Electrical measuring assembly and method for continuous monitoring of the resistance of a protective conductor
By using signal alternating current and leakage capacitors in the power supply system to form a current loop, combined with measuring the loop impedance of the current transformer, the problem of continuous monitoring of loop resistance in grounded and ungrounded systems in the prior art is solved, and the problem of continuous monitoring of loop resistance in grounded and ungrounded systems is realized. High precision and convenient monitoring effects avoid the risks of arcing and high leakage current.
Patent Information
- Application Number
- CN202111080982.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Priority Date
- 2020-09-16
- Filing Date
- 2021-09-15
- Publication Date
- 2024-12-20
- Estimated Expiration
- 2041-09-15
AI Technical Summary
The prior art is difficult to continuously monitor loop resistance in grounded and ungrounded powered systems during activation operations, especially in IT systems, which often lead to arcs and high leakage currents and require additional pilot or test lines for measurements. , affects accuracy and convenience.
By setting up a signal generator in the power supply system to generate a signal alternating current of a specific measurement frequency, using a leakage capacitor and an active conductor to form a current loop, and combining a measurement current transformer to measure the loop impedance, the active conductor of the power supply line replaces the pilot line , realize continuous monitoring without additional lines.
High-precision continuous monitoring of the resistance of the protective conductor connector is achieved, suitable for grounding and non-grounding systems, avoiding the risks of arcing and high leakage currents, and no additional pilot wires are required, improving the convenience and accuracy of measurement.
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Figure CN114264883B_ABST
Abstract
Description
Technical Field
[0001] The invention relates to an electrical measuring assembly and a method for continuously monitoring the protective conductor resistance of a protective conductor connection in a power supply system, the power supply system having a power supply station, a power supply line and an electrical device, the power supply station comprising a first leakage capacitor, the power supply line comprising an active conductor, the electrical device being grounded via the protective conductor connection and comprising a second leakage capacitor and a load. Background Art
[0002] Within the scope of the "automatic disconnection of the power supply" protection measure required for electrical installations, a sufficiently high current in the part of the installation to be monitored is a prerequisite so that the installed overcurrent protection device can initiate a tripping command quickly enough. In grounded power supply systems (TN systems, TN networks), this is achieved in the case of a first fault; in ungrounded power supply systems (IT systems, IT networks), this is achieved in the case of a second fault. Therefore, it must be ensured that the impedance of the loop current (loop impedance), which is generated by the power supply station via the active conductor of the supply line to the load and returned via the protective conductor connection, has a low impedance to induce a sufficiently high loop current as a tripping current.
[0003] The IEC61557-3:2007 standard specifies specific requirements for measuring equipment for measuring loop impedance, the measurement being carried out by reducing the network voltage by loading the network. However, in practice, this measurement method seems to be suitable only for cyclic tests and not for continuous monitoring of the loop resistance during active operation. In particular, this measurement method cannot be applied to IT systems, because in IT systems, faults with negligible impedance at the active conductor have been switched to ground to measure the loop impedance, thereby forming the measurement loop. However, for the first fault with negligible impedance, the existing network leakage capacitance is immediately unloaded. In extensive IT networks with correspondingly large network leakage capacitances, this can lead to arcs and high leakage currents. Therefore, determining the loop impedance in IT systems by means of loop currents is not without danger and implies additional loading for electrical installations.
[0004] Therefore, the prior art discloses other devices for measuring a loop while feeding a measurement signal, which devices are equipped with a pilot line (also called a control line or a test line) as a measurement current return line.
[0005] The pilot conductor is connected to the protective conductor on the load side. Therefore, a simple resistance measurement in the supply station can be used to detect the loop resistance of the pilot conductor and protective conductor connection, which includes the contact resistance of the plug connection.
[0006] For example, DE 10 2017 213 357 B3 describes a method with a pulsating activation of a measurement signal on a control line. If the control line or the protective conductor connection reaches a high impedance via a defect or even both reach a high impedance via a defect, this fault can be easily detected.
[0007] At the same time, the necessity of a pilot line has the disadvantage that only specific supply lines containing a pilot line can be used. If an installed supply line without a pilot line is to be used, the option of retrospective monitoring cannot be added.
[0008] Other proposals in the prior art are based on naturally existing multiple grounds or on intentionally created multiple grounds as pilot wires.
[0009] Specification DE 10 2015 218 764 A1 describes a method for connecting a ship to shore in a port, in which a measurement signal is inductively coupled into a protective conductor connection. To form a current loop, the method makes use of the fact that a plurality of grounds are available via conductive salt water or a metal gangway is available. However, determining the loop impedance and thus the protective conductor resistance is hampered, since returning the measurement current via the gangway may fail if the gangway is arranged on an insulating ground. Furthermore, the resistance of the return measurement current can only be recorded with difficulty and inaccurately via the conductive salt water.
[0010] Another method is described in specification US 2013 / 0191059 A1, in which a measuring signal is inductively induced into the protective conductor connection at a frequency of approximately 2 kHz. The impressed measuring current returns via the soil, the resistance of which is considered to have a low impedance and is regarded as being predominantly ohmic. From the point of view of measurement technology, the soil as a measuring current return line therefore corresponds to a naturally occurring lead line, with the disadvantage that the resistance of the soil is quantitatively unreliable. Summary of the invention
[0011] The object of the present invention is therefore to propose a method and a device for continuously monitoring the protective conductor resistance of a protective conductor connection in a power supply system, which does not require any additionally installed pilot, control or test wires, prevents inaccuracies when recording the measured values, allows time-continuous monitoring, and is suitable for grounded as well as ungrounded power supply systems.
[0012] With regard to the device, this object is achieved by an electrical measuring assembly having the features listed in the embodiments of the present application.
[0013] The baseline for these observations and the requirements for the system environment in which the present invention is applied is a power supply system with a power supply station, which includes a first leakage capacitor, which is usually a filter capacitor for radio interference suppression, switched between the protective conductor connection and the respective active conductor of the power supply line. In addition, it is assumed that the electrical device is grounded via the protective conductor connection and the electrical device also includes a second leakage capacitor. The load arranged in the electrical device is connected to the power supply station of the power supply system via the active conductor of the power supply line as required.
[0014] As desired by the invention, the power supply station comprises a signal generator for generating a signal alternating current with a specific measuring frequency. To prevent EMC interference, the signal alternating voltage is preferably sinusoidal and is preferably provided by the signal generator via an analog-to-digital conversion (followed by an amplifier). The signal generator is controlled by a digital computing unit, which is arranged in the evaluation unit and is preferably implemented as a microcontroller.
[0015] According to the invention, a first measuring current transformer having a first winding is provided in the power supply station, the first winding surrounding the protective conductor connection and being used to inductively couple the signal alternating voltage to the protective conductor connection. This results in a current loop in which a loop current caused by the applied signal alternating voltage flows through a first leakage capacitor in the power supply station, an active conductor of the power supply line, a second leakage capacitor in the electrical device and through the protective conductor connection.
[0016] Since the complex-valued loop impedance is measured based on the electrical variable measured at the guard conductor, the first measuring current transformer comprises a second winding, which also surrounds the guard conductor connection, for a second measurement of the guard conductor voltage.
[0017] The electrical measuring arrangement further comprises a second measuring current transformer having a secondary winding for recording a protective conductor current flowing in the protective conductor connection.
[0018] In addition to controlling the signal generator, the evaluation unit with the digital computing unit is used in the electrical measuring arrangement to determine the loop impedance from the guard conductor voltage and the guard conductor current and to evaluate the real part of the loop impedance.
[0019] The basic idea of the invention consists in a leakage capacitor, which is arranged in the power supply station and the electrical device, respectively, and which is able to generate a loop current, which flows through the active conductor and the protective conductor connection, when the measurement frequency of the coupled signal alternating voltage is high enough. Thus, as desired by the invention, the active conductor of the power supply line is used as a pilot conductor. Thus, the active conductor of the power supply line forms part of the loop and replaces the complex installation of the pilot conductor. Nevertheless, in terms of determining the loop impedance, the active conductor of the power supply line shows the same high measurement accuracy as the installed pilot conductor. Therefore, in terms of measurement accuracy, the electrical measurement component according to the invention and the method based thereon according to the invention are superior to the methods known to date, which are based on measuring current return lines by means of multiple grounding parts switched in parallel.
[0020] Furthermore, the invention can be used in grounded and ungrounded power supply systems, depending on the requirements of the above-mentioned system environment, to obtain the measurement quality of a measurement assembly comprising a pilot conductor in the practical absence of a pilot conductor.
[0021] Advantageously, the measuring frequency (fs) of the signal alternating voltage (Us) is in the range of 100 kHz.
[0022] By choosing such a measuring frequency of about 100 kHz, which is many times higher than the network frequency (e.g. 50 Hz), the sensitivity to faults caused by network voltage distortions or non-harmonic network currents is significantly reduced, since the measuring frequency is about the 2,000th harmonic of the network frequency. Moreover, with the chosen measuring frequency, the resistance of the leakage capacitor assumes small values, which results in a high loop current (corresponding to the measured protective conductor current, apart from negligible deviations) and a high signal / noise ratio.
[0023] Furthermore, the electrical measuring assembly comprises a high-pass circuit for filtering the protective conductor current. The high-pass circuit is preferably implemented as an analog RC element and separates the desired protective conductor current with a high frequency from interferences with a low frequency.
[0024] Advantageously, the evaluation unit comprises an analog-to-digital converter for analog-to-digital conversion of the guard conductor voltage and the guard conductor current.
[0025] Preferably, the measured guard conductor voltage and the measured guard conductor current are further processed after being analog-to-digital converted using methods from digital signal processing.
[0026] The electrical measuring assembly according to the invention is based on the technical teachings described in the embodiments of the present disclosure. In this respect, the above technical effects and the advantages gleaned therefrom apply to the method features.
[0027] In particular, it is emphasized that, in addition to the protective conductor connection, the available leakage capacitors as well as the active conductors of the supply line form part of the measurement loop when using measurement frequencies that are several times higher than the network frequency. Thus, by using the active conductors of the supply line like pilot wires, an actual pilot wire is no longer necessary, while obtaining the same measurement accuracy as using a pilot wire.
[0028] Further advantageous embodiment features emerge from the following description and the drawings, which describe preferred embodiments of the invention using examples. BRIEF DESCRIPTION OF THE DRAWINGS
[0029] Figure 1 shows the measurement of the loop impedance with a pilot wire according to the prior art, and
[0030] Figure 2 An electrical measuring assembly according to the invention is shown. DETAILED DESCRIPTION
[0031] Figure 1 The measurement of the loop impedance Zm in a TN system A with a pilot conductor P as known in the prior art is shown.
[0032] The power supply station V is connected to the electrical device E via a power supply line K. The power supply line K comprises an additional pilot conductor P in addition to the active conductors L and N and the protective conductor connection PE. In the electrical device E, the pilot conductor P is connected to the protective conductor connection PE, thereby forming a current loop in which a loop current Is flows when a signal alternating voltage Us is applied between the protective conductor connection PE and the pilot conductor P in the power supply station V. Therefore, a simple resistance measurement in the power supply station V can be used to detect the loop impedance Zm of the pilot conductor P and the protective conductor connection PE, which loop impedance Zm includes the contact resistance of the plug connection S that may be present.
[0033] Figure 2 A measuring assembly 2 is shown according to the invention in a power supply station V in a TN system A. The power supply station V is connected to an electrical installation E including a load R via a supply line K including active conductors L and N, a protective conductor connection PE and, if applicable, a plug connection S.
[0034] Starting from the active conductors L, N to the protective conductor connection PE, the power supply station V and the electrical installation E comprise a first leakage capacitor C1 , C2 and a second leakage capacitor C3, C4, respectively.
[0035] In this assumed system environment, the electrical measuring assembly 2 according to the invention is arranged in a power supply station V. The electrical measuring assembly 2 comprises a signal generator 4 for generating a signal alternating voltage Us with a measuring frequency fs.
[0036] The electrical measuring assembly 2 further comprises a first measuring current transformer 6 which surrounds the protective conductor connection PE and comprises a first winding 8 for inductively coupling the AC signal voltage Us into the protective conductor connection PE. The first measuring current transformer 6 can be designed as a toroid through which the protective conductor connection PE is guided.
[0037] Thus, a current loop for the alternating current is obtained, which is composed of: a parallel circuit of leakage capacitors C1 and C2 in the supply station V; a parallel circuit of active conductors L and N of the supply line K - if applicable, with the contact resistance of the plug connector S; a parallel circuit of the protective conductor connection PE and leakage capacitors C3 and C4 in the electrical device E - if applicable, with the corresponding contact resistance of the respective plug connector S.
[0038] The first measuring current transformer 6 is equipped with a second winding 10 for a second measurement of the protective conductor voltage Um. The second winding 10 is loaded with a high impedance and generates a protective conductor voltage Um which is fed to an analog-to-digital converter 26 for further processing in a digital calculation unit 28 of the evaluation unit 20.
[0039] The electrical measuring assembly 2 further comprises a second measuring current transformer 12 which surrounds the protective conductor connection PE and comprises a secondary winding 14 for detecting the protective conductor current Im flowing in the protective conductor connection PE. The second measuring current transformer 12 operates as a current transformer, i.e. it is usually provided with low impedance terminations. The second measuring current transformer 12 is also designed as a ring through which the protective conductor connection PE is guided.
[0040] Via an analog high-pass circuit 22 comprising a capacitor Ch and an ohmic resistor Rh, the protective conductor current Im detected by the secondary winding 14 is fed via an amplifier to an evaluation unit 20. The threshold frequency of the high-pass circuit 22 is measured in such a way that the network frequency and, if applicable, the interfering components occurring are sufficiently suppressed below the measuring frequency fs.
[0041] In the digital calculation unit 28, the loop impedance Zm is determined from the protection conductor voltage Um and the protection conductor current Im. In this case, the impedance is calculated in such a way that the real part and the imaginary part of the loop impedance Zm can be represented separately. Preferably, after the analog-to-digital conversion 26, a numerical method for calculating the Fourier coefficients is applied in the digital calculation unit 28.
[0042] The imaginary part of the loop impedance Zm calculated in this way is obtained only by the effect of the leakage capacitor with a purely imaginary impedance Zc=1 / jωC. The real part Zre of the loop impedance Zm of the current loop consists of the following components: the equivalent series resistance (ESR) of the parallel circuit of the leakage capacitors C1 and C2; the ohmic resistance of the parallel circuit of the active conductors L and N of the supply line K - if applicable, with the contact resistance of the respective plug connection S; the equivalent series resistance (ESR) of the parallel circuit of the leakage capacitors C3 and C4; and the ohmic resistance of the protective conductor connection PE - if applicable, with the respective contact resistance.
[0043] The ESR values of most leakage capacitors are so small that they can be neglected. It can be discerned that the ohmic resistance Rpe of the protective conductor connection PE - if applicable, with the respective contact resistance - cannot be measured separately as a component of the measurement loop. However, this does not entail a disadvantage, since the ohmic resistance of the parallel circuit of the active conductors L and N of the supply line K also always leads to negligible values. The real part Zre of the loop impedance Zm therefore corresponds approximately to the protective conductor resistance Rpe to be monitored. If a certain value falls below the protective conductor resistance threshold, an appropriate warning can be issued, which can be optical and / or acoustic.
Claims
1. An electrical measuring component (2), the electrical measuring component (2) being used for continuously monitoring the protective conductor resistance (Rpe) of a protective conductor connection (PE) in a power supply system (A), the power supply system (A) comprising a power supply station (V), a power supply line (K) and an electrical device (E), the power supply station (V) comprising a first leakage capacitor (C1, C2), the power supply line (K) comprising an active conductor (L, N), the electrical device (E) being grounded via the protective conductor connection (PE) and the electrical device (E) comprising a second leakage capacitor (C3, C4) and a load (R), the electrical measuring component (2) comprising: a signal generator (4) for generating a signal alternating voltage (Us) having a measuring frequency (fs), a first measuring current transformer (6), which surrounds the protective conductor connection (PE) and has a first winding (8) and a second winding (10), the first winding (8) being used for inductively coupling the signal alternating voltage (Us) to the protective conductor connection (PE) so that a loop current (Is) flows through the first leakage capacitor (C1, C2), the active conductor (L, N), the second leakage capacitor (C3, C4) and the protective conductor connection (PE), the second winding (10) being used for a second measurement of the protective conductor voltage (Um), a second measuring current transformer (12) which surrounds the protective conductor connection (PE) and has a secondary winding (14) for capturing a protective conductor current (Im) flowing in the protective conductor connection (PE), An evaluation unit (20) has a digital calculation unit (28) for: controlling the signal generator (4); determining a loop impedance (Zm) as a function of the protection conductor voltage (Um) and the protection conductor current (Im); and for evaluating a real part (Zre) of the loop impedance (Zm).
2. The electrical measuring assembly (2) according to claim 1, Features: The measuring frequency (fs) of the signal alternating voltage (Us) is in the range of 100 kHz.
3. The electrical measuring assembly (2) according to claim 1 or 2, Features: A high-pass circuit (22) is provided for filtering the protective conductor current (Im).
4. Electrical measuring assembly (2) according to any one of claims 1 to 3, Features: The evaluation unit (20) comprises an analog-to-digital converter (26) for performing an analog-to-digital conversion of the protective conductor voltage (Um) and the protective conductor current (Im).
5. A method for continuously monitoring the protective conductor resistance of a protective conductor connection (PE) in a power supply system (A), the power supply system (A) comprising a power supply station (V), a power supply line (K) and an electrical device (E), the power supply station (V) comprising a first leakage capacitor (C1, C2), the power supply line (K) comprising an active conductor (L, N), the electrical device (E) being grounded via the protective conductor connection (PE) and the electrical device (E) comprising a second leakage capacitor (C3, C4) and a load (R), the method comprising the following steps: generating a signal alternating voltage (Us) with a measuring frequency (fs) by means of a signal generator (4), The signal alternating voltage (Us) is inductively coupled to the protective conductor connection (PE) by means of a first measuring current transformer (6), which surrounds the protective conductor connection (PE) and has a first winding (8), so that a loop current (Is) flows through the first leakage capacitor (C1, C2), the active conductor (L, N) of the supply line (K), the second leakage capacitor (C3, C4) and the protective conductor connection (PE), using the second winding (10) of the first measuring current transformer (6) to perform a second return measurement of the protective conductor voltage (Um), capturing a protective conductor current (Im) flowing in the protective conductor connection (PE) by means of a second measuring current transformer (12), which surrounds the protective conductor connection (PE) and has a secondary winding (14), and The signal generator (4) is controlled by means of a digital computing unit (28) in an evaluation unit (20) arranged in the power supply station (V), a loop impedance (Zm) is determined from the protective conductor voltage (Um) and the protective conductor current (Im), and a real part (Zre) of the loop impedance (Zm) is evaluated.
6. The method according to claim 5, Features: The measurement frequency (fs) is in the range of 100 kHz.
7. The method according to claim 5 or 6, Features: The protective conductor current (Im) is filtered by means of a high-pass circuit (22).
8. The method according to any one of claims 5 to 7, Features: The protection conductor voltage (Um) and the protection conductor current (Im) are converted from analog to digital (26).