Answer / bikash chandra pal

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

Answer / bikash chandra pal

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

Answer / sunil guru

EXCELLENT 2 GUD ANSWER BY MR.Bikash Chandra Pal(PRESCRIBED AND DISCRIPTIVE)
THNKS......

Answer / abobakkar

NCT selected by based on rating of
transformer


Netural CT - Netural current Transformer it can used in
transformer side only
it purpose when a current comes reverse through netural
side means it should sense by current transformer only.
then CT secondary can be connected to relay (ie.,)Earth
Fault Relay. when a current is high means it will sense
then coil will enegrised and gave trip command to breaker.

it is one of the protection of transformer from fault.

Answer / n. basak

Mr. Manoj the following answer to point no. 7 may give you
some basic idea on type of CTs. Here I should mention that
I am not at all a CT expert:

The type of CTs mentioned are basic types of transformers

1. Wound type CTs a current transformer in which the
primary winding usually consist of more that one turn.Wound
primary transformers have an internal primary winding (in
CT mould)and have no windows for the primary conductor to
pass through. These are normally designed with low current
ratio, therefore are having higher accuracy & burden.

2. RING TYPE CTs : A CT which has an opening in the center
to accommodate a primary conductor which may be busbar or
cable through it. A current transformer where primary
winding is provided by user. They are cheaper.

3. Bar type transformer: The CT in which the primary
conductor is made up of a bar or a set of bars in parallel.
The bar is of suitable size (Copper or aluminium) and is an
integral part of CT.We can also define bar type CT’s are
window CT’s with a solid bar installed.

Answer / manoj sharma

First of all thank u very much MR.B.C.Pal for giving such
precious knowledge. I think u have some what good command
on this matter. But i am cuious to ask some further details.
Please clarify following quarries
1. Why CT construction is not like normal transformers.
2. Why CT ratio is always X/1 or X/5.
3. what is differece betweel CT and CBCT.
4. What is technical difference between protection and
measurng CT(If any).
5. what is difference between standby and restricted earth
fault protection.
6.What is significance of Imag.
7. Explain the details of diffrent types of CT like window
type, BAR type and wound type and these applications in
brief.
I am sure that u will not ignore me...waiting for positive
response

Answer / syed abuthahir

1) in CT why we select 5p20 for class of accuracy and how
the va burden comes 10 and alf is 20

2) alf is selected from class of accuracy but how to calculate

3) when i am calculating knee point voltage i am not getting
700-350-175 (my primary ct ratio is 800-400-200/1A and RCT
is 5-2.5-1.25)

4) how to calculate magnetizing current for above
specification CT

Answer / imran

pls send me the exact answers thanx

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