Method and device for determining leakage current of overvoltage protection device in alternating current power grid

Abstract

The invention discloses a method and a device for determining leakage current of an overvoltage protection device in an alternating-current power grid. The method comprises the following steps of: 100, measuring current and voltage curves of the overvoltage protection device along with time change; the method comprises the following steps: 200, carrying out Fourier transform on a current and voltage curve, 300, determining voltage and current components and phases thereof, 400, determining phase angles of the voltage and the current at a power frequency of a power grid, 500, determining a capacitance component of impedance of the overvoltage protection device, 600, determining capacitance component current with voltage Uf, 700, determining resistance components of the current at different frequencies, and 400, determining the phase angles of the voltage and the current at the power frequency of the power grid. 800, determining the effective value of the resistance component current; the device has a current sensor, a voltage sensor and a computing unit.

Description

Technical Field

[0001] The invention relates to a method and a device for determining leakage current of an overvoltage protection device in an AC power grid. Background Art

[0002] In order to protect electrical equipment, overvoltage protection devices are increasingly being installed. One example is a varistor, which switches on when an overvoltage occurs and bypasses the overvoltage outside the electrical equipment to be protected. Typically, such an overvoltage event is the result of a discharge, for example, caused by a switching operation or a lightning strike, and is conducted as a pulse current through a varistor (or other overvoltage protection device).

[0003] However, when there is no overvoltage event, the overvoltage protection device should essentially behave like an insulator.

[0004] From an electrical point of view, a varistor has a resistive component and a capacitive component. In other words, a varistor behaves as a complex impedance. The capacitive component is formed by the essentially constant capacitance between the contact surfaces of the varistor. The resistive component is formed by the nonlinear current-voltage characteristic curve of the varistor, which allows a small amount of current to flow even at the mains voltage.

[0005] However, due to aging, environmental conditions (temperature, humidity, etc.) and use, varistors may degrade. This degradation usually leads to a current flow even in the absence of an overvoltage. This current is called leakage current. Usually, this leakage current is (initially) only a few hundred microamperes (μA). However, with further aging and use, the leakage current may increase, resulting in severe thermal heating and even possible fire. For this situation, modern overvoltage protection devices are equipped with thermal protection devices that disconnect the varistor from the corresponding circuit.

[0006] Aging manifests itself as a change in the current-voltage characteristic curve, which in turn results in a change in the resistance component of the leakage current. The resistance component of the leakage current is therefore a suitable characteristic for assessing the aging state of a varistor.

[0007] The total leakage current is also a possible indicator of aging, but is less sensitive to aging since the capacitive component of the leakage current dominates in slightly aged varistors.

[0008] Therefore, a method is needed that can determine the resistive leakage current as accurately as possible. This method is difficult due to the fact that the varistor characteristic curve is nonlinear and the voltage signal does not necessarily have to have a precise (50 Hz) sine wave at the rated voltage, but may contain voltage deviations and harmonics.

[0009] Current state of the art: In energy measurement, it is known to use the phase difference between current and voltage to separate power into reactive power and active power. However, those methods and devices are mainly aimed at measuring linear loads with significantly higher currents. For the measurement of small and very small currents and the measurement of strongly nonlinear loads, these methods and devices are not applicable.

[0010] The Chinese patent CN 105182160 B is known in the prior art and describes a system having a current sensor for measuring the total leakage current with time resolution and an optical sensor for detecting the phase of the voltage. The phase angle of the 50 Hz signal between the current and the voltage can be determined by Fourier transformation, and then the resistance component can be expressed as where φ is the phase angle. The proposed method has been shown to be inaccurate in practice.

[0011] The disadvantage of this method is that only the main frequency of 50 Hz is taken into account, while harmonics and voltage amplitude are not taken into account. When deviating from the rated voltage, different levels of leakage current will occur, but this no longer provides reliable information about the aging of the varistor.

[0012] A method for directly measuring the voltage is known from Chinese patent application CN 115494330 A. The aging assessment here is based on the phase angle, which is independent of the voltage amplitude and therefore represents a better aging indicator.

[0013] Methods known from the prior art have proven to be inaccurate. Summary of the invention

[0014] In order to allow for a more accurate determination, the present invention provides a method and apparatus for determining the leakage current of an overvoltage protection device in an AC power grid.

[0015] The present invention provides a method for determining the leakage current of an overvoltage protection device in an AC power grid, wherein the AC power grid frequency is <> 0 Hz, and the method comprises the following steps: 100. Measure the current and voltage curves of the overvoltage protection device that change with time; 200. Performing Fourier transform on the current and voltage curves, 300. Determine the voltage and current components U f , I f and its phase φ U,f ,φ I,f , 400. Determine the phase angle of the voltage and current at the power grid power frequency f, 500. Determine a capacitive component of the impedance of the overvoltage protection device. 600. Determine the voltage U f The capacitive component current: 700. Determine the resistance component of current at different frequencies. 800. Determine the effective value of the resistance component current:

[0016] A device for carrying out the method comprises a current sensor and a voltage sensor and a computing unit.

[0017] A method for calibrating the device is to use a known capacitance for calibration.

[0018] The present invention will be described in detail below by means of the accompanying drawings and embodiments. The accompanying drawings are schematic and not shown to scale. The accompanying drawings do not limit the present invention in any way. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is the external design of the device according to some aspects of the embodiments of the present invention, Figure 2 is a schematic electrical structure of an apparatus according to some aspects of an embodiment of the present invention, Figure 3 is a schematic flow chart of a method according to some aspects of an embodiment of the present invention, and Figure 4 is a schematic diagram of logical or structural elements of an apparatus according to some aspects of an embodiment of the present invention.

[0020] Reference Numbers List SPD N , SPD L1 , SPD L2 , SPD L3 Overvoltage protection device; M-meter; S M Sensor module; S N , S L1 , S L2 , S L3 Current sensor; S U1 , S U2 , S U3 Voltage sensor; S T1 , S T2 Temperature sensor; A socket; CPU Computing Unit. DETAILED DESCRIPTION

[0021] The present invention will be described in more detail below in conjunction with the accompanying drawings. It should be noted that different aspects are described, and these aspects can be used alone or in combination. That is, unless clearly indicated as a pure alternative, any aspect can be used together with different embodiments of the present invention.

[0022] In addition, for simplicity, the following generally only refers to one entity. Unless explicitly noted, the present invention may also include multiple related entities. Therefore, the use of the words "a" and "an" only means that at least one entity is used in a simple embodiment.

[0023] In the methods described below, the various steps can be arranged and / or combined in any order, unless the context clearly indicates otherwise. In addition, these methods can be combined with each other without explicitly noting other situations.

[0024] Notes with numerical values ​​are not generally to be regarded as exact values ​​but include a tolerance of + / -1% to + / -10%.

[0025] The standards, specifications or similar contents mentioned in this application always refer to at least the standards, specifications or similar contents applicable at the filing date. That is, if a certain standard / specification etc. is updated or replaced by a subsequent version, the present invention also applies to this.

[0026] The present invention will be described below in the context of an overvoltage protection device, in particular a varistor. However, the present invention is not limited thereto.

[0027] The inventors have discovered that not taking into account the current voltage and harmonics can lead to inaccurate results.

[0028] Due to the nonlinearity of the varistor, these may cause leakage currents with higher frequency components than the corresponding voltage harmonic itself, but not with lower frequencies. For example, a 150 Hz harmonic may cause leakage currents with a frequency of n*150 Hz, where n is an integer greater than or equal to 1.

[0029] A better, but still error-prone, approach is to determine the phase angle frequency-resolved and then determine the resistive leakage current for each frequency as f refers to the corresponding frequency.

[0030] The effective value of the leakage current can then be determined as Combined with the effective value of the voltage The impedance The resistance component of is used as the aging characteristic.

[0031] However, this approach does not take into account that harmonics in the voltage will generate higher harmonics in the leakage current due to the nonlinearity of the current-voltage characteristic curve.

[0032] Therefore, there will be deviations in the determination of the phase angle, because the leakage current is generated at a certain frequency by different harmonics of the voltage, which may have different phase angles. Therefore, no clear phase difference can be determined for this frequency. If only the 50 Hz component is considered, the leakage current value will be too low, because a significant part of the current may be present in the high-frequency component due to the nonlinearity of the varistor.

[0033] In a method for determining an overvoltage protection device SPD in an AC power grid N , SPD L1 , SPD L2 , SPD L3 In the method for measuring leakage current, the power grid has a network frequency f<>0Hz, and in step 100, the overvoltage protection device SPD is measured. N , SPD L1 , SPD L2 , SPD L3 Current and voltage curves over time.

[0034] To this end, you can Figure 2 and Figure 4 As shown, set a suitable current sensor S L1 , S L2 , S L3 , S N or voltage sensor S U1 , S U2 , S U3 .

[0035] The time-varying current and voltage curves obtained by continuous (periodic or aperiodic) measurement are then Fourier transformed in step 200 .

[0036] Next, in step 300 , the voltage and current components U of the complex spectrum are determined from the Fourier transformation of the current and voltage. f , I f and its phase φ U,f ,φ I,f .

[0037] As a result, in a further step 400 , the phase angle of the voltage and the current at the network power frequency f can be determined.

[0038] In the method of the present invention, in step 500, the overvoltage protection device SPD is determined N , SPD L1 , SPD L2 , SPD L3 The capacitive component of the impedance.

[0039] Then, in step 600, it is determined that the voltage signal U f The capacitive component of the current: for

[0040] Similarly, in step 700, the resistance component of the current at each frequency is determined, and the current corresponding to the resistance component is I R,f =I f -I c,f .

[0041] Finally, in step 800, the effective value of the resistance component current is determined as:

[0042] For determination or calculation, the device of the invention has a computing unit CPU, such as a suitably programmed microprocessor, microcontroller, FPGA, AISC, DSP, etc. Likewise, the device of the invention may have other suitable communication interfaces, such as input devices (switches, buttons, etc.), output devices (displays, signal LEDs, etc.), remote interfaces (local bus, switch outputs, networks), but these will not be described in detail here in order to focus on the present invention.

[0043] The solution of the present invention utilizes the characteristic that the capacitance of the varistor has a linear behavior and that, due to the nonlinearity of the current-voltage characteristic curve, no lower frequencies are generated in the leakage current signal.

[0044] First, the voltage and current components U are determined from the complex spectrum by Fourier transformation f , I f and its phase φ U,f ,φ I,f Then, the phase angle between the voltage and the leakage current is determined on the grid frequency signal f. For simplicity, a grid frequency of 50 Hz is given below, but it is not limited to this.

[0045] The capacitive leakage current at 50 Hz is the current component that is 90° phase shifted relative to the voltage: Here and below, the support points at a frequency of 50 Hz in the Fourier transformed signal are indicated.

[0046] Reactance is And the capacitance is Using this capacitor C, the leakage current of the capacitance component of the entire voltage signal can be determined as

[0047] The resistance component of the leakage current can be determined at each frequency as I R,f =I f -I c,f , so the effective value of the resistance component leakage current is

[0048] The present invention solves the problem of inaccurate phase angle at higher frequencies. To further consider the effect of voltage on leakage current, the real part of impedance is determined by the effective value of the voltage and leakage current of the resistance component as and use it as an aging feature.

[0049] In one embodiment of the invention, the Fourier transform is selected from a group consisting of a real fast Fourier transform, a fast Fourier transform, or a discrete Fourier transform.

[0050] This allows a flexible adaptation to the requirements of the computing unit CPU.

[0051] In another embodiment of the invention, the capacitive and resistive components of the current are determined only at discrete multiples of the grid frequency f.

[0052] This results in fast computing power.

[0053] According to another embodiment of the present invention, the grid frequency is 16 2 / 3 Hz, 50 Hz, 60 Hz or 400 Hz. Therefore, the present invention is applicable to AC power systems commonly used on land, on water and in aerospace.

[0054] The present invention is particularly suitable for determining an overvoltage protection device SPD N , SPD L1 , SPD L2 , SPD L3 However, pulse currents can also be determined.

[0055] According to a further embodiment of the invention, the determined resistive leakage current is used for an aging determination.

[0056] In particular, in one embodiment of the present invention, the aging degree can be determined based on the change of the resistive current-voltage characteristic curve.

[0057] In addition, it is also possible to detect or determine the temperature of the varistor and / or the environment. For example, other temperature sensors can be provided in the S T1 Or environment S T2 to better take the temperature into account. Exemplary temperature sensors include PTC, NTC, PT100. Likewise, the temperature can also be determined via the temperature-dependent resistance component. If the temperature or temperature change is known, the influence of temperature on the characteristic curve can be taken into account.

[0058] Without limiting the generality of the present invention, the present invention is applicable to a varistor as an example of an overvoltage protection device, such as an SPD L1 , SPD L2 , SPD L3 .

[0059] In another embodiment of the present invention, the current of the resistance component is represented by I r,f =I f -I c,f Or it can be determined by the inverse transformed time domain.

[0060] This allows increased flexibility in the design of the method of the invention and allows efficient utilization of limited resources, such as a microcontroller.

[0061] Without limiting the generality, the method can also be used for calibration if known capacitances are used. In particular, known complex (impedance) resistors can also be used, wherein a complex resistor is always understood to be a generally complex resistor which, in addition to a resistance component, can also have an inductive and / or capacitive component. Likewise, the known resistor can also be non-linear, i.e. in particular an overvoltage protection device which has been previously measured.

[0062] In an embodiment of the present invention, the parasitic leakage current can also be used to calibrate the sensor used. However, if the parasitic capacitance of the varistor is known, the expected parasitic current can be determined according to the above formula. If different currents are measured in practice, a calibration factor can be determined based on this difference. By measuring and averaging for a longer time, possible interference (noise, etc.) can be eliminated.

[0063] It should be noted that if there are harmonics in the voltage signal and non-negligible resistive leakage current, these factors may affect the capacitance determination. Therefore, it is possible to monitor the presence of these two interference sources and not make measurements during the period when both conditions are met, or mark the measurement as erroneous.

[0064] However, if the capacitance is known or predetermined (under appropriate conditions), even in this case the resistive leakage current can be accurately determined.

[0065] If the overvoltage protection device under test has a nonlinear current-voltage characteristic curve that is deformed due to aging, using effective values ​​may lead to errors.

[0066] Advantageously, the aging is determined via a change in the current-voltage characteristic curve.

[0067] This can be partly seen from the time curve of the voltage U(t) and the time curve of the leakage current I r (t) is determined as U(I r (t)). The deformation of the characteristic curve due to aging can be represented by a theoretical model, which parameterizes the characteristic curve and includes the aging value as a parameter. The aging parameter can be determined by curve fitting.

[0068] Furthermore, the effect of ambient temperature on leakage current can be taken into account in such a model. Higher temperatures lead to higher leakage currents, even at the same aging state.

[0069] Without limiting the generality of the invention, it is also conceivable to determine the resistance component of the leakage current in an inverse transformation in the time domain.

[0070] Different from the prior art, the present invention can consider harmonics and voltage amplitude at the same time, so that the present invention can more accurately determine the aging condition of the varistor.

[0071] Although the invention is described with respect to varistors, the method can also be applied to other non-linear and linear complex resistors.

Claims

1. Determine the overvoltage protection device (SPD) in the AC power grid N , SPD L1 , SPD L2 , SPD L3 ) leakage current method, wherein the AC power grid frequency f< >0 Hz, is characterized in that, The method comprises the following steps:

100. Measure the overvoltage protection device (SPD N , SPD L1 , SPD L2 , SPD L3 ) Current and voltage curves varying with time; 200. Performing Fourier transform on the current and voltage curves, 300. Determine voltage and current components U f , I f and its phase φ U,f , φ I,f , 400. Determine the phase angle of the voltage and current at the power grid power frequency f, 500. Determine the overvoltage protection device (SPD N , SPD L1 , SPD L2 , SPD L3 ) is the capacitive component of impedance, 600. Determine the voltage U f The capacitive component current: , 700. Determine the resistance component of current at different frequencies.

800. Determine the effective value of the resistance component current: .

2. The method according to claim 1, characterized in that The Fourier transform is a real fast Fourier transform or a fast Fourier transform or a discrete Fourier transform.

3. The method according to claim 1 or 2, characterized in that: The determination of the capacitive component and the resistive component based on the current is determined at discrete multiples of the grid frequency f.

4. The method according to any one of the preceding claims, characterized in that The grid frequency is 16 2 / 3 Hz, 50 Hz, 60 Hz or 400 Hz.

5. The method according to any one of the preceding claims, characterized in that The current is the overvoltage protection device (SPD N , SPD L1 , SPD L2 , SPD L3 ) leakage current.

6. The method according to claim 5, characterized in that The determined resistance current is used to determine the degree of aging.

7. The method according to any one of the preceding claims 5 or 6, characterized in that The determined resistance component is used to determine the degree of aging.

8. The method according to any one of the preceding claims 5 to 7, characterized in that The degree of aging is determined by determining the resistance current-voltage curve.

9. The method according to any one of the preceding claims 5 to 8, characterized in that The overvoltage protection device (SPD N , SPD L1 , SPD L2 , SPD L3 ) has a varistor.

10. The method according to any one of the preceding claims 5 to 9, characterized in that The resistive component of the current is I r,f =I f -I c,f Or it can be determined by inverse transforming the time domain.

11. An apparatus for carrying out the method according to any one of the preceding claims, characterized in that With current sensor (S L1 , S L2 , S L3 ) and voltage sensor (S U1 , S U2 , S U3 ) and a computing unit (CPU).

12. The device according to claim 11, characterized in that The computing unit has a microprocessor or a microcontroller.

13. The calibration method of the device according to any one of claims 11 or 12, characterized in that: The calibration is done using a known capacitance.

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