DESIGN AND CONSTRUCTION
6.0 DESIGN AND CONSTRUCTION
6.1 CORE MATERIALS
All transformer cores shall be fabricated from high grade cold rolled grain oriented steel of low loss characteristics.
6.2 CORE CONSTRUCTION
All parts of the core shall be of robust design capable of withstanding mechanical shocks. Bracing of the core and winding assembly must be adequate to prevent any movement relative to the tank during normal lifting transportation and handling of the transformer. The supporting framework of the core shall be design to avoid the presence of pockets which would prevent complete emptying of the tank through the drain valve, cause the trapping of air during filling or cause the trapping of gases which involve during in service faults. Individual laminations shall be insulated with material which will not deteriorate due to the action of pressure and hot oil.
6.3 WINDING CONDUCTOR MATERIAL
Winding shall be of copper conductors and shall be disposed concentrically around the magnetic circuit limbs. Winding of aluminium conductors shall not be accepted.
6.4 WINDING CLAMPING AND BRACING
The winding and connection shall be adequately braced to withstand mechanical shocks and electromagnetic impulsive forces which may arise during handling, transportation and short circuit conditions.
6.5 WINDING INSULATION
Interturn insulation of winding, shall be of high grade paper.The insulation barriers between windings and to the core shall be cylinders of synthetic bonded paper, pressed board , fibre glass or equivalent, and shall be of high dielectric and mechanical strength.
6.6 OIL DUCTS
Oil ducts be provided where necessary to ensure adequate cooling. The coil clamping arrangement shall be such as will not impede the free circulation of oil through the ducts.
6.7 WINDING TAPPING
Tapping shall be located in such a manner to maintain the electrical balance of the windings and to reduce resulting axial forces to a minimum.
6.8 DRYING PROCESS
Before final assembly the windings shall be subjected to a drying process.
6.9 TRANSFORMER TANKS AND COVERS
The tanks shall be constructed from mild steel sheet having corrugated steel sides with strengthening bars to provide rigidity and mechanical strength. The tank covers shall be welded on. The tanks and covers shall be so designed as to prevent collection of water on the outside surfaces.
The inside of tanks and covers shall preferably be sand or shot blasted or alternatively cleaned thoroughly by some approved means to remove all scales and rust, and shall be protected from rust by permanent coating not affected by hot oil. Skid base shall be permanently welded. Lifting lugs shall be provided on the transformer so that the transformer complete with oil can be lifted.
6.10 COOLING SYSTEM
Cooling tubes or fins shall be fitted on the sides of the transformer. The arrangement of the tubes or fins shall be such that all painted surfaces can be readily cleaned and painted in position. The tubes or fins shall be designed to allow free circulation of oil and to prevent any accumulation of moisture.
6.11 CABLE BOXES
Cable boxes shall be provided at the high voltage terminals of the transformers. Unless otherwise specified in the bill of Quantities and / or Drawings, they shall be vertical bottom entry type. On high voltage side, the bushing assembly shall be integral with the cable box. On the low voltage side the bushing assembly shall be mounted on a plate which shall be flange mounted to a trunk or direct to the transformer side.The cable box shall be separated from the plate in order to facilitate conversion from normal bare bushing type to cable box type or vice versa. The flange fixings on both high and low voltage sides shall be generally in accordance with BS 2562 Pt. 1 or other acceptable authoritative standards subject to the approval of the S.O. Suitable wooden split clamps and galvanised iron channels shall be used to provide supports to the cables.
All gaskets shall be of synthetic rubber-bonded cork or other similar material subject to the approval of the S.O.
All valves shall be attached to the transformer by machined flanges. The outlet shall have female threads. A male threaded plug incorporating a sampler shall be provided with each drain valve. Valves shall be closed by clockwise rotation of the handle and provision for padlocking in the closed and open positions shall be included.
Unless otherwise stated in the Bill of Quantities transformers covered by this specification shall be provided with the following standard fittings :-
(a) Terminal Marking and Rating Plates
(b) Lifting Lugs
(c) Jacking Lugs
(d) Earthing Terminal (two numbers on opposite side)
(e) Oil Level Indicator
(f) Oil Temperature Indicator
(g) Thermometer Pocket
(h) Winding Temperature Indicator
(i) Drain Valve
(j) Filter Valve
(k) Pressure/Vacuum Bleeding Device
(l) Pressure/Vacuum Relief Device
(m) Pressure/Vacuum Gauge
The transformers offered shall be suitable for installation within the room space as shown in Drawings, leaving adequate working space between the compartment walls and cable boxes and/or bushing terminals to facilitate the termination of H.V. and L.V. cables.
6.15 TERMINAL MARKING AND RATING PLATES
The terminal marking plate shall show a plan view of the terminals with the characters of terminal markings as per Appendix D of BS 171 or IEC 76 engraved thereon. It shall also show the tappings in a tabulated form.The rating plate shall indicate the following :-
(a) Manufacturer’s name and address
(b) Transformer Specification Reference
(c) Manufacturer’s Serial Number
(d) Year Of Manufacture
(e) Rating in KVA
(f) Volts at no-load on normal tapping :-
(g) Current at rated load on normal tapping :-
(h) Number of phases
(j) Vector Group Symbol
(k) % Impedance Voltage at Normal Tapping at 75 degrees Celsius
(l) Mass of core and windings (untanking mass) (kg)
(m) Mass of Oil (kg)
(n) Total Mass (kg)
(o) Quantity of Oil (litres)
(p) Maximum Temperature Rise in Oil (degree Celsius)
(q) Tappings in Tabulated form
The terminal marking and rating plates shall be of a durable and corrosion resisting material and the markings thereon shall be permanently legible. The two plates may be combined into a single plate.
6.16 CLEANING AND PAINTING
No painting shall be carried out on the transformers until they have been tested and found satisfactory. All exterior ungalvanised surfaces of tanks and covers shall be thoroughly cleaned of all scales and rust preferably by shot blasting. Following which a priming coat of red lead or other approved paint shall be applied over all metal surfaces liable to rust. After completion of the Works Tests, two further coats of Oil and weather resisting paint shall be applied. The first coat being allowed to dry and thoroughly set before the second is applied.
Transformers shall be supplied filled with oil and hermetically sealed. The oil shall meet the requirements of BS 148 and shall be completely free from PCB.