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how netral CT is selected (including its details
specification)
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Answer Posted / 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
| Is This Answer Correct ? | 27 Yes | 1 No |
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