On rated load, the core losses and copper losses in a transformer are known to b

Question

On rated load, the core losses and copper losses in a transformer are known to be in the ratio 1:3. After a sustained period on no load, the winding temperature is measured to be 20 degree C above ambient. The thermal time constant of the transformer is 1.5 hr. Suppose the transformer is run at full load from a cold start. What will be the winding temperature above ambient after 1 hour? A round rotor synchronous generator which has a synchronous reactance X_s= 1.4 pu is synchronised directly to an infinite bus at a voltage of 1.0 pu. At synchronisation, the generator field current is 800 A. The generator output power is to be set to 0.6 + j0.2 pu by adjusting the prime mover power and the field current. What will be the field current (in Amperes)? The generator has a synchronous reactance of 1.4 pu. All losses and saturation may be ignored.

Explanation / Answer

Solution:

Question(3)

The power is given by,

P = I2 x Z equation(1) .... Where Z is a reactance, Z = 1.4 pu

the power P is = 0.6 + j 0.2 pu (here, pu = pico unit)

so, from equation(1)

To find current we consider real part of the power, because complex part does not comprise current

0.6 =  I2 x 1.4

0.42 =  I2

I = 0.64 A

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