JPH04334977A - Power converter - Google Patents

Abstract

PURPOSE:To prevent the scaling-up of a capacitor having a small current ripple and used for supplying reactive power at a DC terminal by installing a first arm and a second arm connecting a self-arc-extinguishing type switching element and a diode in antiparallel and a third arm connecting the self-arc-extinguishing type switching element in antiparallel. CONSTITUTION:The main circuits 3A-3C of a self-excited inverter are composed of a P side arm 7 and an N side arm 8, in which self-arc-extinguishing devices 1 and diodes 2 are connected in antiparallel, and an intermediate arm 9, in which the devices l are connected in antiparallel. Other P and N side arms 7, 8 except the intermediate arm 9, in which the devices 1 are connected in antiparallel, are constituted in the same manner as a two-level inverter. Consequently, the intermediate arm 9 connected at the intermediate potential of a power supply is replaced with the P side arm 7 and electricity is conducted, thus outputting intermediate potential to a load terminal, then operating a power converter as a three-level inverter. Accordingly, a current ripple is reduced, and a capacitor for supplying reactive power at a DC terminal cannot be scaled up.

Description

[Detailed description of the invention]

[Purpose of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable voltage, variable frequency power converter, and more particularly to a power converter in which a neutral point is clamped to reduce harmonics.

0002

[Prior Art] A power conversion device that outputs variable voltage and variable frequency for variable speed operation of an AC motor uses a self-extinguishing device (transistor, GTO thyristor, etc.) to generate a voltage as shown in FIG. There is a type of self-excited inverter. The output phase voltage of this inverter exhibits two potentials, +Ed/2 and -Ed/2, as shown in FIG. 2(B). Therefore, it is called a two-level inverter here.

On the other hand, there is a self-excited inverter as shown in FIG. 3(A) that utilizes the intermediate potential neutral point created by the voltage division of a capacitor 5. A self-extinguishing device 1 and a diode 2 are connected in antiparallel, and two of them are connected in series to form one arm, and the middle is connected to an intermediate potential obtained by dividing a DC voltage into two by a capacitor. The inverter configured in this way has 0, +E as shown in Fig. 3(B).
It becomes a 3-level inverter that can output three potentials: d/2 and -Ed/2.

PWM (pulse width modulation) control is a method for controlling the output voltage of an inverter, and a two-level inverter has a line voltage waveform as shown in FIG. For it,
Since the three-level inverter can output at three voltage levels as shown in FIG. 3(B), the line voltage waveform becomes as shown in FIG. 5.

Comparing these voltage waveforms, the two-level inverter has a set of pulse voltages with a voltage change of 0-Ed, while the three-level inverter has a set of single and double pulse voltages with a height of Ed/2. It becomes. Therefore, the three-level inverter has many advantages such as smaller current ripple flowing through the motor, smaller torque ripple, and fewer harmonics.

On the other hand, in order to drive the electric motor at variable speed,
The output voltage of the inverter must be controlled from zero voltage to the rated voltage, and in the low speed region, the output voltage of the three-level inverter is as shown in FIG. 5(B). That is,
If only the intermediate voltage is used and only the power is received and received, the main circuit will be used as shown in FIG. When the load motor is under no load, the capacitor 5B supplies reactive power, but when the motor is loaded, the energy is transferred from the power supply to the inverter through the capacitor 5A. In order to transmit energy through a capacitor, the capacitor must be repeatedly charged and discharged, that is, operate as a switched capacitor, so a large capacitance capacitor is required, and a capacitor that can withstand charging and discharging is also required.

[0007]

As described above, in a two-level inverter, the current ripple of the load is large, the harmonic loss increases, and the torque ripple also increases. As a countermeasure to this problem, if a three-level inverter with capacitor division is used, it has the disadvantage that the capacitor becomes large to take the load in the low speed range.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks and to provide a power converter using a self-excited inverter that has small current ripple and does not require a large reactive power supply capacitor at the DC end. [Structure of the invention]

[0009]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a first arm and a second arm in which a self-extinguishing switch element and a diode are connected in antiparallel, and a self-extinguishing switch. It has a third arm in which elements are connected in antiparallel, one end of each of the first arm, second arm, and third arm is commonly connected to serve as an AC output terminal, and the other end of each is connected in common. A main circuit connected to the positive, negative, and intermediate potentials of a DC voltage source, and a DC power supply that connects multiple voltage sources in series to obtain the positive, negative, and intermediate potentials are provided, and the DC voltage is converted to an AC voltage by PWM control. This power conversion device is configured to output a voltage having a peak value of the intermediate potential during conversion.

0010

[Function] With the above configuration, an intermediate potential is obtained on the AC output side by turning on and off the third arm, and it acts as a self-exciting inverter with at least three levels. The potential is output directly without going through a capacitor.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the configuration of an embodiment of the present invention.

Main circuit 3 of the self-excited inverter according to this embodiment
A to 3C are composed of a P-side arm 7 and an N-side arm 8 which are formed by connecting a self-arc extinguishing device 1 and a diode 2 in anti-parallel, and an intermediate arm 9 which is formed by connecting a self-arc extinguishing device 1 in anti-parallel. There is. This intermediate arm 9 uses a reverse blocking type GTO thyristor or the like. P side arm 7, N side arm 8
, one end of the intermediate arm 9 is connected to the load terminal 0, and the other end is connected to P, N and neutral point C of the DC power supply. The main circuits 3A to 3B made up of these three arms constitute a three-phase inverter. For the DC power supply, three-phase Gretz connection rectifiers 4A and 4B using six diodes 2 are connected in series, and reactive power supply capacitors 5A and 5B are connected, respectively. An electric motor 6 is connected to the load terminal 0 of the main circuits 3A to 3C.

According to this embodiment configured as described above, the P and N side arms other than the intermediate arm 9 to which the self-arc extinguishing device 1 is connected in antiparallel have the configuration of the two-level inverter shown in FIG. It's the same. Therefore, by energizing the intermediate arm 9 connected to the intermediate potential of the power supply instead of the P-side arm 7, it is possible to output an intermediate potential to the load terminal, and it operates as a three-level inverter, as shown in FIG. A voltage output of 5 is obtained. Moreover, since the DC power supplies are independent, even if the intermediate arm 9 and the N-side arm are operated as a two-level inverter, the N-side power supply can supply power independently of the P-side power supply. Further, power can also be supplied from the P-side power supply using the P-side arm 7 and the intermediate arm 9.

[0014] In the power conversion device configured in this way,
Since it is possible to configure an inverter with a three-level output, and furthermore, the load power can be extracted directly from the DC power supply, the capacitor at the DC power supply end only needs to have a value that is sufficient to supply reactive power.

[0015]

As described above, according to the present invention, a self-excited inverter with three-level output can be constructed, and the output power supply ripple and harmonic loss can be reduced. In addition, even in a low-speed operation region, even if the load is taken as an intermediate voltage output, the value equivalent to the reactive power supply can be achieved without increasing the capacitor capacity, and a compact power conversion device with good performance can be provided.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an embodiment according to the present invention.

FIG. 2 is a configuration diagram and a phase voltage waveform diagram of a two-level inverter.

FIG. 3 is a configuration diagram and a phase voltage waveform diagram of a three-level inverter.

FIG. 4 is a waveform diagram of an output voltage under PWM control of a two-level inverter.

FIG. 5 is a waveform diagram of output voltage under PWM control of a 3-level inverter.

FIG. 6 is an explanatory diagram of the operation of a three-level inverter in a low voltage output region.

[Explanation of symbols]

1...Self-extinguishing device
2...Diode 3A to 3C...Main circuit
4A, 4B... Rectifier 5A, 5B... Capacitor
6...Electric motor 7...P side arm
8...N side arm 9...Intermediate arm

Claims (1)

[Claims] 1. A first arm and a second arm each having a self-extinguishing switch element and a diode connected in anti-parallel, and a third arm having a self-extinguishing switch element connected in anti-parallel, One end of each of the first arm, second arm, and third arm is commonly connected to serve as an AC output terminal, and the other end of each is connected to a main terminal connected to the positive, negative, and intermediate potentials of a DC voltage source. A circuit is provided with a DC power supply that connects a plurality of voltage sources in series to obtain the positive, negative, and intermediate potentials, and outputs a voltage at the peak value of the intermediate potential when converting DC voltage to AC voltage by PWM control. A power conversion device characterized by: