Question { Bhel, 149809 }

how to calculate knee point voltage of current transformer?

Answer

For calculating knee point voltage there are two methods,
they are
1.Low method
2.High method


1.low impedance method
Vk=24In(Rct+2Rl)

2.High impedance method
Vk=4IsRs

Then, Rs=If/Is(Rct+Rl)
where
Is,In-Rated Sec. Current of CT
Rct-CT resistance
Rl-Max. one way lead reistance from CT to Relay

Question { ABB, 5046 }

1. what is the power factor and whatis his losses
2. what is the working procedure of 51RNB AND 87 relay

Answer

1.Power Factor is defined as the cosine angle between
voltage and current.
From Power triangle cosine angle is defined as the adjacent
by opposite side.
ie., cos angle=kW/kVA


2. 51-AC Time Overcurrent Relay is a relay with either a
definite or inverse time characteristic that functions
when the current in an ac circuit exceed a predetermined
value.

87-Differential Protective Relay is a protective relay that
functions on a percentage or phase angle or other
quantitative difference of two currents or of some other
electrical quantities.

Question { 9753 }

how to select cable size?

Answer

1.Voltage level.
2.Type of cable going to used.
3.Whether the cable is going to laid in air,tray or in
burried.
4.Derating factor, Grouping Factor and ambient temperature.
5.Voltage drop as per the standards.

Question { CTS, 10987 }

what is impedence (Z) of transformer, what indicate in
transformer , which perameter indicate in transformer

Answer

transformer is always interposed between a primary source
of energy and a secondary that makes use of that energy.
Hence, the transformer constitutes an impedance to current
flow from source to load. The higher that impedance, the
more voltage drop will occur for a given load current. If a
secondary fault occurs, its effect on the circuit will be
lessened to the extent that transformer impedance
increases. The circuit designer therefore needs to know the
value of transformer impedance.

Question { 22394 }

how to diffrentiate between ac & dc motors from a distance

Answer

DC MOTOR: The pole shoes are mounted by bolts, the bolts
will be projected in the yoke. In a distance we can easily
identify whether the motor is AC or DC.

AC MOTOR: But in AC Motor we can't see the bolts mounted
in yoke

Question { Hyundai, 28436 }

what is the difference between KVAR & KVA ?

Answer

KVA: For transformer the rating will be mentioned in KVA
b'coz real power are expressed in terms of voltage and
current not dependent on powerfactor.

KVAR: It is reactive apparant power ie., kVAr=kW*TAN PI

Question { Navrachana University, 92466 }

What is the working of AVR in AC generators?

Answer

AVR means Automatic Voltage Regulator.
There are different types of excitation in generators, AVR
is used to regulate the voltage applied to the
generator.Because generator draw enoromous power from the
source in order to regulate this AVR is used.

Question { 50724 }

MCCB 400 Amps 50KA 430Volt AC
What is mean by 50KA

Answer

As per the standards for different voltage level there is
different short circuit fault value.
For ex: 11kV is 31.5kA and for 430V is 50kA.
ie., the breaker can withstand the short circuit of 50kA.

Question { 6021 }

In a Dyn11 transformer, how are the 3 windings of the delta
connected? Assume that R1-R2, Y1-Y2, B1-B2 are the 3
windings on the HV side.

Answer

the delta connection will be at 11'o clock position vector
group