over current and earth fault protection

8/7/2019 OVER CURRENT AND EARTH FAULT PROTECTION

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OVER CURRENT AND EARTH FAULT PROTECTION

The over current protection is needed to protect the transformer from sustained overloads and

short circuits. Induction type over current relays are used which in addition to providing overloadprotection acts as back up relays for protection of transformer winding fault. Fig 10 shows the

combined over current and earth fault protection. The earth fault protection is used to provideprotection against any earth fault in the windings of the transformer. It works on the principle

that when the transformer winding is sound the currents in all the three phases will balance andno current will spill into the earth fault relay. The arrangement is such that the relay does not

respond to any out of balance current between windings caused by tap changing arrangement

Fig 10 illustrates the use of earth fault and over current relays for both star and delta connectionsof the transformer. Instantaneous type of earth relay is used. When the winding is delta

connected the earth relay is operated by the residual current from three C.T.s connected as shownin left hand side of the fig. If the transformer winding has an earthed neutral then the residual

current from the three line current transformers is balanced against the current of the currenttransformers provided in the neutral as on the right hand side.

When the system works normal, the sum of three currents in the C.T.s is zero and no current

flows through the operating winding of the instantaneous earth fault relay and through the neutralof the transformer. However if fault is outside the protection area current flows in the neutral and

lines as well, but the sum of currents in the lines is balanced by the current in neutral and henceearth relay is not operated. Now if earth fault occur within the protected zone say in the winding

itself current will flow only in the neutral of the main transformer and thus there will be nobalancing current in the relay circuit so, the relay is energized and the circuit breaker is opened.

The trip contacts of the over current relay and earth fault relay are in parallel so, with theenergisation of either over current relay or earth fault relay the circuit breaker of the concerned

side will be tripped.


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Merz price circulating current system protects against phase-to- ground and phase-to-phasefaults. It does not protect against turn-to-turn fault on the same phase winding of the stator. It is

because the current that this type of fault produces flows in a local circuit between the turnsinvolved and does not create a difference between the currents entering and leaving the winding

at its two ends where current transformers are applied. It is usually unnecessary to provide inter

turn faults because they invariably develop into earth faults.

In single turn generator there is no necessity of protection against inter turn faults. However it is

provided for multi turn generators such as hydroelectric generators. These generators havedouble winding armatures owing to the very heavy currents, which they have to carry. Fig 7

shows the arrangement of a 3-phase double wound generator. The relays Rc provide protectionagainst phase to ground and phase-to-phase faults whereas relays R1 provide protection against

inter turn faults. S1 and S2 are the stator windings of one phase. Two current transformers areconnected on the circulating current principle. Under normal conditions, the current in the stator

windings are equal and so will be the currents in secondaries of the two C.T.s. the secondarycurrent round the loop then is

same at all points and no current flows through the relay R1. If a short circuit develops betweenthe adjacent turns on S1 the currents in stator windings S1and S2 will no longer be equal.

Therefore unequal currents will be induced in the secondaries of C.T.s and the difference of


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these two current flows through the relay R1. The relay then closes its contacts to clear thegenerator from the system

OVER CURRENT AND EARTH FAULT PROTECTION:-

The over current and earth fault protection is same as the over current and earth fault protectionof transformers which will be discussed later. Fig 6 shows the over current and earth faultprotection scheme for generators.

DIFFERENTIAL PROTECTION:-

This type of protection provides a continuous check on faults within the points, where the C.T.s

are used as in shown in fig 2. There are two sets of C.T.s; each set is mounted on either end ofthe stator phase. The secondaries of these current transformer sets are connected in star and their

ends are connected together through pilot wires. The over current inverse time relay is connectedacross the equipotential points in the pilot wires, which will naturally be in center of these wires

as shown in fig 2. But its convenient to locate these relays adjacent to the main circuit breaker;this can be achieved by inserting balancing resistance in series with the pilot wires to achieve

equipotential point near the main circuit breaker as shown in fig 3. If there is no fault in thegenerator, same current will pass through the C.T.s at both the ends and there will be no current

spilling into the relay.


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Now if earth fault occurs as shown in fig 2, there will be an increase in current through set C.T.1as compared to current through C.T.2. Thus there is no current balance any more resulting into

spilling of current through the relay R hence tripping the circuit. If there is short circuit betweenthe two phases as in fig3 it will also cause operation of relay R and tripping of the circuit

breaker. This scheme does not provide protection for the turns of same phase


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RESTRICTED EARTH FAULT PROTECTION:-

Windings of many smaller transformers are protected by restricted earth fault (REF)systems. Even so, I have come across a lot of installations which would have benefitedfrom this type of protection, but for whatever reason it has not bee installed.

The illustration shows the principal of REF protection. Under normal conditions and byapplication of Kirchhoffs laws the sum of currents in both current transformers (CTs)equals zero. If there is an earth fault between the CTs then some current will bypassthe CT's and the sum of currents will not be zero. By measuring this current imbalancefaults between the CTs can be easily identified and quickly cleared. Fault detection isconfined to the zone between the two CTs hence the name 'Restricted Earth Fault'.

REF protection is fast and can isolate winding faults extremely quickly, thereby limitingdamage and consequent repair costs. If CTs are located on the transformer terminalsonly the winding is protected. However, quite often the line CT is placed in thedistribution switchboard, thereby extending the protection zone to include the maincable.

Without REF, faults in the transformer star secondary winding need to be detected onthe primary of the transformer by reflected current. As the winding fault position movestowards the neutral, the magnitude of the current seen on the primary rapidly decreasesand could potentially not be detected (limiting the amount of winding which can be

protected). As the magnitude of the currents remain relatively large on the secondary(particularly if solidly earthed), nearly the entire winding can be protected using REF.

It should be remembered that the protection as illustrated covers only the secondary ofthe transformer. Sometimes REF protection is added to the primary as well (although ifprimary protection is required I would prefer to consider full differential protection).


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As it is essential that the current in the CTs be balanced during normal conditions (andthrough faults), historically REF has been implemented using High Impedance Relays.CT's have also been specified as matched pairs and the impedance of leads/wires andinterconnecting cables has had a large influence on the functioning of the relay.Measurement errors associated with these issues have been responsible for nuisance

tripping and the system could be difficult to commission. This may be the reason somepeople avoid the use of REF. Recent advances in numerical relay technology have allbut eliminated these issues, making the implementation of REF relatively easy, ensuringno nuisance tripping and simplifying commissioning.

In the diagram 87N is the ANSI Device Number for restricted earth fault.

BACK UP PROTECTION OF TRANSFORMERS

Relay Backup

The primary protection for large transformers,

almost without exception, is high-speed

differential relaying. Relay back-up protection,

when provided, usually consists of a fault-

pressure relay, a tank-to-ground relay, time

overcurrent relays or distance relays. See Figure

10(a). The time overcurrent relay is provided for

system fault back-up protection but gives some

relay back-up protection for moderate to severe

internal transformer faults.

Breaker Backup

To provide breaker backup, a primary relay plus

timer scheme, similar to that shown in Figure

10(b), could be used. Again, it is noted that

selector relays are not adequate but may be

used in addition to breaker a switches for


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selection of the proper back-up breakers to trip.

If the transformer of Figure 10(a) is connected in

a line without a line-side breaker, the remote

breaker would be tripped by transferred-trip

signal initiated by the transformer-differential

relays. If the remote breaker failed to trip for any

reason, the transformer fault would not be

cleared. However, at least one installation is

planned in which a second transferred-trip

channel will be used to trip the remote bus if the

remote line breaker does not operate.

OIL / WINDING TEMPERATUREINDICATORS

Belltek Mechanical Instruments are incorporates proven designfeatures acquired from many years of experience in providing

Temperature Indicators/Controllers for Power & DistributionTransformers

Oil Temperature Indicator : The Oil Temperature Indicator (OTI)measures the Top oil Temperature. It is used for control and

protection for all transformers.

Winding Temperature Indicator : The Winding is the component with

highest temperature within the transformer and, above all, the onesubject to the fastest temperature increase as the load increases.

Thus to have total control of the temperature parameter withintransformer, the temperature of the winding as well as top oil, mustbe measured. An indirect system is used to measure winding

temperature, since it is dangerous to place a sensor close to windingdue to the high voltage. The indirect measurement is done by means

of a Built-in Thermal Image.

Winding Temperature Indicator is equipped with a specially designed

Heater which is placed around the operating bellows through whichpasses a current proportional to the current passing through thetransformer winding subject to a given load. Winding Temperature is

measured by connecting the CT Secondary of the Transformerthrough a shunt resistor inside the Winding Temperature Indicator to

the Heater Coil around the operating Bellows. It is possible to adjustgradient by means of Shunt ResistorIn this way the value of the winding temperature indicated by theinstrument will be equal to the one planned by the transformer


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manufacturer for a given transformer load.

SALIENT FEATURES :

1)No external power supply required for

indication or operation

2)Liquid in metal expansion provides selfpowered system

3)Fully compensated for ambient

temperature changes in line & case

4)Built-in Thermal imaging fortransformer 'Hot Spot' temperature

indication (in case of WTI)

5)Weather proof case for outdoor services(IP55)

6)

Panel mount or Projection mounting

(varies with Different Models)

7)Maximum capillary length of 15 mtrs.(varies with Different Models)

8)Up to 4 control switches are available

(varies with Different Models)

9)Maximum indicating pointer facilitySwitch Test Knob facility

10)Remote Indication & Data acquisition

facility

11)

Sealed system which requires no

trimming or periodic site adjustments.

A) MERCURY SWITCH TYPE OIL TEMPERATURE INDICATORS (OTI) /WINDING TEMPERATURE INDICATORS (WTI) INSTRUMENTS

Belltek Model no: 9002

This is a high performance microswitch instrument with +/- 2Caccuracy. capable of temperature

monitoring, indication & control,for the protection of Transformersfrom 1 MVA to 10 MVA rating.

Technical Data :

Dial Diameter : 118 mm Segment

DialAngular Sweep : 60

Range : 0-120C, 0-150C, 20-140C & 30-150C

Accuracy : +/-2CGraduations : 2C per division

Electrical Connections :All line to body

(Earth) High VoltageBreakdown Tested : 2.5 KV AC,50Hz for 60 seconds

Terminal Type : WAGO Terminals


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Adjustable Range : 10-90% of FullScale RangeSwitch Differential : 8C Fixed

Switch Accuracy : +/-2.5C of SetValueContact Type : Switch Contacts

are Potential FreeInstrument Case : WeatherproofAluminum Alloy Casting with

epoxy coated at all sides. Suitablefor outdoor mounting - Protective

Class IP55 as per IS 13947 Part II.

Capillary Entry : Bottom or Top ofthe caseCap. Length : Up to 6 Meters

Cap. Material : Capillary lineprotected by flexible StainlessSteel Armour

Standard Fittings : 1) SwitchTesting Knob

2) ResettableMaximum Pointer

3) Anti-Vibration Mounting

B) MICRO SWITCH TYPE OIL TEMPERATURE INDICATORS (OTI) /WINDING TEMPERATURE INDICATORS (WT) INSTRUMENTS

Belltek Model No. : 9004

This is a high performanceinstrument with +/- 2C accuracy.capable of temperaturemonitoring, indication & control,for the protection of Transformers

from 1 MVA to 10 MVA rating

Technical Data :

Dial Diameter : 118 mm SegmentDialAngular Sweep : 60

Range : 0-120C, 0-150C, 20-140C & 30-150CAccuracy : +/-2C

Graduations : 2C per division

Electrical Connections :All line to body

(Earth) High VoltageBreakdown Tested : 2.5 KV AC,50Hz for 60 seconds

Terminal Type : WAGO TerminalsPress Fit Type

Switch Configurations :Maximum Number of Switches : 4Switch Type : Heavy Duty MicroSwitchSwitch Rating : 5A at 240V 10%

AC & 0.5 A at 220 V 10% DCSwitches : Normally Open Type

Adjustable Range : 10-90% of Full

Scale RangeSwitch Differential : 5C Fixed

Switch Accuracy : +/-2.5C of SetValueContact Type : Switch Contacts

are Potential FreeInstrument Case : Weatherproof

pressure die-cast Aluminum AlloyCasting with epoxy coated tallsides.

Protective Class IP55 as per IS

13947 Part II.


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Capillary Entry : Bottom or Top ofthe case

Cap. Length : Up to 15 MetersCap. Tubing : Capillary lineprotected by flexible Stainless

Steel Armour and additionalStainless Steel Armour to reinforceat the entry to the case.

Standard Fittings : 1)

Resettable Maximum Pointer2) Anti-

Vibration Mounting Grommets (forProjection

Mounting)

Belltek Model No. : 9031

This is a high performance,

precision micro switch instrumentwith +/- 2.50C accuracy, capableof temperature monitoring,

indication & control, for theprotection of Transformers from 5MVA rating.

Technical Data :Dial Diameter : 150 mm Segment

DialAngular Sweep : 60

Range : 0-150CAccuracy : +/-2.5C

Graduations : 2C per division

Electrical Connections :

Cable Conductor Size : 1 sq. mm.Oil Temperature : 4 Core CopperCable for Switch Connections

(Length as per CustomerRequirement)Winding Temperature Indicator : 4Core for Switch connections and 2Core Copper Cable for CT

Connection Length as perCustomer Requirement)

All Line to Body (Earth) HighVoltage Breakdown Tested : 2.5KV AC, 50Hz. for 60 Seconds

Switch Configurations :Maximum Number of Switches : 2

Switch Type : Heavy Duty MicroSwitchSwitch Rating : 5A at 240V 10%AC & 0.5 A at 220 V 10% DCSwitches : Normally Open Type

Adjustable Range : 10-90% of FullScale RangeSwitch Differential : 5C Fixed

Switch Accuracy : +/-2.5C of SetValue

Contact Type : Switch Contactsare Potential FreeInstrument Case : Weatherproof

Aluminum Alloy Casting withepoxy coated at all sides. Suitable

for outdoor mounting - ProtectiveClass IP55 as per IS 13947 Part II.

Capillary Entry : Bottom or Top ofthe caseCap. Length : Up to 6 Meters

Cap. Material : Capillary lineprotected by flexible Stainless

Steel Armour

Standard Fittings : 1)Resettable Maximum Pointer


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2) Anti-Vibration Mounting

over current and earth fault protection

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