Cooled Single Phase Potential Transformers
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 :
f) Oil Expansion Chamber
g) Secondary connection box
The Core : The core is built from high grade transformer steel
sheet. Shell type construction of core is used in all cases.
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.
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
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.
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.
No other maintenance apart from the occasional cleaning of the
insulator is considered necessary.
Required for Tendering :
- 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