RU2093943C1 - Method for reducing making current during repeated on/off switching of transformer - Google Patents

Abstract

FIELD: electrical engineering; power transformers. SUBSTANCE: during transformer disconnection from supply mains, prior to its next turning-on, switch contacts are opened at definite moment from beginning of supply voltage rise across one of phases and closed during next connection of transformer at moment counted from same initial period of voltage rise, as during preceding disconnection, across same phase of supply mains. EFFECT: improved operating reliability of transformer, switching and protective equipment due to reducing electrodynamic forces and switching surges. 5 dwg

Description

 The invention relates to the field of electrical engineering, in particular to transformers, mainly power, and can be used with their frequent switching on and off.

 Surge arising during switching-on of the magnetizing current of transformers (for example, electric furnaces, i.e., supplying chipboard steel-arc furnaces) leads to dangerous electrodynamic forces in the windings and current-carrying elements; to overvoltages, especially during switching during the continuous switching process; degrade the operation of electric filters, make it difficult to configure and operate relay protection.

 The development of special transformers with small inrush currents (TV) is unpromising for technical and economic reasons, therefore, ways are needed to reduce the inrush currents using "external" devices.

There is a method of reducing TV surges by switching in series with a transformer a starting resistor or reactor [1]
The method has several disadvantages: special resistors or reactors are required, a second set of switches shorting the resistors after the end of the transition mode, that is, for the implementation of the method requires large additional costs. Therefore, it is rarely used, it is especially difficult to use it at the existing substations of metallurgical plants.

 There is also a method of reducing the turn-on currents during repeated switching of a transformer in a three-phase AC network, in which the circuit breaker contacts when connected to the network at a given time, which is determined by the magnitude and polarity of the residual induction in the transformer rods. The necessary residual induction is achieved by preliminarily passing a constant magnetizing current through the transformer windings [2]. This method is the closest to the invention in terms of technical nature and the achieved effect. The disadvantage of this method (prototype) is the need for an additional DC source, special switching equipment, a current-limiting reactor in the magnetization circuit, performing several additional switching operations before each transformer is turned on, which is difficult to implement, especially in the conditions of existing substations of metallurgical plants.

 The purpose of the invention is to reduce inrush currents of transformers, to increase the reliability of electrical equipment (transformers, switching and protective equipment) by reducing electrodynamic forces and switching overvoltages.

,
где f частота сети,
cosΦ_мин и cosΦ_макс- минимальный и максимальный возможные коэффициенты мощности нагрузки и трансформатора относительно его сетевых вводов,
а замыкание контактов выключателя при очередном подключении трансформатора к сети осуществляют в момент времени, который также отсчитывают от такого же, как в предшествующем выключении, начала периода изменения напряжения в той же самой фазе сети и определяют по формуле

Способ поясняется чертежами, где изображено: на фиг.1 принципиальная схема осуществления способа снижения токов включения трансформаторов, на фиг. 2 структурная схема устройства синхронизации выключателя, обеспечивающего настройку постоянных моментов времени его отключения и включения, на фиг. 3 обмотки трехфазного трансформатора, на фиг. 4 зависимость от времени линейных токов включения, на фиг. 5 зависимость максимальных линейных токов включения трансформатора от момента времени включения трансформатора (Т_вкл) при неизменном значении момента отключения (Т_откл).This goal is achieved by the fact that in the known method, according to which the circuit breaker contacts are closed when connected to the network at a given time, determined by the magnitude and polarity of the residual induction in the transformer rods, the circuit breaker contacts are opened when the transformer is disconnected from the network, prior to its next turn on, at a point in time, which is measured from the beginning of the period of change in the voltage of the network in one of its phases and is determined by the formula

,
where f is the frequency of the network,
cosΦ _min and cosΦ _max are the minimum and maximum possible power factors of the load and the transformer relative to its network inputs,
and the closure of the contacts of the switch at the next connection of the transformer to the network is carried out at the time, which is also counted from the same as in the previous shutdown, the beginning of the period of voltage changes in the same phase of the network and is determined by the formula

The method is illustrated by drawings, where: FIG. 1 is a schematic diagram of an implementation of a method for reducing transformer turn-on currents; FIG. 2 is a structural diagram of a circuit-breaker synchronization device for setting constant time instants of its opening and closing, in FIG. 3 windings of a three-phase transformer, in FIG. 4 the time dependence of linear switching currents, in FIG. 5 the dependence of the maximum linear transformer energizing currents from power-on time of the transformer (T _on) at a constant value the time off (T _off).

 The drawings indicate: 1 mains supply, 2 mains switch, 3 - circuit breaker busbars (3 phases), 4 operational breakers, which, upon the command of the operator from the control panel, repeatedly turn on and off the transformer 5, 6 load of the transformer, for example, an electric arc furnace , 7 and 8 are voltage transformers, 9 current transformers, 10 and 11 are the drive coils for opening and closing the circuit breaker 4, 12 and 13 normally closed (n.o.) and normally open (n.o.) block contacts of the circuit breaker 4, 14 circuit breaker timing device 16 regulator for setting the moment the transformer is turned on, 17 terminals for the signal for disconnecting from the control panel of the circuit breaker 4, 18 terminals for the signal for turning on, 19 conclusions to the oscilloscope, 20 synchronization unit with mains voltage, 21 block for setting the moment for turning off the switch, 22 block for setting the moment for turning on , 23 and 24 control signal conditioners, 25 and 26 logic elements I, 27 and 28 contact signal to logic converters, 29 and 30 short-circuit breaker coils and switch-on coils, 31, 32 and 33 network circuits quipment (HV winding) of the transformer, 34, 35 and 36 of the secondary winding (LV) feeding the load.

 The implementation of the method is illustrated by the example of the power supply circuit from the network 1 of the arc steel furnace 6 through an electric furnace transformer 5.

после перехода через нуль напряжения одной из фаз трансформатора (например, фазы А, т.е. напряжения U_ab, см. фиг. 3). Для такой синхронизации работы выключателя в устройство 14 вводят опорное напряжение сети U_ab от трансформатора напряжения Т, которое подают на блок синхронизации 20, который формирует короткий импульс в момент перехода U_ab через ноль с возрастанием напряжения, т.е. с положительной производной. Этот импульс поступает в блок настройки 21, формирующий необходимое время задержки. Блок настройки 21 содержит регулятор (например, многопозиционный переключатель) 15, позволяющий с малым шагом (порядка 1 2 мс для частоты f=50 Гц) изменять момент формирования сигнала управления выключателем относительно переходов через нуль входного напряжения в диапазоне 20 мс (360 градусов). Блок 23 формирует импульс заданной длительности и мощности для управления короткозамыкателем 28. Логический элемента И 25 разрешает прохождение этого импульса управления на короткозамыкатель 29 при готовности выключателя 4 (контролируется по его блок-контакту 12) и наличии команды на его отключение на клеммах 17. Короткозамыкатель 29 подключает постоянное напряжение 220 В к катушке отключения выключателя.Each time the transformer 5 is turned off again, the switch 4 gives a signal to actuate the switch actuator 10 (to short circuit 29), but not directly, but through the synchronization device of the switch 14. This device delays the trip signal so that, taking into account the drive’s own time, the circuit breaker 4 started over time

after zero crossing the voltage of one of the phases of the transformer (for example, phase A, i.e. voltage U _ab , see Fig. 3). For such synchronization of the circuit breaker operation, the mains voltage U _ab from the voltage transformer T is introduced into the device 14, which is supplied to the synchronization unit 20, which generates a short pulse at the moment U _ab passes through zero with increasing voltage, i.e. with a positive derivative. This pulse enters the tuner 21, forming the necessary delay time. The tuner 21 contains a controller (for example, a multi-position switch) 15, which allows you to change the moment of formation of the control signal of the switch relative to the zero transitions of the input voltage in the range of 20 ms (360 degrees) with a small step (of the order of 1 2 ms for the frequency f = 50 Hz). Block 23 generates a pulse of a given duration and power for controlling short-circuit 28. Logic element I 25 allows the passage of this control pulse to short-circuit 29 when switch 4 is ready (controlled by its block contact 12) and there is a command to turn it off at terminals 17. Short-circuit switch 29 connects a constant voltage of 220 V to the circuit breaker coil.

Since the transformer is switched off each time with the same voltage phase, the same residual induction distribution Bra, Brb and Brc appears in the three rods of the core of the three-phase transformer. Due to the fact that when the AC switch is turned off, the transformer is disconnected from the network not at the moment the contacts diverge, but at the moment the current passes through zero (an arc passes between the contacts of the switch along which current flows, breaking off when this alternating current passes through zero) , with a load of a different nature, a slightly different distribution of the residual inductions over the rods is obtained. So, with a purely active load (cosΦ 1.0), a break in the active current when crossing zero corresponds to the instant of induction maximum in the rod, and the maximum residual induction in the rod is obtained. With a purely inductive load (cosΦ0), the breaking of the reactive current circuit corresponds to the minimum residual induction in the rods. Therefore, contact time differences (T _off) is set in accordance with the nature of the load (v arctg r / x), where x and r the inductive resistance and the active load circuit and the transformer. In the formula, T _off, in particular, it is considered that, if the nature of the load is unchanged (e.g., cosΦ 0,71 const), and then consistently _off time T = 2.5 ms _{(arccosΦ max = arccosΦ min = arccos0,71} = π / 4, 2arccos 0.71 / 6.28 / 50 0.025 c 2.5 mc or 45 degrees). If the nature of the load varies within certain limits (from Φ _min to Φ _max ), then the formula determines the average delay time. As a result, with this method, turning off the transformer allows each time to obtain the same ratio of residual induction in the transformer rods, preparing its subsequent subsequent switching on in the same initial state of the magnetization of the rods (this was obtained by calculation studies and experimentally confirmed).

с (±30 градусов) допустим из-за того, что при различных в пределах этого разброса моментах времени начала срабатывания выключателя (начала расхождения его контактов) отсоединение трансформатора от сети (обрыв дуги переменного тока) происходит в один и тот же момент прохождения тока через нуль. Дополнительные расчетные исследования показали, что и при увеличенном разбросе

(±60 градусов) способ по изобретению работает, но с меньшим эффектом.Dispersion of the moment of shutdown

s (± 30 degrees) is permissible due to the fact that at different times within this scatter of the circuit breaker start time (beginning of the divergence of its contacts), the transformer is disconnected from the network (AC arc break) at the same instant of current passage through zero. Additional computational studies showed that with an increased spread

(± 60 degrees) the method according to the invention works, but with less effect.

которое отсчитывают от такого же, как в предшествующем включении, начала периода изменения напряжения в той же самой фазе сети. Любой из двух в течение одного периода переходов переменного напряжения сети через нуль можно рассматривать как начало изменения напряжения, поэтому под "таким же началом изменения напряжения" следует понимать одинаковый переход через нуль с возрастанием напряжения как при отключении, так и при включении. Работа блоков 20, 22, 24, 26, 28 и 30 в устройстве синхронизации на фиг. 2 аналогична ранее описанной работе при отключении трансформатора.At each next turn-on of the transformer by the operational switch 4, the signal is applied to the actuation of the drive of the switch (to the short circuit 30 of the drive coil 11) through the synchronization device of the switch 14 in the same way as when disconnected. The turn-on signal is then delayed so that, taking into account the intrinsic response time of switch 4, the contact closure occurs after a certain time

which is counted from the same as in the previous inclusion, the beginning of the period of voltage change in the same phase of the network. Any of the two during one period of transitions of the AC voltage of the network through zero can be considered as the beginning of a change in voltage, therefore, by "the same beginning of a change in voltage" should be understood the same transition through zero with an increase in voltage both when disconnecting and when turning on. The operation of blocks 20, 22, 24, 26, 28 and 30 in the synchronization device in FIG. 2 is similar to the previously described operation when turning off the transformer.

Let us explain the meaning of the formula for T _incl . When the transformer is arbitrarily connected to a three-phase network, inrush current surges occur, which with an unfavorable combination of residual induction in the rods and the turn-on phase can reach large values. In FIG. 4 shows the type of linear currents of phases A, B and C, each of which has a maximum value of Ia, Ib and Ic in the first period.

соответствует максимальным значениям токов, показанных на фиг. 4. При моменте времени включения

бросков тока включения вообще нет (все три тока нулевые линии), нет их и в "мервой зоне"

что объясняет предлагаемый метод и формулу для Т_вкл этого метода. Важно подчеркнуть, что, судя по результатам расчета, приведенным на фиг. 5, способ действует с меньшим эффектом в расширенной зоне

однако при этом возникают токи включения, но относительно небольшие, вполне приемлемые в эксплуатации (до 10 20% от максимального значения). На фиг. 5 максимальный ток Iа составляет примерно 5 номинальных токов.In FIG. Figure 5 shows the dependences of these maximum linear currents on the turn-on phase for a certain combination of residual inductions in the rods provided by the synchronous shutdown described above (the curves were obtained by computer calculation and confirmed experimentally). From FIG. Figure 5 shows that the instant of switching on

corresponds to the maximum currents shown in FIG. 4. At the time of inclusion

there are no inrush current surges (all three currents are zero lines), they are not in the “first zone” either

which explains the proposed method and the formula for T _on this method. It is important to emphasize that, judging by the calculation results shown in FIG. 5, the method operates with less effect in the extended area

however, this leads to switching currents, but relatively small, quite acceptable in operation (up to 10 20% of the maximum value). In FIG. 5, the maximum current Ia is approximately 5 rated currents.

периода.For carrying out the method of the invention requires customization T _on and T _off for each instance of the circuit breaker, since Each switch has its own individual on and off times. The adjustment is made using the regulators 15 and 16 in blocks 21 and 22 during trial switching on and off of the circuit breaker with voltage and current monitoring by waveforms. Another setting method is also possible when a series of trial cycles of turning on / off a transformer with oscillography of switching currents is carried out. Each cycle is performed at a fixed value T, and _none of changeable values of T _ON period in the range network frequency. The number of cycles T _off due to frequency 3-phase network can be varied only within

period.

где k= (-1) (+1) в зависимости от вида схемы включения сетевых обмоток трансформатора.It is pointed out that the formula given for T _none embodiment incorporating the transformer windings in a triangle. If included in another winding pattern (star zigzag triangle with sides are extended et al.), The formula for T _off must be entered corrective term of the form

where k = (-1) (+1) depending on the type of switching circuit of the transformer network windings.

 The method according to the invention can be used to reduce inrush currents not only in three-phase, but also in single-phase and any multiphase networks.

 The method is confirmed by theoretical calculations for specially developed computer programs, as well as experiments on transformers of the ETIND-160000/35 type installed in the power circuits of furnaces of the type DSP-10046 of the Moldavian Metallurgical Plant. At the same time, mock-ups of a switch synchronization device of the USV-35/1 type were used that implement the method of the present invention. The experiments showed that without the application of the method according to the invention, the maximum value of the inrush current was 7.5 8 kA, which corresponds to five times the rated current. When applying the method, the maximum inrush currents are reduced to 1.3 kA, and in 90% of cases they are completely absent (the occurrence of small inrush currents of the switching current instead of zero values is probably due to the spread of the circuit breaker operating time and other secondary reasons).

Claims (1)

The method of reducing the turn-on currents during multiple switching of the transformer into a three-phase AC network, in which the circuit breaker contacts when connected to the network at a predetermined point in time, which is determined by the magnitude and polarity of the residual induction in the transformer rods, characterized in that the circuit breaker opens when the transformer is turned off from the network, preceding its next inclusion, is also carried out at a certain point in time, which is counted from the beginning of the period voltage rise in one of its phases and is determined by the formula

where f is the network frequency;
cosΦ _min and cosΦ _max - the minimum and maximum possible power factor of the load and the transformer relative to its network inputs,
and the closure of the contacts of the switch at the next connection of the transformer to the network is carried out at the time, which is also counted from the same as in the previous shutdown, the beginning of the period of increase in voltage in the same phase of the network, and is determined by the formula

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