A device for protecting a voltage transformer from ferroresonance phenomena

Abstract

The utility model relates to electrical engineering and can be used to protect electromagnetic voltage transformers from ferroresonant oscillations occurring in voltage transformers in distribution networks with a voltage class of 6-10 kV, alternating current with a frequency of 50 Hz with an isolated neutral.In a device for protecting voltage transformers from ferroresonance phenomena, designed with the possibility of connecting to two terminals of auxiliary secondary windings of three single-phase transformers connected in an open-delta circuit, containing a series-connected RTS thermistor, a damping resistor and an electronic switching circuit with a threshold switching characteristic, which includes a switching element in the form of a triac, the control electrode of which is connected to a control circuit including a dinistor, two resistors and a capacitor, the control electrode of the triac is connected to the control circuit through a normally closed controlled key, the RTS thermistor is equipped with a temperature sensor connected to the controlled key, and the dinistor is selected to be symmetrical.Technical result: increasing the effective operating time of a device for protecting voltage transformers from ferroresonance phenomena in the presence of modes in the network accompanied by prolonged exposure to single-phase short circuits to ground by ensuring restoration of the original operating mode. 2 fig.

Description

[0001] The utility model relates to electrical engineering and can be used to protect electromagnetic voltage transformers from ferroresonant oscillations occurring in voltage transformers in distribution networks for voltage class 6-10 kV, alternating current with a frequency of 50 Hz with an isolated neutral.

[0002] A known circuit for damping oscillations in single-phase grounded inductive voltage transformers in a three-phase network with an isolated neutral (Patent DE 1265836, IPC H01F27 / 34, H02H9 / 00, 1968) is connected to additional secondary windings of a three-phase group of single-phase voltage transformers, connected in an open-delta configuration, via controlled contacts of three relays. These controlled contacts close when the relay is triggered by voltages received from the secondary measuring windings of the voltage transformers. Using this relay configuration allows de-energizing the damping resistor in the event of an emergency network operation mode accompanied by a prolonged short circuit to ground.

[0003] The disadvantage of this solution is the need to use electromechanical elements such as relays and the need for additional involvement of secondary measuring windings of voltage transformers.

[0004] A protection circuit for voltage transformers is known (Patent for Utility Model RU 74014, IPC H02H7 / 05, H02H9 / 00, 2007), adopted as a prototype, in the said protection circuit, connected to the circuit of auxiliary windings of three single-phase voltage transformers arranged in an open delta, containing a quenching resistor and a thermal protection element in the form of an RTC thermistor, which are connected to the circuit of auxiliary windings of three single-phase voltage transformers arranged in an open delta by an electronic switching circuit having a threshold switching characteristic, so that exceeding the threshold voltage at the terminals of the auxiliary windings of three single-phase voltage transformers arranged in an open delta turns the electronic switching circuit into its conducting state,a quenching resistor and a thermal protection element are connected to the open-delta circuit of auxiliary windings of three single-phase voltage transformers by means of a switching circuit that contains a triac having a gate controlled by a dinistor.

[0005] The disadvantage of the prototype is the limited time of effective operation of the device to prevent the development of ferroresonant processes in the presence of modes in the network accompanied by prolonged exposure to single-phase short circuits to ground, due to an increase in the resistance of the RTS thermistor to a value that is not capable of damping possible subsequent ferroresonant oscillations.

[0006] The technical result consists in increasing the effective operating time of a device for protecting voltage transformers from ferroresonance phenomena in the presence of modes in the network that are accompanied by prolonged exposure to single-phase short circuits to ground by ensuring restoration of the original operating mode.

[0007] The technical result is achieved in that in a device for protecting voltage transformers from ferroresonance phenomena, configured with the possibility of connecting to two terminals of auxiliary secondary windings of three single-phase transformers connected in an open-delta circuit, containing a series-connected RTS thermistor, a damping resistor and an electronic switching circuit with a threshold switching characteristic, which includes a switching element in the form of a triac, the control electrode of which is connected to a control circuit including a dinistor, two resistors and a capacitor, the control electrode of the triac is connected to the control circuit through a normally closed controlled key, the RTS thermistor is equipped with a temperature sensor connected to the controlled key, and the dinistor is selected symmetrical.

[0008] The essence of the utility model is explained by the drawings. Fig. 1 shows a diagram of the claimed device, Fig. 2 shows a diagram of the connection of a device for protecting a voltage transformer from ferroresonance phenomena in a network with an isolated neutral.

[0009] A device for protecting voltage transformers from ferroresonance phenomena comprises a series-connected thermistor PTC 1, a damping resistor 2 and a switching element in the form of an electronic switching circuit 3 with a threshold switching characteristic. The electronic switching circuit includes a triac 4, the control electrode of which is connected to the control circuit through a normally closed controlled switch 5. The control circuit includes a connected first resistor 6, a symmetrical dinistor 7, the output of which is connected to a second resistor 8 and a capacitor 9, connected in parallel with the triac 4. The PTC thermistor is equipped with a temperature sensor 10 connected to the controlled switch 5, forming a temperature feedback in the control circuit of the switching electronic circuit.

[0010] The device for protecting voltage transformers from ferroresonance phenomena operates as follows.

[0011] Device connection contacts 11 and 12 are connected to the terminals of the auxiliary secondary windings of three single-phase transformers connected in an open-delta configuration (Fig. 2). During device operation, a current is transmitted between terminal a Д the first transformer TNA and output x Д third transformer TN С voltage 3U0 arises, which is applied to terminals 11 and 12 of the claimed device.

[0012] In the case of symmetrical operation of a three-phase network, the zero-sequence voltage 3U0 has a value that is slightly different from 0 V, and no current flows through the claimed device.

[0013] If there is an acceptable asymmetry in the operation of a three-phase network, the voltage 3U0 has a value that does not exceed 30% of the phase voltage of the network for one hour, which is less than the threshold voltage value of the electronic switching circuit 3. In this case, there is also no current flowing through the claimed device.

[0014] When ferroresonant processes occur in the network, the voltage 3U0 reaches a value exceeding the threshold value of the switching electronic circuit. In this case, current flows through damping resistor 2 and thermistor PTC 1. Due to the combined resistance of damping resistor 2 and thermistor PTC 1, which ranges from 6 to 8 ohms, ferroresonant oscillations immediately attenuate when they occur. The combined resistance of damping resistor 2 and thermistor PTC 1, as well as the heat capacity of the thermistor PTC 1, are selected such that ferroresonant oscillations are completely damped before the temperature of the thermistor PTC 1 reaches the limit, at which point the equivalent resistance of the claimed device increases to 1000 ohms or more.

[0015] If an asymmetrical mode continues for a long time in a three-phase network, which can be caused, for example, by an arc burning for a second or more during a series of arc short circuits to ground of one of the phase conductors, the voltage 3U0 also has a value exceeding the threshold value of the electronic switching circuit 3, the effective value of the current flowing through the claimed device increases to 10-12 A. This leads to heating of the thermistor PTC 1, resulting in a significant increase in the equivalent resistance of the claimed device, in this case, the effective value of the current flowing through the device decreases to a value of no more than 0.1 A. Temperature sensor 10 records the heating of the thermistor PTC 1 and sends a signal to open the controlled switch 5 to close the triac 4, bringing the device to a non-conducting state.The voltage on the thermistor RTS 1 disappears, its temperature decreases, when the temperature drops below the set value, the signal from the temperature sensor 10 to the controlled switch 5 stops, it goes into the closed position, which ensures the restoration of the original operating mode of the claimed device.

[0016] Thus, an increase in the effective operating time of the voltage transformer protection device against ferroresonance phenomena is ensured in the presence of modes in the network that are accompanied by prolonged exposure to single-phase short circuits to ground by ensuring the restoration of the original operating mode.

[0017] Simulation modeling was performed, developing a digital twin comprising an electrical network with variable parameters, a voltage transformer, and the antiresonance device itself. A full-scale physical modeling of electrical network elements with an effective voltage of 6(10) kV was performed, along with a surge arrester simulating an intermittent arc, and a voltage transformer connected to the network with the proposed antiresonance device. The results of mathematical modeling on the digital model and physical modeling of the device over a wide range of simulated parameters confirmed the technical result, operability, and high efficiency of the proposed technical solution.

Claims

A device for protecting voltage transformers from ferroresonance phenomena, designed with the possibility of connecting to two terminals of auxiliary secondary windings of three single-phase transformers connected in an open-delta circuit, containing a series-connected RTS thermistor, a damping resistor and an electronic switching circuit with a threshold switching characteristic, which includes a switching element in the form of a triac, the control electrode of which is connected to a control circuit including a dinistor, two resistors and a capacitor, characterized in that the control electrode of the triac is connected to the control circuit through a normally closed controlled key, the RTS thermistor is equipped with a temperature sensor connected to the controlled key, and the dinistor is selected to be symmetrical.

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