutral point of the transformer 1 from the smoothing capacitors 3A and 3B. A current is passed through the neutral point of the series circuit to increase the DC voltage EB across the negative-side smoothing capacitor 3B. Conversely, when the differential voltage EA-EB is smaller than the second predetermined value, the power device β of the semiconductor switch 6 is turned on, and the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B is turned on in the transformer 1. A current is passed through the sex point to increase the DC voltage EA across the negative smoothing capacitor 3A. By repeating this control, the DC voltage divided by two can be kept constant.

  In the above control, the first predetermined value and the second predetermined value may be the same value, but the first predetermined value is set to be larger than the second predetermined value in order to stabilize the control. Also good. This concept is the same in the following embodiments.

  If the 2-level converter 2 is configured at 3 levels instead of 2 levels, the potential at the neutral point of the series circuit consisting of the smoothing capacitors 3A and 3B is also subject to fluctuations in frequency three times the power supply frequency. Similarly, the DC voltage divided by two can be kept constant by controlling the semiconductor switch 6 of this embodiment in the same manner.

  As described above, according to the present invention, by controlling the semiconductor switch 6 according to the fluctuation of the neutral point voltage, there is no control restriction and the electric power that can suppress the fluctuation of the neutral point potential by simple control. A conversion device can be provided.

  FIG. 2 is a circuit configuration diagram of the power conversion apparatus according to the second embodiment of the present invention. About each part of this Example 2, the same part as each part of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The second embodiment differs from the first embodiment in that instead of the voltage detectors 7A and 7B, there are provided current detectors 9A and 9B for detecting the currents flowing through the smoothing capacitors 3A and 3B, respectively, and the detected signals are neutral points. This is the point that it is used as the input of the control circuit 8A.

  Currents IA and IB flowing into the smoothing capacitors 3A and 3B are detected by current detectors 9A and 9B, respectively, and are compared by a neutral point control circuit 8A to obtain a difference current IA-IB. When the difference current IA-IB is larger than the first predetermined value, the neutral point control circuit 8A turns on the power device α of the semiconductor switch 6 and includes the smoothing capacitors 3A and 3B from the neutral point of the transformer 1. A current is passed through the neutral point of the series circuit to increase the DC voltage EB across the negative-side smoothing capacitor 3B. Conversely, when the difference current IA-IB is smaller than the second predetermined value, the power device β is turned on by the semiconductor switch 6, and the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B is turned into the inside of the transformer 1. A current is passed through the sex point to increase the DC voltage EA across the positive smoothing capacitor 3A.

  Since the currents IA and IB flowing into the smoothing capacitors 3A and 3B are proportional to the DC voltages EA and EB at both ends of the smoothing capacitors 3A and 3B, respectively, the DC voltage divided by two is kept constant by repeating the above control. Is possible.

  FIG. 3 is a circuit configuration diagram of the power conversion apparatus according to the third embodiment of the present invention. About each part of this Example 3, the same part as each part of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The third embodiment is different from the first embodiment in that current detectors 9A and 9B that detect currents flowing through the smoothing capacitors 3A and 3B are provided, in addition to the detection signals of the voltage detectors 7A and 7B that are already provided, The detection signals of the current detectors 9A and 9B are also input to the neutral point control circuit 8B.

  The operation of the neutral point control circuit 8B is as follows.

  First, the voltages detected by the voltage detectors 7A and 7B are set as EA and EB, respectively, and the difference voltage EA−EB is obtained as in the case of the first embodiment. Further, the currents IA and IB flowing into the smoothing capacitors 3A and 3B are detected by the current generators 9A and 9B, respectively, and the difference current IA-IB is obtained in the same manner as in the second embodiment.

  Therefore, when the differential voltage EA-EB is larger than the first predetermined value and the differential current IA-IB is larger than the second predetermined value, the neutral point control circuit 8B turns on the power device α of the semiconductor switch 5, A current is passed from the neutral point of the transformer 1 to the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B so as to increase the DC voltage EB across the negative side smoothing capacitor 3B. When the difference voltage EA-EB is smaller than the third predetermined value and the difference current IA-IB is smaller than the fourth predetermined value, the neutral point control circuit 8B turns on the power device β of the semiconductor switch 6, A current is passed from the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B to the neutral point of the transformer 1 to increase the DC voltage EA across the positive-side smoothing capacitor 3A.

  Thus, if the semiconductor switch 6 is controlled under the AND condition of the differential voltage and the differential current, the neutral point potential fluctuation can be more reliably suppressed.

  FIG. 4 is a circuit configuration diagram of the power conversion device according to the fourth embodiment of the present invention. About each part of this Example 4, the same part as each part of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The fourth embodiment is different from the first embodiment in that a semiconductor switch 6 is provided between the neutral point of the secondary winding of the transformer 1 and the neutral point of the series circuit including the smoothing capacitors 3A and 3B. A semiconductor switch 6A is provided between the neutral point of the primary winding of the AC motor 5 and the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B. The semiconductor switch 6A is controlled by a neutral point control circuit 8C. This is the point to be done with the output of

  Here, as in the case of the first embodiment, the voltages detected by the voltage detectors 7A and 7B are set as EA and EB, respectively, and the differential voltage EA−EB is obtained by the neutral point control circuit 8C. When this differential voltage EA-EB is larger than the first predetermined value, the neutral point control circuit 8C turns on the power device α of the semiconductor switch 6, and comprises the smoothing capacitors 3A and 3B from the neutral point of the AC motor 5. A current is passed through the neutral point of the series circuit to increase the DC voltage EB across the negative-side smoothing capacitor 3B. Conversely, when the differential voltage EA−EB is smaller than the second predetermined value, the neutral point control circuit 8C turns on the power device β of the semiconductor switch 6 and the neutrality of the series circuit including the smoothing capacitors 3A and 3B. A current is passed from the point to the neutral point of the AC motor 5 to increase the DC voltage EA across the positive smoothing capacitor 3A. By repeating this control, the DC voltage divided by two can be kept constant as in the case of the first embodiment.

  FIG. 5 is a circuit configuration diagram of a power conversion device according to Embodiment 5 of the present invention. About each part of this Example 5, the same part as each part of the power converter device which concerns on Example 4 of FIG. 4 is shown with the same code | symbol, and the description is abbreviate | omitted. The fifth embodiment differs from the fourth embodiment in that instead of the voltage detectors 7A and 7B, there are provided current detectors 9A and 9B for detecting the currents flowing through the smoothing capacitors 3A and 3B, respectively, and the detected signals are neutral points. This is the point where the input is made to the control circuit 8D.

  Here again, as in the case of the second embodiment, the currents IA and IB flowing into the smoothing capacitors 3A and 3B are detected by the current output devices 9A and 9B, respectively, and are compared by the neutral point control circuit 8D, and the difference current IA− Find IB. When the difference current IA-IB is larger than the first predetermined value, the neutral point control circuit 8D turns on the power device α of the semiconductor switch 6A, and includes the smoothing capacitors 3A and 3B from the neutral point of the AC motor 5. A current is passed through the neutral point of the series circuit to increase the DC voltage EB across the negative-side smoothing capacitor 3B. Conversely, when the difference current IA-IB is smaller than the second predetermined value, the neutral point control circuit 8D turns on the power device β of the semiconductor switch 6A, and the neutrality of the series circuit composed of the smoothing capacitors 3A and 3B. A current is passed from the point to the neutral point of the AC motor 5 to increase the DC voltage EA across the positive smoothing capacitor 3A.

  Since the currents IA and IB flowing into the smoothing capacitors 3A and 3B are proportional to the DC voltages EA and EB at both ends of the smoothing capacitors 3A and 3B, respectively, the DC voltage divided by two is kept constant by repeating the above control. Is possible.

  FIG. 6 is a circuit configuration diagram of a power conversion apparatus according to Embodiment 6 of the present invention. About each part of this Example 6, the same part as each part of the power converter device which concerns on Example 4 of FIG. 4 is shown with the same code | symbol, and the description is abbreviate | omitted. The sixth embodiment is different from the fourth embodiment in that current detectors 9A and 9B for detecting the currents flowing through the smoothing capacitors 3A and 3B are provided, in addition to the detection signals of the voltage detectors 7A and 7B already provided, The detection signals of the current detectors 9A and 9B are also input to the neutral point control circuit 8E.

  The operation of the neutral point control circuit 8E is as follows.

  First, the voltages detected by the voltage detectors 7A and 7B are set as EA and EB, respectively, and the difference voltage EA−EB is obtained as in the case of the fourth embodiment. The currents IA and IB flowing into the smoothing capacitors 3A and 3B are detected by the current output devices 9A and 9B, respectively, and the difference current IA-IB is obtained in the same manner as in the fifth embodiment.

  Therefore, when the differential voltage EA-EB is larger than the first predetermined value and the differential current IA-IB is larger than the second predetermined value, the neutral point control circuit 8E turns on the power device α of the semiconductor switch 6A, A current is passed from the neutral point of the AC motor 5 to the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B so as to increase the DC voltage EB across the negative-side smoothing capacitor 3B. When the difference voltage EA-EB is smaller than the third predetermined value and the difference current IA-IB is smaller than the fourth predetermined value, the neutral point control circuit 8E turns on the power device β of the semiconductor switch 6, A current is passed from the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B to the neutral point of the AC motor 5, thereby increasing the DC voltage EA across the positive smoothing capacitor 3A.

  Thus, if the semiconductor switch 6 is controlled under the AND condition of the difference voltage and the difference current, the neutral point potential fluctuation can be more reliably suppressed as in the case of the third embodiment.

  FIG. 7 is a circuit configuration diagram of the power conversion device according to the seventh embodiment of the present invention. About each part of this Example 7, the same part as each part of the power converter device which concerns on Example 1 of FIG. 1 is shown with the same code | symbol, and the description is abbreviate | omitted. The seventh embodiment differs from the first embodiment in that a three-level converter 2A is provided in place of the two-level converter 2 and that the three-level inverter 4 and the AC motor 5 are not shown. The three-level converter 2A, like the three-level inverter 4 of the first embodiment, bridge-connects power devices connected in series, and clamps the midpoint of each power device connected in series to a neutral point potential by a diode. It is configured.

  In FIG. 7, the load of the three-level converter 2A is not shown. However, for example, if a DC reactor is connected as a load, a reactive power compensator can be configured.

  As described above, the potential at the neutral point of the series circuit composed of the smoothing capacitors 3A and 3B fluctuates at a frequency three times the power supply frequency. Since the amplitude corresponding to the fluctuation of the neutral point potential corresponds to the difference between the voltages at both ends of the smoothing capacitors 3A and 3B, the voltages detected by the voltage detectors 7A and 7B are respectively used as in the first embodiment. Assuming EA and EB, the neutral point control circuit 8 obtains the differential voltage EA−EB. When this differential voltage EA-EB is larger than the first predetermined value, the neutral point control circuit 8 turns on the power device α of the semiconductor switch 6 and comprises the smoothing capacitors 3A and 3B from the neutral point of the transformer 1. A current is passed through the neutral point of the series circuit to increase the DC voltage EB across the negative-side smoothing capacitor 3B. Conversely, when the differential voltage EA-EB is smaller than the second predetermined value, the neutral point control circuit 8 turns on the power device β of the semiconductor switch 6 and the neutrality of the series circuit composed of the smoothing capacitors 3A and 3B. A current is passed from the point to the neutral point of the transformer 1 to increase the DC voltage EA across the positive smoothing capacitor 3A. By repeating this control, the DC voltage divided by two can be kept constant.

  Similarly to the case of the second embodiment, even if the semiconductor switch 6 is controlled not by voltage detection but by current detection, fluctuations in the neutral point potential can be suppressed.

  Further, as in the case of the third embodiment, even if the voltage detection and the current detection are used in combination to control the semiconductor switch 6, fluctuations in the neutral point potential can be suppressed.

The circuit block diagram which shows the power converter device of Example 1 of this invention. The circuit block diagram which shows the power converter device of Example 2 of this invention. The circuit block diagram which shows the power converter device of Example 3 of this invention. The circuit block diagram which shows the power converter device of Example 4 of this invention. The circuit block diagram which shows the power converter device of Example 5 of this invention. The circuit block diagram which shows the power converter device of Example 6 of this invention. The circuit block diagram which shows the power converter device of Example 7 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Transformer 2 2 level converter 2A 3 level converter 3A, 3B Smoothing capacitor 4 3 level inverter 5 AC motor 6 Semiconductor switch 7A, 7B Voltage detector 8, 8A, 8B, 8C, 8D, 8E Neutral point control circuit 9A, 9B current detector

Claims (8)

A two-level converter for converting alternating current input from an alternating current power source through a transformer into direct current;
The DC voltage at the output of the two-level converter is divided into two, and positive and negative smoothing capacitors connected to obtain a three-level DC voltage;
A three-level inverter that converts the three-level DC voltage into an alternating current;
A bidirectionally energizable semiconductor switch provided between a secondary neutral point of the transformer and a neutral point of the positive and negative smoothing capacitors;
A detector for detecting the amount of electricity of the positive and negative smoothing capacitors;
Control means for controlling the energization direction of the semiconductor switch so as to keep the voltage at the neutral point of the positive and negative smoothing capacitors constant according to the amount of electricity detected by the detector. A power conversion device. A two-level converter or a three-level converter that converts alternating current input from an alternating current power source through a transformer into direct current;
A smoothing capacitor on the positive and negative sides connected to obtain a three-level DC voltage at the output of the two-level converter or the three-level converter;
A three-level inverter that converts the three-level DC voltage into alternating current and drives an alternating-current motor with the output;
A bidirectionally energizable semiconductor switch provided between a neutral point of the primary winding of the AC motor and a neutral point of the positive and negative smoothing capacitors;
A detector for detecting the amount of electricity of the positive and negative smoothing capacitors;
Control means for controlling the energization direction of the semiconductor switch so as to keep the voltage at the neutral point of the positive and negative smoothing capacitors constant according to the amount of electricity detected by the detector. A power conversion device. A three-level converter that converts AC input from an AC power source through a transformer into three-level DC;
Positive and negative smoothing capacitors connected to the output of the three-level converter;
A bidirectionally energizable semiconductor switch provided between a secondary neutral point of the transformer and a neutral point of the positive and negative smoothing capacitors;
A detector for detecting the amount of electricity of the positive and negative smoothing capacitors;
Control means for controlling the energization direction of the semiconductor switch so as to keep the voltage at the neutral point of the positive and negative smoothing capacitors constant according to the amount of electricity detected by the detector. A power conversion device.   The power converter according to any one of claims 1 to 3, wherein the quantity of electricity is a voltage of the positive and negative smoothing capacitors.   4. The power converter according to claim 1, wherein the amount of electricity is a current of the positive and negative smoothing capacitors. 5.   4. The power converter according to claim 1, wherein the amount of electricity is a voltage and a current of the positive and negative smoothing capacitors. 5.   The control means determines whether the direction of energization of the semiconductor switch depends on whether the difference between the amount of electricity of the positive-side smoothing capacitor and the amount of electricity of the negative-side smoothing capacitor detected by the detector is greater than a predetermined value. The power conversion device according to any one of claims 1 to 3, wherein the power conversion device is selected.   The control means determines whether both the voltage difference and the current difference between the positive and negative smoothing capacitors detected by the detector are larger than the first predetermined value and the second predetermined value, respectively. The power converter according to claim 6, wherein an energization direction of the semiconductor switch is selected.

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