DE102022200909A1 - Apparatus for determining a compensated RMS value and method for determining a compensated RMS value - Google Patents

Abstract

The invention relates to a device for determining a compensated effective value Ieff of an alternating current I, comprising a current transformer and a digital evaluation unit, a corrupted effective value Ieff,g being calculated from the series of the secondary current signal measured by the current transformer, the total number of existing Sampled values NTotal and the number of sampled values considered invalid NInvalid are determined by the digital evaluation unit and a correction value k(A) for correcting the falsified effective value Ieff,g is calculated using the proportion of sampled values considered invalid in the total number of available sampled values A = NInvalid/Ntotal and using this correction value k(A) the compensated effective value Ieff.

Description

The invention relates to a device for determining a compensated effective value and a method for determining a compensated effective value.

Typically, effective values of alternating currents can be determined using a current transformer. This is also possible when the current transformer is in saturation and the measured values of the current transformer are then fed to a digital evaluation unit, for example a microcontroller, where an effective value is calculated using a correction method. Typically, such correction methods and devices can process and correct sinusoidal alternating currents in current transformers in saturation; application to other current forms, for example square-wave, is often also possible.

Current transformers can be used to measure alternating currents, since they convert an alternating current flowing on the primary side linearly into a secondary current within a certain working range. This secondary current is well suited for feeding it to an evaluation unit. When using a current transformer in this way, the advantages are the galvanic isolation, the setting of a suitable secondary current level and, for example, current limitation.

The linear working range of a current transformer is limited. Such an upper current limit, up to which the current transformer converts alternating currents linearly, results from the magnetic core of the current transformer reaching saturation at high currents. As soon as the core is saturated during transmission, the current is no longer transmitted linearly. Qualitatively, such behavior is in 2 shown, in which the effective value of the secondary current is plotted against the effective value of the primary current as a characteristic. Ideally, the rms value of the secondary current would be linear over the rms value of the primary current; when the linear range is left, this curve drops down to lower rms values of the secondary current, as is shown in FIG 2 shown to the right of the dashed line. To the left of the dashed line is the linear transmission range of the current transformer, to the right is the non-linear saturation range.

In the saturation range, the rms value of the secondary current is smaller than its set value. The evaluation unit is therefore supplied by the current transformer with a secondary current that is too low, and from this it would be concluded that the primary current was too low. If, for example, a protective function depends on this evaluation, this can lead to a dangerous failure.

In order to be able to understand the buckling of the transfer characteristic, the time course of the secondary current in the saturation range is meaningful. It can be seen that a certain proportion of the current signal is transmitted linearly even in the saturation range. From a certain point in time, however, the secondary signal drops very quickly to a value close to 0. Secondary current then no longer flows until there is a change of sign on the primary side, for example at the start of the next half-wave in the case of sinusoidal alternating currents. Only with the change of sign does the magnetic core become magnetized.

For a sinusoidal secondary current, the behavior is in 3 shown schematically. 3 shows the secondary current with saturation of the magnetic core, resolved over time and ideally as a sinusoidal curve. The very fast drop to the value close to 0 is an example according to the representation in 3 approximately at the extremes of sinusoidal alternating current.

The upper current limit from which the saturation effects begin depends, among other things, on the following properties of the current transformer: These are, for example, the core cross-section, the core material, the resistance of the secondary winding, which is strongly dependent on the temperature, the load on the current transformer in the form of electronic circuitry on the secondary side and the frequency and signal form of the primary current. This means that the saturation behavior differs at different converter temperatures and primary current frequencies, so that no trivial compensation - for example using a fixed factor - is possible, since such a factor would only be valid for a small operating range.

In the EP 1 300 686 A2 discloses a method for determining the current value using a current transformer, the current transformer operating in the area of core saturation, a peak value or crest value of a half-wave of the secondary current being determined by means of a peak value rectifier. The peak or peak value is stored in a storage element for further processing. A comparator detects when the peak or peak value is reached. After that, the comparator sends a switching signal fed to a signal processing unit and this signal starts the signal processing unit. In the case of large currents, the rms value is not calculated using the root mean square of the secondary current, as is usual, but only the peak value is determined and converted to the rms value using the crest factor.

In the paper "Current Transformer Saturation Compensation for Transformer Differential Relay", IEEE Transactions on Power Delivery 2015 (30), pp. 2293 - 2302, a method is described that uses an algorithm to reconstruct measured values that are lost due to saturation operation and then use the reconstructed signal to calculate the effective value.

It is an object of the invention to provide an apparatus for determining a compensated rms value and a method for determining a compensated rms value which overcomes the disadvantages in the calculations of the previous methods.

According to the invention, this object is achieved by the device for determining a compensated effective value according to patent claim 1 . Advantageous refinements of the method according to the invention are specified in subclaims 2 to 8. The object is also achieved according to the invention by the method for determining a compensated effective value according to patent claim 9 . Claims 10 to 13 specify advantageous configurations of the method according to the invention.

The device for determining a compensated rms value I _eff of an alternating current I according to claim 1 comprises a current transformer and a digital evaluation unit, an rms value I _eff,abg falsified by the current transformer saturation being calculated from the series of the secondary current signal measured by the current transformer, with the total number of existing samples N _total and the number of samples considered invalid N _invalid being determined by the digital evaluation unit and by means of the series of the secondary current signal measured by the current converter of the proportion of samples considered invalid in the total number of available samples A = N _invalid/ N _total , a correction value k(A) is calculated to correct the corrupted effective value I eff,abg and the compensated effective value I _eff is calculated using this correction value k(A) _.

The advantage here is that, compared to compensating for the saturation effect via pure peak value evaluation, there are several effects. The device covers an even larger range since it can also compensate if more than half of a half-wave is cut off by saturation. Since a correct peak value cannot be determined in such a case, the peak value evaluation supplies errors. Furthermore, there are time curves such as harmonics, for which a calculation of the effective value via the peak value and the sine crest factor yields a large error; the device according to the invention can also determine better values there. In comparison to methods that implement a computational completion of a half-wave by means of an intelligent look-ahead, the computing effort in the device according to the invention is many times lower and can therefore be implemented in inexpensive and space-saving evaluation units or microcontrollers. With the help of this device, the current converter can be developed even smaller, lighter and more cost-effectively, while at the same time the power loss in the converter and the connected electronics decreases due to the earlier saturation point. Alternatively, the measuring range can be greatly expanded with the same converter size.

In one embodiment of the device according to the invention, the correction value k(A) is multiplied by the corrupted effective value I _eff,abg to calculate the compensated effective value I _eff .

gewählt und bei einem rechteckförmigen Wechselstrom I zu $$k\left(A\right)=\frac{1}{\sqrt{1-A}}.$$

In a further embodiment, the alternating current I is sinusoidal or square-wave. With a sinusoidal alternating current I, the correction value increases $$k\left(A\right)=\frac{1}{\sqrt{\left(1-A\right)-\frac{1}{2\pi }\text{sin}\left(2\pi \left(1-A\right)\right)}}$$

chosen and with a square-wave alternating current I to $$k\left(A\right)=\frac{1}{\sqrt{1-A}}.$$

gewählt werden.With a sinusoidal alternating current I, the correction value $$k\left(A\right)=5.8\ast A^4+1.014$$

to get voted.

In a further embodiment of the device according to the invention, sampled values are considered invalid if they are below a limit value G.

In a further embodiment of the device according to the invention, the number of sampled values N _Invalid considered invalid is adjusted by subtracting an empirical value E.

In a further embodiment of the device according to the invention, the series of the secondary current signal measured by the current transformer is formed by sampling the alternating current I essentially equidistantly over time.

• - Messen einer Reihe des Sekundärstromsignals durch einen Stromwandler;
• - Bestimmen der Gesamtanzahl der vorliegenden Abtastwerte N_Gesamt;
• - Bestimmen der Anzahl als ungültig betrachteter Abtastwerte N_Ungültig;
• - Berechnen des Anteils als ungültig betrachteter Abtastwerte an der Gesamtanzahl vorliegender Abtastwerte A = N_Ungültig/N_Gesamt;
• - Berechnen eines Korrekturwerts k(A) zur Korrektur eines verfälschten Effektivwert I_eff,abg mittels des Anteils als ungültig betrachteter Abtastwerte an der Gesamtanzahl vorliegender Abtastwerte A = N_Ungültig/N_Gesamt; und
• - Berechnen des kompensierten Effektivwertes I_eff mittels dieses Korrekturwerts k(A).

The method according to the invention for determining a compensated effective value I _eff according to patent claim 9 comprises the steps:

• - measuring a series of the secondary current signal by a current transformer;
• - determining the total number of samples present N _total ;
• - determining the number of samples considered invalid N _Invalid ;
• - Calculation of the proportion of samples considered invalid in the total number of samples present A=N _invalid /N _total ;
• - calculation of a correction value k(A) for correcting a corrupted effective value I _eff,abg by means of the portion of the total number of present sample values considered invalid in the sample values A=N _invalid /N _total ; and
• - Calculation of the compensated effective value I _eff using this correction value k(A).

In one embodiment of the method according to the invention, the calculations and determinations are made by a digital evaluation unit.

• - Anpassen der Anzahl als ungültig betrachteter Abtastwerte N_ungüitig durch Abzug eines Erfahrungswerts E.

In a further embodiment of the method according to the invention, after the determination of the number of sampled values N _Invalid considered invalid, the step follows:

• - Adjusting the number of samples considered invalid N _invalid by subtracting an empirical value E.

gewählt und bei einem rechteckförmigen Wechselstrom I zu $$k\left(A\right)=\frac{1}{\sqrt{1-A}}$$

In a further embodiment of the method according to the invention, the correction value is increased in the case of a sinusoidal alternating current I $$k\left(A\right)=\frac{1}{\sqrt{\left(1-A\right)-\frac{1}{2\pi }\text{sin}\left(2\pi \left(1-A\right)\right)}}$$

chosen and with a square-wave alternating current I to $$k\left(A\right)=\frac{1}{\sqrt{1-A}}$$

gewählt werden.With a sinusoidal alternating current I, the correction value can increase $$k\left(A\right)=5.8\ast A^4+1.014$$

to get voted.

The properties, features and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more clearly understood in connection with the following description of the embodiments, which are further explained in connection with the figures.

• 1 erfindungsgemäße Vorrichtung zum Ermitteln eines kompensierten Effektivwerts eines Wechselstroms I mit Stromwandler und digitaler Auswerteeinheit;
• 2 qualitative Kennlinie eines Stromwandlers;
• 3 typische Zeitverläufe des Sekundärstroms im Sättigungsbereich;
• 4 typische Zeitverläufe des Sekundärstroms im Sättigungsbereich mit Größen zur Berechnung des Korrekturwerts k(A); und
• 5 erfindungsgemäßes Verfahren zum Ermitteln eines kompensierten Effektivwerts I_eff.

show:

• 1 device according to the invention for determining a compensated effective value of an alternating current I with a current transformer and a digital evaluation unit;
• 2 qualitative characteristic of a current transformer;
• 3 typical time curves of the secondary current in the saturation range;
• 4 typical time curves of the secondary current in the saturation range with values for calculating the correction value k(A); and
• 5 inventive method for determining a compensated effective value I _eff .

In 1 a device 100 according to the invention for determining a compensated effective value I _eff of an alternating current I is shown. The alternating current I can flow through a conductor 200, for example. The device 100 according to the invention comprises a current transformer 160 which interacts with the conductor 200 and in which the alternating current I as a primary current is converted by the current transformer 160 into a secondary current. Furthermore, the device 100 according to the invention comprises a digital evaluation unit 150. This digital evaluation unit 150 is connected to the current converter 160 and can, for example, from the series of the secondary current signal measured by the current converter 160, calculate an effective value I _eff,abg falsified by current converter saturation.

The evaluation unit 150 can be connected via connections 151 as a protective device with a protective function to an electrical switch or generally to a network for further processing and monitoring.

In 2 the characteristic curve of a current transformer 160 is shown qualitatively. The effective value of the secondary current is plotted against the effective value of the primary current. A linear relationship ideally results as the characteristic curve of the current converter 160 . A saturation effect occurs due to the core cross-section of the current transformer 160, the core material, the resistance of the secondary winding, which is strongly dependent on the temperature, or the load on the current transformer 160 in the form of electrical circuitry on the secondary side, the frequency or the signal form of the primary current I. In 2 the dashed line separates the linear region, which is to the left of this line, from the nonlinear saturation region, which is to the right of the dashed line. The effective value of the secondary current is smaller than its ideal set value in the non-linear range.

In 3 shows the course of the secondary current over time using a sinusoidal alternating current I. When the magnet core is saturated, the signal of the secondary current falls very quickly to a value close to 0, and no more secondary current flows until there is a change of sign on the primary side. For example, this can be the start of the next half-wave in the case of sinusoidal alternating variables.

4 shows a half-wave of a sinusoidal secondary current I _sec over time. On the one hand, the ideal profile of the secondary current is shown as a sinusoidal current, and on the other hand, the real secondary current I _sec running into saturation, which suddenly drops to approximately 0 due to the saturation effects of the current transformer 160 . A threshold G is introduced, below which the samples of the secondary current are considered invalid. According to the representation in 4 this means that in the interval from -G to +G the samples of the secondary current I _sec are considered invalid. This results in a first interval T _Invalid,1 , in which the samples of the secondary current I _sec are considered invalid, and also a second interval T _Invalid,2 , due to the saturation of the secondary current.

In the device 100 according to the invention, the total number of present sampling values N _Total is now determined from the series of the secondary current signal measured by the current transformer 160 and the number of sampling values N _Invalid considered invalid is determined by the digital evaluation unit 150 . The number of samples N invalid considered to be _invalid corresponds to those samples which lie in the intervals T _{invalid ,1} and T _invalid,2 . The portion of the total number of existing samples that is considered invalid is determined as A=N _invalid/ N _total and from this a correction value k(A) for correcting the corrupted effective value I _eff,abg can be calculated by means of which the compensated effective value I _eff can be calculated.

To calculate the compensated effective value I _{eff ,} the correction value k(A) can be multiplied by the corrupted effective value I _eff,abg .

gewählt und bei einem rechteckförmigen Wechselstrom I zu $$k\left(A\right)=\frac{1}{\sqrt{1-A}}$$

The alternating current I under consideration can be sinusoidal or square-wave, for example. With a sinusoidal alternating current I, the correction value increases $$k\left(A\right)=\frac{1}{\sqrt{\left(1-A\right)-\frac{1}{2\pi }\text{sin}\left(2\pi \left(1-A\right)\right)}}$$

chosen and with a square-wave alternating current I to $$k\left(A\right)=\frac{1}{\sqrt{1-A}}$$

The determination of this correction function as a function of the expected signal form is initially to be described in general below.

There is an alternating current signal (ωt) which is repeated with the period T. The fundamental frequency f of the signal then results in: $$ƒ=\frac{1}{T}$$

The angular velocity ω results in: $$\omega =2\pi \ast ƒ=\frac{2\pi }{T}$$

The effective value of this complete current signal is to be determined below over half a period. This corresponds to a phase angle of ωt = π = 180° $$I_{eff}=\sqrt{\frac{1}{\pi }{\displaystyle \underset{0}{\overset{\pi }{\int }}{\left(i\left(\omega t\right)\right)}^2\mathrm{i.e}\omega t}}$$

It is now considered how this effective value I _eff changes if part A of the signal is cut off towards the end of half the period: $$I_{eff,abG.}=\sqrt{\frac{1}{\pi }{\displaystyle \underset{0}{\overset{\pi \left(1-A\right)}{\int }}{\left(i\left(\omega t\right)\right)}^2\mathrm{i.e}\omega t}}$$

The rms value of a signal in which a time portion A was cut off towards the end of half a period is compared with the rms value of a signal that has not been cut off. The result is interpreted as a correction value k(A). This assigns a correction value to a portion A. A correction value k for the effective value I _{eff,abg , which is too small due to the converter saturation, can be determined in a simple manner from the consideration and evaluation of the time curve,} which is supplied by the quantity A: $$k\left(A\right)=\frac{I_{eff}}{I_{eff,abG.}}=\frac{\sqrt{{\displaystyle {\int }_0^{\pi }{\left(i\left(\omega t\right)\right)}^2\mathrm{i.e}\omega t}}}{\sqrt{{\displaystyle {\int }_0^{\pi \left(1-A\right)}{\left(i\left(\omega t\right)\right)}^2\mathrm{i.e}\omega t}}}$$

The correction value k(A) is now to be determined as an example for a square-wave alternating current i _square and a sinusoidal alternating current i _sine .

$$\begin{array}{l}{I}_{Rechteck,eff}=\sqrt{\frac{1}{\pi }{\displaystyle \underset{0}{\overset{\pi }{\int }}{\left(I_{Sp}\right)}^{2}d\omega t=I_{Sp}}}\\ I_{Rechteck,eff,abg}=\sqrt{\frac{1}{\pi }{\displaystyle \underset{0}{\overset{\pi \left(1-A\right)}{\int }}{\left(I_{Sp}\right)}^{2}d\omega t=I_{Sp}\ast \sqrt{1-A}}}\\ I_{Rechteck}\left(A\right)=\frac{I_{Rechteck,eff}}{I_{Rechteck,eff,abg}}=\frac{I_{Sp}}{I_{Sp}\ast \sqrt{1-A}}=\frac{1}{\sqrt{1-A}}\end{array}$$

With square-wave alternating current i _square-wave , the result is: $$i_{RecHteck}\left(\omega t\right)=I_{Sp}f\ddot{\mathrm{and}}\mathrm{right}\text{0}\le \omega t<\pi$$

$$\begin{array}{l}{I}_{RecHteck,eff}=\sqrt{\frac{1}{\pi }{\displaystyle \underset{0}{\overset{\pi }{\int }}{\left(I_{Sp}\right)}^2\mathrm{i.e}\omega t=I_{Sp}}}\\ I_{RecHteck,eff,abG}=\sqrt{\frac{1}{\pi }{\displaystyle \underset{0}{\overset{\pi \left(1-A\right)}{\int }}{\left(I_{Sp}\right)}^2\mathrm{i.e}\omega t=I_{Sp}\ast \sqrt{1-A}}}\\ I_{RecHteck}\left(A\right)=\frac{I_{RecHteck,eff}}{I_{RecHteck,eff,abG}}=\frac{I_{Sp}}{I_{Sp}\ast \sqrt{1-A}}=\frac{1}{\sqrt{1-A}}\end{array}$$

The following is calculated for the sinusoidal alternating current i _sine : $$\begin{array}{l}{i}_{Sin\mathrm{and}s}\left(\omega t\right)=I_{Sp}\ast \text{sin}\left(\omega t\right)\\ I_{Sin\mathrm{and}s,eff}=\sqrt{\frac{1}{\pi }{\displaystyle \underset{0}{\overset{\pi }{\int }}{\left(I_{Sp}\ast \text{sin}\left(\omega t\right)\right)}^2\mathrm{i.e}\omega t}}=\frac{1}{\sqrt{2}}\ast I_{Sp}\\ I_{Sin\mathrm{and}s,eff,abG.}=\sqrt{\frac{1}{\pi }{\displaystyle \underset{0}{\overset{\pi \left(1-A\right)}{\int }}{\left(I_{Sp}\ast \text{sin}\left(\omega t\right)\right)}^2\mathrm{i.e}\omega t}}\\ =\frac{1}{\sqrt{2}}\ast I_{Sp}\ast \sqrt{\left(1-A\right)-\frac{1}{2\pi }\text{sin}\left(2\pi \left(1-A\right)\right)}\\ k_{Sin\mathrm{and}s}\left(A\right)=\frac{I_{Sin\mathrm{and}s,eff}}{I_{Sin\mathrm{and}s,eff,abG.}}=\frac{1}{\sqrt{\left(1-A\right)-\frac{1}{2\pi }\text{sin}\left(2\pi \left(1-A\right)\right)}}\end{array}$$

The calculation of the correction value k(A) can be simplified by an approximation function that is simplified with regard to the computation effort and that supplies sufficiently precise values in the relevant range. For example, sinusoidal currents can be approximated using a polynomial: $$k_{Sin\mathrm{and}s}\left(A\right)=\frac{I_{Sin\mathrm{and}s,eff}}{I_{Sin\mathrm{and}s,eff,abG.}}=\frac{1}{\sqrt{\left(1-A\right)-\frac{1}{2\pi }\text{sin}}\left(2\pi \left(1-A\right)\right)}\approx 5.8\ast A^4+1.014$$

As a result, the device 100 according to the invention can be implemented in firmware on a less powerful and therefore less expensive microcontroller with little effort, since no complex arithmetic operations have to be carried out.

The number of samples considered invalid N _Invalid can be adjusted by subtracting an empirical value E.

The series of the secondary current signal measured by the current transformer 160 can be measured by sampling the alternating current I substantially equidistant in time.

• - Messen 1010 einer Reihe des Sekundärstromsignals durch einen Stromwandler 160;
• - Bestimmen 1020 der Gesamtanzahl vorliegender Abtastwerte N_Gesamt;
• - Bestimmen 1030 der Anzahl als ungültig betrachteter Abtastwerte N_Ungültig;
• - Berechnen 1040 des Anteils als ungültig betrachteter Abtastwerte an der Gesamtanzahl vorliegender Abtastwerte A = N_Ungültig/N_Gesamt;
• - Berechnen 1050 eines Korrekturwerts k(A) zur Korrektur eines verfälschten Effektivwerts I_eff,abg mittels des Anteils als ungültig betrachteter Abtastwerte an der Gesamtzahl vorliegender Abtastwerte A = N_Ungültig/N_Gesamt; und
• - Berechnen 1060 des kompensierten Effektivwerts I_eff mittels des Korrekturwerts k(A).

5 10 shows a method 1000 according to the invention for determining a compensated effective value I _eff . Method 1000 begins at start 1001 and ends at end 1099. After start 1001, the steps follow:

• - measuring 1010 a series of the secondary current signal by a current transformer 160;
• - determining 1020 the total number of present sample values N _total ;
• - determining 1030 the number of samples N _Invalid considered to be invalid;
• - calculating 1040 the proportion of samples considered invalid in the total number of samples present A=N _invalid /N _total ;
• - Calculation 1050 of a correction value k(A) for correcting a corrupted effective value I _eff,abg by means of the portion of the total number of present sample values considered invalid of sample values A=N _invalid /N _total ; and
• - calculating 1060 the compensated effective value I _eff by means of the correction value k(A).

The calculations 1030; 1040; 1050; 1060 and provisions 1010; 1020 can be performed by a digital evaluation unit 150.

• - Anpassen 1035 der Anzahl als ungültig betrachteter Abtastwerte N_Ungültig durch Abzug eines Erfahrungswerts E.

In one embodiment of the method 1000 according to the invention, after the determination 1030 of the number of sample values N _Invalid considered invalid, the step follows

• - Adjusting 1035 the number of samples considered invalid N _Invalid by subtracting an empirical value E.

und bei einem rechteckförmigen Wechselstrom I zu $$k\left(A\right)=\frac{1}{\sqrt{1-A}}.$$

In the case of the method 1000 according to the invention, the correction value k(A) can be selected for a sinusoidal alternating current I $$k\left(A\right)=\frac{1}{\sqrt{\left(1-A\right)-\frac{1}{2\pi }\text{sin}\left(2\pi \left(1-A\right)\right)}}$$

and with a square-wave alternating current I to $$k\left(A\right)=\frac{1}{\sqrt{1-A}}.$$

gewählt werden.In the case of the method 1000 according to the invention, the correction value k(A) can alternatively be $$k\left(A\right)=5.8\ast A^4+1.014$$

to get voted.

The method 1000 according to the invention and the device 100 according to the invention can be used as soon as at least one half-wave of the secondary current signal is present. Compensation of the effective value I _eff takes place via an evaluation of the time profile of the secondary current signal that is easy to implement and meaningful for the effective value.

If one considers not only the effective or peak value of the converter signal, but also its time profile in the saturation range, the procedure according to the invention is: The secondary current signal behaves in such a way that from the point in time at which saturation of the converter core is reached, virtually no more current is converted and the secondary signal is therefore equal to or close to the idle level, for example 0. This is the case until a sign change occurs on the primary side. The method 1000 according to the invention and the device 100 according to the invention are based on determining what time portion of the current signal is actually still being transmitted and what portion is missing due to the saturation effect. For this it is assumed that measured values close to the resting level are caused by the saturation effect and are therefore invalid. A limit value G is set for this decision. These variables can be determined, e.g. in a digital evaluation unit 150 such as in a microcontroller, from a series of equidistant sampled values.

QUOTES INCLUDED IN DESCRIPTION

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Patent Literature Cited

• EP 1300686 A2 [0009]

Claims (13)

Device (100) for determining a compensated effective value I _eff of an alternating current I, comprising a current transformer (160) and a digital evaluation unit (150), a corrupted effective value I _eff,abg being calculated from the series of the secondary current signal measured by the current transformer (160), characterized in that in the series of the secondary current signal measured by the current transformer (160), the total number of present sampling values N _total and the number considered invalid Sample values N invalid are determined by the digital evaluation unit (150) and a correction value k(A) for correcting the falsified effective value I eff, _abg is calculated using the proportion of samples considered _invalid in the total number of available sample values A = N _invalid /N _total and using this correction value k(A) the compensated effective value I _eff . Device (100) according to Claim 1 , where the correction value k(A) is multiplied by the corrupted effective value I _eff,abg to calculate the compensated effective value I _eff . Device (100) according to Claim 1 or 2 , where the alternating current I is sinusoidal or square-wave. Device (100) according to patent claim 3 , where with a sinusoidal alternating current I the correction value increases $$k\left(A\right)=\frac{1}{\sqrt{\left(1-A\right)-\frac{1}{2\pi }\text{sin}\left(2\pi \left(1-A\right)\right)}}$$

is selected and with a square-wave alternating current I to $$k\left(A\right)=\frac{1}{\sqrt{1-A}}.$$

Device (100) according to patent claim 3 , where with a sinusoidal alternating current I the correction value increases $$k\left(A\right)=5.8\ast A^4+1.014$$

is chosen. Apparatus (100) according to any preceding claim, wherein samples are considered invalid if they are below a threshold G. Device (100) according to one of the preceding claims, in which the number of samples considered invalid N _Invalid is adjusted by subtracting an empirical value E. Device (100) according to one of the preceding claims, in which the series of the secondary current signal measured by the current transformer (160) is formed by sampling the alternating current I essentially equidistant in time. Method (1000) for determining a compensated effective value I _eff comprising the steps of: - measuring (1010) a series of the secondary current signal by a current transformer (160); - determining (1020) the total number of samples present N _total ; - determining (1030) the number of samples considered invalid N _Invalid ; - calculating (1040) the proportion of samples considered invalid in the total number of samples present A=N _invalid /N _total ; - Calculation (1050) of a correction value k(A) for correcting a corrupted effective value I _eff,abg by means of the proportion of sampled values considered invalid in the total number of present sampled values A=N _invalid/ N _total ; and - calculating (1060) the compensated effective value I _eff using this correction value k(A). Method (1000) according to Claim 9 , in which the calculations (1030; 1040; 1050; 1060) and the determinations (1010; 1020) are carried out by a digital evaluation unit (150). Method (1000) according to Claim 9 or 10 , in which after determining (1030) the number of samples N Invalid considered _invalid , the step follows: - adjusting (1035) the number of samples N Invalid considered _invalid by subtracting an empirical value E. Method (1000) according to any one of patent claims 9 until 11 , in which the correction value increases with a sinusoidal alternating current I $$k\left(A\right)=\frac{1}{\sqrt{\left(1-A\right)-\frac{1}{2\pi }\text{sin}\left(2\pi \left(1-A\right)\right)}}$$

is selected and with a square-wave alternating current I to $$k\left(A\right)=\frac{1}{\sqrt{1-A}}.$$

Method (1000) according to any one of patent claims 9 until 11 , in which the correction value increases with a sinusoidal alternating current I $$k\left(A\right)=5.8\ast A^4+1.014$$

is chosen

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