Depending up on the total load to select the dg.For example 
180KW load is there 200kva capacity genarator is sufficient.

Depending upon the total load and maximum demand of that
plant...

Copper Cable: Sqmm*1.5, Exam: If we have 50 Sqmm copper 
cabe, The it will take load is 50*1.5=75Amp



Al Cable is multiplying factor of 1.
Exam: 50 Sqmm cable will take 50A load.

In a three phase system p=1.732x3xVxIxCOS@
When load is given 50KW
Then I=50000/1.732x3x415x.8
=86A near about
taking some future expectancy you can take a cable of 
having 100A capacity.SO by this u can take a cable of 70mm 
square in aluminium and 50 mm square in copper

yes by using flux control method

Yes, by injected emf to rotor coils.

Yes. Either by reducing pole or increasing supply frequency.

Yes,by increase the frequency of motor by using variable 
frequency drive(VFD).

no

rated speed means no load speed .at no load, speed of rotor
is approximately equal to the speed of stator magnenetic
field,so slip become zero and hence torque devlop  to rotate
rotor is also equal to zero.

wheninduction motor run over synchoronous speed then it
behave as generator.
this can be done by using flux control or by external means

In Single Phase the Voltage or current will be Zero for 
every Half Cycle

In Two Phase the Votage or current in one phase is zero and 
at the same time other phase also zero and one phase get 
peak and other also get peak becuase the phase diffrence is 
180Dec. so here also no votage of current if one is zero.

In Three Phase the Phase diffrence between phases is 120Dec 
so that if one phase is zero rest of two will get some 
magnetude.So here we get Voltage or current if any one 
phase is zero.

So there is no need to generate 4phase and 5phase systems

BECAUSE THE 3 PHASE SYSTEM FOUND TO BE THE MOST ECONOMICAL 
FROM OTHER TRANSFOMATION , IN SINGLE PHASE THERE WILL 
ALWAYS BE MAGNETIC FORCES TRANSFORMER WALL WILL ALWAYS BE 
HEATED, IN 2 PHASE SYSTEM IF A WIRE BREAK HAPPENS HALF OF 
THE LOAD WILL BE DISCONECTED NOT LIKE THE 3 PHASE, AND IN 5 
OR 4 PHASE THERE WILL BE ALOT OF WIRES CONNECTION AND 
ISOLATORS USED WICH IS NOT ECONOMICAL

In india voltage is transmitted in multiples of 11 because,
we all know insulators(procelain) has with stand capacity 
of 11kv, depending upon the voltage its number 
increases .For example for 33kv transmission no of 
insulators used are 3.So to maintain the exact no of 
insulators so that there is no leakage the voltages r 
transmitted in multiples of 11.

size of the insulator plays an imp role in its breakdown 
voltage. for porcelain the  breakdown voltage is 40 to 200 
kv per inch . so i think insulators are so designed since 
transmission voltages are multiples of 11