Measuring system for measuring current value of conductor and correcting measured current value

Abstract

The invention relates to a measuring system (5) for measuring a current value of an alternating current flowing through a conductor (1) and correcting the measured current value with respect to a capacitive interference input. The measuring system comprises: a Rogowski coil (2) which is designed to induce a voltage by means of an alternating current; the signal processing device (7) is used for acquiring and processing an induced voltage value in the Rogowski coil; means (6) for determining a current value by integrating the voltage processed by the signal processing means; and means (6) for correcting the current value with respect to the capacitive interference coupling input. In this case, the device (6) for correcting the current value is provided for correcting by means of an approximation of the voltage drop caused by the capacitive interference coupling input. By means of the correction according to the invention, the measurement accuracy is improved without additional hardware expenditure. Therefore, the measuring device formed by the Rogowski coil can be realized in a smaller or more compact manner.

Description

Technical Field

[0001] The invention relates to a measuring system comprising a Rogowski coil for measuring the current value of a conductor and correcting the measured current value with respect to capacitive interference coupling. Background Art

[0002] Rogowski coils are used to measure alternating currents (AC current measurement). The voltage at the output of the measuring turns of the Rogowski coil is used as an inductive measurement. In addition to the time-varying magnetic field of the current to be measured (the current in the current conductor to be measured (primary current)), the voltage at the output of the measuring turns of the Rogowski coil is also influenced by electric fields, in particular alternating electric fields. The main source of the alternating electric field is the current conductor (primary current) itself, since the current conductor is at mains potential relative to the measuring circuit (for example, in the case of a voltage of 230 Vrms and a frequency of 50 Hz; Vrms: root mean square of the alternating voltage) (see Figure 1 ).

[0003] Generally speaking, the conductive windings of a Rogowski coil form a more or less complex capacitive structure with all conductive surfaces of the environment. Since the distance between the turns of a Rogowski coil and the conductive surface (e.g., the surface of the primary conductor) is greater than with a discrete capacitor, the corresponding coupling capacitance is usually only very low. However, since the potential difference between these surfaces is very large compared to the measured voltage across the Rogowski coil, even a small coupling capacitance can lead to considerable measurement errors when measuring current using a Rogowski coil under normal operating conditions in low-voltage networks.

[0004] This problem is typically addressed by using shielding surfaces, such as expanded metal, additional shielding turns made of thin wire or strips of shielding material, or shielding the entire sensor housing. If these shielding surfaces are held at the coil's measuring potential via suitable electrical connections to the measuring circuit, they generate the necessary "countercharge" on their surface, thereby shielding the electric field. This means that the surface of the Rogowski coil does not need to be reloaded, and no capacitive interference occurs at the Rogowski coil's measuring output. The use of capacitive shielding in small Rogowski coils is subject to certain limitations, as good shielding should not affect the time-varying magnetic field in the coil and should not significantly increase the space required. When using shielding turns, care should be taken that they are not connected to ground or a reference potential on both sides (to avoid circulating currents).

[0005] In the past, larger Rogowski coils were often used to measure larger currents. For this purpose, Rogowski coils with a significantly larger diameter (ring diameter) but a similar ring height were used. Generally speaking, these larger Rogowski coils can be designed to provide a better signal-to-noise ratio. Simply put, an increase in the distance between the turns of the Rogowski coil and the current conductor (primary current) results in a reduction in coupling capacitance. However, this is only possible if the necessary installation space is available in the power distribution system. Generally speaking, AC currents with effective values ​​of less than 100 A can also be measured using toroidal core transformers in low-voltage applications. In addition to their advantages, toroidal core transformers also have some disadvantages compared to Rogowski coils, such as higher cost, greater weight, a larger structural design, or a significantly larger size. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to effectively correct interference coupling input in a measurement system based on Rogowski coils without increasing the space requirement of conventional systems.

[0007] This technical problem is solved by the measuring system according to the invention. Advantageous further developments are given in the description.

[0008] According to the present invention, a measuring system is proposed for measuring the current value of an alternating current flowing through a conductor and correcting the measured current value with respect to capacitive interference coupling. This can also be a system with multiple phases or conductors and can take into account capacitive interference coupling of other phases into the conductor being observed. The measuring system can be designed as a measuring device, such as a power monitoring device or PMD (Power Monitoring Device). However, the system can also be designed as a multi-component system, for example by transmitting the measured values ​​to a central evaluation location and performing the correction there.

[0009] The system according to the present invention includes a Rogowski coil, which is designed to induce a voltage via an alternating current. Typically, the conductor to be monitored is passed through the Rogowski coil. Furthermore, the system includes a signal processing device for acquiring and processing the value of the voltage induced in the Rogowski coil. The signal processing device, for example, is implemented as a current loop, through which the induced voltage is intercepted, filtered, and possibly amplified for subsequent input to an analog-to-digital converter. Post-processing of the digital signal may also be performed.

[0010] Furthermore, the system comprises means, for example a microcontroller, for determining the current value by integrating the voltage processed by the signal processing means.

[0011] Preferably, the system is constructed with an analog-to-digital converter and digitally integrates the processed voltage. However, the present invention is also applicable in principle to systems with analog integration based on Rogowski coils. It is conceivable that the analog system be supplemented by a (possibly external) device for correcting the current value, which corrects the current value obtained by analog integration.

[0012] According to the invention, there is also a device for correcting the current value with respect to capacitive interference coupling, which is preferably identical to the device for determining the current value by integration. The device is designed or programmed to use an approximation for the voltage drop caused by capacitive interference coupling. Make corrections.

[0013] The approximation For example, it has the following form, in which the approximation is based on at least one calibration coefficient (C K1,1 ) describes the voltage drop caused by capacitive interference coupling into the input, wherein the at least one calibration factor (C K1,1 ) can be determined by one or more calibration measurements.

[0014] Furthermore, the approximation may include at least one term that is related to the calibration coefficient (C K1,1 ), the derivative of the voltage that can be assigned to the monitored conductor and the resistance (R i_RoGo ). In this case, multiple phases can be taken into account, and a corresponding term is provided for each phase considered.

[0015] The approximate form can be where the index μ runs through the considered phase, C K1,μ is the calibration coefficient, is the voltage that can be distributed to the monitored conductor (R i_RoGo ) and R i_RoGo is the resistance that can be assigned to the Rogowski coil (R i_RoGo ).

[0016] With the aid of an approximation for the voltage drop caused by capacitive interference coupling into the input The correction performed can take the form of using the expression obtained by integration (possibly with sign adjustment) in the formula for the current value (I1(t)) of the monitored conductor.

[0017] Generally speaking, the approximation The form of can be configured so that the approximate integral is based on the voltage (U μ(t)) and, if necessary, in the case of approximations of the other phases under consideration, linear functional terms. This has the advantage that, in the calibration measurements for determining the coefficients, a linear equation or a system of linear equations is obtained, which can be solved relatively easily.

[0018] The expression for the conductor current (I1(t)) can be expressed as where the index μ runs through the considered phase, C K1,μ is the calibration factor, U μ (t) is the voltage allocable to the conductor (the conductor being examined or other phase being considered), R i_RoGo is the resistance assignable to the Rogowski coil, and The term "integral of the induced voltage in the Rogowski coil" should be understood as a voltage value that can be processed by a signal processing device after induction in the Rogowski coil.

[0019] The present invention also includes a method for determining a calibration factor (C) of a measuring system according to the present invention by means of a calibration measurement. K1,1 ) method. Here, an expression of the form

[0020]

[0021] Based on this, the calibration factor (C K1,1 ). This includes the use of simplified expressions, such as considering only one conductor or neglecting the secondary diagonal of the matrix. Preferably, the voltage applied to the conductor or other phase Measured at lower levels.

[0022] The invention has the advantage that no additional device elements are required, which are necessary in conventional systems and increase the space requirement. Furthermore, a mathematical expression can be specified by which the correction can be performed simply, efficiently and with minimal effort. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be described in detail below within the scope of exemplary embodiments.

[0024] Figure 1 A simplified diagram showing the generation of capacitive interference coupling input to the signal of a Rogowski coil;

[0025] Figure 2 A schematic diagram showing a measuring device (power monitoring device) for acquiring measurement data related to a current loop;

[0026] Figure 3 A simple equivalent circuit diagram showing the combination of a Rogowski coil and an anti-aliasing low-pass filter;

[0027] Figure 4 A simplified equivalent circuit diagram showing a combination of a Rogowski coil and an anti-aliasing low-pass filter;

[0028] Figure 5 Shown based on Figure 3 The simulation results of the capacitively coupled input in the equivalent circuit diagram are shown in Figure 2. DETAILED DESCRIPTION

[0029] Figure 1 A conductor 1 is shown surrounded by a Rogowski coil 2. Electric field lines 3 are drawn in order to illustrate the influence of the field generated by the conductor 1 on the Rogowski coil 2, whereby the induced voltage signal on the signal line 4 of the Rogowski coil 2 is influenced.

[0030] More precisely, the alternating electric field formed between the primary conductor 1 and the Rogowski coil 2 used to measure current generates potential fluctuations relative to ground on the signal line 4 or the signal output of the Rogowski coil. These potential fluctuations can lead to various interferences in the current measurement. These interferences are hereinafter referred to as capacitive interference coupling. This is a common term, stemming from the fact that the conductor surface and the turns of the Rogowski coil form a capacitor. In addition to overloading the input of the measuring circuit, Furthermore, these capacitive interference couplings lead in particular to series interference in the measuring circuit.

[0031] Power monitoring devices (PMDs) are commonly referred to as PMDs. Compact PMDs are primarily used to measure electrical power, thereby measuring current and voltage at grid frequency.

[0032] Figure 2 A PMD 5 for acquiring current loop-related measurement data is shown. The PMD can also be designed to determine consumption data. The determination of consumption data can be used, for example, for standardized billing of consumed energy (e.g., according to MID guidelines EN50470-1 / 3 or standards IEC TR 63213 or IEC 61557-12).

[0033] The PMD 5 is connected to the current loop or measurement network to be monitored and receives measurement data, which is acquired by a sensor (in this example, a Rogowski coil) and processed by a measurement electronics 7. The measurement electronics typically include a low-pass filter 71 (anti-aliasing), an analog-to-digital converter (ADC) 72, and a filter 73, such as a high-pass filter, for post-processing the digitized data. These components process the sensor data in the order described above (see also DE 102015216981 B4). The processed measurement data is further processed by a microcontroller or MCU 6 (e.g., calculation of consumption data). The MCU 6 is also equipped with an interface 9 through which data can be transmitted or read. A power supply device or power supply circuit 8 is provided for powering the MCU 6. The power supply device is powered by the monitored current loop and provides a 3.3V DC voltage to the MCU 6. In PMDs, an additional energy supply for the power supply circuit (power supply circuit) is typically provided to ensure power supply.

[0034] If a Rogowski coil is used for current measurement at the grid frequency, the Rogowski coil is typically designed so that its internal impedance is dominated by the ohmic series resistance of the coil at the grid frequency. At the same time, the impedance of the anti-aliasing filter 71 should be selected so that it is significantly greater than the ohmic series resistance of the Rogowski coil. To prevent adverse effects on the Rogowski coil signal quality, for example due to temperature drift, the value of the "ohmic component" of the anti-aliasing low-pass filter is typically selected to be significantly greater than the ohmic series resistance of the Rogowski coil (RAA > Ri_Rogo → 1 / (ωmess*C) > RAA > Ri_Rogo) (see Equation 1). The capacitance CK of the capacitive coupling and the values ​​RAA and CAA of the anti-aliasing low-pass filter result in the following relationship:

[0035]

[0036] If the impedance of the capacitive coupling between the primary conductor and the Rogowski coil is much greater than the ohmic series resistance of the Rogowski coil within the measurement frequency range (for PMD or energy monitoring equipment, measurements are mainly made at the grid frequency), then Figure 3 The equivalent circuit diagram in can be greatly simplified. The greatly simplified equivalent circuit diagram is as follows Figure 4 shown.

[0037] use Figure 4 The equivalent circuit diagram and the approximate value of equation 1 can be used to derive the approximate formula for the current through the coupling capacitor as follows:

[0038] ICK=U·jωCK Equation 2

[0039] The reference to plural quantities is indicated by underscores.

[0040] Therefore, the capacitive coupling between conductor 1 and Rogowski coil 2 produces approximately the following voltage drop across analog-to-digital converter 72 (ADC):

[0041]

[0042] where Ri_RoGo is the ohmic series resistance value of the Rogowski coil. Figure 5 The simulation results shown clearly confirm the relationship of Equation 3 through its frequency response.

[0043] The relationship in Equation 3 can be extended to include the effects of all phases of the distribution system (since these phases are also typically located in close proximity relative to each other):

[0044]

[0045] Thus, the following relationship for the voltage at the input of the measurement system or analog-to-digital converter (ADC) is generated for the time domain:

[0046]

[0047] in:

[0048] C K1,1 >>C K1,2 ; C K1,1 >>C K1,3 Equation 6

[0049] For the total voltage at the output of the combination of the Rogowski coil 2 and the anti-aliasing low-pass filter 71 and the total voltage at the input of the analog-to-digital converter (ADC) 72 , Equation 7 is as follows.

[0050]

[0051] To analyze the current to be measured by Rogowski coil 2, in most cases, the output signal of Rogowski coil 2 requires analog or digital integration. Equation 7 directly demonstrates that integrating the output signal of Rogowski coil 2 or the input signal of analog-to-digital converter ADC 72 is useful. Assuming that inductive interference coupling into the input can be neglected, this integration yields the following relationship for the current:

[0052]

[0053] As can be seen from equations 7 and 8, in order to correct the capacitive interference coupling input, the voltage values ​​of the conductors in the direct vicinity of the Rogowski coil 1 in a multiphase system must be known (hereinafter, the equations are based on a three-phase system, i.e., for example, three conductors, which can each be surrounded by a Rogowski coil, as Figure 1As shown in Figure 1, a conductor is shown in Figure 2). Due to geometrical reasons, the coupling capacitance values ​​from, for example, phase 2 to coil 1 are generally much lower than the coupling capacitance values ​​from, for example, phase 1 to coil 1, so these couplings can be ignored. Since the value of the capacitance CK1,μ sometimes depends on manufacturing tolerances, it is expedient to determine the value of the coefficient used for digital correction during calibration, and in particular when calibrating the voltage measurement of the PMD. During operation of the PMD, the voltage measurement results and the calibration coefficients cK1,μ stored in the measuring system are used to correct the current value obtained from the integration of the output voltage of the Rogowski coil. In order to facilitate the clear allocation of the individual effects, the calibration coefficients cK1,μ should be determined as much as possible at zero current (when calibrating the measuring channel for voltage measurement, only very small currents should flow, and the measurement should be performed based on the rated current). Therefore, when calibrating the measuring channel for current measurement, only very low voltages should be used.

[0054]

[0055] Here, "*" is used to represent multiplication to indicate that this is a matrix operation. is the vector of the voltages of each individual phase.

[0056] It is also important to note that the values ​​of the sub-diagonal elements in the matrix CK and the matrix S are often much smaller than the values ​​of the main diagonal elements and can therefore be ignored. This greatly simplifies the calculation of the capacitive coupling correction and can be easily performed on the microcontroller 6 or similar embedded hardware.

[0057] The following describes the determination of the coupling coefficient using the simplest case, i.e., a single conductor. Equation 9 simplifies to

[0058]

[0059] A predetermined generated test current I(t) is applied. The voltage U(t) of the conductor is measured or known. The voltage U(t) induced and processed in the Rogowski coil is detected by the microcontroller 6. ADC Integrate to get the value for time point t Two equations are needed to determine the two coefficients c K and S. To this end, the variables of equation 10 are determined for different time points t1 and t2, or the calculation can also be performed using different test currents I(t).

[0060] The correction according to the invention allows an increase in measurement accuracy without requiring additional hardware expenditure. As a result, a measuring device formed with Rogowski coils can be made smaller or more compact.

Claims

1. A measuring system (5) for measuring the current value of an alternating current flowing through a conductor (1) and correcting the measured current value with respect to capacitive interference coupling input, the measuring system comprising - a Rogowski coil (2) designed to induce a voltage via an alternating current; - a signal processing device (7), for acquiring and processing the induced voltage value in the Rogowski coil; - means (6) for determining the value of the current by integrating the voltage processed by the signal processing means; and - a device (6) for correcting the current value with respect to capacitive interference coupling, wherein The device (6) for correcting the current value is provided for using an approximation for the voltage drop caused by the capacitive interference coupling Make corrections.

2. The measuring system according to claim 1, It is characterized by: The approximation According to at least one calibration coefficient (C K1,1 ) describes the voltage drop caused by capacitive interference coupling into the input, wherein the at least one calibration factor (C K1,1 ) can be determined by calibration measurements.

3. The measuring system according to claim 2, It is characterized in that The approximation includes at least one term that is related to the calibration coefficient (C K1,1 ), the derivative of the voltage that can be assigned to conductor (1) and the resistance (R i_RoGo ) is proportional to the product of .

4. The measuring system according to claim 3, It is characterized by: A plurality of phases is considered and an item according to claim 3 is provided for each considered phase.

5. The measuring system according to claim 4, It is characterized in that The approximate form is where the index μ runs through the considered phase, C K1,μ is the calibration coefficient, is the voltage assignable to conductor (1) The derivative of R i_RoGo is the resistance that can be assigned to the Rogowski coil (R i_RoGo ).

6. The measuring system according to claim 1, It is characterized in that With the aid of an approximation for the voltage drop caused by capacitive interference coupling into the input The correction is performed in such a way that an expression for the current value (I1(t)) of the conductor (1) is used which includes the integral of the approximation.

7. The measuring system according to claim 6, It is characterized in that The approximation The form is configured so that the approximate integral is based on the voltage (U μ (t)) and, if appropriate, linear function terms in the approximation of the other phases considered.

8. The measuring system according to claim 7, It is characterized by: The expression for the current value (I1(t)) of conductor (1) is of the form where the index μ runs through the considered phase, C K1,μ is the calibration factor, U μ (t) is the voltage assignable to conductor (1), R i_RoGo is the resistance assignable to the Rogowski coil, and is the integral of the induced voltage in the Rogowski coil.

9. A method for determining a calibration coefficient (C) of a measuring system according to any one of claims 2 to 8 K1,1 ) method, It is characterized by: -Based on an expression of the form and - Determine the calibration factor (C) by measurements with known currents applied to the conductor (1) and, if applicable, other phases K1,1 ).

10. The method according to claim 9, It is characterized in that The voltage applied to the conductor (1) and / or other phases Measured at lower levels.

Images