transformer proection

TATA CONSULTING ENGINEERS TCE. M6-EL-PI-GTF-6508

SECTION: TITLE

SHEET ( i ) OF (iii)

PROTECTION OF TRANSFORMERS

PROTECTION OF TRANSFORMERS

TATA CONSULTING ENGINEERS 73/1, ST. MARK’S ROAD BANGALORE 560 001

FLOPPY NO FILE NAME

REV.NO

: TCE.M6-EL-FP-DOC-005 : M6-6508.DOC

RO

R1

ISSUE

INITIALS

SIGN

INITIALS

SIGN

PPD.BY

PVD

Sd/-

DB

Sd/-

CKD.BY

CPS

Sd/-

SJH

Sd/-

APP.BY

DKB

Sd/-

UAK

Sd/-

DATE

1982-10-30

INITIALS

SIGN

INITIALS

SIGN

R1

1999-01-22 FORM NO. 020R2

SECTION:CONTENTS

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

SHEET (ii) OF (iii)

PROTECTION OF TRANSFORMERS

CONTENTS

CLAUSE NO.

DESCRIPTION

SHEET NO.

1.0

SCOPE

1

2.0

RECOMMENDED PRACTICE

1

3.0

DISCUSSIONS

18 TABLES

TABLE-1

TABLE-2

PROTECTION OF TWO WINDING TRANSFORMERS OF 500 TO 5000 KVA RATING

2

PROTECTION OF TWO WINDING TRANSFORMERS OF 5000 TO 10,000 KVA RATING

6

PROTECTION OF TWO WINDING TRANSFORMERS OF MORE THAN 10,000 KVA RATING

10

PROTECTION OF THREE WINDING TRANSFORMERS

14

TABLE-5

PROTECTION OF AUTO TRANSFORMERS

16

TABLE-6

PROTECTION OF EARTHING TRANSFORMERS

18

TABLE-3

TABLE-4

APPENDIX APPENDIX-1

TRANSFORMERS OF 500 KVA AND BELOW PROVIDED WITH FUSES ON H.V. SIDE

24

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:REV. STAT.

SHEET (iii) OF (iii)

REVISION STATUS SHEET REV NO.

DATE

DESCRIPTION

R1

1998-12-15

DOC. GENERALLY REVISED

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

1.0

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 1 OF 24

SCOPE This guide gives recommended protections for two winding or three 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.

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 2 OF 24

TABLE – 1 PROTECTION OF TWO WINDING TRANSFORMERS OF 500 TO 5000 kVA RATING SR. NO. 1.1

DESCRIPTION

1.2

Backup overcurrent protection for phase faults on HV side

Instantaneous high set over current protection for phase faults on HV side (source side)

DEVICE NO. ‘50’

‘51’

DETAILS Instantaneous over current relay should be provided on all three phases, o/c relay should have low transient overreach of less than 5% and high drop off to pick up ratio

Relay should be provided on all three phases. Relay should have very inverse normal time characteristics

SETTING REMARKS RANGE Range 500 In case of low ratio CTs – to 2000% 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 Range 50 Though very inverse time to 200% 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

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 3 OF 24

TABLE – 1 (cont’d) SR. NO. 1.3

DESCRIPTION

1.4

Backup ground fault protection on HV side

Ground fault protection on HV side

DEVICE NO. ‘50N’

‘51N’

DETAILS 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. Applicable for star connected HV winding. Normal inverse time characteristic relay connected in residual mode is recommended

SETTING RANGE Range 2080% for effectively earthed system. 10-40% for system with ground fault current limited to rated current / impedance earthed system

REMARKS Consider 3 phase fault on secondary terminals of transformer for selection of stabilising resistor

Range 2080% for effectively earthed system and 10-40% for system with ground fault current limited to rated current

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 4 OF 24

TABLE – 1 (cont’d) SR. NO. 1.5

DESCRIPTION

1.6

Backup ground fault protection for LV systems with neutral directly earthed

‘51N’

1.7

Standby ground fault protection for LV system with neutral earthed through low resistances

‘515N’

Backup over current protection on LV side for phase faults

DEVICE NO. ‘51’

DETAILS

SETTING RANGE Protection should 50-200% be provided on all three phases. Very inverse time characteristics recommended for co-ordination with relay protected outgoing feeders and extremely inverse time characteristics to be used when co-ordination with fuse protected feeders is required Protection to be 20-80% connected to CT in Neutral CT the transformer ratio shall neutral connection. be same as of Normal inverse time that characteristic relay phase CTs is recommended. This protection is required even where all the outgoing feeders are fuse protected. Relay to be 10-40% connected to CT ratio neutral CT. Long same as inverse time phase CTs characteristic relay if Ig ≥ Irated is recommended. In other (Relay 51N above cases may either be select CT deleted or ratio based connected to on ground residual connection fault current of phase CTs if Ig < value IR Range 10 to 40%

REMARKS See discussions for use of breaker series trips in place of these relays in case of 415V systems

See discussion for 415V systems where outgoing feeders are provided with fuses and LV circuit breaker is provided with series trip device

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

ISSUE R1 FORM NO. 120 R1

SECTION:WRITE -UP.

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

SHEET 5 OF 24

PROTECTION OF TRANSFORMERS

TABLE – 1 (cont’d) SR. NO. 1.8

DESCRIPTION

1.9

Transformer oil temperature trip

‘OTI’

1.10

Transformer winding temperature trip

WTI

Oil pressure trip

PRD

1.11

Gas protection

NOTE:

1.

DEVICE NO. ‘63G’

DETAILS 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) 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” 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’ Pressure relief device supplied with the transformer shall be provided with auxiliary relay with four pairs of hand reset contacts for tripping

SETTING RANGE

REMARKS 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.

These are optional items depending on transfomer rating and applications

- DO -

This will be provided for transformers above 2500 kVA.

Items 1.8, 1.9 & 1.11 are not applicable for dry type transformers.

ISSUE R1 FORM NO. 120 R1

SECTION:WRITE -UP.

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

SHEET 6 OF 24

PROTECTION OF TRANSFORMERS

TABLE – 2 PROTECTION OF TWO WINDING TRANSFORMERS OF 5000 TO 10,000 kVA RATING SR. NO. 2.1

DESCRIPTION Differential protection

DEVICE NO. ‘87T’

DETAILS

SETTING RANGE

REMARKS Refer discussions regarding harmonic restraints for magnetic inrush currents.

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.

2.2

2.3

Instantaneous short circuit protection for phase faults on HV side Backup over current protection for phase faults on HV side

‘50’

51

(c) Auxiliary CTs on both sides for ratio matching and compensation of mismatch of line CTs Same as item 1.1 Range 500- Same as in 1.1 2000%

Same as item 1.2

Range 200%

50- Same as in 1.1

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 7 OF 24

TABLE – 2 (cont’d) SR. NO. 2.4

DESCRIPTION

2.5

Backup earth fault protection on HV side

51N

2.6

Backup overcurrent protection on LV side for phase faults Backup ground fault protection for LV side

51

2.7

2.8

Earth fault protection on HV side

Restricted earth fault protection of LV side when LV neutral is grounded through low resistance

DEVICE NO. 50N

51N

‘64’

DETAILS Applicable only for delta connected HV. Protection same as item 1.3

Applicable only for star connected HV side. Same as item 1.4

Same as item 1.5

SETTING REMARKS RANGE 20-80% for effectively earthed systems and 10-40% for low resistance earthed system 20-80% for effectively earthed system and 10-40% for system with low resistance earthing 50-200%

20-80% for effectively earthed system and 10-40% for system with low resistance earthing CT ratio for phase and 10-40% neutral CTs should be range, Stand. same values of stabilising resistor are 50 ohm for 5 A CT and 200 ohm for 1 A CT Relay to be connected to neutral CT. CT ratio same as phase CTs when Ig ≥IR

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%.

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 8 OF 24

TABLE – 2 (cont’d) SR. NO. 2.9

DESCRIPTION

2.10 2.11

Gas protection Winding temperature trip Transformer oil temperature high Oil pressure trip

2.12

2.13

Standby earth fault protection for LV side when neutral is grounded through low resistance

DEVICE NO. ‘515N’

DETAILS 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)

63G

Same as 1.8

WTI

Same as item 1.10

DTI

Same as item 1.9

PRD

Same as item 1.11

SETTING RANGE CT ratio same as phase CTs if Ig ≥ IR. In other cases select CT ratio based on designed ground fault current value. Range 1040%

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

ISSUE R1 FORM NO. 120 R1

SECTION:WRITE -UP.

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

SHEET 9 OF 24

PROTECTION OF TRANSFORMERS

TABLE – 3 PROTECTION OF TWO WINDING TRANSFORMERS OF RATING MORE THAN 10,000 KVA SR. NO. 3.1

DESCRIPTION Differential protection

DEVICE NO. ‘87T’

DETAILS

SETTING RANGE

REMARKS

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

3.2

Instantaneous short circuit protection for phase faults on HV side

‘50’

(d) High set instantaneous unit in differential circuit Same as item 1.1 Range 5002000%

(i)

This protection shall not provided for generator transformers (ii) Same as in 1.1

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 10 OF 24

TABLE – 3 (cont’d) SR. NO. 3.3

3.4

DESCRIPTION Backup overcurrent protection for phase faults on HV side Instantaneous ground fault protection on HV side

DEVICE NO. ‘51’

‘50N’

3.5

Backup ground fault protection on HV side

51N

3.6

Backup phase overcurrent protection for phase faults on LV side

‘51’

DETAILS Same as item 1.2

SETTING RANGE Range 50200%

REMARKS

Applicable only when Range 20HV is delta connected 80% for Same as item 1.3 effectively earthed system and 1040% for system with ground fault currents limited to rated current/ impedance earthed systems Applicable for star Range 20connected HV 80% for effectively winding earthed Same as item 1.4 system and 1040% for systems with limited ground fault current Same as item 1.5 Range 50200%

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 11 OF 24

TABLE – 3 (cont’d) SR. NO. 3.7

DESCRIPTION

3.8

Restricted earth fault protection of LV side when LV neutral is grounded through low resistance Standby earth fault protection of LV side when LV neutral is grounded through low resistance Gas protection Winding temperature trip Transformer oil temperature trip Oil pressure trip

3.9

3.10 3.11 3.12 3.13

Backup overcurrent protection for ground fault on LV side

DEVICE NO. ‘51N’

DETAILS Same as item 2.7

‘64’

Same as item 2.8

‘51SN’

Same as item 2.9

63G WTI

Same as 1.8 Same as item 1.10

DTI

Same as item 1.9

PRD

Same as item 1.11

SETTING RANGE Range 2080% for effectively earthed system and 1040% for system with low resistance earthing 10-40% range

REMARKS

Relay time setting to be co-ordinated with short time rating of resistor

ISSUE R1 FORM NO. 120 R1

SECTION:WRITE -UP.

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

SHEET 12 OF 24

PROTECTION OF TRANSFORMERS

TABLE – 4 PROTECTION OF THREE WINDING TRANSFORMERS SR. NO. 4.1

DESCRIPTION

4.2

Short circuit protection with instantaneous o/c relays on HV side (source side) Backup overcurrent protection for phase faults on HV side Instantaneous ground fault protection on HV side

4.3

4.4

Differential protection

DEVICE NO. 87T

‘50’

‘51’

‘50N’

DETAILS

SETTING RANGE

REMARKS

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 Same as item 1.1 Range 5002000%

Same as item 1.2

Range 200%

50-

Applicable only for delta connected HV winding Same as item 1.3

Range 2080% for effectively earthed system and 10-40% for systems with restricted ground fault current

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 13 OF 24

TABLE – 4 (cont’d) SR. NO. 4.5

DESCRIPTION

4.6

Backup o/c protection on LV side, for phase faults Backup o/c protections for ground fault on LV side

4.7

4.8

4.9

4.10

4.11 4.12 4.13

Backup ground fault protection on HV side

Restricted E/F protection of LV side when LV is grounded through a resistance Standby E/F protection when LV is grounded through a resistance Gas protections

Winding temperature trip Oil temperature trip Oil pressure trip

DEVICE NO. 51N

‘51’

‘51N’

‘64’

‘51SN’

DETAILS

SETTING REMARKS RANGE Applicable for star Range 20-80% connected HV for effectively winding earthed system Same as item 1.4 and 10-40% for systems with limited ground fault current Same as item 1.5 Range 50-200% Both LV windings shall have separate protections Same as item 2.7 Range 20-80% or - do 10-40% depending on available fault current Same as item 2.8 10-40% range - do -

Same as item 2.9

63G

Two stage Buchholz protection is part of transformer Same as 1.8

WTI

Same as item 1.10

OTI PRD

Same as item 1.9 Same as item 1.11

- do -

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 14 OF 24

TABLE – 5 PROTECTION OF AUTO-TRANSFORMERS SR. NO. 5.1

5.2

5.3

DESCRIPTION

DEVICE NO. ‘87T’

Differential protection

Restricted protection

E/F

Backup overcurrent protection for phase faults (Directional) on HV side

‘64’

67/5150

DETAILS

SETTING RANGE

REMARKS

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 Alt:1 7CT method – See fig.1 All CT ratios should be identical

Single phase instantaneo us current operated relay. Alt-2 9CT method – Range 2080% See fig.2 All CT ratios should be equal. One CT is Three phase required on each instantaneo current phase at neutral end us of common winding. operated This alternative also relay covers phase faults Range 2080% inside transformers Protection shall be 3 relays – provided on all one per phases. phase. Directionalisation is Range 50required as power 200% for flow can be both inverse ways. Additional high current set instantaneous unit element and can be provided to 500-2000% clear internal faults for high set unit

Specify value of stabilising resistor taking RCT into account

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 15 OF 24

TABLE – 5 (cont’d) SR. NO. 5.4

5.5

5.6

5.7

5.8

5.9

DESCRIPTION Backup o/c protection for ground faults (Directional) on HV side Backup o/c protection for phase faults on LV side (Directional)

Backup o/c protection for ground faults (Directional) Overload relay (for alarm) for HT winding

Overload relay (for alarm) for common winding Gas protection

DEVICE NO. 67/51N

DETAILS Directionalisation is required as current flow can be in both directions. Adopt potential polarisation

67/5150

Similar to 5.3 above

67/51N

Same as item 5.4

50/OL

50/OL

63G

To be connected to HT line CTs to monitor HT winding current

SETTING RANGE

REMARKS

Directional o/c relay Range 2080% Directional o/c relay - 3 relays Range 50200% for inverse time o/c & 5002000% for high set unit Range 2080%

Definite time o/c relay. Range 50200% Time 1-10 sec. High DO/Pu ratio

To be connected to CTs at the neutral - do end of phase winding Two stage Buchholz protection is part of transformer Same as 1.8

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 16 OF 24

TABLE – 5 (cont’d) SR. NO. 5.10 5.11 5.12 5.13

5.14

5.15

DESCRIPTION Winding temperature trip Oil temperature trip Oil pressure trip For very large transformer (above 100 MVA) comprehensive numerical protection in one group & conventional protection in another group shall be provided Over fluxing protection (for grid transformers) 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.

DEVICE NO. WTI OTI PRD

DETAILS

SETTING RANGE

REMARKS

Same as item 1.10 Same as item 1.9 Same as item 1.11

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 17 OF 24

TABLE – 6 PROTECTION OF EARTHING TRANSFORMERS SR. NO. 6.1

DESCRIPTION

6.2

Gas protections

Instantaneous o/c protection

DEVICE NO. ‘50’

‘63G’

DETAILS Relay should be provided on all three phases of the earthing transformer Two stage gas protection is a part of transformer. Same as item 1.8

SETTING RANGE Instantaneous o/c relay Range 20-80%

REMARKS The CTs should be connected in Delta

ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 18 OF 24

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 such systems the outgoing transformer feeders should be ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 19 OF 24

provided with core balance CTs and instantaneous over current relays for ground fault protection in place of ‘50N’ and ‘51N’ relays. A small time delay of 50 – 100 min is desirable. 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 builtin facility. Even with built-in ratio matching feature 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 ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

3.10

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 20 OF 24

of completeness. These trips also should be routed through auxiliary relays to obtain flag indication / remote indication. 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 or fuses on the outgoing feeders but should have adequate ISSUE R1 FORM NO. 120 R1

TATA CONSULTING ENGINEERS TCE.M6-EL-PI-G-TF6508

PROTECTION OF TRANSFORMERS

SECTION:WRITE -UP.

SHEET 21 OF 24

time delay to prevent its operation during magnetising inrush current. 3.7

(a)

Differential & restricted earth fault protection for star connected 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.

ISSUE R1 FORM NO. 120 R1