Title of Invention	METHOD FOR DETERMINING THE CURRENT ZERO-CROSSING IN A CONVERTER
Abstract	ABSTRACT A converter is operated with at least two static converter branches, wherein the current alternately flows via at least a first and via at least a second current converter branch. The timing of a current zero-crossing in the converter is determined in that the current is measured by means of a device for determining the timing of the current zero-crossing, the measured current value is compared with a current threshold value in the device and by means of the time at which the current value reaches the value of the current threshold value before the current zero-crossing, a probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing is calculated by means of the device.

Title of Invention

METHOD FOR DETERMINING THE CURRENT ZERO-CROSSING IN A CONVERTER

Abstract

ABSTRACT A converter is operated with at least two static converter branches, wherein the current alternately flows via at least a first and via at least a second current converter branch. The timing of a current zero-crossing in the converter is determined in that the current is measured by means of a device for determining the timing of the current zero-crossing, the measured current value is compared with a current threshold value in the device and by means of the time at which the current value reaches the value of the current threshold value before the current zero-crossing, a probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing is calculated by means of the device.

Full Text	Method for determining the current zero-crossing in a converter Description Technical area The invention relates to the area of power electronics. It concerns a method for determining the timing of the current zero-crossing in a converter with the characteristics of the preamble of Claim 1, a device for performing this method and a converter. Prior art Current-controlled converters serve to convert an input current into an alternating current having one or multiple phases. The converters contain two static converter branches via which a current l(t) can flow. Each of the static converter branches can be switched to a conducting state or to a blocking state through a current valve. A first current valve in the first static converter branch is switched to conductive state during a half-wave of the current where the current is either positive or negative throughout. When the current changes its sign the first current valve is switched to blocking state and a second valve in the second static converter branch, which, during the period, in which the first current valve was in conductive state, was in the blocking state, is now switched to conductive state. However, if the transition from one static converter branch to the other does not occur exactly at the time at which the current changes its sign, an overlap range occurs in which both static converter branches carry current or a range, in which no current flows in any of the static converter branches. This overlap range or the range in which no current flows in any of the static converter branches however leads to major losses of energy and transients. To determine the time at which the current valves are to be switched, complex methods have been developed which include measurement of the current. Such a method is described in a document by S. Valiviita, Ovaska S.J., Kyyra J. ("adaptive signal processing system for accurate zero-crossing detection of cycloconverter phase currents"; IEEJ 1997 - PCC Nagoka, p. 467-472). in the method, a low frequency prefilter is utilised to filter undesirable low-frequency components from the current signal and a sinusoidal curve of the current signal is reconstructed, from which the time of the current zero-crossing can subsequently be predicted. However, this method is involved and costly. Errors in the current measurements and the current offset reduce the reliability and accuracy of determining the time of the current zero-crossing. US 6,535,402 describes a method for determining the current zero-crossing for a dead time compensation in a converter, wherein a constant additional current is added to the current from which the current zero-crossing is to be determined. The addition of the additional current creates an asymmetry of the periods of the positive and negative half-wave. The difference of the periods of the two half-waves is used to correct from the measured current zero-crossing to the presumed true timing. Presentation of the invention The object of the invention is to create a method and a device of the kind mentioned at the outset, which allow accurate and simpler prediction of the timing of the current zero-crossing in a converter. This object according to the invention is solved through a method for determining the current zero-crossing in a converter with the features of patent claim 1, a device for determining the timing of the current zero-crossing with the features of patent claim 8 and a converter with the features of patent claim 9. With the method according to the invention the current in the converter is measured by means of a device for determining the timing of the current zero-crossing, the measured current value compared in the device with a current threshold value and, by means of the timing, at which the current value reaches the value of the current threshold value before the current zero-crossing, a period At from the reaching of the current threshold value to the reaching of the current zero-crossing determined by means of the device. Since only one measurement has to be determined and only simple calculations are necessary, this method can be carried out easily. With a sinusoidal or approximately sinusoidal curve of the current, the current can be divided into two half-waves of which each half-wave comprises the period in which the current is either continuously greater or continuously smaller than zero. From the currents measured in a positive half-wave or the currents measured in a negative half-wave a maximum amplitude of the current of the half-wave lmax is determined and the maximum amplitude of the current is used for determining the timing of the current zero-crossing. Through determining the maximum amplitude of the current of a half-wave the accuracy of the method is increased. The probable period At from the reaching of the current threshold value lset to the reaching of the current zero-crossing can be determined upon the reaching of the current threshold value before the current zero-crossing from an amount of the current threshold value lset\| , the maximum amplitude of the current lmax and a set frequency co of the current by means of the equation: At = u> * , wherein a 'max negative current threshold value lsetn is substituted as current threshold value lset in the negative half-wave and a positive current threshold lset, p in the positive half-wave. The advantage of this embodiment is that only few easily determined measurements enter the determination of the current zero-crossing and the determination of the timing of the current zero-crossing therefore takes place by means of a simple equation and thus quickly. Additional advantageous versions and embodiments are evident from the dependent patent claims. Brief description of the drawings The method according to the invention and the object of the invention are explained in the following by means of a preferred exemplary embodiment which is shown in the enclosed drawing. The figure shows a curve of the current over the time in a converter and switching characteristics of a first current valve in a first static converter branch. Ways for carrying out the invention In the following example a converter with two static converter branches is described, however the invention is also applicable to single or multi-phase converters with at least two static converter branches. The figure shows the current curve over the time and the switching characteristics of a first current valve in a first static converter branch. The current in the converter is measured by means of a device for determining the timing of the current zero-crossing of the converter. By means of curve 1 it is shown that starting from the negative half-wave the maximum amplitude of the current lmax of the negative half-wave is determined from the measured current values of the converter (not shown in the figure for reasons of clarity). Establishment is performed with a popular calculation method. Upon the reaching a negative current threshold value lset, n after passing the maximum amplitude of the current and before the current zero-crossing 2 a probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing is determined. In the positive half-wave the maximum amplitude of the current lmax is determined and upon the reaching of a positive current threshold value lset, p, after passing through the maximum amplitude of the current and before the current zero-crossing 3, the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing is determined. The instantaneous current I(t) is obtained from the maximum amplitude of the current lmax, the frequency u) and the time t according to the following equation: l(t) = lmax sin(urt) With small values for (u>t) it is possible to substitute sin(tot)\| «cot .This simplifies the equation for the instantaneous current and the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing can be determined for an amount of the current value \| l(t)\|: Upon the reaching a given current threshold value lset before the current zero-crossing, the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing can be determined with the above stated equation and an amount of the current threshold value lset as follows. Where as current threshold value lset, lSet, n \|S substituted in the negative half-wave and lset, p in the positive half-wave. Curve 1 shows that due to establishing the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing, the switching over between two static converter branches is triggered with a delay by the period At upon the reaching of the given current threshold value. For reasons of clarity, only the state of the first static converter branch is shown in the figure. In the negative half-wave the first static converter branch is in conductive state 11 and the second static converter branch in blocking state. After expiration of the period At after the reaching of the given negative current threshold value lset, n the first static converter branch is switched to blocking state 12 and the second static converter branch into conductive state. In the positive half-wave, after expiration of the period At after the reaching of the given positive current threshold value lseti p , the first static converter branch is again switched to conductive state 11 and the second static converter branch to blocking state. The current threshold value Iset, n for the negative half-wave and Iset, p for the positive half-wave can be given as a function of operation. If the current is affected by noise the value can advantageously be selected so large that the current value can be clearly distinguished from noise. On the other hand, if the current value should be so low that the error made through the approximation of sin(u>t)\| « u)t remains small. The maximum amplitude of the current can be re-determined within each half-wave but it is also conceivable to re-determine a maximum amplitude of the current only for every second or a maximum of every second half-wave. If the maximum amplitude is determined within a half-wave and this value utilised for determining the period to the directly following or next but one current zero-crossing, one obtains a particularly accurate result If the maximum amplitude of the current is only used for determining the period to a later current zero-crossing or if the maximum amplitude is not re-determined for each half-wave, the method can be carried out particularly rapidly. The method also becomes particularly simple if the. amount of the negative current threshold value l-set n is equal to the amount of the positive current threshold value lsetp . Then, a constant can be determined from the product of the frequency to and the current threshold value and in each half-wave, only the quotient from the constant and the maximum amplitude of the current of a half-wave need to be calculated to determine the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing. The method for determining the current zero-crossing according to the invention in a converter can be carried out by means of a device comprising a means for measuring the current and a data processing system for comparing the current threshold values and for determining the probable period from the reaching of the current threshold value to the reaching of the current zero-crossing. With the method, the current is measured by means of the device, the measured current value compared in the data processing system with a current threshold value lset and by means of the timing, at which the current reaches the value of the current threshold value before the current zero-crossing, a probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing determined by means of the data processing system. A computer program for determining the current zero-crossing can be loaded into the data processing system and executed in the data processing system. When the computer program is executed, it compares the measured current value with a current threshold value lset and, by means of the timing at which the current reaches the value of the current threshold value before the current zero-crossing, determines a probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing. In another version of the method the current zero-crossing in a converter is determined by means of a device which comprises a means for measuring the current and an electric switching circuit for comparing the current threshold value and for determining the probable period from the reaching of the current threshold value to the reaching of the current zero-crossing. In converters, a valve can be installed before each static converter branch through which a static converter branch can be switched to a blocking state or a conductive state. If such a valve performs the switching over of the respective static converter branch with a time delay from the time at which switching over is triggered, the time delay to switching over can be taken into account such that the current threshold value is selected so large that the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing is greater or equal to the time delay to switching over. The time delay can be taken into account in determining the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing such that the respective valve at a time before a current zero-crossing, which corresponds to the time delay, is switched and the valve at the time of the current zero-crossing blocks or transmits. The method for operating a converter described above can be applied in a converter in which a continuous current flow is demanded and in which the current flow is not to be interrupted through an actively performed operation such as the switching of a valve. A further application of the method occurs in a current-controlled converter. In a current-controlled converter the current which is fed into the converter behaves like a current source and a current quantity independent of the output voltage is supplied at the output of the converter (like a power source). The method can also be applied in a direct converter in which alternating current of a given frequency is directly converted into alternating current of another frequency, without the alternating current being converted into direct current in the meantime. PATENT CLAIMS 1. A method for determining the current zero-crossing in a converter with at least two static converter branches, wherein the current alternately flows via at least a first and via at least a second static converter branch and with a device for determining the timing of the current zero-crossing, characterized in that the current is measured by means of the device, a maximum amplitude of the current lmaxis determined from the measured current values in a positive and / or a negative half-wave, the measured current value in the device is compared with a current threshold value lset and, by means of the timing at which the current reaches the value of the current threshold value lset before the current zero-crossing and the value of the maximum amplitude of the current lmaxof a half-wave, a probable period At from the reaching of the current threshold value lset to the reaching of the current zero-crossing is determined by means of the device. 2. The method according to claim 1, characterized in that the current has a sinusoidal curve. 3. The method according to claim 1 or 2, characterized in that the probable period At from the reaching of the current threshold value lset to the reaching of the current zero-crossing is determined within the same half-wave from which the maximum amplitude of the current lmax is determined. 4. The method according to any one of the claims 2 or 3, characterized in that the probable period At from the reaching of the current threshold value lset to the reaching of the current zero-crossing is determined from an amount of the current threshold value \| iset \| , the maximum amplitude of the current lmax and a frequency w of the current by means of the equation: wherein a negative current threshold value lset n is substituted as current threshold value lset in the negative half-wave and a positive current threshold value lset, p in the positive half-wave. 5. The method according to claim 4, characterized in that an amount of the negative current threshold value lset^n \| and an amount of the positive current threshold value j \seXp are equal in size. 6. The method according to any one of the claims 1 to 5, characterized in that after expiration of the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing in at least a static converter branch a current valve each is switched, each of which switches over a static converter branch between a conductive state and a blocking state. 7. The method according to claim 6, characterized in that the at least one current valve blocks or transmits the respective static converter branch after a switching operation only after a time delay and the current threshold value lset is so great that the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing is greater or equal to the time delay to the blocking or transmitting and the time delay is taken into consideration in establishing the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing such that the respective valve at a time before a current zero-crossing, which corresponds to the time delay, is switched and the valve at the time of the current zero-crossing, blocks or transmits. 8. A device for determining the timing of the current zero-crossing, characterized in that the device for determining the timing of the current zero-crossing comprises means for measuring the current at the input of a static converter branch, means for comparing the measured current value with a current threshold value and means for establishing the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing by means of the time at which the current reaches the value of the current threshold value before the current zero-crossing and the value of the maximum amplitude of the current lmaxOf a half-wave. 9. A converter, at least containing two static converter branches, wherein the current alternately flows via a first and via a second static converter branch, characterized in that the converter comprises a device for determining the timing of the current zero-crossing according to claim 8.

Full Text

Method for determining the current zero-crossing in a converter
Description Technical area
The invention relates to the area of power electronics. It concerns a method for determining the timing of the current zero-crossing in a converter with the characteristics of the preamble of Claim 1, a device for performing this method and a converter.
Prior art
Current-controlled converters serve to convert an input current into an alternating current having one or multiple phases. The converters contain two static converter branches via which a current l(t) can flow. Each of the static converter branches can be switched to a conducting state or to a blocking state through a current valve. A first current valve in the first static converter branch is switched to conductive state during a half-wave of the current where the current is either positive or negative throughout. When the current changes its sign the first current valve is switched to blocking state and a second valve in the second static converter branch, which, during the period, in which the first current valve was in conductive state, was in the blocking state, is now switched to conductive state. However, if the transition from one static converter branch to the other does not occur exactly at the time at which the current changes its sign, an overlap range occurs in which both static converter branches carry current or a range, in which

no current flows in any of the static converter branches. This overlap range or the range in which no current flows in any of the static converter branches however leads to major losses of energy and transients.
To determine the time at which the current valves are to be switched, complex methods have been developed which include measurement of the current. Such a method is described in a document by S. Valiviita, Ovaska S.J., Kyyra J. ("adaptive signal processing system for accurate zero-crossing detection of cycloconverter phase currents"; IEEJ 1997 - PCC Nagoka, p. 467-472). in the method, a low frequency prefilter is utilised to filter undesirable low-frequency components from the current signal and a sinusoidal curve of the current signal is reconstructed, from which the time of the current zero-crossing can subsequently be predicted.
However, this method is involved and costly. Errors in the current measurements and the current offset reduce the reliability and accuracy of determining the time of the current zero-crossing.
US 6,535,402 describes a method for determining the current zero-crossing for a dead time compensation in a converter, wherein a constant additional current is added to the current from which the current zero-crossing is to be determined. The addition of the additional current creates an asymmetry of the periods of the positive and negative half-wave. The difference of the periods of the two half-waves is used to correct from the measured current zero-crossing to the presumed true timing.
Presentation of the invention
The object of the invention is to create a method and a device of the kind mentioned at the outset, which allow accurate and simpler prediction of the timing of the current zero-crossing in a converter.
This object according to the invention is solved through a method for determining
the current zero-crossing in a converter with the features of patent claim 1, a

device for determining the timing of the current zero-crossing with the features of patent claim 8 and a converter with the features of patent claim 9.
With the method according to the invention the current in the converter is measured by means of a device for determining the timing of the current zero-crossing, the measured current value compared in the device with a current threshold value and, by means of the timing, at which the current value reaches the value of the current threshold value before the current zero-crossing, a period At from the reaching of the current threshold value to the reaching of the current zero-crossing determined by means of the device. Since only one measurement has to be determined and only simple calculations are necessary, this method can be carried out easily.
With a sinusoidal or approximately sinusoidal curve of the current, the current can be divided into two half-waves of which each half-wave comprises the period in which the current is either continuously greater or continuously smaller than zero. From the currents measured in a positive half-wave or the currents measured in a
negative half-wave a maximum amplitude of the current of the half-wave lmax is
determined and the maximum amplitude of the current is used for determining the timing of the current zero-crossing. Through determining the maximum amplitude of the current of a half-wave the accuracy of the method is increased.
The probable period At from the reaching of the current threshold value lset to the
reaching of the current zero-crossing can be determined upon the reaching of the current threshold value before the current zero-crossing from an amount of the
current threshold value lset| , the maximum amplitude of the current lmax and a
set
frequency co of the current by means of the equation: At = u> * , wherein a
'max negative current threshold value lsetn is substituted as current threshold value lset
in the negative half-wave and a positive current threshold lset, p in the positive half-wave. The advantage of this embodiment is that only few easily determined
measurements enter the determination of the current zero-crossing and the

determination of the timing of the current zero-crossing therefore takes place by means of a simple equation and thus quickly.
Additional advantageous versions and embodiments are evident from the dependent patent claims.
Brief description of the drawings
The method according to the invention and the object of the invention are explained in the following by means of a preferred exemplary embodiment which is shown in the enclosed drawing. The figure shows a curve of the current over the time in a converter and switching characteristics of a first current valve in a first static converter branch.
Ways for carrying out the invention
In the following example a converter with two static converter branches is described, however the invention is also applicable to single or multi-phase converters with at least two static converter branches.
The figure shows the current curve over the time and the switching characteristics of a first current valve in a first static converter branch. The current in the converter is measured by means of a device for determining the timing of the current zero-crossing of the converter. By means of curve 1 it is shown that starting from the
negative half-wave the maximum amplitude of the current lmax of the negative half-wave is determined from the measured current values of the converter (not shown in the figure for reasons of clarity). Establishment is performed with a popular
calculation method. Upon the reaching a negative current threshold value lset, n
after passing the maximum amplitude of the current and before the current zero-crossing 2 a probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing is determined. In the positive half-wave
the maximum amplitude of the current lmax is determined and upon the reaching of
a positive current threshold value lset, p, after passing through the maximum
amplitude of the current and before the current zero-crossing 3, the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing is determined.

The instantaneous current I(t) is obtained from the maximum amplitude of the current lmax, the frequency u) and the time t according to the following equation: l(t) = lmax sin(urt)
With small values for (u>t) it is possible to substitute sin(tot)| «cot .This simplifies
the equation for the instantaneous current and the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing
can be determined for an amount of the current value | l(t)|:

Upon the reaching a given current threshold value lset before the current zero-crossing, the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing can be determined with the above stated
equation and an amount of the current threshold value lset as follows.

Where as current threshold value lset, lSet, n |S substituted in the negative half-wave and lset, p in the positive half-wave.
Curve 1 shows that due to establishing the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing, the switching over between two static converter branches is triggered with a delay by the period At upon the reaching of the given current threshold value. For reasons of clarity, only the state of the first static converter branch is shown in the figure. In the negative half-wave the first static converter branch is in conductive state 11 and the second static converter branch in blocking state. After expiration of the
period At after the reaching of the given negative current threshold value lset, n the
first static converter branch is switched to blocking state 12 and the second static converter branch into conductive state. In the positive half-wave, after expiration of
the period At after the reaching of the given positive current threshold value lseti p ,

the first static converter branch is again switched to conductive state 11 and the second static converter branch to blocking state.
The current threshold value Iset, n for the negative half-wave and Iset, p for the
positive half-wave can be given as a function of operation. If the current is affected by noise the value can advantageously be selected so large that the current value can be clearly distinguished from noise. On the other hand, if the current value
should be so low that the error made through the approximation of sin(u>t)| « u)t remains small.
The maximum amplitude of the current can be re-determined within each half-wave but it is also conceivable to re-determine a maximum amplitude of the current only for every second or a maximum of every second half-wave. If the maximum amplitude is determined within a half-wave and this value utilised for determining the period to the directly following or next but one current zero-crossing, one obtains a particularly accurate result If the maximum amplitude of the current is only used for determining the period to a later current zero-crossing or if the maximum amplitude is not re-determined for each half-wave, the method can be carried out particularly rapidly.
The method also becomes particularly simple if the. amount of the negative current threshold value l-set n is equal to the amount of the positive current threshold
value lsetp . Then, a constant can be determined from the product of the
frequency to and the current threshold value and in each half-wave, only the quotient from the constant and the maximum amplitude of the current of a half-wave need to be calculated to determine the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing.
The method for determining the current zero-crossing according to the invention in a converter can be carried out by means of a device comprising a means for measuring the current and a data processing system for comparing the current threshold values and for determining the probable period from the reaching of the current threshold value to the reaching of the current zero-crossing. With the method, the current is measured by means of the device, the measured current

value compared in the data processing system with a current threshold value lset
and by means of the timing, at which the current reaches the value of the current threshold value before the current zero-crossing, a probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing determined by means of the data processing system.
A computer program for determining the current zero-crossing can be loaded into the data processing system and executed in the data processing system. When the computer program is executed, it compares the measured current value with a
current threshold value lset and, by means of the timing at which the current
reaches the value of the current threshold value before the current zero-crossing, determines a probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing.
In another version of the method the current zero-crossing in a converter is determined by means of a device which comprises a means for measuring the current and an electric switching circuit for comparing the current threshold value and for determining the probable period from the reaching of the current threshold value to the reaching of the current zero-crossing.
In converters, a valve can be installed before each static converter branch through which a static converter branch can be switched to a blocking state or a conductive state. If such a valve performs the switching over of the respective static converter branch with a time delay from the time at which switching over is triggered, the time delay to switching over can be taken into account such that the current threshold value is selected so large that the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing is greater or equal to the time delay to switching over. The time delay can be taken into account in determining the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing such that the respective valve at a time before a current zero-crossing, which corresponds to the time delay, is switched and the valve at the time of the current zero-crossing blocks or transmits.

The method for operating a converter described above can be applied in a converter in which a continuous current flow is demanded and in which the current flow is not to be interrupted through an actively performed operation such as the switching of a valve.
A further application of the method occurs in a current-controlled converter. In a current-controlled converter the current which is fed into the converter behaves like a current source and a current quantity independent of the output voltage is supplied at the output of the converter (like a power source).
The method can also be applied in a direct converter in which alternating current of a given frequency is directly converted into alternating current of another frequency, without the alternating current being converted into direct current in the meantime.

PATENT CLAIMS
1. A method for determining the current zero-crossing in a converter with at
least two static converter branches, wherein the current alternately flows
via at least a first and via at least a second static converter branch and with
a device for determining the timing of the current zero-crossing,
characterized in that the current is measured by means of the device, a
maximum amplitude of the current lmaxis determined from the measured
current values in a positive and / or a negative half-wave, the measured
current value in the device is compared with a current threshold value lset
and, by means of the timing at which the current reaches the value of the
current threshold value lset before the current zero-crossing and the value
of the maximum amplitude of the current lmaxof a half-wave, a probable
period At from the reaching of the current threshold value lset to the
reaching of the current zero-crossing is determined by means of the device.
2. The method according to claim 1, characterized in that the current has a sinusoidal curve.
3. The method according to claim 1 or 2, characterized in that the probable period At from the reaching of the current threshold value lset to the reaching of the current zero-crossing is determined within the same half-wave from which the maximum amplitude of the current lmax is determined.
4. The method according to any one of the claims 2 or 3, characterized in that the probable period At from the reaching of the current threshold value lset to the reaching of the current zero-crossing is determined from an amount of the current threshold value | iset | , the maximum amplitude of the
current lmax and a frequency w of the current by means of the equation:
wherein a negative current threshold value lset n is

substituted as current threshold value lset in the negative half-wave and a positive current threshold value lset, p in the positive half-wave.
5. The method according to claim 4, characterized in that an amount of the
negative current threshold value lset^n | and an amount of the positive
current threshold value j \seXp are equal in size.
6. The method according to any one of the claims 1 to 5, characterized in that after expiration of the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing in at least a static converter branch a current valve each is switched, each of which switches over a static converter branch between a conductive state and a blocking state.
7. The method according to claim 6, characterized in that the at least one current valve blocks or transmits the respective static converter branch after a switching operation only after a time delay and the current threshold
value lset is so great that the probable period At from the reaching of the
current threshold value to the reaching of the current zero-crossing is greater or equal to the time delay to the blocking or transmitting and the time delay is taken into consideration in establishing the probable period At from the reaching of the current threshold value to the reaching of the current zero-crossing such that the respective valve at a time before a current zero-crossing, which corresponds to the time delay, is switched and the valve at the time of the current zero-crossing, blocks or transmits.
8. A device for determining the timing of the current zero-crossing,
characterized in that the device for determining the timing of the current
zero-crossing comprises means for measuring the current at the input of a
static converter branch, means for comparing the measured current value
with a current threshold value and means for establishing the probable
period At from the reaching of the current threshold value to the reaching of
the current zero-crossing by means of the time at which the current

reaches the value of the current threshold value before the current zero-crossing and the value of the maximum amplitude of the current lmaxOf a half-wave.
9. A converter, at least containing two static converter branches, wherein the current alternately flows via a first and via a second static converter branch, characterized in that the converter comprises a device for determining the timing of the current zero-crossing according to claim 8.

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Patent Number

268267

Indian Patent Application Number

1270/CHENP/2007

PG Journal Number

35/2015

Publication Date

28-Aug-2015

Grant Date

24-Aug-2015

Date of Filing

27-Mar-2007

Name of Patentee

ABB RESEARCH LTD.

Applicant Address

AFFOLTERNSTRASSE 52, CH-8050 ZURICH, SWITZERLAND

Inventors:

#	Inventor's Name	Inventor's Address
1	AKDAG, ALPER	MOOSSTRASSE 11, CH-5406 BADEN-RUTIHOF , SWITZERLAND
2	STEFANUTTI , PHILLIPPE	IMPASSE DES IRONDELLES 15, F-74330 CHOISY, FRANCE
3	MEYSENC, LUC	14 RUE CHANCELIERE ,F-38120 LE FONTANIL , FRANCE
4	HUGO, NICOLAS	RUE DES GARES 11, CH-1201 GENEVE, SWITZERLAND

PCT International Classification Number

H02M 7/42

PCT International Application Number

PCT/CH05/00556

PCT International Filing date

2005-09-27

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	04405635.6	2004-10-08	EUROPEAN UNION