CN1062991C - PWM converter - Google Patents

Abstract

A PWM rectifier that compensates for the common-mode voltage generated so that no unwanted action occurs in the leakage current detection section. It includes: a filter composed of an input reactance and a grounding capacitor; a DC conversion unit composed of a plurality of switching elements and a smoothing capacitor; a rectifier command value generation unit for controlling the conduction ratio λ of the switching element; a device for modulating the command signal Modulation signal generation unit; drive circuit. The PWM rectifier is equipped with a pair of bridge arm inverter circuits in its DC conversion unit. The inverter circuit outputs a compensation voltage of the opposite phase to the common-mode voltage generated by the PWM rectifier, and the machine is grounded through the compensation voltage.

Description

Pulse Width Modulated Rectifier

The present invention relates to a pulse width modulation (PWM) rectifier for AC/DC conversion of commercial power, in particular to a compensation device that can reduce the leakage current generated by the PWM rectifier based on the common-mode voltage when the commercial power of the grounding system is connected .

For traditional thyristor rectifiers, PWM rectifiers are suitable for many power supply devices because they can reduce high-order harmonic currents and reactive power. However, since most rectifier circuits are subharmonic modulated by the modulating wave signal, if the intermediate voltage of the DC circuit is set as the electrical neutral point, then a common-mode voltage of the modulating wave component will be generated on the AC input side.

Figure 5 is a schematic circuit diagram of a PWM (Pulse Wide Modularion) rectifier connected to a three-phase AC power supply, and Figure 6 is an explanatory diagram of the waveforms of each part of the PWM rectifier. Use Figure 5 and Figure 6 to illustrate the common mode voltage generated by the PWM rectifier connected to the mains power supply with a grounding system and the mechanism of leakage current caused by the common mode voltage and three-phase unbalanced voltage.

In FIG. 5(A), a three-phase (UR, US, UT) AC power source 1 having a grounding system supplies AC power to a PWM rectifier 2 through a line impedance Zn. The PWM rectifier 2 is composed of the following parts: a filter F composed of an input reactance Zc and a grounding capacitor Cn; a DC power conversion unit 3 composed of switching elements 31-36 and a smoothing capacitor 3B; controlling the conduction ratio of the switching elements 31-36 The rectifier instruction value generating unit 4A of the rectifier; the modulating wave signal generating unit 4B for generating the modulating wave signal Vc modulated to the command signal; the command value of the rectifier and the modulating wave signal Vc are compared by a comparator 4C, and then each switching element 31-36 are the rectifier drive circuit 4D for switch control.

All electrical, electronic components and the grounding system are coupled through stray capacitance to form a grounding current. Here, assuming that the coupling with the grounding system is balanced, and removing the differential (differential mode) coupling factor, we can study it In-phase (common mode) coupling factor with the ground system, that is, the state of coupling from the neutral point of the smoothing capacitor 3B to the ground system through the stray capacitance Cx. Cx in Figure 5 represents this state.

The filter F composed of the input reactance Zc and the ground capacitance Cn can prevent the high-frequency noise generated by the switching action of the switching elements 31-36 of the DC current conversion unit 3 from circulating in the AC power supply 1 .

(A), (C) and (E) in Fig. 6 illustrate the form of the switching control signal generated in the rectifier drive circuit 4D according to the comparison result of the modulation wave signal Vc and the rectifier command value (UR', US', UT'). through the process. In Fig. 6, the horizontal axis is taken as the time axis, and the thick line sine waves in (A), (C), (E) in Fig. 6 represent the rectifier command values of the R, S, and T phases of the three-phase PWM rectifier (UR ', US', UT'). The thin-line triangular wave on the same figure is the modulated wave signal Vc generated from the modulated wave signal generating unit 4B. When the modulation wave signal Vc is higher than the rectifier command value (UR', US', UT'), the corresponding switching elements 31-33 are turned on, and the switching elements 34-36 are turned off. On the contrary, when the modulated wave signal Vc is lower than the rectifier command value (UR′, US′, UT′), the corresponding switching elements 31-33 are turned off, and the switching elements 34-36 are turned on.

(B), (D) and (F) in Fig. 6 represent the bridge arm switching element pairs (31, 34), (32, 35), (33, 36) that constitute the DC power conversion unit 3 shown in Fig. 5 Voltages VR, VS, VT formed between the middle point and the middle point of the smoothing capacitor 3B. In order to simplify the description, the relationship of the R phase will be described using (A) and (B) in FIG. 6 . When the modulated wave signal Vc is lower than the rectifier command value UR', the switch element 34 is turned on. Therefore, the voltage VR formed by the neutral point of the smoothing capacitor 3B and the middle point of a pair of bridge arms (31, 34) is +E _d / 2. E _d is the charging voltage value at both ends of the smoothing capacitor 3B. Next, when the modulation wave signal Vc is higher than the rectifier command value UR', the switch element 31 is turned on, and the voltage VR formed between the neutral point of the smoothing capacitor 3B and the middle point of a pair of bridge arms (31, 34) is -E _d /2. Similarly, (C) and (D) in FIG. 6 show the relationship of the S phase, and (E) and (F) of FIG. 6 show the relationship of the T phase.

Figure 6(G) shows the relationship of the common mode voltage. The voltages VR, VS, and VT represented by (B), (D), and (F) in Figure 6 are the input reactance ZC and the ground capacitance Cn of the filter F, in The voltage synthesized on the stray capacitance Cx as the non-inverting voltage Vn. Usually, the impedance of the stray capacitance Cx is very high, and the common-mode voltage Vn is generated by superimposing the above-mentioned voltages VR, VS, and VT in essence.

Figure 5(B) shows the equivalent circuit of the grounding current flowing in the grounding system. If the unbalanced voltage of the three-phase AC power supply 1 is Vcn, and the line impedance is Zn, then the unbalanced voltage Vcn of the three-phase AC power supply 1 The generated ground current Icn flows through the ground capacitance Cn and the line impedance Zn. In addition, the ground current Icn generated by the common-mode voltage Vn passes through the stray capacitance Cx, the input reactance Zc, the ground capacitance Cn and the line impedance Zn form a loop, and the current flowing through the line impedance Zn that constitutes the mains power supply 1 is The ground current Icn, the high-frequency component of the common-mode voltage Vn of this ground current Icn, is shunted through the ground capacitor Cn.

The modulated wave signal Vc shown in FIG. 6 is 6 times the frequency of the rectifier command signal, but the frequency of this modulated wave signal is not necessarily limited to 6 times.

However, like the above-mentioned PWM rectifier circuit in the prior art, in order to carry out subharmonic modulation by the modulating wave signal, if the intermediate voltage of the DC circuit is set as the electrical neutral point, the frequency of the modulating wave as the fundamental frequency will be generated on the AC input side. common-mode voltage. Therefore, especially when the mains power supply is a grounded system, the above-mentioned common mode voltage passes through the power line, the power ground line, the earth, and passes through the stray capacitance from the machine ground line, causing the ground current to flow in the DC intermediate circuit. Because this ground current passes through the earth, there is a problem that it needs to be detected by the leakage detection link.

In order to reduce this ground current, for example, change the ground capacitance, input reactance, and stray capacitance to reduce the ground current, but two power supplies are required, and the adjustable range is limited, so there is no good effect. For this reason, countermeasures such as installing an isolation transformer between the AC power supply and the PWM rectifier have been adopted in many cases.

The present invention is made in view of the above disadvantages, and its purpose is to solve the above problems and provide a PWM rectifier that compensates the common-mode voltage generated by the PWM rectifier, reduces leakage current, and does not generate unnecessary actions in the leakage detection link.

In order to achieve the above purpose, the first invention adopts the following technical means: a PWM rectifier, including: a filter composed of an input reactance and a grounding capacitor; a DC power conversion unit connected to the filter and composed of a switching element and a smoothing capacitor ; control the conduction ratio of the above-mentioned switching elements, and generate a rectifier command value generation unit for the rectifier command value; generate a modulation signal generation unit for modulating the modulation signal of the above-mentioned rectifier command value; A rectifier drive circuit in which components are switched and controlled; the PWM rectifier inputs alternating current from an alternating current power source and converts it into direct current. Outputting a compensating voltage with an opposite phase to the common mode voltage generated by the above-mentioned PWM rectifier is grounded from the middle point of the switching elements of the above-mentioned pair of bridge arms through a series circuit of a reactance and a grounding capacitor.

The second invention is that the driving signal of the inverter circuit uses the modulation signal of the PWM rectifier.

The third invention is that the driving signal of the inverter circuit is modulated by a high-frequency modulation wave with the modulation signal of the PWM rectifier as the command value.

The fourth invention is that when the PWM rectifier has a three-phase input, the driving signal of the inverter circuit is realized by detecting the neutral point voltage of the AC power supply and superimposing it on the command value of the modulation signal of the PWM rectifier.

The fifth invention is that, for the drive signal of the inverter circuit, the value obtained by multiplying the modulation signal of the PWM rectifier by the reciprocal of the conduction ratio of the PWM rectifier is used as the command value.

According to the above structure, the function of the first invention is to generate an AC output of opposite phase with respect to the common mode voltage generated by the PWM rectifier, and by grounding the AC output, the common mode voltage of the PWM rectifier is used by the AC output of the inverter. to offset.

The role of the second invention is to drive the inverter circuit with the modulated wave signal of the PWM rectifier, and the inverter circuit generates a rectangular wave AC output with the phase opposite to the common mode voltage of the PWM rectifier and having the frequency component of the modulated wave.

The effect of the third invention is that the driving signal of the inverter circuit is modulated by the modulation wave signal of the PWM rectifier and the high-frequency modulation wave signal, which can cancel the common-mode voltage of higher frequency components.

The effect of the fourth invention is that when the PWM rectifier is a three-phase input, the neutral point (unbalanced) voltage of the commercial power supply is detected, and the neutral point voltage is superimposed on the command value of the modulation wave signal of the PWM rectifier. As the driving signal of the inverter circuit, it can offset the ground potential including the unbalanced voltage of the commercial power supply.

The function of the fifth invention is that the product of the inverted modulation signal of the PWM rectifier multiplied by the reciprocal of the conduction ratio of the PWM rectifier is used as the drive signal of the inverter circuit, which can adapt to changes in the output voltage of the DC power conversion circuit and offset Changes in potential to ground due to changes in the conduction ratio.

Fig. 1 is a functional block diagram of one embodiment of the PWM rectifier of the present invention.

FIG. 2 is a functional block diagram of another embodiment of a PWM rectifier that can cancel common-mode voltages of higher frequency components.

Figure 3 is a functional block diagram of a PWM rectifier that can offset the potential to ground including the unbalanced voltage of the mains power supply.

Fig. 4 is a functional block diagram of a PWM rectifier that can offset the ground potential caused by the conduction ratio of the PWM rectifier.

5A and 5B are schematic circuit diagrams of a PWM rectifier connected to a three-phase AC power supply.

FIG. 6 is an explanatory diagram of waveforms of various parts of the PWM rectifier.

FIG. 7 is an explanatory diagram of a compensation voltage waveform of an embodiment for compensating the common-mode voltage of a PWM rectifier.

The embodiment will be described in detail below in conjunction with the accompanying drawings.

Fig. 1 is the functional block diagram of the PWM rectifier of an embodiment of the present invention, Fig. 2 is another embodiment, is the functional block diagram of the PWM rectifier that can offset the common mode voltage of higher frequency components, Fig. 3 is the functional block diagram that can offset the common mode voltage including mains power supply The functional block diagram of the unbalanced voltage to ground potential. Figure 4 is a functional block diagram that can offset the ground potential caused by the PWM conduction ratio. Figure 7 is an explanatory diagram of the compensation voltage waveform for compensating the common mode voltage of the PWM rectifier. 5. The same functional components in Fig. 6 use the same symbols.

In FIG. 1, the PWM rectifier 2 is supplied with AC power by a three-phase (UR, US, UT) AC power source 1 with a grounding system through a line impedance Zn not shown in the figure. The PWM rectifier 2 includes the following parts: a filter F composed of an input reactance Zn and a grounding capacitor Cn; a DC power conversion unit 3 composed of switching elements 31-36 and a smoothing capacitor 3B; a rectifier for controlling the conduction ratio of the switching elements 31-36 Command value generation unit 4A; modulation wave signal generation unit 4B that generates modulation wave signal Vc that modulates the command value signal; rectifier drive circuit 4D and inverter circuit 5 that connects a pair of bridge arms in parallel to DC power conversion unit 3 . PWM rectifier 2 is grounded from an intermediate point between switching elements 51 and 52 of inverter circuit 5 through a series circuit of reactance 5A and ground capacitance 5B.

According to the above configuration, the inverter circuit 5 outputs a compensation voltage with an opposite phase to the common-mode voltage Vn generated by the PWM rectifier 2, and the compensation voltage connects the machine to ground through a filter circuit composed of a reactance 5A and a grounding capacitor 5B in series. Accordingly, It can offset the common-mode voltage generated by the PWM rectifier 2, and can also reduce the zero-sequence voltage of the PWM rectifier 2 grounded to the machine, reducing the leakage current Icn. In addition, the reactance 5A and the grounding capacitance 5B are selected at a fixed value that has little influence on the common-mode voltage of the high-order harmonic.

Next, please refer to the waveform explanatory diagram of each part of the PWM rectifier 2 in FIG. 6 . FIG. 6(G) is a waveform of the common mode voltage Vn generated by the PWM rectifier 2. As explained in the prior art, it is synchronized with the modulation wave signal Vc and the waveform is reversed in steps. Furthermore, according to experimental data, the effective value of the common-mode voltage Vn has a negative linearity with respect to the conduction ratio λ of the PWM rectifier 2 .

Therefore, as the driving signal of the inverter circuit 5 of FIG. 1, the modulation wave signal Vc of the PWM rectifier 2 is converted into a rectangular wave signal by the comparator 6A, and input to the inverter driving circuit 6. Accordingly, it is not difficult to form a common mode The voltage Vn is in the opposite phase and has the compensation voltage of the same fundamental wave. FIG. 7 is a part of the waveforms of each part in FIG. 6 extracted and enlarged to illustrate the compensation voltage waveform for compensating the common mode voltage of the PWM rectifier. Fig. 7 (G) is the waveform of the common-mode voltage of the above-mentioned PWM rectifier, and Fig. 7 (H) is the compensation voltage output by the inverter 5 when the modulating wave signal Vc is converted into a rectangular wave by the comparator 6A to drive the inverter 5 waveform.

In addition, in FIG. 2, the difference from FIG. 1 is that a high-frequency modulation waveform signal generating unit 6B is also added to the input circuit of the comparator 6A. Increase the high-frequency modulation wave signal generation unit 6B, use the modulation wave signal Vc of the PWM rectifier 2 as the command value, and modulate it by the high-frequency modulation wave signal Vn, thus, the compensation voltage of the intermediate circuit of the inverter circuit 5, as shown in Figure 7 The average view shown in (I) is close to the triangular wave, and the compensation voltage ratio of the rectangular wave in Figure 7(G) is closer to the common-mode voltage Vn of the PWM rectifier, except for the common-mode voltage of the fundamental component of the modulation wave signal Vc, the higher order Harmonics of the common-mode voltage can also be canceled out.

In addition, in Fig. 3, the difference from Fig. 2 is that when the PWM rectifier 2 is a three-phase input, the neutral point voltage generated by the unbalance of the mains power supply 1 is detected by the insulating transformer, and the detected voltage is added to Based on the command value of the modulated wave signal Vc of the PWM rectifier 2, the ground potential including the unbalanced voltage of the commercial power supply can be canceled accordingly.

In addition, in FIG. 4, the difference from FIG. 2 is that the value obtained by multiplying the modulated wave signal Vc of the PWM rectifier 2 and the reciprocal of the conduction ratio λ of the PWM rectifier 2 in the multiplier 8A is used as the inverter drive circuit 6, according to which, when the conduction ratio changes with the change of the output voltage of the DC power conversion unit 3, the ground potential can also be offset.

In addition, not shown in the figure, the insulating transformer 7 of FIG. 3 is used together with the reciprocal of the conduction ratio λ and the multiplier 8A of FIG. The potential to ground that changes with the output voltage of the DC power conversion unit.

According to the present invention described above, the common-mode voltage generated by the PWM rectifier is compensated and reduced, thereby reducing the leakage current (ground current) on the AC power supply side, and eliminating unnecessary actions in the leakage detection link.

Claims (6)

1 A PWM rectifier comprising: A filter composed of input reactance and ground capacitance; A DC power conversion unit connected to the above-mentioned filter and composed of a switching element and a smoothing capacitor; A rectifier command value generation unit that controls the conduction ratio of the above-mentioned switching elements and generates a rectifier command value; a modulation signal generating unit that generates a modulation signal that modulates the command value of the rectifier; A rectifier driving circuit that compares the above-mentioned rectifier command value and modulation signal with a comparator, and performs switching control on the above-mentioned switching element; The PWM rectifier inputs AC power from the AC power supply and converts it into DC, which is characterized by: The DC power conversion unit is provided with an inverter circuit comprising a pair of switching elements of the bridge arms, and the inverter circuit outputs a compensation voltage with an opposite phase to the common-mode voltage generated by the above-mentioned PWM rectifier, from the middle of the switching elements of the above-mentioned pair of bridge arms The point is connected to ground through a series circuit of reactance and capacitance to ground. 2. The PWM rectifier according to claim 1, wherein the driving signal of the inverter circuit is a modulation wave signal of the PWM rectifier. 3. As claimed in claim 1, the PWM rectifier is characterized in that: the drive signal of the inverter circuit uses the modulated wave signal of the PWM rectifier as the command value, and is modulated by the high-frequency modulated wave signal. 4. The PWM rectifier according to claim 2, characterized in that: the above-mentioned PWM rectifier is a three-phase input, and the driving signal of the inverter circuit detects the neutral point voltage of the AC power supply, and the neutral point voltage is superimposed on the PWM rectifier The command value of the modulated wave signal is realized. 5. The PWM rectifier according to claim 3, characterized in that: the above-mentioned PWM rectifier is a three-phase input, and the driving signal of the inverter circuit detects the neutral point voltage of the AC power supply, and superimposes the neutral point voltage on the PWM The command value of the modulated wave signal of the rectifier is realized. 6. The PWM rectifier according to any one of claims 2 to 5, characterized in that: the drive signal of the inverter circuit uses the product of the modulated wave signal of the PWM rectifier and the reciprocal of the conduction ratio of the PWM rectifier as the command value.

Images