transformer proection

REV.NO RO R1 ISSUE

INITIALS SIGN INITIALS SIGN INITIALS SIGN INITIALS SIGN

PPD.BY PVD Sd/- DB

Sd/-CKD.BY CPS Sd/- SJH Sd/- R1

PROTECTION OF TRANSFORMERS

TATA CONSULTING ENGINEERS

73/1, ST. MARK’S ROAD

BANGALORE 560 001

FLOPPY NO : TCE.M6-EL-FP-DOC-005 FILE NAME : M6-6508.DOC

CLAUSE NO.

DESCRIPTION

SHEET NO.

1.0

SCOPE

1

2.0

RECOMMENDED PRACTICE

1

3.0

DISCUSSIONS

18

TABLES

TABLE-1

PROTECTION OF TWO WINDING

TRANSFORMERS OF 500 TO 5000 KVA

RATING

2

TABLE-2

PROTECTION OF TWO WINDING

TRANSFORMERS OF 5000 TO 10,000 KVA

RATING

6

TABLE-3

PROTECTION OF TWO WINDING

TRANSFORMERS OF MORE THAN 10,000 KVA

RATING

10

TABLE-4

PROTECTION OF THREE WINDING

TRANSFORMERS

14

TABLE-5

PROTECTION OF AUTO TRANSFORMERS

16

TABLE-6

PROTECTION OF EARTHING TRANSFORMERS

18

APPENDIX

APPENDIX-1

TRANSFORMERS OF 500 KVA AND BELOW

PROVIDED WITH FUSES ON H.V. SIDE

REVISION STATUS SHEET

REV NO.

DATE

DESCRIPTION

winding transformers, auto transformers and earthing transformers

which are controlled by circuit breakers and protected by relays.

Protection of transformers provided with fuses on HV side is

covered separately in Appendix-1.

2.0

RECOMMENDED PRACTICE

Transformers are classified into the following groups for the

purpose of defining their protection requirements (which are given

in the tables to follow)

2.1

Two winding transformers

(a) Transformers of rating 500 to 5000 kVA – Table-1

(b)

Transformers of rating 5000 to 10000 kVA – Table-2

(c) Transformers of rating 10,000 kVA and above – Table-3

2.2

Three winding transformers of all ratings – Table-4.

Generally such transformers do not have ratings below 10,000 kVA

in practical applications.

2.3

Auto-transformers of all ratings – Table-5.

These are generally used as interconnecting transformers between

two system voltages such as 220 kV to 110 kV.

2.4

Earthing transformers of all ratings – Table-6

Ratings of earthing transformers are decided by desired ground

fault level of the system to which they are connected.

TABLE – 1

PROTECTION OF TWO WINDING TRANSFORMERS OF

500 TO 5000 kVA RATING

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 1.1 Instantaneous

high set over current protection for phase faults on HV side (source side) ‘50’ Instantaneous over current relay should be provided on all three phases, o/c relay should have low transient over-reach of less than 5% and high drop off to pick up ratio

Range 500 to 2000%

In case of low ratio CTs – say 200/1A & less in a high fault level system of say 40 kA, (i.e. CT sec. side fault current ≥200 XIs) a separate protection CT core with higher ratio to be used for ‘50’ to avoid CT saturation Alternatively inst. over current relays may be connected directly to the main CT whose ratio is selected with due regard to sensitivity to terminal faults. Time delayed over current relays may be connected to sec. side of an aux. CT of suitable ratio & knee point. The pry of aux. CT will be in services with the instantaneous overcurrent relays 1.2 Backup overcurrent protection for phase faults on HV side ‘51’ Relay should be provided on all three phases. Relay should have very inverse normal time characteristics Range 50 to 200%

Though very inverse time characteristic is preferred, for proper coordination, normal inverse curve may have to be used if the same has been used on the LV side incomer.

In genral the characteristic of the relays should be selected to give fast & selective tripp-ing through proper gradation. As far as possible use relays with similar characteristic on both sides of the transformers

1.3 Ground fault protection on HV side

‘50N’ Applicable only for delta connected winding on HV side Instantaneous overcurrent relay connected in residual mode should have series stabilising resistor to ensure stability for magnetising inrush currents and LV through fault currents. For high impedance earthed system the relay shall be with adjustable time delay of 01. – 1 sec. and shall be connected to core balance C.T. Range 20-80% for effectively earthed system. 10-40% for system with ground fault current limited to rated current / impedance earthed system

Consider 3 phase fault on secondary terminals of transformer for selection of stabilising resistor 1.4 Backup ground fault protection on HV side

‘51N’ Applicable for star connected HV winding. Normal inverse time characteristic relay connected in residual mode is recommended Range 20-80% for effectively earthed system and 10-40% for system with ground fault current limited to rated current

TABLE – 1 (cont’d)

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 1.5 Backup over current protection on LV side for phase faults ‘51’ Protection should be provided on all three phases. Very inverse time characteristics recommended for co-ordination with relay protected out-going feeders and extremely inverse time characteristics to be used when co-ordination with fuse protected feeders is required

50-200% See discussions for use of breaker series trips in place of these relays in case of 415V systems 1.6 Backup ground fault protection for LV systems with neutral directly earthed ‘51N’ Protection to be connected to CT in the transformer neutral connection. Normal inverse time characteristic relay is recommended. This protection is required even where all the outgoing feeders are fuse protected.

20-80% Neutral CT ratio shall be same as that of phase CTs

See discussion for 415V systems where outgoing feeders are provided with fuses and LV circuit breaker is provided with series trip device 1.7 Standby ground fault protection for LV system with neutral earthed through low resistances ‘515N’ Relay to be connected to neutral CT. Long inverse time characteristic relay is recommended. (Relay 51N above may either be deleted or connected to residual connection of phase CTs if Ig < IR 10-40% CT ratio same as phase CTs if Ig ≥ Irated In other cases select CT ratio based on ground fault current value Range 10 to 40%

Relay time setting should be co-ordinated with short time rating of grounding resistance

1.8 Gas protection ‘63G’ Two stages buchholz relay with one alarm stage and one tripping stage. This is supplied along with the transformer. Each stage will actuate an auxiliary relay with four pairs of self reset contacts (Alarm) or Hand reset contacts (Trip)

For transformers protected by fuse, trip stage without any switching device cannot be used as tripping device is not available. Only alarm stage may be used.

1.9 Transformer oil temperature trip

‘OTI’ Oil temperature indicators are usually provided with two contacts, one for alarm on “Oil temperature high” and other for trip on “Oil temperature very high”

These are optional items depending on transfomer rating and applications

1.10 Transformer winding

temperature trip

WTI Winding temp. indicators are provided with two stages, one for alarm on ‘winding temp. high’ and the other for trip on ‘winding temperature very high’

DO

-1.11 Oil pressure trip PRD Pressure relief device supplied with the transformer shall be provided with auxiliary relay with four pairs of hand reset contacts for tripping

This will be provided for transformers above 2500 kVA.

TABLE – 2

PROTECTION OF TWO WINDING TRANSFORMERS OF

5000 TO 10,000 kVA RATING

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 2.1 Differential protection ‘87T’ Relay should be connected on all three phases. Relay should incorporate

(a) Adjustable

percentage biasing for through fault stability and tap changing

conditions.

(b) Harmonic restraint / zero current sensing to provide stability against magnetising inrush currents. (c) Auxiliary CTs on

both sides for ratio matching and compensation of mismatch of line CTs Refer discussions regarding harmonic restraints for magnetic inrush currents. 2.2 Instantaneous short circuit protection for phase faults on HV side

‘50’ Same as item 1.1 Range 500-2000% Same as in 1.1 2.3 Backup over current protection for phase faults on HV side

51 Same as item 1.2 Range 50-200%

2.4 Earth fault protection on HV side

50N Applicable only for delta connected HV. Protection same as item 1.3 20-80% for effectively earthed systems and 10-40% for low resistance earthed system 2.5 Backup earth fault protection on HV side

51N Applicable only for star connected HV side. Same as item 1.4 20-80% for effectively earthed system and 10-40% for system with low resistance earthing 2.6 Backup overcurrent protection on LV side for phase faults 51 Same as item 1.5 50-200% 2.7 Backup ground fault protection for LV side 51N Relay to be connected to neutral CT. CT ratio same as phase CTs when Ig ≥IR 20-80% for effectively earthed system and 10-40% for system with low resistance earthing 2.8 Restricted earth fault protection of LV side when LV neutral is grounded through low resistance

‘64’ CT ratio for phase and neutral CTs should be same 10-40% range, Stand. values of stabilising resistor are 50 ohm for 5 A CT and 200 ohm for 1 A CT Design stabilising resistance value on the basis of 3 phase through fault. RCT should be kept low to limit value of Vk & stabilising

resistor.

Recommended setting is 10%.

TABLE – 2 (cont’d)

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 2.9 Standby earth fault protection for LV side when neutral is grounded through low resistance ‘515N’ Relay connected to neutral CT. Long inverse time characteristic relay is recommended. (Relay ‘51N’ in 2.8 above should be connected to residual connection of phase CTs in this case)

CT ratio same as phase CTs if Ig ≥ IR. In other cases select CT ratio based on designed ground fault current value. Range 10-40% Relay time setting should be co-ordinated with short time rating of grounding resistance

2.10 Gas protection 63G Same as 1.8 2.11 Winding

temperature trip WTI Same as item 1.10 2.12 Transformer oil

temperature high

DTI Same as item 1.9 2.13 Oil pressure trip PRD Same as item 1.11

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 3.1 Differential protection ‘87T’ Relay should be connected on all three phases. Relay shall be high speed type incorporating

(a) 2nd & 5th harmonic restraint / zero current sensing for magnetising inrush currents (b) Adjustable percentage bias for through fault stability

(c) Auxiliary CT on both sides for ratio matching and compensation for line CT mismatch (d) High set instantaneous unit in differential circuit 3.2 Instantaneous short circuit protection for phase faults on HV side

‘50’ Same as item 1.1 Range 500-2000%

(i) This

protection shall not provided for generator transfor-mers

TABLE – 3 (cont’d)

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 3.3 Backup overcurrent protection for phase faults on HV side

‘51’ Same as item 1.2 Range 50-200%

3.4 Instantaneous ground fault protection on HV side

‘50N’ Applicable only when HV is delta connected Same as item 1.3 Range 20-80% for effectively earthed system and 10-40% for system with ground fault currents limited to rated current/ impedance earthed systems 3.5 Backup ground fault protection on HV side

51N Applicable for star connected HV winding Same as item 1.4 Range 20-80% for effectively earthed system and 10-40% for systems with limited ground fault current 3.6 Backup phase overcurrent protection for phase faults on LV side

‘51’ Same as item 1.5 Range 50-200%

3.7 Backup overcurrent protection for ground fault on LV side

‘51N’ Same as item 2.7 Range 20-80% for effectively earthed system and 10-40% for system with low resistance earthing 3.8 Restricted earth fault protection of LV side when LV neutral is grounded through low resistance ‘64’ Same as item 2.8 10-40% range 3.9 Standby earth fault protection of LV side when LV neutral is grounded through low resistance

‘51SN’ Same as item 2.9 Relay time setting to be co-ordinated with short time rating of resistor

3.10 Gas protection 63G Same as 1.8 3.11 Winding

temperature trip

WTI Same as item 1.10 3.12 Transformer oil

temperature trip

DTI Same as item 1.9 3.13 Oil pressure trip PRD Same as item 1.11

TABLE – 4

PROTECTION OF THREE WINDING TRANSFORMERS

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 4.1 Differential protection 87T Relay should be connected to all three phases. Relay shall have bias from all the three windings. If tertiary is not loaded and brought out, then the bias shall be taken from HV & LV sides only. Other requirements same as 3.1 4.2 Short circuit protection with instantaneous o/c relays on HV side (source side)

‘50’ Same as item 1.1 Range 500-2000% 4.3 Backup overcurrent protection for phase faults on HV side

‘51’ Same as item 1.2 Range 50-200%

4.4 Instantaneous ground fault protection on HV side

‘50N’ Applicable only for delta connected HV winding Same as item 1.3 Range 20-80% for effectively earthed system and 10-40% for systems with restricted ground fault current

NO. NO. RANGE 4.5 Backup ground

fault protection on HV side

51N Applicable for star connected HV winding Same as item 1.4 Range 20-80% for effectively earthed system and 10-40% for systems with limited ground fault current 4.6 Backup o/c protection on LV side, for phase faults

‘51’ Same as item 1.5 Range 50-200% Both LV windings shall have separate protections 4.7 Backup o/c protections for ground fault on LV side

‘51N’ Same as item 2.7 Range 20-80% or 10-40% depending on available fault current do -4.8 Restricted E/F protection of LV side when LV is grounded through a resistance ‘64’

Same as item 2.8 10-40% range

do -4.9 Standby E/F protection when LV is grounded through a resistance ‘51SN’ Same as item 2.9 do

-4.10 Gas protections 63G Two stage Buchholz

protection is part of transformer

Same as 1.8 4.11 Winding

temperature trip WTI Same as item 1.10 4.12 Oil temperature

trip OTI Same as item 1.9 4.13 Oil pressure trip PRD Same as item 1.11

TABLE – 5

PROTECTION OF AUTO-TRANSFORMERS

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 5.1 Differential protection ‘87T’ Relay to be connected on all three phases. High speed, harmonic restraint, percentage bias differential relays are to be used.

If tertiary winding is brought out for loading, additional bias from this side also to be provided 5.2 Restricted E/F

protection ‘64’

Alt:1 7CT method – See fig.1

All CT ratios should be identical

Alt-2 9CT method – See fig.2

All CT ratios should be equal. One CT is required on each phase at neutral end of common winding. This alternative also covers phase faults inside transformers Single phase instantaneo us current operated relay. Range 20-80% Three phase instantaneo us current operated relay Range 20-80% Specify value of stabilising resistor taking RCT into account 5.3 Backup overcurrent protection for phase faults (Directional) on HV side 67/51-50 Protection shall be provided on all phases. Directionalisation is required as power flow can be both ways. Additional high set instantaneous unit can be provided to clear internal faults

3 relays – one per phase. Range 50-200% for inverse current element and 500-2000% for high set unit

5.4 Backup o/c protection for ground faults (Directional) on HV side 67/51N Directionalisation is required as current flow can be in both directions. Adopt potential polarisation Directional o/c relay Range 20-80% 5.5 Backup o/c protection for phase faults on LV side (Directional) 67/51-50 Similar to 5.3 above Directional o/c relay - 3 relays Range 50-200% for inverse time o/c & 500-2000% for high set unit 5.6 Backup o/c protection for ground faults (Directional) 67/51N Same as item 5.4 Range 20-80%

5.7 Overload relay (for alarm) for HT winding 50/OL To be connected to HT line CTs to monitor HT winding current Definite time o/c relay. Range 50-200% Time 1-10 sec. High DO/Pu ratio

5.8 Overload relay (for alarm) for common winding

50/OL

To be connected to CTs at the neutral end of phase winding

do -5.9 Gas protection

63G

Two stage Buchholz protection is part of transformer

TABLE – 5 (cont’d)

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 5.10 Winding

temperature trip WTI Same as item 1.10 5.11 Oil temperature trip OTI Same as item 1.9 5.12 Oil pressure trip PRD Same as item 1.11 5.13 For very large

transformer (above 100 MVA) compre-hensive numerical protection in one group & conventional protection in another group shall be provided 5.14 Over fluxing protection (for grid transformers)

5.15 In cases where trans-formers are continuously

operated at full load, and in addition, are likely to be subjected to short time overloads, (which may raise the winding temp. to more than 100°C temp. time integrators should be provided to enable estimation of degree of aging.

SR. NO. DESCRIPTION DEVICE NO. DETAILS SETTING RANGE REMARKS 6.1 Instantaneous o/c protection ‘50’ Relay should be provided on all three phases of the earthing transformer Instantaneous o/c relay Range 20-80% The CTs should be connected in Delta

6.2 Gas protections ‘63G’ Two stage gas protection is a part of transformer.

3.0

DISCUSSIONS

3.1

Transformers below 500 kVA are generally used for distribution

systems and are usually protected by fuses. Hence this range of

power rating is not included in the recommendations. Appendix-1

gives such a typical system for general guidance. For particular

applications where H.V. circuit breakers are provided table I may

be followed. LV side of the transformers may have circuit breakers

or fuses. In case of fuses, LV backup overcurrent and ground fault

protections are not applicable.

3.2

Ground fault protections to be provided for the transformers depend

on the type of earthing of the systems to which the transformer is

connected and also the way in which transformer neutrals are

grounded. Following types of system earthing are possible.

(a)

Effective earthing

(b) System neutral earthed through a resistance to limit the fault

current to a value equal to the rating of the transformer. In

some cases this limit may be further reduced to provide only

adequate current for ground fault relaying.

(c) System neutral earthed through resistance loaded distribution

transformer to limit ground fault current to very small values.

Earthing discussed in (c) above is usually adopted in power

station for unit and station transformer earthing. In this case

two types of relaying are possible.

(i) To provide ground fault alarm by a voltage relay

connected across the resistor of grounding transformer

secondary. No further discriminative ground fault relaying

is done on outgoing feeders.

(ii) To provide sensitive core balance CTs on the outgoing

feeders in addition to the voltage relay in (i) above and

ensure time discrimination on the voltage relay. This

scheme is adopted in TCE power station practice. The

ground fault current available depends on the total

capacitance in the connected system. Core balance CTs

are specified to give a primary sensitivity of 5 to 10 A. In

3.3

Most of the indigenous make differential relays do not incorporate

ratio matching taps in the relay. Hence separate auxiliary CTs

should be provided wherever adopted relays do not have this

built-in facility. Even with built-built-in ratio matchbuilt-ing feature built-in therelay

auxiliar CTs are required for vector correction.

3.4

Protections recommended in tables 1 to 4 are on the basis that the

source is connected to HV side of transformer only. Most of the

applications fall under this category and hence the generalisation.

If particular applications have additional source on the LV side also

then backup over current protection for phase faults on the LV side

should be made directional over current protection. Additional

directional ground fault protection should be added and connected

to residual connections of the phase CTs.

3.5

When two transformers are operated in parallel, and they are not

provided with differential protection, directional over current phase

and ground fault relaying mentioned in 3.4 should be provided for

selective isolation of faulty transformer (internal fault).

3.6

Transformers of smaller rating usually have percentage impedance

value of about 5%. The current for secondary three phase fault in

this case will be 20 times rated current. The instantaneous phase

over current relay on HV side should be set atleast 1.5 times this

value. Since the maximum setting is 2000%, CT ratio will have to

be increased to reach this setting. Phase over current elements

have 50-200% range thus permitting the raised CT ratio.

3.7

A common high speed, hand reset trip relay should be used for

simultaneous tripping of both HV and LV breakers whenever the

protections operate.

3.8

Buchholz trip should be routed through a shunt connected auxiliary

relay with hand reset contacts. The auxiliary relays shall provide

necessary flag indication.

3.9

Winding temperature, alarm and trip, oil temperature alarm & trip

normally form part of transformer and are covered here for the sake

of completeness. These trips also should be routed through

auxiliary relays to obtain flag indication / remote indication.

3.10

In case of transformers feeding to 415 V auxiliary supply system,

the LV side may be provided with time delayed electromagnetic

releases for short circuit protection and thermal overload relays in

place of CT connected time over current protections. In such

systems where secondary neutral is directly grounded, no separate

ground fault relaying is envisaged. Under the presumption that the

phase connected releases will operate for ground faults also, since

fault current is not limited by neutral resistances. Even with all fuse

protected O/G feeders it is desirable to have E/F relay 51N.

(a)

For transformers with solidly earthed neutral residually

connected earth fault relay shall be provided.

(b) For transformers with resistance earthed neutral to restrict

earth fault current to about rated current of the transformer,

residually connected earth fault relay or earth fault relay

connected to transformer neutral CT or core balance CT shall

be provided.

(c) For transformers with resistance earthed neutral to restrict

earth fault current to a very low value (5 A or 10 A).

Earth fault relay connected directly across the NGR or across

the loading resistor on the secondary of the earthing

transformer shall be providwed . Relay may be wired only for

alarm, but in cases where transformer feeder extensive

cabling, each outgoing feeder should be protected additionally

by an earth fault relay connected to a core balance CT. In this

case the earth fault relay of each feeder is wired to trip the

feeder.

All the earth fault relays should be either inverse time or

definite time relays and where connected to trip are to be

graded with earth fault relays or fuses provided for the

outgoing feeders.

(d)

For Delta connected transformers either an instantaneous

residually connected earth fault relay with a suitable

stabilising resistor or a time delayed earth fault relay shall be

provided. This relay need not be graded with earth fault relays

windings can be combined for transformer upto 63 MVA or 132

kV. If earthing transformers are provided these can be included

in the REF scheme. Earthing transformer can also be covered

by the differential protection.

(b) Differential & restricted earth fault protection should be

separately provided for transformer of rating greater than 63

MVA or for all 220 kV / 400 kV transformers. REF protection

uses bushing CTs while differential protection uses transformer

yard CTs. Double wound transformer should have REF

protection for both the windings.

(c) Large auto transformer should be provided with REF protection

using nine (9) CTs.