bikash chandra pal


{ City } kolkata
< Country > india
* Profession * manager -electrical engg
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Total Answers Posted # 12

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Questions / { bikash chandra pal }
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Answers / { bikash chandra pal }

Question { 23629 }

what is a relay?


Answer

It is a device which generate & send some desired output
signal on receipt of desired input signal & comparision of
the same with some preset value/Data, based on it's inbuilt
logic.

Is This Answer Correct ?    0 Yes 1 No

Question { 35531 }

what is the difference between earthed system and unearthed
system.. where do we use unearthed system because in
electrical catalogues always cable for earthed and unearthed
system is given..pls elaborate


Answer

WAHT IS EARTHED SYSTEM??

when the source(it may be feeding generator neutral point
or feeding transforme neutral or a bus through a zig-zag
transformer etc)neutral point of a perticular voltage level
of a system is earthed(may be solidly or through a
resistance/inductance), then that perticular voltage level
of the system is called as earthed system.

WAHT IS UNEARTHED SYSTEM??

When the above said neutral point is not earthed then that
perticular voltage level of the system is called as
unearthed system.

ADVANTAGES OF EARTHED SYSTEM

The system eathing(solidly or through a resistance or
impedance) is done to provide a returne path of earth fault
current, which in turne helps

1)To isolate the faulty part of the system due to earth
fault by sensing earth fault current.

2)To Reduce the insulation requirement of phase elements of
equipments connected in that voltage level & cost of the
equipment/s in turn. VIZ: Earthed grade cables are less
costlier than unearthed grade cables.

3)Fenomennon like ARCHING GROUND can be avoided.

So,
System with lesser Equipment cost(refer SL.NO.(2)above),
health of the concern part of the system(Refer sl no.(1) &
(3)above) remain protected.


DISADVANTAGES OF EARTHED SYSTEM

Possibility of interruption of power supply due to earth
fault,is more in EARTHED SYSTEM than UNEARTHED SYSTEM.

ADVANTAGES OF UNEARTHED SYSTEM

1)Possibility of continuation of power supply is more in
UNEARTHED SYSTEM, as tripping of system due to first earth
fault is not happens.

2)sensing of presence of earth fault through sensing of
concern system's neutral displacement is possible & planned
maintenance/earth fault clearence of the system is
possible,which supports better availity of the system &
lesser interruption of process.

DISADVANTAGES OF UNEARTHED SYSTEM

1)Isolation of the faulty part of the system due to earth
fault by sensing earth fault current is not possibele due
to first earth fault.

2)Insulation requirement of phase elements of equipments
connected is higher Hence cost of the equipment/s is
higher. VIZ: unearthed grade cables are costlier than
earthed grade cables.

3)Fenomennon like ARCHING GROUND is there.

So,
Capital investment is higher for unearthed System, But
availability of system is better.

Is This Answer Correct ?    41 Yes 1 No


Question { 35531 }

what is the difference between earthed system and unearthed
system.. where do we use unearthed system because in
electrical catalogues always cable for earthed and unearthed
system is given..pls elaborate


Answer

The following is in reference to Mr.S Bhattacharrya's
answer. The following I wish to point out for readers who
are learning.
1) It is correctly indicated that VOLTAGES 415VOLT SYSTEM
GENERALLY NEUTRAL IS DIRECTLY GROUNDED AND THIS IS CALLED
EARTHED SYSTEM. But, it is to note "33KV & above system" is
normally grounded solidly (not that "above 33kv system" is
grounded solidly as indicated in mr. Bhattacharrya's reply)

2) Midium voltage level 3.3kv to 11kv(not upto33 kv) are
normally grounded through Impedance or resistance to limit
earth fault current. there are so many terms to describe
such system e.g. "high resistance(or impedance)
earthed" / "low resistance (or impedance)
earthed / "effectively earthed" /"non effectively earthed"
etc. but not termed as " UNEARTHED SYSTEM" (as indicated in
Mr. Bhattachrrya's reply.)

3)For information,"high resistance(or impedance)
earthed" / "low resistance (or impedance)
earthed / "effectively earthed" /"non effectively earthed"
terms are defined based on "co-efficient of earthing". What
is co-efficient of earthing??---- go through some text book
like "sunil s Rao" it will be clear to you.

4) For information "unearthed system " is that system,
which is not earthed at all, ie.nither solidly or through
resistance.

5) Please note, Earthing through Resistance or impedance ,
limits the "Earth fault current" to desired value as per
system requirement, But it is having no effect or
connection with system's Short circuit current(STC) or
Fault level, as indicated in Mr. Bhattacharyya's reply.

6)"----CALLED UNEARTH SYSTEMS AND THIS SYSTEM IS VERY
STABLE COMPARED TO DIRECTLY GROUND SYSTEM "---- is not a
true statement again given by Mr. Bhattachryya in his
reply. Both the system is quite stable. System stability is
not depended on system grounding. To know about system
stability you may refer any TEXT book for example "NAGRATH
KOTHARI".
systems "earthed" , "resitance or impedance earthed"
or "unearthed", having their own advantage / disadvantage
already explained to some extent in my earlier reply no.1
above.

Is This Answer Correct ?    24 Yes 3 No

Question { ABC, 40360 }

how netral CT is selected (including its details
specification)


Answer

Neutral CT is generally used on earth connector connecting
the earth or ground & the star point of a Generator or
Transformer.
Neutral CT is selected based on the value of designed the
earth fault current which is supposed to return to
transformer or generator star point from earth.
Process of selection:-
primary rating : Designed E/F current x 1.2
Secondery rating : normally selected as 1A(reason being,
the distance between the relay & NCT which is located on
transformer bushing is generally longer),However, if the
distance is lesser & approximately within 25M, 5A secondery
also may be selected.
Accuracy class: To be selected based on application,eg: for
Stand by earth fault protection(51G) it may be 5P10. For
restricted earth fault protection(64R), PS calss CT is
applicable Vk to be calculated,Imag to be chosen based on
relay mnf's reccommendation. For Neumerical CT,mnf's
guideline need to be adhered.
BIL: based system requirement & CT construction(eg. for
window type CT system BIL is not applicable, as by virtue
of it's construction it is LT CT as primary is full voltage
isolated.But for BAR & wound type epoxy cast CT system BIL
is applicable.
Trust the above is clear. feel free to ask for
clarification if any

Is This Answer Correct ?    31 Yes 3 No

Question { ABC, 40360 }

how netral CT is selected (including its details
specification)


Answer

Dear Mr.Manoj Sharma,I am not an expert on the subject but
having a general idea on the same. I am trying to reply
your questions to the extent possible for me.

1)CT construction: Constructionwise Ct basically same as
normal transformer. that is, it is having a primary wdg. a
secondery wdg & the core.Primary & secondery wdg is coupled
magenetically.
Functionally, there is a difference of course. the
difference is:
In normal transformer LOAD draws current from secondery wdg.
(secondery dwg. magnetically coupled with primary
wdg.),which in turn draws current from primary through
magnetic couplling.
In CT, current flows through primary wdg.(as LOAD connected
on primary wdg draws the current), due to that primary
current, flux get linked on sec.wdg. through mag.coupling &
induce EMF on sec. This is the reason,why CT sec. should
not be kept open, oterwise a high voltage will generate in
sec.wdg.(in absence of counter flux),subsequently the
insulation will puncture.
There is a big difference between CT & normal TX. in view
of their accuiracy class(ie designwise). reason is very
simple: CT is a measuring instrument(it's accuracy
matters), which is not at all that important for power
transformer.
CT is considered as current source wheras power tx is
considered as voltage source.

2) As per standard Relay /meter(to which CT is supposed to
be connected) as available in the market is of 1A & 5A
rated. otherwise, theoritically no issue to design a CT
other than 1A or 5A

3)Differebce between CT & CBCT:
Basic Construction wise : NIL
But application wise is as follows:-
CT is connected one on each phase. CT measures Phase
current.
CBCT is connected one on combined three phase.CBCT measures
only the resultant of 3-phase current, which is normally
zero(even if each phase individually carries substential
current). That is CBCT come in action only when there is a
unbalance among 3 phases.

4) the difference between measuring & protection CT lies in
their accuracy level & saturation level. to explain,
measuring CT accuracy level cl-1 means-- the output of CT
is 99% correct within it's operating limit. it's saturation
limit is low to protect the connected meters. it is very
obvious , Measuring CT is supposed to work under normal
operating condition & need not to work work under fault
condition.
Wheras the protection CT need not to work under normal
operating condition but must act during fault condition.
saturation level of such CTs is high. for example class of
accuracy 5P20 means : 5% error till 20 times Ct primary
couuent.
5) stand by protection: is last line of defence for earth
fault protection of a transformer.wheras, REF is 1st line
of protection for the wdg of TX to which it ic
connected.It's a differential protection.
6) if I mag of connected Ct to a relay, is more that
recommended value for the relay, then the relay may
malfunction.

point 7 i will reply later. I hae to go for a meeting now
OK ? be happy , remain connected

Is This Answer Correct ?    27 Yes 1 No

Question { 6073 }

AT THE TIME OF INSTALLING THE POWER PLANT POWER IS TAKEN
FROM NEARBY GRID AND GIVING THE SUPPLY TO THE GENERATOR
WHICH ACTS AS MOTOR. WHAT IS THE NEED OF RUNNING THE
GENERATOR AS MOTOR?


Answer

I think The question who have asked, wanted to ask that "AT
THE TIME OF START-UP A POWER PLANT....." & NOT "AT THE TIME
OF INSTALLING THE POWER PLANT ...."-------------If my above
assumption is true, then the answer WILL BE as follows :-

TO BRING THE GENERATOR NEAR TO SYNCHRONUS SPEED DURING
START UP.

Is This Answer Correct ?    2 Yes 1 No

Question { Areva, 22486 }

what happens when a DC supply is given to AC motor?


Answer

Earlier reply is OK, some more elaboration given below
No rotating magnetic field is generated and hence the motor
will not rotate,
No back EMF will be generated,
reactance(XL)will not come effect as frequency is "ZERO",
Rotor impedance will be remain isolated from equivalant ckt
of stator.
Only resistance of stator ckt(which is very low) will be
there. Hence stator current will be very high,Motor will
burn.

Is This Answer Correct ?    74 Yes 0 No

Question { Bhel, 81775 }

What is vector group? What is meant by DYN11?Why is it needed?


Answer

Abobe two reply explained correctly "what is vector group".
The answer to the last question "why it is needed" is as
follows:-
By virtue of constrction & terminal connections of the
transformer, a difference between primary vector &
secondery vector occurs. We need to know the same for the
following reasons:-

1)Matching of vector group is required (to avoid
circulating current/ fault ), when 2 transformer need to be
operate parallal on a common bus/system.

2) to provide transformer differential protection correctly.

etc

Is This Answer Correct ?    37 Yes 23 No

Question { Essar, 9013 }

what is the diffrance between earthing and grounding...?


Answer

There is no difference.Term "EARTHING" & "GROUNDING" means
same.

Is This Answer Correct ?    10 Yes 23 No

Question { Essar, 9013 }

what is the diffrance between earthing and grounding...?


Answer

some of our friends not agreed to my earlier reply . let me
clarify "earthing" & "grounding" are the terminology used
one in UK & other in USA. But the finally it mean the same
thing. ie . when some point of electrical system is made
fixed to ground (may be for safty purpose like equipment
body earthing or for system protection purpose like
transformer star point earthing. Hope the thing is clear.
if you are still in doubt, you may refer diffrent TEXT
BOOKs to check the same or search GOOGLE.

Is This Answer Correct ?    25 Yes 4 No

Question { 5516 }

As we know,charged clouds(from which lightning stroke
initiates)may be charged with either +ve charge or -ve
charge.
Now say one cluster of cloude is fully charged with +ve
charge & another cluster is fully charged with -ve charge.
In the sky these two cluster of charged clouds have come in
such a distance that, if they come clouser by some few
yeards then the lightning discharge will take place between
these two cluster of clouds. Under such a situation an
Aircraft came inbetween. what happend??


Answer

I am not convinced with previous answer,Because, as we
know, number of lightning happens between charged clouds,
which are visible also during thunderstrom ( correct me if
i am wrong). let me place another relevant question --"is
there any instrument used by aircraft to sense existance of
nearby charged cloud??"

Is This Answer Correct ?    0 Yes 2 No

Question { NTPC, 44887 }

why we use only sine wave in ac why we do not use other wave
forms


Answer

By virtue of construction & operation of generator, it
possible to generate Sine wave only.
When a non varying flux (fild flux) rotates & cuts the
stator winding(a winding of conductor ) , a sinusoidal
voltage wave induced in stator in turn current.

The appliences(eg Motor etc) has been designed to utilise
the same in the most easiest way with minimum losses.

of course we can generate voltage wave(in turn current)
which is not sine wave simply by variying the field flux.
But what will be the result of that?? Hermonics. which
will contribute increased losses, lesser life span of
equipments because of more heating, if we go in details of
the same , it will be seen that effect of non-sinusoidal
waves is not +ve. it is -ve .
Hence we only use sinusoidal wave(voltage & hence current)

I have just given a try to explain ,saying just whatever
came in my min in first go. if somebody can add some more
value to it please. we all will learn. please

Is This Answer Correct ?    69 Yes 25 No