CA2074260A1

CA2074260A1 - Filter arrangement

Info

Publication number: CA2074260A1
Application number: CA 2074260
Authority: CA
Inventors: Gerhard Titscher; Gunter Moraw; Walter Richter
Original assignee: Individual
Current assignee: Andritz Hydro GmbH Austria
Priority date: 1989-12-07
Filing date: 1990-12-07
Publication date: 1991-06-08
Also published as: HUT61140A; HU9201884D0; EP0502929A1; FI922617A0; FI922617A; BR9007897A; WO1991009451A1; JPH05502364A; CS611090A3

Abstract

A filter arrangement is useful for filtering and damping harmonics and for shifting resonance points in electric networks. To this end, a first energy-absorbing damping element (Rp21) is connected in parallel with a series arrangement consisting of a first inductance (Lp2), a first capacitance (Cp21) and a fourth capacitance (Cp22). This parallel arrangement is connected in series with a parallel path consisting of a second inductance (Lp3) and a second capacitance (Cp3), as well as with a third capacitance (C1). In addition, a second energy-absorbing damping element (Rp41) is connected in parallel with the series arrangement consisting of the fourth capacitance (Cp22) and the second capacitance (Cp3). The use of this filter arrangement in the case of two tuning frequencies permits strong damping of harmonic currents with frequencies close to one of the two tuning frequencies, without significant losses due to currents with the operating frequency.

Description

20~ ~2~3 FILTER ARRANGEMENT
" ~
The invention relates to a filter arrangement for filtering and damping harmonic oscillations or higher-frequency oscillation packages and for shifting resonance points in electrical power grids.
Low-frequency harmonic oscillations in electrical power grids, which are generated for example during sudden releases because of transformer saturation in connection with resonance points, can lead to considerable surge voltages and voltage distortion, which represent severe disturbances in respect to normal power grid operation.
The nature of the a.c. current system only has a small effect in regard to the appearance of these disturbances.
This means that the disturbances cannot be prevented by a particular structure of the a.c. current system.
German Published, Non-Examined Patent Application DE-OS 35 39 950 deals with a high-pass filter with a filter arrangement for a rotary current power grid connected with frequency converters. Here, a condenser and a choke are connected in series with a parallel circuit of ohmic, inductive and capacitive connecting elements which are grounded in case of high voltage and free duriny medium voltage, and is connected with it to the power grid. The two resonance frequencies and qualities of the dual high-pass filters are adjusted in such a way, that only a small resonance increase can occur in the critical harmonic oscillation range.
The disadvantage of this circuit lies in that the losses because of operating frequency currents are comparatively large in this arrangement if the resistances are selected to be correspondingly low for achieving rapid damping of low-frequency harmonic oscillations.
A high-pass filter of a filter arrangement for a line connected with rectifiers is proposed in European Patent Disclosure EP-O 318 790. A C-section is connected in 207~2~

series with an L-section and a parallel path, where an L-section of a series circuit consisting of an R-section and a C-section is disposed par~llel in the parallel path. A
further R-section is disposed parallel to the series connection element of the L-section and the parallel path.
With this circuit it is also not possible to dampen harmonic oscillations strongly without the losses caused by the operating frequency currents remaining small.
The problem of filtering out harmonic oscillations is also treated in Chapter 200-15 entitled "Characterization and Control of Harmonic Overvoltages at HVDC Stations~, by E.V. Larsen and R. A. Nalling, published in the Proceedings ~Section 2, pp. 1 to 6) of the "CIG~E"
symposium conducted in September 1987 in Boston, USA.
Here, in a first circuit arrangement, a first and a second C-section connected in series are connected in series with a parallel path consisting of a surge arrester and an L-section, and an R-section is disposed parallel to the series connection part of the second C-section and the L-section.
The disadvantage in this circuit lies in that only the harmonic oscillation currents of only one tuned frequency can be balanced in the power grid and damped in the process.
In a second circuit in the same Proceedings, a first C-section is connected in series with a parallel path which is constructed of a series circuit comprised of a second C-section and a first L-section and of a series circuit consisting of a voltage-dependent resistor and an R-section; a surge arrester is disposed parallel with the series connection part of the two C-sections.
The disadvantage of this circuit also lies in the limitation of the balancing and damping effect to only one tuned frequency.

2 ~3 ~
It is the object of the invention to provide a filter arrangement by means of which adequate damping can be achieved for two tuned frequencies and where the losses ~, , caused by operating frequency cùrrents are limited to a minimum.
This object is attained by means of the invention.
The latter is characterized in that a first inductor and a first capacitor are disposed in a series circuit, and that this series circuit is connected with a first damping element to a first parallel path, and that the first parallel path is connected in series with a second parallel path, consisting of a second inductor and a second capacitor, as well as with a third capacitor.
In connection with two tuned frequencies, it is possible for the first time by means of the invention to damp very strongly harmonic oscillation currents with frequencies in the vicinity of these tuned frequencies and to prevent substantial losses because of operating frequency currents.
A further embodiment of the invention consists in that the first inductor, together with the first capacitor and a fourth capacitor, is arranged into a series circuit in such a way, that the first capacitor is disposed between the first inductor and the fourth capacitor, and that this series circuit is connected with the first damping element to form an expanded first parallel path, and that this first expanded parallel path is connected in series with the second parallel path and with the third capacitor in such a way, that the third capacitor is connected on the one side to a line terminal and on the other side to the connecting line from the first damping element to the first inductor, and where the second parallel path is connected on the one side to the connecting line from the first damping element to the fourth capacitor and on the other side to a ground terminal, and that a second damping element is ` 2074~
connected on the one side to the connecting line from the first capacitor to the fourth capacitor and on the other side directly or via current transformers to ground potential.
An increased damping effect of the filter arrangement is achieved by this development.
An em~odiment of the invention provides that the first inductor is disposed between the first capacitor and the fourth capacitor, and that the third capacitor is connected on one side to the line terminal and on the other to a connecting line from the first damping element to the first capacitor.
In this connection variant, in which the first inductor and the first capacitor are exchanged, there is the advantageous possibility of arranging the first and third capacitor in the same frame, if required.
Further advantageous embodiments and developments of the invention ensue from additional dependent claims.
In this way it is possible to take special properties of electrical grid systems into consideration.
The invention will be described in detail by means of exemplary embodiments. Fig. 1 illustrates the simplest form of a filter arrangement according to the invention, Fig. 2 a filter arrangement with increased damping. A
filter arrangement with a specific charàcteristic curve is shown in Fig. 3, while the circuit shown in Fig. 4 assures optimization of all important parameters.
In connection with the employment of static compensators in electrical power grids the necessity arises to damp the free oscillations of the system, which are excited in the course of each equalization process and which are superimposed on the grid frequency base oscillation, rapidly enough so that at the time when the individual voltage values of all these forms of oscillations are superimposed in the same sense at the time `-` 297 ~2~
of their greatest value, the amplitudes of these free oscillations are already greatly reduced.
Something similar is v~lid for the magnetizing currents of transformers, which àre driven into their saturation range in case of large overvoltages or switching operations. With asymmetric saturation, the second and third harmonics are very defined and also result in considerable overvoltage in case of grid resonances present in the vicinity of twice or three times the base frequency of the grid. Balancing and simultaneous damping of these currents results in the rapid return to the normal operating range of the operating range of the transformers.
Thus the use of filters which have, on the one hand, a balancing effect and on the other strong damping effects, is practical in connection with both the above actions.
The property of such filters of having very small losses of the operating-frequency base oscillation of the grid is of great technical and economic importance.
The simplest basic form of a filter arrangement in accordance with the invention is shown in Fig. 1. In this case an ohmic resistor (Rp21) has been connected parallel as the first damping element to a series circuit consisting of a first inductor (~ 2) and a first capacitor (Cp21).
This parallel connection is connected in series with a parallel path, consisting of a second inductor (~3) and a second capacitor (Cp3), and of a third capacitor (Cl).
This basic form already has all characteristic properties of the invention. By means of it it is possible in the case of two tuned frequencies to damp very strongly harmonic oscillations having frequencies in the vicinity of these tuned frequencies without substantial losses occurring in this arrangement because of operating-frequency currents.
Fig. 2 represents an expansion of the circuit of Fig.
1. As indicated in Fig. 2, a fourth capacitor (Cp22), 2 ~ ' connected in series to the first capacitor (Cp21), has been connected to the series circuit comprised of a first inductor (Lp2) and a first çapacitor (Cp21). In addition, a second ohmic resistor (Rp41) is connected parallel with the series circuit of the fourth capacitor (Cp22) and the second capacitor (Cp3).
Damping of the filter arrangement is considerably increased by this simple expansion of the circuit by means of the capacitor (Cp22) and the resistor (Rp41).
Fig. 3 shows a further development of the circuit illustrated in Fig. 2. In this case the first ohmic resistor (Rp21) has been expanded into a series circuit by means of a first varistor (Vp2), and this series circuit is connected parallel to the series circuit consisting of a first inductor (Lp2), a first capacitor (Cp21) and a fourth capacitor (Cp22). In addition, the second ohmic resistor (Rp41) has been expanded into a series circuit by means of a second varistor (Vp4). This series circuit has been connected parallel to the fourth capacitor (Cp22) and the second capacitor (Cp3).
The filter arrangement illustrated in Fig. 3 shows a very steep impedance characteristic curve with the impedance minimum at two tuned resonances. Greatest possible damping occurs at the largest content of harmonic oscillations.
In the circuit in accordance with Fig. 4 - in the form of a further development of the circuit in accordance with Fig. 3 - a third ohmic resistor (Rp22) is connected parallel to the series circuit consisting of a first ohmic resistor (Rp21) and a first varistor (Vp2). In addition, a fourth ohmic resistor (Rp42) is connected parallel to the series circuit consisting of a second ohmic resistor (Rp41) and a second varistor (Vp4).
It is possible with the aid of this further development to take small harmonic oscillations into 2~7~2~
consideration and to affect the characteristic curve to a greater extent in regard to damping in case of a small proportion of harmonic osci~lations. Also, a further reduction of losses occurs.

Claims

1. A filter arrangement for filtering and damping harmonic oscillations or higher-frequency oscillation packages and for shifting resonance points in electrical power grids, characterized in that a first inductor (Lp2) and a first capacitor (Cp21) are disposed in a series circuit, and that this series circuit is connected with a first damping element (Rp21) to a first parallel path, and that the first parallel path is connected in series with a second parallel path, consisting of a second inductor (Lp3) and a second capacitor (Cp3), as well as with a third capacitor (C1).
(Fig. 1)

2. A filter arrangement in accordance with claim 1, characterized in that the first inductor (Lp2), together with the first capacitor (Cp21) and a fourth capacitor (Cp22), is arranged into a series circuit in such a way, that the first capacitor (Cp21) is disposed between the first inductor (Lp2) and the fourth capacitor (Cp22), and that this series circuit is connected with the first damping element (Rp21) to form an expanded first parallel path and that this first expanded parallel path is connected in series with the second parallel path and with the third capacitor (C1) in such a way, that the third capacitor (C1) is connected on the one side to a line terminal (1) and on the other side to the connecting line from the first damping element (Rp21) to the first inductor (Lp2), and where the second parallel path is connected on the one side to the connecting line from the first damping element (Rp21) to the fourth capacitor (Cp22) and on the other side to a ground terminal (2), and that a second damping element (Rp41) is connected on the one side to the connecting line from the first capacitor (Cp21) to the fourth capacitor (Cp22) and on the other side directly or via current transformers to ground potential.
(Fig. 2)

3. A filter arrangement in accordance with claim 2, characterized in that that the first inductor (Lp2) is disposed between the first capacitor (Cp21) and the fourth capacitor (Cp22), and that the third capacitor (C1) is connected on one side to the line terminal (1) and on the other to a connecting line from the first damping element (Vp2) to the first capacitor (Cp21).

4. A filter arrangement in accordance with claims 1, 2 or 3, characterized in that a varistor is employed as the first damping element ((Rp21) and/or as the second damping element (Rp41).

5. A filter arrangement in accordance with claims 1 or 2, characterized in that a series or parallel circuit of an ohmic resistor with a varistor (Vp2, Vp4) is employed as the first damping element ((Rp21) and/or as the second damping element (Rp41).

6. A filter arrangement in accordance with claim 5, characterized in that a third ohmic resistor (Rp22) is connected parallel to the first damping element (Rp21).

7. A filter arrangement in accordance with claim 5, characterized in that a fourth ohmic resistor (Rp42) is connected parallel to the second damping element (Rp41).

8. A filter arrangement in accordance with claims 1 to 7, characterized in that a silicon-carbide surge arrester without a discharger is employed as the first damping element ((Rp21) and/or as the second damping element (Rp41).

9. A filter arrangement in accordance with claims 1 to 7, characterized in that a silicon-carbide surge arrester with a discharger is employed as the first damping element ((Rp21) and/or as the second damping element (Rp41).

10. A filter arrangement in accordance with claims 1 to 7, characterized in that a series or parallel circuit of an ohmic resistor with a silicon-carbide surge arrester without a discharger is employed as the first damping element ((Rp21) and/or as the second damping element (Rp41).

11. A filter arrangement in accordance with claims 1 to 7, characterized in that a series or parallel circuit of an ohmic resistor with a silicon-carbide surge arrester with a discharger is employed as the first damping element ((Rp21) and/or as the second damping element (Rp41).

12. A filter arrangement in accordance with one of claims 4 to 11, characterized in that the first and/or the second varistor (Vp2, Vp4) is replaced by antiparallel-connected thyristor arrangements.

13. A filter arrangement in accordance with one of claims 4 to 12, characterized in that a tuned discharger is connected in series to the first and/or the second varistor (Vp2, Vp4).

14. A filter arrangement in accordance with at least one of claims 1 to 13, characterized in that antiparallel-connected thyristor arrangements are connected in series to at least one of the ohmic resistors employed in the first damping element (Vp2) and in the second damping element (Rp41).

15. A filter arrangement in accordance with at least one of claims 1 to 14, characterized in that at least one of the varistors employed in the first damping element (Vp2) and in the second damping element (Rp41) is replaced by a parallel circuit of a plurality of varistors.