JPH09103078A - Power converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a leakage current flowing through a ground caused by switching a switching element in high frequency by installing a bypass filter consisting of a capacitor connected between an AC line and the ground and a capacitor connected between a DC line and the ground. SOLUTION: A capacitor 10 is added between an LC filter circuit on the AC side and a frame 8 and a capacitor 11 between the negative pole side of a filter circuit and the frame 8. The output voltage of an inverter contains high frequency, but the high-frequency component of the output voltage is removed by the LC filter circuit on the AC side, and output voltage between AC lines is formed in a sine wave in low frequency. Voltage containing the high-frequency component can be generated between an AC line and a ground and between the ground and a DC line, but the multiple wave component of the frequency of a triangular wave is absorbed by the reactors 6a-6c of the LC filter circuit on the AC side and the filter circuit 10 and the capacitor 11, and only the low frequency of a fundamental-wave component is left.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power conversion device that performs forward conversion or inverse conversion.

[0002]

2. Description of the Related Art In a power converter comprising a forward converter for converting AC power into DC power and an inverse converter for converting DC power into AC power, a high frequency pulse width using a high speed switching element has been recently used. A modulation (PWM) control method is widely adopted.

In these power converters, in order to remove high frequency components generated between AC lines, an AC reactor is inserted in the AC line of each phase of the forward converter and the inverse converter, and a capacitor is provided between the AC phases. And an LC filter circuit formed by connecting the.

FIG. 13 is a block diagram showing an example of a conventional converter. In FIG. 13, a power converter 2 which is connected to an AC power supply 1 and has a switching connection of a switching element for converting AC power into DC power, a DC capacitor 3 for smoothing the converted DC power, and this DC capacitor 3
A load 4 connected to the AC power supply 1, a control circuit 5 for performing PWM control by applying a gate signal to the switching element of the power converter 2 based on the voltage and current of the AC power supply 1 and the voltage of the DC capacitor 3, and the AC power supply 1. An LC filter circuit including AC reactors 6a to 6c and AC capacitors 7a to 7c is provided between the AC line and the power converter 2.

Further, the configuration other than the AC power source 1 and the load 4 is as follows.
It is housed in a conductive frame 8 as a converter device. The frame 8 is connected to the ground 9 in order to improve safety and electromagnetic shield effect.

[0006]

However, according to the prior art, even though the high frequency component between the AC lines can be removed, the high frequency component generated between the AC line and the ground and between the DC line and the ground can be removed. Can not.

Therefore, when the switching element of the inverter or the converter is switched at a high frequency, a voltage including a high frequency is generated between the AC line and the ground as indicated by v _E in FIG. 14, and the voltage between the AC line and the ground and between the ground and the DC line is generated. A ground current containing a high frequency was flowing between them as indicated by i _E in FIG.

This is because when the converter of FIG. 13 is actually constructed, stray capacitance C _E of several tens of PF / m to several hundreds of PF / m exists between each circuit element and wiring and ground.
FIG. 15 is a block diagram in which stray capacitance is added to the converter circuit block diagram shown in FIG.

In FIG. 15, the AC power source 1 has a single line (for example, V
Phase), and a stray capacitance C _3E exists between the DC portion of the power converter 2 and the frame 8 and a stray capacitance C _4E exists between the DC line and the ground.

As a result, due to the voltage v _E of the high frequency component generated in the power converter 2, the AC line of the power converter 2 → the stray capacitance C _5E of the AC reactor 6 → AC power source 1 → ground line →
Ground → Frame → Stray capacitances C _3E and C _4E → A current path to the DC line of the power converter 2 is formed, and the above-mentioned current i
_E flows.

Since this current i _E is a leakage current flowing through the ground line, if there is a leakage breaker or the like in the AC line, it will cause an erroneous trip, and even for other equipment connected to the AC line. There is a problem that causes malfunction or noise disturbance. Even if the AC power supply 1 is not grounded, the stray capacitance C _1E between the AC line and ground causes the current i _E to flow.

For example, assuming that the switching element of the power converter is switching in about 1 μs, if the harmonic voltage of the 1 MHz component is 100 v and the total stray capacitance is 10 nF in a 400 v circuit, 6.3 A Leakage current flows.

Therefore, it is an object of the present invention to provide a power conversion device in which the leakage current flowing through the ground generated by high-frequency switching of a switching element is reduced.

[0014]

In the power converter of the present invention, a capacitor connected between an AC line and ground,
By providing the bypass filter including the capacitor connected between the DC line and the ground, the high frequency voltage generated between the AC line and the ground and between the DC line and the ground due to the switching of the switching element is absorbed by the bypass filter. Therefore, the voltage acting between the AC line and the ground corresponds to the fundamental wave of the AC line, and the leakage current due to the stray capacitance can be reduced.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of the present invention. The converter shown in FIG. 1 has an LC on the AC side as compared with the conventional converter described in FIG.
The configuration is such that a first capacitor 10 is added between the V phase of the filter circuit and the frame 8 and a second capacitor 11 is added between the negative side of the filter circuit on the DC side and the frame 8.

FIG. 2 shows the characteristics of the first and second capacitors of the present invention. This characteristic is a characteristic diagram of the frequency (F) -internal impedance (Zc) of the capacitor, and has the characteristic that the internal impedance becomes the smallest at the switching time t of the switching element of the power converter.

The control circuit 5 receives an AC voltage, an AC current, and a DC voltage as input, controls the AC input current as a sine wave, and controls the phase and magnitude thereof to keep the power factor at 1. In this state, the switching elements T1 to T6 are PWM-controlled so as to maintain the DC output voltage at the target value. Incidentally, such a waveform improving control of the input current is publicly known and is described, for example, in “Semiconductor Power Conversion Circuits”, pages 211 to 212 (published by The Institute of Electrical Engineers of Japan), and therefore detailed description thereof is omitted here. To do.

Here, a phenomenon in which a high frequency voltage is generated between the AC line and the ground will be described with reference to FIGS. 3, 4A to 4G, and 5A to 5H. still,
Here, a single-phase inverter will be described as an example, but the same applies to a three-phase inverter and a converter.

In FIG. 3, an inverter circuit includes insulated gate bipolar transistors (hereinafter referred to as IGBTs) T1 to T4 capable of high-speed switching, and a forth recovery diode D connected in anti-parallel thereto.
1 to D4 are bridge-connected, and an LC filter circuit including reactors L1 and L2 and a capacitor C is provided on the output AC line.

A DC voltage Ed is supplied between the DC lines P and N of the inverter configured as described above, and each IGBT T1
When T4 to T4 are controlled by the PWM signal obtained by comparing the sine wave corresponding to the AC output current with the triangular wave of the carrier wave as shown in FIGS. As described above, the output voltage Vo of the inverter includes a high frequency, but the high frequency component is removed by the LC filter circuit, and the output voltage V _AB between the AC lines A and B becomes a low frequency sine wave.

Here, when the circuit states when the IGBTs T1 to T4 are switched by the PWM signal as described above are classified according to the modes, during the positive half-wave, the simplified circuit shown in FIGS. During the negative half-wave, the simple circuit mode shown in FIGS. 5E to 5H is set. At this time, in modes (a) and (c), assuming that the voltages across the reactors L1 and L2 are V _L1 and V _L2 , respectively,

[0022]

## EQU1 ## V _L1 + V _AB + V _L2 = Ed (1) holds. In modes (b) and (d),

[0023]

[Formula 2] V _L1 + V _AB + V _L2 = 0 (2) holds. Similarly, Formula 3 is established in modes (e) and (g), and Formula 4 is established in modes (f) and (h).

[0024]

(3) V _L1 + V _AB + V _L2 = −Ed (3) V _L1 + V _AB + V _L2 = 0 (4) Next, consider the voltage V _BN between the DC line N and the AC line B in each mode. . Here, V _L1 = V _L2
And In modes (a) and (c), FIG.
And from equation 1

[0025]

(Equation 4) Becomes Further, in the mode (b), according to FIG.

[0026]

(Equation 5) Becomes Further, in the mode (d), according to FIG.

[0027]

(Equation 6) Becomes Similarly, in modes (e) and (g), equation 8 is established, in mode (f), equation 9 is established, and in mode (h), equation 10 is established.

[0028]

(Equation 7)

The voltage V _BN thus obtained is represented in a waveform as shown in FIG. 4 (g), which contains high frequencies. The high frequency component is a very high frequency component f = 1 / t due to the time (switching time) t when each of the IGBTs T1 to T4 is switched from ON to OFF or from OFF to ON, and a multiple wave component of the triangular wave frequency fc. Mainly included, this high frequency voltage causes a leakage current to flow between the AC line and the stray capacitance between the DC line and ground.

Similarly in the first embodiment, a voltage including a high frequency component as shown in FIG. 4 (g) can be generated between the AC line and the ground and between the ground and the DC line, but the frequency fc of the triangular wave is generated. The multiple wave components of the LC filter circuit reactors 6a to 6c and the first filter circuit 10
Are absorbed by the second capacitor 11 and the low frequency of the fundamental wave component (power supply).

Further, the very high frequency component f = 1 / t due to the switching time t passes the stray capacitance C _SE of the reactor 6 of the LC filter circuit, but has the characteristic that the internal impedance is low at the frequency f. Bypassed by capacitor 10 and second capacitor 11.

Therefore, the voltage between the AC line and the ground, that is, the voltage across the first capacitor becomes a sinusoidal low frequency voltage waveform v _E as shown in FIG. 6, and the voltage between the ground and the DC line is DC. The voltage becomes 1/2 of the voltage Ed.

As a result, a stray capacitance C _3E exists between the frame 8 and the DC line as in a simple circuit obtained by adding stray capacitance to the circuit of FIG. 1 shown in FIG. 7, and the stray capacitance C of the cable to the load 4 is present. _{Even when 4E} is present and the V phase of the AC power supply 1 is grounded, the leakage current flowing from the low frequency voltage v _E is i _E ′ = 2πf ′ × Co × v _E. However, Co
Is the stray capacitance.

Now, v _E = 116 v, f '= 50 Hz, Co = 10
Assuming nF, i _E ′ is a very small value of 3.3 μA. Even if the system of the AC power source 1 is a non-grounded system, the stray capacitance C _1E determined by the length of the AC line (for example, cable length) and the distance between the ground and the stray capacitance C between the frame 8 and the AC line. _{Since 2E} exists, the leak current i _E flows as in the case where the high frequency component is included in v _E , but even in this case, the value thereof becomes a very small value.

Next, a second embodiment of the present invention will be described with reference to FIG. The second embodiment is a modification of the first embodiment, and is different only in that the second capacitor 11 is connected between the positive side of the filter circuit on the DC side and the frame 8 of the converter. different.

Even with such a configuration, the very high high frequency component f = 1 / t due to the switching time t is caused by the first capacitor 10 connected between the AC line and the ground and the filter circuit on the DC side. Positive side and converter frame 8
By bypassing with the second capacitor 11 connected between and, the same effect as the first embodiment is obtained.

Next, a third embodiment of the present invention will be described with reference to FIG. The third embodiment is a modification of the first embodiment, in which the DC capacitors forming the DC side filter circuit are configured as two DC capacitors 3a and 3b, and the second capacitor 11 is replaced by the DC capacitor 3a. And 3b
The only difference is that it is connected between the neutral point and the frame 8 of the converter.

Even in such a configuration, the high frequency component due to the switching time t can be bypassed by the first capacitor 10 and the second capacitor 11, so that the same effect as that of the first embodiment can be obtained.

Next, a fourth embodiment of the present invention will be described with reference to FIG. The fourth embodiment is a modification of the first embodiment, and is different only in that the first capacitor is connected to each phase of the AC line.

With this structure, the same effects as those of the first embodiment can be obtained, the capacity of the first capacitor can be reduced to 1/3, and the high frequency of the AC line can be reduced. It has the advantage of being decided independently.

Next, a fifth embodiment of the present invention will be described with reference to FIG. The fifth embodiment is an example in which the present invention is applied to an inverter. In FIG. 11, a DC power supply 21 is used as an input, and an inverter 23 formed by switching elements T1 to T6 in a bridge configuration is connected via a DC capacitor 22 that constitutes a DC side filter circuit, and an AC side reactor is provided. 24a to 24c and AC capacitor 25a to
An LC filter circuit composed of 25c is connected to supply sine wave AC power to the load 26 via the LC filter circuit.

The AC line and the frame of the inverter
The first capacitors 28a to 28c are connected between 27, the second capacitor 29 is connected between the AC line and the frame 27 of the inverter, and the control circuit 30 controls the AC output voltage and the AC voltage. The switching elements T1 to T6 are controlled so that the output voltage becomes a desired voltage and frequency according to the output current.
Is PWM controlled.

With this configuration, the LC filter circuit including the reactors 24a to 24c and the AC capacitors 25a to 25c allows the fundamental wave AC component to pass and removes the high frequency component due to the switching of the switching elements T1 to T6.

Further, high frequency components generated between the AC line and the ground and between the DC line and the ground are also stored in the first capacitor 28.
By bypassing with a to 28c and the second capacitor 29, the high-frequency leakage current flowing due to the stray capacitance determined by the AC line portion (for example, the cable) and the ground is reduced as in the first embodiment. be able to.

Next, a sixth embodiment of the present invention will be described with reference to FIG. The sixth embodiment is an example in which the present invention is applied to a converter section and an inverter section of an uninterruptible power supply.

In FIG. 12, the converter explained in FIG. 10 is connected to the inverter explained in FIG. 11 as a load,
Further, a storage battery 33 is connected between the converter and the inverter.

When the AC power supply 1 is normal, the converter portion charges the storage battery 33 and supplies power to the inverter portion. When the AC power supply 1 fails, the converter part is stopped and the storage battery 33 supplies power to the inverter part. As a result, a constant AC voltage can be supplied from the inverter unit to the load 26 even when the AC power supply 1 is normal and during a power failure.

In the case of this embodiment, since the storage battery 33 is provided in the DC line portion in addition to the DC capacitor 3, the stray capacitance determined between the DC line portion and the ground becomes large.

Therefore, the first capacitors 10a to 10
c, the second capacitor 11, and the third capacitor 28a to
The effect of reducing the leakage current by 28c is extremely large. Although the embodiments of the present invention have been described using the three-phase converter, the inverter, and the uninterruptible power supply, the same effect can be obtained with the single-phase converter, the inverter, and the uninterruptible power supply.

Further, the present invention is a VVV for inputting AC power and outputting a variable voltage and a variable frequency to drive various motors.
Even when applied to the F inverter device, the leakage current can be similarly reduced. Further, the same effect can be obtained by replacing the reactor forming the LC filter with a transformer.

[0051]

In the power converter of the present invention, by providing a bypass filter composed of a capacitor connected between the AC line and the frame ground and a capacitor connected between the DC line and the frame ground, the switching element The high frequency voltage generated between the AC line and the ground and between the DC line and the ground due to the switching is absorbed by the bypass filter. Therefore, the voltage acting between the AC line and the ground corresponds to the fundamental wave of the AC line, and the leakage current due to the stray capacitance can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram of a three-phase converter according to a first embodiment of the present invention.

FIG. 2 is a characteristic diagram of the first and second capacitors of the present invention.

FIG. 3 is an explanatory diagram of a high frequency generation phenomenon due to switching.

FIG. 4 is a waveform diagram of each part of FIG.

FIG. 5 is a diagram of each switching mode in FIG.

FIG. 6 is a voltage and current waveform diagram between the AC line and ground of FIG.

FIG. 7 is a simplified block diagram considering the stray capacitance of FIG.

FIG. 8 is a block diagram of a three-phase converter according to a second embodiment of the present invention.

FIG. 9 is a block diagram of a three-phase converter according to a third embodiment of the present invention.

FIG. 10 is a block diagram of a three-phase converter according to a fourth embodiment of the present invention.

FIG. 11 is a block diagram of a three-phase converter according to a fifth embodiment of the present invention.

FIG. 12 is a block diagram of an uninterruptible power supply according to a sixth embodiment of the present invention.

FIG. 13 is a block diagram of a conventional three-phase capacitor.

14 is a voltage / current waveform diagram between the AC line and ground of FIG.

FIG. 15 is a simplified block diagram considering the stray capacitance of FIG.

[Explanation of symbols]

 1 ... AC power supply 2 ... Converter 3 ... DC capacitor 4 ... Load 5 ... Control circuit 6a, 6b, 6c ... Reactor 7a, 7b, 7c ... AC capacitor 8 ... Frame 9 ... Ground 10, 28 ... First capacitor 11, 29 … Second capacitor

Claims (6)

[Claims] 1. A power conversion circuit comprising switching elements connected in a bridge, a control circuit for applying a PWM control signal to the switching element, and a reactor and a capacitor provided on the AC side of the power conversion circuit. A filter circuit and a second filter circuit including a capacitor provided on the DC side of the power converter circuit, the power converter device being connected between at least one phase of the first filter circuit and ground. And a second capacitor connected between any one of the positive and negative polar sides of the second filter circuit and the ground. 2. A first power conversion circuit comprising a switching element bridge-connected, a control circuit for applying a PWM control signal to the switching element, and a reactor and a capacitor provided on the AC side of the power conversion circuit. A filter circuit and a second filter circuit including a capacitor provided on the DC side of the power converter circuit, the power converter device being connected between at least one phase of the first filter circuit and ground. A power converter comprising a first capacitor and a second capacitor connected between a neutral point of the capacitor of the second filter circuit and ground. 3. The power converter according to claim 1, wherein a first capacitor is connected between each phase of the first filter circuit and ground. 4. The power conversion device according to claim 1, wherein the first filter circuit includes a transformer and a capacitor. 5. The power conversion circuit is a voltage source converter, and the control circuit controls the waveform of the AC input current into a sine wave while maintaining the power factor of the AC input current and the AC input voltage at 1. At the same time, the power conversion device according to any one of claims 1 to 4, wherein the switching element is PWM-controlled so that the DC output voltage is controlled to be constant. 6. The power conversion device is a voltage type inverter, and the control circuit PWM-controls the switching element so as to output a desired AC output voltage and frequency. The power conversion device according to claim 4.