1) What is the difference between a High Voltage Power Transformer and High Voltage Testing Transformer?
Power transformers have a high kVA/MVA rating and they are intended to carry the rated load current continuously. The Testing transformer on the other hand has a low Power rating and is intended for short term use, typically the duration of the High Voltage test. In addition, these transformers invariably cater to reactive loads which are primarily capacitive in nature.
2) Why is it not possible to have a single transformers to generate High Voltages?
Normally, a single unit of High Voltage Testing Transformer has at most a maximum voltage rating of 350 kV. Beyond this rating, the transformer design becomes bulky and unwieldy.
3) How many units can be connected in Cascade?
Not more than four units are connected in cascade, usually. As the number of stages increases, the cascading error also increases.
4) How is the Cascading enabled to achieve high voltages?
The circuit diagram below illustrates cascading:
The first unit is normally grounded. The subsequent units are mounted on insulating supports rated for (n-1)V volts where n is the stage number and V is the voltage rating of the first stage.
5) What is the advantage of Cascading Arrangement?
All transformers have identical ratings. Transformers for all the stages have to be rated only for V kV.
6) How do the individual stages get the input voltage in a Cascade arrangement?
The low voltage winding of each unit is supplied from the excitation winding of the previous unit. The excitation winding is a portion of the High Voltage Winding of the previous unit having appropriate voltage rating. Alternatively subsequent stages are fed from the same source through adequate number of Isolation Transformers i.e. stage “n” will have “n-1” isolation transformers. Each isolation transformer rated for the secondary voltage of one stage
7) What is the main power supply to the first stage of the Cascade Transformer Unit?
Supply to the units can be obtained from a motor-generator set or through an induction regulator for variation of the output voltage. The rating of the primary or the low voltage windings is usually 230 or 400 V for small units up to 100 kVA. For larger outputs the rating of the low voltage winding may be 3.3 kV, 6.6kV or 11 kV.
8) What is Cascading error?
The measured voltage at the output is higher than the actual voltage because of the stray capacitances, which become considerable as the number of stages increase because of the increase in height. This is called cascading error.
9) What is a meter winding in a Cascading arrangement?
The meter winding is a portion of the secondary winding of the first stage which is connected to a voltmeter for measuring the output voltage. From this measured voltage the final output voltage is obtained by multiplying the tap ratio (typically set as 1 V across this corresponding to 1 kV) and the number of stages used.
