WO2009027373A2

WO2009027373A2 - Measuring and/or switching device

Info

Publication number: WO2009027373A2
Application number: PCT/EP2008/061089
Authority: WO
Inventors: Thomas Kiendl
Original assignee: Siemens Aktiengesellschaft
Priority date: 2007-08-27
Filing date: 2008-08-25
Publication date: 2009-03-05
Also published as: WO2009027373A3; CN101790689A; DE102007041176A1; CN101790689B

Abstract

The application discloses methods how to arithmetically calculate a corrected effective value from a real effective value of a secondary current of an iron-cored current transformer when magnetization of the iron core reaches the saturation point during generation of the secondary current from a primary current. The corrected value simulates the case that the saturation point is not reached, thus making it possible to use a smaller than normal iron-cored current transformer in measuring and/or switching devices comprising an iron-cored current transformer, e.g. in a low-voltage circuit breaker.

Description

description

Measuring and / or switching device

The invention relates to a device that makes use of a current transformer with iron core. The current transformer with iron core transforms a primary current into a secondary current. Accordingly, the device includes means for coupling the primary current. The secondary current is evaluated in some way. Either the acquisition of a measured value is sufficient for the purpose of the device, or the secondary current is used as a benchmark for switching. Accordingly, the device is a measuring and / or switching device. A typical application is a low voltage power line holder.

By the means for evaluation is deduced from the secondary current to the primary current. So far, it is assumed that the effective value of the secondary current in Wesentli ^¬ chen is linear to the effective value of the primary current. However, this is only possible if the primary current provides for a proportional to its value magnetization of the iron core. The iron core may therefore not achieve the Sättigungsmagneti ^¬ tion. The area where the saturation uses ^¬, can not be achieved. For this reason, a sufficiently large-sized current transformer is usually used in the devices of the type mentioned. It would be desirable, one might smaller current transformer USAGE ^¬.

It is therefore an object of the invention to build measuring and / or Schaltge ^¬ devices of the type mentioned compact.

This is effected in that the means are designed for evaluating the secondary flow, it must be borne in during the evaluation, when the primary current is so high that the magnetic field generated by it causes the region of the Saetti ^¬ supply in the magnetization of the Iron core is achieved. The ^¬ ser case is exactly the case that the magnetization not more (essentially) is linearly correlated with the primary stream. The consequence of this has, that the curve shape of the seconding ^¬ därstroms differs from that of the primary current.

The fact that the means for evaluating the secondary flow be ^¬ into account that the area of saturation is achieved in the Magneti ^¬ tion of the iron core, the area of saturation must not be bypassed in the magnetization of the iron core. As a result, the current transformer can be smaller than usual.

In the center of interest is in a measuring and / or switching device with current transformer of the effective value of the secondary ^¬ current. By using the smaller current transformer is in certain primary current values, the magnetization of the iron core into saturation, and this effective value is too small compared to the linear behavior at lower primary currents between the effective value of the secondary current and the effective ^¬ value of the primary current. The taking into preferably takes into account this effect, characterized in that the means for Auswer ^¬ th a corrected rms value of the secondary current detect (in particular calculate), which would be obtained in the real seconding ^¬ därstrom when the magnetization curve of the iron core so would look like that saturation would not be achieved in the magnetization of the iron core. It is simulated by the corrective effective value so the case that such a large-sized current transformer with iron core is used, that the secondary current is still (substantially) correlated linearly with the primary current.

As already mentioned, it is a preferred application of he ^¬ inventive principle that the device according to the invention is a low-voltage circuit breaker in which a magnetic actuator causes an interruption of the primary circuit. Instead of using the real RMS value as the basis for a tripping ^¬ criterion, now the corrected RMS value is used, that is, the magnetic actuator is then acti ^¬ fourth when the corrected RMS value, the set values of the low-voltage circuit breaker (eg at least one limit).

Generating a corrected rms value is based on the fact that the iron core when it becomes saturated, a defined behavior shows so that can be-closed back to the primary current based on the secondary ^¬ stream fact, even if the secondary current no longer the same waveform like the primary stream has. It turns out that one can view the secondary current as phasenabschnitt- controlled secondary ^¬ current: So the secondary current follows a specific period of time the primary current and falls abruptly in the magnetization of the iron core when reaching the saturation from. It is now possible to define a threshold value such that the time duration of the threshold value being exceeded by the magnitude of the secondary current is substantially exactly equal to the time duration during which the secondary current follows the primary current linearly.

In one embodiment, the comparison with the

Threshold by analog electronics, and it is generated a right ^¬ eckpulssignal, such that the duration of the square ^¬ pulse is equal to the duration of the crossing of the threshold. This rectangular pulse signal can then be evaluated by a microcontroller after transformation by an analog-to-digital converter by simply measuring the length of the rectangular pulses.

Alternatively, the entire evaluation can be done digitally. The comparison of the secondary current values with the threshold value then also takes place in the microcontroller after transformation of the secondary current through an analog-to-digital converter. A rectangular pulse curve does not have to be generated in this case.

The easiest way to handle it mathematically is if the product of the actual effective secondary current is used as a corrected effective secondary current with one of the determined time intervals. duration of exceeding the threshold dependent factor. This correction factor K (a) with the determined duration α is preferably given by the following formula:

For a sinusoidal primary current, this correction factor κ (a) becomes 1, when the real secondary current is also the

Sinus form approaches, then α is exactly π. In this case, the real effective secondary current does not have to be corrected by a factor other than 1.

A duration of exceeding a threshold by the amount of secondary current is not the only quantity that can be used to determine a correction factor for the real effective secondary current. Thus, alternatively, a form factor for the secondary flow can also be defined. This form factor is the ratio of the rms value of the secondary ^{current to the mean} value (also called rectification value) of the secondary current. This measure form factor can be assigned by means of a map of a correction factor by which the corrected value of the effective secondary ^¬ stream from the real effective value of the secondary current calculation ^¬ net is.

Hereinafter, a preferred embodiment of the inven ^¬ tion is described with reference to the drawing, in which

1 shows an electronic trip unit of a low-voltage power switch according to the invention,

FIG. 2 illustrates the time course of a secondary current when an iron core of a current transformer does not saturate and FIG 3 illustrates the time course of a secondary current when an iron core of the current transformer saturates,

4 illustrates the application of a threshold value criterion to the curve from FIG. 3,

FIG. 5 shows a curve with rectangular pulses obtained on the basis of the threshold value criterion,

FIG. 6 shows a characteristic field showing the relationship between a

Correction factor and a measured form factor,

7 illustrates the derivation of the corrected effective value from the time profile of the secondary current according to an embodiment of the invention.

In a low-voltage circuit breaker, the flow of this should start at a certain rms value of a current

Current enabling circuit be interrupted. A he ^¬ inventive low-voltage circuit breaker comprises an electronic trip unit, which is shown in FIG 1 and designated as a whole by 10. It is now not a current flowing in the circuit to be interrupted current used directly as a criterion for breaking the circuit, but this is supplied as a primary current i _p (t) to the current transformer 12, which should have an iron core. The current transformer generates a secondary current i _s (t) on the secondary side, and this secondary current serves as a criterion for triggering the circuit. The secondary current i _B (t) falls on a ^¬ From terminating resistor (load) 14, and an analog to digi tal converter 16 is supplied. In this way, digital values are obtained, which are fed to a microcontroller 18, which carries out the actual evaluation. After a certain criterion, the microcontroller 18 analyzes the digitized secondary current and, optionally via a magnetic actuator 20, the switching mechanism of the low-voltage circuit breaker, the Circuit in which the primary current i _p (t) flows to open mechanically.

Conventional low-voltage circuit breakers use such a current transformer in which the secondary current has the same shape for the primary currents to be expected. 2 shows in this case a curve 22 of the temporal course of the secondary current i _s (t) for a sinusoidal primary current: The secondary current is also sinusoidal, because the iron ^¬ core of the current transformer does not reach saturation.

FIG. 3 shows a different case: Here, the current transformer is smaller in comparison to the current transformer used to generate the secondary current curve 22 of the current transformer used in FIG. With the same primary current, the magnetization of the iron core is in Saetti ^¬ supply. Then obtained instead of the curve 22 to see overall in FIG 3 showed secondary current curve 24. As shown in FIG 3, is a 26 designated in FIG 3 part of the curve 24 almost fully ^¬ forms as a corresponding portion of the curve 22, ie will go through part of a sine wave. From attain one be ^¬ voted primary current, the magnetization of the iron core is saturated, and the curve falls rapidly, see Section 28. You can view the secondary current of FIG 3 as phasenabschnitt- controlled secondary current.

The width of the by curve portions 26 and 28 DEFINE ^¬ th peak is now a measure of the degree of influence of the magnetization curve due to the fact that its non-linear ^¬ saturation region is reached. Now, a threshold value 30 set and a width α of the peak ermit ^¬ telt. The unit of the width α is that of a time. However, this can also be considered as a phase and α can be called the current flow angle. Analogous to the threshold value 30 can be also its negative value 30 set ', and below which also the width of the associated peaks, α, he ^¬ averages are.

One can now summarize the result of the comparison of the curve 24 with the thresholds 30 and 30 'to a curve, the is shown in FIG. The curve is logically high ("comparison result = 1") if the threshold value is exceeded, and it is logically low ("comparison result = 0") as long as the threshold value 30 or 30 'is undershot. In FIG 5, the width α is thus the

Duration of a rectangular pulse. The less the magnetization of the iron core saturates, the greater is α, and the more the curve approaches logically high throughout, and as the magnetization of the iron core becomes more saturated, α becomes smaller.

It is now possible from the real effective value of the secondary ^¬ current, / _{# real} and the size α a corrected effective value

Calculate I _{seff corr} for the secondary current, where the corrected rms value is the value that would result if the

Current transformer would be larger and the range of saturation in the magnetization of the iron core would not be reached when excited by the same primary current. If the threshold value 30 or 30 'is sufficiently low, the following formula can be derived:

* Seff, corr * Seff, real K {a)

It is provided that the microcontroller 18 determines the quantity α by comparing the digitized data with the threshold values 30 and 30 'there, that I _{Seff re (} ) is also determined in the microcontroller and that I _{Seff corr is} determined Magnetic actuator 20 will now pass through the microcontroller

18 then triggered when the corrected RMS value for the secondary _{current intensity} I _{Seffykorr exceeds} a predetermined threshold. Because the corrected RMS value is greater than the RMS value of the secondary current, tripping takes place faster than usual, as with a purely linear relationship between primary and secondary would occur.

Instead of defining the quantity α by the definition of the threshold values 30 and 30 ', then determining it and using it to calculate a correction value K (a), a correction value K can also be determined by other means. In this alternative solution, the form factor of the secondary current is used. This form factor is defined as the ratio of RMS

heffreal

to equal value, al so the magnitude mean li (t) l μ

This form factor can be related to the current flow angle α approximately as follows:

Now using the same correction value as in the alternative K \ a) described above

K {a) = ^l

1 . sin (2 a). - («) π 2

then, a relationship between the shape factor F and the correction factor K can be obtained, for example, by calculating the above formulas for different α. It a map or a characteristic curve can be ermit ^¬ stuffs, in which the correction factor K (a) to form factor F =

F (α) is related. This characteristic curve is shown in FIG. 6 and denoted by 32 there. Using the characteristic curve 32, the value must α no longer determines who ^¬. The characteristic curve 32 can in particular also be determined differently than via the above equations, for example on the basis of experimental tests on current transformers or by means of simulations with the aid of finite elements.

7 summarizes the procedure when using the map

32 together: i _s (t) becomes the RMS value

I _{Seff τeal} and on the other hand the rectifier li _s (t) l determined, these are set in relation to each other to determine the form factor F, and from the form factor F, the correction factor K is derived based on the map 32. The ^¬ ser correction factor K is then multiplied with the _real previously ermit ^¬ telten I _{Seff ^} to obtain I _Seffkorr.

The two embodiments have in common that, starting from the actual effective value of the secondary current, a Corrected ^¬ ter effective value by means of a correction factor K is calculation ^¬ net. In the method according to the invention, other methods are also conceivable, such as can be taken into account in any form that the range of saturation magnetization of the iron core is achieved. The consequence of this consideration is that the current transformer with the iron core does not have to be excessively large. The existing in the prior art ^¬ He fordernis, with an iron core as large auszufüh- the current transformer ren that the secondary current is linearly correlated to the primary current is eliminated.

Claims

claims

1. Measuring and / or switching device with means for coupling a primary current (i _p (t)) with a current transformer (12) with iron core, which transforms the primary current into a secondary current (i _s (t)), and means (14 , 16, 18) for evaluating the secondary current, characterized in that the means (14, 16, 18) for evaluating the secondary current (i _s (t)) adapted to it when evaluating to into account ^¬ term, when the primary current ( i _p (t)) is so high that the magnetic field generated by it causes the region of the Saetti ^¬ supply is achieved in the magnetization of the iron core.

2. measuring and / or switching device according to claim 1, characterized in that the means (14, 16, 18) for evaluating determine a corrected RMS {I _Seffkorr) ^ ^for ^ en secondary current would be obtained at the real secondary current when the magnetization curve of the iron core would look like that

Saturation in the magnetization of the iron core would not be achieved.

3. Measuring and / or switching device, characterized in that it is a low-voltage circuit breaker in which a magnetic actuator (20) causes an interruption of the primary circuit when the corrected effective value (I _{Seff! Corr} ) the Ein ^¬ adjusting parameters of the low-voltage Circuit breaker exceeded.

4. Measuring and / or switching device according to claim 2 or 3, characterized in that a threshold value (30, 30 ') is defined, the time duration of exceeding this threshold value is determined by the amount of the temporally variable secondary current and from this period then the corrected RMS value (/ _{& Λtorr} ) is derived.

5. Measuring and / or switching device according to claim 4, characterized in that one of the secondary side of the current transformer downstream circuit generates a square-wave pulse in which the duration of the rectangular pulse is equal to the time duration of the crossing of the threshold value, the square-wave ^¬ curve an analogue Digital converter is supplied and the durations of the rectangular pulses are detected by a microcontroller.

6. Measuring and / or switching device according to claim 4, characterized in that the secondary side of the current transformer (12) with an analog-to-digital converter (16) is connected and the comparison of

Secondary current values with the threshold value (30, 30 ') takes place in a microcontroller (18) arranged downstream of the analog-to-digital converter (16), which thus derives the time duration (α) of exceeding the threshold value.

7. Measuring and / or switching device according to one of claims 4 to 6, characterized in that

^ Seff, korr ^~ 'SeJf, real K [OC) where i s _e ff _rea i is the e ffect value of the secondary current i st and l _se ff _{koπ is} the corrected rms value of the secondary _current , where K (a) is an on the determined time period α dependent correction factor, which is preferably based on the formula

is determined.

8. Measuring and / or Schaltgerat according to claim 2 or 3, characterized in that a form factor (F) to the secondary current (i _a (t)) as a ratio of the effective value of the secondary _current I _{Seff real} to

5 magnitude average value li _s (t) l of the secondary flow is determined and a correction factor K is assigned to the form factor (F) on the basis of a characteristic map 32 and the corrected effective value of the secondary _flow l _{Seff corr}

- U * Seff corr ^~ * Seff real ^

is calculated .

5