WO1994028615A1

WO1994028615A1 - Frequency converter output filter

Info

Publication number: WO1994028615A1
Application number: PCT/DE1994/000363
Authority: WO
Inventors: Walter Springmann; Roland Schwab
Original assignee: Siemens Aktiengesellschaft
Priority date: 1993-05-24
Filing date: 1994-03-31
Publication date: 1994-12-08
Also published as: DE9307806U1

Abstract

The invention relates to a circuit arrangement (2) connected between a power converter (16) and the leads (8) of an inductive load (18) consisting of a limiting circuit (28), a capacitor network (30) and a plurality of chokes (32), in which one choke (32) connects one input (34, 36, 38) of said circuit arrangement (2) on the one hand via a limiting resistor (42) to one input of the limiting circuit (28) and on the other to an output (46, 48, 50) and an input (40) of a capacitor network (30). According to the invention, a capacitor (64) is connected between a choke (32) and a limiting resistor (42). In this way the freewheeling currents, especially in generator operation, flow via the freewheeling diodes of the inverter (24) of the power converter (16) and hence, during operation, a substantially lower power loss is converted into heat in the limiting resistors (42) of the circuit arrangement (2).

Description

Inverter output filter

The invention relates to a circuit arrangement for reducing voltage peaks and high voltage rise speeds, generated by line inductances and capacitances of a plurality of lines which connect the outputs of a converter device to an inductive load, characterized by the features of the preamble of the claim 1.

The "electrical pollution" caused by a frequency converter is directly related to its operating principle. Inverters with a fixed DC link voltage alternately switch the outputs to the positive or negative rail of the DC link voltage and thus generate a three-phase system. First, a square-wave voltage is created, the mean value of which can be influenced by varying the pulse-pause ratio. As with any inductive load, the current in the three-phase motor does not follow the rapid rectangular voltage. It is smoothed sinusoidally. Almost all modern converters work according to this principle of pulse width modulation (PWM).

However, the thyristors, GTO thyristors, transistors, MOSFET or IGBT in the power section of a converter, which generate this square-wave voltage, are not ideal switches. Switchover losses occur with every transition from the conductive to the non-conductive state and vice versa. They can only be minimized if the switching process runs as quickly as possible. However, ever faster switching processes have disadvantages that can be combated with suitable means.

Modern MOSFET or IGBT only need 200 ns to 50 ns for a switching process. With a usual intermediate circuit voltage of 500 V, the voltages change at a speed of 2500 V / μs to 10 kV / μs. If these potential jumps are passed unfiltered to the output terminal. they have a direct effect on the motor cable. Due to line inductances and capacitances, unpredictable resonant circuits are created, which are repeatedly triggered by the square-wave output voltage of such a frequency converter and can build up in an uncontrolled manner. The undesirable consequences of this behavior can take on considerable proportions: on the one hand, the motor supply line emits electromagnetic fields that can significantly disrupt other electronics. Of course, this is particularly critical if signal-carrying lines are laid nearby. If a shielded cable is used for the motor supply line, this problem can be dealt with, but this also involves additional capacities, which makes the resonant circuits inside the line even more unpredictable. The second impact of voltage buildup and rapid voltage change rates is damage to motor insulation. In the long run, the voltage peaks caused by the oscillating circuit and in particular the high rates of voltage rise can destroy the insulating varnish. These dangers are particularly great in the area of the winding overhangs, where the different phases lie directly one above the other. A third effect is the high-frequency current components that can flow to earth via the winding capacity of the motor. The currents are only very small, but they are sufficient to disrupt capacitive level meters, for example.

To counteract the effects described, output filters are available commercially, which can be connected downstream of a frequency converter.

These filters are constructed as LC low-pass filters, which leave the low-frequency component of the PWM spectrum, that is to say the actual output frequency of, for example, 0 to 100 Hz, unaffected. The switching frequency and its harmonics are blocked. The voltage curve at the output of the Low pass is then no longer rectangular, but follows the sine curve almost ideally. Interference radiation from the motor cables, capacitive leakage currents or damage to the winding are thereby excluded.

However, such output filters are only an economically justifiable protective measure if the frequency inverters operate with a relatively high switching frequency. The inductances and capacitances required for the filters have to be greater the smaller the switching frequency of the frequency converter, and the filter structure becomes more complicated the closer the frequencies to be separated are. In other words, in the case of frequency converters that only work with switching frequencies of, for example, 2 to 4 kHz, the filters become too large and too expensive. In principle, however, the problems are the same, since the interference spectrum does not depend on the clock frequency, but on the slope. As a protective measure, however, only emergency solutions are possible, such as chokes on the motor side or shielded cables. As already mentioned, the problems cannot really be solved in this way.

From EP 0 473 192 A2 a circuit arrangement for reducing oscillation circuit-related voltages is known. This circuit arrangement consists of a multi-pulse diode bridge and several chokes. Each choke connects an input of this circuit arrangement to an input of the diode bridge branch and an output of this circuit arrangement. A limit resistor is assigned to each diode in each bridge branch. Each bridge side of the diode bridge provided with resistors forms a direct current connection of the circuit arrangement. In a further embodiment of this circuit arrangement, these many limiting resistors are replaced by two resistors, each of which connects a direct current connection to a common connection on one side of the bridge. In addition, a capacitor can also be provided, which is electrically parallel is connected to the output of the diode bridge. This circuit arrangement is connected between a frequency converter and the motor leads of a motor in such a way that the phase outputs are connected to the inputs of the circuit arrangement, the DC outputs of the circuit arrangement are connected to the intermediate circuit of the frequency converter, and the motor leads are linked to the outputs of the circuit arrangement.

By means of the diode bridge, this circuit arrangement limits the overvoltage that arises by means of feedback to the converter intermediate circuit. The resistors are provided in the circuit arrangement for damping the existing resonant circuit and for limiting the current.

This known circuit arrangement has the disadvantage that the diodes are loaded with the output voltage of the circuit, which despite the limitation can still be significantly higher than the intermediate circuit voltage. In addition, this circuit arrangement cannot reduce the rate of voltage rise, since no elements, such as an LC low-pass filter, are provided. This circuit arrangement can be used if the motor leads are long enough.

A circuit arrangement equipped according to the preamble for reducing oscillation-related voltage peaks and high voltage rise speeds, generated by line inductances and capacitances of lines which connect the outputs of a converter device to an inductive load, is commercially available. This circuit arrangement is also referred to as line attenuation and is supplied as an option for the Simovert P 6SE48 converter. This line attenuation limits the voltage to the level of the DC link voltage of the frequency converter. The converter works at a pulse frequency of 8 kHz and 4 kHz. In contrast to the circuit arrangement of EP 0 473 192 A2, this line attenuation can also reduce the voltage rise rate. In both of the circuit arrangements shown, the problem arises that freewheeling currents flow through the limiting circuit and not, as intended, through the freewheeling diodes of the inverter of the frequency converter. This results in high losses in the limiting resistors, which can jeopardize the application of the circuit, particularly in generator mode.

The invention is based on the object of improving an arrangement known according to the preamble in such a way that the stated problem is solved.

This object is achieved in that a capacitor is connected in each case between a choke and a limiting resistor.

The problem of the additional power loss is solved by using the capacitors, which are each electrically connected in series with the limiting resistor. This capacitor allows the desired short-term limiting current to continue to flow, but forces the freewheeling current to flow via the freewheeling diodes of the inverter. Another advantage of this circuit is that with each switching operation of the positive valve of an inverter phase, the capacitor is pre-charged by the previous switching operation of the negative valve. As a result, the limitation process starts earlier and the overvoltage at the motor terminals is further reduced.

In an advantageous embodiment of the circuit arrangement, a discharge resistor is electrically connected in parallel with the capacitor. The use of this discharge resistor limits the following effect: since the choke causes the phases to be coupled to one another, the capacitor, which is electrically connected in series with the limiting resistor, is also charged by switching processes in other phases, although the associated phase is not switches. The charge of this capacitor is reduced by the discharge resistor, which is connected electrically in parallel with the capacitor.

In a further advantageous embodiment of the circuit arrangement, a plurality of single-phase chokes decoupled from one another are provided instead of the chokes. As a result of these decoupled single-phase chokes, the capacitors can no longer be charged by switching processes in other phases if the associated phase does not switch.

For a further explanation of the invention, reference is made to the drawing, in which exemplary embodiments of a circuit arrangement for reducing voltage peaks caused by an oscillating circuit and high voltage rise speeds are illustrated schematically.

Figure 1 shows a known circuit arrangement according to the preamble of claim 1, Figure 2 shows a first embodiment of the circuit arrangement according to the invention and in

FIG. 3 shows a further embodiment of the circuit arrangement according to the invention.

FIG. 1 shows a known circuit arrangement 2 for

Reduction of voltage peaks caused by the oscillation circuit and of high voltage rise rates, generated by line inductances 4 and capacitances 6 of lines 8, which connect the outputs 10, 12, 14 of a converter device 16 to an inductive load 18. This circuit arrangement

2 is known in trade under the name line attenuation. In addition, such circuit arrangements 2 are also referred to as converter output filters according to their use. The converter device 16, also called a frequency converter or frequency converter, consists of an uncontrolled rectifier 20 on the input side, an intermediate circuit 22 and an inverter 24 on the output side, in particular a pulse inverter. The pulse inverter 24 is constructed as a six-pulse bridge circuit, a so-called B6 circuit, and uses insulated gate bipolar transistors (IGBT) as converter valves 26. Metal oxide layer field effect transistors (MOSFET) or bipolar transistors can also be used as converter valves 26. As already mentioned at the beginning, such converter valves 26 require a time period between 200 ns and 50 ns for a switching operation. With an intermediate circuit voltage Uς * of, for example, 500 V, this means that the voltage changes at a speed between 2500 V / μs and 10 kV / με. Such potential jumps are given to the output terminals 10, 12 and 14 of the inverter 24.

The line attenuation 2 is connected between the converter device 16 and the feed lines 8 of the inductive load 18, for example an AC motor, in particular a three-phase machine. This line attenuation 2 can also be part of the frequency converter 16. This line attenuation 2 consists of a limiting circuit 28, a capacitor network 30 and several chokes 32. The limiting circuit 28 is a multi-pulse diode bridge, which is designed like the bridge circuit of the pulse-controlled inverter. The capacitor network 30 contains a plurality of limiting capacitors which are connected together as a capacitor bridge, two limiting capacitors in each case forming a bridge branch 40. In addition, these limiting capacitors of the capacitor network 30 can also be connected in a star or a triangle. Since the frequency converter 16 has three outputs 10, 12 and 14 here (three-phase), the line attenuation 2 is also constructed in three phases. This means that this circuit arrangement 2 has three inputs 34, 36 and 38, each via a choke 32 with an input (40) of the capacitor network 30 and via a limiting resistor 42 with an input of a bridge branch 44 of the limiting circuit 28 are linked. The circuit arrangement 2 also has three outputs 46, 48 and 50 and two DC connections 52 and 54. The motor feed lines 8 of the three-phase machine 18 are connected to the output terminals 46, 48 and 50. The DC connections 52 and 54 are on the one hand electrically connected via connections 56 and 58 to the intermediate circuit 22 of the frequency converter 16 and on the other hand each to a further connection 60 and 62 of the limiting circuit 28 and the capacitor network 30.

This circuit arrangement 2, which is also known as line attenuation, has the following advantages:

- reduces the tendency of the load circuit to oscillate

- reduces the voltage peaks - reduces the voltage steepness (du / dt)

- also works against PE (protective earth)

- inexpensive

and the following disadvantages:

- Connectable line length is limited

- Depending on the length of the line, principle-related losses

- increased resistance losses at 500 V.

- problematic for large outputs (size)

on

In this known line attenuation 2 there additionally arises a problem that freewheeling currents flow through the diode bridge 28, which is also called limiting circuit because of its function, and not as provided via the freewheeling diodes of the converter valves 26 of the inverter 24 (FIG. 2). This results in high losses in the limitation resistors 42, particularly in generator operation.

FIG. 2 shows a first embodiment of the converter output filter 2 according to the invention, same reference numerals as in Figure 1 are used. Compared to the circuit arrangement 2 according to FIG. 1, a few simplifications have been made in order not to overload the illustration. From the converter device 16, only the inverter 24 is shown. All connecting terminals 10, 12, 14, 56, 58 and 34, 36, 38, 46, 48, 50, 52, 54 of the converter device 16 and the circuit arrangement 2 are no longer shown in detail. In addition, this embodiment of the circuit arrangement 2 according to the invention differs from the circuit arrangement 2 according to FIG. 1 in that a capacitor 64 is connected between a choke 32 and a limiting resistor 42. This capacitor 64 is dimensioned such that it continues to allow the desired brief (1 με to 10 μs range) limiting current to flow, but forces the freewheeling current (100 μs range) via the freewheeling diodes of the converter valves 26 deε Inverter 24 to flow. Thus, the diodes of the limiting circuit 28 are no longer loaded with the freewheeling currents of the load 18 and a significantly lower power loss occurs in the limiting resistors 42.

This converter output filter 2 has yet another advantage: with each switching operation of a positive or negative converter valve 26 of an inverter phase, the capacitor 64 is precharged by the preceding switching operation of the negative or positive converter valve 26 of the same inverter phase. As a result, the limitation process starts earlier and the voltage at the motor terminals is further reduced.

FIG. 3 shows a second embodiment of the converter output filter 2 according to the invention, a discharge resistor 66 being connected electrically in parallel with the capacitor 64 in each case compared to the embodiment of the circuit arrangement 2 according to FIG. This discharge resistor 66 limits the voltage across the capacitor 64. The three Chokes 32 can also be designed as a three-phase output throttle. The three doses 32 or the three-phase output choke cause the phases to be coupled to one another. As a result of this coupling, the capacitor 64 of each phase is also charged by switching processes in other phases, although the associated phase does not switch. In order to limit this effect, a discharge resistor 66 is electrically connected in parallel to each capacitor 64.

This problem can also be solved by using three decoupled single-phase chokes as the choke 32 or instead of the three-phase output choke. Due to the decoupling, the capacitors 64 in each phase are no longer charged by switching processes in other phases.

This inventive design of the converter output filter 2 according to FIG. 2 or 3, the limiting capacitors of the capacitor network 30 also being able to be connected in star or delta, prevents the operation of three-phase machines on pulse converters, in particular with long motor leads 8, voltage peaks occur at the motor terminals, which can be up to twice the DC link voltage Uς-. As a result of this voltage load, the winding insulation of the machines is usually subjected to an impermissibly high stress, so that motor failures due to insulation defects can occur. In addition, it is prevented that the converter 16 acts on the AC motor 18 with very steep voltage edges due to its switching operations. These voltage edges lead to an uneven distribution of the voltage to the individual coils of the motor winding, so that individual coils are subjected to a much higher load. In addition, these voltage edges in the winding insulation result in faster aging. In addition, it is achieved that, in particular in generator operation, the freewheeling currents flow through the freewheeling diodes of the converter valves 26 of the inverter 24 and thus Circuit arrangement 2 has a substantially lower power consumption, which increases the efficiency of the drive. In addition, it is not necessary to dissipate as much power loss as heat. This makes it much easier to integrate such a converter output filter 2 in an existing converter device 16.

Claims

1. Circuit arrangement (2) for reducing voltage peaks and high voltage rise speeds, generated by line inductances and capacitances (4, 6) of a plurality of lines (8) which also carry the outputs (10, 12, 14) of a converter device (16) Connect an inductive load (18), consisting of a limiting circuit (28), a capacitor network (30) and several chokes (32), one choke (32) each having an input (34, 36, 38) of this circuit arrangement (2) by means of a limiting resistor (42) with an input of the limitation circuit (28) and on the other hand with its output (46, 48, 50) and an input (40) of the capacitor network (30), and in each case a further connection (60 , 62) of the limiting circuit (28) is connected to a direct current connection (52, 54) of this circuit arrangement (2), characterized in that in each case between a choke (32) and a limiting device gs- resistor (42) a capacitor (64) is switched.

2. Circuit arrangement (2) according to claim 1, d a ¬ d u r c h g e k e n n z e i c h n e t that in each case a discharge resistor (66) is electrically connected in parallel to the capacitor (64).

3. Circuit arrangement (2) according to claim 1 or 2, so that a plurality of single-phase decoupled decouplers are provided instead of the droplets (32).

4. Circuit arrangement (2) according to one of claims 1 to 3, characterized in that a multi-pulse diode bridge is provided as a limiting circuit (28).

5. Circuit arrangement (2) according to one of the claims 1 to 4, so that the capacitor network (30) consists of several limiting capacitors which form a bridge circuit.

6. Circuit arrangement (2) according to claim 5, so that the limiting capacitors of the capacitor network (30) are connected in star.

7. Circuit arrangement (2) according to claim 5, so that the limiting capacitors of the capacitor network (30) are connected in a triangle.