Method and evaluation unit for calculating a fault current between a grounded secondary side of a transformer and a switching device

Abstract

A method is proposed for calculating a fault current between a grounded secondary side of a transformer and a switching device using an evaluation unit (ETU) of the switching device. This method is based on a switching device for switching and interrupting an AC power supply to a load, consisting of three phase conductors and a neutral conductor (N). The switching device includes identical sensors for measuring the phase currents and is configured to transmit corresponding measurement signals to an evaluation unit (ETU) of the switching device. It is also configured to receive measurement signals from a sensor located at the grounded secondary side of the transformer for measuring the current between the secondary side of the transformer and earth (PE) and transmitting these signals to the evaluation unit (ETU).In the inventive method, the measurement signals of the sensor arranged at the grounded secondary side of the transformer for measuring the current between the secondary side of the transformer and earth (PE) are adapted according to the measurement signals transmitted by the similar sensors for measuring the line currents, the measurement signals of the three line currents and the adapted measurement signal from the sensor arranged at the grounded secondary side of the transformer are added for a defined time and a fault current between the grounded secondary side of the transformer and the switching device is calculated for that time using the added measurement signals.

Description

[0001] The invention relates to a method for calculating a fault current between an earthed secondary side of a transformer and a switching device and an evaluation unit designed for carrying out the method.

[0002] Protective and switching devices are used for the distribution of electrical energy. In the event of a fault, the switching device, located on the power source side immediately upstream of the fault, should interrupt the fault current.

[0003] Circuit breakers are used for power distribution. Circuit breakers can not only switch operating currents but also interrupt the circuit in the event of a fault. They are usually designed for high switching capacities but low switching frequencies. In low-voltage applications, open circuit breakers (air circuit breakers - ACBs) and compact circuit breakers (moulded case circuit breakers - MCCBs) are primarily used.

[0004] In Fig. Figure 1 shows an exemplary scenario. A fault current on the outgoing side can be disconnected by the switching device 4. Fault currents on the outgoing side can be detected by the current sensors 5. The electronic trip unit 7 monitors the measured current and compares it with an adjustable limit value. If this limit is exceeded, the electronic trip unit 7 sends a tripping command to the circuit breaker 4.

[0005] Fault currents or differential currents can occur due to an overload, a short circuit, or a ground fault. Because a ground fault can be detected without restriction, the area on the outgoing side or the consumer side is referred to as the "unrestricted zone."

[0006] The circuit breaker 4 typically includes the current sensors 5 and the electronic trip unit 7. Circuit breaker 4 is, for example, a low-voltage circuit breaker. Typical low-voltage circuit breakers are compact circuit breakers or MCCBs (moulded case circuit breakers) and open circuit breakers or ACBs (air circuit breakers). A fault between a transformer 8 and the low-voltage supply circuit breaker 4 can only be disconnected by the switching device 1 on the high-voltage side of the transformer.

[0007] Current transformers 2 and 3 are used for fault current detection. The electronic trip unit 6 compares the measured signal with an adjustable limit value. If this limit is exceeded, the electronic trip unit 6 sends a switch-off command to the switching device 1.

[0008] Due to the arrangement of current transformers 2 and 3, earth faults can only be detected in the area between transformer 8 and circuit breaker 4. Therefore, this area is referred to as the restricted zone.

[0009] Different current sensors 5 and current transformers 2 and 3 are used to detect the fault currents in the two areas. The output signals of elements 2, 3, and 5 are very different. Current transformers 2 and 3 are relatively large and require complex integration into the switching device. Low-voltage circuit breakers such as the Siemens 3WA (ACB) can process and evaluate the measurement signals from 2 and 3 and send a trip command to the switching device 1.

[0010] Current transformers 2 and 3 are connected to the circuit breaker with high impedance. Additional components such as a varistor or voltage dividers made of power resistors are required to protect the measurement input.

[0011] There is a need for cost-effective solutions for protecting the restricted area or "restricted zone" with a circuit breaker in application scenarios comparable to that of Fig. 1. The object of the invention is to contribute to this.

[0012] The problem is solved by a method according to claim 1.

[0013] Beneficial further training opportunities are listed in the sub-requirements.

[0014] The invention proposes a method for calculating a fault current between a grounded secondary side of a transformer and a switching device or a defined point in a monitored power supply (typically the point where phase currents are detected by sensors in the switching device). The method is carried out by an evaluation unit (e.g., ETU or Electronic Trip Unit) of the switching device. The invention assumes a switching device (e.g., a circuit breaker) for switching and interrupting an AC power supply to a load based on a grounded three-phase system with three phases. This switching device includes identical (typically structurally identical) sensors for measuring the phase currents and is designed to transmit corresponding measurement signals to an evaluation unit of the switching device.The switching device is also designed to receive measurement signals from a sensor located on the grounded secondary side of the transformer. This sensor measures the current between the secondary side of the transformer and ground and transmits these signals to the evaluation unit (ETU). This measurement is typically performed at the exposed, grounded neutral point of the transformer. If the AC power supply has a neutral conductor (the term "neutral conductor" here also includes a PEN conductor), the measurement can be performed between the phases and the neutral conductor (N), between the phases and the protective earth conductor (PE), or between the phases and the PEN conductor, depending on the network configuration. For an architecture with separate neutral conductors (N) and protective earth conductors (PE), the measurement can be performed between the phases and the neutral conductor (N) or between the neutral conductor (N) and protective earth conductor (PE).

[0015] The method according to the invention involves adjusting the measurement signals of the sensor located at the grounded secondary side of the transformer for measuring the current between the secondary side of the transformer and ground or at the brought-out, grounded neutral point, according to the measurement signals transmitted by similar sensors (e.g., sensors formed with a Rogowski coil) for measuring the line currents, and adding (e.g., vectorial addition) the measurement signals of the three line currents and the adjusted measurement signal from the sensor located at the grounded secondary side of the transformer for a defined time. This can involve either the direct addition of measured values ​​acquired at that time or the addition of averaged values ​​(e.g., RMS values) (here, the term "time" also includes a time interval – it is important that all measurement signals are temporally correlated).Finally, the calculation of a fault current between the grounded secondary side of the transformer and the switching device for the given time is carried out using the added measurement signals.

[0016] By adjusting the measurement signals of the sensor located at the grounded secondary side of the transformer for measuring the current between the secondary side of the transformer and ground, or at the grounded neutral point brought out, according to the measurement signals transmitted by similar sensors, all measurement signals can be processed directly together. This simplifies the fault current calculation. Adjusting the measurement signals of the sensor located at the grounded secondary side of the transformer can include scaling the amplitude and / or a phase shift.

[0017] According to a further development of the inventive method, the adjustment of the measurement signals of the sensor arranged at the grounded secondary side of the transformer for measuring the current between the secondary side of the transformer and ground, or at the brought-out, grounded neutral point, is determined by means of an injection of known test signals, based on the measurement signals transmitted by the similar sensors for measuring the line currents. For example, the adjustment is then carried out by means of scaling and / or phase shifting such that corresponding values ​​are obtained for a test signal that is injected at the sensor arranged at the transformer and at a sensor for measuring a phase.

[0018] According to a further development of the inventive method, the measurement signals of the sensor arranged at the grounded secondary side of the transformer are digitized on the sensor side, transmitted to the switching device in digital form and converted back into analog signals on the device side.

[0019] According to a further development of the inventive method, a further fault current between the switching device and a consumer is calculated by the evaluation unit using the added measurement signals of the three line currents.

[0020] According to one configuration for an AC power supply with a neutral conductor, the neutral conductor current is detected by a sensor similar to those used to measure the line currents, and corresponding measurement signals are transmitted to the evaluation unit of the switching device. For the calculation of the fault current and / or the subsequent fault current (i.e., on the supply side or the consumer side), a measurement signal for the specified time is then added to the measurement signals of the three line currents (for the specified time).

[0021] According to one embodiment of the method according to the invention, time points are specified by the evaluation unit, and the method is carried out for these time points, i.e., a fault current calculation is performed (continuously) for these time points.

[0022] The invention also relates to an evaluation unit (e.g. ETU) designed to carry out a method according to the invention, and a computer program product with a computer program which carries out a method according to the invention when it runs on an evaluation unit, as well as such a computer program.

[0023] The invention is described in more detail below using an embodiment as an example. The figures show... Fig. 1: a conventional switching device, Fig. 2: a switching device with which a method according to the invention is carried out, Fig. 3: the signal path of the current transformer (4) from the switching device accordingly Fig. 2, Fig. 4: Signal waveforms of a sensor (2) formed with a Rogowski coil from the switching device accordingly Fig. 2, Fig. 5: the primary current, the voltage of the Rogowski coil and the secondary current of the current transformer (4) normalized to the Rogowski coil signal from the switching device accordingly Fig. 2, Fig. 6: a schematic diagram of a circuit breaker, Fig. 7: a metrological determination of the correction factors during the commissioning of the circuit breaker, and Fig. 8: a method designed according to the invention for determining fault currents on the supply side and on the consumer side with an adaptation according to the invention of the measuring signals of the sensor arranged at a grounded secondary side of a transformer.

[0024] Fig. Figure 2 shows an example of the use of a circuit breaker in a typical distribution network configuration in a TN-S system, i.e., separate neutral and protective conductors run from the transformer to the loads in the distribution network. As shown, an earth connection (the neutral point) is provided on the low-voltage side of the supplying transformer. Current sensors (1) (usually Rogowski coils) are used in the circuit breaker for current measurement. A current sensor (2) identical in construction to the internal Rogowski coils of the circuit breaker is used to measure the neutral conductor current. This neutral current sensor is located in the circuit breaker or is connected externally to the circuit breaker. A "classic" current transformer (4) can be installed in the neutral point conductor of the transformer for direct measurement of an earth fault current.

[0025] The output signals of the (e.g., inductively operating) current transformer (4) and the Rogowski coils (1), (2) differ in magnitude and angle. For this reason, the electronic trip unit has different input circuits for processing the current signals. Therefore, the current measurement signals of the Rogowski coils and the external current transformer or a current transformer combination are conventionally evaluated separately; that is, the vectorial addition of the phase currents to determine a ground fault current by the electronic trip unit (3) is performed only with the current signals of the Rogowski coils (1) and (2) for the output side and the load side, respectively ("unrestricted zone"). Currently, there is no mixing of different types of input signals. This is where the invention comes in.

[0026] The output signal of the in Fig. The output signal of the externally connected current transformer (4) shown in the diagram can be modified by the electronic trip unit using a mathematical algorithm and is then equivalent to a Rogowski coil output signal. This signal can then be used, for example, for vector addition to calculate an earth fault. Earth fault currents in the range of 100 A to max. 2000 A should be detected and disconnected.

[0027] Fig. Figure 3 shows the signal waveform of the current transformer (2). The secondary current Isek depends on the primary current Iprim and the turns ratio. There is no perceptible phase shift between the primary current Iprim and the secondary current Isek.

[0028] In contrast, a Rogowski coil provides an output voltage Usek as an output signal, which corresponds to the derivative of the primary current. This signal, corresponding to the derivative, is integrated in the electronic triggering unit. Only then is a phase-coherent image Usek_integrated of the primary current Iprim generated. This is in Fig. 4 shown.

[0029] In order for the output signal of the current transformer to be processed together with the signals of the Rogowski coils, it must be adjusted in magnitude and, if necessary, angle.

[0030] Fig. Figure 5 shows the primary current Iprim, the voltage of the Rogowski coil Usek and the secondary current Isek_normalized of the current transformer, which is normalized to the Rogowski coil signal.

[0031] To adjust the signal supplied by the current transformer, the magnitude and, if necessary, the angle must be corrected. The determination and processing of correction factors can be carried out in the electrical trip unit of a circuit breaker.

[0032] Fig. Figure 6 schematically shows elements of a typical low-voltage circuit breaker (LS). The circuit breaker (LS) is designed to interrupt electrical conductors L1, L2, L3 of an electrical circuit, for example, a three-phase AC circuit, where the first conductor L1 is the first phase, the second conductor L2 is the second phase, and the third conductor L3 is the third phase of the three-phase AC circuit. It is designed according to... Fig. 2. Furthermore, a neutral conductor is provided. This is in Fig. 6 not shown. In general, the sensor for the neutral conductor can also be located outside the circuit breaker; the signal is then transmitted to the circuit breaker.

[0033] In the example according to Fig. 6. The third conductor L3 is connected to the energy converter EW such that at least a portion of the current, i.e., a partial conductor current, or the entire current of the third conductor flows through the primary side of the energy converter EW. The energy converter EW is typically a core-type transformer. An energy converter EW can also be provided in each phase or conductor of the electrical circuit. The secondary side of the energy converter EW is connected to a power supply unit NT, which provides power, for example, in the form of a supply voltage, for a control unit or evaluation unit ETU (e.g., Electronic Trip Unit). A sensor unit SE is provided, which is formed with at least one sensor element to determine the magnitude of the electric current. This is, for example, a Rogowski sensor, which consists of a Rogowski coil and an analog integrator.

[0034] The sensor unit (SE) is connected to the control unit (ETU) and transmits the magnitude of the electric current of at least one or more conductors in the electrical circuit. For example, alternating current with a mains frequency of 50 Hz is transmitted. A current measurement is then taken with each half-cycle, i.e., every 10 ms (samples). The display of values ​​takes into account the physiological characteristics of the human eye. For example, a value is displayed every 200 ms (display value). The display value is calculated, for example, as the root mean square (RMS) of the samples taken within a 200 ms interval. These values ​​are often also referred to as RMS values.

[0035] The transmitted current values ​​are compared in the control unit ETU with current limits and / or current-time interval limits, which determine the triggering criteria. If these limits are exceeded, the electrical circuit is interrupted. This can be achieved, for example, by using an interruption unit UE, which is connected to the control unit ETU and has contacts for interrupting conductors L1, L2, L3, or other conductors of the electrical circuit. In this case, the interruption unit UE receives an interruption signal to open its contacts.

[0036] The ETU control unit is equipped with a display AZ, which can show values ​​for system-relevant parameters such as current, voltage, energy, power, phase angle, etc. Some of these values ​​are measured, while others are calculated from measured values. A communication interface KS (e.g., Zigbee, Wi-Fi, or BLE wireless interface, or a wired interface, e.g., for a LAN cable) is also shown, through which the recorded system-relevant values ​​can be transmitted to a monitoring station for display or analysis.

[0037] Regarding the determination of a correction factor for the amount, the following procedure can be used. The ETU600 electronic trip unit of the Siemens 3WA series circuit breakers has an input for connecting a current transformer for directly measuring an earth fault current. A current transformer with a rated current of 150 A to 2000 A and a secondary rated current of 1 A can be connected to this input. The transformation ratio is set during the commissioning of the circuit breaker.

[0038] The correction factor for the magnitude can be derived from the translation ratio and the level reduction of the integrator of the Rogowski coils.

[0039] Regarding the correction factor for the angle, the following approach is possible. Initially, the angular error could be neglected or set to a constant value.

[0040] Determining the correction factors through measurement during commissioning of the circuit breaker would be one option. Alternatively, the correction factors can be determined during an adjustment (calibration mode). For this, the current transformer and one phase of the circuit breaker must be connected in series and a test current must be applied. This in Fig. 7 shown.

[0041] The RMS value of the current transformer, measured by the electronic trip unit, is compared with the RMS value of the phase current measured via Rogowski coils. The quotient corresponds to the magnitude correction factor. The angle correction factor can be determined from the shift of the zero crossings.

[0042] The solution described above introduces, for the first time, the combined processing of different signal inputs for a protective function. The current transformer can be installed at a considerable distance of 100 to 250 m from the circuit breaker without signal loss. In contrast, a Rogowski coil may be installed no more than 10 m from the circuit breaker.

[0043] Fig. Figure 8 shows the sequence of a method with a procedure according to the invention for determining differential currents for the supply-side and consumer-side areas. Values ​​for a supply-side differential current I,Diff,Ref and a consumer-side differential current I,Diff,UREF are continuously determined. This determination is performed at a clock frequency. This clock frequency corresponds, for example, to a clock used to obtain measured values ​​and is specified by an MCU or a CPU of the evaluation unit ETU of the switching device. The corresponding clock frequency then corresponds, for example, to the operating frequency of the MCU or CPU or is derived from it. In a first step S11, the current values ​​of the phases I,L1(j), I,L2(j), and I,L3(j) and the neutral conductor I,N(j) are obtained. The value I,PE(j) of the current sensor 4 is obtained from another input of the evaluation unit. Fig. 2. The evaluation unit receives the differential current I,Diff,UREF from the consumer side of phases I,L1(j), I,L2(j), and I,L3(j) and the neutral conductor I,N(j). It uses this information to determine a value for the differential current I,Diff,UREF by vector addition (I,Diff,UREF = ΣI,LN = I,L1(j)+I,L2(j)+I,L3(j)+I,N(j)) (step S13) and compares it to a threshold SW(1) for tripping (step S14). If necessary, tripping occurs at this point (step S15). To determine the differential current I,Diff,REF from the supply side, the value I,PE(j) received from current sensor 10 is adjusted (step S16: I,pe(j) = f(I,PE(j))). Typically, this adjustment includes scaling and, if necessary, a phase shift, so that the adjusted value I,pe(j) is comparable to or processable with the values ​​I,L1(j), I,L2(j) and I,L3(j) of the phase conductors and the neutral conductor I,N(j).Subsequently, in step S17, the feed-side differential current I,Diff,REF is determined by vector addition (I,Diff,REF = ΣI,LNPE = I,pe(j) + ΣI,LN) (step S17), compared with a threshold value SW(2) (which can correspond to the first threshold value SW(1)) in step S18, and triggered if necessary (step S19). If no trigger occurs, the procedure is repeated in the next iteration (see step S20). This procedure is for illustrative purposes only. In real-world implementations, further optimizations will be made, such as using two parallel programs to simultaneously calculate the feed-side differential current I,Diff,REF and the consumer-side differential current I,Diff,UREF.

[0044] For certain network configurations, it may be that due to the sensor position of sensor 4... Fig.2. No neutral conductor signal is required to perform a fault current calculation for the restricted area.

Claims

[1] Method for calculating a fault current between an earthed secondary side of a transformer and a switching device by an evaluation unit (ETU) of the switching device, wherein - the switching device is designed for switching and interrupting an alternating current power supply of a consumer based on an earthed three-phase system with three outer conductors (L1, L2, L3), - the switching device includes identical sensors for measuring the line currents and is designed to transmit corresponding measurement signals to an evaluation unit (ETU) of the switching device, and - the switching device is designed for receiving measurement signals from a sensor arranged at the earthed secondary side of the transformer for measuring the current between the secondary side of the transformer and earth (PE) and transmitting them to the evaluation unit (ETU), comprising - Adapting the measurement signals of the sensor located on the grounded secondary side of the transformer for measuring the current between the secondary side of the transformer and earth (PE) according to the measurement signals transmitted by the similar sensors for measuring the line currents, and - Adding the measurement signals of the three line currents and the adapted measurement signal from the sensor located at the grounded secondary side of the transformer for a specified time, and - Calculating a fault current between the grounded secondary side of the transformer and the switching device for the time using the added measurement signals. [2] Method according to claim 1, characterized by , that adjusting the measurement signals of the sensor located at the grounded secondary side of the transformer includes scaling the amplitude. [3] Method according to one of claims 1 or 2, characterized by, that adjusting the measurement signals of the sensor located at the grounded secondary side of the transformer involves a phase shift. [4] Method according to any one of the preceding claims, characterized by , that the adjustment of the measurement signals of the sensor arranged at the earthed secondary side of the transformer for measuring the current between the secondary side of the transformer and earth (PE) is determined according to the measurement signals transmitted by the similar sensors for measuring the line currents by means of an injection of known test signals. [5] Method according to any one of the preceding claims, characterized by , that the measurement signals of the sensor located on the grounded secondary side of the transformer are digitized on the sensor side, transmitted to the switching device in digital form and converted back into analog signals on the device side. [6] Method according to any one of the preceding claims, characterized by, that a further fault current between the switching device and a consumer is calculated by the evaluation unit (ETU) using the added measurement signals of the three line currents. [7] Method according to any one of the preceding claims, characterized by , that - the alternating current power supply is formed with a neutral conductor (N), - the neutral conductor current is detected by a sensor similar to the sensors used to measure the line currents, and corresponding measurement signals are transmitted to the evaluation unit (ETU) of the switching device, and - for the calculation of the fault current and / or the further fault current, a measurement signal for the specified time is added to the measurement signals of the three line currents. [8] Method according to any one of the preceding claims, characterized by , that - times are specified by the evaluation unit (ETU), and - the procedure is carried out for these times. [9] Evaluation unit (ETU) configured to carry out a method of one of claims 1 to 8. [10] Computer program product comprising a computer program which performs a method according to any one of claims 1 to 8 when it runs on an evaluation unit (ETU) according to claim 9. [11] Computer program which performs a method according to any one of claims 1 to 8 when it runs on an evaluation unit (ETU) according to claim 9.

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