Method for verifying the suitability of residual current devices for differential circuit breakers

Abstract

Method for verifying the suitability of residual currents of differential circuit breakers to residual currents passing through it Method for verifying the suitability of a differential circuit breaker arranged at the head of an electrical installation to residual currents passing through it, by means of a leakage current measuring clamp clamping the active conductors leaving the differential circuit breaker to supply a plurality of electrical devices, the method consisting of an acquisition over a total acquisition period, at a determined sampling frequency, of samples of residual currents during successive acquisition periods; in the frequency analysis by FFT of these samples of residual currents in predetermined frequency bands;in determining for each frequency band and for each successive acquisition period, a maximum effective current and, at the end of the total acquisition period, in recording the maximum value of the maximum effective currents thus determined; and in disqualifying or not the differential circuit breaker depending on whether this maximum value of the maximum effective currents meets or does not meet a predetermined compatibility condition and in displaying this disqualification or not in binary mode by a pictogram on the leakage current measuring clamp. Figure for the abstract: Fig. 2.;

Description

Title of the invention: Method for verifying the suitability of differential circuit breakers for residual currents Technical field

[0001] The present invention relates to the field of monitoring electrical devices and installations by differential protection consisting of comparing the currents entering and leaving these devices and installations, in order to ensure the protection of people against direct electrical contact resulting from a fault in these devices or installations and it relates more particularly to a method making it possible to verify the correct adequacy of this differential protection with the leakage or residual currents emanating from these devices and installations. Prior art

[0002] Differential protection is implemented in electrical installations through two families of devices, namely differential switches and differential circuit breakers, the latter being by nature very selective (i.e. sensitive but not too much) also integrating the detection of overcurrents in order to ensure both protection against earth faults and overloads.

[0003] The increasingly widespread use of power electronics in power sources, in their control but also in the powered products has led to the appearance of residual currents of a complex nature and form, with, depending on the case, a continuous component, low frequency components but also high frequency components.

[0004] Also, the international standard IEC 60755 and its German equivalent VDE 0664-100 mainly define four families of differential circuit breakers according to the nature of their tripping: type AC when this tripping is ensured by an alternating current without a direct component, type A when a pulsed current superimposed on a direct component of at most 6mA can also ensure this tripping, type F when this direct component is at most 10 mA and composite currents can also ensure this tripping and type B / B+ when a direct current or a high-frequency residual current (possibly greater than 420mA for type B+) can also ensure this tripping. By high frequency, we mean a frequency of up to 1 kHz.

[0005] In certain cases, disturbances resulting from the network or its environment can induce an untimely tripping of the differential circuit breaker which leads to cuts in the electrical supply in the absence of a dangerous situation. This type of tripping, often repetitive, is very detrimental to the quality of the supply energy and causes operating losses for the user. These disturbances can also lead to non-tripping in the presence of a fault and therefore danger due to a reduction in sensitivity in the detection of dangerous fault currents. This situation must not be neglected because it affects safety.

[0006] Among the main types of disturbances that can cause untimely triggering, we will particularly note: • The permanent leakage currents are higher as the electrical installation is large. In any electrical installation, there is a permanent leakage current to earth due either to imbalances in the natural leakage capacities of the active conductors to earth (three-phase circuits), or to capacities between a phase and earth for single-phase circuits which may originate from the filter capacitors connected to the ground of certain electronic equipment (automation, communication systems, computer networks, etc.). • High-frequency leakage currents present in the form of harmonics or transients (resulting, for example, from switching on when power is applied) and which may originate from computer equipment power supplies, frequency converters, variable speed motor controls, fluorescent lighting systems. They may also come from proximity to medium-voltage switching devices and capacitor banks for reactive energy compensation.

[0007] Among the main types of disturbances which can lead to non-triggering, there are in particular: • Permanent currents with a direct component as well as permanent currents with a very low frequency component (typically less than a few Hz). If these currents, by nature, represent low or lesser risks with regard to the protection of people, their capacity to saturate the magnetic core which is the active element in the detection of differential currents, leads to the ineffectiveness by blinding of certain differential circuit breakers. • The temperature which can impact the mechanical elements of the circuit breaker.

[0008] Thus, to operate in optimal safety conditions without untimely tripping or non-tripping, differential circuit breakers must be crossed by residual currents of very specific forms depending on their type (AC, A, F, B / B+).

[0009] However, to date, the only existing measuring devices are not capable of simply determining this adequacy. In fact, they are either simple devices portable devices, such as leakage current clamps without DC component management, or fixed devices, such as insulation monitors, installed at the head of the electrical installation into which a control signal is injected, or even analysis and expertise centers that are particularly complex to use, even for an experienced technician. Statement of the invention

[0010] The invention therefore aims to overcome this shortcoming by proposing a method and portable equipment that is easy to use and makes it possible to analyze the correct match of differential circuit breakers with the residual currents flowing through them in the absence of faults.

[0011] These aims are achieved by a method for verifying the suitability to residual currents of a differential circuit breaker arranged at the head of an electrical installation, by means of a leakage current measuring clamp surrounding the active conductors leaving the differential circuit breaker to supply a plurality of electrical devices, method consisting of: - acquisition over a total acquisition duration (tfin - tstart), at a determined sampling frequency, of samples of residual currents during successive acquisition periods, - frequency analysis by FFT of these residual current samples in predetermined frequency bands, - determination for each frequency band and for each of the successive acquisition periods, of a maximum effective current and, at the end of the total acquisition period, recording of the maximum value of the maximum effective currents thus determined in each frequency band, and - disqualification or not of the differential circuit breaker depending on whether this maximum value of the maximum effective currents meets or does not meet a predetermined compatibility condition and display of this disqualification or not in binary mode by a pictogram on the leakage current measuring clamp.

[0012] Thus, this method implemented by a measuring clamp whose processing module is specially configured for this purpose, can be used in complete safety for the user, requires neither mechanical intervention (disassembly, disconnection, etc.), nor interruption of the power supply and therefore presents no risk of potential degradation for the electrical installation.

[0013] According to a preferred embodiment, the predetermined frequency bands are the following four: DC; ]DC - 50Hz[; ]60Hz - 1kHz] and ]1kHz - 10kHz].

[0014] Preferably, the calculation of the FFT is reduced by determining it only up to 1kHz, the frequency band ]1kHz - 10kHz] being calculated by quadratic subtraction between the total effective current obtained over the entire frequency range [DC - 10kHz] and the sum of the residual current samples obtained in the frequency band ]DC - 1kHz],

[0015] Advantageously, the FFT calculation is preceded by a Hanning or Hamming windowing applied to a determined number of residual current samples.

[0016] Preferably, the use of a differential circuit breaker of type AC, A, or F which is not recommended is displayed on the leakage current measuring clamp respectively by the following pictograms:

[0017] Advantageously, the maximum current value in each of the frequency bands defining the predetermined compatibility condition and leading to the disqualification of the differential circuit breaker is given by the following table: DC ]DC - 50Hz[ ]60Hz - 1kHz] >lkHz Icons displayed >lmA and <6mA indifferent indifferent indifferent 0 > 6mA and <10mA indifferent indifferent indifferent > 10mA indifferent indifferent indifferent indifferent >lmA indifferent indifferent indifferent indifferent >lmA<IAn indifférent indifférent indifférent > The indifferent year

[0018] Preferably, the total effective current is calculated by simple quadratic addition of all the residual current samples.

[0019] Advantageously, the DC current is calculated by taking an average of the residual current samples.

[0020] Preferably, each acquisition period has a fixed duration of 100 ms, the sampling frequency is 81.92 kHz and the determined number of samples for the calculation of the FFT is 512.

[0021] The invention also relates to a leakage current measuring clamp comprising an AC+DC current sensor capable of measuring AC or DC currents from 1mA, over a frequency band between 0Hz and 10kHz minimum, and a processing module specially configured to implement the aforementioned method. Brief description of the drawings

[0022] Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate an exemplary embodiment thereof without any limiting character and in which:

[0023] [Fig-1] [Fig.l] illustrates a schematic example of an electrical installation of technique to which the method of verifying differential circuit breakers according to the invention is applied,

[0024] [Fig.2] [Fig.2] shows the different stages of the verification process of dis differential junctions according to the invention, and

[0025] [Fig.3] [Fig.3] shows in detail the step of frequency analysis by FFT of the residual currents of the process of [Fig.2]. Description of the embodiments

[0026] The principle of the invention is based on a method for qualifying a standard differential protection of type AC, A, F, B / B+ implemented in an electrical installation, i.e. for diagnosing or highlighting in this installation free of faults, an inadequacy of this differential protection with regard to the residual currents present in this installation.

[0027] This verification method consists, by enclosing all the active conductors at the head of a differential circuit breaker, in identifying the differential circuit breakers not compatible with the measured residual currents and in providing, in the form of a pictogram, a simple binary indicator, a verdict of inadequacy (or non-compatibility) of the installed differential protection. It may also integrate the visualization of the main causes of inadequacy observed or / and measured.

[0028] [Fig.l] shows an example of a domestic electrical installation whose electrical panel 10 includes a differential circuit breaker 12 to be tested.

[0029] The differential circuit breaker is connected to various electrical appliances which it protects, for example: an electric oven 14, a refrigerator 16, a washing machine 18 all supplied in single phase and an inverter heat pump 20 supplied in three phase. The installation is assumed to be in operation and without fault as mentioned previously (the invention has no meaning in a defective installation).

[0030] According to the invention, the method for verifying the suitability of this differential circuit breaker 12 for the residual currents flowing through it is implemented in this installation by means of a measuring device 30 enclosing all of the active conductors (phase(s) + neutral) connected to the output of the differential circuit breaker. Such a measuring device is for example a leakage current measuring clamp as described in application FR2206239 filed in the name of the applicant, comprising a leakage current sensor. AC+DC leakage current capable of accurately measuring AC or DC currents from 1mA, over a frequency band between 0Hz and 10kHz minimum, whose measurement processing module is specially configured to implement this innovative process.

[0031] The different stages of this process are illustrated in [Fig.2].

[0032] Once the measuring clamp 30 has been put in place by the operator so as to clamp all of the active conductors (phase(s) + neutral) coming out of the differential circuit breaker 12, the first step 40 of the method consists of carrying out, over a total acquisition duration defined by the operator (as will be detailed later), successive acquisitions, for example per period of 100 ms, of the residual current present in the installation and circulating in this differential circuit breaker.

[0033] In a second step 42, the measured residual currents are subject to a frequency analysis in four predetermined frequency bands: DC; ]DC - 50Hz[; ]60Hz - 1kHz] and ]1kHz - 10kHz]. DC meaning the frequency 0Hz and the open or closed brackets respectively meaning an exclusion or an inclusion of the boundary frequency of the associated frequency range.

[0034] Then, in a following step 44, for each acquisition period and in each frequency band, a maximum effective current (ImaxDC, ImaxJDC_50HzL, ImaxJ6oHz-ikHzj, ImaxjikHz- iokHzj) corresponding to the maximum value of the residual currents measured in a determined frequency band and for a determined acquisition period, is calculated and recorded in memory. At the end of the total acquisition duration, the maximum value (maximum maximorum) of these maximum effective currents is recorded in turn.

[0035] Finally, in a fourth and final step 46, this maximum value of the maximum effective currents obtained in each frequency band is used as a compatibility condition to segregate the different types of differential circuit breakers in accordance with the aforementioned standards and therefore disqualify this or that circuit breaker by informing the operator by a simple binary signal which can typically be materialized by a pictogram or an icon. However, in order not to make the operator wait if, for example, it is already known that the AC circuit breaker is not compatible, it is possible to carry out this segregation step and the step of displaying the pictograms which will be detailed further at the same time as the calculation of the maximum of the maximums, that is to say in particular every 100ms.

[0036] To avoid a problem of blinding on certain differential circuit breakers in the presence of current >lkHz, it is also possible to inform the operator about the value of the current in this band and, if this current is significant, to recommend that he does not use an AC type circuit breaker.

[0037] The total duration of acquisition of the measurements (tfin - tstart) and therefore of the fre- The corresponding quantity depends on the nature of the loads connected to the differential circuit breaker 12. It is up to the operator to estimate this duration because it is he who has knowledge of the nature of the loads connected to the differential circuit breaker. The goal is to be able to record all operating modes of the devices. Some illustrative examples are given in the table below: Type of load Total acquisition time for home automation load (computer, light, roller shutter, etc.) < 1 minute Compressor system: - Refrigerator, freezer - Old generation heat pump 10 minutes Variable frequency controlled system: - New generation heat pump with inverter - New generation refrigerator with inverter - Steam oven with inverter - Car battery charger < 1 minute Household appliances - Washing machine 60 minutes

[0038] The first step of acquiring current measurements is detailed more precisely below.

[0039] The analog signals from the current sensor are sampled using an analog-to-digital converter in the processing module of the measuring clamp. The sampling frequency must be set so as to be able to acquire the highest frequency. For example, to accurately measure 10kHz, a sampling frequency of at least 50kHz should be chosen. Similarly, the acquisition duration must be at least greater than one period of the electrical network (i.e. 20ms for 50Hz and 16.66ms for 60Hz). This duration can be variable depending on the measured network period, but it is preferably fixed for reasons of simplicity and robustness. Typically, a fixed duration equal to 100ms is preferred because it represents an integer number of periods for both 50Hz (5 periods) and 60Hz (6 periods). Thus, sampling at 50kHz would allow 5000 measurement samples to be collected over 100ms.

[0040] The second step of frequency analysis is now detailed with reference to the

[0041]

[0042]

[0043]

[0044]

[0045] [Fig.3]. This is achieved by a Fast Fourier Transform (FFT) algorithm 50 which requires, for correct operation, a number of samples multiple of n=2p. The sampling frequency is therefore preferably chosen at 81.92kHz, thus allowing 8192 samples of the residual current (n = 213) to be obtained over the 100ms period. A 52 window, such as Hanning or Hamming, precedes the FFT calculation 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 this calculation is also proportional to n. Also, to reduce the size of the memory required for storing samples and save on calculation time, it is chosen to perform this FFT calculation only up to 1Khz (low-pass filtering block 56) and also to take only 1 sample out of 16 (decimation block 54), i.e. only 512 samples out of the 8192 collected. Since the FFT calculation is only to be performed on four frequency bands, this simplification of the calculation by the decimation module 54 and the use of low-pass filtering 56 determining the FFT only up to 1kHz, makes it possible to calculate the frequency band ]1kHz - 10kHz] by quadratic subtraction between the total effective current (calculation block 58) obtained over the entire frequency range [DC - 10kHz] and the sum of the FFT samples (calculation block 60) obtained in the frequency band ]DC - 1kHz]. The total effective current is calculated by simple quadratic addition of all the samples. The DC current is also obtained from the FFT but can be calculated more precisely by averaging the samples (calculation block 62). This innovative technique allows to reduce by approximately 23 the calculation time of the FFT (to the initial gain of 170 in calculation time of the FFT due to the decimation, it is necessary to add the calculation of the low-pass filter at 1kHz and the quadratic subtraction which brings this ratio back to approximately 23) and therefore as indicated previously to reduce the size of the memory by 16. The last step of the process, which provides information to the operator on whether or not the tested RCD is disqualified, is detailed below. In fact, the calculation of the maximum value of the maximum effective currents in the four frequency bands allows, depending on its level, to segregate the 4 types of RCD according to, for example, the IEC 60755 standard (AC / A / F / B). To do this, and thus simplify the analysis result for the operator, it can be displayed in binary or all or nothing mode, the following pictograms: - : the use of an AC type differential circuit breaker is not recommended order the use of type A differential circuit breaker is not recommended n: the use of type F differential circuit breaker is not recommended

[0046]

[0047]

[0048] The table below defines the maximum current value in each frequency band leading to the disqualification of certain differential circuit breakers: DC ]DC - 50Hz[ ]60Hz - 1kHz] >lkHz Icons displayed >lmA and <6mA XXX XXX XXX 0 > 6mA and <10mA XXX XXX XXX > 10mA XXX XXX XXX XXX >lmA XXX XXX XXX XXX >lmA<IAn XXX 00 XXX XXX > IAn XXX XXX = indifferent and IAn = uncertainty - An exceeded threshold on the DC current leads to a potential safety problem with blinding and non-triggering of the differential protection in the event of an insulation fault, - A threshold exceeded on the frequency bands ]DC - 50Hz[ and ]60Hz - 1kHz] does not cause a safety risk but potential unwanted triggering, - Too high a current in the band > 1kHz can cause a potential safety problem on differential circuit breakers with blinding and non-tripping of the differential protection in the event of an insulation fault. It should be noted that, although in the above example the frequency band above 1kHz is not used for the selection of differential circuit breakers, there is nothing to prevent it from being used if the need arises, particularly in the context of standard VDE 0664-100 or any other to come.

Claims

Claims

1. Method for verifying the suitability of a differential circuit breaker (12) disposed at the head of an electrical installation (10) for residual currents flowing through it, by means of a leakage current measuring clamp (30) clamping the active conductors leaving the differential circuit breaker to supply a plurality of electrical devices (14, 16, 18, 20), method consisting of: - acquisition over a total acquisition duration (tfin - tstart), at a determined sampling frequency, of samples of residual currents during successive acquisition periods, - frequency analysis by FFT of these samples of residual currents in predetermined frequency bands, - determination for each frequency band and for each of the successive acquisition periods, of a maximum effective current and, at the end of the total acquisition duration,recording of the maximum value of the maximum effective currents thus determined in each frequency band, and - disqualification or not of the differential circuit breaker depending on whether this maximum value of the maximum effective currents meets or does not meet a predetermined compatibility condition and display of this disqualification or not in binary mode by a pictogram on the leakage current measuring clamp.,

2. The method of claim 1, wherein the predetermined frequency bands are the following four: DC; ]DC - 50Hz[; ]60Hz -1kHz] and ]lkHz - 10kHz].

3. A method according to claim 2, wherein the calculation of the FFT is reduced by determining it only up to 1kHz, the frequency band ]1kHz - 10kHz] being calculated by quadratic subtraction between the total effective current obtained over the entire frequency range [DC -10kHz] and the sum of the residual current samples obtained in the frequency band ]DC - 1kHz].

4. Method according to claim 3, in which the FFT calculation is preceded by a Hanning or Hamming windowing applied to a determined number of residual current samples.

5. A method according to any one of claims 1 to 5, wherein the use of a type AC, A, or F differential circuit breaker that is not recommended is displayed on the leakage current measuring clamp. respectively by the following pictograms

6. Method according to claim 4, in which the maximum current value in each of the frequency bands defining the predetermined compatibility condition and leading to the disqualification of the differential circuit breaker is given by the following table: DC ]DC-50Hz[ ]60Hz - 1kHz] >lkHz Icons displayed >lmA and <6mA indifferent indifferent indifferent > 6mA and <10mA indifferent indifferent indifferent > 10mA indifferent indifferent indifferent indifferent >lmA indifferent indifferent indifferent indifferent >lmA < lAn indifferent indifferent indifferent > lAn indifferent

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

8.

9.

10. A method according to any one of claims 1 to 7, wherein the DC current is calculated by averaging the residual current samples. Method according to any one of claims 1 to 8, in which each of the successive acquisition periods has a fixed duration of 100 ms, the sampling frequency is 81.92 kHz and the determined number of samples for calculating the FFT is 512. Leakage current measuring clamp comprising an AC+DC current sensor capable of measuring AC or DC currents from 1 mA, over a frequency band between 0 Hz and 10 kHz minimum, and a processing module specially configured to implement the method according to any one of claims 1 to 9.

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