RU2794204C1 - Method for detecting interturn short circuits in the windings of three-phase consumer transformers - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention can be used to detect interturn short circuits in the windings of three-phase consumer transformers during operation. The method for detecting turn-to-turn short circuits in the windings of three-phase consumer transformers consists in measuring the phase currents of the transformer in idle mode using measuring current transformers. The first measurement is made before starting operation on a known-good transformer. The signals coming to the measuring current transformers from the measuring current transformers are processed, and the result is stored. During the operation of a three-phase transformer, during a scheduled inspection, measurements of the phase currents of the transformer are repeated with the load disconnected and the signals are re-processed. The processing of signals coming from measuring current transformers consists in the fact that after removing the signals from the ammeters during the first measurement, the direction of the current vector of one of the phases is selected, the angles between the vectors of the phase currents are determined, and the value of the negative sequence current vector for the selected phase is calculated. The result obtained is compared with the result of the first measurement, and the presence of interturn short circuits is judged by the absolute value of the vector difference between the calculated negative sequence current vectors, which, in the presence of an interturn short circuit, will be greater than zero.

EFFECT: increasing the sensitivity to the presence of short circuits in the windings of a three-phase power transformer by taking into account negative sequence currents.

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Description

The invention relates to the electric power industry and can be used to detect coil faults in the windings of three-phase consumer transformers during operation.

A known method for detecting turn-to-turn short circuits in the windings of three-phase transformers (RU 2645811 publication date 28.02.2018; IPC G01R 31/06), measurements are carried out in the absence of connections between the windings of a three-phase transformer, except for a common point in the "star" circuit, measurements are performed alternately by applying an operational power frequency voltage from an adjustable power source to only one primary or secondary winding at the beginning of the first phase of the transformer, for example, phase A, while measuring the amount of active power, simultaneously measuring and recording the voltage values on both windings of the first phase at idle, then the operating voltage is applied of the previous value on the primary or secondary winding of the second phase of the transformer, measure and record the value of the active power for the second phase and the voltage values on the windings of the second phase, similarly measure and record the active power and voltages for the third phase, after identifying the damaged phase, the recorded values of the winding voltages are calculated the values of the actual transformation ratio between the windings of this phase for two probable cases of a turn circuit in the primary or secondary winding, then the calculated values of this actual ratio are compared with the passport transformation ratio for this transformer, and if the actual ratio is less than the passport one, then a conclusion is made about the fact of a turn circuit in the primary winding, if the actual coefficient is greater than the passport one, then a conclusion is made about the presence of a turn short circuit in the secondary winding.

The main disadvantage of this method is that it cannot be applied in the operating conditions of consumer transformers, since the diagnosis requires disconnecting the transformer from the network, and this is associated with significant difficulties, since in the operating mode the power transformer is always connected to a high-voltage power line (6- 10 kV).

A known method for detecting turn-to-turn short circuits in the windings of three-phase transformers according to the current principle (Petrov G.N. Electrical machines. In 3 parts. Part 1. Transformers. - M. Energy, 1974. - 240 s; Woldek A. Electrical machines. - L .: Energy, 1974. - 840 s). The method is based on the fact that with a short circuit of a certain number of turns of the winding, the current in this winding increases at a constant voltage. On this basis, if a three-phase transformer is a symmetrical system in the form of a group of single-phase transformers, then in the idle mode, by measuring the current in three phases, and then by comparing the measurement results with each other by the highest current value, it is possible to identify the phase of the transformer with a turn circuit.

The disadvantage of the analogue is the unreliability of the results due to the fact that for transformers with a three-rod magnetic circuit, due to the asymmetry of the magnetic system, the value of the no-load currents in phases is not the same even in the absence of turn short circuits. The occurrence of a turn short circuit in the winding of any of the three phases will change the degree of asymmetry of the no-load currents of the transformer.

It was also noted that negative sequence currents change phase when an interturn fault occurs (Ivanchenko D.I., Shonin O.B.. Identification of interturn short circuits of a power transformer based on the analysis of the amplitude-phase relationships of reverse sequence currents. - Journal of the Notes of the Mining Institute. T 196. St. Petersburg 2012, pp. 240-243). This method is proposed to be used in the differential protection of transformers, which, according to the PUE, is used on transformers with a capacity of 6.3 MVA and above, and is not used in consumer transformers. In this case, the phases of the negative sequence currents at the input to the transformer and at the output from it are compared. The studies of differential relay protection in this article were carried out using the SimPowerSystemsSimulinkMatlab environment. A transformer with a voltage of 35/6 kV, a power of 5.6 MVA was used, the comparison of the signals was carried out using current sensors installed at the input and output of the transformer. To identify signs of interturn short circuits, an analysis was made of the symmetrical components of the differential current and currents of the primary and secondary windings of the transformer, the amplitude criterion for detecting interturn faults is based on the calculation of differential currents of the positive and negative sequence, which showed that in the case of an internal short circuit in the transformer, this criterion takes values close to to zero. However, this article is intended to use the criterion used in the single-phase earth fault protection device. Differential protection works when the transformer is under load, but the interturn fault current is usually a very small part of the transformer load current and it is impossible to detect it with great sensitivity with this method, the protection only works if the windings are significantly damaged. Meanwhile, it is important to detect turn faults at an early stage of damage, and if they are detected when the damage affects a significant part of the winding, the cost of repairing such a transformer will be commensurate with the cost of a new transformer. In connection with this, the disadvantage of the method is that it is impossible to detect an interturn short circuit with greater sensitivity.

Significantly greater sensitivity can be achieved if inter-turn short circuits are determined in the idle mode of the transformer. In this case, the current in the phases caused by the interturn short circuit will be a significant proportion of the no-load current of the transformer, which greatly simplifies the task of detecting the interturn short circuit.

The prototype of the claimed invention is a method for detecting interturn short circuits, described in the utility model patent "Device for diagnosing interturn short circuits in the windings of a three-phase transformer" (RU 205505, publication date 07/19/2021; IPC G01R 31/62) and described in detail in (Belov A. V., Ilyin Yu.P., Skorodumova N.V. A device for diagnosing interturn short circuits in the windings of a three-phase transformer. - Journal of Electrical equipment: operation and repair, No. 1, 2022, pp. 18-21). The method is based on measuring the phase currents of the transformer at no load. The first measurement is made before the start of operation on a known-good transformer. The measurement is made using measuring current transformers. The signals received from measuring current transformers are processed. The processing consists in the fact that the AC signals are rectified using a full-wave three-phase rectifier and the received DC signals are summed, obtaining a signal proportional to the sum of the absolute values of the phase currents of the transformer in idle mode. The resulting value is stored. It is assumed that this sum of phase currents, measured in the idle mode, in a serviceable transformer is practically a constant value. Then, during the operation of the transformer, the measurement is periodically repeated. If at some point an interturn short circuit occurs in the transformer windings, then the sum of the absolute values of the phase currents will increase. The sum of the absolute values of the phase currents is compared with that previously obtained during the first measurement. The presence of interturn short circuits in the transformer windings can be judged by the excess of the sum of the absolute values of the phase currents measured during operation, compared with the value obtained during the first measurement.

The disadvantage of the prototype is the lack of sensitivity to the presence of interturn circuits.

It is possible to increase the sensitivity by using negative sequence currents as an indicator of interturn short circuits.

Negative sequence currents are absent in a symmetrical three-phase system and appear only when there is an unbalance in the phase currents. Since the unbalance of the phase currents of the transformer is caused by turn-to-turn faults, the negative sequence currents are the most sensitive indicator of turn-to-turn faults in the windings of a three-phase transformer. Therefore, taking into account negative sequence currents provides the greatest sensitivity to the presence of interturn short circuits.

The purpose of the present invention is to increase the sensitivity to the presence of short circuits in the windings of a power three-phase transformer by taking into account negative sequence currents.

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This goal is achieved by the fact that the method for detecting turn-to-turn short circuits in the windings of three-phase consumer transformers, which consists in measuring the phase currents of the transformer in idle mode using measuring current transformers, the first measurement is made before starting operation on a known-good transformer, the signals arriving at ammeters from measuring current transformers are processed, and the result is stored, then during the operation of a three-phase transformer during a scheduled inspection, measurements of the phase currents of the transformer are repeated with the load disconnected and the signals are re-processed; the result obtained is compared with the result of the first measurement, and the presence of interturn short circuits is judged by the magnitude of the difference between the measurement results. Unlike the prototype, the processing of signals from measuring current transformers consists in the fact that after removing the signals from the ammeters during the first measurement, the direction of the current vector of one of the phases is selected, the angles between the vectors of the phase currents are determined, the value of the negative sequence current vector for the selected phase is calculated , and the presence of interturn short circuits is judged by the absolute value of the vector difference between the calculated negative sequence current vectors. If the value of the vector difference is greater than zero, this will indicate the presence of an interturn short circuit. The method is carried out in the following sequence: The phase currents of a three-phase transformer are measured in idle mode using measuring current transformers and ammeters, the first measurement is made before starting operation on a known-good transformer, after removing the signals from the ammeters during the first measurement, the direction of the current vector of one of the phases is selected, determine the angles between the phase current vectors. Determining the angles can be done by jointly solving the equations:

And

Полученный результат запоминают.Calculate the value of the negative sequence current vector for the selected phase, for example, using the formula:

The result is remembered.

. Если значение векторной разности будет больше нуля, это будет свидетельствовать о наличии межвиткового замыкания.Then, during the operation of a three-phase transformer during a scheduled inspection, measurements of the phase currents of the transformer are repeated with the load disconnected and the signals are processed again in the same way. The result obtained is compared with the result of the first measurement, and the presence of interturn short circuits is judged by the absolute value of the vector difference between the calculated negative sequence current vectors, which can be determined by the formula

. If the value of the vector difference is greater than zero, this will indicate the presence of an interturn short circuit.

между векторами токов обратной последовательности фазы А после повреждения и до него.The proposed method is illustrated by the figures: Fig. 1 - Scheme for measuring the phase currents of a three-phase transformer; fig. 2 - Location of phase current vectors on the complex plane; fig. 3 - Obtaining a vector difference

between phase A negative sequence current vectors after and before the fault.

An example of the implementation of the method: In Fig. 1 shows the connection diagram of a three-phase transformer T, which, for example, has a “star with isolated neutral” higher voltage winding connection scheme (other higher voltage winding connection schemes are possible). Measuring current transformers TA1, TA2, TA3 connected to ammeters are connected to the phase conductors.

Consider the operation of the transformer in idle mode. In this mode, no-load currents of the transformer flow through the higher voltage windings. In this case, if the phase voltage vectors in a three-phase system have, as a rule, the same value (module), and are located at 120 degrees relative to each other, then the phase current vectors can have different modules and be located at angles other than 120 degrees from each other .

на комплексной плоскости некоторым образом, представленным на фиг. 2. Поскольку нейтраль трансформатора со стороны высшего напряжения (6-10 кВ) потребительского трансформатора изолирована, то, при условии прямой последовательности фазных токов (А-В-С), взаимное расположение векторов фазных токов имеет единственное решение.Readings are taken from ammeters, they provide information about the absolute values (modules) of phase currents. Imagine the location of the phase current vectors

on the complex plane in a manner shown in Fig. 2. Since the neutral of the transformer on the side of the higher voltage (6-10 kV) of the consumer transformer is isolated, then, under the condition of a direct sequence of phase currents (A-B-C), the relative position of the phase current vectors has a unique solution.

Modules of phase current vectors are denoted as A, B, C. Then the current vectors in complex form can be written:

от действительной оси абсцисс на комплексной плоскости.where ϕ _A , ϕ _B , ϕ _C - deviation angles of the current vectors

from the real x-axis in the complex plane.

совпадает с осью абсцисс (т.е. ϕ_A=0).The direction of the current vector is chosen, for example, phase A. Let us set the condition that the current vector of the phase

coincides with the abscissa axis (i.e. ϕ _A =0).

Then

составляет ϕ_B, а угол между осью абсцисс и вектором

составляет ϕ_C.Angle between x-axis and vector

is ϕ _B , and the angle between the x-axis and the vector

is ϕ _C .

Since the transformer on the higher voltage side (6-10 kV) has an isolated neutral, the vector sum of the phase currents must be equal to zero (in accordance with the Kirchhoff law), i.e.

- фазные токи, направление векторов которых зависит от переменных углов ϕ_B и ϕ_C.Where

- phase currents, the direction of the vectors of which depends on the variable angles ϕ _B and ϕ _C .

на действительную ось (ось абсцисс) дадут в сумме величину вектора

взятого со знаком «минус».In addition, it follows from the same law that the vector sum of any two currents is equal to the third current, taken with the opposite sign. In other words, the sum of the projections of the vectors

on the real axis (abscissa) will sum up the value of the vector

taken with a minus sign.

на ось абсцисс рассчитывают, перемножив модули векторов на косинусы углов и ψ_B и ψ_C по выражениям:Vector projections

on the x-axis is calculated by multiplying the modules of the vectors by the cosines of the angles and ψ _B and ψ _C according to the expressions:

and further, considering that ψ _B =ϕ _B - 180° and ψ _C =180°-ϕ _C , write the second equation to find the phase angles:

В⋅cos(ϕ _B - 180°)+С ⋅ cos(180° - ϕ _C )=А (3)

Equations (2) and (3) are jointly solved, the angles ϕ _B and ϕ _C are obtained

определяют в соответствии с выражениями (1).Then the current vectors

determined in accordance with expressions (1).

The positive and negative sequence currents for phase A (and, if necessary, for other phases) are found according to the well-known formulas of the theory of symmetrical components:

where a is the rotation vector used to rotate other vectors by 120 degrees:

The value of the negative sequence current in idle mode remains approximately constant as long as the transformer is in good condition; for transformers of the same power and the same design, these values are almost the same. The absolute values (modules) of negative sequence currents, in accordance with the theory of symmetrical components, are the same for all phases, and differ only in direction (they are at an angle of 120 degrees from each other). Therefore, it does not matter for which of the phases the negative sequence current is calculated - it is only important that the calculations are made for the same phase during subsequent measurements.

The method was implemented as follows:

The tests were carried out in laboratory conditions, a three-phase transformer TS 2.5 kVA was chosen. The first measurement is made on a known-good transformer, before the start of operation, we measure the phase currents of a three-phase transformer in idle mode using measuring current transformers and ammeters.

поворотный вектор

сигналы, снятые с амперметров каждой фазы трехфазного трансформатора составили A:=125, В:=99, С:=125 (единицы измерения условно опускаем). Выберем направление вектора тока фазы А. Пусть вектор тока фазы

совпадает с осью абсцисс (т.е. ϕ_A=0).For the calculation, you can use any computer program, for example, the calculation in the MathCad program is presented, all given values \u200b\u200bare used with a sign to assign, you must specify the imaginary unit,

turning vector

the signals taken from the ammeters of each phase of the three-phase transformer were A:=125, V:=99, C:=125 (we conditionally omit the units of measurement). Let us choose the direction of the phase A current vector. Let the phase current vector

coincides with the abscissa axis (i.e. ϕ _A =0).

Введем служебное слово Given присвоим приближенное значение начальных углов ϕ_B:=- 2 и ϕ_C:=2; Решим совместно уравнения

В ⋅ cos(ϕ_B - 180°)+С ⋅ cos(180° - ϕ_C)=А, используя символ логическогоравенства, знак жирное равно (=); Напишем функцию

перечислив неизвестные переменные в качестве параметров функции. В результате расчетов получаем необходимые углы между векторами фазных токов ϕ_B=-1,967, ϕ_C=2,331, значения углов получены в радианах. Тогда векторы токов каждой из фаз в комплексном выражении будут равны:We express the current vectors of each phase in absolute value and angle according to the formula (1). We write the current vectors in a complex form:

Let's introduce the official word Given and assign the approximate value of the initial angles ϕ _B :=- 2 and ϕ _C :=2; Let's solve the equations together

B ⋅ cos(ϕ _B - 180°)+C ⋅ cos(180° - ϕ _C )=A, using the symbol of logical equality, bold equal sign (=); Let's write a function

by listing unknown variables as function parameters. As a result of calculations, we obtain the required angles between the phase current vectors ϕ _B =-1.967, ϕ _C =2.331, the angle values are obtained in radians. Then the current vectors of each of the phases in the complex expression will be equal to:

находим ток обратной последовательности. Например, ток обратной последовательности для фазы А в комплексном алгебраическом выражении будет составлять:

или в абсолютном значении

Найденное значение вектора тока обратной последовательности необходимо запомнить. Если в ходе эксплуатации трехфазного трансформатора при плановом осмотре произошло замыкание витков в обмотке фазы С, повторяем измерения фазных токов трансформатора в режиме холостого хода. При, очередном измерении токов холостого хода, модули сигналов, снятых с амперметров, оказались равными: А=160; В=19 и С=174 (данные эксперимента). Видно, что при возникновении межвиткового замыкания фазные токи, измеренные таким же образом в режиме холостого хода, изменяются. Изменяются углы между векторами фазных токов: ϕ_B=-1,633; ϕ_C=2,671.(the index j denotes the imaginary unit). Then we calculate the negative sequence current vector value for the selected phase. We substitute the found values of the current vectors into the formula

find the negative sequence current. For example, the negative sequence current for phase A in a complex algebraic expression would be:

or in absolute terms

The found value of the negative sequence current vector must be remembered. If during the operation of a three-phase transformer during a scheduled inspection, the turns in the winding of phase C are short-circuited, we repeat the measurements of the phase currents of the transformer in the idle mode. At the next measurement of no-load currents, the modules of the signals taken from the ammeters turned out to be equal: A \u003d 160; B=19 and C=174 (experimental data). It can be seen that when an interturn fault occurs, the phase currents, measured in the same way in the idle mode, change. The angles between the phase current vectors change: ϕ _B =-1.633; φ _C =2.671.

Similarly, we calculate the negative sequence current vector for the same phase. The calculation of the negative sequence current for phase A (with an interturn short circuit in phase C) in a complex algebraic expression gave the following result:

In this case, a vector difference is detected between the current vector of the negative sequence of phase A obtained after the turn circuit in phase C and the same current vector obtained before the fault in the first measurement and previously stored.

In FIG. 3 shows the difference between the phase A negative sequence current vectors before and after the fault, which can also be obtained graphically.

The presence of an interturn short circuit is judged by the absolute value of the vector difference between the negative sequence current vector of phase A with a damaged winding and the same current vector of phase A obtained before the damage:

можно судить не только о факте межвиткового замыкания, но и о величине повреждения.At the same time, in terms of size

it is possible to judge not only the fact of the interturn circuit, but also the magnitude of the damage.

Let's compare the proposed method with the method taken as a prototype in terms of sensitivity to interturn short circuits.

As you know, the basis of the prototype is a comparison of the sum of the absolute values of the phase currents (ie, ammeter readings) before and after the turn-to-turn circuit, measured in the idle mode of the transformer.

We sum up the absolute values of the phase currents during the first measurement:

During the next measurement of no-load currents (during operation with an interturn circuit in the winding of phase C), the sum of the modules of phase currents was:

The sum of the phase currents measured in no-load mode increases, although some individual phase current may even decrease.

Let us evaluate the sensitivity of the method in this case, taking the sensitivity coefficient as the ratio of the sum of the signal modules. It is:

Now consider the proposed method, using the same ammeter readings.

или в абсолютном значении

So, during the first measurement, the following values of phase currents were obtained: A=125; B=99 and C=126. So, during the first measurement, the stored negative sequence current vector was:

or in absolute terms

During operation during the next measurement (with an interturn short circuit in the winding of phase C), the calculated negative sequence current vector was:

The absolute value of the negative sequence current difference was:

The sensitivity coefficient of the method is defined as the ratio of the absolute value of the negative sequence current difference, calculated during operation, to the absolute value of the negative sequence current, calculated during the first measurement, then:

which is 1.9 times higher than the sensitivity factor for detecting interturn short circuits according to the prototype (factor K).

The above calculation proves that the proposed method in comparison with the prototype for any method of calculation provides a greater sensitivity factor to interturn short circuits.

The method involves periodic repetition of measurements during the operation of the transformer.

This method has the following advantages over the prototype:

1. increasing sensitivity to the presence of short circuits in the windings of a power three-phase transformer by taking into account negative sequence currents;

2. The method is well combined with digital diagnostic technologies, since no mechanical devices are used in its implementation, except for current transformers.

The technical result achieved as a result of applying the proposed method is to increase the sensitivity to the presence of short circuits in the windings of a power three-phase transformer.

Claims (1)

A method for detecting turn-to-turn short circuits in the windings of three-phase consumer transformers, which consists in measuring the phase currents of the transformer in idle mode using measuring current transformers, the first measurement is made before starting operation on a known-good transformer, the signals coming to the ammeters from measuring current transformers, process, and the result is stored, then during the operation of a three-phase transformer during a scheduled inspection, repeat the measurements of the phase currents of the transformer with the load off and re-process the signals; the result obtained is compared with the result of the first measurement, and the presence of turn-to-turn short circuits is judged by the magnitude of the difference between the measurement results, which differs in that the processing of signals from measuring current transformers consists in the fact that after removing the signals from the ammeters during the first measurement, the direction of the vector is chosen current of one of the phases, determine the angles between the vectors of the phase currents, calculate the value of the negative sequence current vector for the selected phase, and the presence of interturn short circuits is judged by the absolute value of the vector difference between the calculated negative sequence current vectors, which in the presence of an interturn short circuit will be greater than zero.

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