GB2310553A

GB2310553A - Noise filter for inverter

Info

Publication number: GB2310553A
Application number: GB9603817A
Authority: GB
Inventors: Kiyoaki Sasagawa; Masato Mochizuki
Original assignee: Fuji Electric Co Ltd
Current assignee: Fuji Electric Co Ltd
Priority date: 1995-02-28
Filing date: 1996-02-22
Publication date: 1997-08-27
Also published as: GB9603817D0; DE19607201A1; JPH08237936A

Description

1 NOISE FILTER FOR VOLTAGE SOURCE INVERTER 2310553 The present invention

relates to noise filters, and particularly concerns a noise filter for a voltage source inverter, which is designed to eliminate high-frequency noise during operation of a voltage source inverter including self-arc-extinguishing semiconductor switching elements.

A main circuit of a conventional example of a voltage source inverter is shown in the circuit diagram in Figure 8. In the circuit shown in Figure 8, a rectifier 2 serving as a first power converter is connected to an AC power supply (not shown), such that the rectifier 2 converts AC power supplied from this power supply to DC power, and generates the DC power as its output to a direct-current intermediate circuit. A smoothing condenser 3 connected to the direct-current intermediate circuit functions to absorb and eliminate any ripple component that is included in the DC power generated by the rectifier 2. An inverter 4 serving as a second power converter is connected to the direct-current intermediate circuit, to receive the smoothed DC power, and reconvert the DC power back into AC power having the desired voltage and frequency, for example by pulse width modulation control, so as to operate an induction motor 5 at a variable speed.

The inverter 4 is constructed by connecting switching circuits in a threephase bridge arrangement, each switching circuit consisting of a self-arcextinguishing semiconductor switching element in the form of an insulated gate bipolar transistor (hereinafter abbreviated to 9G137') 4T and a feedback diode 4D which are inversely connected in parallel. Each IGI3T 4T is provided with a separate gate driving circuit 4G. Recently, a high speed switching element, such as an IGI3T, has been widely used so as to achieve a sufficiently high carrier frequency using pulse width modulation control.

When a high-speed switching element, such as an IGI3T, is used, a considerably high rate of change of voltage is seen upon switching of the element (large dV/dT). With floating capacitance 6 present between the inverter 4 and ground, a leakage current IE may flow to ground through 2 the floating capacitance 6, depending upon the above-indicated rate of change of voltage (dV/dT).

In the known circuit of Figure 8, the leakage current]E flows from the AC power supply (not shown) to ground, through the rectifier 2, the directcurrent intermediate circuit, the inverter 4 and the floating capacitance 6, in that order, as indicated by the one-dot chain line in Figure 8. The value of this leakage current IE is represented by the following mathematical expression 1, where CO is value of the floating capacitance 6.

E = CO ' (dV/dT) (1) As is apparent from the expression 1, the leakage current IE increases with an increase in the rate of change of voltage (dV/dT).

If this leakage current flows toward the AC power supply, other apparatus (in particular, electronics) connected to the same AC power supply is adversely influenced by the leakage current. In view of this, a noise filter, which is not shown in the circuit of the known example of Figure 8, is provided with respect to each phase on the AC input side of the rectifier 2, for preventing the above leakage current IE from flowing toward the AC power supply.

The provision of the noise filter at each phase on the AC input side of the rectifier 2, however, may cause problems as follows:

(1) While a three-phase AC power supply is often used to generate AC input for the rectifier 2, the provision of the noise filters for the three phases undesirably increases the number of components of the inverter. Further, the size and cost of the resulting inverter device are increased due to the increased number of filters, which include bulky, expensive ferrite cores.

(2) The size of the voltage source inverter is increased if the noise filters are integrated with the rectifier 2, smoothing condenser 3, and the inverter 4. It is, on the other hand, difficult to add the noise filters to a conventional voltage source inverter.

3 ft is therefore an objective of the present invention to provide a noise filter which is small in size but is nevertheless able relia61y to prevent high-frequency noise, which occurs upon switching of the voltage source inverter, from flowing toward a power supply.

To accomplish the above-described object, in accordance with a first aspect of the invention, there is provided a noise filter for a voltage source inverter consisting of a first power converter connected to an AC power supply for generating DC power as output to a direct-current intermediate circuit, a second power converter connected to the directcurrent intermediate circuit, and having switching elements, preferably self-arc-extinguishing semiconductors, as constituent elements for generating AC power having a predetermined voltage and frequency, and a smoothing condenser connected to the direct-current intermediate circuit, wherein the noise filter comprises a first low-pass filter consisting of a zero-phase-sequence reactance and condensers provided between a DC side of the first power converter, and the smoothing condenser.

In accordance with a second aspect of the invention, the noise filter further comprises a second low-pass filter circuit consisting of a zerophase-sequence reactor, a secondary winding provided on the zero-phasesequence reactor, a resistance for short-circuiting the secondary winding, and condensers, provided between a DC side of the first power converter, and the smoothing condenser.

In accordance with a third aspect of the invention, a third low-pass filter circuit consisting of a zero-phase-sequence reactor, a secondary winding provided on the zero-phase-sequence reactor, a current detector for detecting current flowing through the secondary winding, and condensers is provided between a DC side of the first power converter and the smoothing condenser, and a ground detector is provided which detects the occurrence of a ground fault if the current of the secondary winding which is detected by the current detector exceeds a predetermined value.

The low-pass filter circuit may be constructed by a combination of the zero-phase-sequence reactor and the converters, in which the zero-phasesequence reactor has a common magnetic core around which are c 4 wound a line through which positive-side current, i.e., direct current generated by the first power converter, flows, and a line through which negafive-side current flows. This low-pass filter circuit is provided between the direct-current side of the first power converter, and the smoothing condenser. Although the zero-phase-sequence reactor of the lowpass filter circuit does not influence the normal current concurrently flowing through the positive-side and negative-side lines of the directcurrent intermediate circuit, any leakage current flowing to ground through the floating capacitance only flows in either one or the other of these two lines wound on the zero-phase-sequence reactor. In this case, the zero-phase-sequence reactor functions as an inductance wkh respect to the leakage current. Accordingly, the leakage current can be suppressed if the low-pass filter circuit is constructed in such a way that the zerophase-sequence reactor is combined with condensers each having one end grounded.

The present invention will now be explained in detail, with reference to the accompanying drawings, in which:

Figure 1 is a main circuit diagram representing a first embodiment of the present invenflon.

Figure 2 is a view indicating a current path through which leakage current flows through floating capacitance in the circuit of Figure 1.

Figure 3 is an equivalent circuit diagram representing an equivalent circuit of a first low-pass filter circuit included in the circuit of Figure 1.

Figure 4 is a main circuit diagram representing a second embodiment of the present invention.

Figure 5 is an equivalent circuit diagram representing an equivalent circuit of a second low-pass filter circuit included in the circuit of Figure 4.

Figure 6 is a main circuit diagram representing a third embodiment of the present invention.

Figure 7 is a time chart representing a ground detecting operation of the circuit of Figure 6.

1 Figure 8 is a main circuit diagram showing a conventional example of voltage source inverter.

Referring now to the drawings, there is seen in Figure 1 a main circuit diagram representing the first embodiment of the present invention. The circuit of the first embodiment includes a rectifier 2 serving as a first power converter, a smoothing condenser 3, an inverter 4 serving as a second power converter and including feedback diodes 4D, gate driving circuits 4G and IGI3Ts 4T, and an induction motor 5. The names, purposes and functions of these components 2, 3. 4 and 5 are the same as those of the corresponding components of the circuit of the conventional example as described above in relation to Figure 8, and thus for brevity will not be explained again.

In the circuit of the first embodiment of Figure 1, a first low-pass filter circuit 10 is inserted between the rectifier 2 and the smoothing condenser 3 of the direct-current intermediate circuit.

The first low-pass filter circuit 10 consists of a zero-phase-sequence reactor 11, and filter condensers 12, 13, 14 and 15. The zero-phasesequence reactor 11 takes the form of a toroidal iron core having a central opening, through which the positive-side line P and the negativeside line N of the direct-current intermediate circuit are passed. Respective first filter condensers (capacitors) 12 and 13 are connected at one of their ends to the positive-side and negative-side lines P and N at points between the rectifier 2 and the zero-phase-sequence reactor 11, and second filter condensers (capacitors) 14 and 15 are connected at one of their ends to respective sections of the positive-side and negative- side lines P and N at points between the zero-phase-sequence reactor 11 and the smoothing condenser 3. The other ends of each of the filter condensers 12 to 15 are connected to an earth. With the positive-side line P and the negative-side line N passing through the central opening of the core of the zero-phase-sequence reactor 11 shown in the figure, this reactor 11 forms a one turn coil. The zero-phase-sequence reactor 11, however, may be constructed such that the positive-side line is wound a plurality of times around the core, and the negative-side line is wound the same number of times, and in the same 4 6 sense, around the same core. Further, the use of a material (ferfte core, for example) having a good saturation characteristic for the core leads to an improved effect of preventing the leakage current IE.

Figure 2 is a view showing a current path of the leakage current flowing through floating capacitance 6 in the circuit of the first embodiment shown in Figure 1. In Figure 2, the one-dot chain line indicates the current path taken in the case where the leakage current IE flows to earth, through the positive-side line of the direct-current intermediate circuit and the floating capacitance 6. The leakage current IE is a high frequency component of current, since the voltage repeatedly applied to the floating capacitance 6 at a high speed has a high rate of change, due to repeated high-speed switching of the IGBTs 4T of the inverter 4. Since a low-pass filter is effective to suppress the high-frequency leakage current IE, the first low-pass filter circuit 10 is provided in the direct-current intermediate circuit, so as to suppress the leakage current IE.

Figure 3 is an equivalent circuit diagram representing an equivalent circuit of the first low-pass filter circuit 10 of the first embodiment of Figure 1. The low-pass filter circuit is formed such that the zerophase-sequence reactor 11 provides inductance L, and filter condensers having respective capacitance levels of Cl and C2 are provided to connect the positive-side and negative-side lines P and N to earth at points on either side of the inductance L It is to be noted that reference numeral 18 denotes the positive-side or negative-side line P or N of the directcurrent intermediate circuit, and reference numeral 19 denotes the earth.

Figure 4 is a main circuit diagram representing the second embodiment of the present invention. The circuit of the second embodiment includes a rectifier 2 serving as a first power converter, a smoothing condenser 3, an inverter 4 serving as a second power converter and including feedback diodes 4D, gate driving circuits 4G and IGIBTs 4T, an induction motor 5, a zero-phase-sequence reactor 11, and four condensers 12 to 15.

7 The names, purposes and functions of these components 2, 3, 4, 5, 11 and 12 to 15 are the same as those of the corresponding components of the circuit of the first embodiment of Figure 1, and thus their explanation will not be repeated. The circuit of the second embodiment is different from that of the first embodiment of Figure 1 in that a second embodiment of a low-pass filter circuit 20 is inserted between the rectifier 2 and the smoothing condenser 3. The second low-pass filter circuit 20 includes the four filter condensers 12 to 15, a secondary winding 21 wound around the core of the zero-phase-sequence reactor 11, and a short-circuit resistance 22 connected between the ends of the secondary winding 21, short-circuiting the secondary winding 21.

The characteristics of the low-pass filter circuit deteriorate if resonance phenomena occur due to the wiring inductance of the main circuit portion and the capacitance of the filter condensers. With the secondary winding 21 on the core of the zero-phase-sequence reactor 11 being short-circuited by the short-circuit resistance 22 which has a resistance value R, the resonance phenomena can be attenuated, and the deterioration of the characteristics of the low-pass filter circuit can be thus avoided.

Figure 5 is an equivalent circuit diagram representing an equivalent circuit of the second low-pass filter circuit 20 of the second embodiment shown in Figure 4. The low-pass filter circuit 20 mainly consists of the zero-phase-sequence reactor 11 that provides an inductance L, the secondary winding provided on the reactor 11 and short-circufted by the short-circuit resistance having the resistance value R, and the filter condensers having capacitance levels of Cl and C2, which are provided on either side of the inductance L. It is to be noted that reference numeral 18 denotes the positive-side or negative-side line P or N respectively of the direct-current intermediate circuit, and reference numeral 19 denotes the earth.

Figure 6 is a main circuit diagram representing the third embodiment of the present invention. The circuit of the third embodiment includes a rectifier 2 serving as a first power converter, a smoothing condenser 3, an inverter 4 serving as a second power converter and including feedback diodes 4D, gate driving circuits 4G and lGBTs 4T, an 8 induction motor 5, a zero-phase-sequence reactor 11, four pairp of film condensers 12 to 15, and a secondary winding 21. The names, purposes and functions of these components 2, 3, 4, 5, 11. 12 to 15 and 21 are the same as those of the corresponding components of the circuit of the second embodiment of Figure 4, and thus will not be explained.

The circuit of the third embodiment is different from that of the second embodiment in that the circuit between the ends of the secondary winding 21 is completed by a current detector 31, instead of the above-indicated short-circuit resistance 22. Thus, a third low-pass filter circuit 30 including the current detector 31 is inserted between the rectifier 2 and the smoothing condenser 3. Similarly to the circuit of the second embodiment of Figure 4 as described above, occurrence of resonance phenomena can be avoided by controlling the impedance of the current detector 31 to an appropriate value. Further, K a ground fault occurs at a point A in Figure 6, for example, the current in the positive-side line P of the direct-current intermediate circuit differs from that in the negative-side line N of the same circuit, whereby current corresponding to the difference between the currents in the positive-side and negativeside lines P and N flows through the secondary Wncling 21 on the zerophase-sequence reactor 11. The current flowing through the secondary winding 21 is detected by the current detector 31, and, if the detected current exceeds a predetermined value, an earth detector 32 is operated to raise an alarm to indicate the occurrence of the ground fault.

Figure 7 is a time chart representing the earth detecting operation of the circuit of the third embodiment of Figure 6. In the time chart, (1) indicates a change in the earth current IL flowing through the secondary winding 21, which current IL is detected by the current detector 31, and (2) indicates the operation of the earth detector 32, which operates to give the alarm to inform occurrence of the ground fault, when the ground current IL exceeds a predetermined level at a certain point of time T.

According to the present invention, the low-pass filter circuit consisting of the zero-phase-sequence reactor and the condensers is inserted in the direct-current intermediate circuit of the voltage source 9 inverter. This arrangement prevents high-frequency leakage current from flowing toward the AC power supply through the floating capacitance, when the switching circuits of the second power converter operate to convert DC power into AC, power. This eventually prevents other apparatus connected to the AC power supply from being adversely influenced by the above leakage current. Further, installation of only one set of the low- pass filter circuft in the direct-current intermediate circuit does not result in a signfficantly increased number of components as encountered in the conventional device having a noise filter with respect to each phase of the AC power supply. Thus, the voltage source inverter of the present invention can be relatively small in size, and low in cost.

9 the zero-phase-sequence reactor is provided with the secondary winding, which is short-circuited by a resistance having an appropriate resistance value, occurrence of resonance phenomena caused by the circuit inductance and filter condensers is prevented, resulting in no reduction in the effect of the low-pass filter circuit due to resonance. If the second winding is connected by the current detector, instead of the shortcircuit resistance, a ground fault of the voltage source inverter can be detected, while at the same time avoiding the occurrence of the resonance as described above.

Claims

1. A noise fifter for a voltage source inverter comprising a first power converter connected to an AC power supply for generating DC power as an output to a direct-current intermediate circuit having positive and negative power lines, a second power converter connected to the directcurrent intermediate circuit for generating AC power having a desired voltage and frequency, and a smoothing condenser connected between the positive and negative power lines of the direct-current intermediate circuit, wherein between the DC output side of said first power converter and said smoothing condenser a noise fifter comprising a zero-phasesequence reactor is provided and condensers connect the positive and negative power lines to ground at points on either side of the zero-phasesequence reactor.

2. A noise fifter according to claim 1, wherein the noise filter circuit further comprises a secondary winding provided on the zero-phase-sequence reactor, and a resistance connected between the ends of the secondary winding.

3. A noise filter according to claim 1, wherein the noise filter further comprises a secondary winding provided on the zero-phase-sequence reactor, a current detector for detecting current flowing through the secondary winding, and a ground detector that compares the current in the secondary winding detected by said current detector with a predetermined reference value, and determines the occurrence of a ground fault if the current exceeds the predetermined value.

4. A noise filter according to any preceding claim wherein the zero-phase-sequence reactor is a toroidal iron core having a central opening, through which the positive and the negative power lines of the direct-current intermediate circuit pass.

5. A noise filter according to any preceding claim wherein the second power converter has self-arc- extinguishing semiconductor switching elements.

I

6. A noise filter for a voltage source inverter which consists of a first power converter connected to an AC power supply for generating DC power as an output to a direct-current intermediate circuit, a second power converter connected to the direct-current intermediate circuit and having self-arc-extinguishing semiconductor switching elements as constituent elements, for generating AC power having a desired voltage and frequency, and a smoothing condenser connected to the direct-current intermediate circuit, wherein the noise filter comprises a first low-pass filter circuit consisting of a zero-phase-sequence reactor and condensers is provided between the DC output side of said first power converter, and said smoothing condenser.

7. A noise filter for a voltage source inverter which consists of a first power converter connected to an AC power supply, for generating DC power as output to a direct-current intermediate circuit, a second power converter connected to the direct-current intermediate circuit and having self-arc-extinguishing semiconductor switching elements as constituent elements, for generating AC power having desired voltage and frequency, and a smoothing condenser connected to the direct-current intermediate circuit, wherein a second low-pass filter circuit consisting of a zero-phase-sequence reactor, a secondary winding provided on the zero-phase-sequence reactor, a resistance for short-circuiting the secondary winding, and condensers is provided between a DC side of said first power converter, and said smoothing condenser.

8. A noise filter for a voltage source inverter which consists of a first power converter connected to an AC power supply, for generating DC power as output to a direct-current intermediate circuit, a second power converter connected to the direct-current intermediate circuit and having self-arc-extinguishing semiconductor switching elements as constituent elements, for generating AC power having desired voltage and frequency, and a smoothing condenser connected to the direct- current intermediate circuit, wherein a third low-pass filter circuit consisting of a zero-phase-sequence reactor, a secondary winding provided on the zero-phase-sequence reactor, a current detector for detecting current flowing through the secondary winding, and condensers is provided between 12 a DC side of said first power converter, and said smoothing cor)denser, and that a ground detector is provided that determines occurrence of a ground fault if the current of the secondary winding which is detected by said current detector exceeds a predetermined value.

9. A noise filter for a voltage source inverter substantially as described herein, with reference to Figures 1 to 3, Figure 4, Figure 5 or Figure 6 of the accompanying drawings.