Advanced Differential Protection

7/26/2019 Advanced Differential Protection

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VIII Seminrio Tcnico de Proteo e Controle28 de Junho a 1ode Julho de 2005

Rio de Janeiro RJ

Artigo: ST-3

ADVANCED POWER TRANSFORMER DIFFERENTIAL

PROTECTION

Zoran Gaji, Ivo Brni, Birger [email protected],[email protected],[email protected]

ABB Power Technologies AB, Vsters, Sweden

1. ABSTRACT

Three most typical weaknesses of the analogue

differential protection relays for power transformersand autotransformers have been:

1) Long operating time in case of heavy internalfaults followed by main CT saturation. Longdelays for heavy internal faultsthey can be of the

order of several tens of milliseconds are aconsequence of the harmonic distortion of the faultcurrents as they are seen by the differential relay.

The harmonic distortion is due to initial heavysaturation of the current transformers under faultconditions. Harmonic restrain criterion preventsimmediate operation of the differential protection.

2) Unwanted operations for external faults andtransformer inrush. Analogue differentialprotection relays for power transformers show atendency to unwanted operations for faults external

to the protected zone with the power transformerparticularly for external earth faults.

3) Bad sensitivity for low-level internal faults,

such as winding turn-to-turn faults. The low-level turn-to-turn faults typically cannot be detectedwith differential relay due to limited sensitivity ofthe relay operaterestraint characteristic. Even therelatively high sensitivity in the first section of the

differential relay characteristic of typically 30%may not be enough to detect a minor turn-to-turnfault, which initially only causes a differential

current of 10%, until it evolves into a more severefault with higher differential currents.

All these weaknesses can be successfully avoided if

the position of the fault (i.e. internal or external to

the differential protection zone) is quickly andcorrectly determined for all cases. The newprotection principle is based on the theory of

symmetrical components [1] and [2], or more exact,on the negative-sequence current theory.

Key Words: Protection, Transformer Protection,Differential Algorithm.

2. INTRODUCTION

The above-described problems can be effectivelysolved by the application of directional comparisonprinciple between the negative sequence currents

from all power transformer sides. Existence ofrelatively high negative-sequence currents is initself a proof of a disturbance on the power system,possibly a fault in the power transformer. Thenegative-sequence currents are measurableindications of abnormal conditions, similar to the

zero-sequence currents. One of the severaladvantages of the negative-sequence currentscompared to the zero-sequence currents is however

that they provide coverage for phase-to-phase andpower transformer turn-to-turn faults [4] as well,not only for earth-faults. Theoretically the negative

sequence currents do not exist during symmetrical

three-phase faults, however they do appear duringinitial stage of such faults [3] for long enough timefor the differential relay to make the properdecision. Further, the negative sequence currentsare not stopped at a power transformer of the Yd, or

Dy connection. The negative sequence currents arealways properly transformed to the other side ofany power transformer for any external disturbance.

Finally, the negative sequence currents are typicallynot affected by through-load currents.

The new algorithm for the internal/external fault

discriminator is based on the theory of symmetrical

components, or more exact, on the negative-sequence currents. Already in 1933, Wagner andEvans [1] stated that:
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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1) Source of the negative-sequence currents is atthe point of fault, (ENS= -INS* ZNS)

2)

Negative-sequence currents distribute throughthe negative-sequence network

3)

Negative-sequence currents obey the first

Kirchhoff's law

Similar statements are as well re-confirmed inreference [2].

The internal/external fault discriminator simplydetermines the position of the source of the

negative sequence currents with respect to theprotected zone. If the source of the negativesequence currents is found to be outside the zone,then the fault is external. If the source is found to be

inside the zone, the fault is internal.

The internal/external fault discriminator only worksif the protected power transformer is connected tosome load, so that currents can flow through the

protected power transformer, or at least through twowindings in case of a three-winding powertransformer. Thus, at an initial current inrush, theinternal/external fault discriminator algorithm

declares neither internal, nor external fault.

3. PRINCIPLES OF OPERATION

In order to avoid misunderstandings about what ismeant by the same direction and opposite

direction, an explanation of relay internally usedCT reference directions is shown in Figure 1.

Relay

E1S1

Z1S1

E1S2

Z1S2

IW1

IW2

IW1

IW2

Figure 1: Used reference connections of CTs, anddefinition of positive direction of currents

As shown in Figure 1 relay will always measure theprimary currents on all sides of the powertransformer with the same reference direction

towards the power transformer windings.

For an external fault the fictitious negativesequence source will be located outside thedifferential protection zone at the fault point. Thusthe negative sequence currents will enter the

healthy power transformer on the fault side, andleave it on the other side, properly transformed.According to the current direction definitions in

Figure 1, the negative sequence currents on therespective power transformer sides will have

opposite directions. In other words, the

internal/external fault discriminator sees thesecurrents as having a relative phase displacement ofexactly 180oas shown in Figure 2.

Relay

ENS

ZNSS1

ZNSS2

Yy0; 1:1INS

S1 INS

S1 INS

S2

INSS1

INSS1

Negative Sequence

Zero Potential

Figure 2:Flow of negative sequence currents forpower transformer external fault

For an internal fault (with the fictitious negativesequence source within protected power

transformer) the negative sequence currents willflow out of the faulty power transformer on both

sides. According to the definitions in Figure 1, thenegative sequence currents on the respective powertransformer sides will have the same direction. Inother words, the internal/external fault

discriminator sees these currents as having arelative phase displacement of zero electricaldegrees, as shown in Figure 3. In reality, for aninternal fault, there might be some small phase shiftbetween these two currents due to possible differentnegative sequence impedance angles of the source

equivalent circuits on the two power transformersides.

Relay

ENS

Yy0; 1:1

ZNSS1

ZNSS2

INSS2

INSS1

INSS2INSS1

Negative Sequence

Zero Potential

Figure 3: Flow of negative sequence currents for

power transformer internal fault

3.1 Negative sequence differential current

Modern numerical transformer differential relaysuse matrix equations to automatically compensatefor any power transformer vector group and turnsratio [6]. This compensation is done automaticallyin the on-line process of calculating the traditional

differential currents. It can be shown, that thenegative-sequence differential currents can be

calculated by using exactly the same matrixequations, which are used to calculate thetraditional differential currents. However, the sameequation shall be fed by the negative-sequence

currents from the two power transformer sidesinstead of individual phase currents, as shown in

the following matrix equation for a case of two-winding, Yd5 power transformer.


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2 2

_ 1 _ 2 1 1 _ 1 0 1 _

1 _ 1_ 2 _ 1 2 1 _ 1 1 0 _

3 _ 3_ 3 _ 1 1 2 _ 0 1 1 _

Id L N S IH V N S IL V N S

U r L V

Id L N S a IH V N S a IL V N S

U r H V

Id L N S a IH V N S a IL V N S

(0.1)

where:

Id_L1_NS is the negative sequence differentialcurrent in phase L1 (in HV side primary amperes).IHV_NS is HV side negative sequence current in

HV side primary amperes (phase L1 reference).ILV_NS is LV side negative sequence current inLV side primary amperes (phase L1 reference).

Ur_HV is transformer rated phase-to-phase voltageon HV side (setting parameter).Ur_LV is transformer rated phase-to-phase voltageon LV side (setting parameter).

a is the well-known complex operator for sequence

quantities; a=0,5+j*0,866.

In reality only the first negative sequence

differential current, e.g. Id_L1_NS, needs to becalculated, because the negative sequence currentsalways form the symmetrical three phase current

system on each transformer side. Consequentlythree negative sequence differential currents willalways have the same magnitude and be phase

displaced for 120 electrical degrees from eachother.

As marked in matrix equation, the first term on theright hand side of the equation, represents the totalcontribution of the negative sequence current from

HV side compensated for eventual powertransformer phase shift. The second term on theright hand side of the equation, represents the totalcontribution of the negative sequence current from

LV side compensated for eventual powertransformer phase shift and transferred to the powertransformer HV side.

When above compensation is made, then the 0-180

degree rule is again valid between negativesequence current contributions from the two sides.For example, for any unsymmetrical external fault,the respective negative sequence current

contributions from the HV and LV powertransformer sides will be exactly 180 degrees apart

and equal in magnitude, regardless the powertransformer turns ratio and phase displacement, asin example shown in Figure 4.

0.1 kA

30

210

60

240

90

270

150

330

180 0

Contribution to neg. seq. differential current from HV side

Contribution to neg. seq. differential current from LV side

0.2 kA0.3 kA

0.4 kA

"steady state"

for HV side

neg. seq. phasor

"steady state"

for LV side

neg. seq. phasor

10ms

10ms

Figure 4: Trajectories of Negative SequenceCurrent Contributions from HV and LV sides of

Yd5 power transformer during external fault

Figure 4 shows trajectories of the two separate

phasors representing the negative-sequence currentcontributions from HV and LV sides of an Yd5power transformer (e.g. after the compensation of

the transformer turns ratio and phase displacementby using previous matrix equation) for anunsymmetrical external fault. Observe that the

relative phase angle between these two phasors is180 electrical degrees at any point in time. There isnot any current transformer saturation for this case.

3.2 Internal / external fault discriminator

The internal/external fault discriminator is based onthe above-explained facts. Its operation is based onthe relative position of the two phasors representing

HV and LV negative-sequence currentcontributions, defined by matrix expression. Itpractically performs directional comparisonbetween these two phasors. First, the LV sidephasors is positioned along the zero degree line.After that the relevant position of the HV side

phasor in the complex plain is determined. Theoverall directional characteristic of theinternal/external fault discriminator is shown in

Figure 5.

Figure 5: Operating characteristic of theinternal/external fault discriminator

Neg. Seq. currentcontribution from

HV side

Neg. Seq. currentcontribution from

LV side


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In order to perform directional comparison of the

two phasors their magnitudes must be high enoughso that one can be sure that they are due to a fault.On the other hand, in order to guarantee a goodsensitivity of the internal/external fault

discriminator, the value of this minimum limit must

not be too high. Therefore this limit value, calledIminNegSeq, is settable in the range from 1% to

20% of the differential protections base current,which is in our case the power transformer HV side

rated current. The default value is 4%. Only ifmagnitudes of both negative sequence currentcontributions are above the set limit, the relative

position between these two phasors is checked. Ifeither of the negative sequence currentcontributions, which should be compared, is toosmall (less than the set value for IminNegSeq), no

directional comparison is made in order to avoid thepossibility to produce a wrong decision. This

magnitude check, as well guarantee stability of thealgorithm, when power transformer is energized.

The setting NegSeqROA represents the so-calledRelay Operate Angle, which determines theboundary between the internal and external faultregions. It can be selected in the range from 30

degrees to 90 degrees, with a step of 1 degree. Thedefault value is 60 degrees. The default settingsomewhat favours security in comparison to

dependability.

If the above condition concerning magnitudes is

fulfilled, the internal/external fault discriminatorcompares the relative phase angle between thenegative sequence current contributions from the

HV side and LV side of the power transformerusing the following two rules:

If the negative sequence currents contributionsfrom HV and LV sides are in phase, the fault isinternal (i.e. both phasors are within internal fault

region)

If the negative sequence currents contributionsfrom HV and LV sides are 180 degrees out of

phase, the fault is external (i.e. HV phasors isoutside internal fault region)

Therefore, under all external fault condition, therelative angle is theoretically equal to 180 degrees.

During internal fault, the angle shall ideally be 0degrees, but due to possible different negativesequence source impedance angles on HV and LV

side of power transformer, it may differ somewhatfrom the ideal zero value. However, during heavyfaults, CT saturation might cause the measured

phase angle to differ from 180 degrees for external,and from about 0 degrees for internal fault. See

Figure 6 for an example of a heavy internal faultwith transient CT saturation.

0.5 kA

30

210

60

240

90

270

120

300

150

330

180 0

HV side contribution t o the t otal negative sequence differential current in kA

Directional limit (within the region delimited by 60 degrees is internal fault)

1 .0 kA

1 .5 kA

definitely

an internal

fault

Internal

fault

declared

7 ms after

internal

fault

occurred

trip command

in 12 ms

excursion

from 0 degrees

due to

CT saturation

external

fault

region

35 ms

Directional Comparison Criterion: Internal fault as seen from the HV s ide

Figure 6:Operation of the internal/external faultdiscriminator for internal fault with CT saturation

4. IMPROVEMENT OF THE PROTECTION

The internal/external fault discriminator is a verypowerful and reliable supplementary criterion to thetraditional power transformer differential

protection. It detects even minor faults, with a highsensitivity and a high speed, and at the same timediscriminates with a high degree of dependability

between internal and external faults. When goodproperties of traditional power transformerdifferential protection are combined together with

advanced features of internal/external faultdiscriminator a high performance differential

protection for power transformers andautotransformers is achieved.

4.1 No extra delays at heavy internal faults

As the newly introduced internal/external faultdiscriminator has proved to be very reliable, it has

been given a great power. If, for example, a faulthas been detected, i.e. start signals set by ordinarydifferential protection, and at the same time the

internal/external fault discriminator characterisedthis fault as internal, then any eventual blocksignals produced by either the harmonic or the

waveform restraints, are ignored. This assures theresponse times of the new and advanced differentialprotection below one power system cycle (i.e.below 20ms for 50Hz system) for all internal faults.Even for heavy internal faults with severelysaturated current transformers new differential

protection operates well below one cycle becausethe harmonic distortions in the differential currentsdo not slow down the differential protection

operation. Practically, an unrestrained operation isachieved for all internal faults.

4.2 Stability against external faults

External faults happen ten to hundred times moreoften than internal ones. Many power transformer


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differential protection relays have a rather poor

stability against external faults. If a disturbance hasbeen detected and the internal/external faultdiscriminator characterised this fault as externalfault, the additional criteria are posed on the

differential relay before its trip is allowed. This

assures high stability against external faults.However, in the same time the differential relay is

still capable to trip for evolving faults. Example ofsuch evolving fault is shown in Figure 7.

0.2

0.4 kA

30

210

60

240

90

270

120

300

150

330

180 0

Magnitude of contribution from the HV (Y) side (in kA)

Directional limit (within 60 degrees is internal fault)

only

ext.

fault

ext.

and

int.

faults

int.

only

internal -

external

fault

boundary

A

B

C

internal

faults

external

faults

Figure 7:Operation for evolving fault

Point A in Figure 7 corresponds to the external faultonly. Point B corresponds to simultaneous external

and internal faults. The internal fault occurred 20msafter the external one. Point C corresponds to thesituation after the external fault has been cleared by

some other protection in 128 ms, while the internalfault persists. The advanced differential protectionwould actually operates already at point B and

disconnects the power transformer, in spite of thefact that the point B is deep in the external faultarea because of the more dominant (heavier)external fault.

4.3 Detection of minor internal faults

The internal/external fault discriminator has shownextreme capability to detect low-level faults such as

winding turn-to-turn faults. For more information

on this subject please refer to reference [4].

5. OPERATING PRINCIPLES FOR THREE-

WINDING TRANSFORMERS

The principle of the internal/external faultdiscriminator can be extended to powertransformers and autotransformers with three

windings. If all three windings are connected totheir respective networks, then three directionalcomparisons can be done, but only twocomparisons are necessary in order to positively

determine the position of the fault with respect to

the protected zone. The directional comparisons,which are possible, are: primary - secondary,

primary - tertiary, and secondary - tertiary. The rule

applied by the internal / external fault discriminator

in case of three-winding power transformers is:

If all comparisons indicate an internal fault,then it is an internal fault.

If any comparison indicates an external fault,

then it is an external fault

If one of the windings is not connected, thealgorithm automatically reduces to the two-windingversion. Nevertheless, the whole power transformer

is protected, inclusive the non-connected winding.

5.1 Example of unsymmetrical internal fault for

three-winding transformer

An internal fault L2-L3-Ground on the secondary

winding (d1) of a three-winding power transformer,connection group Yd1d5, has been simulated by

ATP [7].

0 10 20 30 40 50 60 70 80-4

-2

0

2

4

PrimarycurrentsinkA

iA

iB

iC

iN

0 10 20 30 40 50 60 70 80-50

0

50

100

SecondarycurrentsinkA

ia

ib

ic

0 10 20 30 40 50 60 70 80-5

0

5

10

Time in ms, internal fault at t = 13 ms

Inst.diff.curr.in

kA

inst diff L1inst diff L2

inst diff L3

int. fault

int. faultint. fault

int. fault

iB = primary (Y) line current L2

ib = secondary (d1) line current L2

instaneous diff. curr. L2

Figure 8:Currents for an L2-L3-E internal fault onthe secondary winding (d1) of an Yd1d5 power

transformer. Differential currents are in primary kA

The currents on the primary- and secondary sides,

and the instantaneous differential currents, areshown in Figure 8. The two directional

comparisons, made by the internal/external faultdiscriminator on the contributions to the totalnegative sequence differential current from theprimary, secondary and tertiary are shown in Figure9 and Figure 10.


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Figure 9:Internal/external fault discriminator

operation between primary and secondary windings

Figure 10:Internal/external fault discriminatoroperation between primary and tertiary windings

Obviously both of them steadily indicate that the

fault is internal. Deviations of the relative phaseangle from zero degrees in Figure 9 and Figure 10were mainly due to current transformer saturation.Severe current transformer saturation is actually the

most dangerous enemy of the internal/external faultdiscriminator. However very effective means tocounteract the negative effects of main CTsaturation have been integrated in advanceddifferential protection algorithm.

0 10 20 30 40 50 60 70 80

0

2

4

6

8

10

12

14

16

18

Time in ms, internal fault at t = 13 ms

Binary signals of the power transformer differential protection

fault

start-L1

start-L2

start-L3

trip

tripRestrained

tripUnrestrained

tripNegSeqUnrestrained

tripNegSeqSensitive

blockDueToCurr2ndHarm-L1


blockDueToCurr2ndHarm-L3blockDueToCurr5thHarm-L1

blockDueToCurr5thHarm-L2


blockDueToWaveAnalysis-L1



InternalFault

ExternalFaultint. fault declared

trip in 15 ms

Figure 11: Binary output signals of the advanceddifferential protection during internal fault in 3-

winding power transformer.

It shall be noticed that in Figure 11 the usual

restrained differential protection (signal namedtripRestrained) was delayed due to harmonic andwaveform block criteria. Besides, this signal wasunstable (onoffon, etc). The usual unrestraineddifferential protection limit, which had been set to10 times transformer rated current, was not

exceeded due to heavy ct saturation, and thus nohelp from the unrestrained differential protectionwas obtained either. Only the new advanced

differential protection was capable to quickly detectand trip the faulty power transformer in 15 ms afterthe fault inception.

5.3 Example of two simultaneous external faults

A case with two simultaneous external faults, the

first one L1Ground fault on the secondary side andthe second fault L1Ground on the tertiary side of

an Yd1d5 power transformer, is presented here.

0.2 kA0.4 kA

30

210

60

240

90

270

120

300

150

330

180 0

Contribution to t otal neg. seq. diff. current from tertiary

Contribution to total neg. seq. diff. current from primary

Contribution to t otal neg. seq. diff. current from secondary

0 .6 kA

fromsecondary

side

from tertiary

side

Figure 12. Trajectories of the contributions to thetotal negative sequence differential current for thefirst 25 ms during two simultaneous external faults

From Figure 12 it is obvious that the new

internal/external fault discriminator will securely

0.2

0.4

0.6 kA

30

210

60

240

90

270

120

300

150

180 0

Comparison Between Contributions: Primary - Secondary

Negative sequence differential current phasor (in kA)


57 ms

after

fault

internal

fault

declared

here

external

fault

zone

external

fault

zone

0.2

0.4

0.6 kA

30

210

60

240

90

270

120

300

150

330

180 0

Negative sequence differential current phasor (in kA)


Comparison Between Contributions: Primary - Tertiary

57 ms

after

fault

trip

external

fault

zone

external

fault

zone

internal

fault

zone


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declare this fault as external for this complicated

test case.

5.4 Example of three-phase internal fault

The negative-sequence-current-based directional

principle yields a fast and reliable discriminationbetween external and internal faults. This is easy to

understand in case of unsymmetrical faults, wherethe negative sequence system is expected to exist.

But the principle is just as efficient in case ofwholly symmetrical three-phase faults as well. Thereason is that when a (symmetrical) three-phase

fault occurs, the negative sequence current sourceappears at the fault for a while, more exactly, untilthe dc components in the fault currents die out [3].As far as advanced power transformers differential

algorithm is concerned, this interval of time is longenough for the directional criterion to declare either

an internal or an external fault.

Figure 13 shows magnitudes of the negative

sequence differential current, and its components,for an absolutely symmetrical internal three-phasefault on the Y side of an Yd1d5 power transformeras they were calculated by the differential relay. It

took in this example about 20 ms for currenttransformers to reach heavy saturation. Theexistence of the false negative sequence currents

after CT saturation was not a surprise. However,much more interesting was that the negativesequence system appeared immediately following

the inception of the internal symmetrical fault.

Figure 14 shows that during 22 ms after the fault

inception, both directional tests correctly indicatedan internal fault, which was long enough time todisconnect the faulty power transformer in 14 ms

(output relay make time not included) as shown inFigure 15.

0 10 20 30 40 50 60 70 800

0.2

0.4

0.6

0.8

1

1.2

1.4The total negative sequence differential current and its three components

CurrentsinkA

(transformerratedcurrentI1=0.5

23kA)

Time in ms, internal symmetrical fault at t = 13 ms

IdifNegSeqTotal

IdifNegSeqContrPri

IdifNegSeqContrSec

IdifNegSeqContrTer

total negative

sequencedifferential

current

current transformer

saturation sets in

heavy ct saturation

3-phase

internal

fault

rated current

Figure 13:Magnitudes of the negative sequencedifferential current, and its components, for an

internal three-phase fault on an Yd1d5 transformer.

210

240

90

270

120

330

150

180

Primary - Secondary Primary - Tertiary

Magnitude of negative sequence differential current (in kA)


60

30

0

300

0

30

60

90

120

150

180

210

240270

300

330

Figure 14: Directional tests for an internal three-phase fault on an Yd1d5 transformer, first 25 ms.

0 10 20 30 40 50 60 70 80 90

0

2

4

6

8

10

12

14

16

18

Time in ms, internal fault occured at t = 13 ms

Binary output signals of the differential protection for 3-phase internal fault

fault

start-L1

start-L2

start-L3

trip

tripRestrained

tripUnrestrained

tripNegSeqUnrestrained

tripNegSeqSensitive

blockDueToCurr2ndHarm-L1blockDueToCurr2ndHarm-L2








InternalFault

ExternalFaultint. fault found, 9 ms

trip in 14 ms

12 ms

Figure 15: Output signals for an internal whollysymmetrical three-phase fault.

As shown in Figure 15 the internal fault was

declared in 9 ms, the negative sequence differentialprotection issued a trip request after 12 ms, and the

final trip command to the power transformer circuitbreakers was given in 14 ms after the faultinception by the advanced differential protection.

6. CONCLUSIONS

This paper shows, that by using advanced

numerical technology, it is now possible to protectpower transformers with advanced differentialprotection principle, which has much higheroperation speed, security and sensitivity thantraditional transformer differential protection.

Operation of new internal/external faultdiscriminator for power transformers has been

successfully tested, by using simulation filesproduced by ATP [7], disturbance recording filescaptured during independent transformerdifferential protection testing on the analogue

network simulator [5] and finally from thedisturbance recordings captured in the field. Allthese tests indicate excellent performance of the

internal/external fault discriminator for power

transformers and autotransformers. It detects evenminor faults, with a high sensitivity and a highspeed, and at the same time discriminates with a


8/8

high degree of dependability between internal and

external faults. The only shortcomings of this newdirectional comparison algorithm are that it onlyoperates when power transformer is loaded and itdoes not provide indication of the faulty phase(s).

However, for internal faults power transformers are

always tripped three-phase, while from the captureddisturbance record at the moment of tripping the

faulty phase(s) can be identified.

7. REFERENCES

[1] C.F. Wagner, R.D. Evans, Book:Symmetrical Components", McGraw-Hill,New York & London, 1933

[2] J.L. Blackburn, Book: SymmetricalComponents for Power System Engineering,

Marcel Dekker, New York, Basel, HongKong, 1993; ISBN: 0-8247-8767-6

[3] Jonas Johansson, Master Thesis: FastEstimation of Symmetrical Components,Department of Industrial ElectricalEngineering and Automation, Lund

University, Sweden 2002.

[4] Z. Gaji, I. Brni, B. Hillstrm, I. Ivankovi"Sensitive Turn-to-Turn Fault protection forPower Transformers", CIGRE SC B5Colloquium, September 2005, Calgary-

Canada

[5] Z. Gaji, G.Z. Shen, J.M. Chen, Z.F. Xiang,"Verification of utility requirements onmodern numerical transformer protection bydynamic simulation" presented at the IEE

Conference on Developments in PowerSystem Protection, Amsterdam, Netherlands,2001

[6] F. Meki, Z. Gaji and S. Ganesan, "AdaptiveFeatures of Numerical Differential Relays,"presented at the 29th Annual Conference for

Protective Relay Engineers, Spokane,Washington, USA, October 2002

[7] ATP is the royalty-free version of theElectromagnetic Transients Program (EMTP).

For more info please visit the following websites: http://www.eeug.de/ orhttp://www.ee.mtu.edu/atp/
http://www.eeug.de/http://www.eeug.de/http://www.ee.mtu.edu/atp/http://www.ee.mtu.edu/atp/http://www.ee.mtu.edu/atp/http://www.eeug.de/

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Advanced Differential Protection