Electrical Engineering Q/A 20 To 30

 21: Why skin effect exist?
Skin effect exists because of unequal flux distribution. The flux which is at the center is linking the inner part only and the flux at the outer part is linking both outer & inner part. That is, flux at the surface is more as compared to flux at the center and we know that flux is inversely proportional to inductive reactance.
The more the flux, the lesser be the inductive reactance & vice versa. Therefore inductive reactance will be less at outer surface because of more flux and it will be more at inner surface because of less flux.
This is why, since inductive reactance is less at outer surface so current passes more from outer surface and less from inner surface and this is why skin effect exists.

22: Why EHV busbars are hallowed and circular?
To avoid corona effect 

23: What are the different electrical clearances in Substation?
Ground Clearance: This is the minimum distance between any live conductor and the earth or ground.
Phase Clearance: It is the clearance between the different phases in a circuit or same phases in different circuits
Sectional Clearance: This is the minimum clearance between the live conductor and the limits of work, where spacial clearance is related to the safety of the operating personnel. The limits of work section may be ground or the platform on which the person. is working.

24: Which type of bus arrangement is costlier and more reliable?
One and half breaker scheme is most reliable and costlier scheme. In one and half breaker scheme 3 circuit breakers are provided for 2 bus-bars. Hence the name is coined as One and half breaker scheme.

25: What is the speed variation of the induction motor from no load to full load?
The decrement in the speed of the small rating Induction Motor from no load to full load will be around 4 to 5 percent and 2 to 2.5 percent in the case of large size induction motors. 

26: What type of cooling is provided for Generator in power plant?
Hydrogen gas cooling is employed for large size generators because of better heat carrying ability of the hydrogen. Hydrogen cooling is provided for rotors and core of the generator. Water cooling is provided for the stator of the alternator. 

27: What is Cavitation and disadvantages of Cavitation?
The formation of air bubbles and water vapour on the water surface due to the reduction in the pressure is called "Cavitation". When the pressure on the water reduces below the saturation pressure corresponding to the temperature of the water, the rapid formation of the water vapour and air bubbles starts. The bubbles suddenly collapse with the violent action and this collapsing pressure will be very high. The rapid formation and collapsing of bubbles causes the pitting of the metal surface. Cavitation also reduces the efficiency of the hydraulic prime mover causing the honey-combing of runner and blade contours which reduces the power output. 

28: What are the major electric systems in Thermal Power Plant?
Major electrical equipment in thermal power plant are
Turbine Generator
Exciter System
Generator Protection System
Generator Transformer
HT/LT switch gear
Electrical Switch-yard 

29: What is difference between operating voltage and input voltage?
Input voltage is something which you are feeding from power source to that equipment to make it work.
Whereas operating voltage is related to the product capabilities. It means the product can give you desired output even if the input voltage varies b/w +/- 5%.
e.g. Let say if a motor is having a rated voltage of 230V, then its operating voltage is from 219 V to 242 V. i.e. motor can operate satisfactorily even if the input voltage changes from 219 V to 242 V.

30: What is the largest pump in the thermal power plant?
In thermal power plant Boiler Feed Pump (BFP) is the largest pump, it delivers the water to the boiler