fundamentals of transformer protection

7/28/2019 Fundamentals of Transformer Protection

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Fundamentals of

TransformerProtection

By:

Bhuvanesh Oza



Fundamentals of Transformer Protection:FAULTS IN TRANSFORMERS:a) Faults in the auxiliary equipments of the transformer. b) Internal faults in the transformer windings. c) External faults.

Faults in Auxiliary Equipment (Minor or Incipient Faults):i. Oil leakage in the transformer tank ii. Deterioration of dielectric strength of oiliii. Failure of ventilation systemiv. Weakening of insulation between laminations of core and core bolt insulationv. Improper joints or connectionsvi. Inter-turn faults



Buchholz Relay:

To Alarm

Circuits

Oil From

Main Tank

To Conservator

To Trip Circuit

4

3

2

1

2

1: Baffle Plate

2: Mercury Switches

3: Floats

4: Accumulated Gas



Drawbacks of Gas Actuated Relays :

The vibrations and shocks caused by some reasons may mal-operate the mercury contacts resulting in unwanted tripping of transformer.

As the minimum operating time of Buchholz relay is about 0.1second, it is considered slower.

Sudden pressure relays are faster, in operation, only for largefaults. On the other hand electrical relays can be used for largerfaults where high speed is necessary. They can also be used forbushing flashovers and faults on leads which are outside the oiland hence do not create oil surge.

The Buchholz relay is limited to application for protectionagainst incipient faults and non-electrical faults.



Overcurrent and Earth Fault Protection:

1. PS and TMS of overcurrent (phase) relays on secondary side is to be

decided based on downstream relays

2. Phase relays on primary side are to be graded with phase relays onsecondary side and PS should be at least equal to 110 to 125% of

rated primary current of transformer.

3. Earth fault relay on primary side is not to be graded with the same on

secondary side.

4. Earth fault relay on primary side provides restricted earth fault

protection to primary windings.

5. The high set instantaneous phase unit on primary side is to be set

considering two facts

Worst possible magnetizing inrush current of transformer

Fault current reflected to primary side when there is a bolted 3-phase

short circuit on secondary side with worst assymmetry.

External Faults or Through Faults :



External Fault

Restricted earth fault protection:

R

Y

B

R

R s I f

I f

i f

i f I f



R

Y

B

R

R sI f i f I f

I f

i f

I f

Internal Fault




For relay setting calculations, let us consider a case with following data:

Transformer: 250 MVA, 15.75/240 kV, DY-11, % Impedance=14%, solidly earthed.

Current transformers: Line and neutral C.T.s with ratios 1000/1 amp, KPV>550 V andR CT<5 ohms.

Relay: Instantaneous overcurrent relay. Relay rating: 1 amp. Relay Burden: 0.9-1.0 VA.Setting range: 10 - 40% of 1 amp

Lead resistance: 1.36 ohms.

As the transformer is solidly earthed at its secondary (240 kV side), the fault current forthe earth-fault will be high and pick-up setting of 50% of full-load rating of transformer,for the relay can be selected. The lower setting may make the relay too sensitive with

the result that the relay may operate for through earth-faults, because of mismatch of the C.T.s.

Rated current of the transformer,

C.T. secondary equivalent of this full load current will be 0.6014 amp. Therefore pick-upsetting of the relay can be 0.3007 amp. Hence the setting of 30% is selected. The relayshould be immune to external earth-faults. Hence, such stability requirement demands

the use of stabilizing resistance. Such a calculation has been already shown for the caseof generator differential protection.

amp4.6012403

1000250



The earth-fault current for the earth-fault at the terminal of the transformer (after theline C.T.) can be calculated, based on the values of Z1, Z2 and Z0; the positive, negativeand zero sequence impedances of the transformer. Considering Z0=10% fault currentwill be about 4.75 kA. This when reflected to C.T. secondary,

if = 4.75 amp

= 4.75 (5 + 1.36)

= 30.21 volts

This suggests that KPV of the C.T.s should not be less than 60.42 volts.

= Burden/is2 = 1.0/(0.3)2

= 11.11 ohms

= 89.59 ohms

The stabilizing resistance of one third of this value can be selected.

)R R (iV LCTf r

r stabr s V)R R (i

r

s

r

stabR

i

VR



Differential protection:Problems in Application of Differential Protection:

non-identical C.T.s may cause high spill current to flow through the relay in case of heavy external fault.

The full load currents of the transformers on primary and secondary sides are different.

The primary current of the transformer is given by vectorial summation of KIs and I0.The C.T. ratios are selected considering the nominal transformation ratio and hencesome spill current will always flow through the relay because of the no-load currentcomponent of primary current.

The ratio errors of the C.T.s on either sides differ during such conditions due to (i)inherent difference in C.T. characteristic and (ii) unequal d.c. components in the short-

circuit currents. Inherent phase-shift of currents in the transformers.

C.T. Ratio Errors

Tap-changing

Magnetizing inrush current of the transformer

Saturation of transformer core



Differential protection:

Problems in Application of Differential Protection:

non-identical C.T.s may cause high spill current to flow through the relay in case of heavy external fault.

The full load currents of the transformers on primary and secondary sides are different.

The primary current of the transformer is given by vectorial summation of KIs and I0.The C.T. ratios are selected considering the nominal transformation ratio and hencesome spill current will always flow through the relay because of the no-load currentcomponent of primary current.

The ratio errors of the C.T.s on either sides differ during such conditions due to (i)inherent difference in C.T. characteristic and (ii) unequal d.c. components in the short-

circuit currents. Inherent phase-shift of currents in the transformers.



r I

yI

bI

RLI

YLI

BLI

RI

YI

BI

1R 2R

2Y

2B

1Y

1B

2r 1r

2y

1y

2b 1b

r I

yI

bI

Transformer Winding Connection



RI

R-I

Y-I

BI

BLI

RLI

B-I

YI

YLI

r I

yIbI

Primary Currents

Secondary Currents

r I

30°

RLI

(a) (b)

Comparison of Primary and Secondary Current Vectors



C.T. Connections in Differential Protection



RI

R-I

Y-I

BI

BLI

RLI

B-I

YI

YLI

Primary (Transformer)

r I

bIyI

RLi

YLi

BLi

r i

yibi

rli

yli

bli

Primary (C.T.)

Secondary (Transformer)

Secondary (C.T.)

Vector Diagrams



Mal-operation of Differential Protection Scheme for External E/F



CT ratio errors

Tap-changing

Magnetising inrush current of the transformer

Solutions are even harmonic cancellation, harmonic restraint and wave-shapemonitoring

saturation of transformer core



Wave-shape monitoring:Some of the solutions proposed are as follows:•Blocking of differential protection for one cycle by sensing of current behavior in the regionof zero-crossing. This can be achieved with or without a microprocessor.

•

Allowing the relay to operate only if the residual current exceeds a certain level for morethan 2/3 of a period in one cycle. (It can be demonstrated that the interval is smaller than2/3 of the period of a cycle for inrush magnetizing condition).

•Fast extraction of the second and fifth harmonics which are predominant in inrushmagnetizing current and comparison of these with the fundamental during the first cycle.This can be achieved with a digital signal processor and specially developed software.



Block diagram of static differential relay:



Relay Setting Illustration :Data:Transformer:250 MVA, 15.75/240 kV Taps: -5% to +7.5% on h.v. side Connection: DY-11

% Impedance: 14% Current Transformers:L.V. side: 10000/5 amp star connected H.V. side: 1000/1 amp star connected Interposing C.T.s on h.v. side: 1/4.4 amp Primary: Star connected Secondary: Delta connected Ratio error of all C.T.s: 3%

Relay: Biased Differential RelayRating: 5 amp Sensitivity setting: 15% of 5 amp (fixed) Bias setting: 15%, 30%, 45% Instantaneous High-set unit: 10 times rated current (fixed) 2nd harmonic restraint: Operation is prevented when 2nd harmonic content in the differential circuitexceeds 15%.

5th

harmonic bypass: This is provided to avoid possible mal-operation under over excited conditions.



Relay Setting Illustration :Solution:Referring to figure, primary rated current of the transformer, IP = |IR | = |I Y | = |IB| = 9164 amp Reflecting this current to secondary of C.T.s iP = |iR | = |i Y | = |iB| = 4.582 amp

This is the pilot current on the delta side of transformer.Now, secondary rated current of the transformer, Is = |Ir| = |Iy| = |Ib| = 601.4 amp Reflecting this current to secondary of C.T.s is = |ir| = |iy| = |ib| = 0.6014 amp This current, when transformed by C.T.s, is1 = |ir1| = |iy1| = |ib1| = 2.646 amp Hence pilot current on star side of transformer will, in turn be,

is2 = |ir2| = |iy2| = |ib2| = is1 = 4.583 amp Comparison of these currents with ip proves that the two pilot currents are equal and in phase, as isrequired. One can appreciate the need of I.C.T.s here as 240 kV C.T. with ratio 1000/4.4 amp is non-standard. Now, for deciding the relay setting, highest tap is to be considered, i.e. 7.5%.

At this tap, the turns ratio will be or 258000/15750.



Relay Setting Illustration :Percentage bias is, now, to be selected such that the relay remains stable for three phase bolted short-circuit taking placeafter C.T.s on the secondary side. While selecting this bias setting, the consideration is to be given for the tap-changing,possible C.T. saturation, mismatching of C.T. saturation characteristic and C.T. ratio error.

At the highest tap under consideration, the fault current on primary and secondary side for the three phase fault at thelocation stated above can be calculated on the basis of percentage impedance. i.e. Isf = 601.4/0.14 = 4.3 kA

C.T. secondary equivalents of these currents are, ipf = 35.22 amp isf = 4.30 amp Considering maximum and cumulative C.T. errors at +3% for C.T.s on primary side and -3% for C.T.s on secondary side. ipf = 36.27 amp isf = 4.17 amp This when reflected on secondary of ICT,

Once again considering -3% error of ICT, isf1 = 17.80 amp Thus, isf2 = = 30.83 amp Differential current,

and Restraining current,

Pick-up ratio,

Ak 43.70)15750/258000(3.4I pf

mpa35.184.417.4i 1sf

mpa44.5ii 2sf pf

mpa55.352/)ii( 2sf pf

%21.16=2/)i+i(

i-i

sf2 pf

sf2 pf



Relay Setting Illustration :Giving consideration to possible C.T. saturation and C.T. mismatching at this high throughfault current, bias can be set at 30% which is the next higher available setting. For in-zone fault, the operation of the relay is assured because for three phase fault or two-phase fault in transformer secondary winding, differential current will be high leading tohigher pick-up ratio than set bias of 30%. As such the operating coil current will be muchhigher because the internal fault will be fed from both the sides since generally thetransformer is connected to the infinite bus. For faults on primary winding, the operatingcoil current will be still higher and perhaps higher than the setting of the high-set unitleading to a one-cycle operation of the relay. The earth-faults are taken care of by earth-fault protection scheme on primary side and restricted earth-fault protection on secondaryside.

The C.T.s will not saturate for large external fault-currents if their knee point voltage is high.The relay manufacturer will give the C.T. requirements for the application. Generally, the KPV is given by the formula, Where, I = Rated relay current = 5 amp R ct = C.T. secondary resistance = 1.5 ohms R l = Lead resistance = 1.6 ohms



Differential protection of 3 winding transformer:



• Protection against overfluxing

• Protection of grounding transformers



Protection against overheating

Switch on Air Fans : 600C Switch on Oil Pumps : 700C

Audible and Visual Alarm : 850

C Trip Signal to Circuit Breaker : 950C

Oil thermometer

Winding thermometer



Transformer differential protection using a numerical relay:Differential protection:

M

i1 i2

I1 I2

i + i1 2

Principle of Differential Protection



In figure, if there is no fault in the transformer, current I2 leaves the transformer. In this case thecurrents I1 and I2 are same. Hence the secondary currents i1 and i2 are also same. Currents i1 and i2 justcirculate in the pilots and no current flows through the operating element M. No doubt, the current mayflow through this element due to C.T. mismatch when heavy through fault occurs. This has to be avoidedby proper stabilization.

In case of an internal fault, current flows through the operating element M as shown in figure. Relayoperates in this case, tripping the transformer by tripping breakers on both sides. A numerical relay usually is required to be fed by data of transformer to be protected and instrumenttransformer data. MVA rating, primary and secondary voltages, vector group, etc. of transformer areentered in the relay. Relay calculates the full load currents on both the sides and finds out the secondaryequivalents. All these calculations are possible to be made by some form of microprocessor within therelay. When numerical relay is used, C.T.s on both sides are connected in star only and no ICTs are required to

be used. Relay continuously takes the samples of C.T. secondary currents on both the sides and beforefeeding these currents to the pilots, the pilot currents are vectorially and arithmetically matched.

Where, [Im] = Matrix of the pilot currents k = Constant factor to match the pilot currents arithmetically [M] = Coefficient matrix depending on vector group to take care of vectorial inherent phase shift of line

currents of transformer.[In] = Matrix of phase currents of R, Y and B phases available from C.T. secondaries.

]I][M[k ]I[ nm



Thus proper pilot currents are fed to the relay. i1 is the pilot current on primary side, i2 is that on thesecondary (and i3 is the same on tertiary in case of three winding transformer). The relay will calculatethe differential current idiff and stabilizing current istab given by,

, i.e. the vectorial sum And , i.e. arithmetic sum

If , a tripping signal is issued. The characteristic is shown in figure.

a b

c

I diff

I stab

Special Stabilization

d e

Block AreaTrip Area

Characteristic of Numerical Transformer Differential Relay

|iii|i 321diff

|i||i||i|i 321stab

stabdiff IK I



A straight horizontal line „ab‟ shows the basic setting. Curve „bc‟ gives the first slope to take care of C.T.errors and tap changing. Curve „cd‟ causes stronger stabilization to take care of C.T. mismatch whichoccurs for heavy through faults. If Idiff is higher than the value given by portion „de‟, relay will alwaysissue trip command and no harmonic restraint or stabilization is effective. If a bolted three phase short-circuit occurs, C.T.s may badly saturate and operating point may lie in the trip area. The fact that the C.T.

does not saturate during first cycle is made use of in this case. If, during a first cycle, a point moves inthe area defined by special stabilization and then moves to trip area in subsequent cycles, the operationcan be blocked for a selectable period within which some primary relay may operate avoiding uncalledtripping of differential protection. Second harmonic stabilization facility is provided in the relay to avoid unnecessary tripping due tomagnetizing inrush which occurs while switching ON the transformer. If second harmonic content in idiff ismore than a preset percentage of fundamental, the relay issues the blocking command. Fifth harmonic stabilization is also generally available in modern numerical relays. This feature takes care

of over-excitation of transformer and avoids possible mal-operation of the relay. If fifth harmonic contentin idiff is more than certain preset percentage of fundamental, tripping is blocked. But if heavyovervoltage is found on the primary, it will cause large magnetizing current and iron losses will be veryhigh. If, in this case, idiff is higher than certain preset value, blocking is withdrawn and trip commandissued.




IB

i + i + iR Y

IY

IR

Bi =2

iNi =1

Numerical

RelayIN



Samples of currents IR , I Y , IB and IN are taken as seen in secondaries of C.T.s. The principle is similar todifferential protection. Current i2 will exist only for internal or external earth-fault.i1 = iN, the neutral current

, the residual current Now,

When tripping command is issued.

BYR 2 iiii

|ii|I 21ref

|i||i|I 21stab

,I×k >i stabref



Back-up Overcurrent Protection:Back-up overcurrent protection can be provided for line faults. Usually four standard characteristics areavailable in modern numerical relays:

Normal inverse IDMT characteristic Very inverse IDMT characteristic Extremely inverse IDMT characteristic Long time inverse IDMT characteristic

Facility of “Switch ON to fault” is provided; i.e. if the fault exists while closing a breaker, the relay tripsinstantaneously and associated time delay of IDMT characteristic is bypassed.Thermal Overload Protection:Temperature rise in the windings can be calculated by mathematical formula based on current flowingand thereafter alarm and/or trip signal can be issued if it exceeds a set limit. Thermal memory is alsoavailable in the relay. This gives the option of considering pre-load temperature rise of the transformer

winding. External Trip Functions:If cooling fans or cooling pumps fail or a buchholz relay operates, the operation of a buchholz relay ordeenergization of fan contactor can be routed through the relay and relay would issue trip command.This can be done through the binary input, i.e. (say 110 V) d.c. voltage can be applied to designatedterminals of the relay through the contact of buchholz relay etc.



Appendix:INFORMATION REQUIRED FOR DESIGNING PROTECTIVE SCHEMEWITH RELAY SETTINGS FOR POWER TRANSFORMER:Transformer:

MVA rating

Nominal transformation ratio Rated primary and secondary voltage Vector group of transformer (i.e. DY-11, DY-1, DY-5, etc.) Percentage impedance Type of neutral earthing (i.e. effective/non-effective) Value of earthing impedance, if neutral is non-effectively grounded.

Indoor or outdoor With or without conservator tank Zero sequence impedance No-load current Tap-changer details Overfluxing withstand



Current Transformer:Class of C.T. Knee Point Voltage (KPV) Magnetizing current Secondary resistance C.T. ratio Details of interposing C.T.s, if used. Accuracy limit factor of C.T.s used for overcurrent protection Burden (VA rating)

Pilot wire resistance for differential protection Type of relays used for overcurrent protection, REF protection, Differential protection,

overfluxing protection, etc. and the technical particulars of the relays particularly burdenand C.T. requirements. Potential Transformer:

Voltage ratio VA rating

Power system particulars:Network diagram showing the position of transformer

Fault level at transformer terminals

fundamentals of transformer protection

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