PROTECTION OF HT &PROTECTION OF HT & L L TT TRANSFORMERTRANSFORMER

(A) INTRODUCTION: (A) INTRODUCTION:

• Power transformer is a very vital link in Power transformer is a very vital link in power transmission system. The power transmission system. The development of modern power system development of modern power system has been reflected in the advances in has been reflected in the advances in transformer designs. This has resulted transformer designs. This has resulted in a wide range of transformers with in a wide range of transformers with sizes ranging from few KVA to Several sizes ranging from few KVA to Several hundred MVA being available for use in hundred MVA being available for use in a wide variety of applications.a wide variety of applications.

– The consideration for a transformer The consideration for a transformer protection vary with application & protection vary with application & importance of transformer. To reduce the importance of transformer. To reduce the effect of thermal stress & effect of thermal stress & electrodynamics forces, it is advisable to electrodynamics forces, it is advisable to ensure that the protection package used ensure that the protection package used minimizes the time for disconnection minimizes the time for disconnection in .the event of a fault occurring within in .the event of a fault occurring within transformer.transformer.

– Small distribution transformer can be Small distribution transformer can be protected using fuses and over current protected using fuses and over current relays. Time delayed fault clearance is relays. Time delayed fault clearance is not accepted on large power transformer not accepted on large power transformer due to system operation / stability and due to system operation / stability and cost of repair or length of outages.cost of repair or length of outages.

(B) TYPES OF (B) TYPES OF TRANSFORMER FAULTS:TRANSFORMER FAULTS:1 Transformer faults are generally 1 Transformer faults are generally

classified into five categories.classified into five categories.

• i) Winding and terminal faultsi) Winding and terminal faults• ii) Core faultsii) Core faults• iii) Tank and transformer accessory faultsiii) Tank and transformer accessory faults• iv) ON load tap changer faultsiv) ON load tap changer faults• v) Abnormal operating conditionv) Abnormal operating condition• vi Sustained or un cleared external faultsvi Sustained or un cleared external faults

• Out of these faults, faults from (i) to (iv) are Out of these faults, faults from (i) to (iv) are faults originating in transformer itself while faults originating in transformer itself while faults ( v) & (vi) are attributed as the system faults ( v) & (vi) are attributed as the system faults.faults.

(C) WINDING FAULT :(C) WINDING FAULT :• . . A FAULT IN TRANSFORMER A FAULT IN TRANSFORMER

WINDING IS CONTROLLED IN WINDING IS CONTROLLED IN MAGNITUDE BY THE FOLLOWING MAGNITUDE BY THE FOLLOWING FACTORS. FACTORS.

(i) Source Impedance(i) Source Impedance

(ii) Neutral Earthing impedance(ii) Neutral Earthing impedance

(iii) Transformer leakage reactance(iii) Transformer leakage reactance

(iv) Fault voltage(iv) Fault voltage

(v) Winding connection.(v) Winding connection.

Several distinct cases are as belowSeveral distinct cases are as below

a) Star connected winding with Neutral a) Star connected winding with Neutral earthed through an impedance: earthed through an impedance:

The winding earth fault current depends on The winding earth fault current depends on earthing impedance and is proportional to earthing impedance and is proportional to the distance of the fault from neutral, since the distance of the fault from neutral, since fault voltage is directly proportional to this fault voltage is directly proportional to this distancedistance

b )Star connected winding with Neutral b )Star connected winding with Neutral solidly earthed: solidly earthed:

The fault current is controlled mainly by The fault current is controlled mainly by leakage reactance of winding which varies leakage reactance of winding which varies in a complex manner with position of fault. in a complex manner with position of fault. For faults close to neutral end of winding, For faults close to neutral end of winding, the reactance is very low and results in the reactance is very low and results in highest faults current.highest faults current.

c) Delta connected windings : c) Delta connected windings : The range of fault current magnitude is The range of fault current magnitude is

less than for a star connected winding. less than for a star connected winding. The actual value of fault current will The actual value of fault current will depend on the method of system depend on the method of system earthing.earthing.

d) Phase to phase faults : d) Phase to phase faults : Faults between phases within a Faults between phases within a

transformer are relatively rare. If such transformer are relatively rare. If such fault occurs it will give rise to fault occurs it will give rise to substantially high current comparable to substantially high current comparable to the earth fault currents discussed earlier.the earth fault currents discussed earlier.

((e) Interturn Faults : e) Interturn Faults :

. . In a low voltage transformer In a low voltage transformer interturn insulation interturn insulation breakdown is unlikely to breakdown is unlikely to occur, unless the mechanical occur, unless the mechanical force on the winding due to force on the winding due to external short circuit has external short circuit has caused insulation degradation caused insulation degradation or insulating oil has become or insulating oil has become contaminated by moisture.contaminated by moisture.

• A high voltage transformer connected A high voltage transformer connected to transmission system will be to transmission system will be subjected to a impulse voltage, arising subjected to a impulse voltage, arising from lighting strikes, faults & switching from lighting strikes, faults & switching operation. A line surge which is several operation. A line surge which is several times the rated voltage will times the rated voltage will concentrate on the end turns of concentrate on the end turns of winding. Partial winding flashover is winding. Partial winding flashover is therefore more likely. The subsequent therefore more likely. The subsequent progress of fault, if not detected in progress of fault, if not detected in early stage, may destroy the evidence early stage, may destroy the evidence of the true cause.of the true cause.

(D) CORE FAULTS(D) CORE FAULTS

• . . A conducing bridge across the laminated A conducing bridge across the laminated structures of the core can permit structures of the core can permit sufficient eddy current to flow to cause sufficient eddy current to flow to cause serious overheating. The bolt that clamp serious overheating. The bolt that clamp the core together are always insulted to the core together are always insulted to avoid this trouble. If any portion of the avoid this trouble. If any portion of the core insulation becomes defective, the core insulation becomes defective, the resultant heating may reach a magnitude resultant heating may reach a magnitude sufficient to damage the winding.sufficient to damage the winding.

• In an oil immersed transformer, core In an oil immersed transformer, core heating sufficient to cause winding heating sufficient to cause winding insulation damage will also cause insulation damage will also cause breakdown of oil with evolution of gas. breakdown of oil with evolution of gas. This gas will escape to the conservator, This gas will escape to the conservator, and is used to operate a mechanical and is used to operate a mechanical relay.relay.

(E) TANK FAULTS(E) TANK FAULTS• Loss of oil through tank leaks will Loss of oil through tank leaks will

produce a dangerous condition, either produce a dangerous condition, either because of a reduction in winding because of a reduction in winding insulation or because of overheating due insulation or because of overheating due to loss of cooling.to loss of cooling.

EXTERNALLY APPLIED EXTERNALLY APPLIED CONDITIONSCONDITIONS

•Sources of abnormal stress in Sources of abnormal stress in a transformer area transformer are

i) Overloadi) Overload

ii) System faultsii) System faults

iii) Over voltageiii) Over voltage

iv) Reduced system frequency.iv) Reduced system frequency.

• OVER LOAD:-OVER LOAD:-

Overload causes increased "copper loss" Overload causes increased "copper loss" and a consequent temperature rise. and a consequent temperature rise.

• SYSTEM FAULTS :- SYSTEM FAULTS :-

System circuit produce a relatively System circuit produce a relatively intense rate of heating of feeding intense rate of heating of feeding transformer. Copper loss in such cases transformer. Copper loss in such cases increases in proportion to the per unit increases in proportion to the per unit fault current. Maximum mechanical stress fault current. Maximum mechanical stress on winding occurs during the first cycle of on winding occurs during the first cycle of the fault.the fault.

OVER VOLTAGEOVER VOLTAGE OVER VOLTAGE CONDITION ARE OF OVER VOLTAGE CONDITION ARE OF

TWO KINDSTWO KINDS• a) Transient surge voltages.a) Transient surge voltages.• b) Power frequency over voltagesb) Power frequency over voltages. .

a) Transient over voltages arise from a) Transient over voltages arise from faults, switching; and lightning faults, switching; and lightning disturbances and are, liable to cause disturbances and are, liable to cause interturn faults. These over voltage are interturn faults. These over voltage are usually limited by providing surge usually limited by providing surge diverters.diverters.

b) Power frequency over voltage cause b) Power frequency over voltage cause both an increase in stress on the both an increase in stress on the insulation and a proportionate increase in insulation and a proportionate increase in working flux. The latter effect causes an working flux. The latter effect causes an increase in iron loss and large increase in increase in iron loss and large increase in magnetizing current.magnetizing current.

REDUCED SYSTEM REDUCED SYSTEM FREQUENCYFREQUENCY

• Reduction of system frequency has an Reduction of system frequency has an effect with regard to flux density, effect with regard to flux density, similar to that of over voltage. A similar to that of over voltage. A transformer can operate with some transformer can operate with some degree of over voltage with a degree of over voltage with a corresponding increase in frequency, corresponding increase in frequency, but operation must not be continued but operation must not be continued with a high voltage input at a low with a high voltage input at a low frequency.frequency.

TRANSFORMER TRANSFORMER OVERHEATINGOVERHEATING• The rating of a transformer is based on The rating of a transformer is based on

the temperature rise above an assumed the temperature rise above an assumed maximum ambient temperature. Under maximum ambient temperature. Under this condition no sustained overload is this condition no sustained overload is usually permissible.usually permissible.

• Therefore winding must not overheat. A Therefore winding must not overheat. A rise of 8 to 10 degrees in temp will rise of 8 to 10 degrees in temp will halve the insulation life of the unit. halve the insulation life of the unit. Protection against this is based on Protection against this is based on winding temperature which is winding temperature which is measured by thermal imaging measured by thermal imaging technique.technique.

OVER VIEW OF OVER VIEW OF TRANSFORMER TRANSFORMER PROTECTIONPROTECTION• Faults relating to transformers Faults relating to transformers

require some means of protection. require some means of protection. Electromechanical relays protects Electromechanical relays protects against single type of fault while against single type of fault while modern numerical relay provide all of modern numerical relay provide all of the required protection functions in a the required protection functions in a single package.single package.

Fault typeFault type Protection usedProtection used

Primary winding Ph - Primary winding Ph - ph fault ph fault

Differential, over Differential, over currentcurrent

Primary winding ph Primary winding ph to earthfaultto earthfault

Differential, over Differential, over currentcurrent

Secondary winding Secondary winding ph- ph faultph- ph fault

DifferentialDifferential

Secondary winding Secondary winding ph to earth faultph to earth fault

Differential, Differential, Restricted Earth faultRestricted Earth fault

Inter turnInter turn Differential, Differential, Restricted Earth Restricted Earth faultfault

Core FaultCore Fault Differential; Differential; BuchholzBuchholz

Tank fault Tank fault Differential; Differential; BuchholzBuchholz

Over fluxingOver fluxing Over fluxing relay.Over fluxing relay.

Over heatingOver heating ThermalThermal

TRANSFORMER OVER CURRENT TRANSFORMER OVER CURRENT PROTECTIONPROTECTION

• Fuses may adequately protect small Fuses may adequately protect small transformer, but larger one require over transformer, but larger one require over current protection using a relay & circuit current protection using a relay & circuit breaker as fuses do not have the required breaker as fuses do not have the required fault breaking capacity.fault breaking capacity.

• . . Fuse commonly protect small distribution Fuse commonly protect small distribution transformer. For protection of large transformer. For protection of large transformers over current relays are used. transformers over current relays are used. These relays are connected to current These relays are connected to current transformers. The contacts of these relays transformers. The contacts of these relays are used in circuit breaker control.are used in circuit breaker control.

• Typical examples of E. E. make relay is Typical examples of E. E. make relay is CDG 63. This is a IDMT (Inverse Definite CDG 63. This is a IDMT (Inverse Definite mean time) protection. The time delay mean time) protection. The time delay should be chosen to discriminate with should be chosen to discriminate with circuit protection on secondary side. High circuit protection on secondary side. High set instantaneous relays element is also set instantaneous relays element is also provided which enables high speed provided which enables high speed clearance of primary terminal short clearance of primary terminal short circuits circuits

RESTRICTED EARTH F AUL T RESTRICTED EARTH F AUL T PROTECTIONPROTECTION

• For star connected winding with impedance For star connected winding with impedance earthed neutral, earth fault protection using over earthed neutral, earth fault protection using over current element is not adequate. The degree of current element is not adequate. The degree of protection is very much improved by REF protection is very much improved by REF protection.protection.

• . . The residual current of three line CT's is The residual current of three line CT's is balanced against the output of a CT in neutral balanced against the output of a CT in neutral conductor. The system is operative for faults conductor. The system is operative for faults within the zone ofwithin the zone of

CT's. The system will remain stable for all faults CT's. The system will remain stable for all faults outside this zone. REF is also often applied even outside this zone. REF is also often applied even when the neutral is solidly grounded. Since fault when the neutral is solidly grounded. Since fault current then remains at a high value even to the current then remains at a high value even to the last turn of winding virtually complete cover for last turn of winding virtually complete cover for earth fault is obtained.earth fault is obtained.

DIFFERNTIAL DIFFERNTIAL PROTECTIONPROTECTION

• This protection is usually employed This protection is usually employed for large power transformers and is for large power transformers and is very effective & clears the fault very very effective & clears the fault very speedily. Current transformers on the speedily. Current transformers on the primary and secondary sides are primary and secondary sides are connected to form a circulating connected to form a circulating current system.current system.

FAULT OUT OF ZONEFAULT OUT OF ZONE

P1

P1 P2

P2

S1 S2 S1S2R

FAULT WITHIN ZONEFAULT WITHIN ZONE

P1

P1 P2

P2

S1 S2 S1S2R

OVER FLUXING OVER FLUXING PROTECTIONPROTECTION

• The over fluxing arises principally The over fluxing arises principally from following system conditions.from following system conditions.

a) High system voltagea) High system voltage

b) Low system frequencyb) Low system frequency

• A time delayed tripping is provided A time delayed tripping is provided since momentary system disturbances since momentary system disturbances can cause transient over fluxing that can cause transient over fluxing that is not dangerous. The relay is is not dangerous. The relay is triggered if a defined V/ F threshold is triggered if a defined V/ F threshold is exceeded.exceeded.

OIL AND GAS DEVICESOIL AND GAS DEVICES

• All faults below oil in an oil immersed All faults below oil in an oil immersed transformer result in localized heating transformer result in localized heating and breakdown of oil. This ultimately and breakdown of oil. This ultimately results in release of gases. results in release of gases.

• When fault is minor - Gas releases slowlyWhen fault is minor - Gas releases slowly

• When fault is severe - Rapid release of When fault is severe - Rapid release of high volume of Gases.high volume of Gases.

Buchholz Protection Buchholz Protection ::• Buchholz protection is normally provided Buchholz protection is normally provided

on all transformer fitted with conservator. on all transformer fitted with conservator. A typical buchholz relay will have two sets A typical buchholz relay will have two sets of contacts, Alarm and trip.of contacts, Alarm and trip.

• When a major winding fault occurs, this causes When a major winding fault occurs, this causes a surge of oil which displaces the trip float & a surge of oil which displaces the trip float & causes isolation of transformer by the tripping causes isolation of transformer by the tripping of circuit breaker. of circuit breaker.

• Buchholz will operate forBuchholz will operate for a) All severe winding faults, either to earth or a) All severe winding faults, either to earth or

inter phaseinter phase.. b) Loss of oil if allowed to continue to a b) Loss of oil if allowed to continue to a

dangerous level.dangerous level.

Pressure Relief Devices: Pressure Relief Devices:

• . . Simplest form of pressure relief device is Simplest form of pressure relief device is the widely used frangible disc located at the widely used frangible disc located at the end of on oil relief pipe protruding the end of on oil relief pipe protruding from the top of tank. Surge of oil caused from the top of tank. Surge of oil caused by serious faults burst the disc and allow by serious faults burst the disc and allow the oil to discharge rapidly. This avoids the oil to discharge rapidly. This avoids the explosive rupture of the tank and the explosive rupture of the tank and consequent fire risk.consequent fire risk.

• For large power transformers instead of For large power transformers instead of disc, a pressure relief valve is provided disc, a pressure relief valve is provided which opens to allow discharge of oil if which opens to allow discharge of oil if the pressure exceeds a set level, but the pressure exceeds a set level, but closes automatically as soon as internal closes automatically as soon as internal pressure falls below this level. If the pressure falls below this level. If the abnormal pressure is relatively high, PRV abnormal pressure is relatively high, PRV can operate within few milliseconds & can operate within few milliseconds & trips the switchgear with the help of trips the switchgear with the help of suitable contacts fitted on it.suitable contacts fitted on it.

OTHER PROTECTIONSOTHER PROTECTIONS

• OTI OTI & & WTI PROTECTION.WTI PROTECTION.

• CONSERVATOR OIL LEVEL CONSERVATOR OIL LEVEL PROTECTIONPROTECTION

FIRE PROTECTION.FIRE PROTECTION.