Current transformer steady-state
performance can be calculated from ANSI accuracy classification for C type or
excitation data supplied by the manufacturer for C and T types. The secondary
voltage as given by the maximum fault current reflected on the secondary side
multiplied by the connected burden should not exceed the assigned C accuracy
class.
Conversely, the permissible current
transformer burden for a given accuracy class can be calculated. Performance
using excitation data calculates the excitation current at the secondary voltage,
which should be a small percentage of the operating current.
For T-type transformers, the manufacturer
should supply overcurrent ratio curves from 1 to 22 times normal current and
for all standard burdens up to the one that causes a ratio error of 50%. The
transient performance should consider the dc component of the fault current
because it has far more effect in producing severe saturation of the current
transformer than the ac component.
Before a current transformer reaches its
saturation flux, it may still accurately reproduce the offset fault current for
a short duration. The time to saturate is a function of magnitude of the
short-circuit current and its offset, secondary burden, system time constant,
and current transformer time constant and its characteristics.
The current transformers may saturate
within the first half cycle at large fault currents. The remanence in the core
is the result of a previous current interruption, at other than current zero.
This can force the current transformer into saturation sooner than expected.
Time to desaturate is dependent upon the
same parameters as the time to saturate and is on the order of five cycles,
largely because of the power system time constant. If a current transformer is
chosen based upon steady-state performance, current transformer saturation due
to dc offset will not cause problems on the operation of time overcurrent
relays, as long as the dc component decays to zero in the time the relay is
expected to operate.
For high-speed relays, such as
instantaneous relays, differential and distance relays, and other relay types
operating in less than two cycles, the current transformers should be selected
so as not to saturate in less than 2 cycles. The rms value of the distorted
output current of a current transformer can be calculated.
Relays that respond to only the fundamental
may be more seriously affected than what the rms value indicates. Selecting as
high a ratio as an application will permit and series and parallel connections
of current transformers are some of the means to reduce saturation.
Current transformers with an air gap have a
fairly high exciting current and low residual flux and can be used in some
differential schemes; however, their use is limited in the modern protective
schemes.
Current transformer saturation, waveform of
secondary current output for various degrees of saturation
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