FORM 2
THE PATENTS ACT, 1970
(39 OF 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF INVENTION
Outdoor dry transformer
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 025, Maharashtra, India, an Indian Company
INVENTORS
Bhatia Anilkumar Brijlal, Technology Department, Transformer Division (Tl), Crompton Greaves Ltd. Kanjur (E), Mumbai 400 042, Maharashtra, India, Yargole Arun Dattatraya, Condition Moniotoring and Diagnostic Research Centre, CG Global R&D Centre, Crompton Greaves Ltd. Kanjur (E), Mumbai 400 042, Maharashtra, India, and Roy Deosharan, Analytical Lab, CG Global R&D Centre, Crompton Greaves Ltd, Kanjur (E), Mumbai 400042, Maharashtra, India, all Indian Nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed :

FIELD OF INVENTION
This invention relates to an outdoor dry transformer.
BACKGROUND OF INVENTION
Dry transformers are advantageous over oil filled transformers in several respects. For instance, dry transformers do not require periodic maintenance and oil replacement as in the case of oil filled transformers. They are also free from oil leakages and oil pilferation. Oil is environmentally polluting and capable of causing health hazards besides being susceptible to fire accidents. Dry transformers have excellent insulation characteristics and are also flame proof. Dry transformers generally operate at higher temperatures of the order of 120 to 180°C and are preferred in hazardous areas such as mines, densely populated residential areas or hospitals. Dry transformers also have reduced weight and size as compared to oil filled transformers which are heavy and bulky.
An outdoor dry transformer generally comprises a core and a coil assembly mounted on the core. The coil assembly comprises a low voltage winding (secondary winding) and a high voltage winding (primary winding) with electric insulation material between the layers of conductors of each of the windings and between the windings. The windings are optionally impregnated with a resin and the core and coil assembly as a whole is encapsulated with a resin. The outer surface of the resin encapsulation is plain. In order to provide for dissipation of heat being generated within the core and coil assembly during operation of the transformer air ducts are provided within the core and coil assembly. Heat sinks also provided within the core and coil assembly for heat dissipation (PCT Publication No WO2006/016377 based on Indian
2

Patent Application No 307/MUM/2003). The air ducts and heat sinks increase the size and weight of the transformer besides the cost. The heat sinks also render the construction of the transformer complex and complicated and reduce productivity. Dry transformers for outdoor applications generally require a protective metallic tank or shelter so as to prevent water and foreign matter getting into the transformer through the air ducts. Metallic tank or shelter also increases the cost of the transformer besides the space required for its operation.
OBEJCTS OF INVENTION
An object of the invention is to provide an outdoor dry transformer which has reduced size and weight and which is cost effective besides having all the other advantages of a dry transformer.
Another object of the invention is to provide an outdoor dry transformer which is simple in construction and increases productivity.
Another object of the invention is to provide an outdoor dry transformer which eliminates the protective metallic tank or shelter.
DETAILED DESCRIPTION OF INVENTION
According to the invention there is provided an outdoor dry transformer comprising a core and a coil assembly mounted on the core, the coil assembly comprising a low voltage winding and a high voltage winding with electric insulation material between the layers of conductors of each of the windings and between the windings, the core and coil assembly including the poles of the transformer being encapsulated with a
3

resin having good thermal conductivity of at least 1 Watt per meter Kelvin, a metallic screen being disposed around the core and coil assembly and embedded in the resin encapsulation, the outer surface of the resin encapsulation being provided with fins or convolutions so as to increase the effective outer surface area of the transfer and the heat dissipation by radiation.
The following is a detailed description of the invention with reference to the accompanying drawing, in which :
Figs 1 and 2 are elevation and plan view of a single phase outdoor dry transformer according to an embodiment of the invention; and
Figs 3 and 4 are vertical crossection and horizontal crossection at A-A and B-B of the transformer of Figs 2 and 1, respectively.
The transformer 1 as illustrated in Figs 1 to 4 of the accompanying drawings comprises a magnetic material core 2 and a coil assembly 3 mounted on the core. The coil assembly comprises a low voltage winding 4 and a high voltage winding 5 with electric insulation material 6 between the windings. The coil assembly also comprises electric insulation material (not shown) between the layers of conductors of each of the windings. The electric insulation material comprises electric grade plastic films commonly used for such purpose. The core and coil assembly is fully encapsulated with a resin (encapsulation marked 7) with the outer surface of encapsulation being formed with fins or convolutions 8. The poles marked 9 of the transformer are also encapsulated with the resin simultaneously. A metallic screen 10 is disposed
4

around the core and coil assembly and embedded in the resin encapsulation to ensure a zero potential on the surface of the coil assembly. The screen is made of metals such as copper, brass or phosphor bronze. The fins or convolutions on the outer surface of the encapsulation increases the effective outer surface area of the transformer so as to increase dissipation of heat being generated within the core and coil assembly by radiation during operation of the transformer. According to the invention air ducts and heat sinks provided within the core and coil assembly have been eliminated and the size and weight of the transformer is reduced besides the cost. The transformer construction is rendered simple and productivity is also increased. As the air ducts have been eliminated, the transformer also does not require protective metallic tank or shelter for outdoor applications. Any resin having good thermal conductivity of least 1 W/mk (Watt per meter Kelvin) can be used for the encapsulation. Preferably, the resin is polycrete having a thermal conductivity of 1.1 W/mk.
Temperature rise in a single phase outdoor transformer of 25 KVA capacity according to Fig 1 and in an equivalent conventional dry transformer comprising air ducts was found to be of the order of 90°C. The transformer of Fig 1 was encapsulated with polycrete resin having thermal conductivity of 1.1 W/mk. The weights of the transformers were as shown in the following Table 1.
5

Table 1

Weights in kgs
Core Low Voltage Winding High Voltage Winding
Transformer of invention 97 34 35
Conventional Transformer 140 35 20
It is quit clear from the above experimental data that the cooling efficiency of the transformer of the invention was comparable with that of the conventional transformer and that there was reduction in the material and size of the transformer of the invention.
It is to be clearly understood that the transformer of the invention can be 2, 3 or more phases. The shape of the fins or convolutions can vary.
6

We claim:

1. An outdoor dry transformer comprising a core and a coil assembly mounted on the core, the coil assembly comprising a low voltage winding and a high voltage winding with electric insulation material between the layers of conductors of each of the windings and between the windings, the core and coil assembly including the poles of the transformer being encapsulated with a resin having good thermal conductivity of at least 1 Watt per meter Kelvin, a metallic screen being disposed around the core and coil assembly and embedded in the resin encapsulation, the outer surface of the resin encapsulation being provided with fins or convolutions so as to increase the effective outer surface area of the transfer and the heat dissipation by radiation.
2. An outdoor transformer as claimed in claim 1, wherein the core and coil assembly including the poles of the transformer is encapsulated with polycrete resin having thermal conductivity of 1.1 watt per meter Kelvin.
3. An outdoor dry transformer as claimed in claim 1 or 2, which is single phase.
4. An outdoor dry transformer as claimed in anyone of claims 1 to 3, wherein the metallic screen is made of copper, brass or phosphor bronze.
Dated this 15th day of March 2007

7

ABSTRACT
An outdoor dry transformer (1) comprising a core (2) and a coil assembly (3) mounted on the core. The coil assembly comprises a low voltage winding (4) and a high voltage winding (5) with electric insulation material (6) between the layers of conductors of each of the windings and between the windings. The core and coil assembly and the poles (9) of the transformer are encapsulated with a resin having good thermal conductivity of at least 1 Watt per meter Kelvin. A metallic screen (10) is disposed around the core and coil assembly and embedded in the resin encapsulation. The outer surface of the resin encapsulation is provided with fins or convolutions (8) so as to increase the effective outer surface area of the transfer and the heat dissipation by radiation (Fig 3).