Cascade two or more transformers is desired to use for voltages higher than 400 kV. This transformer is subdivided into single units of the weight of the whole unit. Also with this, the transformer cost may be reduced and therefore transport and erection become easier. So that it is found that the cost of insulation for such voltages for a single unit becomes proportional to the square of operating voltage.
Figure (2.8) shows a basic scheme for cascading three transformers. A low voltage supply is connected to the primary of the first stage transformer. Therefore a voltage is available across the secondary of this transformer. Now the excitation winding of the first stage feeds the primary winding of the second stage. The number of turns in both windings of the first stage are some. This is same in the case of the second and third stages. Now as shown in figure (2.8) the potential of tertiary is fixed to the potential V of the secondary winding. The voltage of 2V is available between ground and terminal of secondary of the second stage. Same with the case of III stage transformer the voltage is 3V. It is to be noted that except the uppermost transformer, others has three winding transformers.
Metal tank construction of the transformer is shown in figure (2.8).The tank of the stage~l transformer is earthed. The tank of stage-II and stage-III transformers have potentials of V and 2V.
However, if the high voltage windings are of mid-point potential type, the tanks are held at 0.5V, 1.5V and 2.5V respectively.
The main disadvantage of cascading the transfers is that the lower stages of the primaries of the transformers are loaded more as compared with the upper stages.
The total short circuit impedance of a cascaded transformer for individual stages can be obtained. The equivalent circuit of an individual stage is shown in figure (2.9).