Answer Happy

Consider a power component of 1 M2 insulation resistance designed to withstand up to 245 kV r.m.s value over the long period of its operation say 50 years shall be tested by connecting it to 460 kV rms a.c. voltage for one-minute period. In the laboratory there are ten transformer units. The available variable voltage source can give the highest rms value of 25 kV across the primary winding of each transformer with the transformation ratio of 3. When all transformers are loaded, the voltage drop for each transformer unit can be about 9 kV. Assume that when transformers are connected in cascade, the resulting voltage drop in the output will be the sum of voltage drops of all transformers connected in cascade: (a) What is the minimum number of transformers which shall be connected in cascade to give the desired testing voltage? Your answer shall be supported with calculations. (b) What will be the value of current in the primary winding of the transformer of the lowest stage?