An Improved Algorithm for Variable Slope

Differential Protection of Distribution

Transformer using Harmonic Restraint

B S Shruthi

National Institute of Technology Karnataka, Surathkal, India

Email: shruthibs123@gmail.com

K Panduranga Vittal

National Institute of Technology Karnataka, Surathkal, India

Email: vittal.nitk@gmail.com

Abstract—In the context of reliable protection of distribution

transformer, differential protection is considered to be the

best. The infiltration of Distributed Energy Resources into

the grid has necessitated the security of transformer to be

increased many fold. In this paper, an improved variable

slope differential method is proposed for a 1 MVA, 11/0.433

kV /Y connected distribution transformer considering

provision for accounting challenges of DES penetration. By

using second and fifth harmonic restraint, along with

negative sequence component, it is possible to effectively

differentiate between various conditions like inrush, over

excitation and internal fault including inter turn fault with

few shorted turns. Apart from these, cases for remanence

and external fault are also taken up to test the reliability of

the scheme. Effective simulation studies have been

performed to demonstrate the efficiency of the proposed

scheme using PSCAD/EMTDC.

Index Terms—differential protection, distributed generation,

Distribution transformer, harmonic restraint.

I. INTRODUCTION

Distribution transformers are generally considered to

be those which provide transformation from 11kV or

lower voltages down to the level of final distribution. The

main intention of transformer protection is to provide the

ability to detect internal faults with high sensitivity, along

with a high degree of immunity to operation on system

faults for which tripping of transformer terminal breaker

is not required. It is based on the fact that the differential

current will be high only in the case of faults internal to

the zone. Security of transformer differential protection

schemes is dependent on detecting the magnetizing inrush

currents of the protected transformer and associated

blocking of differential operation due to inrush related,

non-fault and other unbalance currents. Variable slope

differential protection is one of the schemes that provide

restraint against inrush and under overexcitation.

Manuscript received March 1, 2013; revised June 10, 2013.

With the connection of DG, the system loses radial

configuration. Since the DG sources also contribute to the

fault, the system coordination could be lost. M. Mañana

et. al. [1] concluded that magnetizing inrush currents of

power transformers have to be limited, especially when

they share a common bus with DG. With the proliferation

of DG installed at the customer load site, the delta-wye

distribution transformers become sources of fault current

that can be difficult to detect and isolate. Under worst-

case scenarios, without proper protection, the customer

generation may keep the faulted utility circuit energized

even after the utility source breakers have opened.

Several schemes to detect and isolate the transformer

under such condition are discussed in [2].

Current differential relaying is the most commonly

used type of protection for transformers of approximately

10 MVA three-phase and above. Although utility practice

for protection of distribution transformers as of now is

only through remote back up or fuses local to transformer,

it is envisaged in future with DG penetration that

differential protection becomes inevitable. General

protection issues with DG proliferation are gaining

importance [3]-[5] but its impact on distribution

transformer protection is not sought much. Detailed

analysis and publications are not available for reliable

transformer protection and hence the main objective of

this work is to address these concerns. Improvement of

the existing protection scheme is taken up as the first

phase and is the focus of this paper.

In the context of protection, transformer inrush is of

large importance and appropriate models for these

transient behaviors needs to be chosen. Unified magnetic

equivalent circuit (UMEC) model in PSCAD/EMTDC

has the ability to accurately reproduce the unbalance

operations. A 1 MVA, 11/0.433 kV /Y connected

transformer is opted for the purpose of this study. The

following section discusses the general differential

protection for a transformer along with the issues related

to the same followed by variable slope differential

protection. Section 3 briefs about the proposed relay logic.

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Few simulation results to support the developed scheme

are presented in the last section.

II. TRANSFORMER DIFFERENTIAL PROTECTION

Transformer is one of the most important equipment in

power systems and it is important that adequate measures

are taken for protection. Differential protection is a fast,

selective method of protection against short circuits in

transformers. In a basic differential protection scheme,

currents on both sides of the transformer are compared.

Under normal conditions, primary and secondary currents

are equal and hence no difference current flows.

There has been renewed interest in devising a reliable

scheme for the protection of transformer using various

methods. B. Kasztenny et. al. [6] reviewed the principles

of protection against internal short circuits in

transformers of various constructions. A new approach

for transformer differential protection using current-only

inputs is described in [7]. Various protection schemes

with neural network algorithms and wave-shape

recognition techniques have also been proposed [8–11].

Sequence components have also been proved useful. H.

Khorashadi-Zadeh [12] describes the complete design of

a prototype digital protective relay for three-phase

transformers using positive sequence current whereas

usage of negative-sequence currents in order to both

detect and determine the position of the fault with respect

to the protected zone is dealt with in [13].

The nature of power transformers creates several

complications for the application of such differential

relays.

Magnetizing inrush currents created by

transformer transients

Phase shift between primary and secondary for

delta-wye connections

Variable ratio of the power transformer caused by

a tap changer

High exciting currents caused by transformer

overexcitation

Mismatch between the CT ratios and CT

Saturation

Differential relays are prone to maloperation due to

one or more of the above mentioned reasons. Harmonic

restraint or blocking methods ensure relay security for a

very high percentage of inrush and overexcitation cases.

The selection of CTs along with choice of percentage

slope for differential relaying and the effects of

magnetizing inrush are discussed in detail in [14].

A variable-percentage or dual-slope characteristic,

further increases relay security for heavy CT saturation.

Several common methods for defining restraint and slope

characteristics are examined and guidance on selection of

the correct slope setting is provided in [15].

Modern day transformers make use of IEDs designed

for protection, control, measurement and. Parameter

setting and DFR records can be accessed using the IEC

61850 protocol. Oscillographic files are available to any

Ethernet-based application in the standard COMTRADE

format. Flexible user settings afford programming for

different CT ratios and transformer phase shifts [16].

Digital relaying technique thus provides value addition to

the well-established percentage differential relays.

III. DEVELOPMENT OF RELAYING SCHEME

In transformer differential applications, CTs are

selected to accommodate a maximum fault current along

with preserving low current sensitivity. As a minimum

goal, CT saturation should be avoided for the maximum

symmetrical external fault current. The CT ratio and

burden capability should also permit operation of the

differential instantaneous element for the maximum

internal fault. Detailed procedure for selection of CT is

given in [17].

Inrush or overexcitation conditions of a power

transformer produce distorted currents because they are

related to transformer core saturation. The distorted

waveforms provide information that helps to discriminate

these conditions from internal faults. The restrained

differential is designed to sensitively clear internal faults

while remaining secure under other unbalance conditions.

Digital relays use unrestrained differential that

provides a very fast clearance of severe internal faults

with a high differential current regardless of their

harmonics. The second harmonic restraint, together with

the waveform based algorithms, ensures that the

restrained differential does not trip due to the transformer

inrush currents. The fifth harmonic restraint ensures that

the restrained differential does not trip on apparent

differential current caused by a harmless transformer

over-excitation [16].

Among the several research work that describe similar

relaying schemes, fault discriminants for differential

protection of transformer have been defined in [18] using

the information available from harmonic analysis. The

second and fifth harmonics along with transformer

primary voltage amplitude provides accurate distinction

between over excitation and internal fault condition. A

parabolic characteristic digital differential relay has been

developed based on the above discriminants.

But the traditional transformer differential percentage

protection is not sensitive enough to detect minor internal

winding faults. A short circuit of a few turns of the

winding will give rise to a heavy fault current in the

short-circuited turns, but changes in the transformer

terminal currents will be very small, because of the high

ratio of transformation between the whole winding and

the short-circuited turns.

Addition of DG sources might cause significant effects

to the existing scheme. Issues like sympathetic inrush

could add to problems such as increase in fault current

due to the additional source and directionality of flow.

This calls for certain improvements in the conventional

relay logic, including the capability to detect internal

winding faults.

Instantaneous differential and bias currents are

calculated from the CT outputs and harmonic analysis is

carried out. The fundamental, second and fifth harmonic

of the differential current is extracted along with its

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negative sequence component and the fundamentals of

restraining current and the primary voltage.

The threshold values for each of these are chosen

based on the simulation results. For severe internal faults,

unrestrained operation is desired. If the fundamental

harmonic of differential current is higher than a set value,

and second harmonic component is below a threshold,

immediate trip decision is given. Parabolic characteristics

are used in other cases and trip signal is issued when

where Id1 and Ir1 are the fundamental components of

differential and restraining currents.

The values of a and b in this case are 0.3 and 0.08

respectively. Fault discriminant K is used to give a stable

discrimination between inrush and internal fault condition.

If K and Id2 are greater than their threshold values,

inrush indicator flag is set. To check for overexcitation

condition, verify whether the voltage level is greater than

times normal voltage (say =1.1), and also whether

fifth harmonic component, Id5 has increased. Finally if the

voltage at primary terminals has collapsed below times

normal voltage (say = 0.9), and the negative sequence

component is higher than a set threshold, increment the

trip counter for inter-turn fault. In any case the trip

counter is incremented up to 3 before giving the trip

decision. This approach ensures security under external

faults, inrush and overexcitation, and dependability for

internal faults along with maintaining sensitivity.

There can be several ways to use the harmonic restraint

scheme. The approach generally in practice is to consider

the second and the fifth harmonic as a percentage of

fundamental. The restraint can be applied by considering

each phase separately or all the three phases together. In

case of choosing a common restraint, considering the sum

of harmonics of each of the three phases gives an

additional advantage of setting a higher value of restraint

compared to averaging over the three phases. Adaption of

the appropriate technique depends on the application.

IV. SIMULATION RESULTS

The performance is evaluated through extensive

simulations using the PSCAD/EMTDC software. Few

typical results obtained are given in this section. A 1

MVA, 11/0.433 kV common core, three limb transformer

was taken up for study, the primary being connected in

delta and secondary in star. The system simulated is as

shown in the figure 1 below.

Figure 1. PSCAD model used for simulation.

Harmonic analysis of the differential current leads us

to the result shown below. Table I shows the maximum

value of various harmonic components as a percentage of

fundamental under inrush and internal fault condition. It

can be seen that second harmonic is highly predominant

in the case of inrush.

Armando Guzmán et. al. [19] summarize the existing

methods for discriminating internal faults from inrush and

overexcitation conditions along with comparative studies

between harmonic restraint and blocking methods.

Following are the various cases that were analyzed to

check the dependability of the above mentioned

protection scheme.

TABLE I. DIFFERENTIAL CURRENT AS A PERCENTAGE OF

FUNDAMENTAL

Harmonic Differential Current Component (%)

Inrush Internal Fault

Fundamental 100 100

Second 46.64 28.92

Third 21.22 16.64

Fourth 17.37 10.89

Fifth 14.52 8.58

Sixth 16.85 8.53

Seventh 18.64 10.98

A. Magnetizing Inrush

During transformer energization, the inrush current can

be as high as 8 – 30 times the rated current for few cycles,

depending upon the transformer core and system

resistance. Other factors that affect the inrush current are

the point on voltage waveform during switching and

residual flux in core.

Following figure 2shows the harmonic components of

Phase A primary current under typical energization.

Figure 2. Harmonics of Phase A primary current after energization.

Sympathetic inrush phenomenon occurs when a

transformer is switched on in a power system network

containing other transformers which are already

energized. Results show that the inrush current in

transformers being connected to system where there are

other transformers already in operation, decays slower.

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B. Remanent Flux In Core

When a transformer is de-energized, the magnetizing

voltage is taken away, the magnetizing current goes to

zero while the flux follows the hysteresis loop of the core.

This results in a residual flux in the core. Remanence may

be as high as 80-90% of the rated flux, resulting in large

peak values and heavy distortions of magnetizing current.

Simulations were carried out with various amounts of

remanent flux in core. The Fundamental and Second

harmonic extracted from FFT analysis with a remanence

of -60% in phase A, 50% and -20% in phase B and C

respectively is as shown in the figure 3 below. Figure 4

shows the inrush flag set after energization.

Figure 3. Fundamental and 2nd harmonic under remanence condition.

Figure 4. Inrush flag with remanent flux.

C. Internal Fault

The internal electrical faults that can occur are winding

short circuits, winding to ground or turn to turn faults.

Various types of temporary and permanent faults were

simulated on both the windings of the transformer. It was

observed that in every case, the trip signal was issued at

around 20ms from the time of fault inception.

As an example, a temporary phase A to C to ground

fault on the primary winding of the transformer is

considered. Fault is applied from 0.2 sec to 0.3 sec.

Figure 5. Differential and restraining current under ACG fault.

Figure 5 shows fundamental of differential current

rising to an extremely high value and quick trip decision

should be given before the flow of this high current

causes damage to the winding or insulation. The trip

decision is given at 0.221 sec, which is considerably good

from the transformer safety point of concern.

Figure 6. Inrush and trip signal under ACG fault.

Turn to ground and turn to turn faults were simulated

by creating new components with the help of FORTRAN

programming. The change in inductance matrix under

fault was derived using the basic equations based on

consistency, leakage and proportionality. Satisfactory

results were obtained for various levels of shorted turns

which justify the necessity of using negative sequence

component as an additional discriminant.

D. External Fault

From the point of view of differential protection, it is

also important to study the effects of out of zone faults. In

some cases, there might be unusual maloperation of the

differential protection after removal of external fault. The

reason is that saturation states of CTs on either side are

different after an external fault occurs [20].

An example of phase A to C double line fault on the

secondary side considered. The main difference is that

restraining current becomes higher than the differential

current which is seen in figure 7. Inrush and trip indicator

is as seen in figure 8.

Figure 7. Differential and restraining current under AC fault.

Figure 8. Inrush flag under AC external fault.

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E. Over Excitation

The magnetic flux inside the transformer core is directly proportional to the applied voltage and inversely proportional to the system frequency. Overvoltage and/or under frequency conditions can produce flux levels that saturate the transformer core.

Over excitation condition was simulated by increasing

the voltage in steps. It was observed that for every step

increase in voltage, a new level of inrush current is

reached and it also causes a rise in the fifth harmonic.

Figure 9. 5th harmonic during overexcitation.

Figure 10 shows the inrush and over excitation flag.

Though the transformer core saturated, the relay was

found to be stable even up to 150% of rated voltage.

Figure 10. Inrush and overexcitation flag.

V. CONCLUSION

Internal electrical faults in transformers are very

serious and cause immediate damage. Short circuits and

ground faults in windings and terminals are normally

detected by variable slope differential protection. Second

and fifth harmonic restraint provides discrimination

against inrush and overexcitation condition. Under inter

turn fault, lesser the number of turns shorted, lesser will

be the current and hence detection through harmonic

analysis is not always feasible. Under such situations

negative sequence restraint provides better sensitivity. A

relay logic combining all these along with voltage

restraint is proposed in this paper. Simulation results have

proved that the relay gives acceptable decisions under

various operating and fault conditions of the transformer.

Addition of DER to a distribution or sub-transmission

system has the potential to impact relay systems well

beyond the point of common coupling. Distribution

systems are basically designed as radial networks and

revision of the existing protection schemes become

necessary with DG integration. Different DG sources

show different characteristics towards their ability to

provide short circuit current needed to trip the relay.

Distribution transformers, in particular face problems like

inrush, ferroresonance, etc. along with CT saturation.

Sympathetic inrush phenomenon also becomes more

evident and sensitivity of the relays becomes important.

Hence the work carried out here will aid in taking

adequate measures for distribution transformer protection

in networks where there is flow of negative sequence

current and intermittency.

REFERENCES

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Pérez, “Effects of Magnetizing Inrush Current on Power Quality

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Short-Circuit Protection for Transformers,” Proc. of 63rd Annual

Conference for Protective Relay Engineers, College Station, TX, March 2010.

[7] A. Guzmán, S. E. Zocholl, G. Benmouyal, and H. J. Altuve,

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[10] H. Khorashadi-Zadeh, “Power Transformer Differential Protection

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Electrical Engineering, IEEE Sep 23-25, 2004.

[11] H. Khorashadi-Zadeh, M. Sanaye-Pasand, “Power transformer differential protection scheme based on wavelet transform and

artificial neural network algorithms,” Proc. of the 39nd

International Universities Power Engineering Conference, 2004, pp. 747-753.

[12] H. Khorashadi-Zadeh, “Transformer Differential Protection Using

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[13] I. Brncic, Z. Gajic, and T. Einarsson, “Transformer Differential

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[14] Russell C. Mason. "The Art and Science of Protective Relaying,"

General Electric, John Wiley & Sons, Jan 1956, ch. 11, pp. 211-239.

[15] M. J. Thompson, “Percentage restrained differential, percentage of

what?”, 64th Annual Conference for Protective Relay Engineers, April 2011, pp. 278-289.

[16] 615 series ANSI technical manual, ABB, July 2011.

[17] S. E. Zocholl, A. Guzmán, D. Hou, “Transformer Modeling as Applied to Differential Protection,” 22nd Annual Western

Protective Relay Conference, Spokane, WA, Oct 24-26, 1995.

[18] K. P. Vittal, “Numeric Protection Relays and Development

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Current-Based Solution for Transformer Differential Protection—

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Part I: Problem Statement,” IEEE Transactions on Power Delivery, Vol. 16, No. 4, Oct 2001, pp. 485-491.

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Delivery, Vol. 23, No. 3, July 2008, pp. 1374-1379.

B S Shruthi was born in Bangalore, in 1989. She

received her Bachelor of Engineering degree in

Electrical and Electronics Engineering from The National Institute of Engineering, Mysore in 2011.

She is currently perusing M.Tech by Research in the

field of Power and Energy Systems at The National Institute of Technology Karnataka (NITK). Area of

Research is Development of Integrated Protection

Schemes for Protection of Transformers Supporting Deregulated Distribution Systems.

She is a Student Member of IEEE along with IEEE – Power

Engineering Society and IEEE – Women in Engineering. Her areas of interest include Power System Protection, Power Transformers,

Distributed Generation and Embedded Systems.

Dr. Panduranga Vittal K was born in Bellary, in 1964. He received his B.E. (E & E) degree from

Mysore University in the year 1985, M.E. (Applied

Electronics) degree in 1989 from PSG College of technology, Coimbatore. Then he earned Ph D.

degree from NITK during the year 1999. Presently,

Dr.Vittal is serving as Professor and Head of Dept. of Electrical & Electronics Engineering, National Inst.

of Technology, Karnataka – Surathkal, Mangalore.

He is a senior member of IEEE, Member of IEEE-Power Engineering Society, Life member of ISTE and IE (India). He has

published 50 technical research papers in various National and

International conferences and 15 papers in International Journals. He has chaired several international conferences in India and abroad.

Recently he has served as chairman for IEEE, ICIIS-2010 an

international conference and NSC-2010 a national conference. He has been principal investigator in various funded projects from Indian

government and University programs of industries. He has guided and

also presently guiding several research scholars for the award Ph.D. degree.

His research areas of interest include power system protection,

power quality and design of embedded systems.

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