Detection of inter turn fault in transformer windings by parameters of their transition processes Текст научной статьи по специальности «Электротехника, электронная техника, информационные технологии»

© Р.Г. Мустафин

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UDC 620.9 DOI: 10.30724/1998-9903-2019-21-3-14-23

DETECTION OF INTER TURN FAULT IN TRANSFORMER WINDINGS BY PARAMETERS OF THEIR TRANSITION PROCESSES

R.G. Mustafin

Kazan State Power Engineering University, Kazan, Russia

ramil.mustafin@gmail.com

Abstract: The proposed method of detection the inter turn fault of transformer windings relates to the area of defectoscopy and allows detecting inter turn faults in a wide range of damaged (closed) turns. Power and instrument transformers with iron-core are widely used in power networks. As the insulation ages or is damaged, the wires between various transformer sections short circuits occur, which inevitably leads to a complete damage of the transformer. Short-circuited part of the transformer forms an additional winding, the outputs of which are short-circuited. The transition process of current increasing when DC voltage is connected to the transformer outputs occurs in diverse ways in undamaged (cut-off) winding section and in the damaged (short-circuited) section. The current growth rate in the undamaged section of the winding is determined by high magnetization inductance. The inductance of the short-circuited part of the winding is much less, so, the current growth rate in the short-circuited part is significantly greater than the current growth rate in the undamaged part of the winding. The article presents observations from computer models and real measurements of the substation auxiliary power transformer, which show the possibility of determining the presence of a turn fault in regards to the transition process parameters, the rate of current increase and decay in the transformer winding. The device aimed to find the inter turn faults in the transformer windings, working according to the proposed method will be quite simple and have a high sensitivity.

Keywords: transformer, ferromagnetic core, inter turn fault, transition process.

For citation: Mustafin RG. Detection of inter turn fault in transformer windings by parameters of their transition processes. Power engineering: research, equipment, technology. 2019; 21(3):14-23. (In Russ). doi:10.30724/1998-9903-2019-21-3-14-23.

ОБНАРУЖЕНИЯ ВИТКОВЫХ ЗАМЫКАНИЙ ОБМОТОК ТРАНСФОРМАТОРОВ ПО ПАРАМЕТРАМ ПЕРЕХОДНОГО ПРОЦЕССА

Р.Г. Мустафин

Казанский государственный энергетический университет, г. Казань, Россия

ramil.mustafin@gmail.com

Резюме: Предлагаемая методика обнаружения витковых замыканий обмоток трансформаторов относится к области дефектоскопии и позволяет обнаруживать витковые замыкания в обмотках трансформатора в широком диапазоне поврежденных (замкнутых) витков. Силовые и измерительные трансформаторы с железным сердечником широко применяются в электроэнергетических сетях. При старении изоляции, при повреждении изоляции, в трансформаторах возникают замыкания между

витками из разных частей обмотки, что неизбежно приводит к полному повреждению трансформатора. Короткозамкнутая часть обмотки трансформатора образует дополнительную обмотку, выводы которой замкнуты накоротко. Переходный процесс роста тока при подключении постоянного напряжения к выводам трансформатора по-разному происходит в неповрежденной (разомкнутой) части обмотки, и в поврежденной (короткозамкнутой) части. Скорость роста тока в неповрежденной части обмотки определяется большой величиной индуктивности намагничения. Индуктивность короткозамкнутой части обмотки значительно меньше, соответственно скорость роста тока в короткозамкнутой части существенно больше скорости роста тока в неповрежденной части обмотки. В статье приводятся наблюдения в компьютерной модели и в реальных измерениях на трансформаторе собственных нужд подстанции возможность определения наличия витковых замыканий по параметрам переходного процесса, по скорости нарастания и спада тока в обмотке трансформатора. Прибор, предназначенный для поиска витковых замыканий в обмотках трансформатора, работающий по предлагаемой методике будет достаточно простым и иметь высокую чувствительность.

Ключевые слова: трансформатор, ферромагнитный сердечник, витковые замыкания, переходный процесс.

Introduction

One of the most frequently-occurring transformer defects is the inter turn fault, when some of the transformer winding turns is short-circuited [1]. Usually, the presence of inter turn faults is detected from the parameters of the transformer normal operation at industrial frequency of 50 Hz [2-6]. Increasing the frequency range during measurements of transformer parameters can increase the sensitivity of detection methods of inter turn faults [7, 8].

In case of inter turn faults, two parts of a winding are formed: the undamaged part (A in Fig. 1), which plays the role of the primary transformer winding, and short-circuited one (B in Fig. 1), which works as a secondary transformer winding.

To analyze the operation of the transformer winding with an inter turn fault, consider the T-shaped model [9] (Fig. 1), where the active resistance of the A turns is Rot; scattering inductance of the A turns is La; active resistance of B turns is Rb; scattering inductance of the B turns is Lb; inductance of saturation is L'o (which is less than the initial inductance Lo due to short-circuiting of B turns); E is the EMF connected to the winding. (All values are given for side A).

Fig. 1. T-shaped model of winding with inter turn fault (B turns)

Note that the scattering inductances of turns La and Lb are significantly less than the saturation inductance of L'o, so the time constant Tab (of the Ra-La-Lb-Rb circuit), equal to, Tab = (La + Lb )/(Ra + Rb), is significantly less than the time constant T'o (of the Ra-La-L'o

chain), which is equal to. T0 = (La + L0)/Ra. Thus, the time constant Tab of the winding with

inter turn fault is significantly less than the time constant of the saturation circui T'o: T(bb T0 This difference governs the principle of detection the inter turn faults of transformer windings: the

© P.r. Mycmafym

time constant is measured in wide range, and the presence of transients with a small transition time clearly indicates the winding short circuits in the transformer winding. Materials and methods

To illustrate the operating principle of method for detection the inter turn fault of transformed windings, the Matlab/Simulink software was used to build the model (Figure 2).

Fig. 2. The model of a transformer with a inter turn short-circuit, which is presented in the form of a short-circuited secondary transformer winding

Fig 3 presents the parameters of the three-phase transformer model.

A short-term DC voltage of 100 V (from DC Voltage Source in Fig. 2), is supplied to the primary winding of transformer (Three-Phase Transformer) through the current limiting resistance (Parallel RLC Branch) of 60 Ohms active resistance. Three-phase breaker (Three-Phase Breaker) connects DC voltage to the primary winding of the transformer (to the BC winding) in 0.02 seconds after the model start-up. The second three-phase switch (Three-Phase Breakerl) closes the primary winding of the transformer (BC winding) in 0.5 seconds after the model start-up. Thus, magnetization of the transformer core occurs in the range of 0.02 - 0.5 seconds (current increase in the primary winding), after 0.5 seconds, the demagnetization of the transformer core occurs (current drop in the primary winding).

Block Parameters: Three-Phase Transformer (Two Windings) access the neutral point of the Wye.

Click the Apply or the OK button after a change to the Units popup to confirm the conversion of parameters.

Configuration Parameters Advanced Units SI

Nominal power and frequency [ Pn(VA) , fn(Hz) ] ![ 16e6 ,_50j

Winding 1 parameters [ VI Ph-Ph(Vrms), Rl(Ohrn), L1(H) ] |[1.15е+05_2.2317_0_.136В1]

Winding 2 parameters [ V2 Ph-Ph(Vrms) , R2(Ohm) , L2(H) ] j[1050_0.0558140.0034216] Magnetization resistance Rm (Ohm)

!6.9225e+05_

Magnetization inductance Lm (H)

2B66

Saturation characteristic [ ¡1(A), phil(V.s) ; ¡2 , phi2 ; ... ]

|_[0_0£Q.89744_343.72;113_6 390,94]_____________________

Initial fluxes [ phiOA , phiOB , phiOC ] (V.s):

[0 0 0]

L3 R3

Fig. 3. Characteristics of transformer used in the model

In the absence of inter turn short circuits (secondary turns of the transformer bc are not closed), the transformer magnetization inductance is high (Magnetization Inductance Lm (H) in Fig. 3), consequently, the rate of magnetization current change is small (Fig. 4).

When a turn short circuit occurs (transformer secondary turns bc are closed), the model has two magnetization inductances: the primary circuit BC and the short circuit of bc turns. Moreover, the number of turns in the primary winding is much larger than the number of turns in the turn faults (as it can be seen in Fig. 3, from Winding 1 parameters V1 and Winding 2 parameters V2). Therefore, the magnetization inductance of the primary BC circuit is much greater than the magnetization inductance of the inter turn short circuit bc. This is reflected in the rate of increase and decrease of the magnetization current (Fig. 5): first (up to a time of the order of 0.1 s), a rapid increase in current occurs in the inter turn short circuit bc.

© P.r. Mycmatfrm

Fig. 4. Current in the primary winding BC in the absence of inter turn short circuits. Horizontal axis is time, s; vertical axis is current, A

Fig. 5. Current in the primary winding BC in the presence of inter turn short circuits (the secondary transformer turns bc are closed). Horizontal axis is time, s; vertical axis is current, A

After current saturation in inter turn short circuit (as shown in Fig. 6), only the slow growth of the magnetization current component in the primary winding BC remains (at times from about 0.2 to 0.5 s). After the primary winding BC closes, a similar process occurs: a rapid decrease in the inter turn short circuit current (Fig. 6) (approximately in the range from 0.5 to 0.7 s), then a slow decrease in the current in the primary BC circuit (see Fig. 5).

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Fig. 6. Current in inter turn short circuits bc. Horizontal axis is time, s; vertical axis is current, A

Results

From the presented model of inter turn failures in transformers (Figs. 2 - 6), it can be seen that, based on the large difference in the rates of current change in the primary winding BC and in the inter turn short circuit bc, it becomes possible to create a device for detecting coil faults in transformer windings [10], using which the search for coil damage is significantly facilitated.

To verify and test the proposed methodology, transients were measured using an auxiliary transformer TSKS-40/145/10-UZ of Askold Electrotechnical Plant LLC (TU 3411-002-656534922010, the transformer idle run current is not more than 12%, power is 38 kVA, number of phases is 3, frequency is 50 Hz, degree of protection is IP 00, rated voltage of HV winding is 10 kV, rated voltage of HV winding is 0.4 kV, rated current of HV winding is 2.19 A, rated current of LV winding is 54.9 A , short circuit voltage is 1.48%, insulation class F, circuit and group connection is Y/Yn-O).

To record transients, the digital oscilloscope 6501 was used; the current in the transformer windings was measured by recording the voltage drop across the active resistance, connected in series with the measured transformer winding. Initially, a constant voltage of 8 V (through an active resistance of 47 Ohms) was applied to the primary HV winding of the transformer (BC pins). The current through the winding was measured by the voltage drop at a resistance of 10 Ohms, while the behavior of the transient process (with open - Fig. 7 and with closed - Fig. 8 turns of the secondary LV transformer windings) was similar to the picture observed in the Matlab/Simulink program (see Figs. 4, 5).

There are several ways for increasing the sensitivity of the proposed method:

1. By increasing DC voltage supplied to the transformer windings, while reducing the measurement time interval (to maintain the same current in the transformer winding).

2. By applying a constant voltage (measuring transient) in the windings of the lower voltage (LV).

3. By obtaining the reference measurements of the transient process in all windings of the undamaged (without winding short circuits) transformer. Further, during periodic measurements, the current transient measurements in the transformer windings should be compared with the saved reference measurements.

© Р.Г. Mycmafym

1000 900 800 700 600 500 400 300 200 100 0

Fig. 7. The transient process in the primary HV winding (BC pins). At specific time (5 s) DC voltage of 8 V was connected to the HV; at time of 11 s HV winding was closed. The X axis is time, s; the Y axis is voltage amplitude, mV, at a resistance of 10 Ohms (current measurement in the HV primary winding)

800 700 600 500 400 300 200 100 0

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Fig. 8. The transient process in the primary HV winding (BC pins), the secondary LV winding (BC pins) are closed. At a time of 33.5 s, a DC voltage of 8 V was applied to the HV

through a resistance of 47 Ohms; at a time of 39.5 s, the HV winding was closed. The X axis is time, s; the Y axis is voltage amplitude, mV, at a resistance of 10 Ohms (current measurement in the HV primary winding)

Further DC voltage of 8 V (through an active resistance of 10 Ohms) was applied to the secondary LV transformer winding (bc pins), the current through the winding was measured by the voltage drop at a resistance of 1 Ohm. A clear difference between the transient process in the absence (Fig. 9) and in the presence (Fig. 10) of inter turn fault (additionally wound one short-circuited turn on the B phase core) was observed.

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Fig. 9. The transient process in the secondary LV winding (bc pins). At time of 0.02 s a constant voltage of 8 V through a resistance of 10 Ohms was applied to the LV; at time of 0.10 s, the LV winding was closed. The X axis is time, s; the Y axis is voltage amplitude, mV, at a resistance of 1 Ohms (current measurement in the LV secondary winding)

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500

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Fig. 10. The transient process in the secondary LV winding (bc pins) in the presence of inter turn fault

(additionally wound one short-circuited turn on the B phase core). At time of 0.02 s a constant voltage of 8 V through a resistance of 10 Ohms was applied to the LV; at time of 0.10 s, the LV winding was closed. The X axis is time, s; the Y axis is voltage amplitude, mV, at a resistance of 1 Ohms (current measurement in the LV secondary winding)

Conclusions

Thus, in the proposed method for detection the inter turn faults of transformed windings, the transients are measured in wide ranges of times, which allows tuning away from transformer magnetization currents, by measuring the transient in the fault itself, namely in the short-circuited turns of the transformer winding. This solves the task of detecting the inter turn faults in the transformer windings in a wide range of damaged (closed) turns.

The author expresses his gratitude to Badretdinov Nail (head of the laboratory of the educational and research center "Electric Power Engineering" of KSPEU) and Zapechelnyuk Eduard (head of the laboratory of the department "Relay protection and automation" of KSPEU) for assistance in measurements.

© Р.Г. Мустафин

References

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Литература

1. Bartley W. Analysis of Transformer Failures // International Association of Engineering Insurers 36th Annual Conference. Stockholm, Sweden, 2003.

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3. Oliveira L. Comparing Power Transformer Turn-to-Turn Faults Protection Methods: Negative Sequence Component Versus Space Vector Algorithms / L. Oliveira, A.J.M. Cardoso // IEEE Transactions on Industry Applications. 2017. Vol. 53. P. 2817-2825.

4. Tytko O. Definition of turn-fault of stator winding induction motor on the basis of the compensation magnetic field / Tytko O., Khudiakov A.V. // Техшчна електродинамжа. 2017. № 1. С. 58-61.

5. Федотов Г.И. Витковые замыкания силового трансформатора и методы их диагностирования / Г.И. Федотов, А.В. Голговских // Общество, наука, инновации (НПК-2016):. 2-е издание, исправленное и дополненное. Вятский государственный университет. 2016. С. 20802083.

6. Шерьязов С.К. Устройство мониторинга силовых трансформаторов 6-10/0,4 кВ / С.К. Шерьязов, А.В. Пятков // Материалы LV Междунар.науч.-техн. конф. «Достижения науки - агропромышленному производству». Челябинск : ЮУрГАУ, 2016. С. 240-244.

7. Иванова З.Г. Разработка устройства для выявления витковых замыканий в обмотках сухих силовых трансформаторов / З.Г. Иванова, Н.Л.Макарова. //Вестник Московского государственного агроинженерного университета им. В.П. Горячкина. 2013. № 1 (57). С. 30-31.

8. Макарова Н.Л. Диагностирование состояния изоляции силовых трансформаторов сельских электрических сетей: монография / Н.Л. Макарова, Р.С. Ахметшин. М.: МГАУ, 2012. 90 с.

9. Stanley H. H. Power System Relaying, Third Edition / Stanley H. Horowitz, Arun G. Phadke // Print ISBN:9780470057124, |Online ISBN:9780470758786. 2008 Research Studies Press Limited.

10. Мустафин Рамиль Гамилович «Устройство обнаружения витковых замыканий обмоток

трансформаторов» Патент РФ 11.07.2017. Бюл. № 20 Ссылка активна на: 02.02.2017.

Authors of the publication

Ramil G. Mustafin- candidate of physical and mathematical sciences, the associate professor at "Relay protection and automation of electrical power systems", Kazan State Power Engineering University.

Received December 29, 2018