negative-sequence based scheme for fault … based scheme for fault protection in twin power...

Resincap Journal of Science & Engineering

Volume 1, Issue 6, July2017

Paper ID: EE6051

166

Negative-Sequence Based Scheme For Fault Protection in Twin

Power Transformer

Ms. Kanchan S.Patil

PG, Student

SSGBCOET, Bhusawal

[email protected]

Prof.Girish K. Mahajan

Associate Professor

SSGBCOET, Bhusawal

[email protected]

Prof.Ajit P. Chaudhari

Associate Professor

SSGBCOET, Bhusawal

[email protected]

Prof.Gaurav P. Tembhurnikar Asst.Professor

SSGBCOET, Bhusawal

[email protected]

ABSTRACT

A power transformer is a vital component in any power

system network and any of its failure may cause disturbance

in the proper operation of electrical power system network.

Negative sequence current flows in transformers primary and

secondary which can be computed by using digital relay along

with their phase differences. Magnitude and phase

information of negative sequence currents can be used to

detect minor transformer windings fault. Negative sequence

current and voltage based algorithms are very useful in the

determination of faults in electrical power systems. Negative

sequence algorithm becomes insensitive when current is not

flowing through the secondary windings of transformer. A

relay system for the protection of transformer using negative

sequence current and voltage is introduced in this paper. The

proposed protection scheme was evaluated with different

cases, which included different numbers of shorted turns of

the transformer, inrush currents. The experimental results

presented in this paper indicate that the algorithm proposed in

this paper is faster and more sensitive and is capable of

detecting turn-to-turn faults occurring during transformer

energization.

Keywords

Negative Sequence Algorithm, Power Transformer.

1. INTRODUCTION The faults occur in a transformer are classified in two types:

external and internal faults. External faults are those that

occur outside of the transformer: over voltage, over-fluxing,

under frequency, and external System short circuits. Internal

faults are those that occur Inside of the transformer: winding

turn-to-turn, turn-to-ground, over-fluxing. From the last few

decades a continuous growth has observed in the power

system and the progress will continue in the upcoming years.

A transformer, being an integrated part of the power system,

is an important link between a generating power station and a

point of power utilization.[1] Due to various kinds of intricate

loads and their control systems, transformer is prone to faults.

Internal winding faults in transformers can cause huge

damages in a very short time, and in some cases the damages

are repairable, and also about 70%-80% of transformer

failures are caused by internal faults. Among these faults,

Winding turn to turn fault is challenging to monitor and

detect, especially at lower magnitude of the fault current.

Internal faults involve a magnitude of fault current which is

low relative to the power transformer base current. This

indicates a need for high speed and high sensitivity to ensure

good protection. According to the IEEE Standard documents,

there is no one standard way to protect all power transformers

against minor internal faults and at the same time to satisfy

basic protection requirements: sensitivity, selectivity, and

speed. The most difficult internal turn-to-turn fault is the fault

which initially involves only a few turns [2].

Turn to turn faults can be calculated by numerous techniques.

Fuzzy logic based techniques are also available to detect turn

to turn faults which involves 16% of transformer windings

[3]. To distinguish between healthy and faulted transformer,

negative sequence algorithm having some assumptions such

as the faults are unlikely to be three phase faults.

Zero sequence current based schemes are also proposed to

detect turn to turn faults [4]. The algorithms used in zero

sequence current protection schemes are very sensitive to

faults and is intended to be used in conjunction with current

differential protection scheme during energization. During

normal operations 1% turn to turn faults can be detected by

using negative sequence current based algorithms [5].

Magnitude and phase difference of primary and secondary

negative sequence current can be used by the algorithms to

detect turn to turn faults.

During energization, the transformer's secondary breaker is

open. Inrush current flows on the primary side of the

transformer while no current flows on the secondary side of

the transformer. Therefore, the phase information of the

negative-sequence current on the secondary side of the

transformer is not useful during energization. [1]Voltage

differential algorithms are used to protect the transformer

from third harmonics voltage which is achieved by negative

sequence algorithm. The transformer protection scheme is



Paper ID: EE6051

167

based on the changes in flux linkages which occur due to turn

to turn faults which is applied to both Y-Y winding

arrangement and Y-∆ winding arrangements. The algorithm

described in [6] was found to operate during transformer

energization and over excitation conditions. The algorithm's

sensitivity was able to detect turn-to-turn faults comprised of

10% of the windings.

Voltages as well as Currents also get affected by the turn to

turn faults. The algorithms introduced in this paper compares

primary and secondary voltage magnitude to detect the turn

to turn faults.[5] Algorithm was developed in order to protect

the transformer during energization and normal operation.

Proposed algorithm was tested using real time simulator.

Relay system is proposed in order to protect the transformer

using real time simulator. The simulated transformer model

was capable of simulating transformer energization current

along with turn-to-turn faults[7].

To recognise value of inrush currents in constructed model

was calculated using iterative process given in [8] and

according to the example given in [9]. The calculations for

inrush currents can be provided for the transformer under

study. A brief discussion on the methods used to detect turn to

turn faults in power transformer is given in section II.

Proposed negative sequence based schemes are discussed in

section III. Section IV describes the relay system and how it

was tested. Section V provides the prototype test results.

2. TURN TO TURN FAULTS

DETECTION For transformer protection, negative sequence current

protection based relays are in use [10]. This relay makes use

of a negative-sequence percentage differential current element

which is calculated by the vector addition of all currents

entering the protection zone. Sensitivity of algorithm is high

which can detect faults involving 2% of the transformer's

windings. The negative-sequence current differential element

blocks the relay during inrush conditions.

Fig:1 Detection of fault using negative sequence currents

The primary and secondary negative-sequence currents along

with their phase differences are used in the relay description

[11] . Figure:1 describes the algorithm visually. Both primary

and secondary negative-sequence current magnitude must be

larger than in order for a phase comparison to occur. If either

or both negative-sequence currents are less than, the phase is

set to 120. This angle is outside the Relay Operating Angle

(ROA) and ensures no trip signal is issued if the negative-

sequence current is too small to obtain an accurate phase

angle. If the negative-sequence currents magnitudes are larger

than , the phase difference between the primary and secondary

negative-sequence currents is examined. This phase difference

must fall within the region described by the ROA.

3. SCHEME BASED NEGATIVE

SEQUENCE

3.1 Scheme based Negative sequence

current A negative-sequence current-based algorithm (NSCA) for

turn-to-turn faults sensing is proposed in [5]. First of all, the

negative sequence current is calculated for both the primary

side and secondary side of the transformer. Form this

calculations we get, two negative-sequence current phasors.

Let Ip and Is denote the negative-sequence current phasors

calculated for the primary and secondary side of the

transformer. The next step of this algorithm is to check the

magnitudes of Ip and Is to ensure that they are both above a

minimum threshold as shown in equation (1) and (2). This is

important not only to prevent false tripping due to minor

imbalances but also to ensure that the phase angle of the

negative-sequence currents are reliable.

│ I2p │> Base current (Primary side) (1)

│ I2s│> Base current (Secondary side) (2)

If the magnitude satisfy above equations then the

phases of Ip and Is are compared.

│ ∠ I2p - ∠ I2

s │< 60o (3)

If equation (3) is satisfied, then there is trip. The current

transformers (CT) are arranged such that negative-sequence

current caused by external faults result in phase differences of

180. Ideally, an internal fault would results in a zero phase

difference. Current Transformer saturation is the main cause

of excursions from the ideal phase difference [5], making it

necessary to allow for a range of angles from 0 to 60o

3.2 Proposed Scheme By using advanced numerical technology, it is now possible to

protect power transformers with new differential protection

principle, which has much higher sensitivity than traditional

transformer differential protection for low-level internal

faults. The basic requirement of the algorithm is to satisfy

phase comparison of equation (1) & (2). Negative sequence

Internal

fault

region

External

fault

region 0o 180

o

90o

270o



Paper ID: EE6051

168

current magnitudes of both primary and secondary currents

must be greater than the threshold values given in equation (1)

& (2). When current is flowing on the primary and secondary

sides of the transformer inrush current can occur during the

removal of a fault or the energization of transformer. When

transformers primary is switch on by keeping secondary open,

current will not flow in secondary windings while inrush

current starts to flow in primary windings causing the

negative-sequence current-based turn-to-turn fault detection

method to block for any severity of fault.

Currents as well as voltages will be affected by the turn to

turn faults occurring in the transformer. Also the

transformer’s turn ratio would be affected by the turn to turn

faults. Due to this voltages between all the phases gets

disturbed causing imbalance between negative sequence

current or voltages in three phase system. When transformer is

loaded the current method of negative sequence fault

detection is effective. But as there is no load current flowing

during energization, this method is blind to turn-to-turn faults.

Voltage exists on the load side of the transformer whether

current is flowing or not. Inrush current affects the terminal

voltages which allow negative sequence voltage algorithm to

react faster than the current differential algorithm. the

negative-sequence voltages for the primary and secondary

side of the transformer can be calculated if the primary phase

voltages and secondary phase voltages are available. The

algorithm for comparing these two negative-sequence

voltages is similar to the differential current algorithm. The

pick-up negative-sequence voltage is set to 1% of the rated

phase voltage.

In order to illustrate how a voltage imbalance is detected, a

single phase transformer with a turn-to-turn fault will be

discussed in detail. It represents one phase of a 3-phase

transformer experiencing a turn-to turn fault. Two scenarios

will be discussed: a turn-to-turn fault on the primary side or a

turn-to-turn fault on the secondary side. A small portion of the

primary windings are shorted causing a small amount of

additional current I2p to be drawn. This does not create a

significant change in ep since the source resistance is assumed

to be low. Therefore the negative- sequence voltage

contributed by the primary side, given by V2p in (4), will be

negligible

[

] =

[

] [

] (4)

The current travelling through the short circuit changes the

mmf contribution of the faulted winding causing a change in

the effective turns ratio from Np to N’p. Therefore the

secondary side contributes a large amount of negative-

sequence voltage, as given by in (6). The two negative-

sequence voltage magnitudes and

are compared in a

manner similar to differential current protection. Equation (4),

(5), (6), and (7) are valid for Y-Y connected transformers.

=

’

Vp (5)

=

’ Vp (6)

[

] =

[

] [

] (7)

Similarly the secondary side turn-to-turn fault will change the

effective turns ratio, from Ns to N’s . A fault on the primary

side causes a decrease in secondary phase voltage while a

fault on the secondary side causes an increase in phase

voltage.

Fig:2 Algorithm for negative sequence current

4. NEGATIVE SEQUENCE

ALGORITHMS First of all primary and secondary side negative sequence

currents are considered. Figure 2 shows the algorithm for

negative sequence currents. After that obtain primary and

secondary side negative sequence current magnitude and also

phase. If negative-sequence current exists only on the primary

side of the transformer but not on secondary side, the primary

side of the transformer is being energized. For transformer

discrete Fourier transform and negative sequence transform

on the primary and secondary side three phase currents was

obtained. Here only the magnitude of the primary and

secondary negative-sequence current is considered. Both the

primary negative-sequence current and the secondary

negative-sequence current, transformed to the primary side of

the transformer, must be larger than 1% of the rated primary



Paper ID: EE6051

169

current. This prevents erroneous tripping due to small

imbalances.

An energization on the secondary side of the transformer also

results in the selection of the negative-sequence voltage

algorithm. Figure 3 shows the algorithm for negative

sequence voltages. the magnitude of the primary and

secondary negative-sequence voltage is of interest in this case.

Notice that the secondary negative-sequence voltage is

transformed to the primary side of the transformer. Both the

primary negative-sequence voltage and the secondary

negative-sequence voltage, transformed to the primary side of

the transformer, must be larger than 1% of the rated primary

voltage which prevents erroneous tripping due to small

imbalances in transformer. The restraining and differential

voltages are calculated using (8) and (9). If the differential

voltage exceeds the restraining voltage equation, a trip is

warranted.

Vr = (Average of primary and secondary voltage magnitude) x

(nominal turns ratio) (8)

Vd = (Difference of of primary and secondary voltage

magnitude) x (nominal turns ratio) (9)

IEEE standard [13] requires that the transformer winding

voltages, at no load, be within 0.5% of the nameplate voltage.

If a 0.5% imbalance is introduced to the otherwise healthy

system, it produces a negative-sequence voltage well below

the threshold.

Fig: 3 Algorithm for negative sequence voltages

5. SIMULATION RESULTS

Fig:4 Voltage and current waveform of transformer on

primary side when 2% windings are shorted

Fig:5 Voltage and current waveform on secondary side of

transformer

Fig:6 Pattern for inrush currents in three phases of



Paper ID: EE6051

170

transformer when 2% windings are shorted on primary

side

Fig:7 Voltage and current waveform of transformer on

primary side when 20% windings are shorted

Fig.8 Voltage and current waveform on secondary side of

transformer

Fig:9 Pattern for inrush currents in three phases of

transformer when 20% windings are shorted on primary

side

Figure 4 shows voltage and current waveforms of transformer

when primary side of transformer is shorted by 2%. Voltage

waveform are shown by upper part and lower part shows

current waveform. Normal operation of transformer is seen till

1second, at the instant of 1 second fault occurs which causes

disturbance in the operation of power transformer. But when

only 2% turns are shorted during turn to turn faults, faults

could not be detected properly and hence there is no trip at

instant of 1second, it is clearly seen in figure 4, 5, 6. Figure 5

shows voltage and current waveform of transformer on

secondary side. Pattern for inrush current is shown in figure 6

when 2% winding are shorted. Inrush currents for all three

phases of transformer on primary side is shown in figure.

Figure 7 shows Voltage and current waveform of transformer

on primary side when 20% windings are shorted. Primary side

of transformer is taken into consideration. Occurrence and

clearance of fault during instant of 1 sec is clearly shown in

figure 7. Fault is cleared within 0.02 seconds in power

transformer when 20% windings are shorted on primary side .

Voltage waveform are shown by upper part and lower part

shows current waveform. The changes in magnitude of

current during, after and before occurrence of fault is to be

noted. Figure 8 shows voltage and current waveforms on the

secondary side of power transformer. The voltage and current

waveforms are shown during energization of power

transformer. The waveforms can be seen before, after and

during energization of the primary side of transformer. Pattern

for inrush currents in all the three phases of transformer is

shown in figure 9 when 20% windings are shorted on primary

side. At the instant of one second fault occurs and cleared

within a very short period of 0.02 seconds and after words

pattern becomes uniform again. The working of transformer

can be observed before occurrence of fault, during fault and

after clearing the fault.

6. CONCLUSION In this paper, an efficient protection scheme based on negative

sequence currents for detecting minor internal turn to-turn

faults in power transformers was described. The proposed

scheme is simple to implement The proposed protection

scheme was evaluated with different cases, which included

different numbers of shorted turns of the transformer,

different values of the system parameters, different

connections of the power transformer, and the inrush current.

By using the proposed algorithm turn-to-turn faults involving

5% of the transformer's windings were detected. The

negative-sequence-based algorithm was seen to be more

sensitive and faster than the current differential algorithms.

The reliability of protection system using negative sequence

algorithm is increased.



Paper ID: EE6051

171

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negative-sequence based scheme for fault … based scheme for fault protection in twin power...

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