Question { L&T, 48456 }

WHY THE FREQUENCY IS 50 HZ IN INDIA.CAN WE USE SOME OTHER
FREQUENCY.IF SO WHAT IS THE RANGE AND WHAT WILL BE THE
EFFECT IN PRESENT POWER SYSTEM IF WE CHANGE THE FREQUENCY
AS YOU SUGGEST ?

Answer

we can change the freqency e.g in USA it is 60 HZ ..
but at the power plant all the machines design would be
changesd as speed of generator is given as
Ns = 120 * f / P
where p is no of poles of machines and we would have to
change the design of all electrical application to new
frequency ..

Question { ABB, 21469 }

what is a lightning arrester???

Answer

Lightning arrester is a device that is used for the
protection of electrcal devices and electrical system e.g at
the power station it is installed at power transformer ,
main purpose is to protect system against lightning impulse
.. it is non linear resistance normally open in case of
lightning impulse it grounds all the voltage and protects
systen against extra high voltage

Question { Aspire, 11015 }

What is meant by power factor correction?

Answer

Active power is the product of cureent , voltager and power
factor as power factor decrease requireq cureent increse so
losses increses and KVA increases , we have to increase it
close to unity by sereis reactors , parrallel capacitors to
make our system n application run at increased efficiency .

Question { HCL, 56944 }

Why we choose 400kv,220kv,132kv,66kv,33kv transmission
voltages for transmitting power. Why not other voltage
levels.

Answer

normally generation voltage is 11kv . if we have to transmit
this power to large distance we must increase voltage and
decrease current to reduce I^2R losses so we increase the
voltage to 33kv if distance is not large and if distance is
very large 400kv , 500kv .. These are only standards by IEC ..
any other cab be used. but we use this bcoz standard
transformer , cables , insulators , arrestors for these
nominal voltages are available

Question { 19321 }

what happens when d cexcitation given to alternator is
removed while it is running or what happens when dc
excitation is failed in alternator while running?

Answer

when excitation is lost due to fault or field breaker
tripping . Reactive power will be supplied by the system and
generator will work as induction generator and its speed
will rise a little . generator which was in over excitation
mode will work in underexcitation .
but there is under excitation limit which should not be
reached so we should detect loss of excitation and trip
generator

Question { 5665 }

whye the current increases in the motor when the load on the
motor increases

Answer

its like law of conservation of energy , as load or output
of motor increases so do input increases .. so current in
motor increases and flux increases to meet the demand