d iscrimination of magnetic inrush current from fault

Discrimination of magnetic inrush current from fault

current in transformer

- A new approach

Yashasvi Tripathi 1, Kushagra Mathur 2, Dr S.V.N.L. Lalitha 3,

Dr M. Ramamoorty 4

Department of EEE, KL University Green fields, Vaddeswaram, Guntur, AP,

522502, INDIA

[email protected],[email protected],

[email protected], [email protected]

Abstract

It is a challenge for the power engineers all around the globe to find a fast and accurate method

of discriminating magnetic inrush currents from fault currents in Power Transformers.

Though the previously known inrush current detection techniques are able to do it but they

are less reliable and slow to respond due to use of filter. A new approach of discriminating

inrush current from fault current in a fast and precise manner is developed. Based on the

asymmetry of inrush current waveform, a unique criteria for discrimination is established.

MATLAB coding is developed to model a transformer for the analysis. Various switching

instants on the supply voltage waveform have been considered at intervals of 900 from 00 to

3600 with different residual flux in magnetic core.

Keywords: Inrush current, Power transformers, Discrimination, Asymmetry, Residual flux.

1. Introduction

Transformer is a static device which transfers power from one electrical circuit to another at a

constant frequency. It is a valuable electrical component in power system both at transmission

as well as distribution end. The power is transmitted at a very high voltage by using step up

transformers to reduce the transmission line losses in the power system which improves the

overall efficiency of the transmission. At the distribution end step down transformers are used

to get the voltage at distribution level, so transformer is having dual action.

International Journal of Pure and Applied MathematicsVolume 114 No. 12 2017, 615-625ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

615

Due to the above properties Transformers are one of the most important part of Power System.

So it is anticipated from the power engineers to have a proper and fast protective scheme for

its safety and to ensure its hassle free operation.

Among many one of the most common reason for mal operation of differential relays used for

transformer protection is high magnetic inrush current. The relay sometimes is unable to

discriminate between an inrush current and fault current within the first cycle hence it issues a

trip signal to the circuit breaker which causes an unwanted interruption in power.

When a transformer is energized at no load or lightly loaded condition for the first few cycles

it draws a very high amount of current known as magnetic inrush current. Magnetising current

is the one which is responsible for the development of rated flux in the transformer. Inrush

current peak is in the order of 5 to 10 times of the full load current. Due to this high current

magnitude it becomes difficult for a relay to distinguish between inrush and fault current which

causes its mal operation by giving tripping signal which causes discontinuity in power

transmission.

So an accurate identification of currents in a transformer is the key necessity for preventing

mal-operation of the protection system under different high current conditions which include

magnetising inrush current, external and internal fault current, etc. The requirement for

transformer protection has become major issue due to the need for precise, quick, and reliable

distinction between magnetic inrush current and internal fault current.

The magnetic inrush current has a good percentage of second harmonics unlike this fault do

not have large second harmonic. The percentage of second harmonic component of the current

to its fundamental component is utilized (also known as SHR) for discriminations .In this

method the relay should block the current when its ratio exceeds the pre-set value [1]. Most of

the viable differential relays are provided by the SHR method to prevent tripping due to inrush

current conditions. The ratio has been usually set in the range of 12–20%. But this method may

fail to differentiate high inrush currents when transformers are energized with some significant

residual flux.

The next method utilises the duration of gap between the zero crossing instants of the current

which is known as gap detection technique [2] is being used by some relays to identify the fault

and inrush currents. But, this gap detection method is liable to mal-operate when saturation of

CT takes place which in general is caused by high DC component present in the inrush current.

A number of algorithms have also been developed to overcome this serious problem which

consists of Wavelet Transform [3], ANN [4] and fuzzy logic [5]. Yet some of these methods

require a huge data for online training purpose, computational liability on the relay is

increased, and are difficult to predict counter to high frequency noise signal [6].

Due to the above mentioned drawbacks these methods have not reached a practical level yet.

So the SHR and gap detection are widely used methods in practise in spite of their limitations

of detecting high inrush currents till date.

2. Transformer Protection

Transformer is a static device which can transfer power from one electrical circuit to another

maintaining constant frequency. It is an important electrical component used in power system

to reduce the transmission losses.

We know from Faraday’s 2nd law emf equation,

E = N 𝒅ɸ

𝒅𝒕 ...( 2.1)

Where, N= number of turns , ɸ= flux linkage ,

E= Em sin 𝝎t , Em = Peak value of voltage, 𝝎 = Angular frequency.

International Journal of Pure and Applied Mathematics Special Issue

616

Flux linkage is,

ɸ = 𝑬𝒎 𝒔𝒊𝒏 𝝎𝒕

𝑵 𝒅𝒕 ...(2.2)

Applying KVL in primary loop,

Em sin 𝝎t – i*r1 – L1 𝒅𝒊

𝒅𝒕 =0 …(2.3)

So current through transformer ,

i = 𝑬𝒎

𝒁{sin (𝝎t +) – α} + c 𝒆(

−𝒓𝟏∗𝒕

𝑳𝟏) ….(2.4)

where α = 𝝎𝐿1

𝑟1 , r1= Primary winding resistance , L1 = Primary winding reactance.

If the magnetic core is not saturated, then the inductance L1 is high and constant. The current

‘i’ is magnetising current with small amplitude. If the core gets saturated then the inductance

L1 is very low and current ‘i’ will have very large amplitude.

Fig. 1: Transformer under no load

The above fig. represents a transformer under no load condition with sinusoidal supply.

The equivalent circuit of transformer is shown in the figure below.

where, N1 = Primary turns , N2 = Secondary turns

Fig. 2: Transformer equivalent circuit

A. Transformer protection using differential relay: In general, for transformer protection differential relays are popularly used. The differential

relay works on the principle of difference current flowing through primary and secondary coils.

If the differential current also known as spill current is zero then the relay does not operate or

remains in the blocking region but if the spill current is non-zero then the relay operates.

Differential relay only works for internal faults and remains un-operative for external (through)

faults. Fig. 2.3 shows a typical differential relay scheme for transformer protection against

internal fault.

Fig. 3: A differential relay for transformer protection


617

In differential protection the difference of two currents is fed to the relay operating coil. So for

the external faults the currents in the two C.Ts must be equal in magnitude and opposite in

phase hence the spill current becomes zero.

B. Cause for mal operation of differential relay:

Differential Relays are widely used in the transformer protection but may maloperate due to

system disturbances, such as:

1. Over excitation: Over excitation [7] of transformer implies the level of magnetic flux is higher than the

designed level. This leads to saturation of the core drawing large current. This can leads

to severe fault and mal-operation of the differential relay.

2. CT saturation: It is a physical phenomenon that happens when all magnetic domain on ferromagnetic

material are already aligned and further flux increment does not takes place. The current

transformer [8] implication on secondary current may be different. Saturated core does not

imply constant flux increase or high current on secondary. Generally improper selection of

CT ratio and high DC offset in line current under fault current result in saturation of CTs.

3. Large magnetizing inrush current:

The large current drawn by transformer on no load when the transformer is energised and

will last only for few cycles. The magnitude of inrush[9] is generally several times more

than rated current .As its magnitude is near to fault current so there is a chance of tripping

of over current relay.

3. Inrush currents and types

A. Inrush current and its types: Magnetic inrush currents are the transient no load current which has high magnitude (5 to 10

times full load current) drawn by the primary winding of transformer. As discussed earlier

primary current develops rated flux in the transformer core rate of change generates counter

emf in the winding. But sometimes its magnitude becomes comparable to the fault current and

hence the relay is unable to discriminate between the inrush and fault current. A typical inrush

current waveform is shown in fig. 3.1.

Fig. 4: An inrush current waveform


618

The inrush currents are mainly classified in three types:

(i) Inrush current during energisation

It occurs when transformer is brought from off state to energised state.

(ii) Inrush current during recovery

It occurs when the voltage is recovered after a small dip or disruption is restored.

(iii) Inrush current during operation or Sympathetic inrush current

It occurs when transformer is energised which is in parallel to an already excited

transformer. The sudden drop in voltage caused due to energisation can cause inrush current

in already excited transformer.

Among the above three types of inrush currents [10] energisation is the most common

which generates the largest current magnitude.. To understand inrush analytically we have

to recognize the relationship between voltage applied to transformer and transformer’s

magnetic core flux .That relation is

E= dλ(t) / dt

Or λ (t) = ∫ e(t) dt + λ (0) ….(3.1)

Where λ = Flux linkage and E =Induced emf λ (0) =Residual flux

It is observed that the extreme possible value of λ after energisation is 2 λm + λ (0). The relation

between inrush current and λ is given by the saturation characteristics of transformer core.

The protection system must be able to discriminate between the inrush current and actual short

circuit.

B. Methods for Discrimination of inrush current from fault current: 1. Second harmonic Restraint Method Inrush current is dominated by second harmonic which is used in most of the differential relays

for transformer, to discriminate inrush current and fault current. The harmonic sensing relays

most commonly block operation if the harmonic(s) exceed a given percentage of the

fundamental component. Some relays use the harmonic to increase the restraint current.

Generally differential relays are aided by second harmonic restraint to block tripping due to

magnetic inrush current. Pickup ratio is in range of nearly 12 to 20 %. An S.H.R. relay is shown

in the figure below.

Rest

rain

ing

coil

Operation coil

Harmonic bias

Through bias

Transformer

High Set Unit

XLXC

Fig. 5: S.H.R. Relay circuit

2. Gap Detection Approach As shown in the figure below, the time difference in each cycle is called as dwell time [11] in

this region differential current is almost zero. So to identify the inrush current check where

current is becoming less then even 5% of the rated current.


619

This criterion is known as the gap detection approach. CT saturation due to both the short

circuit fault and inrush currents adversely affects the detection criteria. When CT saturation

happens after a small period subsequent to an internal fault, output currents of CT may be

distorted so huge harmonic components are also present. Consequently, there is possibility that

second harmonic criteria operates inaccurately and blocking of differential relay takes place.

Fig. 6: Gap detection technique

Following observations about the two methods discussed above,

CT saturation due to both short circuit fault and inrush currents adversely affects inrush current

detection.

3 A variety of algorithms recently presented include artificial neutral network (ANN), fuzzy logic and wavelet analysis ANN is a novel approach which is used in online detection to discriminate the inter turn fault

and magnetizing inrush current, and also the fault location i.e., whether the turn to turn fault

lies in secondary winding or primary winding through the use of discrete wavelet transform

and artificial neural networks.

Wavelet Transform [12] is used to analyse the signal for short duration for its spectral contents.

Another method using operational matrices and Hartley transform [13] is proposed for

evaluation of inrush current and its simulation.

Following observations are made,

These methods need a large data set for training.

Impose a high computational burden on relay.

Depend on the transformer parameters or initial conditions.

Seem to be unpredictable against high frequency noise.

C. Proposed strategy: A new approach of discriminating inrush current from fault current in fast and precise manner

is developed. Based on the asymmetry of inrush current waveform a unique criteria for

discrimination is established.

It is observed that the instant at which the switching takes place with respect to voltage

waveform has a significant role on the peak value of current. Several case studies have been

conducted for various switching instants such as when the voltages waveform crosses zero, at

its positive peak, at its negative peak, etc. Intermediate switching instants have also been

considered at an interval of 900 from 00 to 3600 on the supply voltage waveform which are

thoroughly discussed in results and conclusions.


620

4. Output Waveforms

For the proposed approach single phase transformer is modelled for two different conditions

which are Inrush current with open secondary, Short circuit at secondary with fault resistance

and Internal fault for primary turns shorted. The tool used is MATLAB 2011 version to carry

out all the simulations.

A. Parameters:

1 ɸ Saturable Transformer having 2 winding with following parameters,

Power Rating: 9.6 KVA

Voltage Rating: 300 V/ 150 V

Primary Winding: r1= 0.06 Ω x1 = 0.3 Ω

Secondary winding: r2= 0.03 Ω x2 = 0.15 Ω

Core loss resistance: Rc = 1200 Ω

Unsaturated magnetizing reactance: Xm = 600 Ω

Unsaturated inductance: Lu = 1.91 H

Saturated inductance: Ls = 0.191 H

Peak value of rated magnetising current in steady state: im = 0.452 A

Frequency: 50 Hz

3 separate conditions are considered for various phase angles:

Condition 1: Inrush current with open secondary.

Condition 2: Secondary short circuited with fault resistance.

Condition 3: Internal fault for some primary turns shorted.

300 V (rms) supply is given to the transformer under the conditions mentioned above and the

developed model is run for 0.2 seconds (10 cycles) for various phase angles. Based on that the

waveforms are obtained then FFT is performed to obtain fundamental, 2nd harmonic, 5th

harmonic and 7th harmonic components. On the basis of the results obtained, logic for the relay

is developed. For fault case the fault resistance is varied for 4 different values which are 𝝎𝑳

𝟏𝟎 ,

𝝎𝑳

𝟕 ,

𝝎𝑳

𝟓 and

𝝎𝑳

𝟑 .Where, 𝜔 = 314 𝑟𝑎𝑑/𝑠 and L=0.191 H then for internal fault two cases

were considered i.e., 5% and 10% primary turns were shorted.

B. Output waveforms:

For Magnetising current

For Residual flux = 0


621

Fig. 7 : Magnetising current for zero residual flux

For Residual flux = 2.7 mWb

Fig. 8 : Magnetising current for 2.7 mWb residual flux

For Internal fault: For 5% turns shorted:

Fig. 9: Fault current for 5% turns shorted


622

C. Observation Table : FFT analysis of the waveforms above is done and both +ve and –ve peaks for the first cycle

are noted and tabulated below for various switching instants :

For Residual flux =0

5. CONCLUSION

This paper highlights a simple, fast and cost effective way of distinguishing magnetising current from a fault

current just by checking the positive and negative peaks of the current waveform.

For inrush current case, its magnitude is always less than magnetising current peak value (i.e., 0.452 A) in one

of the half or in either of the halves for first few cycles.

In case of through fault current, its magnitude will be always greater than magnetising current peak value in

either of the halves for first few cycles. In case of an internal fault it takes 3 to 4 cycles for exceeding

magnetising current in both cycles. So for internal fault logic will be similar to external fault if both halves

For α = 900

Current Inrush Internal

Rf =

5.997

Rf =

8.567

Rf =

11.99

DC

comp

(%) 10.25 51.27 67.74 67.05 65.694

Fund

(%) 100 100 100 100 100

2nd (%) 26.51 42.85 50.57 50.69 50.65

5th (%) 11.58 22.14 22.06 22.18 22.18

7th (%) 9.22 17.57 18.05 17.63 17.58

1st peak 0.08 0.85 -3 -2.5 -2

2nd peak -0.4862 -14.42 -14 -13 -12

For α = 00


Rf =

5.997

Rf =

8.567

Rf =

11.99

DC

comp

(%) 0.23 0.51 0.461 0.463 0.467

Fund

(%) 100 100 100 100 100

2nd (%) 27.27 50 47.83 47.83 47.83

5th (%) 9.09 19.23 21.73 21.73 21.73

7th (%) 9.09 15.38 17.39 17.39 17.39

1st peak 1.4 15.41 12 11.5 10.5

2nd peak -3.9e-3 -0.07 -4 -4.7 -5

For α = 1800


Rf =

5.997

Rf =

8.567

Rf =

11.99

DC

comp

(%) 18.65 38.67 34.16 36.51 39.19

Fund

(%) 100 100 100 100 100

2nd (%) 26.44 50 50.56 50.71 50.6

5th (%) 11.51 22.05 22.13 22.13 22.13

7th (%) 9.17 17.54 17.61 17.58 17.62

1st peak 0.06 2.82 7 7 7

2nd peak -0.788 -12.42 -7 -6.5 -6

For α = 2700


Rf =

5.997

Rf =

8.567

Rf =

11.99

DC

comp

(%) 20.86 34.59 37.1 34.33 30.75

Fund

(%) 100 100 100 100 100

2nd (%) 26.26 50.83 50.69 50.96 50.89

5th (%) 11.11 22.5 22.22 21.93 22.15

7th (%) 9.99 17.5 17.36 17.42 17.36

1st peak 1.364 17.09 15 14 12.5

2nd peak -2e-3 1.25 -1.5 -2.5 -4.5


623

exceed 0.452 A value otherwise both halves are of same polarity so it can be properly discriminated from

inrush current. This method can help fast detection of magnetising current whereas previously known methods

utilise filter which causes time delay and increases computational burden on the relay. So just by incorporating

comparator the fault and inrush currents can be discriminated.

REFERENCES

[1] Pei L, Malik OP, Deshu C, Hope GS, Yong G. Improved operation of differential protection of power

transformers for internal faults. Power Deliv, IEEE Trans 1992;7:1912–9

[2] Mekic F, Girgis R, Gajic Z, teNyenhuis E. Power transformer characteristics and their effect on

protective relays. Presented at the 33rd Western protective relay conference, October 17–19, 2006.

[3] Rasoulpoor M, Banejad M. A correlation basedmethod for discrimination between inrush and short circuit

currents in differential protection of power transformer using discrete wavelet transform: theory,

simulation and experimental validation. Int J Electr Power Energy Syst October 2013; 51:168–77.

[4] Puneet Kumar Singh , D K Chaturvedi. Modeling and Simulation of Single-Phase Transformer Inrush

Current using Neural Network. Control Theory and Informatics Vol.3, No.2, 2013- National Conference

on Emerging Trends in Electrical, Instrumentation & Communication Engineering

[5] Wiszniewski A, Kasztenny B. A multi-criteria differential transformer relay based on fuzzy logic. Power

Deliv, IEEE Trans 1995;10:1786–92

[6] Hooshyar A, Afsharnia S, Sanaye-Pasand M, Ebrahimi BM. A new algorithm to identify magnetizing

inrush conditions based on instantaneous frequency of differential power signal. Power Deliv, IEEE Trans

2010;25:2223–33.

[7] https://electrical-engineeringportal.com %2F protection-against-overexcitation-of-a-

transformer&usg=AFQjCNEj7QJq.

[8] https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0ah

UKEwjrmfTo57bQAhXBv48KHfZ_A_sQFgg2MAQ&url=http%3A%2F%2Fwww.gohz.com%2Fwhat-

happens-in-current-transformer-core-during-ct- saturation &usg=AFQjCNGi 5p3ReC9A0uR5bJEnh-

GjnNi4cQ&sig2=cwniyL--lPm-gexXmTgeUQ

[9] http://www.google.com/url?q=http%3A%2F%2Fwww.ece.mtu.edu%2Ffaculty%2Fbamork%2Fee5220

%2FInrush.pdf&sa=D&sntz=1&usg=AFQjCNHcWsfLj7hHRHmh9V9oOMjTk1Lx0wnSKvx0w3RlUV

AdPtrK7EQ&sig2=6PZQp9z9_0cEhaJeG0RvTA.

[10] https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0ahUKEwiRnZ7k_

LbQAhUErI8KHRvlATQQFggqMAM&url=http%3A%2F%2Fwww.electrical4u.com%2Fmagnetizingi

nrush-current-in-power-transformer%2F&usg=AFQjCNHVw6y-4T9pQp

AjymyL4HkpK56fMA&sig2=LXnJaAfouFiXDjpec2QhzQ.

[11] Dashti-Hamed , Mahdi Davarpanah, Majid Sanaye-Pasand, Hamid Lesani. Discriminating transformer

large inrush currents from fault currents electrical power and energy systems 75 (2016) 74-82

[12] Maya.P, S.Vidya shree, Roopasree.K, K.P.Soman. Discrimination of Internal Fault Current and

Inrush Current in a Power Transformer using Empirical Wavelet Transform

[13] M. A. Taghikhani, A. Sheikholeslami and Z. Taghikhani. Harmonic Modeling of Inrush Current in

Core Type PowerTransformers using Hartley Transform. Iranian Journal of Electrical & Electronic

Engineering, Vol. 11, No. 2, June 2015.


624

d iscrimination of magnetic inrush current from fault

Documents