RU2563342C1 - Active earthing method for power transformer neutral line - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: active earthing method for a power transformer neutral line through a protective module consists in the creation of a dead-earthed neutral in case of unavailable geomagnetic activity. In periods of geomagnetic storms a mode of an insulated neutral is created and neutral voltage is controlled by switching the protective module on when voltage reaches the rated value. The protective module is made as back-to-back connected thyristors, to control electrodes of which control pulses are sent when the geomagnetic activity is unavailable and the transmission of the above control pulses is stopped in the periods of geomagnetic storms.

EFFECT: simplified power circuit and increased degree of usage of active earthing equipment for the neutral line.

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Description

The invention relates to the field of electric power and can be used to protect power transformers with a grounded neutral of power plants and substations from the effects of geo-induced currents during periods of geomagnetic storms.

There is a method of capacitive grounding of the neutral of a power transformer through a parallel-connected switching module, a capacitor bank and a protective module, consisting of a high-speed controlled arrester and a high-voltage switch, which consists in the fact that in the absence of geomagnetic activity, the switching module is turned on, creating a dead-grounded neutral mode, and during periods of geomagnetic storms, the switching model is switched on, creating a capacitive grounding mode of the neutral, and the voltage of the condensers is controlled battery, including the protective module, if the voltage reaches the maximum permissible level [1, 2].

The known method requires the implementation of a complex power circuit, since to create a capacitive grounding mode of the neutral, both a capacitor bank and a protective module are required. However, the capacitor bank and the protective module are not used to create an earthed neutral mode. Therefore, the known method is characterized by a low degree of use of power equipment, since the total duration of geomagnetic storms and, accordingly, the operation of a power transformer with capacitive neutral ground does not exceed 1% of the total duration of operation with dead-grounded neutral.

Closest to the proposed one is a method of actively grounding the neutral of the power transformer through a protective module, which consists in the fact that in the absence of geomagnetic activity they create a dead-grounded neutral mode, and during periods of geomagnetic storms create an isolated neutral mode and control the neutral voltage, including the protective module, if the voltage reaches maximum permissible level [3].

The known method has a complex power circuit and a low degree of use of equipment, since to protect the power transformer from the effects of geo-induced current during periods of geomagnetic storms, a protective module is required, consisting of a high-speed controlled arrester and a high-voltage switch, and the duration of the power transformer with an isolated neutral does not exceed 1% the total duration of the power transformer with a grounded neutral.

The purpose of the invention is to simplify the power circuit and increase the degree of use of active neutral grounding equipment.

This goal is achieved by the fact that the protective module is made in the form of counter-parallel connected power thyristors, the control electrodes of which supply control pulses in the absence of geomagnetic activity and stops the supply of control pulses during periods of geomagnetic storms.

Figure 1 shows a diagram of a device that implements the proposed method; figure 2 presents time diagrams explaining the functioning of the active grounding neutral power transformer.

A device that implements the proposed method contains a protective module 1, power thyristors 2, 3, a symmetrical voltage limiter 4, shunt diodes 5, 6, a control unit 7, power transformers 8, 9 connected by an air line 10.

The power circuit of the protective module 1 is formed by counter-parallel connected power thyristors 2, 3, between the control electrodes of which a symmetrical voltage limiter 4 is connected, and shunt diodes 5, 6 and the outputs of the control unit 7 are connected to the control transitions.

A protective module 1 is included in the neutral of the high voltage windings of the power transformer 8. The high voltage windings of the power transformer 9 have a grounded neutral.

The proposed method of active neutral grounding is as follows.

In the absence of geomagnetic disturbances in the power thyristors 2, 3 of the protective module 1, a voltage of the zero sequence of the main frequency (50 Hz) is applied, the value of which is determined by the asymmetry of the phase voltages of the high voltage windings of the power transformer 8 (figure 2)

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 8;

K _{U (0)} is the voltage asymmetry coefficient in the zero sequence.

According to GOST R 54149-2010, the normal allowable value of the asymmetry coefficient is 2%, and the maximum allowable value is 4%. For example, at U _nom = 110 kV, the zero sequence voltage can be U ₍₀₎ = (2.2 ÷ 4.4) kV.

тока нулевой последовательности I₍₀₎ (фиг.2). Через силовой тиристор 3 будет протекать отрицательная полуволна $$I_{(0)}^{(-)}$$

тока нулевой последовательности I₍₀₎.By the command "Grounding", which is given in the absence of geomagnetic disturbances, block 7 generates a continuous control signal supplied to the control electrodes of the power thyristors 2, 3. In accordance with the change in the polarity of the zero sequence voltage U ₍₀₎ , the power thyristors 2 will turn on 3. A positive half-wave will flow through power thyristor 2 $$I_{(0)}^{(+)}$$

current zero sequence I ₍₀₎ (figure 2). A negative half-wave will flow through the power thyristor 3 $$I_{(0)}^{(-)}$$

zero sequence current I ₍₀₎ .

The continuous control signal of the control unit 7 eliminates the possibility of an intermittent current flowing through power thyristors 2, 3. The zero sequence current I ₍₀₎ will not have pauses and therefore will not contain higher harmonics. As a result, the protective module 1 will provide a dead-grounded neutral mode, since the differential resistance of power thyristors 2, 3 in the open state has an extremely low value (not more than units of mOhm), and the threshold voltage does not exceed (1.0 ÷ 1.2) B.

. В результате между заземлителями силовых трансформаторов 8 и 9 возникает разность потенциалов (фиг.1)During periods of geomagnetic storms, variations in the geomagnetic field induce on the earth's surface a quasi-constant geoelectric field with a horizontal component of tension $${\overrightarrow{E}}_0$$

. As a result, between the earthing switches of the power transformers 8 and 9, a potential difference occurs (Fig. 1)

where l is the distance between the earthing switches of the power transformers 8 and 9, which can be taken equal to the length of the overhead line 10;

α is the angle of orientation of the air line 10 relative to the vector of the intensity of the geoelectric field;

- модуль вектора напряженности геоэлектрического поля. $${\mathrm{<figure-callout\; id="0"\; label="E"\; filenames="00000012.png"\; state="\{\{state\}\}">E</figure-callout>}}_0=\left|{\overrightarrow{E}}_0\right|$$

- the module of the vector of the geoelectric field.

Under the action of the potential difference E in a closed circuit: ground electrode - protective module 1 - neutral and high voltage windings of the power transformer 8 - phase wires of the overhead line 10 - high voltage windings, neutral and grounding of the power transformer 9, a quasi-constant geo-induced current will flow (I _GIT to figure 1), causing saturation of the magnetic system of power transformers 8, 9.

The experience of observations shows that typical ranges of values of the intensity and frequency of the geoelectric field at the surface of the earth are (1 ÷ 20) V / km and (0.001 ÷ 0.1) Hz depending on the intensity of the geomagnetic storm. For example, with the length of the overhead line l = 100 km, α = 0, the potential difference between the grounding conductors of power transformers 8, 9 can reach 2 kV, and the value of the geo-induced current, depending on the value of the active resistances of high voltage windings and phase wires, can reach tens or even hundreds ampere.

By the command "Protection", which is submitted before the start of the predicted geomagnetic storm, the control unit 7 stops the formation of the control signal. Power thyristors 2, 3 are locked, blocking the flow of both the zero sequence current I ₍₀₎ and the geo-induced current I _GIT . As a result, for the period of the geomagnetic storm, the isolated neutral mode of the power transformer 8 is provided, since the resistance of the power thyristors 2, 3 in the closed state reaches tens or even hundreds of megohms. To maintain an isolated neutral mode, power thyristors 2, 3 must withstand voltage with an amplitude value

In emergency mode, for example, with a single-phase short circuit at the beginning of the overhead line 10, a voltage equal to the phase voltage of the high voltage winding of the power transformer 8 will be applied to the power thyristors 2, 3, which is many times higher than the value determined by expression (3). To protect the neutral insulation of the power transformer 8 from overvoltage and power thyristors 2, 3 from breakdown, the latter is forcedly switched on with the help of a symmetrical voltage limiter 4 and shunt diodes 5, 6.

, и должен быть на (100÷200)B меньше повторяющегося напряжения силовых тиристоров 2, 3. Величина отрицательного напряжения на управляющем переходе силового тиристора 2 (3) в случае увеличения напряжения на защитном модуле 1 до уровня лавинообразования симметричного ограничителя напряжения 4 ограничивается величиной прямого падения напряжения на шунтирующем диоде 6 (5).The avalanche level of the symmetrical voltage limiter 4 is the same for voltages of positive and negative polarity, i.e. $$\left|U_{(0)}^{(+)}\right|=\left|U_{(0)}^{(-)}\right|$$

, and should be (100 ÷ 200) B less than the repetitive voltage of the power thyristors 2, 3. The value of the negative voltage at the control transition of the power thyristor 2 (3) in the case of an increase in the voltage on the protective module 1 to the avalanche level of the symmetrical voltage limiter 4 is limited to the direct voltage drop across the shunt diode 6 (5).

, через шунтирующий диод 5, симметричный ограничитель напряжения 4 и управляющий переход силового тиристора 2 протекает положительный импульс тока. Происходит включение силового тиристора 2, через который будет протекать положительная полуволна тока однофазного короткого замыкания. После запирания силового тиристора 2 возникнет импульс перенапряжения противоположной полярности. При достижении уровня лавинообразования $$U_{л}^{(-)}$$

через шунтирующий диод 6, симметричный ограничитель напряжения 4 и управляющий переход силового тиристора 3 протекает импульс тока. Происходит включение силового тиристора 3, через который будет протекать отрицательная полуволна тока однофазного короткого замыкания. Далее процессы включения и выключения силовых тиристоров 2, 3 будут чередоваться до тех пор, пока воздушная линия 10 не будет отключена высоковольтным выключателем по сигналу релейной защиты (на фиг.1 не показаны).The most unfavorable, but most likely, is a single-phase short circuit at the time of the maximum phase voltage of the damaged phase. In this case, at the moment of a single-phase short circuit, an almost abrupt increase in the voltage across the power thyristors 2, 3 occurs theoretically up to the amplitude of the phase voltage of the high voltage winding of the power transformer 8. However, upon reaching an avalanche level, for example $$U_l^{(+)}$$

A positive current pulse flows through a shunt diode 5, a symmetrical voltage limiter 4, and a control transition of the power thyristor 2. The power thyristor 2 is turned on, through which the positive half-wave of the single-phase short circuit current will flow. After locking the power thyristor 2, an overvoltage pulse of opposite polarity will occur. Upon reaching the level of avalanche formation $$U_l^{(-)}$$

a current pulse flows through a shunt diode 6, a symmetrical voltage limiter 4, and a control transition of the power thyristor 3. The power thyristor 3 is turned on, through which the negative half-wave of the current of a single-phase short circuit will flow. Next, the processes of turning on and off the power thyristors 2, 3 will alternate until the overhead line 10 is turned off by the high-voltage switch according to the relay protection signal (not shown in FIG. 1).

For example, the trip time of the air circuit breakers is (0.04 ÷ 0.08) s, and the response time of the relay protection is (0.004 ÷ 0.01) s. As a result, during a single-phase short circuit, it is possible to (4 ÷ 5) turn on each of the power thyristors 2, 3.

After disconnecting the overhead line 10, a zero sequence voltage will be applied to the protective module 1, the amplitude of which is less than the avalanche level of the symmetrical voltage limiter 4. The power thyristors 2, 3 turn on and the initial mode of the isolated neutral of the power transformer 8 is restored. As a result, the grounded high voltage windings of the power transformers 8, 9 are exposed to geo-induced current for a very short time interval (not more than 0.1 s), which o not enough to saturate the magnetic system. After a successful automatic re-enable cycle, the operation of the overhead line 10 is restored and the operation of the power transformer 8 in the isolated neutral mode continues until the end of the geomagnetic storm and the “Grounding” command is received to the control unit 7.

Thus, the protective module 1, built on the basis of power thyristors 2, 3, provides protection of the power transformer 8 from the effects of geo-induced current, continuously in operation, and in the absence of geomagnetic activity, and during periods of geomagnetic storm. This achieves a positive effect, which consists in simplifying the power circuit of the active neutral grounding and in increasing the degree of use of power equipment for active grounding due to the continuous operation of the protective module.

Information sources

1. Kappenman J.G., Scott R.N. Neutral blocking devices combats currents caused by geomagnetic storm / Transmission and Distribution, 1992, 44, No. 5, p. 46-54.

2. Atzmann M.A. Alternatives for blocking direct current in AC - System neutrals and the Radisson / LG-2 Complex / IEEE Trans. on Power Delivery, 1992, 7, No. 3, p. 1328-1337.

3. Kappenman J.G. GIC reduction strategies for the Electric Power Grid / NERC, Agenda GMDTF Face to Face Meeting, November, 9-10, 2011.

Claims (1)

The method of active grounding of the neutral power transformer through a protective module, which consists in the fact that in the absence of geomagnetic activity create a dead-grounded neutral mode, and during the period of geomagnetic storms create an isolated neutral mode and control the neutral voltage, including the protective module, if the voltage reaches the maximum permissible value, different the fact that the protective module is made in the form of counter-parallel connected thyristors, on the control electrodes of which they feed the control pulse s absence geomagnetic activity and stopping supplying control pulses during geomagnetic storms.

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