Differential Protection Scheme for Power Transformer Fault ...

Differential Protection Scheme for Power

Transformer Fault Analysis using PSCAD

Priyadharshini M 1*, Gayathri K 2

1Assistant Professor, Department of Electrical Engineering, Annamalai University,Annamalai Nagar, Tamil Nadu, India. 2 Assistant Professor, Department of Electrical Engineering, Annamalai University,Annamalai Nagar, Tamil Nadu, India.

Abstract— A power transformer is the most expensive device in the electrical systems. The transformer failure would result in huge

economic loss and unexpected outage of power system; hence a maintenance mechanism is essential to prevent the transformers from

failures. Components may fail due to poor maintenance, poor operation, poor protection, undetected faults, severe lightening, short

circuits, etc. This paper deals with internal faults in large power transformer. The explanation of such faults is followed by the

description of an analytical method of Differential protection scheme. The Differential protection for power transformer is used to

extract the feature from transient signal to accurately identify the fault in the input side of three phases, winding fault and

magnetizing inrush current. Effectiveness of proposed approach tested on a three phase 100 MVA, 33/230 kV, 60Hz, power

transformer is modeled in PSCAD/EMTDC software . The simulation results clearly show that the proposed scheme is reliable,

accurate and faster than the conventional approach.

.

Keywords— Differential Protection, Power Transformer, Magnetizing Inrush current, Fast Fourier Transform, PSCAD.

I. INTRODUCTION

Transformers are essential and important elements of power systems. In the past few years, there has been an increasing

concern about the occurrence of faults in power transformers due to the abnormal operation. It is not always possible to analyze

the transformer behavior under such faults under rated conditions, since the tests are largely destructive. Therefore, in order to

avoid severe damage to Transformer, mathematical models are used for the analysis [1].. Transformer Inrush current may trigger

the over-current protection of power transformers, and the associated power distribution system may get affected. Over sizing the

transformer is a common approach to avoid the inrush current. However, this would dramatically increase the size and cost of

transformer, therefore this project proposes a new technique for isolating the inrush of the transformer and preserving the ability

of the projection circuit [2].

The basic operating principle of differential protection is to calculate the difference between the current entering and leaving

the protected zone. The phenomenon occurred during removal of external through fault t.3]. The protection operates when the

differential current exceed the set bias threshold value. For external faults, the differential current should be zero but error caused

by the CT saturation and CT ration error leads to non-zero value.. The operating threshold is raised by increasing the relay setting.

Mal-operation of the differential protection of power transformer may occur due to magnetizing inrush current and CT saturation

through fault inrush among all these three magnetizing inrush results during excitation of transformer under no load condition. It

can also come during the energization of parallel connected power transformer [4].

Many researchers have proposed various protection schemes power transformers. Ouahdi Dris Farag. et. Al [5] have been

proposed a differential protection using second harmonic restrain and fifth harmonic block schemes for power

transformer protection is presented. Initially analysis the concept of differential protection, then investigate harmonics restrain

scheme and microprocessor based-protection on power transformer differential protection has been developed. Manoj Tripathy et.

al [1] has been proposed Wavelet and Neural Network based power transformer fault analysis. This approach extracted the

feature transient signal and the neural network has been trained by the extracted features of the transient signal. This method

accurately discriminates between the internal fault and magnetizing inrush current.

The Feed forward Back propagation Algorithm (FFBPN) has been applied to identify the transformer faults by. Raja Pandil T

et. al [6]. This methodology addresses the challenging task of detecting magnetizing inrush from internal fault. The algorithm is

evaluated using simulation performed with MATLAB and analysis of power transformer from differential protection using back

propagation neural algorithm.

JASC: Journal of Applied Science and Computations

Volume 5, Issue 11, November/2018

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Stanley E et.al [9] develops a power transformer model to evaluate differential element performance. In this study analyzes

transformer energisation, over excitation, external fault, and internal fault conditions with this model..

Mudita Mathur and Vinay Barhate [8] develops application of artificial neural network based pattern recognition

approach for differential protection of transformer. The power transformer protective relay should block the tripping during

magnetizing inrush and rapidly initiate the tripping during short circuit.ANN based technique for discrimination between

magnetizing inrush and internal fault currents of transformer. Artificial neural network has been proposed and demonstrated by

Paraskar S P [7]. This approach has capability of solving the transformer monitoring and fault detection problem. The proposed

ANN provides an inexpensive, reliable, and noninvasive procedure. to determine inrush fault in the transformer

This article presents a power Transformer unit fault analysis strategy using differential protection approach. This proposed

approach improves the fault identification scheme of power Transformer unit. This approach efficiently analysis the input of the

Transformer unit under fault condition and determine the inrush current fault and winding fault. The simulations are carried out

using PSCAD software.

II. TRANSFORMER MATHEMATICAL MODEL

A transformer is a static electrical device that transfers energy by inductive coupling between its winding circuits. A varying

current in the primary winding creates a varying magnetic flux in the transformers core and thus a varying magnetic flux through

the secondary winding [1]. This varying magnetic field at the secondary induces a varying electromotive force (emf).

One of the most important and complex system that has been built by human civilization is the power system. The electric

power system plays a key role in modern society. One of the most important components of any power system is the power

transformer.

The development of the first power transformer significantly changed transmission and distribution systems. Before the

power transformer was invented in 1885 by William Stanley, power was produced and distributed as direct current (DC) at low

voltage. The use of electricity was only limited to urban areas because of voltage drop in the lines. Also, the use of DC voltage

would require the supply voltage to be the same as the supply voltage required by all electrical equipment connected to the power

system. However, it is quite difficult to transform DC power to a lower current and high-voltage form efficiently. The alternating

current (AC) system is able to overcome the limitation of the DC system and distributes electricity efficiently over long distances

to consumers. The use of the AC system allows the use of a multi-voltage level energy delivery system. The AC power, generated

at a low voltage, can be stepped up by using the power transformer for the transmission purpose to higher voltage and lower

current. Thus, voltage drops and transmission energy losses can be reduced. The transformer is the link between the generator of

the power system and the transmission lines. A power transformer is defined in reference as a static piece of apparatus with two or

more windings which by electromagnetic induction, transforms a system of alternating voltage and current into another system of

voltage and current usually of different values and at the same frequency for the purpose of transmitting electrical power.

Fig. 1 Transformer Equivalent Circuits

Where:

VP - primary voltage

VS - secondary voltage

IP - primary current

IS - secondary current

RP - primary reactance

RS - secondary reactance



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XP - primary impedance

XS - secondary impedance

NP - number of primary winding

NS - number of secondary winding

IC - current coil

RC - reactance current

IM - magnetizing current

XM - impedance magnetizing

I0 - no load current

A. Basic transformer model

Fig. 2 shows a simple single-phase shell-type transformer with two windings [9].We used a Transformer bank with

three single-phase transformers for testing and modeling purposes. The total flux in Winding 1 is the sum of the mutual flux

(Ф) plus the Winding 1 leakage flux (Ф11). The sum of the mutual flux (Ф) plus the Winding 2 leakage flux (Ф22) determines the

total flux in Winding 2. The following expressions determine the relationship between voltages and mutual flux in the transformer

core.

TNTILIRE .11.11.11 (1)

TNTILIRE .22.22.21 (2)

.2.1.1. NPINP (3)

Fig.2 Single-Phase Two-Winding Transformer

Where;

E1 winding 1 input volt R1 winding of 1 resistance

E2 winding 2 input volt R2 winding of 2 resistance

L1 winding 1 leakage inductance ∆Ι incremental current

L2 winding 2 leakage inductance ∆Φ incremental 1 magnetic flux

I1 winding 1 current ∆Τ incremental of time

I2 winding 2 current N1 winding 1 number of turn

p core permeance N2 winding 2 number of turn

The ratio I / times N determine the leakage inductance (for example, 1

11

I

times N1 determination the

winding 1 leakage inductance) equation 4 shows the matrix representation of equations 1,2 and 3.



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01

0

0

222

111

1

21

2

1

2

1

2

1

IRE

IRE

NPNP

NL

NL

T

T

IT

I

(4)

Winding 1 and 2 voltages are the input quantities to the transformer model. We want to Determine the current

values for different transformer operating conditions [7]. The first matrix in right term of equation 4 is the coefficient matrix

equation 5 is the matrix representation of the incremental values of winding 1 and winding 2 currents and the incremental value

of the mutual flux.

All terms in the coefficient matrix have fixed values except the permeance P. The following expression determines the

presence of a given transformer core:

III. TRANSFORMER DIFFERENTIAL PROTECTION

Differential protection is a unit type protection for a specified zone (or) piece of equipment [1]. It is the based on the fact that it

is only in the case of faults internal to the differential currents (difference between input and output current will be high).

Fig. 3 Simple Diagram for Differential Power Transformer Protection

Percentage restraint differential protective relays have been in service for many years. Fig. 3 shows a typical differential

relay connection diagram. Differential elements compare an operating current with a current. The operating current (also called

differential current), Id, can be obtained from the phaser sum of the currents entering the protected element Id is

proportional to the fault current for internal faults and approaches zero for any other operating (ideal) conditions. There

are different alternative for obtaining the restraining current IRT. The most common ones include the following differential

relays perform well for external faults, as long as the CTs reproduce the primary currents correctly. When one of the CTs

saturates, or if both CTs saturate at different levels, false operating current appears in the differential relay and could

cause relay male-operation.

A. Current Differential Relay

The relay consists of three differential elements. Each differential element provides percentage restrained differential

protection with harmonic blocking and unrestrained differential protection [2].



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Fig. 4 Current Differential Relay

The input currents are the CT secondary currents from Winding 1. The relay reduces the magnitude of these currents and

converts them to voltage signals. Low-pass filters remove high-frequency components from the voltage signals. The three

differential elements use the compensated currents from Winding 1 and as inputs to their logics. For example, the Differential

Element 1 uses the compensated currents I1W1F1 and I1W2F1.

IV. RESULT AND DISCUSSION WITH CASE STUDY OF POWER TRANSFORMER DIFFERENTIAL PROTECTION

An important contribution of the modeling of power transformer is magnetically represented in independent phases. The

principle of duality has been used to take into account the magnetizing currents and core configuration when modeling multi-limb

transformers with the EMTP program. Based on these models PSCAD MASTER Library supports transforms for transient

studies, the same is used to develop the simple power systems given in Fig.5. PSCAD/EMTDC is the simulation tool for

analyzing power system transient developed by Manitoba HVDC Research Centre. Currently version of PSCAD-EMTDC

provides magnetically independent phase models also. Various types of winding to ground (Body of Transformer) have been

investigated; the transients for these cases and the protection system response for the same are given in Fig. 6 and 7.

Fig.5 Transformer One Line Diagram

The Protection system performance under different numbers of shorted turns of power transformer, different connections of the

transformer, different values of the fault resistances, and different values of the system parameters was investigated. The results

indicate that the new technique can provide a fast and sensitive approach for identifying minor internal turn-to-turn faults in

power transformers.

Table 1 Specifications of Transformer

Transformer rating 100 MVA

Source type RL

Primary Transformer 33KV

Secondary Transformer 230KV

Primary Winding Star

Secondary Winding Delta

Frequency 60HZ

Leakage reactance 0.1



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Fig. 6 Proposed simulated model Transformer in R Phase Fault

The system under study is a single machine power system connected through a three phase power transformer. A three

phase transformer bank was constructed using three single phase transformers, each of 100 MVA, 33/230 kV. The simulated

power system model is depicted in Fig.6. To validate the model, the steady state operation and the transient behavior during faults

were compared with theoretical calculations.

Case (i) R Phase

The voltage and current transients due to magnetizing inrush current are simulated in PSCAD using simulink models.. In

this model a timed circuited breaker is added, the same is closed at T=1.0 sec it causes inrush current due to energization of

transformer under R phase fault. The fault should be create at T=2.0 sec at tripping signal.

The shape of the validation plot and test plot are the performance in smooth over the wide range of sample points. The

steady-state operation and transient during fault current flow (at-1.0kA to 1.0kA) and voltage flow (-350 kV to 350 kV).

This process signals represented the first, second and third order harmonics to identifying the fault signal. The 1st

Harmonic range is higher than the 2nd harmonics upto 2.00 amp 3rd harmonic range is lower than the 2nd harmonics upto 0-10amp

in Fig. 6.

For the mho relay scheme given in Fig. 7 and 8 the intersection of Rab.Rbc & Rca on the mho cicle is a indication of

fault. Not intersection of Rab, Rbc &Rca on the mho circle is a indication of no fault. The graph is plotted with time on x-axis

with 1 second difference and on y-axis graph is plotted as trip, fault, current and voltage in graph;

Similarly

1unit=1second –time

1unit=1(KA) -current

1 unit=100(KV) –voltage

Fig. 7Current and voltage of R phase Fault



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The power transform with difference vector on over voltage side the signal is injected to differential relay may be brought from

the ideal current transforms. During the fault the relay should trip the breakers. The current is taken as input signal the processing

system and final trip signal is used to control the Three phase breaker. As carried out for R-phase fault winding faults has been

simulated.

Fig. 8 Processed signals

Fig. 9 Transformer fault in R Phase Fault

For the mho relay scheme given in Fig. 9 the intersection of Rab, Rbc and Rca on the mho circle is a indication of fault.

Fig. 10 show not intersection no fault

Fig. 10 Transformer no fault in R Phase



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Case (ii)Y Phase

The voltage and current transients due to magnetizing inrush current are simulated In PSCAD semolina model and shows

Fig. 11. In this model a timed circuited breaker is added, the same is closed at T=1.0 sec it causes inrush current due to in

energization of transformer under Y phase fault. The fault should be create at T=2.0 sec at triping signal.


steady-state operation and transient during fault current flow at -1.0 kA to 1.0 kA and voltage flow -350 kV to 350 kV.

This process signals represented the first, second and third order harmonics to identifying the fault signal. The 1 st


in Fig. 12.

For the mho relay scheme given in Fig. 13 and 14 the intersection of Rab. Rbc & Rca on the mho cicle is a indication of

fault. Not intersection of Rab, Rbc & Rca on the mho circle is a indication of no fault. The graph is plotted with time on x-axis


Fig. 11 Current and voltage of Y Phase Fault

The power transform with difference vector on over voltage side the signal is injected to differential relay may be brought from

the ideal current transforms. During the fault the relay should trip the breakers. The current is taken as input signal the processing

system and final trip signal is used to control the Three phase breaker Fig. 13 and 14. As carried out for Y-phase fault winding

faults has been simulated.

Fig. 12 Process signal graph



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Fig. 13 Transformer fault in Y Phase

Fig. 14 Transformer no fault in Y Phase

Case (iii) B Phase

The voltage and current transients due to magnetizing inrush current are simulated. In PSCAD software model and shows in fig

15. In this model a timed circuited breaker is added, the same is closed at T=1.0 sec it causes inrush current due to energization of

transformer under B phase fault. The fault should be create at T=2.0 sec at tripping signal.


steady-state operation and transient during fault current flow (at-1.0KA to 1.0KA) and voltage flow (-350 KV to 350 KV).


Harmonic range is higher than the 2nd harmonics up to 2.00 amp 3rd harmonic range is lower than the 2nd harmonics up to 0-

10amp(in Fig. 15).

For the mho relay scheme given in fig 16 and 17 the intersection of Rab. Rbc & Rca on the mho circle is a indication of



The power transform with difference vector on over voltage side the signal is injected to differential relay may be brought

from the ideal current transforms. During the fault the relay should trip the breakers. The current is taken as input signal the

processing system and final trip signal is used to control the Three phase breaker. As carried out for B-phase fault winding faults

has been simulated.



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Fig. 15 Current and voltage of B Phase Fault

Fig. 16 Process signal graph

Fig. 17 Transformer fault in B Phase



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Fig. 18 Transformer no fault in B Phase

Case (iv) winding phase

The voltage and current transients due to magnetizing inrush current are simulated. In PSCAD using simulink models. These

shows in Fig. 19. In this model a timed circuited breaker is added, the same is closed at T=1.0 sec it causes inrush current due to

energization of transformer under winding phase fault. The fault should be create at T=2.0 sec at triping signal.


steady-state operation and transient during fault current flow (at-1.0kA to 1.0kA) and voltage flow (-350 kV to 350 kV).



in Fig. 20.

For the mho relay scheme given in Fig. 21 and 22 the intersection of Rab.Rbc & Rca on the mho circle is a indication of



Fig. 19 Current and voltage of Winding Fault

The power transform with difference vector on over voltage side the signal is injected to differential relay may be

brought from the ideal current transforms. During the fault the relay should trip the breakers. The current is taken as input signal

the processing system and final trip signal is used to control the Three phase breaker. As carried out for winding-phase fault

winding faults has been simulated.



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Fig. 20 Process Signal graph

Fig. 21 transformer fault in Winding

Fig. 22 Transformer no fault in Winding



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Case (v) Magnetic Inrush

Since the magnetizing branch representing the core appears as a shunt element in the transformer equivalent circuit, the

magnetizing current upsets the balance between the currents at the transformer terminals, and is therefore experienced by the

differential relay as a “false” differential current. The inrush currents simulated in PSCAD is given Fig. 23. It is not able for

protection scheme designed to discriminate inrush from fault currents.

Magnetizing inrush current in transformers results from any abrupt change of the magnetizing voltage. Although usually

considered as a result of energizing a transformer, the magnetizing inrush may be also caused by :

Occurrence of an external fault,

Voltage recovery after clearing an external fault,

Change of the character of a fault (for example when a phase-to-ground fault evolves into a phase-to- phase-to-

ground fault).

Out-of-phase synchronizing of a connected generator.

Certain transients can cause a substantial differential current to flow, when there is no fault, and these differential

currents are generally sufficient to cause a percentage differential relay to trip. However, in these situations, the differential

protection should not disconnect the system because it is not a transformer internal fault. The inrush is one such phenomenon

which happens in the transformer. It has been shown by simulations that the proposed differential protection scheme is able to

protect system from damages under various possible types of faults.

Fig. 23 Phase currents Magnetic Inrush

Fig. 24 Process Signal Graph



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Fig. 25 Transformer fault in Magnetic Inrush

V. CONCLUSIONS

In this research work a magnetic equivalent circuit (UMEC) of the three-limb three-phase transformer core type has been

developed. The arbitrary division of magnetizing current as well as uniform core flux assumptions and lumped leakage reactance

approximations are removed using the UMEC concept. This transformer model provides enough accuracy for differential

protection study purposes. Transformer models without hysteresis modeling reduce model complexity and minimize simulation

time. Relay logic and the algorithm that uses Discreet Fourier transformer for extraction of fundamental and higher harmonics

components of differential current have been reported here. The use of digital protection offers the advantage to implement

complexes algorithms such as DFT to ensure better extraction of fundamental and other harmonics components, then the use of

the second and the fifth harmonics for restraining and blocking, by the differential protection will give a possibility to

discriminate between the faulty and the normal state of power transformer.

The PSCAD/EMTDC software can be successfully used to modeling the disturbances with transformers protection scheme.

It has been shown using PSCAD simulations that Digital current differential relays provide fast and reliable transformer

protection. Further research will focus on the analysis of the extracted FFT signals (Waveforms saved to COMTRADE files)

using soft computing tools.

ACKNOWLEDGMENT

The authors gratefully acknowledge the authorities of Annamalai University for the facilities offered

to carry out this work.

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