J. Energy Power Sources Vol. 4, No. 1, 2017, pp. 9-15 Received: February 15, 2017, Published: March 25, 2017

Journal of Energy and Power Sources

www.ethanpublishing.com

Security Enhancement of Differential Protection of

Power Transformers Based on Second Order Harmonics

Usama Khaled1, 2, Mohamed Qais1, Saad Alghuwainem1 and Abderrahmane Beroual3

1. Department of Electrical Engineering, King Saud University, Riyadh 11421, Saudi Arabia

2. Department of Electrical Engineering, Aswan University, Aswan 81528, Egypt

3. Ampere CNRS UMR, Ecole Centrale de Lyon, Ecully 69134, France

Corresponding author: Usama Khaled (ukhaled@ksu.edu.sa)

Abstract: Differential relays are designed to protect the PTs (power transformers) from total damage when faults occur inside the PTs. Differential protection security of PTs is affected by external disturbances such as inrush currents, external faults, and over-excitation which cause undesired PT outage. In this work, a reliable algorithm is presented to improve the speed and security of PT protection. FFT (fast Fourier transform) method is used for taking out the second order harmonic of measured currents by CTs (current transformers) of the primary side and secondary side of PTs. The algorithm produces two signals: operating signal to disconnect PT during internal faults; and restraint signal to block the undesired action of the differential relay through external disturbances. The algorithm is inspected using PSCAD/EMTDC, where various abnormal conditions internal faults and external disturbances are simulated. The achieved results reveal that the proposed algorithm enhanced the speed and security of differential relay with simplest signal processing method compared to other techniques. Keywords: Power transformers, differential protection, harmonics, PSCAD/EMTDC, current transformer.

Nomenclature

Idiff Differential current (A)

Ips Primary current seen by relay (A)

Iss Secondary current seen by relay (A)

I2nddiff Second order harmonic of differential current (A)

I2ndps Second order harmonic of primary current seen by

relay (A)

I2ndss Second order harmonic of secondary current seen

by relay (A)

Ipickup Pickup current (A)

i(n) Discrete signal of current (A)

n Number of samples

N Periodic number of samples

Idiff1 The RMS fundamental component of differential

current (A)

Idiff2 The RMS second harmonic of differential current

(A)

I2nd The sum of RMS second harmonic of primary and

secondary currents (A)

1. Introduction

PTs (power transformers) are an essential and vital

link between two levels of voltages in the electrical

systems. The faulted transformers should be

de-energized as soon as possible to avoid permanent

damage. Protection of power transformers is the most

challenging problem in the power system relaying,

due to its importance and high cost. However, the

energization of transformer not only impacts the

differential protection of the incoming transformer but

also imposes a threat to the other electrical equipment

which has already been operating in the system. The

existence of inrush current due to the energization of

power transformers and over-excitation due to

overvoltage affect the security of differential relay.

Security Enhancement of Differential Protection of Power Transformers Based on Second Order Harmonics

10

The protective relays should avoid the undesired

operation due to external faults, over-excitation and

inrush currents.

Many researchers concerned with the security

enhancements of differential relays to prevent

permanent outage of power transformers. Kennedy [1]

used the harmonic components as a restraint for

electromechanical relays to prevent the false operation

of differential relays on the unbalanced currents due to

any reasons. Some authors have proposed new

numerical algorithms and techniques, excluding the

principle of harmonics restraint to satisfy most of the

design criteria of differential relays such as speed,

security, selectivity, etc. Discrete wavelet transform

method [2-6] is used to extract the features of

differential current to discriminate between internal

short circuits and inrush currents or external short

circuits. Fuzzy logic algorithms [7, 8] are employed in

differential relays to distinguish between inrush

currents and internal faults. ANNs (artificial neural

networks) based methods [9, 10] are used to detect the

inrush currents, CT (current transformer) saturation,

and over excitation to avoid undesired operation of

differential relays. The statistical calculations such as

calculating the differential current gradient [11, 12]

are used to identify the inrush currents.

Other researchers proposed different schemes to

determine the inrush currents, such as phase angle

difference method [13]. The relationship of the PS

(power spectrum) of second harmonic to the PS of

fundamental based on the autoregressive process is

used for inrush identification [14]. Based on the

magnetic characteristics, Lin et al. [15] proposed filter

algorithm to distinguish the external faults from

internal faults. Khalkhali et al. [16] proposed a new

method based on the developed polynomial model for

magnetic characteristics where the average fault

detection delay is 16 ms (in the 50 Hz base) with

accuracy above 97%. Time-time transform is applied

to fault diagnosis of power transformers as revealed in

Ref. [17]. Ramachandraiah et al. [18] used active

band-pass filters to detect the second order harmonic

ratio to avoid tripping during inrush current.

In this paper, FFT (fast Fourier transform)

technique is used to extract the second order

harmonics of secondary currents of CTs located in the

primary and secondary sides of PTs. FFTs technique

is straightforward and reliable method compared to

other methods used for signal processing.

2. The Principle Operation of Differential Power Transformer Protection

Schematic diagram of differential protection of Y-Δ

power transformer is shown in Fig. 1. Current

transformers CTs are connected in opposite manners

(Δ-Y) to eliminate the 30° phase shift between

primary and secondary currents. Electromechanical

relays that are shown in Fig. 1, consist of operating

coil which is responsible for sending a trip signal to

CBs (circuit breakers) during internal faults; and bias

windings which produce electromagnetic force

opposes those generated by operating coil then no trip

signal will be sent during external disturbances.

3. The Concept of Proposed Algorithm

In new numerical relays, many algorithms with

different signal processing techniques are proposed

according to the principle operation of differential

protection of power transformer. In this research, a

new algorithm is proposed for the differential

protection of power transformer as shown in Fig. 2.

Firstly, differential relays receive primary and

secondary current signals that are measured by CTs

Fig. 1 Schematic diagram of differential protection of power transformer.

Security Enhancement of Differential Protection of Power Transformers Based on Second Order Harmonics

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Fig. 2 Proposed algorithm for differential protection of power transformer.

then the differential current is calculated as in Eq. (1).

diff ps ssI I I (1)

Secondly, FFT is used to extract the second order

harmonic from discrete current waveforms as in Eq.

(2).

2 4 /ndn

I i n e j n N

(2)

The decision of differential relay to send trip signal

to the circuit breaker depends on Eqs. (3) and (4).

diff pickupI I (3)

2 2 21

1.2nd nd nd

diff ps ssI I I (4)

During the internal fault, the differential current Idiff

will be greater than the pickup current Ipickup and the

second order harmonic of Idiff is equal the sum of

second order harmonic of Ips and Iss as in Eq. (5) then

the trip signal will be sent to CBs as shown in Fig. 2.

2 2 2 = nd nd nddiff ps ssI I I (5)

During external disturbances such as external fault,

inrush current or over-excitation, the differential

current Idiff could be greater than the pickup current.

But the second order harmonic of Idiff will be smaller

than the sum of second order harmonic of Ips and Iss as

in Eq. (6), and then the relay is restrained from the

undesired operation: 2 2 2nd nd nddiff ps ssI I I (6)

4. Methodology Simulation

The proposed algorithm is applied to protect three

winding (tertiary) power transformer in a national

electrical grid of Republic of Yemen. Tertiary

transformer 400 kV-Y/132 kV-Y/33 kV-Δ, 600 MVA

is simulated using PSCAD as shown in Fig. 3, where

this tertiary transformer is the largest transformer in

the Yemeni grid and considered the most important

part. The 400 kV and 132 kV transmission system of

Yemeni network are modeled by PSCAD/EMTDC,

where the number of 132 kV transmission lines is 19

and one 400 kV transmission line. Also, five major

power plants are modeled with their step-up power

transformers. CTs ratios are assumed to be 1000:5 at

400 kV side, 3000:5 at 132 kV side, and 12000:5 A at

33 kV side. The proposed algorithm is simulated using

PSCAD as shown in Fig. 4. Idiff1 is the RMS

fundamental component of differential current, Idiff2 is

the RMS second harmonic of differential current, I2nd

is the sum of RMS second harmonic of primary and

secondary currents of power transformer seen by the

relay, and Ipick is the pickup current and assumed as a

constant (0.5 A).

5. Results and Discussion

5.1 Inrush Current

The complex inrush current, including the

magnetizing inrush current and the sympathetic inrush

current, is considered to be relevant with transformer

switching-on of mal-operation. It is disclosed that the

Security Enhancement of Differential Protection of Power Transformers Based on Second Order Harmonics

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Fig. 3 Differential protection layout of tertiary transformer.

Fig. 4 PSCAD modeling of proposed scheme.

CT transient saturation or CT local transient saturation,

caused by the long-lasting aperiodic component from

the magnetizing inrush current, leads to the

mal-operation [18]. In this section, the burden of CTs

is set to significant value 5 Ω to make CTs operate

under stressed conditions. The inrush current occurs

during the energization process of transformers; then

the conventional differential relay sends a trip signal

to the circuit breakers CBs because differential current

Idiff1 will be greater than the pickup current (0.5 A) as

shown in Fig. 5. The proposed scheme did not send a

trip signal because the sum of RMS second harmonic

for both primary and secondary currents I2nd is greater

than 1.2 times the RMS second harmonic of

differential current Idiff2. Hence, the proposed scheme

is secure during the starting conditions of power

transformers.

5.2 Internal Fault

In this part, the proper operation of transformer

differential protection is reproduced by

PSCAD/EMTDC based simulation tests for deeper

and more useful analysis. Either symmetrical or

Fig. 5 Tripping during inrush current.

asymmetrical faults could be applied to the tertiary

power transformer as an internal fault having different

values of ground resistance. Fig. 6 indicates that once

the internal fault is initiated at time 1 s, the proposed

scheme send a trip signal at 1.005 s. That means the

protective relay starts operating only after 5 ms,

equivalent to a quarter of 50 Hz cycle, of elapsed time.

5.3 Turn-to-Turn Fault

In this section, turn-to-turn fault of single phase

transformer is simulated in PSCAD by Juan Carlos

Garcia [19]. The leakage impedances between

windings of studied transformer are calculated using

finite element type program. Turn to turn fault is

incepted in High voltage side turns of windings where

10% of turns are short-circuited. According to IEEE

standard C37.91-2000, fewer turn to turn fault harder

to be detectable. Fig. 7 shows that the proposed

scheme sends trip signal during the turn-to-turn fault.

5.4 External-Fault

In this section, an external fault is applied to the

closest location to the tertiary power transformer. Fig.

8 shows that the conventional differential relay will

send trip signal during the external fault case. On the

Security Enhancement of Differential Protection of Power Transformers Based on Second Order Harmonics

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Fig. 6 Tripping during internal fault.

Fig. 7 Tripping during turn to turn fault.

contrast, the proposed scheme will not send trip signal

during external fault because the sum of RMS second

order harmonic of primary and secondary currents of

tertiary transformer I2nd seen by relay will be greater

than 1.2 times the RMS second order of differential

current Idiff2. According to the results, the proposed

method distinguishes between the internal fault and

other abnormal fault conditions.

Fig. 8 Tripping during external fault.

5.5 Overexcitation

The differential current of the transformer, which is

obtained from the secondary current of CTs, consists

of two parts, the error current due to CT overexcitation

and the magnetizing current of the transformer.

Transformer overexcitation is mainly caused by

overvoltage in the power system. In this section, the

lightning stroke, with 1.2/50 μs and peak current 100

kA, hits the substation (close to the transformer), it

induces overvoltage in the system. Fig. 9 shows that a

lightning stroke hits the line conductor (phase

conductor) directly at 1 s. The induced overvoltage

causes overexcitation to the tertiary transformer as

illustrated in Fig. 10. The conventional differential

relay will detect the overexcitation as an internal fault,

and then send a trip signal. On the other hand, the

proposed scheme will not send a signal which means

that the proposed scheme is immune to the

overexcitation.

6. Conclusions

A new and straightforward algorithm to enhance the

security and speed of differential protection of power

transformers is proposed in this research. The proposed

Security Enhancement of Differential Protection of Power Transformers Based on Second Order Harmonics

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Fig. 9 Lightning surge and transient overvoltage.

Fig. 10 Tripping during over-excitation.

algorithm uses FFT technique to extract the second

order harmonic of primary, secondary and calculated

differential currents. The new algorithm has two

conditional terms to make a trip decision: Firstly,

calculates the differential current and compare it with

pre-set or pickup value; secondly, compares the

second order harmonic of differential current with the

sum of second order harmonic of measured primary

and secondary currents of the power transformer.

400/132 kV, 50 Hz Yemeni grid is simulated using

PSCAD then the proposed algorithm is investigated

and applied to the tertiary transformer. The obtained

results reveal that the proposed algorithm operates in

the presence of internal faults and secure to the

external disturbances such as inrush current, external

fault, and over-excitation. The main impact of this

algorithm is its simplicity and reliability because it

uses only FFT technique and the response to the

internal fault is faster which functions within a quarter

of cycle only.

Acknowledgment

The authors extend their appreciation to the ISPP

(International Scientific Partnership Program) at King

Saud University for funding this research work

through ISPP#0047.

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