Power Systems
The inductance of single phase two wire line is given by

 0.4×10-7× ln(d/r') H/m
 4×10-7× ln(d/r') H/m
 0.4×10-7× ln(r'/d) H/m
 4×10-7× ln(r'/d) H/m

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Power Systems
Gross head of an hydroelectric power station is

 the height of water level in the river where the storage is provided.
 the height of water level in the river where the tail race is provided.
 all of above.
  the difference of water level between the level in storage and tail race.

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