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Sustained external faults - heavy throughcurrent, producing heavy mechanicalstresses on windings and insulation.

The protection scheme must be properly de-signed to take care of these faults individually.Hence factors such as magnetizing inrush cur-rent, winding arrangements, winding connec-tions and connection of protection secondarycircuits must be considered.

In electro-mechanical relays, movement ofarmatures or discs is used to operate contactswhich in turn cause the tripping of circuitbreakers. When the input current exceeds the

set value, the disc rotates. The speed of ro-tation depends on the magnitude of the in-put current and the eddy-current braking pro-duced by a permanent magnet. Moving con-tacts are driven by the rotating disc until con-tact is made with stationary contacts. Whenthis happens, the trip coil of the associatedcircuit breaker is energized and the breakerwill open.

Modern electronic relays use digital cir-

cuitry to process incoming signals derivedfrom current and voltage transducers. Ana-logue to digital converter (A/D) is used tosample and encode the input signals whichare then processed to extract relaying infor-mation. Any desired time/current character-istic can be produced in the electronic relayas a result of the inherent flexibility of the cir-cuitry involved; sluggishness and overshootingare eliminated. However Reliable power sup-

plies and immunity to electrical interferencesfrom other power equipment in the vicinitymust be provided for the modern solid-staterelays [2].

2. Protective Devices and Schemes

A typical protection package for a large dis-tribution transformer is shown in figure 1.

The protective devices and schemes are de-scribed as follows:

Figure 1: Typical Transformer Protection Pack-age. Legend: WT - Winding temperature, B -Buchholz, OT - Oil temperature, 87 - Biased Dif-ferential, 64 Restricted earth fault (REF), 51N -Standby earth fault (SBEF), 50N - Instantaneousearth fault, 51- IDMT over-current, ICT - Inter-posing Current Transformer.

2.1. The biased differential relay (87)

This is a high-speed protection schemewhich is used to clear faults on the LV wind-ing as well as the HV winding, using the ba-sic differential principle that HV and LV CTsecondary currents entering and leaving thezone of protection should be equal under loadand through fault conditions, but unequal un-der internal fault conditions, and a differentialcurrent will cause the relay to operate. Cur-

rent transformer locations define the zone ofprotection. In many cases the HV and LVCT primary ratings will not exactly matchthe transformer winding rated currents andwinding connections in the transformer leadto phase shift in the secondary voltage. As anillustration, a 15MVA, 33/11KV, Dyn1 trans-former has 262.4A as the primary windingrated current, and there is no current trans-former with the ratio 262.4/1 to match. Sim-

ilarly there is no current transformer with theratio 787.3/1 to match the secondary current

Nigerian Journal of Technology Vol. 30, No. 3. October 2011.


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Modern Devices/Techniques in the Protection of Transformers 105

rating of the power transformer. The Dyn1connection symbol of the transformer showsthat the secondary voltage is phase-shifted 30

behind the primary voltage. The application

of the well established principles of differen-tial protection to transformers demands thatfactors, such as phase shift across the trans-former, imbalance of current transformer sig-nals on either side of windings have to beconsidered. Interposing current transformers(ICT) [3] are usually provided to correct thephase shift, correct the ratio error of the mainCT signals and trap LV zero sequence cur-rent, otherwise the relay will erroneously op-

erate for external LV earth faults. Also dur-ing inrush conditions or during transient over-fluxing, high level magnetizing current cancause mal-operation of the relay. Therefore,the differential element must be blocked forthese conditions.

Traditionally, phase and ratio correctionsas well as zero sequence current filtering wereachieved by the application of external inter-posing current transformers (ICT), as a sec-

ondary replica of the main transformer wind-ing arrangement. However, modern differen-tial relays have inbuilt software ICTs. Thisfeature gives the relay the flexibility to caterfor line CTs connected in either star or delta[4].

2.1.1. Ratio correction

For example, let us consider a two-winding15MVA, 33/11KV, Dyn1 transformer:

33KV full load current = 15000333 = 262.4A

HV CT ratio = 300/1

Secondary current of CT = 262.4 1300

=0.875A

11KV full load current = 15000311 = 787.3A

LV CT ratio = 800/1

Secondary current of CT = 787.3 1800

=0.984A

The traditional method of applying an ex-ternal ICT is shown in figure 2. The ICT turns

ratio is determined as follows:Relay current = 0.875A

Figure 2: Differential protection circuit.

LV CT secondary current = ICT primarycurrent = 0.984A

ICT secondary current = 0.8753

= 0.505

ICT turns ratio = N1N2

= I2I1

= 0.5050.984

= 12

The phase correction and zero sequence cur-rent filtering is achieved by the star-delta con-nection of the ICT.

But application of a relay with software ICTwill result in the setting of the relay as follows.Each of these secondary currents is correctedto 1A (relay rated current).

HV ratio correction factor is 10.875

= 1.14setting applied to relay)

LV ratio correction factor is 10.984

= 1.02setting applied to relay)

Thus, adjustable ratio correction factor

ranging from 0.05 to 2, in steps of 0.01, isprovided in the relay. This application is il-lustrated in figures 3a and 3b.

2.1.2. Phase correction and zero sequence

current filtering

If a transformer can pass zero sequence cur-rent to an external earth fault, it is neces-sary that zero sequence current filtering isemployed in order to prevent the relay from

mal-operation due to out of zone earth faults.Obviously, zero sequence current will flow in



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the current transformers associated with thestar winding of Dyn11 transformer if an ex-ternal earth fault occurs. However, there willbe no zero sequence current in the current

transformers on the delta winding. So the LVzero sequence current has to be removed oth-erwise it becomes a differential current andcause the relay to operate. Phase shift of theprotected transformer and zero sequence cur-rent are catered for by software ICTs for eachtransformer winding, instead of the scheme infigure 2. The phase correction settings avail-able are Yy0 (0deg), Yd1 (-30deg), Yd2 (-60deg), Yd11 (+30deg), etc. The selection

of any of these will depend on the phase shiftacross the transformer and zero sequence fil-tering requirements. The phase correction isapplied either side of the relay element asshown in figures3a and 3b.

2.1.3.

Magnetizing inrush current is associatedwith a transformer winding being energizedwith no balancing current present in the other

winding or when a load is suddenly discon-nected from the transformer raising the volt-age at the input terminals of the transformerby 10-20% of the rated value - causing an ap-preciable increase in transformer steady stateexcitation current. The magnitude and dura-tion of the current depend on the transformerdesign, size, system fault level etc. Undernormal steady state conditions, the magne-tizing current associated with the operating

flux level is less than one percent of rated cur-rent. But if a transformer winding is energizedat a voltage zero, with no residual flux, theflux level during the first voltage cycle is twotimes normal maximum flux. Consequently,core saturation and high non-sinusoidal cur-rent waveform occur. This current is knownas magnetizing inrush current and may persistfor many cycles, appearing as a large operat-ing signal for the differential protection; andso the relay operates erroneously during in-

rush. In the past, the operation of the relayunder inrush conditions is prevented by a time

delay. When a time delay cannot be tolerated,the second harmonic component of the currentis used to restrain operation. But if the linecurrent transformer becomes saturated relay

operation could be slow [5]. Modern electronicrelays overcome this by a technique which rec-ognizes magnetizing inrush current. A Fouriertechnique is used to measure the level of fifthharmonic in the differential current. The ratioof fifth harmonic to fundamental componentis compared with a setting. If the ratio ex-ceeds the setting, the differential protection isinhibited [6].

2.1.4. Stability RequirementsWhen saturation occurs in one of the main

CTs, the output from the saturated CTreduces, resulting in a differential currentflowing through the relay and causing mal-operation. To overcome this problem, prin-ciples of high and low impedance protectionare employed [7].

In the case of high impedance protection,the impedance of the relay circuit is made

higher than the saturated CT by the addi-tion of a stabilizing resistor Rs. So the spillcurrent resulting from asymmetric saturationof the line CTs will be forced to flow throughthe saturated CT rather than through the re-lay circuit.

Low impedance protection allows the fullspill current to flow through the relay butraises the relay setting in proportion to thelevel of through fault current. Biasing fea-

tures, which effectively increase the currentsneeded to operate the relay when high cur-rents flow are introduced to reduce the degreeof matching needed. The increase in settingis therefore normally based on a percentage ofthe through current and so is usually referredto as percentage biased differential protection.Small amounts of bias are very effective whenhigh currents are flowing to external faults.If, as an illustration, a current of 20p.u. wereflowing to an external fault on the LV side

of the 15MVA transformer referred to above.Let relay operating current be set at 0.15A. If



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(a)

(b)

Figure 3: Differential protection circuits.



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2% bias is to be applied, then:

Through fault current on LV side of trans-former = 20 800 = 16000A.

CT secondary current 16000800

= 20A

2% bias = 0.02 20A= 0.4A

New relay setting = 0.15A + 0.4A = 0.55A.

2.2. Inverse Definite Minimum-Time(IDMT) over-current relay (51)

This is needed to protect the transformeragainst external faults. It provides protectionagainst three-phase faults, inter-phase faults

and overloads. The relay monitors the cur-rent supplied by the associated current trans-former and triggers a timing system wheneverthe current exceeds the set value. Operat-ing time decreases as relay current increases,approaching a definite minimum value. Thisfeature allows adequate discriminating timemargins to be obtained between relays ap-plied to adjacent sections of networks duringshort-circuit conditions. The minimum op-erating current of the relay must exceed the

rated current of the transformer. The mainCT output is fed to a step-down ICT in themicroprocessor-based overcurrent relay. Afterfull-wave rectification, the output of the ICTis fed to a shunt-connected resistors circuitry.This circuit is switched by the user to obtainthe desired current setting and associated in-verse time/current characteristics.[8]

2.3. Restricted earth fault protection(64)

This is a more sensitive, high speed earthfault protection applied to the LV winding.Its operation is limited to detection of earthfaults within the LV winding; hence the namerestricted earth fault protection.

2.4. Standby earth fault, SBEF (51N)

This is a back-up protection device to clearany sustained external LV faults.

2.5. The instantaneous earth fault re-lay (50N)

The instantaneous earth fault relay is aninherently restricted earth fault element as-sociated with the HV over-current. It is in-stalled as an earth fault protection for the HVwinding, which is delta-connected and hencedoes not pass zero sequence current to the up-stream HV when LV earth fault occurs. Sothis relay can be set to operate without anyintentional time delay, since there is no needto grade it with other earth fault protection.

Earth fault relays normally have low set-tings corresponding to 20% or less of the rated

current of protected circuit since they are con-nected in such a way that they are not affectedby normal balanced currents.

As mentioned before, all these relays nowhave solid-state versions which have a numberof time/current characteristics not available inthe old electro-mechanical relays.

2.6. Winding temperature, oil temper-ature, Buchholz and pressure re-

lief deviceThese are protective devices which are con-

nected to directly trip the circuit breaker aswell as operate auxiliary relays for flaggingpurposes. They clear faults that might not bedetected by protection devices operating fromthe line current transformers. Such faults in-clude excessive temperature in the winding orin the oil, winding inter-turn faults or corelamination faults.

2.6.1. Winding temperature device

Any transformer generates a large amountof heat while in operation, due to load currentflowing in the windings, inadequate oil circu-lation arising from faulty pumps or blockagesin ducts or pipes. Traditionally, thermostatsor bulbs containing volatile liquids were po-sitioned in the oil and within the windings.These operate remote pressure switches con-nected to them by small-diameter tubes. In

recent years, a temperature sensitive bimetalstem is placed near the top of the transformer



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tank where the oil tends to be hottest. Thestem is heated by both the surrounding liquidand a heater element which is fed from a cur-rent transformer connected to one of the phase

windings The combination of the two temper-atures is indicated on a device, known as DialHot Spot Thermometer. When an unaccept-able over-heating occurs, resistance bridges,comprising of heat-sensitive resistors (sensors)are used to produce imbalance output signalsthat initiate either alarms or the opening ofappropriate circuit breakers [9]

2.6.2. Buchholz relay

Any fault, such as insulation puncture,shorted turns, poor contact, which occurs in-side a transformer in operation is generally ac-companied by the evolution of gas as a resultof the decomposition of oil or solid insulation.This gas bubbles rise to the surface and finallyfind their way to the conservator. On its waythere, the gas is collected in what is knownas Buchholz relay installed on a stub pipe be-tween the tank and conservator. The Buch-

holz relay has an upper and a lower float. Asgas collects in the relay housing, it displacesoil out of it. The top float drops, and its mer-cury switch completes an alarm circuit. In thecase of more serious fault, such as an inter-turn short, and the like, the gas is usuallylibrated in an explosive fashion and a largeamount of oil is forced from the tank into theconservator. This causes the lower float torise and close its mercury switch, thereby ac-tivating a tripping circuit which disconnectsthe transformer from supply line and averts amajor breakdown [10]

2.6.3.

In order to avoid irreparable damage to thetank in the case of a heavy gas evolution, apressure relief device is installed on a trans-former. This is a low steel pipe communicat-ing with the tank at one end and closed by adisc of thin glass at the other. When the pres-

sure inside the tank rises dangerously, the discbursts, so that excess oil and gas are expelled

Figure 4: Single line diagram of 33 / 11KV substation. Legend: IS- Isolator, CT - Current transformer, PT - Potential transformer, LA -Lightning Arrester, A - Ammeter, AS - Ammeter switch, V - Voltmeter,VS - Voltmeter switch, W - Wattmeter, CB - Circuit breaker.

into the atmosphere before the tank has timeto be deformed.

Figure 3 is the single line diagram of a33/11KV substation, showing the associatedprotective devices.

3. Conclusion

Transformer is one of the most importantequipment in the electrical power system. Inspecifying a protection scheme, the economiceffect of the loss of the transformer and thecost to repair a major breakdown should betaken into account. If the transformer HVvoltage is above 33KV, high-speed protectioncan be justified. If the voltage is less, the costof the protection scheme must be related to

the value of the transformer[10]. Thus the im-portance and ratings of a transformer are the



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factors that determine the type of protectiveequipment applied. Transformers rated up to5MVA and used at voltage levels below 33KVcan effectively and economically be protected

with fuses. The rating of these fuses must beabove the maximum exciting-current surges.As for larger transformers IDMT overcurrentand earth fault relays are used for protection.Current and time settings of these relays mustbe such that they should not operate when themaximum exciting-current surges flow, andalso, correct discrimination with other protec-tive equipment on associated networks mustbe provided for by the settings. These relays

can also serve as back-up protection for alllarge transformers, where the main protectionis the current-differential scheme. This typeof protection works on the principle that thecurrent entering a circuit is equal to the cur-rent leaving it under healthy conditions. Thezone of protection is marked by the positionsof current transformers on the primary andsecondary sides of the transformer. Trans-formers are also protected by other protective

equipment that do not rely on line currenttransformers for operation. Modern protec-tion systems are based on electronic devicesand circuits.

References

1. Microprocessor relays and protection sys-tems, IEEE Tutorial Course, 88EH0269-1-PWR, 1987.

2. Kennedy, L.F.. and Hayward, C.D. Har-monic restrained relays for Differential pro-tection, Trans. AIEE, 1998.

3. Current matching with interposing CTs,Differential Protection Symposium, BeloHorizonte, November, 2005.

4. Publications and Commissioning Manuals,Alstom Limited, Chennai, India, 1999

5. Poljac, M. and Kolibas, N. Computation ofcurrent transformer transient performance,Trans. IEEE, PD-3, 1816-1822.,1996.

6. Rahman, M.A. and Jeyasurya, B. A state ofthe art review of transformer protection al-gorithms, IEEE Trans. On Power Delivery,3, (2) 534-544, 1988.

7. O.N. Okemiri, I.V. Aghaisu-Oti, BasicPower System Protection Training (P1)

for Pupil Engineers/Technologists, PHCN,Ijora, Lagos, December, 2005.

8. I.T. Davies, Protection of industrial powersystems. Newness, Oxford, 2001.

9. A. R. van C. Warrington, Protective Relays Their Theory and Practice - Volume One,Chapman & Hall Ltd., 1971.

10. Liquid Insulated Primary and SecondaryUnit Substation Transformers, Small PowerTransformer Division, South Boston, ABBPower T & D Company Inc., 1990.


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SBEF and REF