In all
electrical measurements and protection purposes Current transformers and
Voltage transformers are used. You can’t measure current upto 100A by directly
connecting to Ammeter as Ammeter required for that purposes will be very much
large in size which could not be installed. So CT’s are used for that purposes
as CT will feed the current to Ammeter or any other measurement devices within limits
of that measurement equipment.
Same is
the case of relays , relays can’t handle very high value of current and
voltages. CT and PT are used for these purposed as these make current and
voltages within limits of relay.
CT and
VT function like ‘ears' and the ‘eyes' of the protection system. Relay
processes these signals and accordingly give commands to circuit breakers and
other protection systems.
Classification of CTs
The CTs
can be classified into following types:
1.
Measurement CTs
2.
Protection CTs
A measurement grade CT has much lower VA capacity
than a protection grade CT.
These
types of CT’s are explained as below:-
1.
Measurement CT’S
A
criteria for measurement CT is that they should be accurate over its complete
range i.e. from 5% to 125% of normal current. Which means that its magnetizing
impedance at low current levels should be very high, which is done in order to
accurate measurement of small currents. Note that due to non-linear nature of
B-H curve, magnetizing impedance is not constant but varies over the CT's
operating range. It is not expected to give linear response during large fault
currents.
2.
Protection CT’s
In
protection type CT’s there is ultimate requirement that these CT’s should have
linear response up to 20 times the rated current, it should be so as that relay
could operate accurately. As in case of measurement CT’s magnetizing impedance
is high at low current levels but for protection grade CT's magnetizing
impedance should be maintained to a large value in the range of the currents of
the order of fault currents.
Sometimes
CTs can be used for both purposes i.e. for measurement and protection in such
cases it has to be of required accuracy class to satisfy both accuracy conditions
of measurement CTs and protection CTs. In other words, it has to be accurate
for both very small and very large values of current.
Most
commonly CT secondary is always rated in range from 1A to 5A.
We are
assuming that CT should have linear response but practically it’s not possible
as linear response is dependent on the net burden on the CT secondary. Net
impedance on the secondary side is assumed as the CT burden.
If
burden increase on CT then this will leads to increase in voltage, if voltage
exceeds the set limits, then the CT core will saturate and hence linear
response will be lost.
Thus we
see that CT will give linear response up to 20 times the rated current, there
is also an implicit constraint that the CT burden will be kept to a low value.
In general, name-plate rating specifies a voltage limit on the secondary (e.g.,
100 V) up to which linear response is expected. If the CT burden causes this voltage
to be exceeded, CT saturation results.
Classification of CTs
CT’s are classified into two types namely:-
Class T
CT
Class C
CT
Class T CTs
One or
more primary turns are wound on a core in Class T CT as Class T CT is a wound
type CT. These CT’s are associated with
high leakage flux in the core. Due to these higher leakage fluxes, the only way
to determine CT performance is by test. Standardized performance curves cannot
be used with these types of CTs.
Figure above
shows one such tested and calibrated curve for a CT. The letter ‘B' indicates
the burden in ohms to which the CT is subjected. From curve we will see that
when when burden is less than say 0.1 ohms, CT meets the linear performance
criterion.
Now you
can see from the curve that as the burden increases to 0.5 ohms, the
corresponding linearity criteria is not met till the end. Now when burden is
increased to 4 ohm there is high deviation from linearity.
Thus it is very clear that keep burden as low as possible so as to
attain linearity.
Ratio Error:
CT
performance Is measured from the ratio error.
Now what
is Ration Error?
The
ratio error is the percentage deviation in the current magnitude in the
secondary from the desired value.
It can
explained as :- Let secondary current is Is, and actual value is Ip/N, where N is nominal ratio and Ip is the primary current then ratio error is given by Ip/N-Is X100.
Is
When the
CT is not saturated ratio error is due to of magnetizing current IE since Ip/N-Is =Ie.
Therefore,
% ratio error = Ie/IsX100 .
When
there is Saturation in CT then coupling between Primary and Secondary get
reduced and Hence large ratio errors are expected in saturation. The current in
the secondary is also phase shifted.
For
measurement grade CTs, there are strict performance requirements on phase angle
errors also.
Error in
phase angle measurement affects power factor calculation and ultimately real
and reactive power measurements.
It is
expected that the ratio error for protection grade CTs will be maintained
within +10%.
Class C CT
In Class
C CT are more accurate CT’s. Where 'C' letter indicates that the leakage flux
is negligible. These type of CT’s are
usually bar type CT’s.
In such
CTs the leakage flux from the core is very small. Performance of such CT’s can
be evaluated from the standard exciting curves.
Ratio
error is maintained within for limits for standard operating conditions for
such CT’s.
Voltage
rating on the secondary is specified on CT’s for which linear response is
guaranteed. For example, a class C CT specification could be as follows: 500:5
C 100.
This
tell us that 500:5 is the CT ratio and C indicate that it’s curve will be
linear up to 100 times rated current provided the burden on the secondary is
kept below 100/(5X100) = 0.40 ohm.
For
class C CTs, standard chart for versus excitation current on the secondary side
is available.
This
provides the protection engineer data to do more exact calculations e.g., in determining
relaying sensitivity.
