Method and device to compensate for an asymmetrical DC bias current in a power transformer connected to a high voltage converter

A method and a device to compensate for an asymmetrical DC bias current in a multi-phase transformer. The transformer is connected between an AC power system and an AC/DC or DC/AC high voltage converter. For each phase of the AC side of the transformer a current quantity is determined. The current quantity reflects the time dependent behavior of the magnetizing current in the phase. Time intervals in the current quantity are determined during which the current quantity reaches a positive or a negative maximum, respectively. A DC magnetizing quantity is determined from a difference between the amplitude of the positive maximum and the amplitude of the negative maximum. An asymmetrical quantity is determined from a difference between the amplitudes of the positive and/or negative maxima of at least two of the phases and a control signal is generated from the asymmetrical quantity and provided to a control device of the converter in order to adjust the generation of the AC or DC voltage in the particular phase of the converter which corresponds to the phase of the AC side of the transformer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase under 35 U.S.C. §371 of PCT/EP2007/059132 filed 31 Aug. 2007.

FIELD OF THE INVENTION

The invention relates to a method and a device to compensate for an asymmetrical DC bias current in a multi-phase transformer, where the transformer is connected between an AC system and a AC/DC or DC/AC high voltage converter and where at least one side of the transformer is star connected and the star point is grounded via a neutral.

BACKGROUND OF THE INVENTION

A transformer coupled between an AC power system and a AC/DC or DC/AC converter is usually subject to DC bias currents which means that a certain DC magnetization in the transformer occurs. The DC bias currents may be caused by a DC component in the AC voltages on the AC side of the converter which is unavoidable due to small asymmetries in the switching control of the converter valves as well as due to small differences in the reaction, i.e. in the switching characteristics of the valves. In case of a grounded star-connected transformer, a DC bias current may also be introduced into the transformer via the transformer neutral, which occurs for example in HVDC systems (High Voltage Direct Current) with ground return. This is called external DC magnetization in the following. The DC magnetization may result in an unwanted saturation of the transformer which can create significant vibration and audible noise or even lead to overcurrents and a burn-out of the transformer.

In an article “Study on Effects of DC Current on Transformers during HVDC Systems Operated in Ground-return Mode”, Proceedings of the XIVth International Symposium on High Voltage Engineering, Bejing, China, Aug. 25-29, 2005, different methods are mentioned of how to mitigate or block a DC current flowing from an HVDC system via earth through a transformer neutral. Such methods are for example to add a resistor or a capacitor in series with the neutral or to inject a reverse compensating DC current via the neutral. In general, a DC current injected via the neutral results in a symmetrical DC magnetization in the transformer, i.e. the same magnetizing current flows in all the phases of the transformer.

In EP 0475 709 B1, an inverter control device is described which is capable of suppressing DC magnetization in a three-phase transformer in order to prevent a transformer burn-out. The transformer is connected to the AC side of an inverter and is not grounded. The inverter control device determines two correcting signals from at least two of the three phase currents on the AC side of the inverter, where the correcting signals are used to correct two reference signals which are input to a control unit to control the AC output voltages of the inverter. The inverter control device thereby regulates the imbalances or asymmetries in the three output AC voltages in such a way that the DC current component of each phase becomes zero.

In modern HVDC systems, voltage source converters are used instead of line commutated converters. In the valves of the voltage source converters, the converter valves are made up of IGBTs in anti-parallel connection with so called free-wheeling diodes while in line commutated converters thyristor valves are used. The free-wheeling diodes commutate freely and with much higher frequency than the line commutated thyristors. Since the commutation of the free-wheeling diodes is not directly controlled, it is more difficult to generate purely symmetrical output voltages with a voltage source converter than with a line commutated converter, which results in asymmetries in the AC voltages on the AC side of an AC/DC or DC/AC converter. This increases the problem with DC bias currents in the transformer connected to the converter which results in higher vibration and increased audible noise. Additionally, harmonics may occur which cause interferences in nearby telephone communication lines.

SUMMARY OF THE INVENTION

It is an object of the present invention to decrease the audible noise and reduce telephone interferences by providing a method and a device to compensate for an asymmetrical DC bias current in a multi-phase transformer, where the transformer is connected between an AC power system and a AC/DC or DC/AC high voltage converter and where at least one side of the transformer is star connected and the star point is grounded via a neutral.

The object is achieved by a method and a device which is arranged to carry out the method.

The invention is based on the recognition of the fact that in case of a star-ground-star connected transformer (Y0/Y), a DC magnetization can be observed as a distinct pattern in the current flowing in the neutral on the transformer side connected to the AC power system, where the pattern shows clear maxima and minima when the magnetic flux in one of the transformer phases reaches a maximum or minimum, respectively. These maxima and minima occur always at the same electrical angle for each of the transformer phases. Asymmetries in the magnetization are seen as differences in the current amplitudes corresponding to the different phases. Further investigations showed that for other transformer configurations with a grounded star connection on the transformer side to the AC power system, different but still distinctive patterns can be seen in the time dependent behaviour of a magnetizing current which is calculated based on measurements of phase currents from one or both transformer sides and, in case a star connected transformer side is grounded, based on the current flowing in the neutral. Accordingly, it is the basic idea of the invention to detect the maxima and minima in the neutral current directly or in the calculated magnetizing current, to assign them to the corresponding transformer phase and to compare if the amplitudes of the maxima and minima for the phases differ or not. From the differences in the amplitudes control signals are then generated phase-wise to adjust the switching of the converter valves in the corresponding converter phase in order to reduce the asymmetries between the phases.

Therefore, according to the invention, for each phase of the AC side of the transformer, i.e. for each phase of the side of the transformer coupled to the AC power system, a current quantity is determined, where the current quantity reflects the time dependent behaviour of the magnetizing current in the phase. In the current quantity, at least a first time interval is determined during which the current quantity reaches a positive maximum and at least a second time interval is determined during which the current quantity reaches a negative maximum. A DC magnetizing quantity is determined from a difference between the amplitude of the positive maximum and the amplitude of the negative maximum and afterwards an asymmetrical quantity is determined from a difference between the amplitudes of the positive and/or negative maxima of at least two of the phases. Finally, a control signal is generated from the asymmetrical quantity and the control signal is provided to a control device of the converter in order to adjust the generation of the AC or DC voltage in the particular phase of the converter which corresponds to the phase of the AC side of the transformer. The phase-wise adjustment of the AC or DC voltage in the converter leads to a reduction or—in the best case—to an elimination of the asymmetries in the magnetizing current flowing in the transformer, which reduces the transformer saturation and thereby the resulting vibration. A reduced vibration decreases on one hand the mechanical stress on the transformer and on the other hand the resulting noise and telephone interferences.

According to a preferred embodiment, the DC magnetizing quantity for each phase is determined by superimposing the positive and the negative maximum of that phase.

According to a further preferred embodiment, the asymmetrical quantity for each phase is determined by superimposing the DC magnetizing quantity of that phase with the DC magnetizing quantities of the other phases. For a transformer with a number of m phases this superimposition is performed by multiplying the DC magnetizing quantity with m reduced by one and by afterwards subtracting the DC magnetizing quantities of the other phases. This ensures that in case of a symmetrical magnetization in all phases, the result of the superimposition will be zero, i.e. all symmetrical DC components will be fully eliminated in the respective DC magnetizing quantity.

In case that the converter is an AC/DC converter, the generation of the DC voltage in the particular phase of the converter is adjusted so that the DC voltage is decreased when the corresponding asymmetrical quantity is positive and increased when the corresponding asymmetrical quantity is negative.

Sine the compensation of asymmetrical DC magnetization is only usefully applied during normal operating conditions and in order not to disturb any protection and control functions in case of a fault in the AC power system, in the transformer or in the converter, it is suggested according to a further embodiment that the control signal is reduced to zero in case of a fault or disturbance.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1shows a typical magnetization curve for a transformer. A usual operating range for the case of no DC magnetization is marked with square points. In comparison,FIG. 2shows how the magnetization curve and with it the operating range ofFIG. 1is shifted when a DC magnetization occurs. It can be clearly seen that the operating range becomes asymmetrical so that the maximum amplitude of the excitation or magnetizing current with positive polarity is much higher than the maximum amplitude of the magnetizing current with negative polarity.

InFIG. 3, a three-phase two-winding transformer1is shown, which is connected on its primary side with the phases R, S and T to an AC power system2and on its secondary side with the phases r, s, and t to an AC/DC high voltage converter42. The transformer1is star-ground-star connected (Y0/Y), i.e. both windings are star connected and the star points are connected to ground, via a first neutral3on the primary side and via a second neutral4on the secondary side. The AC/DC converter42is a voltage source converter which comprises a so called 6-pulse bridge with six converter valves5, where each valve comprises at least one IGBT6in anti-parallel connection with at least one free-wheeling diode7. The 6-pulse bridge is arranged in the commonly known way with three phases8,9and10and two valves5per phase and with the AC side of the converter42connected to the secondary side of the transformer1and with a capacitor11on the DC side of the converter42. On the AC side, the first phase8of the converter42is connected to the first secondary phase r of the transformer, the second phase9of the converter42is connected to the second secondary phase s of the transformer and the third phase10of the converter42is connected to the third secondary phase t of the transformer. A control device12generates six signals, one for each converter valve5, to switch the IGBTs6in the valves on or off in order to generate a desired DC voltage UDC. The control device12transmits the signals via the control lines13to15to the gates of the IGBTs6. The switching signals are generated by a pulse width modulation unit16(PWM). Additionally, a device17to compensate for asymmetrical DC bias current in the transformer1is integrated in the control device12. The device17receives as input via the signal line18measurements of the current Inflowing in the primary neutral3, measured by a current sensor43. If the current in the primary neutral3is not measurable, it can instead be calculated by adding the three phase currents IR, ISand IT. The output of the device17are a first control signal19to adjust the DC voltage generation in the first converter phase8, a second control signal20for the second converter phase9and a third control signal21for the third converter phase10.

The diagram ofFIG. 4shows the current Inflowing in the primary neutral3as well as the magnetic flux FIRand the phase voltage URin the first primary transformer phase R for the case that no DC magnetization occurs in the transformer1. The quantities are depicted over the electrical phase angle in a range between 0° and 360°. As can be seen, a maximum and a minimum occurs in the neutral current Infor each of the three phases R, S and T during the corresponding electrical phase angle ranges R+, R−, S+, S−, T+ and T−. Looking only at the first phase R, it becomes clear that each zero-crossing of the phase voltage URresults in a maximum or minimum in the magnetic flux FIRand that the maxima and minima in the neutral current Inoccur at the same electrical phase angle as the voltage zero-crossings.

FIG. 5shows a device17to compensate for asymmetrical DC bias current in a transformer connected to an AC/DC or DC/AC converter. The device17comprises a first low-pass filter22, a demultiplexer23, six peak detectors24, six sample-and-hold units25, six second low-pass filter26, three first superimposition units27, three second superimposition units28and three controller units29. The device17receives as input signal a current quantity Im, which in case of the Y0/Y connected transformer ofFIG. 3equals to the current Inflowing in the primary neutral3. The current quantity Imis low-pass filtered in the first filter22and the filter output is entered into demultiplexer23. There, the current quantity Im, is demultiplexed into six components according to the six phase angle ranges which correspond to the maxima and minima of the three phases R, S and T. The six current components R+, R−, S+, S−, T+ and T− belong to the following ranges of the electrical phase angle:

R+:60°to 120°,R−:240° to 360°,S+:180° to 240°,S−:0° to 60°,T+:300° to 360°,T−:120° to 180°.

For each of the six current components, the maximum positive or negative amplitude is determined by one of the peak detectors24and saved by one of the sample-and-hold units25. The result is low-pass filtered in one of the second filters26. Afterwards, the signals with the maximum positive and negative amplitudes corresponding to one and the same of the three phases R, S and T are superimposed in one of the first superimposition units27, thereby creating for each phase a DC magnetizing quantity Rin, Sin and Tin, respectively. The three DC magnetizing quantities are then superimposed for each of the three phases in one of the second superimposition units28, where the DC magnetizing quantity of the relating phase is multiplied by two and where the DC magnetizing quantities of the other phases are subtracted afterwards. The second superimposition units28eliminate all symmetrical components in the DC magnetizing quantities Rin, Sin and Tin and deliver for each of the phases a signal AR, ASor AT, respectively, which represents solely the asymmetrical DC magnetization. This signal is also called asymmetrical quantity and is input to one of the controller units29, where a control signal DR, DS or DT, respectively, is generated to influence in the PWM unit16the generation of the six switching signals30for the six converter valves. The control signal DR for the first transformer phase DR thereby adjusts the DC voltage generation in the first converter phase8, and the other two control signals DS and DT function accordingly. In case of the AC/DC converter ofFIG. 3, the control signal is generated in such a way that the DC voltage in the respective converter phase is decreased when the asymmetrical quantity AR, ASor AT, respectively, is positive and increased when the asymmetrical quantity AR, ASor AT, respectively, is negative

InFIG. 6, intermediate signals of device17can be seen for the generation of the DC magnetizing quantity Rin of the first transformer phase R in case of non DC magnetization. It is assumed in this and in all other cases described below that all currents are zero before the electrical phase angle of 0°. Apart from that, the first and second filters22and26are neglected and the controller units29do not show any dynamic or time dependent behaviour. For clarity reasons, the outputs S&HR+ and S&HR− of the sample-and-hold units25corresponding to the first phase R as well as the DC magnetizing quantity Rin are depicted with an offset of −20. Since the maximum positive and negative amplitudes in the current components R+ and R− for the first phase R have the same absolute value, the DC magnetizing quantity Rin becomes zero eventually.

The same applies to the DC magnetizing quantities of the second and the third transformer phases, Sin and Tin. This can be seen inFIG. 7, where the two signals are depicted with an offset of +20. The control signals for the three phases, DR, DS and DT, are shown inFIG. 7with a negative offset of −25. As could be expected from the absence of any DC magnetization, the control signals settle at zero.

FIGS. 8 to 10show the same signals asFIGS. 4,6and7, but for an external DC magnetization when a DC bias current is injected via earth through the transformer neutrals. InFIG. 9, the DC magnetizing quantity Rin and the outputs S&HR+ and S&HR− of the sample-and-hold units25are depicted with an offset of −30.FIG. 10shows the DC magnetizing quantities Sin and Tin with an offset of +20.FIG. 8visualizes again the relationship between zero-crossings in the phase voltage URand corresponding peaks in the neutral current In. For each of the three transformer phases R, S and T, the same amplitude occurs in the neutral current In, which means that the DC magnetization is symmetrical between the phases. Accordingly, the control signals DR, DS and DT settle again at zero (FIG. 10). The DC magnetizing quantities of the three phases Rin, Sin and Tin remain at a constant level different from zero, but they all have the same value.

InFIGS. 11 to 15, the input, intermediate and output signals of device17are shown for the case of a DC magnetization between the phases R and T. The neutral current Innow shows different amplitudes in the different electrical phase angle ranges (FIG. 11), i.e. the DC magnetization is asymmetrical between the phases. For the first phase R a considerable positive current component R+ can be seen (FIG. 12), while the negative current component R− is almost negligible. The magnetic quantity Rin, which as well as the quantities S&HR+, S&HR− and PDR− is depicted with an offset of −35, shows an almost constant characteristics at the maximum positive current amplitude. For the third phase T, the situation is just the opposite, i.e. the positive current component T+ is almost negligible and the negative current component T− dominates the magnetic quantity Tin (FIG. 14). InFIG. 14, the quantities PDT+, PDT−, S&HT+, S&HT− and Tin are shown with an offset of −10. In the second phase S, the DC magnetization is equal in positive and negative direction, which can be seen in the equal amplitudes of the positive and the negative current components S+ and S− (FIG. 13). Accordingly, the magnetic quantity Sin settles at zero value. The quantities PDS+, PDS−, S&HS+, S&HS− and Sin have an offset of −30 inFIG. 13. The resulting control signals for the three phases are shown inFIG. 15. The control signal DR for the first phase R settles at a positive value, which results in a reduction of the DC voltage in phase8of the converter. The control signal DS for the second phase S settles at zero, and the control signal DT for the third phase T settles at a negative value leading to an increase in the DC voltage in phase10of the converter.

FIGS. 16 to 20belong to the case where a DC magnetization occurs between all three phases R, S and T. The first phase R shows a distinct positive current amplitude R+ and the second and third phases S and T both have a higher negative (S− and T−) than positive (S+ and T+) current amplitude. InFIGS. 17 and 18, the offsets are the same as inFIGS. 12 and 13. InFIG. 19, the offset for the quantities PDT+, PDT−, S&HT+, S&HT− and Tin is −20. The three control signals which are shown inFIG. 20request a decrease in the DC voltage of converter phase8and an increase in the DC voltages of converter phases9and10.

The transformer31ofFIG. 21differs from transformer1ofFIG. 3in that the secondary winding r, s and t is delta connected. It is assumed that the three phase currents Ir, Isand Itinside the delta connection are measured as well as the current INflowing in the first neutral32. The primary winding of the transformer31has a number of n1turns and the secondary winding has a number of n2turns. For the transformer31, the current quantity Im, which is input to device17can be determined according to the following equation:

The current Inflowing in the first neutral32is to be reduced by the sum of the three internal delta phase currents Ir, Isand lt, where the phase currents are to be multiplied by the turn ratio n2/n1. This is to be done for compensating the part of the primary zero-sequence current which corresponds to the current circulating in the secondary delta winding. The determination of the current quantity Im, according to equation (1) could either be performed externally or it could be integrated into device17. The compensation of DC magnetization works then in the same way as described above. In case the transformer31is provided with a tap changer, the value of the turn ratio n2/n1needs to be adjusted according to the current tap changer position, i.e. the actual turn ratio is to be used.

If the neutral32of the primary winding of transformer31is not grounded, the current quantity can be determined for each of the phases separately, according to:

The three current quantities IA, IBand lcare then—in original or filtered form—input to the demultiplexer23. The demultiplexer23decomposes the first current quantity IAinto the current components R+ and R−, according to the two phase angle ranges which correspond to the maxima and minima of the first phase R. In the same way are the current components S+ and S− for the second phase S derived from the second current quantity IB, and the current components T+ and T− for the third phase T from the third current quantity IC(see also description ofFIG. 24).

FIG. 22shows a three-phase three-winding transformer33, where the primary side, which is connected to the AC power system2, is again star connected and the star point is grounded via a first neutral34. The number of turns of the first winding is n1. On the secondary side, which is connected to a converter35, the transformer33has two windings. The second winding is delta connected and has a number of turns n2. The third winding has a number of n3turns and it is star connected, with the star point being grounded via a second neutral36. The converter35is an AC/DC converter arranged as a 12-pulse bridge. The 12-pulse bridge consists of two 6-pulse bridges37and38which are connected in series. The 6-pulse bridges37and38comprise the same components as the 6-pulse bridge ofFIG. 3and are connected in the same way to one of the second and third winding of transformer33, respectively.

Device17can be used also for the configuration ofFIG. 22to compensate for DC magnetization in the transformer33. Assuming that again the internal currents Ir, Isand Itare measured in the second winding and that the currents Inand Insflowing in the first and the second neutral34and36, respectively, are determined, too. In that case, the current quantity Im, can be determined by equation (2):

Accordingly, the current Inflowing in the first neutral34is not only to be reduced by the sum of the corrected internal delta phase currents Ir, Isand Itbut also by the corrected current Insflowing in the second neutral36, where the current Insin the second neutral is corrected by multiplying it with the turn ration n3/n1.

In case that, apart from the currents in the first and the second neutral, only the external phase currents Iu, Ivand Iw(FIG. 22) are measurable in the delta connected second winding, additionally the phase currents IR, ISand ITin the first winding and the phase currents Ia, Iband Icin the third winding need to be known in order to determine the magnetic quantities Rin, Sin and Tin.FIG. 23shows a device39to compensate for asymmetrical DC bias current in the transformers31or33, where only external phase currents Iu, Iv, Iwoutside the delta winding are measurable. The device39comprises generally the same elements as device17ofFIG. 5. Only the current quantity Im, from which the positive and negative current components R+ to T− are determined are now replaced by three current quantities IA, IBand IC, one for each phase. Device39receives as input signals measurements from the currents Inand Insflowing in the first and the second transformer neutrals34and36as well as measurements from the phase currents of each winding of the transformer. In case of the two-winding transformer31, the inputs for the current Insin the second neutral and for the phase currents Ia, Iband Icof the third winding are simply omitted and their values in the equations (3) to (5) below are replaced by zero. In general it can be noted that if one or more of the transformer circuits ofFIG. 22are open, the corresponding quantities in equations (3) to (5) are simply set to zero. For example, it is possible that the star point in the primary winding is left unconnected and thereby free floating. Then the corresponding current Inis set to zero. In the current processing unit44of device39, the input currents are used to generate the three current quantities IA, IBand ICbased on the following equations:

FIG. 24shows the magnetic flux and the phase voltage of the first phase R in the first winding of transformer33together with the three current quantities IA, IBand ICin case of a DC magnetization between phases R and T. For each phase R, S and T, it is again possible to detect one positive and one negative current maximum during the corresponding electrical phase angle ranges. The maxima and minima are emphasized by small circles. As becomes clear fromFIG. 24, the demultiplexer45ofFIG. 23creates from each of the current quantities IA, IBand ICtwo current components according to the following phase angle ranges:

From IA:R+:60° to 120°,R−:240° to 360°,From IB:S+:180° to 240°,S−:0° to 60°,From IC:T+:300° to 360°,T−:120° to 180°.

The current components are then used in the same way as described above to generate the magnetic quantities Rin, Sin and Tin, which are then superimposed in order to eliminate all symmetrical DC magnetization components. From the results of the superimposition three control signals DR, DS and DT are created which influence via a PWM control unit the generation of DC voltage UDC1−UDC2in the respective phase39,40or41of converter35(DR—phase39; DS—phase40; DT—phase41).

In case of a two-winding transformer according toFIG. 21with measurement of external phase currents Iu, Ivand Iwon the secondary side, equations (3) to (5) are simplified to:

The configurations and devices ofFIGS. 3,5,21,22and23and their combinations can be used in modern HVDC power transmission systems. Since problems of noise generation and telephone disturbances caused by DC magnetization in the converter transformers are especially observed in HVDC systems with voltage source converters, the method and the device according to the invention can be advantageously used in particular in such an HVDC system.