Note: Descriptions are shown in the official language in which they were submitted.
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FILTER ARRANGEMENT
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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
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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.
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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
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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
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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
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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),
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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
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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.
