DE102023202519A1 - Residual current protection circuit for detecting direct current - Google Patents

Abstract

In residual current protection circuits for detecting a direct current with the aid of a current transformer (1), wherein the action of an oscillating magnetic field on the transformer (1) in the event of a DC residual current occurring in a monitored circuit induces a compensating current which is evaluated with regard to the need to interrupt the circuit, false triggering can occur due to possible magnetization of the transformer core. It is therefore proposed to further develop such protective circuits, which are formed with a current transformer (1), an excitation circuit (31) for generating an oscillating excitation voltage and an inductance (32) for inducing a magnetic field in the current transformer (1) and a measuring resistor (33), with a further resistor (38) and a switch (39), wherein the further resistor (38) is arranged parallel to the measuring resistor (38) and can be switched on by means of the switch (39). This circuit allows a more accurate calculation of the fault current.

Description

The invention relates to a residual current protection circuit for detecting a direct current and a method for detecting a direct current by means of a residual current protection circuit according to the invention.

Residual current circuit breakers or differential current circuit breakers are often also referred to as RCDs (residual current detectors). They are used to protect against residual currents that can be dangerous, especially for operators or users of an electrical system. An RCD that (also) works with direct current is usually referred to as a "RCD type B". It is particularly challenging for type B RCDs to guarantee the correct response or tripping behavior, i.e. on the one hand to switch off safely in the event of all dangerous residual currents, but on the other hand to avoid false tripping (the term "nuisance tripping" is common in English).

Relevant technical improvements for type B RCDs are described, for example, in the publications EN 102018204047 A1 , DE 102019204272 A1 and EN 102021202171 A1 This mainly concerns the effect of high-frequency currents, which can lead to unwanted triggering.

However, there is a need to improve the tripping behaviour of type B RCDs also with regard to other effects.

It is the object of the invention to contribute to this.

The object is achieved by a residual current protection circuit for detecting a direct current according to claim 1 or a method for detecting a direct current by means of a residual current protection circuit according to the invention according to claim 6. Advantageous further developments are specified in the subclaims.

According to the invention, a residual current protection circuit (circuit breaker) is proposed for detecting a direct current using a current transformer. Typically, such a residual current protection circuit is part of a circuit breaker, e.g. a type B RCD. In the event of a DC residual current occurring in a monitored circuit, the effect of an oscillating magnetic field on the converter induces a compensating current, which is evaluated with regard to the need to interrupt the circuit (if necessary after prior processing, e.g. by filtering and amplification). In particular, this residual current protection circuit can work according to the fluxgate principle.

The residual current protection circuit according to the invention comprises a current transformer, an excitation circuit for generating an oscillating excitation voltage of an inductance for inducing a magnetic field in the current transformer and a first resistor (also sometimes referred to as a "measuring resistor"). These can be arranged in series, for example. According to the invention, the protection circuit is further developed with a second resistor and a switch (e.g. a mechanical switch such as a relay or an electronic switch such as a MOSFET, wherein the second resistor is arranged parallel to the first resistor and can be switched on by means of the switch.

The invention aims to compensate for unwanted magnetization of the converter or the converter core. By connecting the second resistor, the strength of the magnetic field acting on the converter can be changed. This makes it possible to calculate a value for a fault current that has occurred using a method according to the invention, in which the effect of unwanted magnetization is at least partially compensated.

Preferably, the protective circuit according to the invention is also formed with a computing unit or logic unit (typically MCU) for evaluating the detected or recorded current, which is additionally designed to cause the switch to open and close by means of a control signal that can be output via an output.

For a method according to the invention, in which the opening or closing of the switch is coupled to a change in the sign of the excitation voltage, the excitation circuit can be connected to an input of the computing unit and the protective circuit can be designed to provide information via the input through the excitation circuit from which a change in the sign of the excitation voltage can be determined. This information can be, for example, a binary signal that encodes the sign or the direction of the change in sign. However, it is also conceivable that an analog signal is transmitted which is a measure of the voltage itself and the change in sign is determined by the computing unit by means of zero crossing detection. However, solutions may also be possible that do not require a connection between the excitation circuit and the input of the computing unit. In this way, the time of a change in the sign of the excitation voltage can be determined via the induced compensating current detected and evaluated by the computing unit.

In a residual current protective circuit designed according to the invention with a computing unit, the latter can be designed to carry out a method according to the invention described below.

1. a) Öffnen des Schalters bei einem ersten Vorzeichenwechsel der Erregerspannung,
2. b) Schließen des Schalters bei dem auf den ersten folgenden Vorzeichenwechsel der Erregerspannung,
3. c) Detektieren eines ersten induzierten Ausgleichstroms,
4. d) Öffnen des Schalters bei einem zweiten Vorzeichenwechsel der Erregerspannung, wobei der zweite Vorzeichenwechsel dem ersten entgegengesetzt ist,
5. e) Schließen des Schalters bei dem auf den zweiten folgenden Vorzeichenwechsel der Erregerspannung,
6. f) Detektieren eines zweiten induzierten Ausgleichstroms und
7. g) Bilden eines Mittelwerts (vorzugsweise des arithmetischen Mittelwerts) des ersten und zweiten induzierten Ausgleichstroms.

A method for detecting a direct current by means of a residual current protection circuit according to the invention is proposed, in which the following steps are carried out:

1. a) Opening the switch at the first change of sign of the excitation voltage,
2. b) Closing the switch at the change of sign of the excitation voltage following the first one,
3. c) detecting a first induced compensating current,
4. d) opening the switch when the excitation voltage changes sign for a second time, the second change of sign being opposite to the first,
5. e) closing the switch at the change of sign of the excitation voltage following the second one,
6. f) detecting a second induced compensating current and
7. g) forming an average value (preferably the arithmetic mean value) of the first and second induced compensating currents.

By connecting the second resistor (closing the switch), the field strength of the magnetic field acting on the converter increases. By choosing the appropriate size of the second resistor, the converter core can be magnetized, which leads to a compensating current when the second resistor is switched off. The magnetization and subsequent determination of the compensating current is carried out for different magnetic field polarities and the average value is then calculated. If there is no fault current, the average value is zero under ideal conditions. If the core is initially magnetized, this method significantly reduces the apparent fault current.

According to one embodiment, the circuit is monitored for a DC fault current in a conventional manner. If the occurrence of a suspected fault current (i.e. an induced current, which may also be caused by converter core magnetization) is detected, the above-mentioned steps a) - g) are then carried out to determine a value for the fault current. The average value obtained from the first and second induced compensating current is then used as the value for the fault current. It can then be provided that a check is carried out to determine whether the value of the fault current exceeds a threshold for the permissible fault current and that, if the threshold is exceeded, a signal is issued to interrupt the circuit.

The circuit can be monitored for a DC fault current by detecting an induced compensating current and checking whether this exceeds a threshold value for tripping. The determination of a value for the fault current using steps a) - g) is then only carried out if the threshold value for tripping is exceeded. Preferably, a check is also carried out to see if a threshold value for immediate tripping is exceeded and the determination of a value for the fault current using steps is only carried out if the threshold value for immediate tripping is not exceeded. This design is based on the consideration that the possible contribution of core magnetization to a detected fault current is limited. If the fault current is high, it can then be concluded that the fault current exceeds the threshold for tripping, even if the contribution from the converter core magnetization is at its maximum. This is represented by a second threshold value ("threshold value for immediate tripping"). In this case, a signal to interrupt the circuit is preferably issued immediately, i.e. no averaging takes place using steps a) - g).

According to a further development, when monitoring the circuit for a DC fault current after carrying out steps a) - g), the effect of any magnetization of the current transformer caused thereby is taken into account.

The invention also relates to a computer program which carries out a method according to the invention when executed on a computer, and to a computer program product comprising such a computer program.

• 1: herkömmlicher RCD Typ B,
• 2: Hysteresekurve eines Wandlerkerns,
• 3: scheinbarer Fehlerstrom bei einer Magnetisierung des Wandlerkerns durch einen Feldstärkeimpuls,
• 4a: scheinbarer Fehlerstrom als Funktion der Zeit bei einer Magnetisierung des Wandlerkerns durch einen Feldstärkeimpuls,
• 4b: Feldstärke als Funktion der Zeit bei einem Feldstärkeimpuls, der zur einer Kernmagnetisierung führt,
• 5: erfindungsgemäße Fehlerstrom-Schutzschaltung,
• 6: scheinbarer Fehlerstrom bei einer Magnetisierung des Wandlerkerns durch einen Feldstärkeimpuls bei Zuschaltung des Widerstands R_mag der Schaltung aus 5,
• 7a: erfasster Fehlerstrom als Funktion der Zeit bei einem erfindungsgemäßen Vorgehen, wenn keine vorhergehende Kernmagnetisierung durch einen Feldstärkeimpuls vorliegt,
• 7b: Magnetfeld als Funktion der Zeit bei einem erfindungsgemäßen Vorgehen, wenn keine vorhergehende Kernmagnetisierung durch einen Feldstärkeimpuls vorliegt,
• 8a: erfasster Fehlerstrom als Funktion der Zeit bei einem erfindungsgemäßen Vorgehen, wenn eine vorhergehende Kernmagnetisierung durch einen Feldstärkeimpuls vorliegt,
• 8b: Magnetfeld als Funktion der Zeit bei einem erfindungsgemäßen Vorgehen, wenn eine vorhergehende Kernmagnetisierung durch einen Feldstärkeimpuls vorliegt,
• 9: scheinbarer Fehlerstrom bei einer Magnetisierung des Wandlerkerns durch einen Feldstärkeimpuls bei erfindungsgemäßem Vorgehen,
• 10a: erfasster Fehlerstrom als Funktion der Zeit, wenn eine Kernmagnetisierung durch einen Feldstärkeimpuls erfolgt und anschließend das erfindungsgemäße Verfahren durchgeführt wird,
• 10b: Magnetfeld als Funktion der Zeit, wenn eine Kernmagnetisierung durch einen Feldstärkeimpuls erfolgt und anschließend das erfindungsgemäße Verfahren durchgeführt wird,
• 11: ein erfindungsgemäßes Verfahren,
• 12 eine Weiterbildung des erfindungsgemäßen Verfahrens.

The invention is explained in more detail below within the framework of an embodiment using figures.

• 1 : conventional RCD type B,
• 2 : Hysteresis curve of a converter core,
• 3 : apparent fault current when the transformer core is magnetized by a field strength pulse,
• 4a : apparent fault current as a function of time when the transformer core is magnetized by a field strength pulse,
• 4b : Field strength as a function of time for a field strength pulse that leads to nuclear magnetization,
• 5 : residual current protective circuit according to the invention,
• 6 : apparent fault current when the transformer core is magnetized by a field strength pulse when the resistor R _mag of the circuit is connected 5 ,
• 7a : detected fault current as a function of time in a procedure according to the invention, if there is no previous nuclear magnetization by a field strength pulse,
• 7b : Magnetic field as a function of time in a procedure according to the invention, if no previous nuclear magnetization by a field strength pulse is present,
• 8a : detected fault current as a function of time in a procedure according to the invention, if a previous nuclear magnetization by a field strength pulse is present,
• 8b : Magnetic field as a function of time in a procedure according to the invention, if a previous nuclear magnetization by a field strength pulse is present,
• 9 : apparent fault current when the converter core is magnetized by a field strength pulse using the inventive procedure,
• 10a : detected fault current as a function of time when nuclear magnetization occurs through a field strength pulse and the method according to the invention is subsequently carried out,
• 10b : Magnetic field as a function of time when a nuclear magnetization is carried out by a field strength pulse and the method according to the invention is then carried out,
• 11 : a method according to the invention,
• 12 a further development of the method according to the invention.

For the sake of simplicity, the implementation of the residual current protection circuit is described in the exemplary embodiment as a circuit breaker, more precisely as a type B RCD. However, the invention is not limited to this case. Not only can the circuit according to the invention be a component of switches that implement other functions (in particular protective functions such as overload protection and short-circuit protection) in addition to the residual current protection function, but it is also conceivable that the circuit according to the invention is combined with other electronic components (e.g. transformers) to create a product in order to implement an additional residual current protection function.

The current transformers used in RCDs usually have a core made of soft magnetic material. This can easily be magnetized by high-frequency differential currents. This weak magnetization of the core can be interpreted as a DC residual current by the RCD's differential current detection means and can lead to the RCD being triggered unintentionally. This is a particular problem with device combinations of RCD and monitoring device - referred to as "RDC-MD", where "MD" stands for "Monitoring Device" - which trigger at low currents. Such devices, which trigger at a maximum amplitude of 6 mA of an AC residual current and at just 4.5 mA of a DC residual current, are used in the field of electromobility, e.g. to meet the requirements of the IEC 62955 standard.

Standard RCDs use the current transformer or ZCT (Zero Current Transformer) to sum the magnetic fields of the load currents in all conductors and transform the differential current (fault current) into the secondary winding of the ZCT. The principle of current conversion does not work for DC currents or direct currents. For this reason, the fluxgate principle is usually used to measure DC fault currents.

A type B RCD is shown schematically in 1 shown. Phase conductors L1-L3 and a neutral conductor N are shown. In the event of a fault current, the RCD trips and interrupts the conductors by means of switches S. An excitation circuit 31 is provided (the power supply of the excitation circuit 31 is not shown), which generates an excitation current, typically by means of an alternating voltage V _exc which drops across the measuring resistor R _sense or 33, i.e. the excitation current is an alternating current. This periodically drives the magnetic core of a ZCT current transformer 1 with the aid of an inductance 32 from one saturation point to the opposite saturation point (saturation detection by means of detector 34). This generates an average magnetic field of 0 A/m. By generating a magnetic field strength of 0 A/m, a compensation current is also generated in the event of a fault current.

This current leads to a voltage drop at the measuring resistor R _sense (reference number 33). A signal processing circuit with the low-pass filter 35 compensates for the influence of the excitation current. An amplifier 36 amplifies the signal and an adder 37 adds an offset so that the signal can be measured with the help of an analog-digital converter (input 41) arranged in a microcontroller 4. The fault current can then be calculated using the following formula: $$i_{\mathrm{\Delta }}\left(t\right)=\frac{\left({\text{V}}_{\text{ADC}}\left(\text{t}\right)-{\text{V}}_{\text{O}}\right)\cdot {\text{N}}_2}{{\text{R}}_{\text{sense}}\cdot \text{S}}$$

Where S is the gain factor, VADC is the digitized voltage at the AD converter, Vo is the added offset voltage, N2 is the secondary winding number of the converter or the winding number of the inductance 32 and Rsense corresponds to the ohmic resistance 33 in 1 .

The microcontroller 4 calculates the value of the fault current. For example, the RMS value is calculated (RMS is an abbreviation for "Root Mean Square" and refers to the effective value of the current). On the basis of the RMS value, criteria for an interruption of the current are checked (e.g. threshold value for RMS value, possibly combined with a time criterion for exceeding the threshold value). If the conditions for an interruption are met, the microcontroller 4 outputs an interruption signal on the output 42, which causes an interruption unit 5 (the English term "trip unit" is also common) to open the switch S.

It should be ensured that magnetization of the core does not lead to an apparent fault current and thus to a tripping of the RCD. The effect of magnetization of the transformer core is the hysteresis curve of 2 shown. The magnetic field strength B is plotted against the magnetic field strength H. The original hysteresis curve is the lower one. If the core of the converter is permanently magnetized by a positive magnetic field with a field strength Hsurge that is significantly greater than the maximum field strength Hexc generated by the excitation generator, the hysteresis loop can no longer be centered around the origin B=0, but can be slightly shifted towards positive field strengths B (upper curve). This results in a small difference between the values of positive and negative remanence: ΔBr.

The magnetization of the core has the same effect on the detection as a DC fault current, which would induce a corresponding magnetic field in the core. The detector circuit automatically compensates for this magnetization by means of a compensation field strength accordingly: $$\mathrm{\Delta }{\text{H}}_{\text{C}}=\mathrm{\Delta }{\text{B}}_{\text{R}}/\mathrm{\mu }$$

The detector circuit then detects an apparent DC current that is not actually present: $${\text{I}}_{\text{app}}={\text{L}}_{\text{FE}}\times \mathrm{\Delta }{\text{H}}_{\text{C}}.$$

3 shows the apparent current as a function of the field strength pulse Hsurge and the subsequent excitation field strength Hexc. The original operating point A is shifted to point B. The subsequent partial demagnetization at the value Hexc above point C results in a residual magnetization at point D and corresponds to the apparent current I_app.

4a and 4b show the same magnetization and its effects in the temporal representation. The transition from point A to D via B and C cannot be represented in this resolution because it is much faster than the period of the excitation field strength Hexc.

Possible scenarios that can lead to nuclear magnetization are: high current pulses caused by a strong current surge during a lightning strike or a short circuit of the circuit during a ground fault. At such currents (many thousands of amps) the field strength can exceed 100,000 A/m, which is comparable to the magnetic field strength of Hexc in the range of 50 A/m.

If the apparent current I_app or the sum of this current I_app together with a technical leakage current (which on its own would not lead to tripping) exceeds the tripping threshold of the RCD, an unwanted tripping of the RCD occurs.

The value of the differential current that can arise from permanent magnetization of the core is generally in the range of a few mA. For an RCD with a nominal tripping of 30 mA, this is usually not a real problem because this apparent current I_app corresponds to a tolerable measurement inaccuracy. On the other hand, for a device with a nominal tripping of 6 mA, this current caused by the magnetization of the core represents a significant problem because it assumes values (a few mA) that can no longer be ignored or significantly changes the tripping behavior.

• - Ein Ausgang „MAG“ (Bezugszeichen 44) der MCU, welcher den Zustand eines Schalters „Switch“ (Bezugszeichen 39) steuert. Dieser Schalter 39 kann eine Verbindung des Widerstands Rmag (Bezugszeichen 38) parallel zu dem Messwiderstand Rsense (Bezugszeichen 33) herstellen.
• - Ein Eingang „POL“ (Bezugszeichen 43) der MCU zum Überwachen der Polarität des Erregerschaltkreises: der Input ist ein binäres Signal, dessen logische Zustände einer positiven Erregerspannung bzw. einer negativen Erregerspannung entsprechen.

5 shows a possible embodiment of the invention. A typical detection circuit of a type B RCD is shown (see 1 ) with the following additions:

• - An output “MAG” (reference 44) of the MCU, which controls the state of a switch “Switch” (reference 39). This switch 39 can connect the resistor Rmag (reference 38) in parallel to the measuring resistor Rsense (reference number 33).
• - An input “POL” (reference 43) of the MCU for monitoring the polarity of the excitation circuit: the input is a binary signal whose logical states correspond to a positive excitation voltage or a negative excitation voltage.

In normal operation, switch 39 is open and resistor 38 is not connected in parallel with resistor 33. The periodic field strength applied to the current transformer is then $$H_{ex\mathrm{<figure-callout\; id="1"\; label="c"\; filenames="DE102023202519A1\_0019.png"\; state="\{\{state\}\}">c</figure-callout>}1}=\frac{V_{exc}\cdot N_2}{R_{sense}\cdot L_{FE}}$$

und die Feldstärke erhöht sich auf: $$H_{exc2}=\frac{V_{exc}\cdot N_2}{R_t\cdot L_{FE}}$$

When switch 39 is closed, the total resistance is: $$R_t=\frac{R_{sense}\cdot R_{maG}}{R_{sense}+R_{maG}}$$

and the field strength increases to: $$H_{ex\mathrm{<figure-callout\; id="2"\; label="c"\; filenames="DE102023202519A1\_0019.png,DE102023202519A1\_0020.png"\; state="\{\{state\}\}">c</figure-callout>}2}=\frac{V_{exc}\cdot N_2}{R_t\cdot L_{FE}}$$

The field strength Hexc2 is substantially greater than the field strength Hexc1, but cannot be as great as the field strength Hsurge. The value of the field strength Hexc2 can, for example, take on a value in the range of 1000 A/m. Based on the input at input 43, the MCU can open the switch 39 after a positive pulse of the positive field strength +Hexc2 or a negative pulse of the field strength -Hexc2.

7a and 7b show that magnetization occurs after a positive pulse and an apparent current Iapp2+ remains afterward. Similarly, a negative apparent current Iapp2- is detected after a negative pulse of field strength Hexc2, as in 7a is shown.

The average value Im of the apparent currents Iapp2+ and Iapp2- is almost zero and corresponds to reality. In the following operation, the MCU must compensate for the remaining apparent current Iapp2- when calculating the differential current. If, for example, in 7b the procedure has ended (ie after the second negative peak) and the RCD has not tripped, the RCD will continue to monitor the fault current for exceeding a tripping threshold. For this threshold monitoring, the induced magnetization is compensated. Let Iapp, for example, be the current value obtained during monitoring. The compensation can then consist of comparing not Iapp, but (Iapp + (Im - Iapp2-)) with the threshold, where Im and Iapp2- are the values obtained during the last determination of Im.

8a and 8b show a scenario where a DC differential current Idiff flows through the RCD. This creates a field strength Hdiff = Idiff / Lfe, which is compensated by the circuit and results in a current Iapp1 that has the same value as the current Idiff. When magnetization is performed at field strengths +Hexc2 and -Hexc2, the average value Im of Iapp2+ and Iapp2- corresponds to the current Iapp1 and the current Idiff respectively. The averaging after magnetization in positive and negative polarities does not affect the result and the measured current corresponds to the actual differential current.

9 as well as 10a and 10b show a scenario where the core of the current transformer at Hsurge has been magnetized by a very high differential current. The MCU detects an apparent current Iapp1 after the high current, which is caused by the unwanted magnetization. At this point, the MCU cannot distinguish between the effect of a magnetization and an actual DC differential current. The MCU then applies the higher excitation field strengths +Hexc2 and -Hexc2 and measures the new apparent currents Iapp2+ and Iapp2-. The average value Im of these currents is considerably lower than the previous apparent current Iapp1. It can be concluded that at least the difference Iapp1 - Im is caused by the magnetization of the core.

The average current Im may not exactly correspond to the actual value of the differential current because the excitation field strength Iexc2 is still considerably lower than the magnetization at Hsurge, i.e. the effect of the unwanted magnetization cannot be fully compensated. The remaining error has an approximate value of Iapp1 - Iapp2+. However, the error in detecting the DC differential current is significantly reduced by this method if the average value Im is interpreted as the DC differential current.

In 11 A flowchart for a method according to the invention is shown. In a first step S1, a monitoring for a possible fault current takes place through a circuit according to 5 At the beginning, the resistor R _mag is not switched on and the induced current Iapp1 is determined. The process continues depending on whether an induced current is present. The level of the determined current Iapp1 can also be used as a criterion for query S2. This apparent current Iapp1 can also be caused by an unwanted magnetization of the converter core. It would be conceivable, for example, to check in query S2 whether the value of the induced current Iapp1 exceeds a threshold value for tripping the RCD. If so, steps S3 to S10 would be carried out to avoid the likelihood of nuisance tripping. In step S3, a change of sign is detected for the excitation voltage Vexc and communicated via the Pol input of the MCU. The MCU then closes switch 39 via the Mag output, whereby the resistor R _mag is connected in parallel with the resistor R _sense . At the next sign change, switch 39 is opened again (step S4). The induced current Iapp2+ is then determined. Steps S3-S5 are then repeated with opposite sign changes (steps S6-S8) and in this way the value Iapp2- for the indicated current is obtained. In step S9, the average value Im of Iapp2+ and Iapp2- is calculated and used as a criterion for tripping (step S10). If a limit value for tripping is exceeded, the RCD interrupts the monitored current path (step S11). Otherwise, monitoring continues. The magnetization of the transformer core is taken into account in steps S6 and S7.

und $${\text{I}}_{\text{app}\text{,max}}={\text{L}}_{\text{FE}}\times \mathrm{\Delta }{\text{H}}_{\text{C}\text{,max}}.$$

In 12 a variant of the method according to the invention is shown. This is based on the following consideration: The procedure according to the invention is intended to improve the tripping reliability by at least partially compensating for a converter core magnetization when evaluating the induced current Iapp1. This compensation is not necessary if the indicated current Iapp1 is so high that the threshold for tripping is exceeded in any case. Let ΔB _R,max be the maximum core magnetization that occurs in operating situations. This leads to an induced current I _app,max according to the equations $$\mathrm{\Delta }{\text{H}}_{\text{C}\text{,max}}=\mathrm{\Delta }{\text{B}}_{\text{R}\text{,max}}/\mathrm{\mu }$$

and $${\text{I}}_{\text{app}\text{,max}}={\text{L}}_{\text{FE}}\times \mathrm{\Delta }{\text{H}}_{\text{C}\text{,max}}.$$

Let I _tripp1 be the tripping threshold for the residual current protection circuit. According to step S2 in 11 the induced current Iapp1 is determined. The following steps S3 - S9 prevent tripping events where the exceeding of the tripping threshold is solely or primarily due to a magnetization of the transformer core. If the determined current Iapp1 is so high that the fault current is definitely too high, tripping can occur immediately, ie without carrying out steps S3 - S9. This condition is met if Iapp1 > I _tripp1 + I _app,max . According to the further development according to 12 a further limit value I _tripp2 is introduced. This is chosen in such a way that if it is exceeded, it can be safely assumed that the fault current is too high, regardless of whether nuclear magnetization is present, e.g. I _tripp2 ≥ I _tripp1 + I _app,max .

The maximum core magnetization and thus also I _app,max can be determined empirically for a given residual current protective circuit. This procedure is described in step S21 in the further development according to. 12 implemented. Immediate tripping occurs when the threshold value I _tripp2 is exceeded. The advantage is faster tripping, especially in the case of high fault currents, which are also potentially more dangerous.

QUOTES INCLUDED IN THE DESCRIPTION

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Cited patent literature

• DE 102018204047 A1 [0003]
• DE 102019204272 A1 [0003]
• DE 102021202171 A1 [0003]

Claims (14)

Residual current protection circuit for detecting a direct current with the aid of a current transformer (1), wherein the action of an oscillating magnetic field on the transformer (1) in the event of a DC residual current occurring in a monitored circuit induces a compensating current which is evaluated with regard to the need to interrupt the circuit, with - a current transformer (1), - an excitation circuit (31) for generating an oscillating excitation voltage, - an inductance (32) for inducing a magnetic field in the current transformer (1), and - a first resistor (33), characterized in that - the protection circuit is further developed with a second resistor (38) and a switch (39), wherein the second resistor (38) is arranged parallel to the first resistor (38) and can be switched on by means of the switch (39). Residual current protection circuit according to Claim 1 , characterized in that the protective circuit is further developed with a computing unit (4) for evaluating the detected current, which is designed to cause the switch (39) to open and close by means of a control signal that can be output via an output (Mag). Residual current protection circuit according to Claim 2 , characterized in that - the excitation circuit (31) is connected to an input (pole) of the computing unit (4), and - the protection circuit is designed such that the excitation circuit (31) provides information via the input (pole), from which a change in sign of the excitation voltage can be determined. Residual current protective circuit according to one of the Claims 1 until 3 , characterized in that the excitation circuit (31), the inductance (32) and the first resistor (33) are arranged in series. Method for detecting a direct current by means of a residual current protection circuit according to one of the Claims 1 until 4 , comprising the steps of - opening the switch (39) when the excitation voltage changes sign for the first time, - closing the switch (39) when the excitation voltage changes sign following the first, - detecting a first induced compensating current (Iapp2+), - opening the switch (39) when the excitation voltage changes sign for the second time, the second change of sign being opposite to the first, - closing the switch (39) when the excitation voltage changes sign following the second time, - detecting a second induced compensating current (Iapp2-), and - forming an average value (Im) of the first and second induced compensating currents (Iapp2+, Iapp2-). Procedure according to Claim 5 , characterized by the steps - monitoring the circuit for a DC fault current, - upon detection of the occurrence of a fault current (Iapp1) carrying out the steps of Claim 5 to determine a value for the fault current, and - using the mean value (Im) of the first and second induced compensating currents (Iapp2+, Iapp2-) as the value for the fault current. Procedure according to Claim 6 , characterised by the additional steps of - checking whether the value of the fault current exceeds a threshold for the permissible fault current, - issuing a signal to interrupt the circuit when the threshold is exceeded. Procedure according to Claim 6 or 7 , characterized in that - the circuit is monitored for a DC fault current by detecting an induced compensating current (Iapp1) and checking whether it exceeds a threshold value for tripping, and - the determination of a value for the fault current according to Claim 6 is only carried out if the trigger threshold is exceeded. Procedure according to Claim 8 , characterized in that - in addition, a check is carried out to see whether a threshold value for immediate tripping is exceeded, and - the determination of a value for the fault current according to Claim 6 is only carried out if the threshold for immediate triggering is not exceeded. Procedure according to Claim 9 , characterized by immediately issuing a signal to interrupt the circuit when the threshold value for immediate tripping is exceeded. Method according to one of the Claims 5 until 9 , characterized in that when monitoring the circuit for a DC fault current after completing the steps of the Claim 5 the effect of any resulting magnetization of the current transformer (1) is taken into account. Computer program that implements a method according to one of the Claims 5 until 11 when run on a computer. Computer program product containing a computer program according to Claim 12 . Residual current protective circuit according to one of the Claims 2 until 4 , characterized in that the computing unit (4) is designed to carry out a method according to one of the Claims 5 until 11 to carry out.

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