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 series connected, so that a voltage of 2V is produced hereby. Similarly, the stage-III transformer is connected in series with the second stage transformer. The tanks or vessels containing the active parts (core and windings) are indicated by dashed lines only. Then the tank of transformer I can be earthed; the tanks of transformers II and III are at high potentials, namely V and 2V above earth, and must be suitably insulated. Through h.t. bushings the leads from the exciting coils ‘3’ as well as the tappings of the high voltage windings are brought up to the next transformer. If the high voltage windings of each transformer are of mid-point potential type, the tanks are at potentials of 0.5V, 1.5V and 2.5V respectively. This connection results in a cheaper construction and the high voltage insulation now needs to be designed for V/2 from its tank potential. The disadvantage of transformer cascading is the heavy loading of primary windings for the lower stages. In Figure this is indicated by the letter P, the product of current and voltage for each of the coils. For this three-stage cascade the output kVA rating would be 3P, and therefore each of the h.t. windings ‘2’ would carry a current of I D P/V. Also, only the primary winding of transformer III is loaded with P, but this power is drawn from the exciting winding of transformer II. Therefore, the primary of this second stage is loaded with 2P. Finally, the full power 3P must be provided by the primary of transformer I. Thus an adequate dimensioning of the primary and exciting coils is necessary. Another important disadvantage is the fact that the short circuit voltage of the cascade is greater as for a single-unit transformer. As for testing of insulation, the load is primarily a capacitive one, a compensation of this capacitive load by low voltage reactors, which are in parallel to the primary windings, is possible. As these reactors must be switched in accordance to the variable load, however, one usually tries to avoid this additional expense. It might also be necessary to add tuned filters to improve the wave shape of the output voltage, i-e to reduce higher harmonics.
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| Figure: Basic circuit of cascaded transformers |