Advances in Digital Protection of Power Transformer

Synopsis

of

Research work undertaken for the award of Ph. D. degree

in

Electrical Engineering

by

Ashesh Mukeshbhai Shah

159997109001

under supervision of

Dr. Rajeshkumar M. Patel (Supervisor)

And

Dr. Bhaveshkumar R. Bhalja (Co-Supervisor)

GUJARAT TECHNOLOGICAL UNIVERSITY

AHMEDABAD

[December - 2020]

CONTENTS

1. Abstract .............................................................................................................................. 1

2. State of the art of research topic ........................................................................................ 2

2.1 Harmonic Restraint Method ......................................................................................... 2

2.2 Transformer Model Based Algorithms ........................................................................ 3

2.3 Current Waveform Identification Based Schemes ....................................................... 4

3. Definition of the problem .................................................................................................. 4

4. Objective and Scope of work ............................................................................................. 5

5. Original contribution by the thesis ..................................................................................... 6

5.1 S-transform and Support Vector Machine based method ............................................ 6

5.2 Superimposed of differential currents based method ................................................... 7

5.3 Sequence component of differential current based method ......................................... 7

5.4 Quartile of differential current based method .............................................................. 8

6. Methodology of Research, Results / Comparisons ............................................................ 8

7. Achievements with respect to objectives ......................................................................... 14

8. Conclusion ....................................................................................................................... 14

9. List of publications .......................................................................................................... 15

10. References ...................................................................................................................... 15

1

1. Abstract

The differential protection is primary protection for power transformers since long time, that

is based on a comparison of the primary and secondary winding currents. These currents are

in proportion under healthy operating conditions of power transformer and deviate from the

predetermined criteria in case of abnormal condition such as internal fault. However, the

stability of the differential protection endangers when the transformer is energized.

Additionally, the differential relay must remain inoperative for other non-internal fault

conditions of power transformer such as over-excitation, Current Transformer (CT)

saturation due to external faults, sympathetic inrush and recovery inrush. Simultaneously, a

high sensitivity to detect low value turn-to-turn type internal fault has also been required by

differential protection.

Conventionally, harmonic restraint percentage differential protection scheme is used for

power transformer protection. According to the said scheme, the differential protection

restrains the relay operation depending on the content of second harmonic component and,

sometimes, the fifth harmonic component in order to avoid unnecessary tripping against non-

internal fault condition. However, it is difficult to achieve proper discrimination between

internal fault and non-internal fault situations in modern power transformers. Now-a-days,

most of the power transformer manufacturers utilise improved magnetic material due to

which harmonic components has been considerably reduced. On the contrary, the operation

of the above schemes may be delayed/ inhibited in case of external fault on the adjacent long

transmission line due to capacitance effect and also at the time of energisation of transformer

with an internal fault. Moreover, internal faults appear near to the end of the winding or

neutral of the winding in case of star connection, turn-to-turn faults and interwinding faults

are also a challenge for researchers and engineers, since these types of internal faults are not

easy to detect properly with the percentage differential restraining protection scheme of

power transformer. It is, therefore, necessary to develop a new improved technique for

differential protection of the power transformer in order to discriminate internal faults and

other abnormal operating conditions of the power transformer.

Thus, in order to overcome the above mentioned shortcoming of the differential protection

of power transformer, a research work has carried out to develop more robust and accurate

digital differential transformer protection scheme. Moreover, in the proposed research,

2

efforts have been made to present differential protection schemes which should not be

dependent on second harmonic components of differential currents. Further, effective

solutions have been proposed that can identify low valued turn-to-turn faults, interwinding

faults and internal fault while energizing transformer. At the same time, the proposed

methods have been shown to work under stable conditions during the disturbing events such

as magnetizing inrush, over-excitation and CT saturation during external faults.

2. State of the art of research topic

In power transformer differential protection, several techniques have been proposed in the

literature to discriminate between internal faults and non-internal fault operating conditions

such as magnetizing inrush current, over-excitation and external faults during current

transformer saturation. Initially, in order to avoid the tripping operation of differential relay

during non-internal fault operating conditions, differential relays have been utilized slow

speed induction type relays or desensitized for few cycles after switching on the power

transformer. However, differential protection becomes insensitive in case of internal fault

during energization of transformer. Hence, in order to solve the said setback, the other

methods have been proposed to improve the differential relay operation in terms of

sensitivity, selectivity and speed. These methods are primarily classified on the basis of (i)

harmonic restraint methods (ii) transformer model based algorithms and (iii) current

waveform identification based schemes.

2.1 Harmonic Restraint Method

The harmonic restraint method is widely adopted for high rating transformer protection. This

is based upon the investigation of harmonic contents of the magnetizing inrush and over-

excitation currents. In case of internal fault conditions, the differential current is sinusoidal

in nature. However, during magnetizing inrush and over-excitation conditions, this current

contains harmonics which distorts the differential current waveform. Henceforth, the

differential protection of power transformer is based on extracting the fundamental

component (1st), second harmonic component (2nd), and sometimes fifth harmonic

component (5th) of differential current. The ratio of the 2nd harmonic component to

fundamental component is found and compared with a predefined threshold value to

discriminate between magnetizing inrush and internal fault conditions. Moreover, the

proportion of the 5th harmonic component into fundamental frequency component is being

3

compared with a predefined threshold value in order to distinguish between internal fault

and over-excitation state. In order to extract fundamental and higher order harmonic contents

from the differential current, various filters, transforms and mathematical functions have

been utilized. The filtered harmonic components are utilized to restrain or operate the

differential relay. Normally, 15% restrain setting in differential current is adjusted to restrict

the operation during magnetizing inrush. It shows that if second harmonic content in

differential current is greater than or equal to 15% of the fundamental frequency component,

the relay restrains the operation of differential protection of the power transformer.

Though, number of methods have been proposed to extract fundamental frequency and

harmonic components of the differential current signal, second harmonic restraint method

by means of traditional Fast Fourier Transform (FFT) in discrete form is most widely used

technique in the current set-up for power transformer differential protection. However, the

weak performance of FFT has been found during electromagnetic transients and impulsive

conditions. It has also been noted in various literatures that the second harmonic component

may be generated during internal faults in power transformer due to CT saturation and

distributive capacitance of long transmission line to which transformer is connected. In these

situations, the magnitude of second harmonic component during an internal fault may be

nearer or greater than that found in the magnetizing current. On contrary, the second

harmonic components in the magnetizing current likely to be small in modern power

transformer due to improvement in design and core material of modern power transformer.

Hence, the conventional harmonic restraint criteria for differential protection of power

transformer may not be fulfilled to discriminate between internal faults and magnetizing

inrush currents/over-excitation conditions. Thus, the new improved technique for

differential protection of power transformer is required to distinguish between internal fault

and above mentioned abnormal operating conditions of the power transformer.

2.2 Transformer Model Based Algorithms

Digital algorithms have also been proposed based on transformer model which do not utilize

harmonic components of differential current to differentiate internal faults and magnetizing

inrush. These approaches include flux restraints and variations, equivalent circuit

parameters, compensated current, two terminal network and inductance of transformer based

methods. These methods based on transformer model are highly dependent on transformer

parameters. However, it becomes necessary to determine equivalent parameters

4

experimentally, as these parameters are not always available from the manufacturer. In

addition, accurate values of transformer parameters are challenging to attain using complex

electromagnetic relations of transformer model. Additionally, few methods based on

transformer parameters utilize transformer winding currents for relay operation. Though, it

is difficult to measure winding currents in case of delta connected windings as terminals are

generally not taken out from the transformer tank. Furthermore, boosting costs of protective

system of differential protection due to prerequisite of special type of sensors like search

coils, quintuplet set on printed circuit board and additional potential transformer are the main

drawbacks of the transformer model based techniques.

2.3 Current Waveform Identification Based Schemes

Current waveform identification based digital relaying techniques have been proposed in

recent literatures to increase sensitivity, reliability and speed of digital relays. These methods

have been established depending on fuzzy logic, artificial neural network, machine learning,

power differential, wavelet and other transforms and sequence components of differential

currents. Moreover, digital differential protection of power transformer has been also

developed using principal component analysis, mathematical morphology, orthogonal

polynomials and statistical tools.

These advanced current waveform identification techniques for the power transformer

protection have utilized properties of power transformer and percentage differential

characteristic. Further, the system response involves the measurement of various electrical

parameters such as voltage, current, frequency, phase angle, energy and active power.

Moreover, the variation in the rate of change of these parameters with respect to the time as

well as with respect to other parameters has been also examined for the identification of

internal fault situations. Afterwards, the patterns of measurements and variations have been

fed to waveform identification based methods to obtain discrimination accuracy between

internal fault and non-internal fault conditions. However, these developed methods still

suffer from standardization, universal threshold, sensitivity towards noise and computational

complexity.

3. Definition of the problem

The techniques, which are reported in preceding section, is representing a substitute or

advancement in the existing power transformer differential protection schemes. However,

5

no novel approach seems to have reached a practical point so far, and the second harmonic

component restraining technique using Discrete Fourier Transform (DFT) is widely utilized

for power transformer protection. It has been observed that the saturation of current

transformer due to external fault on adjoining long transmission line connected with

transformer may develop second harmonic component during internal faults of power

transformer. In these cases, the magnitude of second harmonic component in internal fault

has been found out as high as that of magnetizing current. On the other hand, the

improvement of the core material in modern power transformer has reduced the second

harmonic component. Thus, the conventional harmonic restraint based differential relay may

not be applicable to distinguish internal faults and magnetizing inrush currents. Moreover,

internal faults appear near to the end of the winding or neutral of the winding in case of star

connection, turn-to-turn faults and interwinding faults are also a challenge for researchers

and engineers, since these types of internal faults are not easy to detect properly with the

percentage differential restraining protection scheme of power transformer. Furthermore, the

conventional percentage differential protection combined with harmonic restraint scheme

may fail and affect the relaying operation in several operating conditions of power

transformer, such as sympathetic inrush and recovery inrush. In addition, this protection

approach has shown significant decrease in sensitivity in case of CT saturation conditions

and simultaneous internal fault while energizing transformer.

4. Objective and Scope of work

The main objective of the thesis is to design and develop effective protection schemes for

accurate discrimination of internal fault and magnetizing inrush conditions of the power

transformer. Moreover, the proposed schemes should be able to detect the low value turn-

to-turn faults as well as interwinding faults. On the other hand, the developed scheme should

also remain stable against other abnormal operating conditions such as various types of

magnetizing inrushes (sympathetic inrush and recovery inrush), over-excitation situation and

CT saturation condition due to external faults. Thus, keeping in the view of research gaps,

the following objectives have been set for the proposed research work:

To develop a new digital relaying algorithm for transformer differential protection;

this should discriminate between magnetizing inrush and internal fault currents.

6

To achieve discrimination between magnetizing inrush and internal faults; and

capable to detect minor faults like turn-to-turn and remain stable against over-

excitation and magnetizing inrushes situations such as switching on inrush including

residual magnetism, sympathetic inrush and recovery inrush.

To design self-decision making and fast digital/numerical transformer differential

protection scheme that should capable to operate or block the operating signal in

unseen situation of power system variation due to residual flux, fault inception angle

(in case of faults) or switching angle (in case of magnetizing inrush and over-

excitation), CT parameters, source impedances and loading condition of power

transformer as well as different rating and connection of power transformers.

The current state of the art and the existing gaps provides tremendous opportunities for

further development of intelligent digital/numerical protection for power transformer, many

of which are subjects of modern research.

5. Original contribution by the thesis

The primary objective of the thesis is to design and develop effective transformer protection

strategies for the identification of internal faults. These methods recognize different types of

internal fault such as winding faults, turn-to-turn faults and interwinding faults successfully

and remain stable against magnetizing inrushes (switching on inrush including residual flux,

recovery inrush and sympathetic inrush), over-excitation conditions as well as CT saturation

during external fault. The main contribution of this research has been listed as follows.

5.1 S-transform and Support Vector Machine based method

Recently, with the advent of Artificial Intelligent (AI) technology, Support Vector Machine

(SVM) has emerged as an outperforming classifier. Using this classifier, a combined S-

transform and SVM based method has been presented to distinguish between internal faults

and other disturbances. In order to evaluate the classification of the different operating

conditions of the power transformer, the power system network is modelled using PSCAD /

EMTDC simulation software and differential currents have been achieved. Obtained

differential currents have been utilized in S-transform to extract the correct feature for

classification of internal faults with non-internal faults. To perform the classification, the

Radial Basis Function (RBF) kernel of SVM has been used to train the dataset. The optimal

7

RBF parameters, attained from the five-fold cross-validation process, have been utilized to

testing dataset to obtain the discrimination accuracy of the internal fault classification.

Compared to the existing technique, the proposed combined S-transform and SVM based

scheme has provided better accuracy of the internal fault classification.

5.2 Superimposed of differential currents based method

Digital differential protection algorithm has been proposed for power transformer protection

based on the extraction of positive and negative sequence components from superimposed

differential currents. Initially, different types of internal faults, magnetizing inrush and over-

excitation conditions have been generated to obtain differential currents by modelling the

power transformer of the power transmission system using the PSCAD/EMTDC software

package. Subsequently, the fundamental frequency positive and negative sequence

components have been extracted from the superimposed differential currents using the

Modified Discrete Fourier Transform (MDFT). Such sequence components have been used

to determine the Internal Fault Detection Factor (IFDF) that can differentiate internal faults

from non-internal faults. The proposed method has also been tested for CT saturation,

specific power transformer connection & rating, CT error and change of tap position. Later,

the reliability of the proposed scheme has been evaluated using internal fault and

magnetizing inrush data collected from the actual field. At the end, a comparative study of

the proposed scheme with recent approaches and conventional techniques has been

conducted. The assessment of the proposed method for diversified cases indicates a high

degree of reliability for internal faults and greater stability during other disturbances.

5.3 Sequence component of differential current based method

A novel method based on sequence components of differential currents has been presented

for distinguishing between internal fault and magnetizing inrush of a power transformer. In

this method, the Internal Fault Detection (IFD) ratio has been calculated from the differential

currents of the power transformer to identify internal faults. Initially, more than 1600

simulation cases have been carried out to produce differential current waveforms for the

different operating conditions of the power transformer. The simulation has been performed

using 3-phase power transformer modelling using the PSCAD / EMTDC software package.

Subsequently, MDFT algorithm has been used to extract fundamental frequency positive and

negative sequence components from differential currents. As a result, using these sequence

8

component values, IFD threshold value has been identified which can effectively

discriminate internal (LG, LL, and turn-to-turn) faults from magnetizing inrushes such as

residual and sympathetic inrushes. In addition, this method also remains stable against over-

excitation and CT saturation during external fault events. A comparative assessment with

the recently developed method had also been carried out to indicate the effectiveness of the

proposed method.

5.4 Quartile of differential current based method

Due to the unavailability of negative sequence components during symmetrical internal

faults (which are very rare to occur in transformers), Superimposed and Sequence

components based differential current algorithms may not able to detect internal fault in case

of Triple Line and Triple Line-to-Ground faults. In order to overcome this limitation, a

quartile of differential current based method has been proposed for the protection of the

power transformer. In this method, the Fault Detection Ratio (FDR) has been computed to

detect internal fault from quartiles of the differential current. The threshold value of the FDR

has been determined from the mathematical models of internal fault, magnetizing inrush and

over-excitation conditions, and further, validated with the help of the simulation dataset

(generated using PSCAD/EMTDC software) for the different operating conditions of the

power transformer. The proposed method has been successfully assessed for all types of

internal faults, including large value symmetrical internal fault currents to low value turn-

to-turn fault currents. In addition, the proposed method has been performed reliably against

the different connection and rating of the transformer, real-time field data from the substation

and noise. In the end, the comparative assessment of the proposed method with conventional

differential protection and the recently developed & existing method reveals superior

performance in terms of sensitivity, reliability and speed.

6. Methodology of Research, Results / Comparisons

The research plan had been designed thoroughly and the study has been carried out on the

following points.

Literature Survey.

Collection of data.

Simulation of different conditions for power transformer differential protection

scheme using PSCAD/EMTDC environment.

9

Development of Hypothesis.

Testing of Hypothesis.

Publications.

The results of the proposed methods in thesis in terms of identification of various power

transformer conditions have been depicted as follows:

S-transform and Support Vector Machine based method

In the proposed scheme, internal and non-internal fault conditions have been simulated to

generate the training and testing dataset using PSCAD/EMTDC software package. Total

12864 cases have been generated for different types of internal faults, various types of

magnetizing inrushes, over-excitation, external fault and normal operating conditions of

power transformer. In order to obtain fault classification accuracy, two features viz.

maximum magnitude of frequency components and standard deviation of the phase contour

of differential currents have been derived with the help of S-transform. Afterwards, SVM

technique has been employed in MATLAB environment by means of Lib-SVM toolbox. Out

of the total 12864 cases, 6432 cases (i.e. 50% of total cases) have been selected for training

dataset comprising different system and fault parameters. For testing and validation of the

proposed technique, remaining 6432 cases have been preferred. Thus, for the total test cases,

the fault discrimination accuracy (η) has been calculated using (1),

𝜂 = 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑜𝑟𝑟𝑒𝑐𝑡 𝑡𝑒𝑠𝑡 𝑐𝑎𝑠𝑒𝑠

𝑇𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑒𝑠𝑡 𝑐𝑎𝑠𝑒𝑠 (6432) × 100% (1)

Once training cases have been identified among all simulated cases, the next step is to train

SVM with the most optimum parametric settings. Using researcher’s experience in

various literature, it has been found that the RBF kernel function is the most valuable and

best choice for fault detection and classification algorithms. In order to achieve the optimum

values of the parameters (cost parameter (C) and free parameter (γ)) of the RBF kernel,

fivefold cross validation has been implemented by altering the values of these two

parameters to avoid over-fitting models on training data. It has been perceived that the

highest cross validation accuracy of 99.78% has been retrieved for C = 103 and γ = 10-3 and

hence, it has been chosen for training the SVM. Utilizing these parameters for SVM training,

the overall performance of the proposed scheme for a total 6432 test cases of internal fault

and non-internal fault has been presented in Table 1.

10

Table 1: Different operating conditions which identify using proposed methods

Faults/

Abnormalities

Number

of test

cases

Number

of cases

identified

correctly

Number of

cases not

identified

correctly

η

(%)

Total Internal faults 3240 3201 39 98.80

Total Non-Internal faults 3192 3099 93 97.09

Overall accuracy 6432 6300 132 97.95

It has been observed from Table 1 that the proposed scheme is capable of correctly

distinguishing internal faults with 98.80% accuracy, while non-internal faults with 97.09%,

thus the overall accuracy has been obtained as 97.95%. Furthermore, a comparative

assessment with the existing scheme reveals that the proposed scheme shows more superior

results compared to the existing scheme.

Superimposed of differential currents based method

In this method, different types of internal faults, magnetizing inrush and over-excitation

conditions have been generated, initially, to obtain differential currents by modelling the

power transformer of the power transmission system using the PSCAD/EMTDC software

package. Subsequently, the fundamental frequency positive and negative sequence

components have been extracted from the superimposed differential currents using MDFT.

Such sequence components have been used to determine the Internal Fault Detection Factor

(IFDF) that can differentiate internal faults from non-internal faults. In the proposed method,

if the value of IFDF is greater than threshold value (in this case, threshold is 1.0) for a definite

time (here, it is 5 ms) then internal fault in the transformer is said to be detected and the

proposed method initiates a trip command. On the contrary, if IFDF is less than threshold

value then the proposed scheme remains stable. The simulation results of the proposed

scheme have been shown for internal fault and magnetising inrush conditions as follows. As

shown in Fig. 1 (a), a solid LG fault (on phase-a) has been simulated at 50% of winding from

the terminal of HV side of the power transformer at 30˚ fault inception angle. Fig. 1 (b)

shows the calculated IFDF by the proposed scheme. As observed from Fig. 1 (b) that the

value of IFDF remains above the threshold for more than a quarter cycle (5 ms) after the

inception of fault. Hence, the suggested scheme is capable to detect such type of an internal

fault within half a cycle after the occurrence of fault. Moreover, the proposed scheme has

been also evaluated for the interwinding and turn-to-turn faults and results indicates that the

proposed method is capable to identify these types of internal faults.

11

Fig. 1 Response of LG fault (a) differential current, (b) IFDF; Response during magnetizing inrush (c)

differential current (d) IFDF

In order to assure stability of the proposed scheme, the residual inrush has been simulated at

0˚ incident angle of the voltage source during no-load condition. The simulation results in

terms of magnitude of residual inrush current and IFDF are shown in Fig. 1 (c) and (d),

respectively. It is to be noted from Fig. 1 (d) that the value of IFDF remains well below the

threshold.

Furthermore, the proposed method remains stable for the different types of magnetizing

inrushes, over-excitation and CT saturation due to external fault conditions. At the same

time, the proposed technique is equally applicable to an entirely different rating and type of

power transformer connection. Subsequently, the performance of the proposed technique has

also been evaluated using internal fault and magnetizing inrush data recorded in the actual

field. Finally, the assessment of the proposed technique has also been carried out with the

recently established techniques.

Sequence component of differential current based method

In this method, the Internal Fault Detection (IFD) ratio has been calculated from the

differential currents of the power transformer to identify internal faults. To correctly

differentiate internal fault from other operating condition of power transformer, the

computed IFD is equated with the predetermined threshold (in this case, it is 7.0). If the

calculated IFD is larger than predetermined threshold value, then it is to be said that an

internal fault in the power transformer is existed and trip command has been commenced.

Conversely, if IFD value is smaller than predetermined threshold then the proposed method

12

prevents tripping. The simulation results of the proposed scheme have been shown for turn-

to-turn type internal fault and over-excitation conditions as follows.

Fig. 2 (a) shows waveform of turn-to-turn internal fault on phase ‘b’ of primary winding

with 0.5% shorted turns at 0˚ fault impedance angle. It has been found from Fig. 2 (b) that

value of IFD raises above the threshold after the beginning of fault. Hence, it could be said

that the proposed method is competent to recognize turn-to-turn type internal fault

accurately. Furthermore, performance of presented method has also been verified against

over-excitation condition of power transformer. In case of over-excitation condition, a

simulation test has with rise in voltage value by 10% and reducing system frequency by 2%

Hz with 0° switching angle has been evaluated. The simulated differential current waveform

for over-excitation and corresponding value of IFD are revealed in Fig. 2 (c). It has been

perceived from Fig. 2 (d) that IFD prevails value lower than threshold and henceforth, kept

away the undesirable tripping. Further, the simulation results show that the proposed scheme

is sensitive to detect different types of internal faults, while improves stability during non-

internal fault conditions. At last, a comparative analysis with recently developed technique

provides the capability of the proposed method.

Fig. 2 Response of turn-to-turn fault (a) differential current, (b) IFD; Response during over-excitation

(c) differential current (d) IFD

Quartile of differential current based method

In the proposed method, the Fault Detection Ratio (FDR) has been computed from quartiles

of the differential current to detect internal fault of power transformer. The threshold value

13

(4% in this case) of the FDR has been determined from the mathematical models of internal

fault, magnetizing inrush and over-excitation conditions, and further, validated with the help

of the simulation dataset (generated using PSCAD/EMTDC software) for the different

operating conditions of the power transformer. The simulation results of the proposed

scheme have been shown for internal fault and magnetising inrush conditions as follows.

Waveforms of the differential currents and FDR for all phases, during a LG fault at 95% of

winding from the terminal with 0° FIA on HV side, are shown in Fig. 3 (a) and (b),

respectively. It is noted from Fig. 3 (b) that the value of FDR for phases ‘a’ and ‘b’ increases

from 0% to around 8% within a cycle after the inception of fault. Thus, it is observed from

the aforementioned internal fault case that the proposed algorithm detects an internal fault

condition when the value of FDR crosses the threshold for any of the three phases. Further,

the proposed scheme equally provides the reliable operation during turn-to-turn and

interwinding faults in noise condition too.

Fig. 3 Differential currents (a) and (c) and FDRs (b) and (d) during LG fault and magnetizing inrush,

respectively

Waveform of differential currents and FDRs during transformer energization at no load with

0° switching angle are shown in Fig. 3 (c) and (d), respectively. It is observed from Fig. 3

(d) that the values of FDR stay well below the threshold. Further, the proposed scheme

remains stable for various types of inrushes, over-excitation and CT saturation conditions.

Simultaneously, it provides equally promising results to the different rating and winding

connection of the power transformer. In addition, verification of its performance on actual

14

field data (containing faulty transformer energization and magnetizing inrush condition)

reveals its correctness in detecting internal faults and immunity from nuisance trip during

non-internal faults. Finally, the comparative assessment of the proposed technique in terms

of coverage of different types of internal faults, stability during external disturbances,

average relay operation time and the sampling frequency requirement demonstrates its

advantages compared to other existing methods.

7. Achievements with respect to objectives

The main objective of the thesis is to design and develop effective transformer protection

strategies that identify different internal faults and endure stable operation during all types

of magnetizing inrushes, over-excitation and CT saturation during external fault conditions.

All proposed methods have been able to fulfil above mentioned objective in terms of high

discrimination accuracy and capability to identify various abnormal conditions. Moreover,

the proposed methods have provided equally promising results for different rating and

connection of transformer that points out the applicability of the proposed methods on

generalized form. Furthermore, the superimposed and quartile of differential currents based

methods have been successfully evaluated on real field data of power transformer which

indicates the proposed methods are capable to provide alternative approach for the power

transformer protection.

8. Conclusion

The work has begun with the aim of developing new differential protection schemes capable

of addressing the challenges and issues associated with conventional differential protection

techniques and providing a more reliable protection solution for the differential protection

of power transformers. The work has been focused on the development of efficient

techniques for distinguishing between internal faults and other disturbances, such as

magnetizing inrush, over-excitation and CT saturation due to external faults. Further, these

methods should recognize different types of internal fault such as winding faults, turn-to-

turn faults and interwinding faults. In this context, four different algorithms for differential

protection of the power transformer have been developed based on the classification of

internal and non-internal faults, the superimposed of differential currents, the sequence

components of differential currents and the quartile of differential current. These developed

15

techniques are fast, reliable and stable for the differential protection of the power

transformer. In this way, the objective of the work has been achieved to a large extent.

9. List of publications

Published Journal Papers:

1. A. M. Shah, B. R. Bhalja and R. M. Patel, "New protection scheme for power

transformer based on superimposed differential current," in IET Generation,

Transmission & Distribution, vol. 12, no. 14, pp. 3587-3595, Aug. 2018.

2. A. M. Shah et al., "Quartile Based Differential Protection of Power Transformer," in

IEEE Transactions on Power Delivery, vol. 35, no. 5, pp. 2447-2458, Oct. 2020.

Published Conference Papers:

1. A. M. Shah, B. R. Bhalja and R. M. Patel, "Power transformer differential protection

using S-transform and Support Vector Machine," 2016 National Power Systems

Conference (NPSC), Bhubaneswar, 2016, pp. 1-6.

2. A. M. Shah, B. R. Bhalja, R. M. Patel and Y. M. Makwana, "The Sequence

Components based Identification of Internal Fault in Power Transformer," 2019

IEEE 16th India Council International Conference (INDICON), Rajkot, India, 2019,

pp. 1-4.

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[2] J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications,

Third Ed. Taylor & Francis Group, 2007.

[3] P. M. Anderson, Power System Protection. Wiley - IEEE Press, 1998.

[4] B. R. Bhalja, R. P. Maheshwari, and N. G. Chothani, Protection and Switchgear,

Second Ed. Oxford University Press, 2018.

[5] IEEE Guide for Protecting Power Transformers. IEEE Standard C37. 91-2008

(Revision of IEEE Std. C37.91-2000).

[6] ABB Inc. Limited, Transformer protection RET670 Version 2.1 ANSI: Application

manual. 2019.

[7] R. Hamilton, “Analysis of Transformer Inrush Current and Comparison of Harmonic

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