DK201600338A1

DK201600338A1 - Filtering device for an electrical converter

Info

Publication number: DK201600338A1
Application number: DKPA201600338A
Authority: DK
Inventors: Nicklas Södö; Stefan Strandberg
Original assignee: Vacon Oy
Priority date: 2016-06-09
Filing date: 2016-06-09
Publication date: 2017-12-18

Abstract

The invention provides a filtering device, typically used in conjunction with a switching type power electronics converter. The filtering device comprises an inductive component having two windings around a common ferromagnetic core such that the first winding is connected to an external filtering circuit and the second winding is connected to the main circuit path. By selecting the components of the external filtering circuit properly, the impedance curve of the circuit between the switching type voltage source and the load can be tuned such that high frequencies are attenuated without causing extra losses at the switching frequency range.

Description

FILTERING DEVICE FOR AN ELECTRICAL CONVERTER

Technical field

The present invention relates to a filtering device used in conjunction with an electrical converter. Furthermore, the present invention relates to an electrical converter comprising a filtering device.

Background of the invention

In conjunction with many switching type power electric converters, e.g. in conjunction with a frequency converter, there is a need for a filtering device between the converter and the external electrical system. For example, known harmful effects of the sharp edges of the output voltage pulses of a PWM inverter can be decreased by using an output filter, which decreases the slope (better known as du/dt, which term is used in the following) of the output voltage pulse edges. A known problem in filter design are the power losses; e.g. in order to design a functional du/dt filter between a frequency converter and a motor it needs to generate notably internal power losses in order to avoid high voltage pulse overshoots and voltage ringing in the motor cable. Known ways to generate appropriate amount of power losses is to use proper core material and/or proper core shapes in inductive filter components. The sharp voltage pulse edges of a PWM inverter represent high frequency components in the voltage spectrum, which means that in order to attenuate them and thus decrease the du/dt the highest power loss components would preferably be generated at about 50...100 kHz and even higher frequencies.

In prior art filter design, when the design target is to create remarkable power losses at high frequencies, the losses tend to be remarkable also at lower frequencies. This is because the output voltage spectrum of power converters contain high components also at the normally used switching frequency range, i.e. at 1 -10 kHz and at its multiples, in which frequencies the losses of a traditional filter may be unpleasantly high. This often leads to an unfavorable situation with a high number of filter product portfolio due to the filters need to be designed for a particular switching frequency and for a particular voltage in order to avoid too high total power losses.

Summary of the invention

The present invention provides a new power filter device, which avoids the drawbacks of the known technology by making it possible the frequency range, where the highest power losses inside the filter device take place, to match with the range of the most harmful frequency components in the output voltage spectrum of a power converter. The effect of this focused power loss is a reduced output voltage pulse du/dt, which in turn reduces the voltage overshoot and ringing in the load cable. Because the power losses are focused just on the harmful frequencies but they are low at other frequencies, the total efficiency of the system increases. The reduction of the voltage pulse overshoot decreases the stress of the load insulation materials, thus increasing their lifetime.

The filtering device according to the present invention comprises a phase-specific inductive component having two windings around a common ferromagnetic core, such that the first winding is coupled to an external filtering circuit and the second winding is coupled in the main current path. For making it possible to connect the external filtering circuit to ground potential, the first winding may be insulated from the second one. By selecting the components in the external filtering circuit properly, the impedance curve of the filtering device can be adjusted such that most of the power losses take place only at desired frequencies. This enables the attenuation of output voltage components at most unwanted frequencies, thus reducing the du/dt, overshoot and ringing of the output voltage pulses without producing too much power losses at other frequencies.

The filtering circuit according to the present invention may comprise resistor(s) and/or capacitor(s) and/or inductor(s). These components are normally coupled in series, but other couplings are possible too. The component values are selected according to the desired filtering effect.

The filtering device according to the present invention can be used in conjunction with a switching type electrical device, e.g. a frequency converter, as an external arrangement for reducing the du/dt of the output voltage pulses. It is also possible to integrate the filtering device according to the present invention into an electrical device, e.g. into a frequency converter, as an internal device for improving the output voltage pulse shape to have a reduced du/dt. The optimum filtering effect may be dependent e.g. on the cable length between the voltage source and the load. The filtering device according to the present invention enables the optimization to be car- ried out simply by changing the component values in the external filter, without affecting on the main filter inductor design as is required by the prior art solution.

Description of drawings

Below the invention is explained more detailed by using examples with references to the enclosed figures, wherein

Fig.1 presents an electrical circuit of two intercoupled devices,

Fig.2 illustrates voltage waveforms in an electrical circuit,

Fig.3 presents a prior art filtering arrangement,

Fig.4 illustrates impedance curves of a prior art filtering arrangement,

Fig.5 presents a filtering device according to the present invention,

Fig.6 illustrates impedance curves of a filtering arrangement according to the present invention, and

Fig.7 illustrates voltage waveforms in an electrical circuit comprising a filtering arrangement according to the present invention.

Detailed description of the invention

Fig.1 presents a simplified diagram of an electrical circuit wherein a voltage source Si, comprising of a switching type power unit PUi, is connected to a load U by a power transmission cable MCi. It is to be noted that the use of the invention is not restricted to any particular type of a switching type voltage source. Therefore, the structure of it is not described in more detail in the following. For example, the supply (not shown in the figure) of the voltage source, the load controlled by it, voltage level or the number of phases does not bear significance to the basic idea of the invention either. Just as an example, the switching type voltage source may be a frequency converter and the load an asynchronous motor. Exemplary waveforms of the output voltage usi of Si and the input voltage uu of U are illustrated in Fig.2. The output voltage usi in this example is typical for a switching type power electronics converter, e.g. for a frequency converter, comprising pulses having sharp rising and falling edges and an essentially constant peak value of usi- The voltage pulses repeat at a frequency 1/T, called as the switching frequency. The du/dt of the voltage pulse edges may be higher than 1 kV^s, which in the frequency spectrum of the voltage can be seen as components around 100 kHz and even more. According to the known transmission line theory the input voltage Uu of Li is delayed from the output voltage pulse usi of Si by a time tD which depends on the cable length. The transfer impedance mismatch in the connection point of Li causes an overshoot of the input voltage pulse uli such that the peak value uu may be even double to the peak value usi- After the first peak, the voltage uli normally comprises attenuating oscillations by a frequency which depends on the cable length.

The voltage overshoot spike at the load end of the power cable may be dangerous to the insulations of the load. One way to decrease the problem is to use a du/dt limiting filter between the voltage source and the load. Fig. 3 presents a prior art filter arrangement Fi, comprising an inductor Li and a capacitor Ci, used between the voltage source Si and the power cable (the cable is represented by its equivalent circuit Lmci, Cmci in the diagram). In the figure also the stray resistance Rs, representing the power loss producing resistances of the winding and of the magnetic core, and stray capacitance Οδ, representing the capacitances between winding turns, have been drawn visible. The sum effect of all the above presented components is a total impedance PTi, illustrated in frequency domain in Fig.4 as values of the impedance Zpn (in ohms) and its phase shift angle apn (in degrees). As can be seen, the impedance value increases when the frequency increases, due to the mostly inductive nature of the circuit. At very high frequencies the impedance may start to decrease due to the effect of capacitances in the circuit. The phase shift angle is near 90° at low frequencies due to mostly inductive nature of the circuit. At higher frequencies, starting from less than 10 kHz the angle starts to decrease, which means an increase of the resistance and thus also increase of power losses if significant current components are flowing in the circuit at these frequencies. At about 50 kHz and above the power losses are desirable in order to decrease du/dt of the voltage pulse edges, but at the switching frequency area (below 10 kHz) the power losses are undesirable. Because of the current ripple at the switching frequency is high just at the switching frequency, even small decrease of phase angle below 90° at that frequency may cause high power losses, which is the disadvantage of prior art filter designs. The losses at the switching frequency can be decreased by increasing the resonance frequency of the filter, but this may lead to undesirable voltage oscillations.

Fig.5 presents a new inventive filtering device F2 according to the present invention, intended to replace the inductor L1 in the filtering arrangement of Fig.3 or to be used as an internal filter between the power unit PU1 and the output terminal T1 of the voltage source as presented in Fig.1. The filtering device F2 comprises a first winding W1 and a second winding W2, both wounded around a common magnetic core. The second winding W2 is intended to be coupled to the main circuit path between the voltage source and the transmission cable. The first winding W1 is intended to be coupled to a filtering circuit, comprising in this example a series connection of a capacitor Cf and a resistor Rf. The component values in the filtering circuit affect directly to the total impedance PT2 between the voltage source and load, thus making it possible to optimize the total filtering effect. An advantageous example of how the filter according to the present invention can be optimized is illustrated in Fig.6, wherein the resistive effect of the total impedance PT2 (i.e. decrease of the phase shift curve αρτ2) starts at above 10 kHz, being strongest at about 200 kHz which is the resonant frequency of the system. (In one form of the invention, the phase shift angle is below 60° between 100 kHz and 200 kHz.) The resonant frequency is mainly determined by the inductances and capacitances of the filtering device F2 and of the cable, and the power losses in the resistive effect frequency range are mainly determined by the resistor Rf of the filtering circuit in F2. At frequencies below 10 kHz the phase shift angle is nearby 90°, advantageously above 85°, indicating that at the switching frequency range the generated losses are very low.

Fig.7 illustrates the effect of a filtering device according to the present invention in time domain. The overshoot of the voltage pulse at the load end of cable (ui_2) is low, less than 20% above the peak value of the source voltage us2 peak value, and in practice there does not exist any voltage oscillations.

The embodiments of the invention described above are provided by way of example only. The skilled person will be aware of many modifications, changes and substitutions that could be made without departing from the scope of the present invention. Though a single-phase filtering device is used in the describing examples above, the invention may be applied in many other applications, e.g. in three-phase output filters of a frequency converter, AC filtering chokes, etc. The claims of the present invention are intended to cover all such modifications, changes and substitutions as fall within the spirit and scope of the invention.

Claims

1. A filtering device, comprising a first winding and a second winding such that both windings are wound around a common ferromagnetic core part, the first winding being coupled to a filtering circuit, wherein, in use: a first end of the second winding of the filtering device is connected to a voltage source unit of a power electronics circuit; and a second end of the second winding of the filtering device is connected to a load.

2. A filtering device according to claim 1, wherein the filtering circuit comprises at least one resistor and at least one capacitor in a serial connection.

3. A filtering device according to claim 1 or claim 2, wherein the filtering circuit is dimensioned such that the phase angle of the total impedance between the voltage source unit and load is below 60° between 100 kHz - 200 kHz frequency range.

4. A filtering device according to any one of claims 1 to 3, wherein the filtering circuit is dimensioned such that the phase angle of the total impedance between the voltage source unit and load is above 85° below 10 kHz frequencies.

5. A power electronics circuit comprising: a voltage source unit generating at least one output signal with a pulse shaped voltage waveform, the voltage source unit taking place inside a voltage source device, and a filtering device, comprising a first winding and a second winding such that both windings are wound around a common ferromagnetic core part, wherein the first winding is coupled to a filtering circuit, and wherein the output signal of the voltage source unit is connected to a first end of the second winding of the filtering device and a second end of the second winding of the filtering device is connected, in use, to a load.

6. A power electronics circuit according to claim 5, wherein the filtering circuit comprises at least one resistor and at least one capacitor in a serial connection.

7. A power electronics circuit according to claim 5 or claim 6, wherein the filtering circuit connected to the first winding of the filtering device is dimensioned such that the phase angle of the total impedance between the voltage source unit and load is below 60° between 100 kHz - 200 kHz frequency range.

8. A power electronics circuit according to any one of claims 5 to 7, wherein the filtering circuit connected to the first winding of the filtering device is dimensioned such that the phase angle of the total impedance between the voltage source unit and load is above 85° below 10 kHz frequencies.

9. A power electronics circuit according to any one of claims 5 to 8, wherein the filtering device is connected device externally between an output terminal of the voltage source device and a phase wire of the power transmission cable.

10. A power electronics circuit according to any one of claims 5 to 8, wherein the filtering device is connected device internally between the output terminal of the voltage source unit and an output terminal of the voltage source device.