RU1772862C - Device to protect instrument voltage transformer from damage at ferroresonance processes in insulated neutral network - Google Patents

Abstract

The essence of the invention: the introduction of additional measuring bodies that control both the primary and harmonic components of the current in the grounded neutral of the primary winding and voltage on the first secondary winding of the protected transformer connected to an open triangle, as well as a half-wave rectifier, the inputs of which are through a single-phase the switching device is connected to the first secondary winding, and the outputs to the newly introduced capacitor, and a three-phase adjustable resistor load, through three-phase A second apparatus connected to a second secondary winding connected to a star provides an increase in the efficiency of the protection. 1 ill. next to

Description

The invention relates to high voltage electric networks, in particular to 6-35 kV distribution electric networks operating with insulated neutral, and is intended for automatic protection against current overloads of voltage transformers during arc short circuits to ground and ferroresonant processes.

A protective device is known, the essence of which is. that the first secondary winding of the voltage transformer, connected in an open triangle, is closed to active resistance 1. However, the minimum value of the constant active resistance from the point of view of the thermal effect of currents of 4A on the insulation of the winding cannot be lower than 25 Ohms, and this is a technical requirement does not provide the necessary efficiency of quenching the ferroresonance process, and therefore does not contribute to reducing the damage to voltage transformers with subsequent damage and Insulation of other electrical equipment, especially windings of stators of electric motors.

Moreover, in the absence of compensation of capacitive currents having small values in the networks of the indicated voltage due to the asymmetry of the capacitance between the individual phases and the ground, as well as the nonlinearity of the no-load resistance of the voltage transformers, it is possible to partially compensate these capacitors (not on all three phases) with the transformer inductance for example With various combinations of inductive and capacitive resistances in phase, a neutral bias occurs, which can be accompanied by a significant increase in phase voltages. This can cause simultaneous damage to the insulation in several places.

It should be borne in mind that in a system with low capacitive currents, low-frequency (subharmonic oscillations in the circuit consisting of phase capacitance with respect to the ground and voltage transformer inductance) may also occur. Such subharmonic oscillations are not accompanied by a significant increase in voltage, but dangerous for voltage transformer, as high currents can flow in its primary windings.

In order to prevent subharmonic fluctuations by directives of the USSR Ministry of Energy, it is recommended that the voltage transformer circuit 31 / o in the block diagrams include a generator-transformer and a synchronous compensator-transformer second resistor with a resistance of 25 Ohms per current 4A, automatically shunting the first permanently connected resistor 2 However, the efficiency these technical recommendations did not give, since neutral displacements, ferroresonant processes, oscillatory phenomena and overvoltage of alternating arcs of the closure to the ground continue to reach dangerous voltages and currents.

Moreover, recommendations on shorting the open-circuit diagram of the first and secondary windings of the voltage transformer with a resistor with a resistance value of 25-100 Ohms are not available in the old version of the Technical Maintenance Rules (PTE) in the new version 3. In clause 5.11.13, it is simply mentioned that with networks and at 6-35 kV connections, if necessary, measures should be taken to prevent ferroresonance processes, including spontaneous neutral displacements.

A device for protecting a voltage transformer for controlling insulation against damage during ferroresonance processes, comprising a circuit of a series-connected non-linear reactor and a resistor, in parallel with which an actuator is connected, which can connect a resistor in parallel to the previously existing 4. This device has all of the above disadvantages of the previous devices, in addition to which the following main disadvantages were added: lost on opportunity

to fix the presence of the current of the ferroresonance process in the dead earthed neutral of the high voltage windings of the voltage transformer; complicated scheme

connecting an additional resistor between the terminals of the open triangle of the first secondary winding due to the difficulty of providing the required parameters of a nonlinear reactor in the process

operation taking into account emergency modes in networks; the efficiency of limiting ferroresonance processes only by the factor of the presence of the voltage of the zero sequence 3U0 between the terminals of the triangle of the first secondary winding decreased.

The closest in technical essence and the achieved solution is a device for protecting the voltage transformer for monitoring the insulation from damage during ferroresonance processes in a network with isolated neutral 5, which we adopted as a prototype and containing, unlike the previous device, a unit

converting a sinusoidal voltage to a rectangular voltage, the inputs of which have terminals for connecting to the first secondary winding of the voltage transformer connected to

an open triangle, and the outputs are connected to the mentioned circuit from a series-connected non-linear reactor and resistor.

In the proposed device, although slightly, the efficiency of limiting ferroresonance processes has increased due to a slight increase in the resistance of the zero sequence in the primary windings of the voltage transformer with a grounded neutral.

The main disadvantages of the device are: the lack of proper efficiency of limiting ferroresonance by resistors in the open triangle of the secondary winding; reduction of the already low efficiency of resistors at low short circuit currents to earth; the impossibility of limiting surge

0 alternating arcs of single-phase earth faults; the likelihood of neutral displacements with a significant increase in phase overvoltages and the appearance of low-frequency (subharmonic) oscillations in the circuit, leading to an increase in currents in the primary windings of the voltage transformer; complexity by additional elements, and therefore insufficient reliability

automation circuits for incorporating resistors into an open triangle of a secondary winding.

The aim of the invention is to more effectively limit ferroresonation processes due to timely limitation of overvoltage of an alternating single-phase short circuit to ground, eliminating the occurrence of neutral displacements and low-frequency oscillations.

The object is achieved in that the device for protecting the measuring voltage transformer from damage during ferroresonance processes in an insulated neutral network comprises: an electromagnetic voltage transformer in which the primary winding is connected to a star with a grounded neutral; the first secondary winding is connected according to an open triangle circuit, between the terminals of which two resistors are connected in parallel, and the first resistor is connected tightly, and the second resistor is connected through a single-phase switching device, and a half-wave rectifier, made according to the bridge circuit, is connected in parallel to the second resistor current of which the capacitor is connected, and the output of the first secondary winding from the side of the direct connection of the second resistor is grounded, and W ra secondary winding connected in a star from the derived neutral conductor; three-phase adjustable resistor load, made in the form of adjustable resistors in phases, connected according to the star circuit with a neutral wire and connected to the phase terminals of the second secondary winding through a three-phase switching device, disconnected in normal mode, and the neutrals of the resistor load and the second secondary winding are directly connected and grounded; a first control circuit comprising a current transformer, the primary winding of which is connected to the neutral of the primary winding of the voltage transformer, and to the secondary circuits of which are connected a current frequency filter, a threshold element, a first intermediate relay and a single-phase switching device switching unit between the terminals of the first secondary winding, moreover, in parallel with the frequency filter and the threshold element, a current relay is connected; a second control circuit comprising a frequency voltage filter-relay between the terminals of the first secondary winding, to which the first time relay is connected in series,

the second intermediate relay and the switching unit of the three-phase switching device between the terminals of the second secondary winding, and a voltage relay is connected in parallel with the frequency 5 filter-voltage relay, and a second second relay of the second control circuit is connected between the output of the current relay of the first control circuit and the input of the second intermediate relay of the second control circuit time relay 10.

Compared with the prototype, significant distinguishing features are: the use of the current of the ferroresonance process φ in the neutral of the primary windings 15 and the zero sequence voltage 3Uo in the open triangle of the first secondary winding of the voltage transformer, respectively; the use of current and voltage frequency filters, 0 providing registration of the occurrence of high-frequency component currents in the neutral primary and voltage in the open triangle of the first secondary winding of the voltage transformer; 5 use of a three-phase resistor load connected to the terminals of the second secondary winding of the voltage transformer; application of a half-wave rectifier connected directly parallel to the second resistor, to the DC terminals of which a capacitor is connected; the presence of two control circuits, providing the connection of these loads to the terminals of the first and second secondary windings of the voltage transformer.

The whole set of the main distinguishing features meets the criterion of novelty.

0 The complex interaction between the low-frequency and high-frequency components of currents and voltages in the voltage transformer windings with alternating arcs, ferroresonance 5, resonant and oscillatory processes on the functional and interconnected connections of the auxiliary circuits in its secondary windings due to the combined functioning of the control circuits 0 ensure compliance the proposed technical solution to the criterion of a positive effect.

On the basis of distinctive essential features, a search for known solutions in science and technology was carried out. No known solutions found. Therefore, the proposed solution meets the criterion of significant differences.

The drawing shows an electrical diagram of the proposed device

The primary winding 1 of the voltage transformer is connected according to the star circuit with the output neutral 2 having a dead earth 3 through the primary winding of the current transformer 4 and is connected directly to the supply section 5,

Its low-voltage winding 6 is connected in a star circuit with an output neutral wire 7, to the linear terminals of which, through a normally disconnected three-phase switching device 8, a three-phase resistor load with adjustable resistors 9 in phases is connected, connected according to the star circuit with an output neutral 10 having ground 11, and there is a direct electrical connection between the neutrals of the second secondary winding and the three-phase load 12.

Its first secondary winding 13 is connected according to an open triangle circuit, between the terminals of which are connected directly and parallel to each other the first resistor 14 and an electric circuit consisting of series-connected single-phase switching device 15 and the second resistor 16, and a half-wave rectifier 17 is connected in parallel with the second resistor , to the terminals of the rectified current of which a capacitor 18 is connected, and the output of the first secondary winding from the side of the direct connection is wallpaper x resistors have a grounding 19,

A secondary control circuit is connected to the secondary circuits of the current transformer, comprising a series-connected current frequency filter 20, a threshold element 21, a first intermediate relay 2 and an on-block 23, of a single-phase switching device between the terminals of the first secondary winding, and connected in parallel to the frequency filter and the threshold element current relay control circuit 24.

The second control circuit contains a frequency filter voltage relay 25 between the terminals of the first secondary link, to which the first time relay, 26, the second intermediate relay 27 and the switching unit 28 of the three-phase switching device are connected in series with the terminals of the first secondary winding and the three-phase resistor load, and in parallel the voltage filter-relay voltage is connected to the control circuit of the voltage relay 29, and between the output of the current relay 24 of the first circuit and the input of the second intermediate relay 27 of the second control circuit a second time relay 30 is connected.

In normal operation of electrical networks, single-phase switching

the apparatus 15 is switched off, the current-carrying process 1ph in neutral 2 of the primary winding 1 of the voltage transformer connected to the supply section 5 is absent, and therefore, there is no current in the secondary circuits of the current transformer 4 and the first control circuit containing the current frequency filter 20, threshold element 21, current relay 24, first intermediate relay 22 and switch-on unit 23, not

5 are functioning. The switching device 8 is also disconnected due to the fact that between the terminals of the open triangle of the first secondary winding 13 there is no voltage - 3Uo, and therefore the second

0, the control circuit comprising the frequency filter of the voltage relay 25, the voltage relay 29, the first time relay 26, the second intermediate relay 27 and the switching unit 28 do not function either.

5 However, non-earthing 3 neutrals 2 of the primary winding 1, joint grounding 11 of the neutral wire 7 of the second secondary winding 6 through direct electrical connection 12 and the output neutral

0 10 three-phase resistor load with adjustable resistors 9, as well as direct grounding 19 of the output of the first secondary winding 13 from the direct connection of both

The 5 resistors 14 and 16 already provide an excess of the efficiency of the permanently connected first resistor 14 in terms of a certain slowdown in the development of overvoltages of the intermittent arc and ferroresonant processes at the initial moment of their occurrence, before the functioning of all elements of the protective device.

Moreover, the direct connection of the primary winding of the transformer 4 to the circuit between neutral 2 and ground 3 of the primary winding 1, as well as the direct connection between the terminals of the first secondary winding 13

0 frequency filter-relay voltage 25 and voltage relay 29 ensure the effective operation of the protective device literally at the initial time in case of damage to the phase insulation of electrical equipment in networks.

In the event of an overvoltage of an alternating arc of single-phase earth faults, the magnetic flux increases in the magneto-wires of the voltage transformers, and therefore already in the initial

The stage of increasing the magnetic flux appears, although insignificant in magnitude, in the current of the initial period of development of the ferroresonance process - φ during oscillations and transients in the primary winding of transformer 4, which drives the first control circuit. The current frequency filter 20 is triggered as a result of the fact that during ferroresonis in the primary windings 1 and, therefore, through the electrical connection between neutral 2 and a solid ground 3 through the primary winding of current transformer 4, the seventh harmonic current, experimentally detected, whose value Compared with the normal mode current, it increases from 0.7 to 30% of the resulting current value. Therefore, the threshold element 21 is very successfully triggered, which triggers the first intermediate relay 22 And the switching unit 23, which enables the switching device 15 to connect between the terminals of the first secondary winding 13 a circuit consisting of a second resistor 16 connected in parallel and a half-wave rectifier 17 with the rectified capacitor current connected to its terminals 18. It is also necessary to add that when ferroresonance with subharmonic oscillations occurs, on the order of 25 or 50 Hz a conventional current relay 24, bypasses the current frequency filter 20 and the threshold element 21, which ensures the above operation of the first control circuit.

At the initial moment of time, the resistance Xc of the capacitor 18 is practically zero, which ensures a very effective and safe short-term shunting of the first resistor 14, which leads to the failure of the oscillatory and transient processes preceding the initial stage of development of the ferroresonance process. As the capacitor 18 is charged, its resistance increases to values that ensure the large current to flow through the second resistor 16, and therefore the resulting current flows between the outputs of the first secondary winding 13 through two parallel-connected resistors 14 and 16 directly to the ground through ground 19, which makes it possible to exclude the re-occurrence of oscillatory and transient processes in the windings 1 of the voltage transformer, and therefore between the phases of the supply sec AI 5 and earth. The presence of grounding 19 at this moment contributes to

improving the stabilization of electromagnetic processes preceding ferroresonance processes. When the current 1ph decreases below the level 5 of the actuation of the threshold element 21, the circuit of the protective device returns to its original state.

However, with the insufficient efficiency of the first control circuit of the protective device 10 and maintaining the zero-sequence voltage of 3Uo between the terminals of the first secondary winding 13, shunted in parallel by connected resistors 14 and 16, 15, a frequency voltage filter relay 25 is triggered, leading to the operation of the first time relay 26, the second intermediate relay 27 and the block 28 28 three-phase switching device

0 8, connecting to the phase terminals of the second secondary winding 6 a resistor load with adjustable resistors 9 in phases connected to the star with the output neutral 10, which is grounded with 5 grounding 11. The neutral wire 7 is connected directly to neutral 10 via electrical connection 12 .

The effective operation of the frequency filter of the voltage relay 25 ensures0 that a third harmonic prevails in voltage 3U0, which in normal operation of electric networks is not more than 0.7 percent, and in transient and initial stages

5 the development of ferroresonance of at least 28 percent of the resulting voltage value, which is more experimentally determined. Regarding the presence of the first time relay 26, it is necessary to note that it has been introduced into the second control circuit due to the need to tune the short-circuit currents arising in places of the entire electrically connected network from the time of disconnection. By connecting to the phases of the second secondary winding 6 of adjustable resistors 9, an effective shunting of the magnetization current is ensured - in the magnetic core of the voltage transformer, and

0 consequently, a decrease in the magnetic flux, in the extreme case of stabilization of its magnitude, excluding, at a minimum, the development of the ferroresonance process to the level of dangerous values of currents and overvoltages of 5 voltages, or even ensuring its complete quenching at the initial stage of development.

Moreover, the joint electrical connection 12 between the neutral 10 and the neutral wire 7, as well as their grounding through the ground electrode 11 stabilizes the magnetization current shunting IjW very efficiently.

In the event of emergency processes with the prevailing presence of 3Uo voltage subharmonic components with a frequency of 25 or 50 Hz, voltage relay 29 is activated, followed by the operation of the above elements of the second control circuit.

In conclusion of the description of the protective device operation efficiency, it is necessary to note the cases of emergency conditions developing in a cyclic-cascade sequence due to the combined effect of overvoltages of the alternating arc of the fault on the ground, resonant and ferroresonant processes with superposition of both low-frequency and high-frequency oscillations leading to bias neutral network and the so-called phenomenon of phase reversal with the emergence, in the end, very dangerous for the isolation of all electrical technical equipment of not only phase, but also linear overvoltages.

However, the presence of a second time relay 30, connected between the terminals of the current relay 24 of the first circuit and the input of the second intermediate relay 27 of the second control circuit, and slightly detuned in the direction of increasing its temporary installation compared to the installation of the first time relay 26, enables the switching device 8 through the second intermediate the relay 27 and the switching unit 28. what is the damping of the complex transient in the very initial stage of development, and this. in turn, it completely eliminates the presence of oscillatory processes on the supply section 5 after shutting down the damaged network section, which currently leads in many cases to insulation damage in the switchgear cells and their ignition, resulting in large economic losses.

Thus, the use of the proposed device fully provides the possibility of timely detection and quenching of ferroresonance processes almost at the initial stage of its development based on the functional interaction between elements affecting the timely and efficient quenching of ferroresonant currents and internal overvoltages.

The minimum economic effect in relation to medium power systems is 120-220 thousand rubles per year per one million kW of installed capacity.

Thanks to the proposed protective device, the trouble-free operation of measuring transformers, discharges, is almost completely ensured.

switching devices and reliable operation of relay protection devices. Hence, the additional economic effect of reducing the accident rate in networks and increasing the reliability of their operation is difficult

0 to overestimate, and the minimum amount for power systems taking into account rural electrification is 350-380 thousand rubles per year.

In connection with the detailed and technically justified description, based on the explanatory diagram and the cited information sources, it can be seen that the proposed device is a technical solution to new problems, realized by previously unknown combinations of means and ways.

Claims (1)

The claims reflect the technical solution of this list of new problems, which are disadvantages in the prototype and analogous 5 rax. Moreover, in the features of the distinctive part of the formula, a completely new, in comparison with the prototype, causal relationship is expressed. 0 SUMMARY OF THE INVENTION A device for protecting a voltage measuring transformer from damage during ferroresonant processes in 5 network with an insulated neutral containing the first resistor with leads for connection to the first secondary winding of the protected transformer connected in an open triangle, and a circuit of 0 series-connected second resistor and a single-phase switching device, connected in parallel with the first resistor, characterized in that, in order to increase efficiency 5 operation, a half-wave rectifier is additionally introduced into it, the inputs of which are connected in parallel with the second resistor, the connection point of which with the first resistor has an output for 0 connection to the ground circuit, a capacitor connected to the outputs of a two-half-wave rectifier, a three-phase switching device, the first conclusions of which are designed to connect 5 to the terminals of the second secondary winding of the protected transformer connected to the star, the second terminals are connected to a three-phase adjustable resistor load connected to the star, the neutral of which has a terminal for connection to neutral of the second secondary winding of the protected transformer and to the ground loop, the first control circuit including the current transformer, the primary winding of which is designed to include the primary winding of the protected transformer in the grounded neutral and the current frequency filter, a threshold element, connected in series to the secondary winding of the current transformer , the first intermediate relay and the unit for switching on the single-phase switching device, while parallel to the current frequency filter and the threshold ele entu a current relay is included, a second control circuit comprising a frequency voltage filter-relay connected in parallel with the first resistor, and a first time relay, a second intermediate relay and a three-phase switching device switching unit, in parallel with the frequency filter a voltage relay is connected a voltage relay whose output is connected to the input of the first time relay, a second time relay is connected between the output of the current relay and the input of the second intermediate relay. ± 41 about #