1MAC050144-MB C
Section 6
Supervision functions
615 series ANSI
451
Technical Manual
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 (?I2) in the healthy set. This change in the negative-sequence 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 pickup value.
The internal blocking is thus useful for avoiding false operation during a fault situation.
Содержание 615 Series ANSI
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