For voltages higher than about 300 to 500 kV, the cascading of transformers is a big advantage, as the weight of a whole testing set can be subdivided into single units and therefore transport and erection becomes easier. Also, with this, the transformer cost for a given voltage may be reduced, since cascaded units need not individually possess the expensive and heavy insulation required in single stage transformers for high voltages exceeding 345 kV.It is found that the cost of insulation for such voltages for a single unit becomes proportional to square of operating voltage. The low voltage. supply is connected to the primary winding ‘l’ of transformer I, designed for an high voltage output of V as are the other two transformers. The exciting winding ‘3’ supplies the primary of the second transformer unit II; both windings are dimensioned for the same low voltage, and the potential is fixed to the high potential V. The high voltage or secondary windings ‘2’ of both units are se...