Clean limitless energy has been the ambition of scientists world-wide and Fusion (the process of confining & fusing a combination of Hydrogen, Deuterium & Tritium gases at very high temperatures) is one way forward. A magnetic confinement chamber is needed to contain the high temperature gases - known as a plasma - and researchers have found that the best configuration is a torus-shaped magnetic chamber known as a Tokamak.

There are several differant types of Tokamak and in 1991 the Culham Science Centre at Abingdon, Oxon. constructed the first spherical Tokamak; the project was called START (Small Tight-Aspect Ratio Tokamak) and made use of several existing power supplies and other equipment. START was the first high temperature spherical Tokamak in the world and was operational until March 1998. The successor to START at Culham is the MAST project (Mega Amp Spherical Tokamak) which has just been completed and is a custom built machine with a stainless steel vacuum chamber and modern power supplies.

Tokamak plasmas require external heating systems and, in the case of MAST, a total power of 5 Mega Watts is needed to achieve MAST's objectives. This heating is provided by two Neutral Beam Injector systems and each of these is powered by a series of power supplies; with some components biased to voltages of up to 120kV dc relative to ground. In order therefore to ensure the safety of the staff at the facility, high power isolation transformers are used to isolate the electrical supplies. TEC was asked to tender for these transformers after previous transformers, manufactured by TEC, were incorporated in the START Neutral Beam Injector project

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TEC was faced with a unique problem. On the START project, to save space, TEC used oil filled porcelain bushings with two poles running through the centre however, the oil filled porcelain bushings had reached their safe working limit, and TEC had to cater for a increase in the ac withstand voltage plus the introduction of a new dc withstand test. The answer came from a firm called CAPMART of Cinderford, Glos. After much experimenting and testing the company fabricated a special two pole paper capacitance bushing to suit our needs. The next hurdle to overcome by TEC's design engineers was how to achieve the added creepage distance and yet retain approximately the same physical dimensions as the previous START transformers. The solution put forward was a series of NOMEX wound tubes and fibreglass cheeks on which the secondary coils would be wound and then stood off from the primary via a paxolin mounted framework. On order to prove that the calculations were correct, TEC decided to build a prototype of one of the 10kVA transformers required in the contract and to undertake a series of high voltage tests, TEC had the facilities for the 85kV ac flash test but had no facilities for HV dc flash testing, and so a call was made to the High Voltage laboratory at Southampton University with whom TEC has worked on several occasions. Southampton was able to provide the 150kV dc flash test requirement and so a prototype transformer was built and it passed the tests with honours; following this success, the rest of the project was undertaken and built to the same parameters.

GENERAL SPECIFICATION

The transformers were all double wound, naturally air cooled, varnish vacuum impregnated with two inter winding screens between windings, insulation class "F" with class "B" temperature rise, enclosed in a drip proof floor standing ventilated sheet steel tank with detachable plates for the incoming cables, and output connections via paper capacitance bushings.

Applied Voltage Withstand Test Levels

The high voltage test levels for all the transformers were 85kV ac 50Hz for 1 minute; and 150 kV dc for 2 minutes. These voltages were applied between the secondary winding/s + secondary earth screen and the primary winding/s + primary earth screen with the primary winding bonded to the core, case and earth.

Six types of isolation transformer were built :-

A. Modulator filament transformers

Input 415 volt 1 phase 50 Hz

Output 26 volts at 700 amps centre tapped

B. Modulator auxiliary transformer

Input 415, 450, 480 volts 3 phase 50 Hz delta

Output 208 volts star 4 wire at 19.43 amps

C. Arc  rectifier isolation transformer

Input1  415 volts 3 phase 50 Hz delta all ends out

Input2  415 volts 3 phase 50 Hz delta all ends out

Output1 98 volts 3 phase delta at 150 kVA

Output2 98 volts 3 phase star at 150 kVA

Rated at 5% ie 0.1 secs on in each 2 sec period

The windings were constructed and joined in such a way as to achieve close coupling and equal impedances

D. Arc auxiliary transformer

Input 415, 450, 480 volts 3 phase 50 Hz delta

Output 480 volts 3 phase star 4 wire

Rated at 20 kVA

E. Deceleration supply transformer

Input 415, 450, 480 volts 3 phase 50 Hz delta

Output 480 volts 3 phase star 4 wire

Rated at 25 kVA

F. Area auxiliary transformer

Input 240 volts single phase 50 Hz

Output 240 volts single phase

Rated at 10. kVA