Method for checking adequacy of residual current of differential circuit breaker

Abstract

A method of checking the adequacy of a residual current traveling through a differential circuit breaker provided at the head of an electrical device by means of a leakage current measuring clamp gripping away from an active conductor of the differential circuit breaker in order to power a plurality of electrical appliances, the method comprises: collecting residual current samples during a continuous collection period at a determined sampling frequency within a total collection duration; performing frequency analysis on the residual current samples in a predetermined frequency band through FFT (Fast Fourier Transform); determining a maximum effective current for each frequency band and each of the consecutive acquisition cycles, and at the end of the total acquisition duration, recording a maximum value of the maximum effective current thus determined; and determining whether to cancel the qualification of the differential circuit breaker according to whether the maximum value of the maximum effective current satisfies a predetermined compatibility condition, and displaying the cancel qualification or not in a binary mode through a pictogram on the leakage current measuring clamp.

Description

Technical Field

[0001] The present invention relates to the field of monitoring electrical appliances and electrical equipment by differential protection, which consists in comparing the currents entering and leaving these appliances and electrical equipment in order to ensure the protection of persons facing direct electrical contact due to faults in these appliances and electrical equipment, and more particularly to a method of checking the correct adequacy of this differential protection with respect to leakage currents or residual currents arising from these appliances and electrical equipment. Background Art

[0002] Differential protection is implemented in electrical installations by two types of devices, namely differential switches and differential circuit breakers, which are very selective in nature (that is, sensitive but not too sensitive) and also integrate detection of overcurrents to ensure protection against ground faults and overloads.

[0003] The increasingly widespread use of power electronic devices in power supplies, power control and power supply products has led to the emergence of residual currents of complex nature and form, which have DC components, low-frequency components and high-frequency components according to different situations.

[0004] Furthermore, the international standard IEC 60755 and its German equivalent VDE 0664-100 define four families of differential circuit breakers, mainly according to the nature of their tripping: AC type when tripping is ensured by an alternating current without a DC component, A type when a pulse current superimposed on a DC component of up to 6 mA can also ensure tripping, F type when the DC component is up to 10 mA and compound currents can also ensure tripping, and B / B+ type when a DC current or a high-frequency residual current (which can be greater than 420 mA for the B+ type) can also ensure tripping. High frequency refers to frequencies up to 1 kHz.

[0005] In some cases, disturbances caused by the network or the network environment can lead to nuisance tripping of the differential circuit breaker, which can cause power supply cuts without causing any danger. This type of tripping is often repetitive, very detrimental to the quality of energy supply and causes operational losses for the user. These disturbances can also lead to no tripping in the presence of a fault and thus to dangers due to a dangerous reduction in the sensitivity of fault current detection. This situation should not be ignored as it affects safety.

[0006] Among the main types of disturbances that can cause nuisance tripping, we will pay special attention to:

[0007] · As electrical equipment becomes larger, permanent leakage currents are higher. In any electrical equipment, there is a permanent leakage current to ground due to the imbalance in the natural leakage capacitance of the grounded active conductor (three-phase circuits) or due to the capacitance between the phase and ground of a single-phase circuit, which may originate from the filter capacitors connected to the bulk of some electronic equipment (automatic equipment, communication systems, computer networks, etc.).

[0008] Leakage currents with high-frequency components in the form of harmonics or transients (generated, for example, by switching on switches) and which may originate from computer equipment power supplies, frequency converters, motors controlled by transmissions, lighting systems with fluorescent lamps. They can also come from close to medium voltage disconnect devices and capacitor banks used for passive energy compensation.

[0009] Among the main types of disturbances that can cause non-tripping are, in particular:

[0010] Permanent currents with a DC component and permanent currents with a very low frequency component (usually less than a few Hz). If these currents represent essentially a low or reduced risk with respect to personnel protection, their ability to saturate the magnetic core (which is the active element in detecting the differential current) can lead to ineffectiveness due to malfunction of some differential circuit breakers.

[0011] ·Affects the temperature of the mechanical components of the circuit breaker.

[0012] Therefore, in order to operate in optimal safety conditions without causing nuisance tripping or non-tripping, the differential circuit breaker must be passed by a very specific form of residual current depending on its type (AC, A, F, B / B+).

[0013] However, the only existing measuring devices so far do not allow for a simple determination of this adequacy. These measuring devices are in fact simple portable devices of the leakage current clamp type without DC component management, or fixed devices of the insulation tester type mounted at the head of the electrical device injecting the control signal, or analysis and specialist stations that are particularly complex to use even for experienced technicians. Summary of the invention

[0014] The object of the present invention is therefore to overcome this drawback by proposing a method and a portable device that is easy to use, making it possible to analyse, in the absence of faults, the good margin of a differential circuit breaker with respect to the residual current passing through it.

[0015] These objects are achieved by a method for checking the adequacy of the residual current of a differential circuit breaker arranged at the head of an electrical installation by means of a leakage current measuring clamp which grasps an active conductor leaving the differential circuit breaker in order to power a plurality of electrical appliances, the method comprising:

[0016] - During the total acquisition duration (t end –t start ) collects residual current samples during consecutive collection cycles at a determined sampling frequency;

[0017] - Perform frequency analysis of these residual current samples in a predetermined frequency band by means of FFT;

[0018] - determining a maximum effective current for each frequency band and for each of said consecutive acquisition periods and recording, at the end of said total acquisition duration, the maximum value of the maximum effective current thus determined in each frequency band; and

[0019] - Determining whether to disqualify / deactivate the differential circuit breaker depends on whether the maximum value of the maximum effective current meets a predetermined compatibility condition, and displaying this disqualification or non-disqualification in binary mode by a pictogram on the leakage current measuring clamp.

[0020] The method implemented by a measuring clamp whose processing module is specially configured for this purpose can therefore be used in complete safety for the user, without requiring any mechanical intervention (disassembly, disconnection, etc.) and without interrupting the energy supply and therefore without risk of potential degradation of the electrical equipment.

[0021] According to a preferred embodiment, the predetermined frequency bands are the following four: DC; DC-50 Hz; 60 Hz-1 kHz; and 1 kHz-10 kHz.

[0022] Preferably, the calculation of the FFT is reduced by determining that the FFT is cut off only to 1 kHz, and the frequency band [1 kHz–10 kHz] is calculated by quadratic subtraction between the total effective current obtained over the entire frequency range [DC–10 kHz] and the sum of the residual current samples obtained in the frequency band [DC–1 kHz].

[0023] Advantageously, the calculation of the FFT is performed after a Hanning or Hamming windowing applied to a determined number of residual current samples.

[0024] Preferably, the AC type, A type or F type differential circuit breaker which is not recommended for use is respectively connected as follows Figure 4-6 The pictogram shown is displayed on the leakage current measuring clamp.

[0025] Advantageously, by Figure 7The table shown gives the maximum current value in each frequency band defining the predetermined compatibility condition and leading to disqualification of the differential circuit breaker. Preferably, the total effective current is calculated by a simple quadratic addition of all the residual current samples.

[0028] Advantageously, the DC current is calculated by averaging said residual current samples.

[0029] Preferably, each acquisition period has a fixed duration of 100 ms, the sampling frequency is 81.92 kHz, and the determined number of samples used for the calculation of the FFT is 512.

[0030] The present invention also relates to a leakage current measuring clamp, comprising an AC+DC current sensor and a processing module, wherein the AC+DC current sensor is capable of measuring AC current or DC current from 1 mA in a frequency band between 0 Hz and 10 kHz, and the processing module is specially configured to implement the above method. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other characteristics and advantages of the invention will emerge from the description given below, with reference to the attached drawings, which illustrate an exemplary embodiment without any limitation and in which:

[0032] Figure 1 A schematic example of a household electrical device to which the method for inspecting a differential circuit breaker according to the present invention is applied is illustrated;

[0033] Figure 2 The different steps of the method for checking a differential circuit breaker according to the invention are shown;

[0034] Figure 3 Shown in detail Figure 2 The method of performing frequency analysis on the residual current by FFT; Figure 4-6 a pictogram showing a differential circuit breaker of type AC, type A or type F which is not recommended for use displayed on the leakage current measuring clamp; and Figure 7 A table showing the maximum current values ​​in each frequency band defining the predetermined compatibility condition and leading to disqualification of the differential circuit breaker is shown. DETAILED DESCRIPTION

[0035] The principle of the invention is based on a method for qualifying a standard differential protection of the AC, A, F, B / B+ type implemented in an electrical installation, that is to say, diagnosing or highlighting in a fault-free installation the inadequacy of this differential protection relative to the residual current present in the installation.

[0036] The inspection method consists in making a judgement of the inadequacy (or incompatibility) of the installed differential protection by grasping all active conductors at the head of the differential circuit breaker, identifying the differential circuit breaker that is incompatible with the residual current being measured, and providing a simple binary indicator in the form of a pictogram. It is also possible to integrate the visualization of the main causes of the inadequacy observed and / or measured.

[0037] Figure 1 An example of a domestic electrical appliance is shown, the electrical panel 10 of which comprises a differential circuit breaker 12 to be tested.

[0038] The differential circuit breaker is connected to various appliances for ensuring protection, such as: an electric oven 14, a refrigerator 16, a washing machine 18 all powered in single phase, and a three-phase powered inverter heat pump 20. It is assumed that the equipment is operating and has no defects as mentioned above (the present invention has no meaning in defective equipment).

[0039] According to the invention, a method for checking the adequacy of the residual current passing through the differential circuit breaker 12 is implemented in the device by means of a measuring device 30 which grasps all the active conductors (phase(s)+neutral) connected at the output of the differential circuit breaker. Such a measuring device is, for example, the leakage current measuring clamp described in application FR2206239 filed in the name of the applicant, comprising an AC+DC leakage current sensor capable of accurately measuring AC current or DC current from 1 mA over a frequency band comprised between 0 Hz and 10 kHz, whose measurement processing module is specifically configured for implementing the innovative method.

[0040] exist Figure 2 The different steps of the method are illustrated in FIG.

[0041] Once the operator has positioned the measuring clamp 30 so as to grasp all the active conductors (phase(s)+neutral) leaving the differential circuit breaker 12, the first step 40 of the method consists in continuously acquiring the residual current present in the device and circulating in the differential circuit breaker, for a total acquisition duration set by the operator (as will be described below), for example every 100 ms period.

[0042] In a second step 42, the measured residual current is subjected to a frequency analysis in four predetermined frequency bands: DC; DC - 50 Hz; 60 Hz - 1 kHz and 1 kHz - 10 kHz. DC means a frequency of 0 Hz and an open or closed bracket means respectively the exclusion or inclusion of the boundary frequencies of the relevant frequency range.

[0043] Then, in the following step 44, for each acquisition period and in each frequency band, the maximum effective current (Imax) corresponding to the maximum value of the residual current measured in the determined frequency band and the determined acquisition period is calculated. DC , Imax ]DC–50Hz[ , Imax ]60Hz–1kHz] , Imax ]1kHz–10kHz] ), and record the maximum effective current in the memory. At the end of the total acquisition duration, the maximum value (the largest maximum value) of these maximum effective currents is recorded in sequence.

[0044] Finally, in a fourth and last step 46, this maximum value of the maximum effective current obtained in each frequency band is used as a compatibility condition to segregate the different types of differential circuit breakers according to the aforementioned criteria and thus disqualify a specific circuit breaker by informing the operator with a simple binary signal, which can typically be indicated by a pictogram or icon. However, in order not to keep the operator waiting, for example, if it is already known that the AC circuit breaker is incompatible, this differentiation step and the pictogram display step can be performed, which will be further detailed while calculating the maximum maximum value, that is to say in particular every 100 ms.

[0045] In order to avoid the problem of blinding of some differential circuit breakers when the current is >1kHz, the operator can also be informed of the value of the current in this frequency band. If the current is obvious, the operator is recommended not to use AC type circuit breakers.

[0046] The total acquisition duration of the measured values ​​(t end –t start ) and therefore the total duration of the corresponding frequency analysis depends on the nature of the charge connected to the differential circuit breaker 12. This duration is estimated by the operator, since the operator knows the nature of the charge connected to the differential circuit breaker. The goal is to be able to record all operating modes of the device. Some illustrative examples are given in the following table:

[0047]

[0048] The first step of acquiring current measurement values ​​is explained in more detail below.

[0049] The analog signal derived from the current sensor is sampled using an analog-to-digital converter in the measurement clamp processing module. The sampling frequency should be set to the highest frequency that can be acquired. For example, in order to accurately measure 10 kHz, a sampling frequency of at least 50 kHz should be selected. Likewise, the duration of the acquisition must be at least greater than one cycle of the power grid (i.e. 20 ms for 50 Hz and 16.66 ms for 60 Hz). This duration can vary depending on the power grid cycle being measured, but for reasons of simplicity and robustness, it is preferably fixed. Typically, a fixed duration equal to 100 ms is preferred because it represents an integer number of cycles of 50 Hz (5 cycles) and 60 Hz (6 cycles). Therefore, for 100 ms, sampling at 50 kHz will allow 5000 measurement samples to be collected.

[0050] Reference now Figure 3 The second frequency analysis step is described in detail. This is performed by a Fast Fourier Transform (FFT) algorithm 50 which requires n=2 for correct operation. p Therefore, it is preferred to select a sampling frequency of 81.92 kHz so that 8192 (n=2 13 ) Sample of residual current.

[0051] For example, a Hanning or Hamming type windowing 52 is performed before the calculation of the FFT in order to obtain a smoother FFT result. The duration of the calculation depends on the number of samples and is proportional to n.log (n). The memory of the processing module required for the calculation is also proportional to n. In addition, in order to reduce the size of the memory required for storing samples and save calculation time, the calculation of the FFT is selected to be performed only to 1Khz cutoff (low-pass filtering box 56), and 1 sample is not taken from 16 samples (extraction box 54) to take only 512 samples from the collected 8192 samples.

[0052] Assuming that the calculation of the FFT must relate only to four frequency bands, this simplification of the calculations by the decimation module 54 and the use of a low-pass filter 56 to determine that the FFT is cut off only to 1 kHz makes it possible to calculate the frequency band [1 kHz-10 kHz] by a quadratic subtraction between the total effective current obtained over the entire frequency range [DC-10 kHz] (calculation block 58) and the sum of the samples of the FFT (calculation block 60) obtained in the frequency band [DC-1 kHz]. The total effective current is calculated by a simple quadratic addition of all the samples. The DC current is also obtained from the FFT, but can be calculated more accurately by averaging the samples (calculation block 62).

[0053] This innovative technique makes it possible to reduce the calculation time of the FFT by approximately 23 (due to the decimation, the initial saving in the calculation time of the FFT is 170, it is necessary to add the calculation of a 1 kHz low-pass filter and a quadratic subtraction that reduces this ratio to approximately 23) and therefore reduce the memory size by 16, as previously mentioned.

[0054] The last step of the method allowing the operator to have information about the disqualification or non-disqualification of the differential circuit breaker tested is detailed below. In fact, the calculation of the maximum value of the maximum effective currents in the four frequency bands allows to distinguish, according to their levels, 4 types of differential circuit breakers according to, for example, the IEC 60755 standard (AC / A / F / B).

[0055] For this purpose and therefore to simplify the analysis for the operator, the following pictograms can be displayed in binary or on / off mode:

[0056] Figure 4 : AC type differential circuit breaker is not recommended

[0057] Figure 5 : Type A differential circuit breaker is not recommended

[0058] Figure 6 : F type differential circuit breaker is not recommended

[0059] Figure 7 The maximum current value for each frequency band that causes some differential circuit breakers to be disqualified is set: where XXX = arbitrary and IΔn is uncertain.

[0060] - Exceeding the threshold on DC current leads to potential safety issues with blinding and non-tripping of the differential protection in case of insulation fault;

[0061] - Exceeding the threshold in the frequency bands [DC–50Hz and] 60Hz–1kHz] does not result in a safety risk, but can lead to undesirable potential tripping;

[0062] - Excessive current in the frequency band > 1kHz can lead to potential safety problems in differential circuit breakers, where the differential protection fails and does not trip in case of insulation faults.

[0063] It should be noted that if in the aforementioned example the frequency band above 1 kHz is not used for the selection of differential circuit breakers, nothing prevents it from being done if necessary, especially in the context of the upcoming VDE 0664-100 standard or any other standard.

Claims

1. A method for checking the adequacy of a residual current passing through a differential circuit breaker (12) arranged at the head of an electrical device (10) by means of a leakage current measuring clamp (30), the leakage current measuring clamp (30) grasping an active conductor leaving the differential circuit breaker to power a plurality of electrical appliances (14, 16, 18, 20), the method comprising: - During the total acquisition duration (t end –t dstart ) collects residual current samples during consecutive collection cycles at a determined sampling frequency; - Perform frequency analysis of these residual current samples in a predetermined frequency band by means of FFT; - determining a maximum effective current for each frequency band and for each of said consecutive acquisition periods and recording, at the end of said total acquisition duration, the maximum value of the maximum effective current thus determined in each frequency band; as well as - determining whether to disqualify the differential circuit breaker according to whether the maximum value of the maximum effective current meets a predetermined compatibility condition, and displaying the disqualification or non-disqualification in binary mode by a pictogram on the leakage current measuring clamp.

2. The method according to claim 1, wherein: The predetermined frequency bands are the following four: DC; DC–50 Hz; 60 Hz–1 kHz; and 1 kHz–10 kHz.

3. The method according to claim 2, wherein: The calculation of the FFT is reduced by determining that the FFT is cut off only to 1 kHz, and the calculation of the frequency band [1 kHz–10 kHz] is performed by a quadratic subtraction between the total effective current obtained over the entire frequency range [DC–10 kHz] and the sum of the residual current samples obtained in the frequency band [DC–1 kHz].

4. The method according to claim 3, wherein: The calculation of the FFT is performed after Hanning or Hamming windowing applied to a determined number of residual current samples.

5. The method according to any one of claims 1 to 5, wherein: Differential circuit breakers of the AC type, A type or F type which are not recommended for use are respectively displayed on the leakage current measuring clamp by pictograms.

6. The method according to claim 4, wherein: The maximum current value in each frequency band which defines the predetermined compatibility condition and leads to disqualification of the differential circuit breaker is given by a table.

7. The method according to any one of claims 1 to 6, wherein: The total effective current is calculated by a simple quadratic addition of all the residual current samples.

8. The method according to any one of claims 1 to 7, wherein: The DC current is calculated by averaging the remaining current samples.

9. The method according to any one of claims 1 to 8, wherein: Each of the consecutive acquisition cycles has a fixed duration of 100 ms, the sampling frequency is 81.92 kHz, and the determined number of samples used for the calculation of the FFT is 512.

10. A leakage current measuring clamp comprising an AC+DC current sensor capable of measuring AC current or DC current from 1 mA over a frequency band comprised between 0 Hz and 10 kHz, and a processing module specifically configured for implementing the method according to any one of claims 1 to 9.

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