DE29822425U1 - Filter device - Google Patents

Description

Filter device

The invention relates to a filter device for connection to an electrical alternating current network with at least a first filter branch for filtering a harmonic wave that is harmonic to the mains frequency of the alternating current network and a second filter branch for filtering harmonics of a frequency that is higher than the harmonic wave, wherein the first filter branch has a first and a second impedance element, of which one has a largely capacitive impedance, the other a largely inductive impedance and the first impedance element a

25 variable impedance.

A converter connected to an electrical alternating current network, such as a converter entering a system for high-voltage direct current transmission (HVDC), generates harmonics at the network frequency due to its mode of operation, harmonic currents on its alternating current side and harmonic voltages on its direct current side. In principle, only harmonics of the order numbers η = kp ± 1 on the alternating current side and the order numbers η = kp on the direct current side occur in this context, where ρ is the pulse number of the converter and k is a positive integer. Converters for HVDC systems are

normally with a pulse number of ρ = 12. As a result, the lowest harmonics on the alternating current side generally have the order numbers 11 and 13. In a power converter with a nominal frequency of 50 Hz, these harmonics therefore have the frequency 550 Hz or 650 Hz. In order to reduce the loads caused by the harmonics on the components belonging to the alternating current network and to meet the conditions set for network perturbations, filters are generally required to limit the spread of interference in the alternating current network. HVDC systems are therefore normally equipped with parallel-connected filter devices which are connected between the corresponding phase and earth of the alternating current network. The filter devices sometimes have filter branches which are tuned to the harmonics of the order numbers mentioned, preferably order numbers 11 and 13, and in addition normally at least one filter branch with a high-pass character which diverts currents of higher frequencies to earth.

These filter branches are generally constructed from passive components, including inductive and capacitive impedances. A tuned filter branch contains a capacitor and an inductor connected in series with a capacitance or inductance selected in such a way that the filter branch exhibits a series resonance at a certain harmonic frequency expected in the AC network.

A general description of filter devices in HVDC systems is given, for example, in J. Arrilaga: High Voltage Direct Transmission, London 1988, especially pages 51-65.

However, fluctuations in the mains frequency and the component values generally mean that an exact tuning cannot be maintained. One solution to this problem is to use an inductive impedance element whose inductance is controlled by a control signal

is variable, into the tuned filter branches. A well-known method for this is to determine a voltage occurring in the filter device in the AC network and a current flowing through the filter branch, and to use a bandpass filter to select and forward the components of voltage and current that have a frequency corresponding to the harmonics to which the filter circuit is tuned. These components are fed to a phase detector, which forms an output signal depending on a phase shift between the components. This phase shift should be zero with perfect tuning. The control signal is formed depending on the output signal of the phase detector and influences the inductance of the inductive element so that the phase shift and thus the control signal approaches zero. In this case, it is particularly advantageous to form the control signal as a function of the trigonometric tangent function of the phase shift mentioned, since the property of the function of producing a high gain for large phase shifts and a gain that decreases with the phase shift can be exploited in the closed control loop that the system described above represents.

An advantageous embodiment of an inductor with variable impedance is described in the published international patent application WO 94/11 891. The inductor has a main winding intended for alternating current, which encloses a tubular core wound from a strip-shaped magnetic material. A control winding intended for direct current runs axially through the core. The permeability of the core in the axial direction and thus also the inductance of the inductor is influenced by changing the current through the control winding.

Alternatively, the capacitive impedance element can be used to adjust the resonance frequency of the filter circuit depending on the control signal with a

switchable capacitor bank. Such a switchable capacitor bank is described in the European patent EP 0 645 866. This patent also describes an advantageous embodiment of a bandpass filter and a phase detector for forming a tangent function of the above-mentioned phase shift.

The arrangements described above for maintaining the tuning of the filter device require access to a measured value of a voltage of the AC network. This voltage is normally determined using a voltage transformer integrated into the HVDC system. Depending on the switching position of the system for different operating cases - especially in bipolar systems where various combinations of single-pole operation can occur - not all of these transformers are always live. In general, several voltage transformers are therefore connected to the filter device via a selector. Before the filter device is put into operation, a check must therefore be carried out to see whether voltage is present and, in some cases, the selector must also be switched over.

It also happens that the filter device for generating reactive power is put into operation without the connected converter equipment with associated voltage transformers being connected to the AC network. In the absence of a measured value of the AC network voltage, in such a case the last inductance value of the inductive impedance element is normally maintained at its last value by fixing the control signal.

The object of the invention is to provide a filter device of the type mentioned at the outset, which enables tuning to the desired harmonic frequency independently of access to the voltage of the alternating current network.

According to the invention, a filter device with at least a first filter branch for filtering a harmonic of the mains frequency and a second filter branch for filtering harmonics of a higher frequency than that of the harmonic, the first filter branch having a first impedance element with a variable impedance and a second impedance element, is characterized in that the filter device contains control means for influencing the variable impedance depending on a measured current through the first filter branch and a measured current through the second filter branch in such a way that a series resonance is achieved between the impedance elements in the first filter branch at a frequency equal to that of the harmonic.

In an advantageous further development of the invention, the first filter branch contains an inductor with a magnetic core, a main winding and a control winding, wherein the inductance of the inductor can be varied by influencing the core permeability as a function of a direct current through the control winding.

Further advantageous developments of the invention are apparent from the description and claims below.

The invention achieves the following advantages. All the measured values required for tuning the filter device are accessible in the filter device, which enables the equipment to start functioning immediately after connection to the AC network. Checking that a measured value of the voltage of the AC network is available and possibly switching the selector is not necessary. The ratio of the amplitudes between the fundamental oscillation and the harmonics of order numbers 11 to 13 is an order of magnitude lower for the current through the second filter branch than the corresponding amplitude ratio.

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the voltage in the AC network, which reduces the requirements for filtering the measured values in the bandpass filter.

The invention will be further clarified by describing embodiments with reference to the attached drawings. All drawings are schematic in the form of single-phase equivalent circuit diagrams or block diagrams, and in which

Figure 1 Parts of a known design of a HVDC system with a filter device connected to an AC network, and

Figure 2 Parts of a filter device according to Figure 1 according to the invention

shows.

The filter device contains calculation devices that are in the

Figures are shown as block diagrams, where the input and output signals of the individual blocks can be signals or calculation values. Signal and calculation value are used synonymously in the following.

Although the blocks shown in the figures are referred to as units, organs, filters, etc., it should be noted that these are to be understood as means for achieving the desired functions, especially when their functions are implemented as software in, for example, microprocessors.

In order not to burden the presentation with distinctions that are obvious to the expert, the same terms are used for the voltages and currents that occur in the system and for the measured values and signals/calculated values that correspond to these quantities and that are listed and treated in the control device described below.

Figure 1 shows a three-phase alternating current network Nl, to which a

bipolar converter station for converting between alternating current and high-voltage direct current. The alternating current network has

a nominal network frequency f of normally 50 or 60 Hz. The converter station contains, in the usual way, two 12-pulse thyristor converters SRI, SR2, each with two 6-pulse valve bridges connected in series. The bridges are connected to the AC network via converter transformers Tl and T2, each with two secondary windings, with voltages that are 30° out of phase with each other. The converters are connected on the DC side to a pole line PLl or PL2 and a common electrode line EL. A capacitive voltage transformer MU measures the voltage of the AC network and generates a voltage signal Uac as a measure of this voltage.

A filter device FU, with three filter branches FUIl, FU13 or FUhp is connected to the AC network via a filter breaker CBF. The filter branch FUIl contains a series connection of a capacitor CIl and an inductor LIl, which has an inductance that can be changed by means of a control signal SIl, which is shown in the figure with an arrow on the inductor. A current 111 flows through the filter branch and is measured by a current measuring device Mill. The control signal SIl is generated by a control device SUIl depending on the supplied value of the current 111 and the voltage signal Uac. The control device is of the type that was described in the introduction with reference to the European patent specification EP 0 645 866, is shown in more detail in Figure 2 and will be described in more detail below. The bandpass filters select components with a frequency equal to 11 times the nominal frequency of the AC network and the control signal SIl, which was formed depending on the phase shift between these components, influences the inductance of the inductor LIl in a direction so that the phase shift approaches zero. A phase shift equal to zero means that for the current frequency, that is, for the harmonic with a frequency equal to 11 times the current

Mains frequency, a series resonance is achieved between the capacitor CIl and the inductor LIl. The capacitor CIl and the inductor LIl are of course selected so that, at the nominal mains frequency and the nominal data of the components, their resonance frequency is mainly at a frequency equal to 11 times the nominal mains frequency.

The filter branch FU 13 is constructed in an analogous manner. In the figure, the parts of the filter branch FU13, which were described above for the filter branch FUIl, are shown with reference designations in which "11" has been replaced with "13". The difference to filter branch FUIl is that the bandpass filter selects and passes on components of a frequency equal to 13 times the nominal mains frequency and that the capacitor C13 and the inductor L13 are selected so that, at the nominal mains frequency and the nominal data of the components, they have their resonance frequency mainly at a frequency equal to 13 times the nominal mains frequency. A current 113 flows through the filter branch and is measured with a current measuring device MI13.

The filter branch FUhp contains, in a known manner, a parallel connection of an inductor L24 and a resistor R24 in series with a capacitor C24 to form a second order high-pass filter. As can be seen from the expression η = kp ± 1 for the order number of the harmonics generated by the converter, in addition to the harmonics of order numbers 11 and 13, harmonics of order numbers of at least 23 and 25 are expected. The components of the high-pass filter are therefore normally selected so that the filter branch receives a resonance frequency centered around the latter harmonics, that is, in this case mainly with a frequency equal to twice the product of the mains frequency and the number of pulses of the converter, here 24 times the nominal mains frequency. The resistance is often selected so that the filter has a low impedance for harmonics of order numbers 11 and 13.

atomic number 17 and higher. A current Ihp flows through the filter branch and is measured with a current measuring device Mlhp.

According to the invention, a control signal is now formed as a function of a phase shift between a measured value of the current flowing through the tuned filter branch and a measured value of the current flowing through the filter branch which is intended for filtering harmonics of a higher order number than the order number to which the tuned filter branch is tuned, that is to say in this case the current Ihp through the filter branch FUhp.

An embodiment of the invention is illustrated in Figure 2 and will be described in more detail following filter branch FU13. This description also applies to filter branch FUIl, which is designed in the same way with the exception of the differences in the dimensioning of the bandpass filter and the components CIl and LIl, which were described above following Figure 1. The parts of the filter device which were described following Figure 1 are shown in Figure 2 with corresponding reference numbers. The converter station is not shown in Figure 2.

The control element SU13 contains a bandpass filter 131, to which a current value Ihp measured with the aid of the ammeter Mlhp is fed, and a bandpass filter 132, to which a current value 113 measured with the aid of the ammeter MI13 is fed. The bandpass filters select components of the ordinal numbers 13 from the supplied measured values and feed them as output signals SA or SB to a phase detector 133.

The phase detector forms an output signal SC depending on the phase shift between the signals SA and SB. The output signal is fed to an amplifier 134, the output signal of which is fed to the inductor L13. The amplifier 134 contains a control section with a proportional integrating gain function and a power amplification section.

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In this embodiment of the invention, the inductor L13 is of the type described in the published patent application WO 94/11 891. The coil thus has a magnetic core with a main winding LH and a control winding LS and the inductance of the coil is variable by influencing the core permeability depending on a direct current through the control winding mentioned. The construction of the inductor is indicated in the figure by the control winding being drawn perpendicular to the main winding. The control signal S13 is therefore formed here by a direct current.

The bandpass filters and the phase detector are thus of the same type as those described in European patent specification EP 0 645 866, but with a modification of the phase detector, due to the consequences described below that, according to the invention, the current Ihp and not the voltage Uac is used for the phase comparison.

In the arrangement described in patent specification EP 0 645 866, in which the voltage Uac is used for the phase comparison, the phase detector generates a signal SC depending on an expression ^{φ 8(φωαα} ~φωη) where tg denotes the trigonometric tangent function, </> _Uac a phase angle of the voltage Uac and _{φ ωη} a phase angle of the current 113. In the case of series resonance, the phase shift in this case is zero, i.e. the control signal is formed depending on the said phase shift and influences the inductance of the inductive element so that the phase shift and thus the control signal approaches zero.

The filter branch FUhp has a largely capacitive impedance for a harmonic of order number 13 (and also for a harmonic of order number 11). In the event that the impedance were purely capacitive, the resonance condition for a filter device according to the invention would thus be fulfilled for the tuned filter branch if the component of the current Ihp for the current harmonic, in this example

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the harmonic wave of ordinal number 13, the corresponding component of the current 113, precedes, more precisely, by a quarter period, i.e. 90° electrically, of the period of the harmonic wave.

The deviation between the impedance of the filter branch FUhp and a purely capacitive impedance, expressed as a phase angle, is also the deviation from -90° of the current component Ihp for the current harmonic. This phase deviation, referred to below as _αφ λεπ, can be calculated with knowledge of the data of the components entering the filter branch.

It can be seen from the above that the phase detector according to the invention is intended to generate a signal SC dependent on a phase shift (&Phi;&ngr;,&rgr;-&Dgr;&phgr;&iacgr;/,&rgr;-&phgr;&igr;&eegr;), advantageously dependent on an expression ^ct g(</>ihp -&Dgr;&phgr;»&igr;&rgr;-&Phi;&igr;&eegr;) , where ctg denotes the trigonometric cotangent function, &igr;_{&iacgr;&igr;&rgr;} a phase angle of the current Ihp, and &phi _;&eegr; a phase angle for the current I13. The expression ctg{$ _Ihp -&Agr;&phgr;_{&igr;&EEgr;&rgr;}-&phgr;_&eegr;3 ) approaches zero when the phase shift approaches -90° electrically. Compared with the phase detector in the arrangement described in patent specification EP 0 645 866, the output signal in the phase detector according to the invention is also to be given a different sign in order to achieve negative feedback. The phase deviation &Agr;φ _{&Igr;&EEgr;&rgr;} calculated in the above-mentioned manner can be implemented as a correction signal in the phase detector.

The invention is not limited to the embodiments shown. Accordingly, for example, the inductive element can be an inductor that can be controlled in a known manner by a semiconductor circuit or an inductor with multiple connections.

The filter branch FUhp is described above as a second order high-pass filter with a resonance frequency equal to 24 times the nominal mains frequency. The filter device is often combined with a further filter branch

of the same type but completed with components selected to obtain a resonance frequency equal to 36 times the nominal mains frequency. In such cases it is advantageous to measure the current in the latter filter branch for a phase comparison with the current in the first filter branch, since the phase deviation A<f> _lhp in this case assumes very small values for practical purposes, typically of the order of 0.5° electrical.

Claims (7)

13 Patent claims 1. Filter device (FU) for connection to an electrical alternating current network (Nl) with at least a first filter branch (FUIl, FU13) for filtering a harmonic wave of the frequency (f) of the alternating current network and a second filter branch (FUhp) for filtering harmonics of a higher frequency than that of the harmonic wave, the first filter branch having a first (LIl, L13) and a second (CIl, C13) impedance element, one of which has a largely capacitive impedance, the other a largely inductive impedance and the first element a variable impedance, characterized in that the filter device contains control means (SUlIl, SU13) in order to influence the variable impedance depending on a measured current (111, 113) through the first filter branch and a measured current (Ihp) through the second filter branch in such a way that at a frequency equal to that of the harmonic wave a series resonance between the first and the other impedance element. 2. Filter device according to claim 1, characterized that the control means includes a phase detector (133) for forming a phase shift dependent on components in said measured currents of a frequency equal to the harmonics, and that the control means influences the variable impedance dependent on said phase shift. 3. Filter device according to claim 2, characterized in that the control means influence the variable impedance so that the said phase shift approaches a quarter period of the harmonic overtone, the component of the measured current through the second filter branch being leading. 4. Filter device according to one of claims 1-3, characterized in that the first impedance element has a largely inductive impedance and the second impedance element has a largely capacitive impedance. 5. Filter device according to claim 4, characterized that the first impedance element is formed by an inductor with a magnetic core, a main winding (LH) and a control winding (LS), and that its inductance can be changed by influencing the permeability of said core depending on a direct current (S13) through said control winding. 6. Filter device according to one of the preceding claims, wherein a power converter (SRI, SR2), preferably a power converter with a given pulse number (p) for conversion between alternating current and high-voltage direct current, is connected to the alternating current network, characterized in that the harmonic overtone has the frequency Fl which results from the expression Fl = f (p ± 1), where f is the mains frequency of the alternating current network and ρ is the pulse number of the power converter, and that the second filter branch is formed by a high-pass filter with a resonance frequency which mainly corresponds to a frequency equal to twice the product of the mains frequency and the pulse number of the power converter. 7. Filter device according to claim 6, characterized in that the filter device further comprises a filter branch which is formed by a high-pass filter with a resonance frequency which mainly corresponds to a frequency equal to three times the product of the mains frequency and the pulse number of the power converter.