Oil Cooled Single Phase Potential Transformers

Construction :
Outdoor oil cooled single phase Potential Transformers are made for use on systems ranging from 11kV to 33kV. The transformers are suitable for mounting on structures, and basically consist of the following parts :
a) Core
b) Winding
c) Insulation
d) Tank
e) Bushing
f) Oil Expansion Chamber
g) Secondary connection box

a) The Core : The core is built from high grade transformer steel sheet. Shell type construction of core is used in all cases.

b) The Winding : There are 2 main windings on every transformer. The H.T. Winding and L.T. Winding. Special oil proof enamelled Copper wire is used and wound in layers, each layer being adequately insulated from the adjacent layers. Since the equipment is for outdoor use, it is important that the windings are designed suitably to withstand the surges to which the system may be subjected. This is duly taken care of by proper grading of the insulation in the various layers.
The L.T. winding is placed adjacent to the core and is insulated from it for power frequency voltage test level of 2kV for 1 minute. Depending upon the requirement, two L.T. Windings can be provided; one for metering and the other for protection. In such case they are insulated from each other and also from the core for power frequency voltage test level of 2kV for 1 minute.

c) The Insulation : The insulation is in the form of oil impregnated paper(Class A). The wound coils, duly insulated with paper are thoroughly dried under vaccum before being assembled. The oil used is high quality transformer oil which is fully treated under vaccum before use.

d) The Tank : The tank is made of sheet steel and is given three coats of paint, both inside and outside, to avoid corrosion. Suitable clamping arrangement is provided in the tank for firmly bolting the core and the wound coils. The L.T. winding terminals are brought out of the tank into secondary connection box via suitable bakelite bushings. Two earthing studs, which are welded to the tank are provided for earthing the transformer whenthe connected into the system. The transformer is mounted onto the structure by 4 nos. of suitable size of bolts and nuts.

e) The Bushing : The porcelain bushings used on the potential transformers are as per BSS 223/1956 and suitable for use in normally polluted atmosphere. The high voltage end/ends of the H.T. winding, duly insulated are brought out of the tank into the oil expansion chamber through bushings. The bushings are mounted on the tank by suitable clamps. Neoprene rubber gaskets are used to avoid any leakage of oil.

f) The Oil Expansion Chamber : The oil expansion chamber which is made of aluminium is fixed onto the bushing by using epoxy resin. The high voltage end/ends of the H.T. winding are connected to the oil expansion chamber. The volume of the expansion chamber is designed to permit expansion of oil due to the increase of ambient temperature. At the time of despatch, the oil is filled only upto the top of the bushing and there is no oil in the expansion chamber.

g) The Secondary Connection Box : This box is welded on to the transformer tank and is provided with a suitable cover to protect the L.T. terminals from atmospheric hazards. A suitable hole for entry of the L.T. cable is provided.

The transformers are of fully sealed in construction and are supplied filled with required quantity of oil. Provision of adequate size oil expansion chamber does away with the need for a breather.

There are 2 types of outdoor single phase Potential transformers. These are :
1.) Suitable for connection between phase and nuetral
2.) Suitable for connection between phases

The type designation used for outdoor oil cooled Potential Transformers is VTO. This is followed by the rated voltage of the transformer and the number after the oblique mark indicates whether the transformer is of single bushing type or has both ends of its H.T. winding insulated.

Maintenance : No other maintenance apart from the occasional cleaning of the insulator is considered necessary.

Technical Informations Required for Tendering :
- Ratio
- Number of Secondary Windings
- 'VA' Burden of each windings
- Accuracy Class
- System Conditions;effectively or non-effectively earthed
- Voltage class
- Whether suitable for heavily or normally polluted atmosphere conditions with creepage distance of insulator.

Standards : IEC 60044-2;1997, IS 3156;1992