RU204520U1 - LEAKAGE CURRENT SENSOR - Google Patents

Abstract

The utility model relates to electrical engineering and can be used to measure direct current, including in insulation monitoring systems. The prior art knows various types of current sensors that are used for various purposes, including in systems for protecting networks and monitoring insulation resistance. However, in contrast to alternating current, where current transformers, including low-power ones, can be used for measurement purposes, direct current cannot be converted with direct current. Most often, in the protection and control system, shunts are used to indirectly measure the current by measuring the voltage drop across the shunt proportional to it. Typical voltage drop across the shunt in such current sensors is 75 mV (no more than 150 mV), which is due to the limitation of the shunt power and its heating. Some modern sensors use the Hall effect to measure DC currents, but its sensitivity is limited. The proposed solution presents a simple sensor design that measures small DC currents, which makes it possible to use it in feeder insulation monitoring systems. For this, a double magnetic core made of a ferromagnetic material with a high relative magnetic permeability is used. Currently, there is a wide range of alloys and permalloy with relative permeability values of 80,000 units and even higher. The technical result of the implementation of the claimed solution is to simplify and improve the accuracy of the leakage current sensor. The use of the whole complex of distinctive features of the proposed solution made it possible to select the 2nd harmonic of the frequency of the input signal of the sensor, a previously unattainable level relative to the 1st harmonic. This simplifies filtering and digitizing the signal from the leakage current sensor and increases its reliability. In addition, a special connection between the pairs of windings makes it possible to compensate for pickups from the AC mains frequency, which also improves the measurement accuracy when using the proposed solution. 7 ill.

Description

Technical field to which the utility model belongs

The utility model relates to electrical measuring equipment and can be used to measure direct current, including in insulation monitoring systems.

State of the art

There is a method of measuring and monitoring the insulation resistance isolated from the ground (case) of power electrical networks of alternating current under operating voltage and a device for its implementation [RF patent for invention No. 2377581], containing a source of stabilized constant voltage, the first resistor current sensor, one terminal connected to a common wire, the first additional resistor connected to the first monitored power network, and the second terminal is connected to the second terminal of the first resistor current sensor and to the first filter capacitor, the power supply unit, the first alarm device with the first and second LEDs, the display unit with a switch, characterized by that the first additional resistor is selected k-times larger than the value of the first resistor current sensor, and a stabilized constant voltage source is connected between the common wire and the body (ground), in addition, the n-th additional resistor is introduced into the device, connected by one withdrawal from the n-th end monitored electrical network, and the second terminal is connected to the first terminal of the n-th resistor current sensor, and the n-th resistor current sensor is connected by the second terminal to the common wire, in addition, the n-th resistor current sensor is connected by the first terminal to the n-th filter capacitor , and the n-th additional resistor in value k-times is chosen greater than the value of the n-th resistor current sensor, in addition, the n-th alarm device with the first n-th and second n-th LEDs is introduced into the device, and all the LEDs cathodes are connected to a common wire, and an external light and sound alarm unit with a generalized LED at the output and with sound and light alarm closing contacts is also introduced into the device, and the external light and sound alarm unit is connected directly to the output of each alarm device.

The disadvantage of this solution is the impossibility of monitoring the insulation resistance for individual sections of a galvanically coupled electrical network.

Also known is a device for measuring the leakage current in the load of a single-phase rectifier [RF patent for invention No. 2642127], containing an indication unit and two sensors of the external magnetic field strength, located on the current supply and current drain wires connecting the load to a single-phase bridge rectifier, and the outputs of the sensitive elements of the voltage sensors are connected, respectively, to the first and second inputs of the comparison device through corresponding amplifiers and narrow-band filters, characterized in that the device additionally includes a threshold device and a voltage level adjuster, and the output of the comparison device is connected to the first input of the threshold device, the second and third inputs of which are connected to the corresponding outputs of the voltage level generator, and the output of the threshold device is connected to the input of the indication unit, which displays the value of the leakage current.

This solution is the closest prototype in technical essence to the claimed solution.

The disadvantages of this solution include the presence of narrow-band filters, which complicates its design, as well as the low sensitivity of the tension sensors, which limits the range of measured currents; there is also no possibility of measuring three-phase current in power networks.

Disclosure of a utility model

Various insulation monitoring systems are known in the art. They perform an important function - control of insulation, and hence serviceability, electrical networks and loads. It is known that insulation resistance is the main parameter, which is an indicator of the state of electrical equipment and cable routes. For all electrical networks there are insulation resistance standards, which are not allowed to be violated due to the danger of emergency situations. All this concerns, first of all, networks with a neutral isolated from the ground - for example, in shipbuilding.

In addition to measurements of insulation resistance, carried out when receiving power and carried out with portable megohmmeters, continuous monitoring of insulation resistance is also carried out during operation, under voltage. For this, voltage monitoring systems are serially produced, the essence of the principle of operation of which is to apply a constant measuring voltage to the monitored section of the network. Further, the level of the leakage current of the monitored network is measured using, for example, a shunt or a ballast resistance included in the circuit for current flow. The voltage drop across the ballast will be proportional to the leakage current and the insulation resistance of the network.

However, all such methods, including in one of the prototypes given in the application materials, are provided by measuring the galvanically coupled section of the network as a whole, without highlighting individual consumers.

The number of measuring voltage sources must be equal to the number of galvanically independent network sections, or there must be a switch that ensures their alternate application to different network sections. If any of the consumers (feeders) supplied from the busbars of the main switchboard turns out to be in an emergency state with a reduced insulation resistance, the insulation readings decrease in the entire galvanically connected area. In this case, personnel must further search for the location of the fault by disconnecting one by one the feeders supplying consumers or sections of the network.

However, when using the proposed leakage current sensor, it is possible to measure the insulation resistance directly on consumers and / or galvanically connected sections of the network, without disconnecting them.

FIG. 1 shows the simplest and well-known [1] fluxgate leakage current sensor. In it, on a common magnetic core made of ferromagnetic material, there are three windings, one of which (at the top in Fig. 1) is input and receives power from a voltage of increased frequency, and the other two are switched on oppositely relative to each other (i.e., their polarity is mutually opposite and bifilar to each other). This leads to the fact that the frequency of the input signal is mutually compensated in them, and when passing through the opening inside the magnetic conductor with a constant current, the ferromagnetic material is magnetized and the hysteresis loop is distorted, which leads to the appearance of 2 harmonic of the signal proportional to the leakage current through it. However, this design is characterized by a drawback - the windings connected in the opposite direction will also compensate for the 2nd harmonic of the signal, since they are located on a common magnetic circuit and they are excited by one stream. In addition, in real measurements of insulation resistance, it is required to monitor three-phase networks, with the superposition of an alternating three-phase current on a direct leakage current.

For this, it is proposed to use the structure shown in FIG. 2. The leakage current sensor is made in a housing with an internal cavity through which three phases of the supply network are passed. The sum of currents in a symmetrical three-phase network is equal to zero [1] and does not participate in the creation of magnetization of the magnetic circuit. The figure conventionally shows the path for the leakage current through the sensor opening and to the conditional place of insulation damage on the consumer side. It is this current that will cause the magnetization of the magnetic circuit of the sensor, which gives the appearance of 2 harmonics at the output [1].

FIG. 3 shows the main switchboard through which power is supplied to consumers, and also conventionally shows the path of leakage currents to consumers (dotted line).

Leakage current sensors are installed on the connections going to consumers of electricity, which makes it possible to measure the leakage current separately for consumers - and immediately display both their insulation level and look for the place of damage without disconnecting consumers from the mains.

For this, it is proposed to make the magnetic circuit of the sensor the leakage current double, with two ferromagnetic magnetic circuits, with windings on each of them. In this case, it becomes possible to avoid mutual compensation of the 2nd harmonic and to increase its level relative to the fluxgate sensor known from the prior art, shown in FIG. one.

In order to eliminate the influence of non-symmetry of the three-phase supply network, it is proposed to use two pairs of secondary (output) coils, as shown in Fig. 4. In this case, the pairs of coils realize the removal of different frequencies: the first pair of coils, connected in the opposite direction, compensates for the frequency of the currents of the three-phase supply network; the second pair compensates for the voltage frequency of the primary (input) coils, that is, 1 harmonic. Since, as shown in FIG. 5, the input coils on two magnetic circuits are switched on oppositely and the switching on of a pair of output coils according to compensates for 1 harmonics, since the coils in pairs are taken from different magnetic circuits and the magnetic fluxes in them are opposite.

For a three-phase supply network, the phases of which pass through both magnetic circuits and magnetize them in the same way, switching on a pair of output coils counter-compensates for the influence of supply network currents, eliminating the influence of asymmetries on the sensor output signal, including modulation in the amplitude of its output voltage.

Thus, in the pairs of output coils, 2 voltages are formed - the voltage of the 2nd harmonic modulated by the currents of the supply network in one pair of coils and compensated from the influence of the currents of the supply network in the other pair of coils. Further, as shown in FIG. 5, pairs of coils are turned on in opposite directions to subtract voltages from each other. This reduces the modulation of the output voltage.

Modulation compensation is implemented for both frequencies - 1st harmonic and mains frequency. Since the pairs of coils are connected oppositely, this results in a mutual subtraction of 1 harmonic of the input signal of the leakage current sensor and the frequency of the three-phase supply network in which the insulation level is measured. The amplitude modulation of the voltage of one pair of coils is subtracted from the amplitude modulation of the other pair of coils, eliminating both components of the output signal - both by 1 harmonic of the input signal and by the frequency of the supply network. This made it possible to isolate the 2nd harmonic of the voltage of the input winding (sensor input signal) at the output, which facilitates both the measurements themselves (the number of filters and their cutoff slope decreases), and the measurement accuracy and sensitivity itself can also be increased. FIG. 6 and FIG. 7 shows the shape of the output voltage at different values of the DC leakage current, it can be seen that the level 2 of the harmonic depends on the magnetization of the magnetic circuits by the leakage current.

The claimed solution is simple and industrially applicable, being a leakage current sensor.

The proposed technical solution is new and has the following fundamental differences from the prototype:

on each of the mentioned magnetic circuits, three coils are installed, one of which is input, the other two are output;

output coils are combined in pairs, coils in pairs are located on different magnetic circuits;

one pair of coils is switched on in the opposite direction, the second pair of coils is switched on according to, and the pairs of coils are switched on oppositely, forming an output winding;

there is an opening in the center of each of the magnetic cores;

the magnetic circuits are located on the same axis, so that their openings coincide, and are placed in a common housing, which also has an opening inscribed inside the openings of the magnetic circuits.

Thus, the entire set of essential features of the utility model was previously unknown and leads to a new technical result - the measurement of constant leakage currents.

Brief Description of Drawings

FIG. 1 shows the simplest fluxgate sensor. FIG. 2 shows the proposed leakage current sensor at the installation site on three-phase buses. FIG. 3 shows the main switchboard with leakage current sensors. FIG. 4 shows a pair of secondary coils of the proposed leakage current sensor. FIG. 5 shows a circuit for switching on the coils of the proposed leakage current sensor. FIG. 6 shows the output voltage of the proposed leakage current sensor at a leakage current of 1 ampere. FIG. 7 shows the output voltage of the proposed leakage current sensor at a leakage current value of 0.1 ampere.

List of used literature.

1. Frumkin A.M. Theoretical foundations of electrical engineering: a textbook for technical schools. M .: Higher school, 1982, 407 p.

Claims (1)

A leakage current sensor containing two magnetic circuits with windings on them, and characterized in that three coils are installed on each of the said magnetic circuits, one of which is input, the other two are output, and the output coils are combined in pairs, the coils in pairs are located on different magnetic circuits, one pair of coils is switched on oppositely, the second pair of coils is switched on according to, and the pairs of coils are switched on oppositely, forming an output winding, there is an opening in the center of each of the magnetic circuits, the magnetic circuits are located on the same axis, so that their openings coincide, and are placed in a common housing made of non-magnetic material, which has an opening inscribed inside the openings of two magnetic circuits.

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