Download Power plant transformer protections PDF

TitlePower plant transformer protections
TagsTransformer Relay Electrical Substation Electric Generator
File Size318.2 KB
Total Pages6
Document Text Contents
Page 1

18Electrical Installation Engineer - Newsletter - August 2013

ELECTRICAL PROTECTION OF TRANSFORMERS IN
LARGE POWER PLANTS

1.0 Introduction
A typical power plant is
equipped with following power
transformers:
a) Generator Transformer
(GT); b) Station Transformer
(ST); c) Unit Transformer
(UT)

Generator transformer
evacuates the generated
power. In a plant with
Generator circuit breaker
(GCB), GT can also be used
to feed the auxiliaries via Unit
transformer. Refer Fig.1. Unit
Transformer feeds the unit
auxiliary loads and Station
Transformer feeds the station
auxiliary loads. In a plant
without GCB Station
Transformer draws power
from the grid to provide startup
power.
This article discusses the
protection philosophy for the
above transformers. It
excludes interconnecting
transformer (ICT), HV and LV
Auxiliary transformers.
2.0 Transformer
Protections
The protections provided for
the power transformers are
detailed below:
2.1 Protection of Generator
Transformer (GT)
2.1.1 Electrical protections
2.1.1.1 GT differential
protection (87GT)

a) In case of three phase
units, GT differential
typically covers the
overhead section of the
switchyard in addition to
the transformer. Refer
Fig.2.

b) For a bank of single
phase transformer, the
GT differential generally
covers only the
transformer. A separate

differential protection (87L) covering the HV winding and overhead section
is also provided for bank of single phase transformer. Refer Fig. 3.

Fig.1: Single Line Diagram of Auxiliary System in Power Plant
with Generator circuit Breaker

Fig.2: Three Phase GT Protection

Page 2

19Electrical Installation Engineer - Newsletter - August 2013

c) It may be noted that CTs shown in shaded
area in Fig 2 and Fig 3 are bushing CTs.

2.1.1.2
a) For a bank of single phase transformers,

differential protection is shown in Fig 4.
b) Two CTs, on either side of LV windings

are provided, intrinsically to cover ground
faults in the delta winding. [Ref (1)]. This
scheme is built on the premise that
sufficient current will flow on occurrence
of earth fault.

c) However in a power plant, high resistance
grounding is provided for generator neutral.
The earth fault current on the delta side of
GT is limited within 10A.

d) The differential protection of generator
transformer cannot sense the ground fault
in the delta winding. It can be sensed only
by voltage based earth fault sensing
scheme provided on generator terminals.
(59N and 64G in Fig.2 and Fig.3)

e) In view of the above, provision of one CT
on delta winding is sufficient. Refer Fig.5.
This philosophy is adopted in all recently
engineered power plants in authors’
company.

2.1.1.3
a) Overhang differential protection (87L) or

HV Restricted earth fault (REF) protection
(64) − Overhang differential protection
(87L) is a biased three phase differential
protection covering the HV windings as
well as the overhead conductors from
switchyard to the transformer. This
protection is provided for a bank of single
phase transformers. This protection is
implemented as a differential protection of
each individual HV winding and its EHV
overhead connection. For this purpose one
CT is provided at the HV neutral bushing
and its corresponding CT is provided at the
EHV breaker end. Refer Fig.6. In this case
separate REF protection is not required for
generator transformer. Alternatively a
single pole differential can act as a REF
protection.

b) For three phase generator transformer, high
impedance restricted earth fault protection
(64) is provided. The restricted earth fault
protection usually covers only the HV
windings. Refer Fig.2.

c) The zone covered by differential protection
and REF protection in case of 3 x single
phase GT bank and three phase GT is
summarised in Table 1.

Fig.3: Bank of Single Phase GT Protection

Fig.4: Conventional Differential Scheme for Single phase
transformer bank

Fig.5: Differential scheme with single CT in delta winding

Page 3

20Electrical Installation Engineer - Newsletter - August 2013

Fig.6: Differential Protection for Single Phase Generator
Transformer

2.1.1.4 Overall differential protection
(87OA) − This covers the Generator and the
GT and functions as a backup to the GT
differential protection.
2.1.1.5 Instantaneous over current protection
acts as a backup to GT differential protection
(50GT).
2.1.1.6 GT HV side over-current protection
(51 GT)
2.1.1.7 GT HV side stand-by earth-fault
protection (51 NGT)
2.1.1.8 GT Over-fluxing protection (24) −
sensed from selected EHV bus voltage (not
shown in figure)
Protections listed in 2.1.1.6 to 2.1.1.8 are
provided for protection against uncleared grid
faults/disturbances.

Table 1

Type of GT Protection Zone covered CTs used Remarks
Differential Transformer Currents from LV bushing Ref Fig. 3
protection windings CTs are compared against
(87GT) currents from HV bushing CTs.
Overhang Transformer Currents from HV neutral 1. Ref Fig. 3
differential HV windings bushing CTs are compared 2. Will respond to phase and
protection and EHV against currents from EHV CTs. earth faults.
(87L) connection 3. REF protection (64R) need

not be provided if this
protection is provided.
4. Widely used

REF Transformer HV Summated currents of HV 1.Ref Fig. 3
protection windings and neutral bushing CT is compared 2.Will respond to earth faults only.
(64R) EHV connection against summated currents of 3. 87L protection need not be

EHV CTs. provided if this protection is
provided.
4. Rarely used

Differential Transformer HV Currents from LV bushing CTs Ref Fig. 2
protection windings and are compared against currents
(87GT) EHV connection from EHV CTs.
REF Transformer HV Current on HV neutral bushing Ref Fig. 2
protection windings CT is compared against
(64R) summated currents of HV phase

bushing CTs
2.1.2 Thermal Mechanical protections

a) OTI / WTI - alarm and trip; b) Pressure Relief Device trip; c) Buchholz - alarm and trip; d) Oil level low
alarm

For single phase transformers all the above protections are provided for each of the phase units.

3 x Single
Phase GT
Bank

Three phase
GT

Page 4

21Electrical Installation Engineer - Newsletter - August 2013

2.1.3 In addition to the above, fire protection is also provided for the GT.
2.1.4 The electrical protections mentioned in CI 2.1.1 are suitably grouped in two or three numerical relays. The
grouping is done in such a manner that each acts as a backup to the other.

A typical suggested grouping of functions for bank of
single phase transformers is given in Table-2.

Table -2
Protection function for bank of single phase

transformers
Relay-1 (R1): Trafo Protection Relay

Overall differential protection (87OA)
Relay-2 (R2): Trafo Protection Relay
Overhang differential protection (87L)

GT HV side instantaneous and IDMT phase over
current protection (50/51 GT)

Relay-3 (R3): Trafo Protection Relay
GT Differential Protection (87GT)

GT HV side stand-by earth-fault protection
(51 NGT)

Overfluxing (24)

A typical suggestive grouping of functions for three
phase transformers is given in Table-3

Table -3
Protection function for three phase

transformers
Relay-1 (R1): Trafo Protection Relay

Overall differential protection (87OA)
Relay-2 (R2): Overcurrent and Earth fault relay

REF protection (64)
GT HV side instantaneous and IDMT phase over

current protection (50/51 GT)
Relay-3 (R3): Trafo Protection Relay

GT Differential Protection (87GT)
GT HV side stand-by earth-fault protection

(51 NGT)
Overfluxing (24)

2.2 Protection of Station Transformer (ST)
2.2.1 Electrical protections (Refer Fig.7)

Fig.7: Station Transformer Protection
2.2.1.1 ST differential protection (87ST) - ST
differential covers the overhead section of the switch
yard in addition to the transformer and busduct
connecting the LV winding with the switchgear.

2.2.1.2 HV winding Restricted Earth fault (REF)
protection (64HV) High impedance REF protection is
usually provided for HV side winding.
2.2.1.3 HV phase over-current and directional earth
fault protection (50/51/67N) The instantaneous (50)
stage of phase and earth fault protection is set to clear
HV side faults instantaneously. The IDMT stage of
phase overcurrent protection is coordinated with LV
side relays. The overcurrent protection also acts a
backup to differential protection for HV side faults. The
earth fault element is directionalised to ensure pickup
for ground faults towards ST and not in grid.
2.2.1.4 LV winding Restricted Earth fault (REF)
protection (64LV) Low impedance REF protection is
envisaged for LV side winding. A low impedance REF
scheme provides sensitive protection without provision
of interposing CT and stabilising resistor as required in
high impedance protection scheme.
2.2.1.5 LV Stand-by Earth fault protection (51 NS) −
This protection is provided to clear uncleared earth
faults on LV side. It acts as backup to LV REF
protection also.
2.2.1.6 ST Over-fluxing protection (24) − This
protection is provided to protect the transformer against
grid over voltages. It is sensed from selected EHV bus
voltage (not shown in figure)
2.2.2 Thermal/Mechanical protections

a) OTI / WTI − alarm and trip; b) Pressure Relief
Device trip; c) Buchholz − alarm and trip; d) Oil
Surge Relay − trip; e) Oil level low alarm

Page 5

22Electrical Installation Engineer - Newsletter - August 2013

2.2.3 In addition to the above, fire protection is also
provided for the ST.
2.2.4 It may be noted that CTs shown in shaded area in
Fig 7 are bushing CTs.
2.2.5 The electrical protections are grouped in two
numerical relays. The grouping is done in such a manner
that each acts as a backup to the other. The grouping of
functions can be as per Table-4.
2.3 Protection of Unit Transformer (UT)
2.3.1 Electrical protections (Refer Fig.8)
2.3.1.1 UT differential protection (87UT) − UT
differential covers the transformer and busduct
connecting the LV winding with the switchgear.
2.3.1.2 HV phase over-current protection (50/51)
− The instantaneous stage of phase overcurrent
protection is set to clear HV side faults instantaneously.
The IDMT stage of phase over-current protection is
coordinated with LV side relays. It also acts a backup
to differential protection for HV side faults.
2.3.1.3 LV winding Restricted Earth fault (REF)
protection (64LV) − Low impedance REF protection
is envisaged for LV side. The comments made for ST
are applicable here also.
2.3.1.4 LV Stand-by Earth fault protection (51 NS)
− This protection is provided to clear uncleared earth
faults on LV side. It acts as backup to LV REF
protection also.
2.3.2 Thermal/Mechanical protections

a) OTI / WTI - alarm and trip; b) Pressure Relief
Device trip; c) Buchholz - alarm and trip; d) Oil
Surge Relay - trip; e) Oil level low alarm

2.3.3 In addition to the above, fire protection is also
provided for the UT.
2.3.4 It may be noted that CTs shown in shaded area in
Fig 8 are bushing CTs
2.3.5 The electrical protections are grouped in two
numerical relays. The grouping is done in such a manner
that each acts as a backup to the other. A typical
grouping of functions could be as per Table-5.
3.0 Grouping of Protection functions
3.1 There is usually an apprehension regarding provision
of LV side REF and differential protection in one relay
(Relay 2 in Table-4 and Table-5) as suggested in CI
2.2.5 and 2.3.5.
3.2 The vector group of ST is star/star with EHV side
solidly earthed. UT is a delta/star transformer. On HV
side of UT, earth fault is restricted within 10A. Current
based protections do not respond to earth faults on HV
side of UT. The earth fault is sensed only by voltage
based earth fault scheme provided on generator
terminals. (59N and 64G in Fig.2 and Fig.3)
3.3 LV side of UT and ST is earthed through neutral
grounding resistor (NGR) restricting the earth fault
current to typically 300A. Operation of differential

protection is doubtful for earth faults on LV side
especially for faults within the winding [Ref (2)]. Only
REF and SEF protection will definitely respond to LV
side earth faults.

Table-4
Protection function

Relay-1 (R1): Overcurrent and Earth fault
relay

LV side stand-by earth-fault protection (51 NS)
HV side phase over current and earth fault

protection (50/51/67N)
HV REF protection (64HV)

Relay-2 (R2): Trafo Protection Relay
ST Differential Protection (87ST)

LV REF protection (64LV)
Overfluxing (24)

Fig.8: Unit Transformer Protection

Table-5
Protection function

Relay-1 (R1): Overcurrent and Earth fault
protection

LV side stand-by earth-fault protection (51 NS)
HV side phase over current protection (50/51)

Relay-2 (R2): Trafo Protection Relay
UT Differential Protection (87UT)

REF protection (64)

Similer Documents