Question { HAL, 16090 }

Why Trasformers rated in KVA

Answer

All transformers are rated in KVA or MVA if they are big
size. This is because it has a particular capacity and we
cannot utilise beyond this capacity. suppose you have
100KVA transformer; then you cannot use more than 100kva
capacity. If you do either transformer will burnt out or
the circuit breaker will trip.

Question { 4339 }

why electron not flow in open loop,how they know path is
open ?

Answer

Electron is like water flow. In water flow also you have to
open the tap then only water flows. Similary in electricity
you have to complete the circuit by a switch then electrons
will flow.

Question { 7204 }

what is the operation of UPS ?
can any one answer me?

Answer

UPS is uninterruptible power supply and is widely used by
one and all. It consists of battery backup and charging
facility. diagram can be supplied if you write to me at
williamloboatrediffmaildotcom.

Question { 16891 }

How can we check ELCB trip current sensitivity
How can we check earth resistance value

Answer

Elcb is earth leakage circuit breaker. There are many
sensitivities in India. popular are 30ma, 100ma, 300ma,
500ma. For getting the correct corelation as to which
sensitivity is to be used for what application, it is
necessary to understand the effect of electric current on
the human being. I have the chart which shows the effect of
different ma current flowing through ones body and the
result. Usually below 30ma current is safe and gives only
tingling sensation to the person. But above 300ma can prove
dangerous and heart will stop.
For the second part you can use an earth megger and
connections are provided on any website or send me a mail
williamloboatrediffmaildotcom.

Question { L&T, 3425 }

What is meant by rating error prevention in breaker??

Answer

Security
Rating error prevention ensures that the circuit-breaker of
matching rating only gets fully inserted into the cradle.

This prevents any human error during assembly of the
components.

Question { 7086 }

Is there any option to replace two Genset of capacity 125KVA
with one higher capacity for same office?

Answer

Looking at your new feedback; it is possible to install a
new DG set of higher capacity. Since the DG sets are on
rental then its more easy. Just ask the DG set supplier (
the one whose performance is satisfactory) to come over and
surely they will be able to give you free advice on how
best the conversion should be done.
The the electrician or engineer who did the job of
providing necessary switchgear and cabling etc for the
additional loads could be contacted and surely he too would
know what best could be done.
But this may entail higher capacity switchgear, cables etc
to be installed in lieu of the existing set up.
Or like the suggestion to provide one 400A TPN and from
here the same original cables could be used to the existing
lower capacity switchgears.
Also one higher capacity new cable is to be installed from
the new DG set to the 400ATPN.

Question { 15557 }

1)which height building required lightning arrester?
2)how to calculate the requirement of lightning arrester?
3)what is the pro instrument?
5)how to test protective relay like overload?earth fault relay?

Answer

The height of the building is not a creiteria for
installing of lightning arrester. If this is the only
building in the area then we have to install the lightning
arrester. But generally it should be installed on the
highest building or tower in the immediate vicinity.

It consists of spikes which can be installed on the
perifery of the building at distances of about 15 to 20
feet.

testing of protective relays will be discussed later.

Question { 15523 }

How can you calculate in detail the VA burden of current
transformer used for Restricted earth fault protection?
Please understand that the CT required here is Class PX or
X. Also please show the calculations for knee point vtg,
stabilizing resistor, magnetising current.

Answer

Restricted earth fault protection is a differential scheme,
with CTs on the transformer neutral and phase leads. The
phase CTs are connected together in parallel & then
paralleled with the neutral CT. The resticted earth fault
(REF) relay is connected across the paralled CT circuits.

The relay is either a voltage operated device (eg Alstom
MFAC14)or a current relay with an external setting resistor
(eg Alstom MCAG14). In either case, the scheme is set at a
voltage based on the maximum voltage that is developed at
the relay for an external fault. A practical setting is
50% of the CT knee point voltage, if full info is not
available.

Question { 15523 }

How can you calculate in detail the VA burden of current
transformer used for Restricted earth fault protection?
Please understand that the CT required here is Class PX or
X. Also please show the calculations for knee point vtg,
stabilizing resistor, magnetising current.

Answer

Knee Point voltage is nothing but a point at which the CT
saturates. Thats why its always better to design the CTs
which can operate without saturation even at a voltage
appears in the CT during fault condition. Pls find below
the following formula for calculating the Knee point
voltage.

Vk=2If(Rct+2Rl).

where, If is the current refected to the CT secondary
during fault condition,

Rct is the resistance of the CT which can be adopted by
means of a thumb rule considering the CT ratio,

Rl is the lead resistance i.e. sum of the resistance of the
wire connected between the relay & CT and Impedance offered
by the internal circuitary of the relay

For PS class CT's designed considering the above,burden is
not applicable.

Question { 15523 }

How can you calculate in detail the VA burden of current
transformer used for Restricted earth fault protection?
Please understand that the CT required here is Class PX or
X. Also please show the calculations for knee point vtg,
stabilizing resistor, magnetising current.

Answer

For PX class CTs , the burden is not normally mentioned .
There the CT is defiened based on the CT kneepoint
voltage , magnetising current etc . The burden is mentioned
in class P CTs

In PX class CTs the kneepoint voltage is normally based on
the max fault current and a diamensioning factor based on
system X/R ratio

Question { 6654 }

Why we are not using capacitor in line when load on Genset?

Answer

By putting capacitor on Genset what we are doing is only
loading the genset additionally.
Capacitor is used to improve the power factor of the
system. This is important for the power supply cos; hence
they insist that pf should be 0.9 or thereabout.
Hence it is compulsory to use Capacitors to improve the PF
if below 0.8 or they will fine you.
When using a gen set we have to see what is the type of
load. If it is motor load inductive then we have to switch
on some of the capacitor banks only.

Question { 7146 }

What are the methods/types of distance protection schemes used
in transmission system?

Answer

Line distance protection relay Siemens Energy
The SIPROTEC 4 7SA522 is a distance protection relay for
transmission lines. This relay provides full-scheme
distance protection and incorporates all functions usually
required for the protection of a power line. The relay is
designed to provide fast and selective fault clearance on
transmission and subtransmission cables and overhead lines
with or without series capacitor compensation. The power
system star point can be solid or resistance grounded
(earthed), resonant-earthed via Peterson coil or isolated.
The 7SA522 is suitable for single-pole and three-pole
tripping applications with and without tele (pilot)
protection schemes.

The 7SA522 incorporates several protective functions
usually required for transmission line protection.
• High-speed tripping time
• Suitable for cables and overhead lines with or without
series capacitor compensation
• Self-setting power swing detection for frequencies up to
7 Hz
• Digital relay-to-relay communication for two and three
terminal topologies
• Adaptive auto-reclosure (ADT)

One of the communication interfaces is system interface for
connecting to a control system via IEC 61850 Ethernet
protocol.

Question { 5030 }

what is the difference between Resin Cast CT and Oil cooled
CT?

Answer

Resin Cast CT and PTs are usually used in indoor
switchgeear 11kv or 6.6kv substations etc. whereas Oil
cooled CTs are used in outdoor substations viz 11kv or 33kv
switchyards.
Difference is that Resin cast are solid cast type. Oil
cooled are those immeresed in Transformer oil and sealed.

In larger capacity CTs where cooling is required then oil
cooled CTs are preferred.

Question { 38113 }

What is clean Earth System ?

Answer

Actually this question should be posted in instrumentation
questions. Nevertheless the answer goes like this. Normal
earthing is connected with the electrical apparatus in the
PIB ( Plant interface building) or any such building
wherein instuments or electronic equipment are installed.
The earthing of these electronic equipment are connected to
a different earthing system which consists of a flat plate
with insulators. From here the earthing goes direct to a
seperate earthing pit and thence the other end of the
earthing pit may then be connected to the ground bus. Never
are any electronic earth connected to the electrical earth
or these electronic sensitive equipment will get damaged.

Question { 4045 }

what are the values to be accepted on 3 core 185sq mm for
11000volts on insulation resistance between conductors and
between conductors and earthed?

Answer

A.C. withstand voltage
Wet(ph/ground)
Time duration
KV
Mins
28 KV
1 Min.