mu4

tir

e &

atio

al

nlin

oposed method can unambiguously detect interturn faults even

operation, and improve power supply and service to custo-high fault currents within the short circuited part of the

winding, accompanied by a relatively low value of current

in the remainder of the winding. On the other hand, if for

for fault detection and diagnosis of the electric machines is

EUROPEAN TRANSACTIONS ON ELECTRICAL POWER

Euro. Trans. Electr. Power 2011; 21:196211

Published online 15 April 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/etep.431more severe damage to the power transformer.

The problem of the traditional differential protection

In literature, using the negative sequence as an indicatoraccount for approximately 70% of all the failures in power

transformers. Furthermore, from these reviews, it can also

be understood that protection of the power transformers

against winding interturn faults is still a challenge. One

major problem in protecting large power transformers from

interturn winding faults is that the current equipment being

used to protect the transformers from interturn faults often

fails until minor faults developed into high level faults with

pickup current of the restraint differential protection be set

relatively high, then minor interturn faults cannot be

detected until it evolves into a more severe fault with

higher differential currents. This work focuses mainly on

the low level interturn faults on the windings of power

transformer. Fast and early detection of this type of faults is

critical in preventing a major damage to the power system

and the transformer itself.winding faults takes a great participation. From a number

of surveys [1,2], it can be deduced that interturn faultssome reason, for example, because of uncompensated

movements of an on-load tap changer, the minimummers. Among several of transformer faults, the transformerusing data generated by simulations on a finite element model. Copyright # 2010 John Wiley & Sons, Ltd.

KEYWORDS

power transformer; interturn fault; negative sequence current; finite element model

* Correspondence

Vahid Behjat, Department of Electrical Engineering, Iran University of Science & Technology, Narmak 16846, Tehran, Iran.

E-mail: [email protected]

1. INTRODUCTION

Nowadays, there is an increasing interest in online monitoring

of power transformers because of its potential to provide

early warning of electrical failures, enhance the reliability of

transformers has been that just these low level interturn

faults could not be detected with the overall sensitivity

represented by the percentage restraint differential

protection. A short circuit of a few turns, in spite of veryheavily distorted conditions. The performance of the proposdown to two shorted turns along the winding. The method is not influenced by the supply and load harmonics even under

ed technique was studied for a variety of operating conditionsinto high level faults with more severe damage to the pow

suggested to overcome this problem to a great extent. The prtransformers using the ratio of negative sequence components of the primary and secondary line currents. The ratio is equal

to the turn ratio during external faults as well as in the supply or load imbalance conditions, while it differs from the turn

ratio when interturn winding faults occur. The main feature of the proposed method is its capability to detect low level

interturn faults which typically cannot be detected by the traditional transformer protection devices before they developed

er transformer. In this work, a major improvement has beenRESEARCH ARTICLE

Online monitoring of powerof internal winding short csequence analysis

Abolfazl Vahedi1,2 and Vahid Behjat1,2*

1Department of Electrical Engineering, Iran University of Scienc

2Center of Excellence for Power System Automation and Oper

ABSTRACT

As a recent trend, online monitoring techniques for electric

considered very important. This paper presents a new owhich recognized as the basis protection of the power

196ransformers for detectioncuit faults using negative

Technology, Tehran, Iran

n, Tehran, Iran

machines, mainly including power transformers, has been

e method for detection of interturn faults in the powerprimarily focused on induction motors. The occurrence of a

Copyright 2010 John Wiley & Sons, Ltd.

to test fault diagnostic techniques. Amajor breakthrough in

winding three-phase power transformer has been con-

sidered for the present study. The transformer was

employed in simulations with all parameters and con-

figuration provided by the manufacturer. Ratings of the

transformer are presented in Table I. A transversal section

representation of the power transformer structure is shown

in Figure 1.

The magnetic field inside the transformer is governed by

Table I. Ratings of the transformer.

Rated power 8 MVA

Rated frequency 50 Hz

Primary rated voltage 20 kV

Secondary rated voltage 11 kV

Turns ratio 510/162

Connection D/Y

Figure 1. Geometry of the transformer model (all units are in mm).

A. Vahedi and V. Behjat Online monitoring of power transformersthe detection of stator interturn faults has been achieved by

utilizing the effective negative sequence impedance as an

indicator of these faults [8,9]. In Ref. [10], a comparative

analysis is provided for online detection of stator winding

interturn short fault in induction motors. The authors

compared several detection methods in terms of diagnostic

efficiency and requirements for practical implementation.

Among all the analyzed techniques, those based on the

sequence components are considered the most promising.

It is claimed in Ref. [10] that the negative sequence

components present a high diagnostic efficiency based on

the compensation of non-idealities, good experimental

results, and medium simplicity for practical implementa-

tion. While the negative sequence current is able to detect

and diagnose a stator winding short, the method is unable

to discriminate between an interturn short fault and the

imbalance in the power supply, which is common in the

operation of power systems.

In the case of power transformers, one of the earliest

works was introduced by Sidhu et al. [11,12]. The techniqueuses the arguments of the positive and negative sequence

impedances of the power system in a fault detection

algorithm. Since then, some other authors have taken

advantage of symmetrical component capabilities to develop

protection schemes for power transformers [13,14].

An attempt has been made in this paper to obtain a new

andmodified fault detectionmethod using negative sequence

quantities. Such a modification could bring significant

improvement in the detection of transformer interturn

faults and especially overcome the drawbacks of the

existing fault detection schemes to detect minor interturn

faults. To this end, a numerical simulation of power

transformer is completed firstly by the use of finite element

model. Using transformer FEMmodel, it would be possible

to obtain a comprehensive view of the overall transformer

magnetic and electrical behavior under normal and

disturbance conditions. In addition, it provides a significant

degree of accuracy, which is very useful to explore the

terminal behavior of the transformer. Based on that, this

paper points out that negative sequence components of the

transformer line currents can be viewed as the feature

criterion and utilized to detect interturn faults.

2. NUMERICAL SIMULATION

The fault simulations were carried out by means of solving

a finite element model of a power transformer. Such model

was defined through data sheet obtained from the

transformer manufacturer. An 8MVA, core type, twonegative sequence component in the motor supply currents

is identified when shorted stator windings are present [3].

In Refs. [4,5], online techniques were presented for

detection of stator winding faults in three-phase induction

motors from observation and measurement of negative

sequence supply current. Refs. [6,7] presented an induction

machine model for simulation of stator interturn faults andEuro. Trans. Electr. Power 2011; 21:196211 2010 John Wiley & Sons, LtdDOI: 10.1002/etepthe well-knownMaxwell equations. FEM is applied here in

order to solve electromagnetic field problems described by

the Maxwell equations. When a fault occurs, the magnetic

flux distribution is fundamentally altered as well as the

current in the circuit domain. But, the transformer terminal

behavior still satisfies the governing equations. Thus,

obtaining the faulty transformer behavior is achieved by

solving these equations. We used direct coupling between

the field and circuit equations in order to define and

simulate the transformer behavior under normal and

interturn fault conditions.

Figure 2 represents finite element model of the studied

power transformer. It is composed of 17 598 surface and

7406 line elements and includes 38 827 nodes. We decided

to use first-order elements, since there was no significant

difference in results compared with second-order elements.

The regions in the finite element model are coupled to the

circuit model. Both the primary and secondary windings

are modeled as stranded coil conductors in the circuit

model. In order to verify the models reliability and

precision, the model was validated in the steady-state

magnetic formulation by comparison with values of. 197

Online monitoring of power transformers A. Vahedi and V. Behjatterminal current and voltages, short circuit impedance, etc.,

provided by the manufacturer for the steady state of the

considered transformer. For the transformer model, the rms

values of the terminal voltages and currents and also the

other parameters obtained from the simulation and the

manufacturer data were almost equal. Table II presents the

results of this comparison and demonstrates the excep-

tional ability of the model to reproduce the real behavior of

the machine. Figure 3a and b shows the flux plot (equiflux

lines) of the transformer and color shaded plots of the flux

density on the transformer regions generated by the FEM

model under normal operating conditions and at the rated

load. A detailed description about the finite element model

is given in Appendix.

Figure 4a and b shows the corresponding circuit domain

and FE domain representation of the transformer coil for an

interturn fault on the transformer phase B secondary

winding. When an interturn fault is on the primary or

secondary winding, the fault winding is divided in three

parts a, b, and c in the FE domain as well as in the

circuit domain as shown in Figure 4. A time controlled

Figure 2. 2D Finite element model of the transformer.switch plus a limiting fault resistance was utilized to

initiate an internal fault. Therefore, by means of the

coupled electrical circuit, faults can be introduced at

different locations along the windings. The severity of the

fault can be controlled by different values of fault

resistance between turns. Indeed, the fault resistance

are denoted by the indices pn and sn, respectively (p stands

Table II. Comparison of the values of the transformer.

Magnitude Specified manufacturer Simulation

Primary voltage 20 000 V(D) 20 000

Primary current 230.9 A(Y) 228.97

Secondary voltage 11 000 V(Y) 10925.939

Secondary current 419.9 A(Y) 415.989

Short circuit impedance 2.33% 2.2%

Total power loss 54.92 kW 52.9 kW

198 Euro.for primary, s for secondary, and n for negative sequence).

The two negative sequence currents are expressed asrepresents the resistive component of the dielectric

material in the dielectric equivalent parallel circuit model.

3. DETECTION METHOD

The theoretical foundation of the new detection method is

based on the theory of symmetrical components, or more

exact, on the negative sequence currents. Existence of

relatively high negative sequence currents is in itself an

indication of a disturbance, and quantitatively represents

asymmetries coupled to a specific condition of an electrical

system, in our case, the transformer. The proposed method

for interturn fault detection is based on the principle that

for a healthy (unfaulted) transformer supplied by

symmetrical multiphase voltage sources without turn

faults in its windings, no negative sequence component

of the line and phase current occurs. An interturn fault will

break that symmetry and give rise to a negative sequence

current which may then be used as a measure of fault

severity or to initiate a warning alarm for the monitoring or

protection devices to make proper decision. However,

asymmetries in the three-phase quantities of the transfor-

mer may arise due to some other reasons such as

instrumentation asymmetries and operation with imbal-

anced load or supply voltages. Such asymmetries can be

reflected in negative sequence components and yield a

significant increase in the negative sequence components

of line and phase currents. The proposed method in this

paper can not only detect and diagnose a winding interturn

short circuit fault, but also is able to discriminate between

an interturn fault and other disturbances such as imbalance

in the power supply and load and external fault conditions.

In general, the negative sequence current is obtained

from the measurement of the three-line currents of the

transformer using:

Ian 13Iaf a2Ibf aIcf (1)

In the above equation, the negative sequence component

is denoted by the index an (a stands for the line current and

n for the negative sequence), Iaf, Ibf, and Icf are the phasorsof fundamental components of three-line current signals,

and a is a phase rotation operator equivalent to ej2p=3.The underlying principle for interturn fault detection is

based on the measurement of negative sequence com-

ponent of the line currents from both sides of the

transformer. The ratio of the negative sequence com-

ponents (RNSC)is compared with the turn ratio for the

purpose of fault detection. Defining the RNSC gives:

RNSC Ipn3

pIsn

(2)

In the above equation, the negative sequence com-

ponents of line currents from primary and secondary sidesTrans. Electr. Power 2011; 21:196211 2010 John Wiley & Sons, Ltd.DOI: 10.1002/etep

A. Vahedi and V. Behjat Online monitoring of power transformersphasors. The H3 coefficient on the right hand side of theequation allows the compensation of turn ratio due to delta

winding in the primary.

Theoretically, in case of external fault and imbalanced

load or supply voltage, the RNSC is equal to the turn ratio

(N1/N2) while for an interturn fault it is not. In order toexplain this phenomenon in greater detail, reconsideration

of the symmetrical components theory will be useful.

Figure 3. (a) Equiflux lines under normal operating conditions. (b) C

Euro. Trans. Electr. Power 2011; 21:196211 2010 John Wiley & Sons, LtdDOI: 10.1002/etepAccording to symmetrical components theory [15,16],

the negative sequence currents are transferred through at a

power transformer. Further, for any external disturbance,

such as external fault and imbalanced load or supply, the

negative sequence source will be located outside the

transformer at the fault point, load or supply side,

respectively. Thus the negative sequence currents will

enter the healthy power transformer on the fault side, and

olor shaded plots of the flux density on the transformer regions.

. 199

been presented in the next section.

Online monitoring of power transformers A. Vahedi and V. Behjatleave it on the other side, properly transformed. Hence, the

RNSC is the same as turn ratio. On the contrary, for an

interturn fault with the negative sequence source within the

transformer, the negative sequence currents will flow out of

the faulty power transformer on both sides and con-

sequently, the RNSC is different from N1/N2.From this point, the RNSC of (2) can be used for

detecting interturn faults in a simple and accurate manner.

However, in reality, for an external disturbance, the RNSC

can be a little different from N1/N2 because there might besome small difference between two negative sequence

currents due to possible different negative sequence

impedance values on the respective sides. The Detector

described by (3) is used to detect a fault. The Detector

measures the percentage difference between the two

estimated negative sequence currents:

Detector 3

pIsn RNSC N2N1

Ipn 100% (3)

Figure 4. Circuit coupled FEM model of the power transformer

for modeling interturn faults: (a) circuit domain and (b) FEM

domain.If (3) is less than a threshold, the transformer is not

faulted; if greater, it is. Further, even for normal operating

conditions, small values of the negative sequence currents

can be measured in line currents. Thus, for a trustworthy

decision regarding the interturn fault occurrence, the two

negative sequence currents must be above a certain

minimum value otherwise no comparison is allowed. If

both negative sequence currents exceed the threshold,

which in itself is as a sign that a disturbance must have

happened, as the negative sequence currents are super-

imposed, pure-fault quantities, the directional comparison

is carried out.

For the system studied in this paper, the recommended

value for the detector threshold is 1% and minimum

allowable negative sequence current is 0.1% of the

transformer rated current.

It should be pointed out that the main feature of the

proposed algorithm is its capability to detect low level

response of the detector is above the threshold, con-

sequently a fault alarm is activated.

200 Euro.4.1.2. Fault size 1%, Rf equal to 0.1V, ratedresistive load.Figure 6ad represents the results for the Case B. The

difference between cases A and B lies in the fault size and

severity. In this case, an interturn fault involving 1% of the

turns on the top end of HV3 (primary) winding is studied

which representing a relatively decreased fault size and

severity. Significant decreasing in the values of terminal4.1. Case 1: Interturn fault on HV winding

4.1.1. Fault size 5%, Rf equal to 0.5V, ratedresistive load.Figure 5a and b shows three primary and secondary line

currents of the transformer working at rated load and for an

interturn fault involving 5% of the turns on the top end of

HV3 (primary) winding, respectively. The studied case has

a fault resistance, Rf, equal to 0.5V. Such a fault will giverise to a large increase of primary current and severe

distortion of it; while the secondary current does not

change very much. Figure 5c and d shows the value of

negative sequence currents obtained from this study and

the operating response of the fault detector, respectively.

As expected, the RNSC from primary and secondary

sides is not the same as the turn ratio and thus the operating4. RESULTS AND CASE STUDIES

The performance of the proposed detection method was

verified on various disturbance conditions such as winding

interturn faults, external fault, as well as load or source

asymmetries. To demonstrate the algorithms ability on

detecting even minor interturn fault, a series of case studies

was designated aimed at covering a wide variety of

interturn fault conditions of different fault severities and

locations. Also, the performance of the faulted transformer

was examined under varying conditions of load level and

power factor. In all of the case studies, the disturbance

occurs at t 0.1 second. Although a much larger number ofsimulations were carried out, only some selected cases are

included here.interturn faults. While a short circuit of a few turns of the

winding will give rise to very small changes in the terminal

currents, it results in an increased negative sequence

components in the line currents. Thus, the proposed

algorithm is a good complement to the traditional power

transformer protections for detecting minor faults, with a

high sensitivity and speed.

Some test results verifying the performance of the

technique for different faults and operating conditions haveTrans. Electr. Power 2011; 21:196211 2010 John Wiley & Sons, Ltd.DOI: 10.1002/etep

a very complicated problem for traditional power

the condition is similar to that which occurs when the

transformer is damaged in the HVwinding. Figure 9c and d

respective negative sequence components from the HVand

LV power transformer sides will be nearly equal in

from the results, the primary and secondary line currents

are heavily distorted. However, the detector remains below

A. Vahedi and V. Behjat Online monitoring of power transformerstransformer protection devices. It can be distinctly seen

from Figure 6a that short circuiting of a few turns (1% of

turns on the winding) will give rise to a very small variation

in the terminal currents; so absolutely the differential and

over current protection will be stable in this case. While,

using the negative sequence current as a fault indicator, a

proper alarm can be generated in the fault detection system.

Thus, this study also demonstrates both the proposed

method dependability and sensitivity for detecting minor

interturn faults. The results of much more simulations

demonstrate that the proposed method is capable of

detecting an interturn fault involving even two shorted

turns in an unambiguous manner.

4.1.3. Fault size 5%, Rf equal to 0.5V, ratedload, 0.66 lagging power factor.Figure 7ad shows the results obtained for the Case C; this

is identical to Case A except for the load index and power

factor. The case study is carried out with a load index equal

to 50% of the rated load and 0.8 lagging power factor. The

comparison of thee two cases demonstrates that the

modulus and phase angle of the negative sequence current

are independent of the angle and level of the impedance

load. Similar to Case A, the response of the detector has a

value greater than the operating threshold.

4.1.4. Fault size 5%, Rf equal to 0.5V, ratedresistive load, harmonic condition.The proposed detection method was simulated for an

interturn fault involving 5% of the turns on the top end of

HV3 (primary) winding and harmonic conditions.

Figure 8ad shows the results for Case D where the

primary and secondary currents are heavily distorted and

contain harmonic components. In practice, such harmonics

may be caused by CT saturation or nonlinear BH charac-teristic of the transformer core. The results clearly indicate

that the two calculated negative sequence components of

the fundamental frequency phasor are not the same in

interturn fault conditions even if the primary and secondary

currents contain harmonic components. Thus the method is

completely independent of time harmonics and fault alarm

is issued as shown in Figure 8d.

4.2. Case 2: Interturn fault on LV winding

Figure 9a and b shows three primary and secondary line

currents of the transformer working at rated load and for an

interturn fault between 1% of the turns on the top end of

LV3 (secondary) winding, respectively. The studied casecurrents and negative sequence component is particularly

visible when the size of the interturn fault decreases, as can

be seen by comparing Figure 6a with Figure 5a. Figure 6d

shows the operating response of the detector which meets

the 1% operating threshold.

It is worth pointing out that the proposed detection

method can easily detect a low level interturn fault which isEuro. Trans. Electr. Power 2011; 21:196211 2010 John Wiley & Sons, LtdDOI: 10.1002/etep1% threshold, consequently the fault detection system

would be stable in this condition and no interturn fault

alarm will be issued.

4.5. Case 5: Supply imbalance

Another case is studied when the transformer is loaded

with the rated load, but supplied with asymmetrical three-

phase voltage source. Supply imbalance conditions produce

almost fully asymmetrical current signals as shown in

Figure 12a and b. The negative sequence currents caused

by these asymmetric currents have been shown in

Figure 12c. The stability of the proposed detection method

against supply imbalance can be observed in Figure 12d.magnitude, after the compensation of the transformer turns

ratio, as shown in Figure 10c. Figure 10d shows the

operating response of the detector; remains its value below

1% threshold. This led to the decision that the fault is

outside the protection zone of the transformer and

consequently no interturn fault alarm is activated.

4.4. Case 4: Load imbalance

The proposed algorithm was simulated in the case of an

imbalanced load. Figure 11ad shows the results obtained

for this case when the transformer has rated load at phases

a and b and no load at phase c. As can be seenshows the value of negative sequence currents obtained

from this study and the operating response of the fault

detector, respectively.

4.3. Case 3: External fault

This study is used to prove the proposed fault detection

method stability under external faults. A symmetrical three-

phase fault was simulated on the HV side of the transformer

outside the transformer protection zone. As expected, thehas a fault resistance equal to 0.01V. It can be easily foundout from the simulations (Figures 5 and 9) that for an

interturn fault on the transformer windings, either on the

primary or secondary winding, the primary current will

rise. However, the secondary current does not change;

remain fairly constant, for the fault on the primary winding

and decreases a little when the fault is on the secondary

winding. Anyhow, asymmetries in the primary side caused

by the interturn faults, regardless of the fault location, will

produce significant values of negative sequence com-

ponents. So when the short circuit arises in the LV winding. 201

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90

100

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time(s)

d) O

pera

ting

Resp

onse

of t

he D

etec

tor

(a)

(b)

(c)

(d)


detector for Case 2.



-20000

-15000

-10000

-5000

0

5000

10000

15000

20000

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time(s)

a) P

rimar

y Li

ne C

urre

nts(

A)

iapibpicp

-40000

-30000

-20000

-10000

0

10000

20000

30000

40000

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time(s)

b) S

econ

dary

Lin

e Cu

rren

ts(A

)

iasibsics

-200

-150

-100

-50

0

50

100

150

200

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time(s)

c) N

egat

ive

Sequ

ence

Com

pone

nts(

A)


0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time(s)

d) O

pera

ting

Resp

onse

of t

he D

etec

tor

(a)

(b)

(c)

(d)




207


-400

-300

-200

-100

0

100

200

300

400

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2Time(s)

a) Pri

mar

y Lin

e Cu

rren

ts(A)

iapibpicp

-800

-600

-400

-200

0

200

400

600

800

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2Time(s)

b) Se

con

dary

Li

ne

Curr

ents(

A)

iasibsics

-250

-200

-150

-100

-50

0

50

100

150

200

250

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2Time(s)

c) Neg

ative

Se

quen

ce Co

mpo

nen

ts(A)


0

0.002

0.004

0.006

0.008

0.01

0.012

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2Time(s)

d) Op

erat

ing

Resp

onse

of t

he D

etec

tor

(a)

(b)

(c)

(d)





-400

-300

-200

-100

0

100

200

300

400

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time(s)

a) P

rimar

y Li

ne C

urre

nts(

A)

iapibpicp

-800

-600

-400

-200

0

200

400

600

800

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time(s)

b) S

econ

dary

Lin

e Cu

rren

ts(A

)

iasibsics

-200

-150

-100

-50

0

50

100

150

200

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time(s)

c) N

egat

ive

Sequ

ence

Com

pone

nts(

A)


0

0.002

0.004

0.006

0.008

0.01

0.012

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Time(s)

d) O

pera

ting

Resp

onse

of t

he D

etec

tor

(a)

(b)

(c)

(d)




209


5. CONCLUSION

sequen

6. LIST OF SYMBOLS AND

REFERENCES

Online monitoring of power transformers A. Vahedi and V. BehjatABBREVIATIONS

6.1. Symbols

HV high voltage side of the transformerIaf phasor of the fundamental component of the phase

a line current signal

Ibf phasor of the fundamental component of the phaseb line current signal

Icf phasor of the fundamental component of the phasec line current signal

Ian negative sequence component of the line currentsIpn negative sequence component of the line currents

from primary side

Isn negative sequence component of the line currentsfrom secondary side

LV low voltage side of the transformerN1 turns number of the primary windingN2 turns number of the secondary windingRf fault resistance between shorted turnsa phase rotation operator equivalent to ej2p=3.

6.2. Abbreviation

RNSC Ratio of negative sequence componentscurrent data. Also, no information concerning the

transformer and power system parameters is needed for

the application of the technique.method a

impleme210nd no additional measurements are required to

nt the technique since it only needs the terminalbeen preue for different faults and operating conditions have

sented. The proposed method is a non-invasiveSome ca

techniqsting power transformer fault detection methods.

se studies verifying the performance of thefaults. H

the exiotection schemes in detecting low level interturn

ence, it is found to be a very good complement toconquers

mer prthe limitations of the traditional power transfor-heavily dtely independent of time harmonics even under

istorted conditions. Further, the proposed methodvariation

complen fault and an imbalance due to load and supply

s and external fault conditions. Thus the method isof depen

interturdability, between an asymmetry caused by aninterturn faults, but also differentiate, with a high degreefault detection method can not only detect windingcaused serious imbalance in the line currents. The proposedfound toce components of the transformer line currents are

be sensitive to interturn fault occurrence whichmethod for detecting interturn faults of power transformers

has been presented in this paper. The effective negativeA negative sequence current-based sensitive detectionEuro.1. Bartley W. Analysis of transformer failures. Inter-national Association of Engineering Insurers 36thAnnual Conference, Stockholm, Sweden, 2003.

2. Stigant SA, Franlin AC. The J&P Transformer Book:

A Practical Technology of the Power Transformer;

10th edn, Wiley: New York, 1973.

3. Cruz SMA, Cardoso AJM. Multiple reference frame

theory: a new method for the diagnosis of stator faults

in three-phase induction motors. IEEE Transactionson Energy Conversion 2005; 20: 611619. 10.1109/TEC.2005.847975.

4. Arkan M, Unsworth PJ. Stator fault diagnosis in

induction motors using power decomposition. Pro-ceedings of the IEEE Industry Applications Confer-ence 34th Annual Meeting, Vol. 3, Phoenix, USA,1999; 19081912.

5. Arkan M, Kostic-Perovic D, Unsworth PJ. Online

stator fault diagnosis in induction motors. IEE Pro-ceedings of the Electric Power Applications 2001;148(6): 537547.

6. Shuo C, Rastko Z. Modelling and simulation of stator

and rotor fault conditions in induction machines for

testing fault diagnostic techniques. European Trans-actions on Electrical Power 2009; Published online inWiley InterScience. 10.1002/etep.342.

7. Arkan M, Kostic-Perovic D, Unsworth PJ. Modeling

and simulation of induction motors with inter-turn

faults for diagnostics. Electric Power System Research2005; 75: 5766. 10.1016/j.epsr.2004.08.015.

8. Kohler JL, Sottile J, Trutt FC. Alternatives for asses-

sing the electrical integrity of induction motors. IEEETransactions on Industry Applications 1992; 28(5):11091117. 10.1109/28.158836.

9. Kohler JL, Sottile J, Trutt FC. Condition monitoring of

stator windings in induction motors. I. Experimental

investigation of the effective negative-sequence impe-

dance detector. IEEE Transactions on Industry Appli-cations 2002; 38(5): 14471453. 10.1109/TIA.2002.802935.

10. Albizu I, Zamora I, Mazon AJ, Tapia A. Techniques

for online diagnosis of stator shorted turns in induction

motors. Electric Power Components and Systems2006; 34(1): 97114. 10.1080/15325000691001359.

11. Sidhu TS, Gill HS, Sachdev MS. A transformer pro-

tection technique with immunity to CT saturation and

ratio-mismatch conditions. IEEE Canadian Confer-ence on Electrical and Computer Engineering, Vol. 1,Waterloo, Ontario, Canada, 1998; 2428.

12. Sidhu TS, Gill HS, Sachdev MS. A numerical tech-

nique based on symmetrical components for protect-

ing three-winding transformers. Electric PowerSystem Research 2000; 54: 1928. 10.1016/S0378-7796(99)00069-3.

13. Guzman D, Ignacio D, Pablo A, Javier G-A. Zero-

sequence-based relaying technique for protectingTrans. Electr. Power 2011; 21:196211 2010 John Wiley & Sons, Ltd.DOI: 10.1002/etep

power transformers and its performance assessment

using unsupervised learning ANN. European Trans-actions on Electric Power 2006; 16: 147160.10.1002/etep.72.

14. Guzman D, Javier G-A, Pablo A. Diagnosis of a turn-

to-turn short circuit in power transformers by means of

zero sequence current analysis. Electric Power Sys-tems Research 2004; 69: 321329. 10.1016/j.epsr.2003.07.014.

15. Wagner CF, Evans RD. Symmetrical Components,

McGraw-Hill: New York & London, 1933.

16. Blackburn JL. Symmetrical Components for Power

System Engineering, Marcel Dekker: New York,

Basel, Hong Kong, 1993.

Appendix

conductors. From the physical point of view, stranded

conductors are characterized by a value of the skin depth

much greater than the dimensions of the conductor cross-

section and, as a consequence, by an almost uniform

distribution of the current density over all the conductor

cross-section. Solid conductors are characterized by a

value of the skin depth comparable to or smaller than the

dimensions of the conductor cross-section. The density of

supplied or induced currents is non-uniform in the cross-

section of such conductors. In this study, both of the

primary and secondary windings, which have a small

cross-section and high number of turns and consequently

negligible eddy current losses in the windings, are modeled

as stranded coil conductors.

To analyze the transient dynamic behavior of the

transformer, the equations of electromagnetic and electric

circuit fields are directly coupled and solved simul-

taneously at each time step. The method of weighted

residuals has been applied to the coupled field-circuit

A. Vahedi and V. Behjat Online monitoring of power transformersThe 2D transient magnetic solver in the FLUX software

package was used to implement the transformer finite

element model. It is possible to carry out a 2D plane study

if the magnetic flux of the device is supposed to concentrate

on the cross-section plane and there may be no extremity

effect or magnetic flux leakage in the third direction. A 2D

study is recommended in modeling of the transformers,

because the magnetic fluxes, created by the primary and

secondary conductors, is strongly confined in the magnetic

circuit and therefore in the cross-section plane.

In general, the magnetic field model of the transformer

can be distinguished into three parts: the core, the

windings, and the oil surrounding the core and the

windings. Neglecting the stray losses in the transformer

tank walls, the transformer tank has excluded in the

computation domain. The core and surrounding oil has

been entirely included in the model. Representation of the

winding is related to the modeling of skin effect.

Ordinarily, the conductors concerned by the field-circuit

coupling are of two types: stranded conductors and solidEuro. Trans. Electr. Power 2011; 21:196211 2010 John Wiley & Sons, LtdDOI: 10.1002/etepequations to yield the finite element matrix equations.

Also, the finite element time stepping scheme has been

used to discretize the equations in the time domain. To

obtain a unique solution for the governing equation based

on AVA formulation, the zero Dirichlet boundary

condition is applied on the external border of the

computation domain.

The mesh of the regions of the transformer should be

created depending on the physics of the problem, since the

quality of the results depends on the quality of the mesh.

The mesh is much more refined in zones of strong variation

and high intensity of the magnetic field than in the zone

close to the computation domain boundary. A mesh

generator that creates first-order rectangular elements has

been used for meshing of the windings. The core,

surrounding oil, and the boundary domain were fully

discretized by triangular first-order elements. The finite

element model of the studied power transformer is

composed of 17 598 surface and 7406 line elements and

includes 38 827 nodes as illustrated in Figure 2.. 211

mu4

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