Resolution:At present there are three hvdc transmission lines in india
1)chandrapur to padghe(mumbai)–(1500 MW at ±500 kV DC)
2)rehand to delhi (1500 MW at ±500 kV DC)
3)talchal to kolar (2500 MW)
Query:What is meant by regenerative braking?
the supply is cutt off for a running motor, it still continue running
due to inertia. In order to stop it quickly we place a load(resitor)
across the armature winding and the motor should have maintained
continuous field supply. so that back e.m.f voltage is made to apply
across the resistor and due to load the motor stops quickly.This type of
breaking is called as “Regenerative Breaking”.
Query:Why is the starting current high in a DC motor?
DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage,Eb =
Back emf in Motor,Ia = Armature current,Ra = Aramture resistance).At
starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra ,where Ra is very
less like 0.01ohm.i.e, Ia will become enormously increased.
The main advantage of using the star delta starter is reduction of
current during the starting of the motor.Starting current is reduced to
3-4 times Of current of Direct online starting.(2). Hence the starting
current is reduced , the voltage drops during the starting of motor in
systems are reduced.
lighting loads, neutral conductor is must and hence the secondary must
be star winding. and this lighting load is always unbalanced in all
three phases. To minimize the current unbalance in the primary we use
delta winding in the primary. So delta / star transformer is used for
depends upon R=rho l/a where area(a) is inversely proportional to
resistance (R), so if (a) increases, R decreases & if R is less the
leakage current will take low resistance path so the earth pin should be
thicker. It is longer because the The First to make the connection and
Last to disconnnect should be earth Pin. This assures Safety for the
person who uses the electrical instrument.
is used to detect earth leakage fault. Once the phase and neutral are
connected in an ELCB, the current will flow through phase and that much
current will have to return neutral so resultant current is zero. Once
there is a ground fault in the load side, current from phase will
directly pass through earth and it will not return through neutral
through ELCB. That means once side current is going and not returning
and hence because of this difference in current ELCB wil trip and it
will safe guard the other circuits from faulty loads. If the neutral is
not grounded, fault current will definitely high and that full fault
current will come back through ELCB, and there will be no difference in
the generation of elect power we need a prime mover which supplies
mechanical power input to the alternator, can be steam turbines,or hydro
turbines .When poles of the rotor moves under the armature conductors
which are placed on the stator ,field flux cut the armature conductor
,therefore voltage is generated and is of sinusoidal in nature…due to
polarity change of rotor poles(i,e) N-S-N-S.
Q:Why an ac solenoid valve attract the plunger even though we interchanges the terminal? Will the poles changes?
A:Yes because the poles changes for every half-cycle of ac
voltage so the polarity of AC voltage is continuously changing for every
half cycle. so, interchanging of terminals in ac system does not show
any difference. That’s why the ac solenoid attract the plunger even
though it’s terminals are interchanged.
current currying of cables will change depending upon the site
temperature (location of site), type of run (it will run through duct,
trench, buried etc.), number of tray, depth of trench, distance between
cables. Considering this condition actual current currying capacity of
cable reduce than current currying capacity (which given to cable
Catalogue) this is called derating.
Q:Why temperature rise is conducted in bus bars and isolators?
bars and isolators are rated for continuous power flow, that means they
carry heavy currents which rises their temperature. so it is necessary
to test this devices for temperature rise.
Q:When voltage increases then current also increases then what is the
need of over voltage relay and over current relay? Can we measure over
voltage and over current by measuring current only?
A:No.We can’t sense the over voltage by just measuring the
current only because the current increases not only for over voltages
but also for under voltage(As most of the loads are non-linear in
nature).So,the over voltage protection & over current protection are
completely different. Over voltage relay meant for sensing over
voltages & protect the system from insulation break down and firing.
Over current relay meant for sensing any internal short circuit, over
load condition ,earth fault thereby reducing the system failure &
risk of fire.So, for a better protection of the system.It should have
both over voltage & over current relay.
Q:If one lamp connects between two phases it will glow or not?
A:If the voltage between the two phase is equal to the lamp
voltage then the lamp will glow. When the voltage difference is big it
will damage the lamp and when the difference is smaller the lamp will
glow depending on the type of lamp.