GENERATOR STATOR WATER COOLING SYSTEM

The losses occuring in the generator stator winding components viz stator winding, main brushes & phase connetors are dissipated directly through water. The cooling water being in direct contact with voltage winding, must have an electrical conductivity of order of 0.5 to 1 µΩ .

The stator winding is cooled by closed circuit circulation of low conductivity water ( Demineralised water) through the hollow rectangular conductors which from the cooling system for higher rating (200 MW & above) Generator.
The cooling water from the expansion tank is circulated by two 100 % duty AC motor driven , centrifugal pumps , one working and the other standby. The pump are electrically interlocked between each other in such a manner that the standby pump automatically starts up in case of water drops in the circuit. In case of the pressure is still less, an alarm signal appears and machine should be tripped.

Ferranti Effect

In a medium or ling transmission line when open-circuited or lightly loaded the receiving end voltage is found more than the sending-end voltage. This phenomenon of rise in voltage at the receiving-end of the open-circuited or lightly or loaded line is called the ferranti effect owing to its first being observed on the depot ford mains laid down by S.Z. de Ferranti in 1890. The effect is due to voltage drop across the line inductance, due to charging current, being in phase with the applied voltage at the sending end of the line. Thus both capacitance and inductance are necessary to causes phenomenon. The capacitance, and therefore, charging current is negligible in short line but significant in medium and long transmission line . On a 3000 km line operating on 50 hz supply, the receiving-end voltage on open circuit is usually  found to be roughly 5% higher than the sending end voltage.

Advantage on HVDC Transmission

i. Cheaper Costa: in DC transmission only 2 conductor is for positive'+' and negative '-' terminal less cross arms then 3 phase 3 wire  AC system.

ii. No skin effect: No frequency in DC system i.e. no skin effect in dc, so there unifirm distribution of current over the section of conductor.

iii. Lower Transmission losses: HVDC transmission system needs only two conductors and , therefore the power loss in a dc line are lesser than the losses in ac line of the same power transfer capacity.

iv. Voltage regulation: There is no inductance , hence the voltage drop due to inductive rectance does not exist in DC transmission line. Thus voltage regulation is better in case of dc transmission.

v. Greater Reliability:  A two-conductor bipolar is more reliable then a 3 wire 3 phase ac line  because the dc line may be operated in a mono-polar mode with ground return when the other line develops a fault.

vi. No stability limit: There is no stability limit in dc system and transmission line may not length.

vi. lesser dielectric loss: The cables has lesser di-electric losses.