ABB RELION REX640 Technical Manual page 1256

Section 6
Supervision functions
1250
in the absence of a zero-sequence voltage is an indication of the current transformer
secondary failure. However, both methods have disadvantages as they require an
additional set of current transformer, or a voltage channel is needed for detecting a
zero-sequence voltage.
The methods may not be applicable where additional current channels or voltage
channels are not available. This CT secondary circuit supervision presents an
algorithm that can be used as an example for detecting the CT secondary failure used
for the unit protection of a two-winding or three-winding transformer. However, the
function has a limitation that it cannot detect failure in case of equipment under
protection in no-load condition or when two simultaneous secondary CT failures
occur.
The detection of a zero current in any one phase is a partial indication of failure in the
current transformer secondary. Furthermore, if this current zero is due to the failure in
the current transformer secondary, it results in a change in the magnitude of the
negative-sequence current in the group only where current zero has been detected.
However, changes in the negative-sequence current in other groups of three-phase
current transformers at the instance of zero-current detection is an indication of a
system problem. Also, it may happen that after the detection of a failure in the current
transformer secondary, a fault may occur in the system. During such condition,
functions are internally blocked.
Phase discontinuity
A zero current detected due to the phase discontinuity results in an asymmetry in all
the sets of the current transformer, which then results in a change in the negative-
sequence current (ΔI ) in the healthy set. This change in the negative-sequence current
2
on the healthy sides, that is, other than where a zero current has been detected, blocks
the function.
In case of a lightly loaded transformer (up to 30%) the change in the negative-
sequence current may be very negligible. However, a phase discontinuity results in a
change in the phase angle difference between two healthy phases in the set of CTs
where a zero current has been detected as well as on the primary side of the
transformer. This change in the value of the angle blocks the function internally.
Overload / System short circuit condition
It is required that any overload or short circuit conditions after a CT failure should
block the function. During overload or short circuit condition, the phase current
increases beyond its rated value; if any phase current on any set of current transformer
exceeds the set limit, the function is blocked internally. Also in case of an
unsymmetrical fault, the negative-sequence current increases. If the negative-
sequence current increases beyond the set limit, the function is blocked internally. The
overcurrent and negative-sequence current setting both can be set equal to the
overcurrent and negative-sequence protection function start value.
The internal blocking is thus useful for avoiding false operation during a fault
situation.
1MRS759142 C
REX640
Technical Manual