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what is CT saturate?and which effect on CT after its saturate?
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It is possible that relatively low-ratio CTs are applied for
protective relaying of small loads
fed from switchgear and motor controllers of relatively high
short-circuit capacity. Assume
the worst-case scenario of 64kA available fault current from
bus feeding a small motor load
of normal current below 50A. In theory, CTs rated as lows as
50:5 and relay class C10 may
be applied for protection purposes.
Realizing that 64kA of fault current is 1080 times the rated
current of the 50:5 CT, the
magnitude of the problem is evident. Protection class CTs
are designed to work in the linear
range, with minimal errors and minimal waveform distortion,
only up to 20 times the rated
nominal current with the burden as defined by the relay
class (saturation voltage) of the CT
per IEEE Std. C57.13.
Well-established and relatively accurate equations are
available for calculation of the
actual maximum primary current for saturation-free operation
under any specific burden,
any specific X/R ratio, and any specific residual flux in
the CTs. This engineering practice is of
little help here: A CT fed with a primary current hundreds
of times its rated current will
saturate severely - only relatively short duration peaks of
limited current will be observed
from the secondary of the CT. These peaks can be as low as
5-10% of the ratio current, and
will last a small fraction of the half-cycle, down to 1-2ms
in extreme cases. As a result only a
very small portion of the actual ratio current is presented
to protective relays fed from such
severely saturated CTs. In terms of the true RMS value, the
secondary current may be as low
as 1-2% of the expected RMS secondary current.
On the surface it may seem that a severe problem takes place
here – the fault current is
so high that it virtually stops the CT from passing the
signal to the relay. The relay does not
see enough proportional secondary current during severe
faults in order to operate its short
circuit protection. The upstream relay, using CTs of a much
higher ratio, measures the fault
current more accurately and trips. Zone selectivity is lost
because the poor low-ratio CT was
“blinded” to the fault.
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