Cascade Transformers
Q. What is Cascaded Transformer? Explain why cascading is necessary? With neat diagram, explain a three stage Cascaded transformer system.
Necessity of Cascade Transformers :
- For higher voltage greater than 700 kV` single unit constructions becomes difficult and costly due to insulation problems.
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| Figure A |
Also, transportation and erection of large transformers become difficult. The HV supply is connected to the primary winding “1” of transformer 3 designed for a HV output of V. The other two transformers too are connected in the same fashion. The excitation winding “3” of Transformer I supplies the primary voltage for the second transformer unit II; both windings are dimensioned for the same low voltage, and the potential gain is fixed to the same value V. The HV or Secondary windings “2” of both units are connected in series, so that a voltage of 2 is produced at the output of 2 unit. The unit III is added in the same way.
The tanks or vessels containing the active parts (core and windings) are indicated by dashed lines. For a metal tank construction and the HV windings shown in this basic scheme, the core and the tank of each unit would acquire the HIV level of the previous unit as indicated. Only the tank of transformer I is earthed. The tanks of transformers II and III are at high potentials, namely V and 2 V above earth , and must therefore be suitably insulated, hence raised above the ground on solid insulators . Through HV bushings the leads from the excitation windings “3”, as well as the tapings of the HV windings “2”, are brought to the next transformer. For voltages higher than about 600 kV, the cascade of such transformers is a big advantage.
The weight and the size of the testing set is sub-divided into single units of smaller size and lower height.
The transportation and erection of the test set in cascade becomes simpler.
However, there is a disadvantage that the primary windings of the lower stages are more heavily loaded with higher current in such sets.
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| Figure B |
- There are several methods of designing the cascade test sets.
- In schematic diagram of another power frequency test set cascaded 3 x 750 = 2250 kV rating is shown.
- This circuit has a third winding, known as “Balancing Winding”. These windings are designed to acquire the intermediate potentials between two stages.
- In this circuit , the transformers of the upper stages have their excitation windings arranged over the HV windings of the transformers of the lower potential.
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| Figure C |
Cascade transformer Application :
- Cascaded transformer 2 x 800 kV (Total rating is 1600 kV, 9600 kVA ) to carry out, dielectric tests under dry and wet conditions and pollution test.
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| Figure D |
- Energizing the experimental transmission line.
- Oscillatory surge ( switching ) upto 2000 kV(p).
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| Figure E |