ABB RELION REX640 Technical Manual page 1356

Supervision functions
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
current 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.
6.9.7 Signals
1356
) in the healthy set. This change in the negative-sequence
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1MRS759142 F
REX640
Technical Manual