HK1064221B - Earth leakage protection device and electrical switchgear unit comprising such a device - Google Patents

Description

Earth leakage protector and electric switch device comprising same

Technical Field

The invention relates to an earth leakage protector comprising an input designed to be connected to an annular iron core for measuring earth leakage current, a comparison device for comparing a signal representative of earth leakage current with a reference threshold, and a processing device controlling a tripping relay for disconnecting the feeder contacts when an earth leakage fault is detected. The invention also relates to an electrical switching apparatus comprising a feeder conductor and contacts in series, an earth leakage current measuring annular core surrounding the feeder conductor, a trip relay for breaking the feeder contacts, and an earth leakage protector connected to the annular core and the trip relay.

Background

Known earth leakage protectors, such as those shown in figures 1 and 2, generally comprise a measuring toroidal core 1 surrounding a primary conductor 2 of the line to be protected. These protectors also comprise an electronic circuit 3 connected to the secondary winding 4 of the toroidal core to control a trip relay 5 for opening the contacts of the feeder.

Typically, earth leakage protectors operate by signal peak detection. Thus, in the example of fig. 1, the amplifier 6 connected to the winding 4 of the toroidal core 1 receives a signal 7, the signal 7 representing the earth leakage fault current present in the primary conductor circuit. At the output of the amplifier, the amplified signal 8 is applied to a comparator 9. The number of the comparators can be one or two, as shown in fig. 1. If the signal is an AC signal, the first part 9A of the comparator compares the positive half-wave with a first reference and the second part 9B compares the negative half-wave with a second reference, the polarity of which is opposite to the first reference. The output of the comparator is applied to the input of the control circuit 10 to control the trip during times when the reference in the comparator is significantly exceeded. However, with signal peak detection, the trip threshold may vary greatly depending on the waveform of the signal. A large difference may occur because the trip threshold is represented by a valid (rms) value. The reference center of the comparator can be re-centred to maximize the signal waveform passage, but in this solution the detected pulse waveform can result in a too low trip threshold.

In the protector shown in fig. 2, the detection is performed at rms values, enabling the problems of the peak detection embodiment to be partially overcome. In this case, a block 11 connected between the amplifier 6 and the comparator 9 enables a signal 12 to be supplied to the comparator 9, the signal 12 representing the rms value of the signal 8 at the amplifier output. This system can overcome the problems associated with pulsed current waveforms, but can cause detection errors when the feeder fault current includes a DC component. This DC component occurs in the following cases: when the feeder signal is a rectified signal or has a single half-wave, or when a direct current is superimposed on an alternating or rectified current or a single half-wave current.

These detection problems are highlighted by the fact that the toroidal core 1 lets the variable signal pass but blocks the DC component. The detection of the rms value of such a correction signal then adversely affects the detection accuracy. A fault with a DC component is thus detected, which has the effect of increasing the trip threshold.

Thus, known protectors operating from peak and rms values can produce excessive threshold differences to enable the passage of very different signal waveforms, particularly pulse waveforms and waveforms containing DC components.

Disclosure of Invention

The object of the present invention is to obtain an improved earth leakage protector which is capable of high tripping accuracy according to several types of earth leakage fault current waveforms.

The earth leakage protector according to the invention comprises a rectifying device receiving at least one signal representative of earth leakage current and a filtering device connected to the rectifying device to filter the rectified signal provided by the rectifying device and to provide the filtered rectified signal to said comparing device.

Preferably, the filter device comprises a low-pass filter having a cut-off frequency which is between 2 and 4 times the fundamental frequency of the power system.

In a preferred embodiment, the protector comprises an integrated circuit comprising:

-an amplifier receiving an input signal,

-a signal rectifier connected to the output of the amplifier,

at least a first part of a filter device connected to the output of the amplifier, comprising a filter output,

-a comparator connected to a first part of the filter device,

-a control device comprising a time delay to monitor the number of trips and non-trips connected to the output of said comparator, and

-a trip control output connected to the output of the control device.

In a particular embodiment, the filter device comprises a first internal part having a resistive element arranged in the integrated circuit and a second external part having a capacitive element arranged outside said integrated circuit and connected to the internal part via the filter output.

Preferably, the filter device comprises a first internal part comprising two buffer circuits for commanding a control current mirror circuit designed to reference the filtered signal to a power line or a reference line.

An electrical switching apparatus according to one embodiment of the invention comprises a feeder conductor and contacts in series, a measurement annulus to earth leakage around the feeder conductor, a trip relay to disconnect the feeder contacts and an earth leakage protector connected to the measurement annulus and the trip relay as described above.

Drawings

Further advantages and characteristics of the present invention will become clearer from the following description of a specific embodiment thereof, given purely by way of non-limiting example and represented in the attached drawings, wherein:

figures 1 and 2 show schematic views of a ground leakage protector of the prior art;

figure 3 shows a schematic view of a ground leakage protector according to a first embodiment of the invention;

figures 4A, 4B, 4C, 4D and 4E represent curves that can be used in a protector according to the invention;

figure 5 shows a schematic view of a switchgear comprising a ground leakage protector according to an embodiment of the invention;

figure 6 shows a schematic diagram of an electric circuit used in the earth leakage protector according to an embodiment of the invention;

figure 7 shows a schematic view of a ground leakage protector according to a second embodiment of the invention.

Detailed Description

In fig. 3 is shown an earth leakage protector according to an embodiment of the invention, which can have a high accuracy of the trip threshold for several types of earth leakage current curves. The protector includes a ring-shaped iron core 1 surrounding a feeder conductor 2 in which earth leakage fault current Id can flow. The secondary winding 4 of the toroid is connected to an amplifier 6 to provide a signal 7 representative of the earth leakage current Id. The amplifier 6 amplifies the input signal and provides the amplified signal 8 to the electronic rectifier 20. The rectifier 20 rectifies the signal 8 to provide a rectified signal to the filter 22. the filter 22 matches a defined set of fault signal curves and frequencies of the distribution system to which the feeder conductors are connected. The filter 22 supplies the filtered signal 23 to the comparison circuit 9, and the comparison circuit 9 compares the signal 23 with a reference and supplies the detection signal to the control circuit 10.

Preferably, the filter 22 is a low-pass filter with a cut-off frequency between 2 and 4 times the fundamental frequency of the circuitry. Preferably, the cut-off frequency is between 100Hz and 200Hz for a 50Hz power system and between 120Hz and 240Hz for a 60Hz power system.

Figures 4A, 4B, 4C, 4D and 4E show curves that can be used in a protector according to the invention, with a very good accuracy of the detection threshold. In fig. 4A, the curve 25 is a sinusoidal curve serving as a reference, the accuracy of which must be the highest. The period T of the curve corresponds to the frequency of the power distribution system. In fig. 4B, curve 26 is a rectified half-wave sinusoid. In fig. 4C, curve 27 is a rectified half-wave sinusoid to which an additional DC component 28 is added. Curves 26, 27 exhibit a DC component due to their single polarity. These curves cause large detection errors in the protector in which the toroidal core is associated with the rms value block.

In fig. 4D, curve 29 is a portion of a rectified single half-wave or a curve truncated at a 90 degree sinusoid. In fig. 4E, curve 30 is a quarter-wave rectified partial curve or a curve truncated at a 135 degree sinusoid. Curves 29 and 30 exhibit a large peak. These curves will cause large detection errors in the peak detector.

In a protector according to an embodiment of the invention, associating a rectifier with a suitable filter allows to take into account the DC component and to attenuate the pulse peaks for a large number of curves.

Figure 5 shows a schematic view of a switchgear comprising an earth leakage protector according to an embodiment of the invention. The toroidal core 1 surrounds a feeder conductor 2, the feeder conductor 2 being in series with a feeder contact 31. The secondary winding 4 of the toroidal core is connected end-to-end with the charging resistor 32 and the protection diode. The AC signal at the output of the winding is also applied to an optional physiological (physiological) filter 34. The filter has a frequency response close to that of the human body. At the output of the filter 34 or directly from the toroidal core, the signal 7 is applied to the input of a processing circuit 35. The processing circuit comprises an amplifier 6 receiving a signal 7, a rectifier 20 connected to the amplifier, a matched filter 22 connected to the rectifier, a comparator 9 connected to the filter 22, and a control circuit 10 connected to the comparator 9. The control circuit 10 has an output to command the thyristor 36 and supply power to the trip relay 5. The circuit 35 may obviously be an integrated circuit. A power supply connected to the feeder conductor 2 supplies power to the electronic circuit. The power supply is controlled by a relay 5, which relay 5 also forms a voltage drop. A variable resistor 38 connected downstream of the coil of the relay 5 protects the electronic circuit from voltage surges. The resistor 39, the rectifier 40 and the capacitor 41 thus enable the voltage reduction, the rectification of the alternating current and the filtering of the supply voltage V1 of the electronic circuit 35, respectively.

Fig. 6 shows a detailed schematic diagram of the circuit 35 used in the earth leakage protector according to an embodiment of the invention. The circuit 35 may be an integrated circuit comprising an amplifier 6 receiving an input signal 7, a signal rectifier 20 connected to an output of the amplifier 6, at least a first part 42 of a matched filter 22 connected to an output of the rectifier and comprising a filter output 43, a comparator 9 connected to the first part of the matched filter, and processing circuits 44, 45 and 46. In the processing circuit, the delay 44 comprises a resistive part and a comparator inside the circuit 35 and an external capacitor 48 to which the capacitive part is connected, for example via an output 49, to the circuit 35. Programmable circuit 45, coupled to the delay circuit, can handle the number of trips and non-trips and control output circuit 46, coupled to output 50.

Circuit 35 also includes a power regulator 51 to provide regulated power to various circuits within circuit 35. The first section 42 of the matched filter 22 includes buffer amplifiers 52 and 53 and a current mirror circuit that references the filtered signal 23 to the power supply line 66 or reference line 67 of the power supply. The matched filter 22 comprises a second external capacitive part 55, the capacitive part 55 being represented by a capacitor 56 connected to the filter output 43. The value of the capacitor enables the cut-off frequency of the filter to be adapted according to the frequency of the power system or according to the type of fault current curve to be detected.

Figure 7 shows a schematic diagram of a ground leakage protector with an integrated circuit according to an embodiment. Integrated circuit 35 comprises two inputs for receiving signal 7, a filter output 43 connected to a filter capacitor 56, and an output 49 connected to a delay capacitor 48.

In this schematic diagram, the earth leakage protector comprises a test circuit comprising a conductor 61 inserted in a loop inside the toroidal core and in series with current limiting resistors 62 and 63 and a push button 64. This assembly is connected to a power supply circuit 37, for example, downstream of the trip relay 5.

In the above embodiments, the matched filter is represented by a resistance-capacitance first order filter, but other types of filters, in particular second order filters or digital filters, may also be applicable.

Claims (5)

1. An earth leakage protector comprising:

-an input arranged to be connected to a measuring toroid (1) for the earth leakage current (Id),

-comparison means (9) for comparing a signal (8) representative of the earth leakage current with a reference threshold, and

-processing means (10) instructing control of a trip relay (5) for opening the feeder contacts (3) when an earth leakage fault is detected,

the protector is characterized in that it comprises an integrated circuit (35), the integrated circuit (35) comprising:

-an amplifier (6) receiving an input signal,

-a rectifying device (20) connected to the output of the amplifier (6) receiving at least one signal (8) representative of the current drain (Id),

-a filtering device (22, 43, 54, 55, 56) connected to the rectifying device (20), filtering the rectified signal (21) provided by the rectifying device (20) and providing a filtered rectified signal (23) to the comparing device (9),

-a first part (42) of the filter device (22) is connected to an output of the rectifying device (20) and comprises a filter output (43),

-the comparison means (9) are connected to a first part (42) of the filter means (22),

-the processing device (10) is connected to the output of the comparison device (9) and comprises a delay (44) to monitor the number of trips and non-trips associated with the output of the comparison device (9), and

-a trip control output (50) connected to an output of the processing device (10).

2. Protector according to claim 1, characterized in that the filter means (22, 43, 54, 55, 56) comprise a low-pass filter with a cut-off frequency which is between 2 and 4 times the fundamental frequency of the power system.

3. Protector according to any one of claims 1-2, characterized in that the filter device (22) comprises a first inner part (42) with a resistive element (54) arranged in the integrated circuit (35) and a second outer part (55) with a capacitive element (56) arranged outside said integrated circuit and connected to the inner part via the filter output (43).

4. A protector according to claim 3, characterised in that the filter device (22) comprises a first internal part (42) comprising two buffer circuits (52, 53) for commanding a control current mirror circuit designed to reference the filtered signal (23) to a supply line (66) or a reference line (67).

5. An electrical switching apparatus comprising a feeder conductor (2) and a contact (31) in series, a measuring toroid (1) of earth leakage current (Id) around the feeder conductor, and a trip relay (5) to open the feeder contact, characterised in that it comprises an earth leakage protector according to any of claims 1 to 4 connected to the measuring toroid (1) and the trip relay (5).