Literature Review: Power Transformer Differential Protection Methods

This literature review delves into the critical area of power transformer differential protection, essential for maintaining the integrity of power distribution networks. The paper examines various methods for protecting transformers from internal and external faults, with a primary focus on differential protection techniques utilizing current and voltage ratios. It explores the challenges of differentiating between inrush currents and internal faults, and reviews several approaches, including harmonic restraint, harmonic blocking, and network and fuzzy logic techniques. The review highlights the effectiveness of wavelet transforms and percentage differential protection. The paper analyzes the strengths and weaknesses of each method, offering insights into the evolution of protection strategies and the importance of robust and secure relay systems. The findings underscore the significance of selecting the most appropriate protection scheme based on the specific characteristics of the power system and the need for continuous advancements in protection algorithms to address emerging challenges.

Literature Review
(Electrical
Engineering)

TABLE OF CONTENTS
Power Transformer Differential Protection Using Current and Voltage ratios..............................1
ABSTRACT.....................................................................................................................................1
1. INTRODUCTION.......................................................................................................................1
2. LITERATURE REVIEW............................................................................................................2
3. EVALUATION............................................................................................................................8
4. CONCLUSIONS..........................................................................................................................8
5. REFERENCES..........................................................................................................................10

Power Transformer Differential Protection Using Current and Voltage ratios.
By '___'
Affiliation () Study no
ABSTRACT
In a power distribution network, transformers play a crucial role and any failure in it
leads to disruption of power supply to consumers. To prevent any interruption in the system, it is
essential to keep transformers protected from the external and internal threats. The main hurdle
in protection of transformer is to create a fast and efficient differential algorithm that can keep
the system isolated from the damaging agents. In this report, revised and robust differential
protection method for power transformers is explained. Various approaches of using differential
protection method for protection of the power transformers will be discussed in this paper. The
findings that are obtained from this research includes the most effective method for protecting
power system is the “Wavelet Transforms”. However, it may be recognised that there are some
limitations in the tactic which are to be filled.to be overcome.
1. INTRODUCTION
Power transformer is a static machine which is used for transforming high voltage power
from one circuit to another (Sarimuthu and et. al., 2016). The frequency of the voltage can be
altered using step-up and step-down transformers and this application makes it more effective
than traditional ones. However, there are many subjects of failure likewise other power systems.
There can be two types of failures of faults in the system, internal or external. The major reason
behind the faults includes short circuits in the windings. Also, some failures occur inside the
transformers. Hence, to deal with these issues it is necessary to render proper protection of the
power system (El Arroudi and Joos, 2017). There are various parameters on which the selection
of the protection depends such as on the cruciality of load, size of the system on the basis of the
total load, etc. For transformers rated above or equals to 10 MVA, the most widely used
protection technique is Percentage differential. However, it may be recognised that the tactic can
fail due to various process linked with the transformer cord such as non-linearities. The main
principle of this scheme is one simple conceptual technique and that is the differential passage
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actually compares the magnitudes of current of primary and secondary components of the power
transformer.
The main objective behind protection of power transformer is to prevent malfunctioning
of protective relays. This issue arises when the phase difference of two or more similar electrical
quantities exceeds some predetermined level. This mal-operation of the passage may be caused
due to any of the reason among the following: magnetic inrush current, external faults with
current transformer saturation, simultaneous inrush with internal faults (Ashraf and et. al., 2017).
Inrush current in the power transformer interferes with the operation of the system and causes
various issues such as arcing of primary circuit components, fuses, injection of noise, etc. These
problems become more severe when a simultaneous inrush occurs with internal faults. Besides
this, the internal faults may cause due to the energisation of the primary or secondary component
of the power transformer. In this paper, some approaches are addressed to overcome the
challenges faced in context to differential protection of three-phase power transformers. These
methods of protect power system is based on the current and voltage ratios. These ratios help in
discriminating between the energisation and internal faults due to inrush current. This approach
is useful in getting robust, secure and dependable relay for power transformers (Babu, and
Gargava, 2017).
There are numerous approaches that are been proposed to distinguish between the inrush
and internal current faults are explained. The differential protection scheme detects the faults on
the basis of three general methods which are time over current relay, percentage differential
relays on the basis of the current going and coming from the protection zone and percentage
differential relays with restraint by harmonics method. However, it may be recognised that there
are some techniques which are less effective as their discrimination of the faulty sector is not
clear and satisfactory. Thus, to overcome these limitations some methods are modernised (Ali
and et.al., 2018)
2. LITERATURE REVIEW
Here, the concept of transformer differential protection is reviewed. There are various
approaches introduced by different researchers for the inrush and internal fault currents.
Differential protection principles are based on the ratios of current and voltages. According to
Murugan and et. al. (2017), the transformer related faults can be easily identified with the help of
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these ratios. Further, it is stated as the most selective, fast and reliable method for protection of
the power transformer. This tool is the practical implication of the Current law proposed by the
Kirchhoff. Thus, it is based on the basic principle which is " The sum entering the power circuit
should be equal to the current leaving it". Hence, in protection of transformers using the
differential method, the current that is coming and going out of the transformer is calculated.
Basically, there are three ways of evaluating the internal faults. These are the increase of
the phase currents, increase of the differential current and engender gas by the fault arc. There
are several differential methods that are been used for the protection of the transformers by using
current and voltage rations. Some of them are explained in this paper (Jarman and et. al., 2017).
Out of them all, one of the most commonly used approach is the Harmonic Restraint. It is
however noted that this method is not effective as there are many losses associated with it. Also,
this tool is not effective enough to differentiate between the internal fault and the energisation.
For checking the mal-operations of the power transformer the ration, of current ε can be
calculated as:
ε = (I1- I2 / I1+I2)
Where, I1 and I2 are the magnitude per unit of the fundamental component of primary and
secondary current respectively.
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Illustration 1: Differential protection during different faults
(Source: Anishek and et. al., 2016)

In case when there is no fault and the power system is normal the value of ε will be
nearly or equal to zero. In case when some energisation occurs in the circuit then value of the
ration of the current changes (Ali, E. and et. al., 2016). As the energisation leads to the entry of
inrush current, the current on the primary side flows whereas no current flows on the secondary
side, this situation leads ε to reach its value to 1.
In addition to this, the internal faults also affect the terminal voltages besides currents.
Similar to the current ration, the voltage ration can be given as:
ε.= (V1- V2 / V1+V2).
Where, V1 and V2 are the magnitudes of the fundamental components of the primary and
secondary voltages respectively. If the value of ε. is equal to the zero then there is no fault in the
power system. In case, if the value of ε. is 1 then there is some fault present during transformer
energisation.
In order to overcome the limitations of the Harmonic restraint method, a revised process
was developed and that is Harmonic Blocking. According to Mirsalim and Masoum (2017), the
discrimination between inrush current and internal faults in this method is done on the basis of
second components harmonic restraint techniques. However, this method fails with the
improvement in the designs of transformer which leads in reduction of second harmonic
component. The computation of second and fifth harmonic components gives, IDiff= I1- I2.
According to this technique, if there is no fault in the power system then, different restraining
operations of differential protection that will be used are:
1). In case when no load magnetisation current is there, 1 2| |I I
 
2). In case of standard differential protection, 22
2121 IIII 



Here, I1 is the second harmonic current and I2 is fifth harmonic current.
β is the slope of the differential characteristics.
To overcome the challenges of harmonic restraint technique, network and fuzzy logic
techniques has been developed. As explained by Mostafaei and Haghjoo (2016), this method is
improvised form of the harmonic restraint method but there are also some problems associated
with it such as large training set, more time consumption, designing of new neutral network for
other transformer system, etc.
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According to Hashemnia, Abu-Siada and Islam (2016), voltage restraint is the most
suitable method of protection of transformer. This process is an integrated substation protection
system. The main function of it is to suppress the tripping action. However, the main reason
behind choosing this is its low cost. Similar, Olivares-Galvan and et. al. (2016), states that flux
restraint can be used. For distinguishing the no fault from the faulted region, this technique can
be used. In this method, the slope curve is used for evaluation rather than the actual numerical
values. In case if the voltage signal is high then the relay will be restrained if,
| |dI
 || V
And ptv ||
Here, vt is the transient monitor function.
As per the view of Rajpurohit and et. al. (2017), Wavelet transforms and PNN is a strong
signal processing tool that is used in the analysis of power systems. This tool is similar to short
time Fourier transform in allowing time localisation of variable frequency components of given
signal. However, the major difference is that the signals in case of Fourier are decomposed using
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Illustration 2: Retraining-differential current curve
(Source: Marks and et. al., 2016)

sines and cosines whereas in case of wavelet transform, the signals are transmitted in both the
real and Fourier space. The expression for the wavelet transform can be given as:
F(ab) = ∫ f(x) € (a, b) (x) dx
This phenomenon works on the energy vector for discrimination. It has been observed
that the discrimination was satisfactory even in the presence of noise. The wavelet algorithm is
used for analysing power system transients. The element that are used in distinguish between
magnetising inrush current and internal fault current are the detailed coefficients of PNN and
Wavelet transforms (Sendilkumar, Mathur and Henry, 2010). The propose protection algorithm
using wavelet transform is summarised below in four simple steps:
First step: It is about to gather the current and voltage signal from both the terminals of the
transformer for the calculations of the orthogonal basis scale and mother wavelet vectors (Vn and
Wn).
Second step: Calculation of the coefficient vectors, am and dm . They can be calculated using the
formulas present below:
am = (discreet signal) Vm∗
dm . = (discreet signal) Wm∗
Third Step: Calculations of the approximation and detailed coefficients Am and Dm.
Am = am V m N∗
Dm. = dm W m N∗
Fourth Step: Generating inverse wavelet transform:
synthesised signal =. Am + Dm + _ _
As stated by the Ahmad and etal. (2016), Percentage differential protection using
Current and Voltage ratios is the most selective and fast method of protection against short
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Illustration 3: Fault identified outside the protection zone.

circuits in transformers. It aids in different situations such as inrush current, CT saturation, etc.
In this it is assumed that the power transformer ratio is 1:1 and there is same ratio and core class
in both the CT. This method is helpful in computing that either the fault has occurred inside or
outside of the protection zone. This difference can be understood directly with the help of below
figures. If the fault takes place inside the circuit, then the resistance reads the same magnitude
but in opposite direction.
In case if the fault occurs outside the protection zone, then the resistance will measure the
same amplitude of current but in opposite direction (Marks and et. al., 2016)
The magnitudes of the fundamental components of voltages V1 and V2and currents I1
and I2 are extracted for the once cycle of DFT. After that, the operating characteristics of
percentage differential relay is calculated using the formula:
Id > Iop and ID > K (Ir - Irmin) = Iop
Where, Iop is the minimum operating current that is 0.2 per unit.
Irmin is the minimum restraining current that is 0.6 per unit and K is the coefficient of restraining.
The percentage differential relay criterion is checked to ensure the operating condition of
the relay. In case if the percentage is observed to be satisfactory then there will be a condition of
either an inrush current or internal fault at inlet or outlet (Anishek and et. al., 2016). It can be
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Illustration 4: Fault identified in protection zone
(Source: Hernandez and Labib, 2017 )

evaluated on the basis of the threshold value. If the ratio of current is greater than the threshold
value and less than 0.9, then there is a condition of loaded energisation or fault in the power
system.
Thus, these are some methods of different protections of the power transformers using
current and voltage ratios. These methods can help in identifying and distinguishing faults on the
basis of their place and reason of occurrence.
3. EVALUATION
From the above research that has been done on the power transformer it may be evaluated
that the differential protection method using current and voltage rations is the best protection
method for the system. Numerous threats to the power transformer are identified in this research
paper which are essential to be resolved for the effective working of the system. Different
approaches of the protection are discussed. It can be evaluated that all of them are based on the
current and voltage ratio for identifying the threats or faults in the transformer (Cui and et. al.,
2016). Further, out of all the method, it has been observed that the most effective is the Wavelet
transforms.
The factors that contributes in the effectiveness of this method includes the accurate and
satisfactory results. It has been observed that the wavelet decomposition breaks up the signal into
the time and the frequency which allows the method to provide a complete description of current
of each phase (Patel, Mistry and Chothani, 2016). Also, it aids in the accurate fault detection.
This method is moreover used for the discontinuity analysis of the signals and thus, the faults can
be observed even at the lowest time space. However, there is a gap in this method which is
required to be filled by the research. In order to resolve the loophole, a bridge that can assist in it
is artificial neutral network. This addition can be used for the classification of the faults with the
detection (Liu and Dinavahi, 2016) This classification will help in proper sorting of the faults
and thus, various methods to resolve errors in groups can be developed.
4. CONCLUSIONS
In this paper, different internal and external threats to the power transformers are
identified so that effective methods of protection can be identified. From the above research on
the protection of the system using differential methods it can be concluded that voltage and
current ratios are very helpful in identification of the faults and distinguishing their area of fault.
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In this report, different approaches of power transformer protection are evaluated. From the
above research, it may be concluded that the wavelet transform is the best technique that can
protect the power system. It has been observed that this process is used for the discontinuity
analysis of the signals and thus, it states about the faults even at the lowest time space. However,
there is a flaw identified in this method and that is its lack of classifying the faults according to
their categories. Furthermore, a more accurate method is also identified in this study and that is
neutral network which can classify the faults along with the detection.
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5. REFERENCES
Books and Journals
Ahmad, A. and et. al., 2016. Short circuit stress analysis using FEM in power transformer on HV
winding displaced vertically & horizontally. Alexandria Engineering Journal.
Ali, E. and et. al., 2016. Power transformer differential protection using current and voltage
ratios. Electric Power Systems Research.154. pp.140-150.
Anishek, S. and et. al., 2016. Performance Analysis and Optimisation of an Oil Natural Air
Natural Power Transformer Radiator. Procedia Technology. 24. pp.428-435.
Ashraf, H. M. and et. al., 2017, March. Modeling and simulation of optical current transformer
using operational amplifiers. In Electrical Engineering (ICEE), 2017 International
Conference on (pp. 1-6). IEEE.
Babu, K. D. and Gargava, P. K., 2017. Transformer Life Enhancement Using Dynamic
Switching of Second Harmonic Feature in IEDs. World Academy of Science, Engineering
and Technology, International Journal of Electrical, Computer, Energetic, Electronic
and Communication Engineering.11(4). pp.381-386.
Cui, Y. and et. al., 2016. Moisture-Dependent Thermal Modelling of Power Transformer. IEEE
Transactions on Power Delivery. 31(5). pp.2140-2150.
El Arroudi, K. and Joos, G., 2017. Performance of Interconnection Protection Based on Distance
Relaying for Wind Power Distributed Generation. IEEE Transactions on Power Delivery.
Hashemnia, N., Abu-Siada, A. and Islam, S., 2016. Detection of power transformer bushing
faults and oil degradation using frequency response analysis. IEEE Transactions on
Dielectrics and Electrical Insulation. 23(1). pp.222-229.
Hernandez, M. D. O. C. and Labib, A., 2017. Selecting a condition monitoring system for
enhancing effectiveness of power transformer maintenance. Journal of Quality in
Maintenance Engineering.
Jarman, P. and et. al., 2017. Reliable, optimised power transformers with heat recovery for urban
areas. Transformers Magazine.4(2). pp.84-90.
Liu, J. and Dinavahi, V., 2016. Detailed magnetic equivalent circuit based real-time nonlinear
power transformer model on FPGA for electromagnetic transient studies. IEEE
Transactions on Industrial Electronics. 63(2). pp.1191-1202.
Marks, J. and et. al., 2016, September. An analysis of Australian power transformer failure
modes, and comparison with international Surveys. In Power Engineering Conference
(AUPEC), 2016 Australasian Universities (pp. 1-6). IEEE.
Mirsalim, M. and Masoum, M. A., 2017. Application of a Recursive Phasor Estimation Method
for Adaptive Fault Component Based Differential Protection of Power Transformers.
IEEE Transactions on Industrial Informatics.13(3). pp.1381-1392.
Mostafaei, M. and Haghjoo, F., 2016. Flux-based turn-to-turn fault protection for power
transformers. IET Generation, Transmission & Distribution. 10(5). pp.1154-1163.
Murugan, S. K. and et. al., 2017. An Empirical Fourier Transform-Based Power Transformer
Differential Protection. IEEE Transactions on Power Delivery.32(1). pp.209-218.
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Olivares-Galvan, J.C. And et. al., 2016, November. Detection of interturn faults during
transformer energization using wavelet transform. In Power, Electronics and Computing
(ROPEC), 2016 IEEE International Autumn Meeting on (pp. 1-5). IEEE.
Patel, D. D., Mistry, K. D. and Chothani, N. G., 2016, December. Digital differential protection
of power transformer using DFT algorithm with CT saturation consideration. In Power
Systems Conference (NPSC), 2016 National (pp. 1-6). IEEE.
Rajpurohit, B. S. and et. al., 2017. A Case Study of Moisture and Dust induced Failure of Dry
type transformer in Power Supply Distribution. Water and Energy International, 60(2).
pp.43-47.
Sarimuthu, C. R. and et. al., 2016. A review on voltage control methods using on-load tap
changer transformers for networks with renewable energy sources. Renewable and
Sustainable Energy Reviews.62. pp.1154-1161.
Online
Sendilkumar, S., Mathur, B. L. and Henry, J., 2010. Differential Protection for Power
Transformer Using Wavelet Transform and PNN [Online]. Available through:
<http://waset.org/publications/9442/differntial-protection-for-power-transformer-using-
wavelet-transform-and-pnn> [Accessed on 12th September, 2017].
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