chap8

CHAPTER-8

Distribution Transformers above 100 kVA

1.0 Outdoor Type Three-Phase 11 kV/433-250 V Distribution Transformers upto and including 100 kVA.

1.1 Scope:

This section covers oil-immersed, naturally cooled, three-phase, 50 Hz, double-wound, outdoor type distribution transformers of capacities above 100 kVA for use on systems with nominal voltages of 33 kV, 11 kV.

kVA Voltage Ratio

250 500 11000/433

630

630

1000 33000/433

1600

2000

2.0 STANDARDS:

2.1 The materials shall conform in all respect to the relevant Indian / International Standard Specification, with latest amendments thereof, some them are listed below:

Indian Standard Title International &

Internationally recognized standard

ISS-2026/1977 Specification for Power IEC 76

Transformer

ISS-1866/10593/9434

Outdoor distribution

6792/2362 above 100 kVA

ISS-335/1983 Specification for Oil BS148

REC Specification ASTM D-1275

39/1993 D-1473, D-1533-1934

IEC Pub 296-1969

ISS-2099-1973 Specification for High voltage

Porcelain Bushings

ISS-7421/1976 Specification for Low voltage

bushings

ISS-3347 Specification for Outdoor Bushings

DIN 42531 to 33

ISS-12444 Specification for Cu Wire rods

ASTM B – 49

ISS-5484 Specification for Al Wire rods

ASTM B – 233

ISS-5/1961 No. 632 Specification for Colors for ready

mixed paints

ISS-6600/1972 Guide for loading of oil immersed

Transformers

Material conforming to other internationally accepted standards, which ensure equal or higher quality than the standards mentioned above would also be acceptable. In case the Bidders who wish to offer material conforming to the other standards, salient points of difference between the standards adopted and the specific standards shall be clearly brought out in relevant schedule. Four copies of such standards with authentic English Translations shall be furnished alongwith the offer.

3. SERVICE CONDITIONS

The distribution transformers to be supplied against this Specification shall be suitable for satisfactory continuous operation under the following climatic conditions as per IS:2026 (Part – I) Latest Revision

i. Location: At various locations in the

country

ii. Max ambient air

temperature (Deg. C) : 50

iii. Min ambient air

temperature (Deg. C) : -5

iv. Max average daily ambient

air temperature (deg. C) : 40

v. Max. Yearly weighed average

ambient temperature (Deg. C) : 32

vi. Max. Altitude above mean sea level (Meters) : 1000

4.0 Type of Cooling

The transformers shall be oil immersed with natural air circulation type-ONAN.

5.0 Insulation Levels

Voltage Impulse Power Frequency

Voltage Voltage

(kV Peak) (kV)

433 - 3

11000 75 28

33000 170 70

6.0 Tappings and Tap Changing

6.1 Tappings shall be provided on the higher voltage winding for variation of HV voltage as under:

Voltage ratio Range of variation

11000/433 + 3 to -9 percent

33000/433 + 3 to -9 percent

6.2 Tap changing shall be carried out by means of an off circuit externally operated self-positioning switch and when the transformer is in deenergised condition. Switch Position No. 1 shall correspond to the maximum plus tapping. Each tap change shall result in variation of 3 percent in voltage. Provision shall be made for locking the tap changing switch handle in position.

7.0 OIL

The insulating oil shall comply with the requirements of relevant standards IS:335 / 1993 or BS:148 or REC 39/1993. Oil shall be filled under vacuum.

8.0 Winding Connections

HV----------------Delta, LV---------star

Vector Symbol----------Dy 11

9.0 Winding Conductor Material

Winding conductor material shall be Aluminium or copper, Statutory directives prohibiting use of copper shall also be taken into account. The copper/aluminium used shall be double paper covered with 50% percent overalap.

10.0 Losses and Impedance

(Subject to tolerance as per IS 2026)

Rating Voltage No-load Load Impedance*

(kVA) ratio loss loss* (percent)

(Volts) (Watts) (Watts)

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250 620 3700 5

500 11000/433 1100 6500 5

630 1200 7500 5

630 1450 7500 5

1000 33000/433 2200 11500 5

1600 3000 16000 6.25

2000 3500 21000 6.25

*Correspond to reference temperature of 75o C.

However, the manufacturer can offer losses less than above. The offer will be evaluated as per the loss evaluation formula given in REC, K-5 Standard – as amended up to date.

Loss capitalization factor for 2001-2002 may to taken as follows:

1. Iron loss Rs.2, 17, 515/kW 2. Copper loss Rs.28, 772/kW

11.0 Overfluxing

The transformer shall be suitable for overfluxing (due to combined effect of voltage and frequency) up to 12.5 percent on any tapping without injurious heating at full load conditions.

The maximum flux density in any part of the core under such condition shall not exceed 1.9 Tesla. The supplier shall furnish necessary design data in support of this stipulation.

12.0 Core Material : CRGO or Amorphous Metal

13.0 Terminals

The terminals shall be provided as under:

Notes: (i) Alternatively 433 volt terminal could be provided with 1.1 kV bushing as per IS : 3747 for normally polluted atmosphere.

i. Alternatively 433 volts terminal case provided with 1.1 kV bushing

suitable for bus duct connections.

ii. P and G denote `Pole' and `Gland' respectively. iii. Epoxy may also be used as the filling medium instead of bitumen compound

14.0 Loading

In accordance with IS : 6600 ---1972, the transformer shall be capable of being overloaded as per guidance given in IS: 6600---1972.

15. 0 Terminal Clearances

The minimum clearances shall be as under :

16.0 Limits of Temperature Rise

16.1 The temperature rise shall not exceed the limits of 55ºC (measured by resistance) for transformer windings and 45ºC(measured by thermometer) in top oil when tested in accordance with IS: 2026.

17.0 Fittings

a. Conservator with drain plug and oil filling hole with blanking plate. b. Plain oil gauge. c. Silica gel breather- to contain minimum 0.5 kg dehydrated silica gel. d. Pressure relief vent:

11 kV - 500 kVA and above

e. Pocket on tank cover for thermometer f. Valves: (i)Drain valve with plug or cover plate. Upto and including 1600 kVA- 25mm.

(ii) Top filter valve with plug or cover plate 1600 kVA- 25 mm.

(iii)Isolation valves between tank and radiators if the radiators are to be detached for transport.

g. Earthing terminals- 2 Nos. h. Rating and terminal marking plate i. Plain rollers j. Lifting plugs. k. Air release plug. l. Platform lugs. m. Oil temperature indicator- 1000 kVA and above.

n. Buchholz relay- 1600 kVA. o. Inspection cover- 1600 kVA.

Note: (i) Conservator may be without detachable end plate. The sump in the conservator will be 25 mm minimum.

(ii) Breather will be fitted at height not exceeding 1500 mm.

(iii) Rating and terminal marking plate will be fitted at the height of about 1750 mm above ground level .

17.1 Surface preparation and painting of tank

17.2 Steel surface shall be prepared by sand/shot blast or chemical cleaning including phosphating as per IS:3618.

17.2 Heat resistance paint (Hot oil proof) shall be provided inside the tank.

17.3 On external surface one coat of Thermo setting powder paint or two coats of zinc chromate followed by two coats of synthetic enamel paint of dark admiral gray shade conforming to No. 632 of IS:-5 of 1961 shall be provided.

18.0 Tests and Inspection

18.1 Routine Tests: All transformers shall be subjected to routine tests at the manufacturer’s works in accordance with IS : 2026 and IS : 1180 (Part I).

The following tests are to be carried out:

a. Measurement of winding resistance b. Measurement of voltage ratio and check of voltage vector relationship c. Measurement of impedance voltage/short circuit impedance and load-loss d. Measurement of no-load loss and current e. Measurement of insulation resistance f. Induced over-voltage withstand test g. Separate-source voltage withstand test.

18.2 Type Tests: In addition to the routine tests, the following type tests may be made as per details in IS 2026.

a. Lightning impulse-test. b. Temperature rise test. c. Short-circuit test d. Air pressure test e. Unbalanced current test: the value of unbalance current shall not be more than 2% of the full load

current.

Note: Purchaser reserves the right to get all or any type test carried out on one sample per 100 unit of transformer at the cost of supplier from any recognized laboratory / government test house.

18.3 The supplier shall furnish calculations in accordance with IS : 2026 to demonstrate the thermal ability of the transformers to withstand short-circuit.

19.0 Inspection

19.1 All tests and inspection shall be made at the place of manufacturer unless otherwise especially agreed upon by the manufacturer and purchaser at the time of purchase. The manufacturer shall afford the inspector representing the purchaser all reasonable facilities, without charge to satisfy him that the material is being furnished in accordance with this specification.

19.2 The purchaser has the right to have the tests carried out at his own cost by an independent agency whenever there is dispute regarding the quality of supply.

GUARANTEED & OTHER PARTICULARS FOR DISTRIBUTION TRANSFORMERS

(To be furnished by the Manufacturer)

________________________________________________________________________

Sl. No. Description

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1. Make & Manufacturer 2. Place of Manufacture 3. Rated Ratio

i. HV (kV) ii. KV (kV)

4. Rated current


5. Rating in kVA 6. Connections


7. Connection symbol 8. Tapping (if applicable)

i. Number of steps ii. Range of Tapping

9. Core Material used and Grad

a. Flux density b. Over fluxing without saturation (curve to be furnished by the

Manufacturer in support of his claim)

10. Maximum temperature rise of

a) Windings by resistance method

b. Oil by Thermometer 11. Magnetizing (No Load) Current at

a. Normal voltage b. Maximum voltage

12. Core loss in watts at 75 deg. C a. Normal voltage b. Maximum voltage

13. Resistance of Windings at 20 deg. C

(with 5% tolerance)

a. HV Winding (ohms) b. LV Winding (ohms)

14. Full load losses (watts) at 75 deg. C 15. Current density used for

a. HV Winding b. LV Winding

16. Clearances a. Core & LV b. LV & HV c. HV Phase to Phase d. End insulation clearance to Earth e. Any point of winding to tank

17. Efficiency at 75 deg.C a. Unity PF b. 0.8 PF

i. 125% Load ii. 100% Load

iii. 75% Load iv. 50% Load v. 25% Load

18. Regulation at a. Unity PF b. 0.8 PF at 75 deg.C

19. % Impedance at 75 deg. C 20. Flash test

HV 28 kV/50Hz for 1 minute

LV 3 kV /50 Hz for 1 minute

21. Over potential Test Double

Voltage & Double frequency for 1 minute

22. Impulse Test 23. Weight content of

a. Core lamination (min) b. Windings (min) c. Tank & Fittings d. Oil e. Oil qty (min) f. Total weight

24. Oil Data 1. Qty for first filling (min) 2. Grade of oil used 3. Maker's name 4. BDV at the time of filling

25. Transformer:

i. Overall length x breadth x height ii. Tank length x breadth x height

1. Thickness of plates for

a. Side plate (min) b. Top & bottom plate (min)

26. Radiation: i. Heat dissipation by tank walls exclusive top & bottom

ii. Heat dissipation by cooling tube iii. Dia & thickness of cooling tube iv. Whether calculation sheet for selecting cooling area to

ensure that the transformer is capable of giving continuous rated output without exceeding temperature rise is enclosed.

27. Inter layer insulation provided in design for i. Top & Bottom layer

ii. In between all layer iii. Details of end insulation iv. Whether wedges are provide at 50% turns of the HV coil

28. Insulation materials provided

a. Or conductors (1) HV (2) LV b. For core

29. Size of the wire used i. HV a)SWG/mm

b)Dia\

ii. LV a)Strip size

b) No. of conductors in parallel

c) total area of cross section (sq.mm)

30. Is the name plate gives all particulars as required in Tender 31. Particulars of Bushings HV/LV

i. Maker's Name ii. Type IS-3347/IS-1180

iii. Rating as per IS iv. Dry power frequency voltage withstand test v. Wet power frequency voltage withstand test

32. Medium of free space above oil level for sealed Transformer 33. Particulars of Metal oxide lightning arrester for self protected transformer 34. Particulars of LT circuit breaker for self protected transformers 35. Particulars of 11 kV Fuse arrangement for self protected transformers

chap8

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