Download - 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.
Pressure Relief Device
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.
Pressure Relief Device
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
Expulsion Drop-out (EDO) Fuses
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 (EDO) Fuses
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
Overcurrent (O/C) Protection
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
Overcurrent (O/C) Protection
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
UNBALANCE CURRENTS CAUSED BY - 2
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
UNBALANCE CURRENTS CAUSED BY - 3
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
UNBALANCE CURRENTS CAUSED BY - 4
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