RU2586326C1 - Method for efficient earthing of power transformer neutral line - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to electrical engineering and can be used for protection of power transformers of electric stations and substations that operate in electric networks with rated voltage of 110 kV and higher against geo-induced currents in periods of geomagnetic activity at space weather perturbations. Method for efficient earthing power transformer neutral line through current-limiting resistor and thyristor switch consists in fact that in case of abnormal and emergency situations is switching thyristor switch, which is connected in parallel with current-limiting resistor and control pulses, thus providing continuous conductivity thyristor switch. Fixed appearance of constant component of currents and second harmonic phase current high-voltage winding of power transformer. In case said currents permissible values of supply of control pulses to the thyristor key. Resumed supply of control pulses only after termination of constant component currents.

EFFECT: technical result consists in reduction of losses of active power in current limiting resistor and reactive power in power transformer.

1 cl, 1 dwg

Description

The invention relates to the field of electric power and can be used to protect power transformers of power plants and substations operating in electric networks with a nominal voltage of 110 kV and above, from the effects of geo-induced currents during periods of geomagnetic activity during disturbances in space weather.

A known method of effective grounding of the neutral power transformer through a current-limiting resistor [1].

The known method allows to limit the amount of geo-induced currents in the high voltage windings of the power transformer during periods of geomagnetic activity during space weather disturbances to a safe level at which there is no one-sided saturation of the magnetic system and an increase in reactive power losses. However, additional losses of active power appear in the current-limiting resistor, the magnitude of which is greater, the greater the asymmetry of the phase voltages in the adjacent electrical network. In addition, the total duration of geomagnetic activity, which requires resistive neutral grounding, does not exceed 1% of the total duration of the power transformer in the electrical network. With voltage fluctuations in the electric network and a constant value of the resistance of the current-limiting resistor, the possibility of one-sided saturation of the magnetic system of the power transformer remains.

Closest to the proposed one is a method of effectively grounding the neutral of the power transformer through a current-limiting resistor and a thyristor switch, which consists in the fact that when anomalous and emergency situations occur, the thyristor switch is switched [2].

The known method allows you to control the neutral mode of the power transformer by switching from resistive grounding to isolated neutral mode and vice versa. If the resistive grounding of the neutral allows only to limit the amount of geo-induced currents, the isolated neutral mode completely excludes the possibility of the flow of geo-induced currents through the high voltage windings of the power transformer during periods of geomagnetic activity during disturbances in space weather. However, this mode is not permissible in electrical networks with a rated voltage above 110 kV. This is because the neutral voltage in this case can exceed the permissible value, for example, due to the asymmetry of phase voltages, since existing power transformers with a rated voltage of 110 kV and higher have a neutral insulation class of no higher than 35 kV. In addition, the neutral grounding mode is changed without taking into account the state of the magnetic system of the power transformer during periods of geomagnetic activity with disturbances in space weather and therefore does not exclude an increase in power losses in the current-limiting resistor and losses of reactive power in the power transformer.

The purpose of the invention is to reduce the loss of active power in a current-limiting resistor and reactive power in a power transformer during periods of geomagnetic activity during disturbances in space weather.

This goal is achieved by the fact that the thyristor switch is connected in parallel with the current-limiting resistor and the control pulses are supplied, ensuring the thyristor switch is continuously conducting, the appearance of the DC component of the neutral current and the second harmonic of the phase current of the high voltage winding of the power transformer is detected, and if the indicated currents exceed the permissible values, they block the supply control pulses to the thyristor switch, and resume the supply of control pulses only after the cessation of the thawed component of the neutral current.

In FIG. 1 shows a diagram of a device that implements the proposed method.

The device contains a current-limiting resistor 1, a thyristor switch 2 connected in parallel with one of the common conclusions with the neutral of the high voltage windings of the power transformer 3. The thyristor switch 2 is formed by counter-parallel connected power thyristors 4, 5, the control electrodes of which are connected to the control unit 6 The second common output of the current-limiting resistor 1 and thyristor switch 2 is grounded through a DC sensor 7, the output of which is connected to the input of a non-inverting Schmitt trigger 8. Direct the output of the non-inverting Schmitt trigger 8 is connected to one of the inputs of the two-input connector 9, and the inverse output is connected to the installation input "S" of the asynchronous RS-trigger 10. The output of the two-input connector 9 is connected to the installation input "R" of the asynchronous RS-trigger 10. The output of the second sensor the harmonics 11 of the phase current of the high voltage winding of the power transformer 3 is connected through a non-inverting Schmitt trigger 12 to the second input of the two-input connector 9.

The proposed method for effectively grounding the neutral of the power transformer is as follows.

Under normal conditions, when the geomagnetic field is calm, the constant component of the neutral current I _{n of the} power transformer 3 is zero, since there are no geo-induced currents in the high voltage windings. It is geo-induced currents, whose frequency does not exceed 0.1 Hz, that remain practically constant for tens, hundreds of periods of mains voltage. Therefore, the output signal U ₍₌₎ of the DC sensor 7 has a zero value

where I _{n (=)} is the constant component of the neutral current of the power transformer 3; To ₍₌₎ is the input-output coefficient of the DC sensor 7.

In the absence of geo-induced currents of one-sided saturation of the magnetic system of the power transformer 3 does not occur and in the phase current I _{f of the} high voltage windings there are no even harmonics, including the most significant second harmonic. Therefore, the output signal U ₍₂₎ of the second harmonic sensor 11 of the phase current has a zero value

where I _{f (2)} is the effective value of the second harmonic of the phase current of the high voltage winding of the power transformer 3; K ₍₂₎ is the input-output transmission coefficient of the second harmonic sensor 11 of the phase current.

.Under these conditions, at the direct output of the non-inverting Schmitt trigger 8 there is a logic signal “0”, and at the inverse output, a signal of logic “1”. At the output of the non-inverting Schmitt trigger 12 there is a logical signal “0”. Thus, the inputs of the two-input connector 9 receive signals of logical "0", setting the output level of the logical "0". The result is a combination of signals S = 1, R = 0, which sets the asynchronous RS-trigger 10 in a single state Q = 1, $$\overline{Q}=0$$

.

асинхронного RS-триггера 10 служит командой «Пуск» для блока управления 6, разрешая формирование управляющих импульсов силовыми тиристорам 4, 5 и обеспечивая непрерывную проводимость тиристорного ключа 2. Дифференциальное сопротивление силовых тиристоров 4, 5 в проводящем состоянии имеет крайне низкое значение - не более десятков мОм, а пороговое напряжение не превышает 1,0 В. В результате тиристорный ключ 2 в проводящем состоянии шунтирует токоограничивающий резистор 1 и создает режим глухозаземленной нейтрали силового трансформатора 3. Через силовые тиристоры 4, 5 будет протекать ток нулевой последовательности под воздействием напряжения нулевой последовательности, величина которого определяется несимметрией фазных напряжений обмоток высокого напряжения силового трансформатора 3The combination of output signals Q = 1, $$\overline{Q}=0$$

Asynchronous RS-flip-flop 10 serves as the “Start” command for control unit 6, allowing the formation of control pulses of power thyristors 4, 5 and providing continuous conductivity of thyristor switch 2. The differential resistance of power thyristors 4, 5 in the conductive state is extremely low - no more than tens mOhm, and the threshold voltage does not exceed 1.0 V. As a result, the thyristor switch 2 in the conductive state shunts the current-limiting resistor 1 and creates a dead-grounded neutral mode of the power transformer 3. Cher of power thyristors 4, 5 will be zero sequence current flow under the influence of the residual voltage, the magnitude of which is determined by the asymmetry of the power transformer phase voltages of high voltage winding 3

where U ₍₀₎ is the effective value of the voltage of the zero sequence; U _nom - the nominal interphase voltage of the high voltage windings of the power transformer 3; K _{U (0)} - voltage unbalance factor of zero sequence normally allowable value which is 2%, and the limit value - 4%.

For example, if U = 110 kV _rated residual voltage can reach values U ₍₀₎ = (2,2 ÷ _4,4) kV. In accordance with a change in the polarity of the instantaneous values of the zero sequence voltage, the power thyristors 4, 5 will alternately turn on. In accordance with the one selected in FIG. 1, the direction of the neutral current through the power thyristor 5 will flow a positive half-wave of the current of the zero sequence, and through the power thyristor 4 - the negative half-wave.

During periods of geomagnetic activity with a weakly perturbed or perturbed geomagnetic field, variations of the latter cause 3 geoinduced currents of I _GIT in the grounded high voltage windings of the power transformer. In the neutral of the power transformer 3, the geo-induced currents of the individual phase windings are summed. Therefore, the DC component of the current flowing through the DC sensor 7 will be determined by

I _{H (=)} = 3 · I _GIT ,

and the output signal of the DC sensor 7 reaches

U _{(=) =} 3 · K ₍₌₎ · I _HIT.

If the level of the output signal U ₍₌₎ of the DC sensor 7 exceeds the threshold for switching on the non-inverting Schmitt trigger 8, then a logical “1” signal will be established at the direct output of the latter and at the corresponding input of the two-input connector 9. At the same time, the logic signal “0” is set at the inverse output of the non-inverting Schmitt trigger 8 and, respectively, at the input “S” of the asynchronous RS-trigger 10.

If the output signal of the second harmonic 11 of the phase current of the signal level is insufficient to switch the non-inverting Schmitt trigger 12 (the intensity of the geo-induced currents is insufficient for one-sided saturation of the magnetic system of the power transformer 3), the logic signal “0” is stored at the output of the two-input connector. In this case, the combination of signals S = 0, R = 0 is set at the inputs of the asynchronous RS-trigger 10. However, this combination will not lead to the switching of the asynchronous RS-flip-flop 10, which remains in a single state, continuing to generate the “Start” command for the control unit 6. Therefore, the supply of control pulses to the power thyristors 4, 5 continues and the dead-earth neutral mode of the power transformer is maintained.

During periods of increased geomagnetic activity, the intensity of the geo-induced currents is sufficient for one-sided saturation of the magnetic system of the power transformer 3, which is accompanied by a multiple increase in the magnetization current due to only one half-wave. Therefore, even harmonics appear in the magnetization current, of which the second harmonic is the most significant. The second harmonic also appears in the composition of the phase current of the high voltage winding of the power transformer 3, causing a proportional increase in the output signal U ₍₂₎ of the second harmonic sensor 11 of the phase current.

. Комбинация выходных сигналов Q=0, $$\overline{Q}=1$$

асинхронного RS-триггера 10 служит командой «Стоп» для блока управления 6, запрещая подачу управляющих импульсов на силовые тиристоры 4, 5. В результате тиристорный ключ 2 запирается, а нейтраль силового трансформатора 3 заземляется через токоограничивающий резистор 1.If the level of the output signal U ₍₂₎ of the second-harmonic sensor 11 of the phase current exceeds the threshold for switching on the non-inverting Schmitt trigger 12, then the output of the last and the corresponding input of the two-input connector 9 are set to logic 1. As a result, the setup signals R = 1, S = 0 are generated, which reset the asynchronous RS-trigger 10 to the zero state Q = 0, $$\overline{Q}=\mathrm{one}$$

. The combination of output signals Q = 0, $$\overline{Q}=\mathrm{one}$$

Asynchronous RS-flip-flop 10 serves as a “Stop” command for control unit 6, prohibiting the supply of control pulses to power thyristors 4, 5. As a result, thyristor switch 2 is locked, and the neutral of power transformer 3 is grounded through a current-limiting resistor 1.

The power transformer 3 starts to work in the neutral resistive grounding mode, in which the value of the geo-induced currents is reduced by the current-limiting resistor 1. The efficiency indicator of the current-limiting resistor 1 is the ratio of the values of the geo-induced currents in the case of resistive grounding of the neutral and the grounded neutral, which is determined by the expression

where I _{GIT (R)} is the value of the geo-induced current with resistive grounding of the neutral of the power transformer 3; I _{GIT (⊥)} - the value of the geoinduced current with a grounded neutral of the power transformer 3; R is the resistance of the current-limiting resistor 1; r _Σ is the total active resistance of the high voltage windings of the power transformer 3, phase wires of the adjacent overhead line, grounding device; R ^* = R / r _Σ is the relative value of the resistance of the current-limiting resistor 1.

As can be seen, even at R ^* ≥2, the value of the geoinduced current can be reduced by more than 3 times. Thus, choosing the relative value R ^{* of the} resistance of the current-limiting resistor 1, it is possible to reduce the value of the geo-induced current to any predetermined safe level.

After turning off the thyristor switch 2 and the transition of the power transformer 3 to the resistive grounding mode of neutral, the value of the geo-induced current will decrease. This will lead to a decrease in the DC component of the neutral current flowing through the DC sensor 7, and to a decrease in the second harmonic of the phase current of the high voltage windings of the power transformer 3, flowing through the second harmonic sensor 11 of the phase current. The signal levels U ₍₌₎ , U ₍₂₎ at the outputs of the DC sensor 7 and the second harmonic sensor 11 of the phase current will also decrease proportionally. However, due to the hysteresis of the transfer characteristic, the shutdown of non-inverting Schmitt triggers 8 and 12 will not occur and at the inputs of the two-input connector 9, the signal levels of logical “1” will be preserved. It is enough that the lower thresholds for the operation of non-inverting Schmitt triggers 8 and 12 are set with a slight excess of the zero level. Then, the resistive grounding mode of the neutral of the power transformer 3 will continue until the termination of space weather disturbances.

When the intensity of the geo-induced currents is reduced to a level at which the one-sided saturation of the magnetic system of the power transformer 3 is stopped, the output signal of the second harmonic sensor 11 of the phase current decreases to zero. The non-inverting Schmitt trigger 12 is turned off, and a logical “0” signal is set at the corresponding input of the two-input conjunctor 9. At the input R of the asynchronous RS-flip-flop 10, a logical “0” signal is also set. However, the new combination of setting signals R = 0, S = 0 does not change the state of the asynchronous RS-trigger 10, and the Stop command for the control unit 6 is not removed. The resistive earth mode of the neutral of the power transformer 3 is maintained.

, возобновляя команду «Пуск» для блока управления 6. Силовые тиристоры 4,5 начинают поочередно включаться и тиристорный ключ 2 вновь создает режим глухозаземленной нейтрали силового трансформатора 3.With a further decrease in the intensity of geo-induced currents until the complete cessation of the output signal of the DC sensor 7 also decreases to zero. The non-inverting Schmitt trigger 8 is turned off, and the logic signal “1” is set at the inverse output. The result is a combination of signals S = l, R = 0, which sets the asynchronous RS-trigger 10 in a single state Q = 1, $$\overline{Q}=0$$

, resuming the “Start” command for the control unit 6. Power thyristors 4,5 start to turn on alternately and thyristor switch 2 again creates a dead-grounded neutral mode of the power transformer 3.

Thus, the proposed method allows you to create a resistive grounding neutral neutral power transformer 3 only during periods of geomagnetic activity with disturbances in space weather to limit the magnitude of the geoinduced current. The transition to resistive grounding occurs when the magnitude of the geoinduced current is sufficient for one-sided saturation of the magnetic system of the power transformer 3. Returning to dead-grounded neutral occurs when the geo-induced currents cease. This achieves a positive effect, which consists in reducing the loss of active power in the current-limiting resistor 1 by minimizing the duration of the resistive grounding mode and reducing the loss of reactive power by limiting the value of the geo-induced current to a safe level at which one-way saturation of the magnetic system of the power transformer 3 does not occur.

Information sources

1. Kadomskaya K.P. A systematic approach to the selection of resistors in the neutrals of power transformers in electric networks 110-750 kV / K.P. Kadomskaya, B.K. Maximov, A.A. Chelaznov // Electric stations. - 1997. - No. 10.

2. Sadigov G.S. Grounding neutral of 6-10 kV networks using a controlled high-voltage thyristor switch and resistor / G.S. Sadigov, H.I. Nabiev, N.I. Orujov // Industrial Energy. - 1998. - No. 3.

Claims (1)

A method of efficiently grounding the neutral of a power transformer through a current-limiting resistor and a thyristor switch, which consists in the fact that, in the event of abnormal and emergency situations, the thyristor switch is switched, characterized in that the thyristor switch is connected in parallel with the current-limiting resistor and supply control pulses, providing continuous conductivity of the thyristor switch , fix the appearance of the DC component of the neutral current and the second harmonic of the phase current of the high voltage winding power transformer and if the indicated currents exceed the permissible values, they block the supply of control pulses to the thyristor switch, and resume the supply of control pulses only after the DC component of the neutral current stops.

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