Transformer Protection

TRANSFORMER PROTECTION

Transformer Technology Design and Operation

University of Queensland July 2009

Transformer windings damaged by excessive through fault current

Fault Rate

In the order of 1 fault per 100 years per transformer

TYPES OF TRANSFORMER FAULT

Phase-ground faults - from winding to core or winding to tankPhase-phase faults - between windings Interturn faults - between single turns or adjacent layers of the same winding Arcing contactsLocal hotspots caused by shorted laminationsExternal faults causing thermal or mechanical damageoverloads

TYPES OF TRANSFORMER FAULT

Low level internal partial discharges (moisture ingress or design problems)Bushing faults (internal to the tank)Tapchanger faults (often housed in a separate tank)Terminal faults (external to the tank, but inside the transformer zone)

Protection Grouping

As far as possible, select one protection type in each protection group (X/Y or Main/Backup) to cover each type of fault.

This will achieve the best diversity of fault coverage.

BUCHHOLZ RELAY

provides very sensitive protection for oil-filled transformers and reactorsonly suitable for transformers fitted with an oil conservatorarguably the best overall transformer protection for internal faultscan be sensitive to accidental bumps or pump starts

Buchholz Relay(EMB Germany)

BUCHHOLZ – alarms for:

Local winding overheating - alarmLocal core overheating (short circuited laminations)Bad contacts or jointsPartial dischargeBroken down core bolt insulation

BUCHHOLZ – trips for:

Detection of loss of or low oil due to1. Leaky pipe joints2. Tank faults3. Contraction of oil under low

temperatures and light load

major internal faults (inter-turn faults or faults involving earth) which result in oil surges to the conservator.

BUCHHOLZ PRINCIPLE

There are two floats in the relay:upper float - detects accumulation of gas –generates alarm onlylower float - detects surge in oil - trips in less than 100msoptional “loss of oil” trip, associated with trip float

Normal state

to transformer

to conservator

alarm float

screw removal for low oil trip

mercury wetted relayfor alarm

mercury wetted relay for trip

trip float

to ground level gas receiver

adjustable tripping vane

reverse flow oil guard

contacts

contacts

Gas build-up alarm

GASOIL

Oil surge trip

oil surge

Pressure Relief Device

“Qualitrol” brand - a spring assisted pressure relief devicerelieves pressure impulses due to massive internal fault conditions. helps prevent the tank bursting or splitting relay contacts are also connected to trip the transformer.


Since pressure waves travel with a finite velocity, they may rupture the tank locally before the pressure wave has reached the pressure relief device, if it is some distance away. Several units are therefore often used on larger transformers.


Spring resets after pressure is relieved -this prevents excessive oil lossuses hydraulic amplification to achieve fast operation - several ms

Qualitrol™

Other pressure relief devices

On-load tap changer tanks may be fitted with a simpler gas impulse relay to protect against tapchanger failure

Overtemperaturegenerally regarded as overload protectionalso deals with failure of or interference with pumps and fans or shutting of valves to pumpsWinding hot spot temperature is the main issue, but both oil and winding temperatures are usually measured and used to:

initiate an alarmtrip circuit breakerscontrol fans and pumps

OvertemperatureTwo temperatures must be monitored

Winding temperature (‘WTI’) - (short thermal τ) this can rise rapidly, without much of an increase in oil temperature Oil temperature (‘OTI’) - (long thermal τ) this can rise slowly to a critical point without an unacceptable winding temperature increase

Temperature Measurementmost common device uses a Bourdon Tube (invented 1849) to measure temperatures

probe placed in oil-filled pocket at the top of transformer - mercury expansion in the probe causes the spiral Bourdon tube to try to straighten, rotating a mechanical arm

Typically two of these are used per transformer - one for winding (including load compensation) and one for oil

Conventional Bourdon tube based temperature indicator(Kihlstrom)

Winding temperature calculation

a calculated value of winding hot spot is made from measurements of oil temperature and load currenta heater, driven by a load current CT, and together with a matching unit, is used as a thermal model of the main winding. heater can be located in the oil pocket placed in the top oil, (the hottest place) or in the WTI itself.

Typical bourdon tube scheme with winding temperature compensation

CTheaterMatching

unit

(transformer dependent)

Alternativesembedded fibre optic sensors for direct measurement of winding hot spot temp are also popular, especially during factory testsan RTD (resistance temperature detector) can also used to measure top oil tempwinding temperature can also be calculated, (in e.g. a PLC or RTU) from measured top oil temp and load currentthese approaches have not displaced the proven, economical and robust Kihlstrom

Typical alarm and trip levels(dependent on asset management policy)

winding alarm - 90ºC to 110ºCwinding trip - 110ºC to 135ºCoil alarm - 80ºC to 95ºCoil trip - 95ºC to 115ºC

Oil trip may be disabled if transformer is readily accessible bymaintenance crews – on the grounds they can visit sub and may be able to remedy problem. This is a controversial practice.

Temperature vs life

economic gains are possible from short time overloads - “life used” calculations may permit higher temperatures for short periods, but WTI trip needs to be more complex or monitored110 ºC winding hot spot temperature gives ‘standard’ 20-25 year life of insulationRoughly every 7 ºC increase in temperature doubles the rate of loss of life for paper in oil insulation

Fuses for Transformers

Used in transformers up to a rating of typically 1MVA, but no higher than10MVAfuses should be rated continuously for emergency overload – this is a fundamental limit to their sensitivity to faults

Fuses provide reasonable protection at low cost – good for low cost (small) transformersSome (HRC) fuses are actually better than a relay/circuit breaker combination in limiting the amount of damage to plant (and personnel)

Advantages of Fuses

The cost of replacement, including timeThey often do not offer protection at currents just above fuse rating - often failing explosively. This means – fuses are for protection against faults, not protection against overloadsNo better sensitivity to earth faults than to inter-phase faults (c.f. O/C and E/F relays)Time-current characteristics are influenced by ambient temperature and pre-fault load current

Disadvantages of Fuses

fuse must be able to withstand the magnetizing inrush current that occurs on energization

6x rated current for up to 1s10x to 12x rated current for 100ms25x rated current for 10ms

Inrush Current Withstand

lightning-caused overvoltages may cause transient line charging and transformer inrush, leading to fuse deterioration or even spurious operation

Lightning Performance

Fuses Types for Transformers

High rupturing capacity (HRC) fuses for ground level (padmount) transformers –these are always also current limitingfusesExpulsion drop-out fuses for pole mounted transformers

For these current limiting fuses, the prospective peak fault current is not reached, except for low level faultsFully enclosed in a ceramic body with quartz filling and metal end capsElements are quite expensive (up to hundreds of dollars)Only ‘Full Range’ fuses guaranteed to safely break all currents which melt the element

High Rupturing Capacity (HRC) Fuses

Current limiting fuse

prospective current

peak voltage

cut-off current

recovery voltage

Current Limiting Characteristic

For external use only, on distribution circuitsnot of the current limiting variety – these interrupt at a current zeroUsed on distribution systems at 11 to 33kV and up to 3MVA

Expulsion Drop-out (EDO) Fuses

elements are low cost – in order of $10“drop-out” action prevents tracking across burnt sections of the fuse and provides a visual indication of operationHave a limited upper breaking current capability


Be aware there are two typestype ‘K’ – fast type ‘T’ – slow

Type ‘K’ can sometimes blow spuriously, hence the development of type ‘T’Don’t mix the two types


Expulsion

drop-out

fuse

Expulsion drop-out

fuse after operation

a margin between the maximum clearing time of the downstream fuse and theminimum melting time of the upstream fuse is requiredHRC fuses - charts used EDO fuses

‘75% of min. melting time’ rule tables of max coordination current

Co-ordination of fuses

Fuse Rating

16050403532 1006 3 12580 200

2 50

maximumtotal I2t

minimumpre-arcingI2t

Chart for grading HRC Fuses

Graphically grading EDO Fuses(method 1)

Fuse A’s max clearing time to be less than 75% fuse B’s min melting time at max fault current

Fuse grading chart

0

1

2

3

4

5

6

7

8

9

10

10 100 1000

Current (A)

time

(s)

Max clearing time fuse B

Min melting time fuse B

Max clearing time fuse A

Min melting time fuse A

OVERCURRENT & EARTH FAULT PROTECTION RELAYS

Used in transformers up to approximately 50MVAFor 10MVA tx – provides main protectionFor 50MVA tx– provides backup protection onlyCommon at voltages up to about 66kV

Overcurrent (O/C) Protection

An overcurrent relay sees phase currents and hence all types of faultOvercurrent relay settings must be above transformer emergency overload – as with fuses, this determines the fundamental limit to their sensitivity


A suitable margin should also be allowed in the current setting for:

growth in load - alwaysrelay reset ratio - optionalcold load pick-up - optional (often a relay feature)transformer taps - optional


An instantaneous O/C element can usually be used to provide very fast clearance for faults close to the HV terminalMust be set such that LV faults are not seen - discrimination

Coping with load growth

allow for a number of years of forecast growth and review after this time …….or base setting on transformer emergency rating

safer option, but slower and less sensitive if transformer capacity not fully utilised yetreview needed only when transformer replaced

Cold load pickup – two aspects

1. Starting current of motors – lasts about 10s

2. Restarting of heating, air-conditioning, or refrigeration plant after prolonged outage – lasts many minutes

Cold load pickup – motor starting current

Short term increase in load following energisation (from Areva NPAG)

Earth Fault (E/F) Protection

An earth fault (E/F) relay sees either transformer neutral or residual (sum of three phases) current, depending on CT locationhence sees earth faults onlyE/F relays can be set well below load –10% of load typical.

RelayLocations HV O/C & E/F

LV NEF

LOAD

HV NEF

current transformer

circuit breaker

NEF = Neutral Earth Fault relay

Physical Arrangements

Older installations often economically configured as 2 x O/C relays + 1 x E/F relaywhere a 2:1:1 current distribution is possible, 3 x O/C + 1 x E/F is betterThis improves sensitivity and speed

33kV

c

b

11kV

c

LV PHASE-PHASE FAULT

a b a

2:1:1 Current distribution - example

2 x O/C + 1 x E/F arrangement

O/C

E/F

O/C

C

B

A

Winding earth fault

From Network Protection and Automation Guide -Areva

Winding fault current is not easily seen at primary terminals (i.e. residual earth fault connection).

A NEF relay, on the other hand, sees actual fault current, and so is a better option

Grading Relays

Each O/C or E/F relay must be time graded with its neighbouring O/C or E/F relayThere must therefore be a time margin between successive relay settings, typically around 0.4sfor the highest fault currentrelays need to be graded only for highest fault current – this ensures discrimination at all lower fault currents

Grading Relays

The requirement for time grading means that overcurrent and earth fault relays can be quite slowNext relays up in the hierarchy are differential relays

Relay grading chart

0.0

0.5

1.0

1.5

2.0

2.5

3.0

10 100 1000

Current (relay Amps)

time

(s)

Time margin between relay curves at max fault current (100A here) must be ≥ 0.4s

Contribution of delta winding to earth fault current – example 1

only positive and negative sequence current fromthis side

generatorunearthed transmission line

impedance = zero

HEALTHY PHASE CURRENT FLOW DURING AN EARTH FAULT

star-star transformerimpedance Z1 = Z2 = Z0 = Z

star-delta transformerimpedance Z1 = Z2 = Z0 = Z

only zero sequencecurrent from this side

fault point

Contribution of delta winding to earth fault current – example 2

positive, negative and zerosequence current flowing onthis side

generatorunearthed transmission line

impedance = zero

CONTRIBUTION OF TRANSFORMER DELTA TERTIARY TO FAULTCURRENT DURING AN EARTH FAULT WITH UNEARTHED GENERATOR

star-star-delta transformerimpedance Z1 = Z2 = Z0 = Z

fault point

only positive and negativesequence current flowing on this side

DIFFERENTIAL PROTECTION

two types, operating on very different principles:Biased differential relaysbased on the balance of ampere-turnsHigh impedance differential relaysbased on Kirchhoff’s Current Law

DIFFERENTIAL PROTECTION

Sensitive – down to <10% of ratingfast operating (20 - 40 ms)Depending on CT location, will also detect terminal faults (a snake across a bushing, for example)

High Impedance Differential Protection

Especially sensitive, very fastOne scheme required for each galvanically connected set of windings i.e. one for HV windings and one for LV windings if galvanically separate

High Impedance Differential Protection

ideal for auto-transformers, as HV and LV are galvanically connected – thus requires only one three phase scheme for transformer (note: delta winding must be separately protected)Not usually applied to delta windings –many CTs required for overlap

relay is stable for thru faults and load

RELAY

Principle of ‘Hi-Z Diff’

relay operates for faults to other windings or earth

RELAYFAULT

but relay does not operate for inter-turn faults!

RELAY

INTER-TURNFAULT

Why high impedance?

The relay must have a high impedance to prevent CT magnetising current from spilling into the relay for heavy through faultsThis approach was empirically derived in the 1950sThe spill current arises because the CTs are not ideal current sources, but draw magnetizing current

CT Equivalent Circuit

RCT

Z mag

lead

s +

rela

y

Setting the relayThe relay is a simple, low impedance, attracted armature O/C relay, to which we must add a high resistanceAssume each CT in turn goes short circuit (saturates) for external fault and calculate voltage across relay when this happensSet relay/resistor combination such that this voltage just operates relay

Vrelay setting = ICT1 . ( R CT1 + R LEADS1 ) = 10A x (8Ω + 2Ω) = 100V

RELAY

TO OTHER CT's IN SCHEME

This CT saturates due to the fault current flowing through it.It now looks like a short circuit!

CT1

12000A

CT2

3600A V = 100V10A x (8ohm + 2 ohm)

8 ohm

Rct

10 A

R leads

7 A

2 ohm

ALL CT's 1200:1

SETTINGRESISTOR(ca. 1000 ohm)

<10 ohm

R leads

3 A

Rct

Setting the relay - example

HIGH IMPEDANCE DIFFERENTIAL PROTECTION

two possible schemesfull scheme for interphase and earth faultsRestricted Earth Fault (REF) scheme

Detects faults where current flows from inside to outside the CT defined zoneDoes not detect intra-winding faults, (shorted turns)

Full Hi Z Diff scheme

Hi Z

Diff

Hi Z Diff

A

BC

detects winding to earth faultsand interphase faults, but not interturn faults

Hi Z Diff

Restricted Earth Fault (REF)

detects winding earth faults onlynot interphase or interturn faults

A

BCREF

BIASED DIFFERENTIAL PROTECTION

based on the balance of ampere-turns between windingsdetects faults down to about 10% of ratingNot quite as sensitive as Hi Z diff, but provides more comprehensive protectionSome, especially older relays, prone to tripping spuriously on inrush current when energised

BIASED DIFFERENTIAL PRINCIPLE -but without bias

10A

PRINCIPLE OF DIFFERENTIAL PROTECTION(LOAD CONDITION ILLUSTRATED - STABLE)

RELAY

1A

1A

10:1

1A

1:1 10:1

BIAS WINDINGS

OPERATING WINDING

BIAS WINDINGS

BIAS WINDINGS

introduced to compensate for undesired unbalance current flowing in the operate winding

Electro-mechanical biased differential relayMetropolitan-Vickers Type DT circa 1950Moving coil design - 3.5VA and 2 x 0.2VA at Inoperating time: <1 cycle to 3.5 cycles

1 operate and 2 bias coils

Electro-mechanical biased differential relay

UNBALANCE CURRENTS CAUSED BY - 1

Mismatch between actual transformer turns ratio (tap changer range) and turns ratios of the CT’s.

The CT ratios are selected to balance on the middle tapuser must calculate this and allow for it in setting the relay


Transformer inrush current on energization.

Inrush current produces a current from the energizing side only, appearing as an internal fault. This current is characterized by the appearance of second harmonics, so additional restraint is requiredno setting calculations required

Inrush current

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-1.5

-1

-0.5

0

0.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-0.5

0

0.5

1

1.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-0.6

-0.4

-0.2

0

0.2

0.4


Magnetizing current in the CT’s, especially as some saturation due to DC fault current sets in.

The amount of bias is increased under heavy through fault conditions to compensate for possible CT saturationno setting calculations required, but an adequate CT class must be selected


Overfluxing, caused by too high a voltage, or too low a frequency.

This is characterized by fifth harmonics. Fifth harmonic restraint is therefore addedno user calculations or settings are required

Multiple CT inputs

relays with up to five bias windings, (to accommodate transformers connected to five other circuits) are available

Biased DifferentialFault coverage

protects every winding on the transformer (remember: each high impedance differential protects only one galvanically connected entity)

detects shorted turns (remember: high impedance differential doesn’t - the ampere turns balance principle is required for this)

CT connections and ratios for older type relays (pre early 90’s)

CT’s for a delta connected primary must be star connectedCT’s for a star connected primary must be delta connectedThe vector group of the protected transformer must be taken into account or the scheme won’t balance

example:Vector group for Yd11

CT connections and ratioscompensates for the phase shift across a star-delta transformer.

The vector group of the transformer must be taken into account in connecting the CT’s to ensure that through currents balance.

prevents any zero sequence currents flowing in the star winding from entering the relay

since they are not present in the line on the delta side.

CT ratio selection

The CT ratios must be opposite to the transformer ratioCT ratios must allow for the fact that current flowing into the relay from the delta connected CT's is root 3 times the CT secondary current

CT's with ratios such as 1000/0.577 are, for this reason, quite common.

CT connections

BIASED DIFFERENTIAL PROTECTION ARRANGEMENTFOR A STAR-DELTA TRANSFORMER

O - OPERATING WINDINGB - BIAS WINDING

O

OB

BB

OB

B

B

A

B

C

a

b

c

A1

B1

C1

A2

B2

C2

a2

b2

c2

a1

b1

c1

A2

B2C2

a2

b2

c2

A B C a b cn

N

Yd1

A

B

C

a

b

c

A1

B1

C1

A2

B2

C2

a2

b2

c2

a1

b1

c1

A2

B2C2

a2

b2

c2

A B C a b cn

N

Yd11

OPERATING CHARACTERISTICS

if currents into the two sides of a relay are I1& I2, then relay is constructed so that there are two counteracting forces:-

1. A RESTRAINT or BIAS QUANTITY = (|I1| + |I2|)÷2essentially, restraint is defined as ∝ |I1| + |I2|

2. An OPERATE QUANTITY = |I1 - I2|

BIAS CURRENT (I1+I2)/2

maximum slope of 'throughcurrent' curve depends ontapping range and CT mismatch

OPERATEREGION

DIFF

EREN

TIAL

CUR

RENT

I1

- I2

0.1In

0.5In

Typical setting range0.1In to 0.5In

margin

In

operating point

constant slope (typically 20%) consta

nt slop

e (typi

cally 8

0%)

CT saturation causesline to tip up

Bias increases here toallow for CT saturation

Typical internal fault curve

RESTRAIN REGION

settingrange

GE T60 relay

TAP CHANGER POSITION

For any setting of tap changer and through current, and given the CT ratios, the values of bias current and differential current can easily be calculated.

SETTINGStypical setting allows:15% margin above the line representing

the worst mismatch of transformer ratio & CT ratios (remember root 3 for delta CT’s!)

to decide worst case - consider the overall scheme

at the top tap position .......... & thenat the bottom tap position.

Tapping Factor & Tapping Range

•There is a tapping factor for each tapping

•the tapping factor is the ratio Ud/UN where

•UN is the rated voltage of the tapped winding on the principal tapping (nominal tap)

•Ud is the open circuit voltage of the tappedwinding on the tap under consideration

Tapping Range = extreme values of tapping factor

Example:

132/66kV 80MVA auto Transformer with a delta tertiary winding is protected by a biased differential relayTransformer tapping is on 132kV winding (just above the LV tap)Tapping range is -15 to +5% (ie 85% to 105% of 132kV = 112.2 to 138.6 kV)

CT's HV 600/1 delta connectedLV 1200/1 delta connected

What is the mismatch at the extremes of the tapping range?

Example:

B B

R

132kV

600/1 1200/1

66kV

Mismatch Calculation formula

⎥⎦⎤

⎢⎣⎡ ×++

××

⎥⎦⎤

⎢⎣⎡ ×+−

××

×=NnomT

kCTkCT

NnomTkCTkCT

Mismatch

tappedwdg

guntappedwd

tappedwdg

guntappedwd

)1(21

)1(21

2

Where k1 = √3 for delta connected CTsk1 = 1 for star connected CTsT = tapping range (consider both extremes)Nnom is transformer ratio on nominal/principal tap

Bottom tapFor T = -0.15

⎥⎦

⎤⎢⎣

⎡×−++

××

⎥⎦

⎤⎢⎣

⎡×−+−

××

×=

2)15.01(360031200

2)15.01(360031200

2Mismatch

=16.2%

Top tapFor T = +0.05

⎥⎦

⎤⎢⎣

⎡×++

××

⎥⎦

⎤⎢⎣

⎡×+−

××

×=

2)05.01(360031200

2)05.01(360031200

2Mismatch

=4.9%

B B

R

132kV

600/1 1200/1

66kV

1. calculate voltages at extremes of tapping range

132kV x 0.85 = 112.2kV bottom tap132kV x 1.05 = 138.6kV top tap

OR – we can easily calculate the mismatch manually

B B

R

132kV

600/1 1200/1

66kV

2. Select a convenient current to work with –same answer for any current, (load or fault), as we are working out a ratio (i.e. the slope Idiff ÷ Ibias)

So assume 600A at 132kV

A85.0

1200kV66

kV2.112A600

CTkV66

kV2.112A600 Ibias66

=

÷×=

÷×=

A00.11200A600

CTA600Ibias132

=÷=

÷=

On 132kV On 66kV

Irestraint = {|Ibias132|+|Ibias66|}/2 = 1.85/2 = 0.925A

Idiff = Ibias132 - Ibias66 = 1.00 - 0.85 = 0.15

Slope of mismatch = Idiff ÷ Irestraint =0.15 ÷ 0.925 = 16.2%

3. Calculate currents in windings of relay on the bottom tap

Idiff

(|Ibias_132 |+ |Ibias_66| ) ÷2

Slope = 16.2%

Slope = 20%Slope = 1.2x16.2%=19.4%

Allow a 20% margin above mismatch line, whose slope is 16.2%. This is simply a line with slope 16.2% x 1.2 = 19.4%

Slope = 50%

Plenty of margin

Repeat the process for the top tap, which is clearly not as onerous in this case

UNRESTRAINED ELEMENT

separate, less sensitive function, providing faster operation for HV terminal faults onlydifferential element only - no bias of any type, fundamental or harmonicmust be set to remain stable on the heaviest through fault and on energization – see manufacturer’s manual

CT REQUIREMENTS

some CT saturation is permissible for through faults, mainly due to the DC component of the fault current

Most manufacturers provide simple equations to determine CT class - no nasty calculations required

More than two circuits

Fundamental principle is…...No pair of CT’s should be paralleled if either’s circuit is capable of supplying fault current into the circuit to which the other CT is connected

separate restraint windings are required here for each set of CT’s feeding the relay

More than two circuits

If neither can supply fault current to the other ...... they may be paralleled, as there is no possibility of spurious circulating current in the paralleled CT’sRecommended practice, nevertheless, is to use a separate input winding for each CT

Overfluxing protection

Caused generally by too high a voltage or too low frequencyMay cause magnetizing current to increase to unacceptable levels/durationGenerally provided in modern biased differential relays

Putting it all together - example

1MVA transformer – fuses only10MVA transformer – O/C and E/F relays20MVA transformer – biased diff with back-up O/C and E/F50MVA transformer - duplicate biased diff or biased diff plus high Z diffAll with Buchholz, Pressure Relief Device and Overtemperature where possible

EARTHING TRANSFORMERS

operationprotection

Earthing transformers

provides a good earth reference for a delta winding during earth faultsrestricts the voltage rise on the healthy phase during earth faults inoperative during balanced voltage conditionscarry significant current only during earth faults (unless tertiary supply) - I0 onlyearthing transformer and associated power transformer always tripped together

earth fault currents

Earthing Transformer

LOAD

Technical Ratings

per phase impedance is equal to zero sequence impedanceshort time rating (typically 3 sec)continuous rating (typically 30A)

Calculation of fault current

I IV

ZZ ohms phaseV phase to ground volts

fault etet

et

= × =×

=

= − −

33

φφ

φ

φ

φ

__

_ /_

N

N

N

V

Z0et=9ohms F

Z2=0

Z0=0

Z1=0

F

F

Construction

not supplied with conservators, but instead use diaphragms to accommodate oil expansionno conservator means no Buchholz protectionno overtemperature protection either!

Protection of Earthing Transformers

two types of faults we need to consider:internal faults - faults inside the earthing transformer, the result of insulation breakdown. external faults - faults on the system outside the earthing transformer. These can cause overheating of the earthing transformer

Internal Faults -Overcurrent Protection

interturn, interwinding or winding-to-core faultsfed from delta-connected current transformers, so that earth faults on the system, which generate a lot of zero-sequence current, are not seensince inter-phase faults also not seen, setting can be very low

O/C relay does not operate for externalearth faultsDef Time and IDMT E/F relays operate for

Earthing Transformer

external earth faults IDMT E/F relayDef Time E/F relay

O/C relay

LOAD

overcurrent setting must be

greater than the magnetising currentgreater than the maximum inrush current. This depends on

earthing transformer’s B-H characteristicsthe point-on-wave of the energisationthe remanence of the core.one common estimate of upper bound is 50x the magnetising current

Earth Fault Protection

detects long term residual voltage, which may cause thermal damage

remember - no overtemperature sensor is provided

need to consider continuous and short-time ratings, and set earth fault below these curvescombination of IDMT and definite time relays used to do this

thermal protection

10 100 1 103 1 1040.1

1

10

100

1 103

1 104

1 105

EARTHING TRANSF THERMAL PROTECTION

EARTH FAULT CURRENT - AMPS

TIM

E - S

E CO

ND

S

30 230 0

contrating30A

max E/Fcurrent2300A

adiabatic thermal limit

actual thermal limit

earthing transformer E/F relay - Definite Time

downstream E/F relay

earthing transformerE/F relay - IDMT

biased differential protection

Earthing transformers are always included in the biased differential zone of their power transformercurrent transformer connections important

stability for external earth faults.

all 1600/0.333

OVERALL BIASED DIFFERENTIAL ARRANGEMENT FOR 132kV/33kV STAR-DELTATRANSFORMER WITH EARTHING TRANSFORMER

N

N

N

C

400/0.577

B

A

externalearthfault

0

0

N

1600/1

b

cN

a

Documents

Transformer Protection