8 service transformer

PIONEER GAS POWER LTD.

380-425MW COMBINED CYCLE POWER PLANT APSIPL-05- EPC-3-B8

ELECTRICAL SYSTEMS- SERVICE TRANSFORMERS

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8.0 SERVICE TRANSFORMER

The service transformer shall conform to the latest editions of

national and international standards indicated in Section B-2

Generator Transformers also, the service transformer shall comply

with the requirements indicated in Section B-2 and specific

requirements indicated below.

8.1 The number and rating of service transformers shall be decided

based on the geographical location of loads without sacrificing the

unit and common load concept.

8.2 The service transformers shall be two winding, three phase, 50 Hz,

6600 / 433 Volts, ONAN cooled with off-circuit taps of + 5% in steps

of 2.5%.

8.3 Impedance

The impedance of the transformers shall be selected such that the

fault level on the switchgear do not exceed its short circuit capability

including the motor contributions and also voltage dip while starting

any auxiliary does not exceed the limit acceptable for the equipment

concerned but in no case shall exceed 20%.

8.4 MVA Rating

Each transformer shall be rated to feed the entire load on the

respective switchgear considering the outage of another

transformer. An appropriate load factor with a minimum of 0.9 for

continuously running loads and 0.4 for intermittent loads like

cranes, plug points, sump pumps, etc., shall be considered. An

additional margin of 20% over the rating thus established shall be

provided for load variations during the course of engineering.

8.5 Specific Requirements

8.5.1 The transformer shall generally comply with specific requirements

indicated for generator transformers.




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8.5.2 The tank cover shall be of conventional type with bolted joint at the

top.

8.5.3 The transformers shall be provided with detachable built on

radiators of adequate number.

8.5.4 Off-circuit tap changers with lockable operating handle, mechanical

tap position indicator, and with a variation of 2.5% per step and a

range of + 5% shall be provided.

8.5.5 Pressure relief device shall be provided.

8.5.6 The neutral of the LV side of the transformer shall be brought out in

a separate cable box. If the same cable box is used for neutral

side, then a metallic partition between phases and neutral shall be

provided.

8.6 Cast Resin Auxiliary Transformers

8.6.1 Instead of outdoor oil filled auxiliary transformer, a cast resin dry

type transformer is acceptable.

8.6.2 The cast resin transformer shall be housed in naturally ventilated

floor mounted cubicle with louvres backed by fine brass wire mesh.

Degree of protection for enclosure shall be atleast IP 32.

8.6.3 The transformer cubicle can be located right next to respective

415V switchgears and LV side shall be connected with solid bus bar

connections, or through bus ducts. The HV connections shall be

through cables.

8.6.4 The dry type transformers shall be provided with core and winding

temperature indicators, with temperature high alarm, and very high

trip, contacts.

8.6.5 The insulation class for cast resin transformer shall be class-F.

11.6.6 The core and windings shall be provided with embedded

temperature detectors for remote indication in CRTs.




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11.6.7 Noise level shall not exceed values specified in NEMA TR-1.

S.NO DESCRIPTION Service Transformers

UNITS VALUES/REQUIREMENTS

1.0 Type Outdoor or indoor (Cast resin), two

winding 2.0 No. load voltage - HV Volts 6600 - LV Volts 433 3.0 Cooling method ONAN 4.0 Overfluxing capability 110% continuous 5.0 Winding connections 5.1 HV Delta 5.2 LV Star 5.3 Vector group Dyn11 6.0 Impulse (1.2 / 50 micro second) /

Power frequency withstand voltage windings

6.1 HV kVp,

kV(rms)

60 / 20

6.2 LV kV(rms

) - / 3

7.0 Maximum temperature rise of oil 0C 50 ( Design Ref. Ambient 500C ) 8.0 Maximum temperature rise of

winding

0C 55 ( Design Ref. Ambient 500C )

9.0 Tapping particulars




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9.1 Type Off-load 9.2 Tap range % + 5 9.3 Tapping step % 2.5 9.4 Category of voltage variation CFVV 9.5 Tapping power Full 10.0 Bushing particulars 10.1

Voltage class – HV / LV

kV

7.2/1.1

10.2 Creepage distance mm /

kV 25, based on system line to line highest system voltage

11.0

Codes and Standards

11.1 Service transformer IS : 2026 / IS:11171 11.2 Fittings and accessories IS : 3639 11.3 Loading of oil immersed

transformers IS : 6600

11.4 Oil IS : 335 11.5 Bushings IS : 2099 11.6 Degree of protection IS : 2147 11.7 Tests IS : 2026 / IS:11171 11.8 Tolerance on guaranteed

particulars IS : 2026

11.9 Buchholz relay IS : 3637




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11.10 Electrical insulation classified by thermal stability

IS : 1271

8 service transformer

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