Power Systems
Critical disruptive voltage is a voltage at which

 corona is just initiated
 both 1 and 2
 nether 1 nor 2
 corona is visible with blue or violet colour

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Power Systems
The draft tube is provided to

 reduce the effect of water hammer.
None of these
  increase the acting head on the water wheel.
 raise the water surface of the stream to create an artificial head.

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