JP3246224B2 - PWM converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PWM converter for AC / DC conversion of a commercial power supply, and more particularly, to reduce a leakage current based on a common mode voltage generated by the PWM converter when connected to a grounded commercial power supply. It relates to a compensator.

[0002]

2. Description of the Related Art PWM converters have been applied to many power supply devices because they can reduce harmonic current and reactive power compared to conventional thyristor rectifiers. However, since many converter circuits perform subharmonic modulation using a carrier wave, a common mode voltage of a carrier wave component is generated on the AC input side when an intermediate voltage of the DC circuit is set to an electrical neutral point.

FIG. 5 shows a PWM (P) connected to a three-phase AC power supply.
FIG. 6 is a principle circuit diagram of a converter (ulse wide module), and FIG. 6 is an explanatory diagram for explaining waveforms at various parts of the PWM converter. The common mode voltage generated by the PWM converter connected to the grounded commercial power supply and the mechanism of the leakage current based on the common mode voltage and the three-phase unbalanced voltage will be described with reference to FIGS.

In FIG. 5A, three phases (UR, U
S, UT) From the AC power supply 1 via the line impedance Zn
AC power is supplied to the WM converter 2. The PWM converter 2 includes a filter F including an input reactor Zc and a ground capacitor Cn, a DC power conversion unit 3 including switching elements (31 to 36) and a smoothing capacitor 3B, and a control rate of the switching elements (31 to 36). A converter command value generator 4A for controlling the operation of the converter and a carrier wave signal generator 4B for generating a carrier wave signal Vc for modulating the converter command value, and the converter command value and the carrier wave signal Vc are compared by a comparator 4C, and each switching element (31-36), and a converter drive circuit 4D for ON-OFF control of (31-36).

[0005] All electric and electronic components are coupled to the ground system by stray capacitance, and a ground current flows. Here, if the coupling with the ground system is balanced,
Except for the elements that are differentially (cross-mode) coupled, the elements that are coupled in-phase (common mode) with the ground system are coupled from the neutral point of the smoothing capacitor 3B to the ground system via the stray capacitance Cx. You should consider it. Cx in FIG. 5 shows this state.

A filter F composed of an input reactor Zc and a ground capacitor Cn prevents high-frequency noise caused by the ON-OFF operation of the switching elements 31 to 36 in the DC power converter 3 from returning to the AC power supply 1. is there. In FIG.
(A), (C), (E) are the carrier signal Vc and the converter command value.
(UR ', US', UT ')
2 illustrates the formation of the ON-OFF control signal in the first embodiment.
In FIG. 6, the time axis is taken on the horizontal axis, and (A), (C),
The sine wave shown by the bold line in (E) indicates the converter command values (UR ', US', UT ') for each of the R, S, and T phases of the three-phase PWM converter. The waveform shown by the triangular thin line on the drawing is the carrier wave signal Vc from the carrier wave signal generator 4B. When the carrier wave signal Vc is higher than the converter command value (UR ', US', UT '), the corresponding switching elements 31 to 33 are turned on and the switching elements 34 to 36 are turned off. vice versa,
When the carrier wave signal Vc is lower than the converter command value (UR ', US', UT '), the corresponding switching elements 31 to 33 are turned off, and the switching elements 34 to 36 are turned on.

FIGS. 6 (B), (D), and (F) show arm pairs (31, 34), (32, 35), and (2) of the switching elements constituting the DC power converter 3 shown in FIG. (33,36) intermediate point and smoothing capacitor
3B shows voltages VR, VS, and VT formed between the neutral point and 3B. The relationship in the R phase will be described with reference to FIGS. When the carrier wave signal Vc is lower than the converter command value UR ', the switching element 34 conducts,
The voltage VR formed between the neutral point of the smoothing capacitor 3B and the intermediate point of the arm pair (31, 34) is + Ed / 2. Here, Ed is a voltage value charged at both ends of the smoothing capacitor 3B. Next, when the carrier wave signal Vc is higher than the converter command value UR ', the switching element 31 conducts and the smoothing capacitor
The voltage VR formed between the neutral point of 3B and the intermediate point of the arm pair (31, 34) is -Ed / 2. Similarly, (C) and (D) in FIG. 6 show the relationship in the S phase, and (E) and (F) in FIG. 6 show the relationship in the T phase.

FIG. 6 (G) shows the relationship between the common mode voltages, and the voltages VR, VS, VT shown in FIGS. 6 (B), (D), (F) show the relationship between the input reactor Zc of the filter F and the ground. Via the capacitor Cn, a voltage is combined with the stray capacitance Cx as the common mode voltage Vn.
Usually, since the stray capacitance Cx has a high impedance, the common mode voltage Vn is substantially generated by adding the voltages VR, VS, and VT described above.

FIG. 5B shows the relationship of the ground current flowing in the ground system by an equivalent circuit. Assuming that the unbalanced voltage of the three-phase AC power supply 1 is Vcn and the line impedance is Zn,
Ground current Icn due to unbalanced voltage Vcn of three-phase AC power supply 1
Flows through the ground capacitor Cn and the line impedance Zn. Also, the ground current Ic by the common mode voltage Vn
n forms a circuit network with the input reactor Zc, the ground capacitor Cn, and the line impedance Zn via the stray capacitance Cx, and the current flowing through the line impedance Zn configuring the commercial power supply 1 becomes the ground current Icn. The high frequency component of the common mode voltage Vn is equal to the ground capacitor Cn.
Is bypassed through.

Although the carrier wave signal Vc shown in FIG. 6 is illustrated at a frequency six times the converter command value, the frequency of the carrier wave signal is not necessarily limited to six times.

[0011]

However, in the PWM converter circuit of the prior art as described above, since the sub-harmonic modulation is performed by the carrier wave signal, if the intermediate voltage of the DC circuit is set to the electrical neutral point, the AC A common mode voltage having a carrier frequency component as a fundamental frequency is generated on the input side. Therefore, especially when the commercial power supply is a grounding system, the above-mentioned common mode voltage is grounded to the DC intermediate circuit via the power supply line, the power supply grounding line / ground, and from the equipment grounding line via the stray capacitance. Electric current flows. Since this ground current passes through the ground, there is a problem that it is detected by the earth leakage detecting element.

In order to solve the problem of reducing the ground current, for example, the ground current is reduced by changing a ground capacitor, an input reactor, and a stray capacitance. However, there are two voltage sources. However, there were no effective countermeasures due to restrictions on the practical range of adjustment.
For this reason, many countermeasures such as equipping an insulating transformer between the AC power supply and the PWM converter have been taken.

The present invention has been made in view of the above-mentioned points, and an object of the present invention is to solve the above-mentioned problems and to provide a PWM
An object of the present invention is to provide a PWM converter in which a leakage current is reduced by compensating for a common mode voltage generated by a converter and a leakage current detection element does not perform unnecessary operation.

[0014]

According to a first aspect of the present invention, there is provided a filter comprising an input reactor and a ground capacitor, a DC power converter comprising a switching element and a smoothing capacitor, and A converter command value generator for controlling the control rate of the switching element, a carrier wave signal generator for generating a carrier wave signal for modulating the converter command generator, and a converter drive circuit. In a PWM converter for conversion, a DC power converter is provided with an inverter circuit of one arm pair, and this inverter circuit outputs a compensation voltage having a phase opposite to that of a common mode voltage generated by the PWM converter. It shall be grounded.

According to the second aspect of the present invention, the drive signal of the inverter circuit uses the carrier wave signal of the PWM converter. Further, according to the third aspect, the drive signal of the inverter circuit uses the carrier wave signal of the PWM converter as a command value and modulates the signal with the high-frequency carrier wave signal.

According to the fourth invention, when the PWM converter has a three-phase input, the drive signal of the inverter circuit detects a neutral point voltage of the AC power supply, and converts the neutral point voltage of the PWM converter to a neutral point voltage. It shall be added to the command value of the carrier wave signal. Further, according to the fifth aspect, the drive signal of the inverter circuit includes the carrier wave signal of the PWM converter as the P signal.
A value obtained by multiplying the reciprocal of the control rate of the WM converter is set as a command value.

[0017]

According to the first aspect of the present invention, a PWM is provided.
The common mode voltage of the PWM converter is canceled by the AC output of the inverter by generating an AC output having a phase opposite to that of the common mode voltage generated by the converter and grounding the AC output.

According to the second aspect of the present invention, the inverter circuit is driven by the carrier wave signal of the PWM converter, so that the inverter circuit has a phase component opposite to that of the common mode voltage of the PWM converter and the frequency component of the carrier wave signal. Generates a rectangular AC output. According to the third aspect,
By modulating the drive signal of the inverter circuit with the carrier wave signal of the PWM converter and the high-frequency carrier wave signal, it is possible to cancel the common mode voltage of a higher-order frequency component.

According to the fourth invention, when the PWM converter has three-phase input, the neutral point (unbalanced) voltage of the commercial power supply is detected, and this neutral point voltage is detected as the carrier wave signal of the PWM converter. By using the drive signal of the inverter circuit in addition to the command value, the ground potential including the unbalanced voltage of the commercial power supply can be canceled. Further, according to the fifth aspect, the value obtained by multiplying the inverted signal of the carrier wave signal of the PWM converter by the reciprocal of the control rate of the PWM converter is used as the drive signal of the inverter circuit, so that the output voltage of the DC power conversion circuit is obtained. Accordingly, it is possible to cancel the fluctuation of the ground potential due to the fluctuation of the control rate.

[0020]

FIG. 1 is a functional block diagram of a PWM converter according to an embodiment of the present invention. FIG. 2 is a functional block diagram of a PWM converter which cancels higher-order frequency component common mode voltages as another embodiment. Is a functional block diagram of a PWM converter that cancels the ground potential including the unbalanced voltage of the commercial power supply, FIG. 4 is a functional block diagram of a PWM converter that cancels the ground potential including the control ratio of the PWM converter, and FIG. FIG. 4 is an explanatory diagram for explaining a compensation voltage waveform for compensating a common mode voltage of the PWM converter according to one embodiment;
The same functional members corresponding to FIGS. 5 and 6 are denoted by the same reference numerals.

In FIG. 1, a three-phase (UR, US, UT) AC power supply 1 of a grounding system has a line impedance Zn (not shown).
AC power is supplied to the PWM converter 2 via the.
The PWM converter 2 includes a filter F including an input reactor Zc and a ground capacitor Cn, and switching elements 31 to
A DC power conversion unit 3 comprising 36 and a smoothing capacitor 3B;
Converter command value generator 4A for controlling the control rate of switching elements 31 to 36, carrier wave signal generator 4B for generating carrier wave signal Vc for modulating the same, and converter drive circuit 4D
And one arm pair of inverter circuits 5 in the DC power converter 3.
And the switching element 51 of the inverter circuit 5,
It is configured to be grounded from the midpoint of 52 via a series circuit of the reactor 5A and the grounding capacitor 5B.

With the above configuration, the inverter circuit 5
Is the common mode voltage Vn generated by the PWM converter 2.
By outputting a compensation voltage having the opposite phase to that of the common mode voltage generated by the PWM converter 2 by grounding the compensation voltage through a filter circuit of a series circuit of the reactor 5A and the ground capacitor 5B. Thus, the zero-phase voltage of the PWM converter 2 with respect to the equipment ground can be reduced, and the leakage current Icn can be reduced.
The constants of the reactor 5A and the ground capacitor 5B are selected so that the influence of the higher-order harmonic common mode voltage is reduced.

Once again, look at the explanatory diagram explaining the waveforms of the respective parts of the PWM converter 2 of FIG. 6 described above. In FIG.
(G) Common mode voltage generated by PWM converter 2
As described in the section of the related art, the waveform of Vn is a stepped voltage waveform synchronized with the carrier wave signal Vc and having inverted amplitude. According to the experimental data, the effective value of the common mode voltage Vn has negative linearity with respect to the control rate λ of the PWM converter 2.

Therefore, the carrier wave signal Vc of the PWM converter 2 is converted into a rectangular wave signal by the comparator 6A as a drive signal of the inverter circuit 5 of FIG. The compensation voltage of the fundamental frequency of the phase can be easily formed. FIG. 7 illustrates a compensation voltage waveform for enlarging and extracting a part of the waveform of each part in FIG. 6 and compensating for the common mode voltage of the PWM converter. FIG. 7 (G) shows the waveform of the common mode voltage Vn of the above-described PWM converter, and FIG. 7 (H) shows the waveform when the carrier wave signal Vc is converted into a rectangular wave signal by the comparator 6A and the inverter 5 is driven. 5 shows a compensation voltage waveform output from the inverter 5.

FIG. 2 is different from FIG. 1 in that
The point is that a high-frequency carrier wave signal generator 6B is added to the input circuit of the comparator 6A. By adding the high-frequency carrier wave signal generation unit 6B and using the carrier wave signal Vc of the PWM converter 2 as a command value and performing modulation with the high-frequency carrier wave signal Vh, the compensation voltage of the intermediate circuit of the inverter circuit 5 becomes As shown in FIG. 7 (I), the triangular wave is closer to the average, the common mode voltage Vn of the PWM converter is closer than the square wave compensation voltage of FIG. 7 (G), and the fundamental frequency of the carrier wave signal Vc. Higher-order harmonic common mode voltages other than the component common mode voltage can be canceled.

FIG. 3 differs from FIG. 2 in that
When the PWM converter has a three-phase input, a neutral point voltage based on the unbalance of the commercial power supply 1 is detected by the insulating transformer 7, and the detected voltage is added to the command value of the carrier wave signal Vc of the PWM converter. It is possible to cancel the ground potential, including the unbalanced voltage of the commercial power supply. 4 differs from FIG. 2 in that a value obtained by multiplying the carrier wave signal Vc of the PWM converter 2 by the reciprocal of the control rate λ of the PWM converter 2 in the multiplier 8A is a driving signal of the inverter driving circuit 6. By doing so, it is possible to cancel the ground potential even if the control rate changes according to the output voltage of the DC power conversion unit 3.

Although not shown, by using the insulating transformer 7 of FIG. 3 together with the reciprocal of the control ratio λ and the multiplier 8A of FIG. The point voltage and the ground potential that fluctuates according to the output voltage of the DC power conversion unit 3 can be offset.

[0028]

As described above, according to the present invention, the PW
By compensating and reducing the common mode voltage generated by the M converter, leakage current (ground current) on the AC power supply side
And the unnecessary operation of the leakage detection element can be eliminated.

[Brief description of the drawings]

FIG. 1 is a functional block diagram of a PWM converter according to an embodiment of the present invention.

FIG. 2 is a functional block diagram of a PWM converter that cancels a higher-order frequency component common mode voltage as another embodiment.

FIG. 3 is a functional block diagram of a PWM converter that cancels a ground potential including an unbalanced voltage of a commercial power supply.

FIG. 4 is a functional block diagram of a PWM converter that cancels a ground potential including a control rate of the PWM converter.

FIG. 5 is a principle circuit diagram of a PWM converter connected to a three-phase AC power supply.

FIG. 6 is an explanatory diagram for explaining waveforms of various parts of the PWM converter.

FIG. 7 is an explanatory diagram for explaining a compensation voltage waveform for compensating a common mode voltage of the PWM converter according to one embodiment;

[Explanation of symbols]

 1, UR, US, UT AC power supply 2 PWM converter 3 DC power conversion circuit unit 31 to 36, 51, 52 Switching element 3B Smoothing capacitor 4A Converter command value generation unit 4B Carrier wave signal generation unit 4C, 6A Comparator 4D converter drive Circuit 5 Inverter 5A, Zc Reactor 5B, Cn Ground capacitor 6 Inverter drive circuit 6B High frequency carrier wave signal generator 7 Isolation transformer 8 Control rate 8A Multiplier Zn line impedance F Filter Vc Carrier wave signal Vh High frequency carrier wave signal Cx stray capacitor UR ', US', UT 'Converter command value

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 7/219 H02M 7/48

Claims (5)

(57) [Claims] 1. A filter comprising an input reactor and a ground capacitor, a DC power converter comprising a switching element and a smoothing capacitor, a converter command value generator for controlling a control ratio of the switching element, and a carrier for modulating the converter command value. A PWM converter comprising a carrier wave signal generating unit for generating a wave signal and a converter driving circuit and receiving AC power supplied from an AC power supply and converting it to DC power, a DC power conversion unit is provided with one arm pair of inverter circuits The inverter circuit outputs a compensation voltage having a phase opposite to the common mode voltage generated by the PWM converter, and grounds the device via the compensation voltage. 2. The PWM converter according to claim 1, wherein a drive signal of the inverter circuit uses a carrier wave signal of the PWM converter. 3. The PWM converter according to claim 1, wherein the drive signal of the inverter circuit uses a carrier wave signal of the PWM converter as a command value and performs modulation by a high-frequency carrier wave signal. 4. The PWM converter according to claim 2, wherein when the PWM converter has a three-phase input, a drive signal of the inverter circuit detects a neutral point voltage of the AC power supply, Wherein the point voltage is added to a command value of a carrier wave signal of the PWM converter.
M converter. 5. The PWM converter according to claim 2, wherein the drive signal for the inverter circuit is a PWM signal which is a carrier wave signal of the PWM converter.
The command value is the value obtained by multiplying the reciprocal of the control rate of the converter.
A PWM converter characterized by the above-mentioned.