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Answer Posted / ramesh kumar k
CT & PTs are called as INSTRUMENT TRANSFORMERS as these are
used to measure Currents (CT) and Voltages (PT it is also
called as VT) in circuit mainly in HT lines.
There are nothing but Ammeter/Voltmeters. But the
difference is... Ammeter/Voltmeter are used to measure the
quantities (Current/Voltage) in a 220V line or house hold
applications as these instruments are insulated upto that
level only.
But how to measure the I or V in High Voltage Lines...
there CTs/VTs are used... These have mainly 2 advantages...
There are Primary & Secondary coils in CTs/VTs as in case
of a normal transformer...
1. These Ammeter/Voltmeters can't be used there....
because they are insulated for 230V only (or 3kV at
most)... in we connect these in HT lines... they will burn
out and also dangerous to the persons who are handling
these instruments... So these will give the I or V values
at a reduced voltage levels...
2. As the I or V in the HT lines are heavy... (i.e upto
some thousands of Amps/Volts) we can't manufacture the
general Ammeters/Voltmeters upto that level.. as it makes
costlier.. so we reduce the line values to a measurable
limits (i.e. 5A or 1A or 110V or 100V) by using Induction
principle.
we connect the primary to HT lines and we will get the I or
V at the secondary with a reduced current/voltage level...
so we can connect our normal Ammeters/Voltmeters directly
to measure the I or V in HT Lines...
Ex.:
A CT of 200/1A shows that.. if ammeter connected in
Secondary of a CT indicates 1A that means there 200A is
flowing in the Primary of CT (i.e. in HT line)
in a 132kV / 110V VT, if voltmeter in the secondary of a VT
indicates 110V that means there is 132kV in the primary.
And then these can be used for different purposes like
metering, Protection, Special Protection etc..
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