Power Systems
The value of off diagonal elements is

 which is connected between bus i and bus j with negative sign
 sum of admittances connected at bus j
 which is connected between bus i and bus j with positive sign
 sum of admittances connected at bus i

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

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