ABB RELION 620 Series Technical Manual page 606

Section 6
Supervision functions
6.4.5
600
Application
Open or short-circuited current transformer secondary can cause unwanted operation in
many protection functions, such as earth-fault current and differential. The simplest
method for detecting the current transformer secondary failure is by comparing currents
from two independent three-phase sets of CTs or the CT cores measuring the same
primary currents. Another widely used method is the detection of a zero-sequence current
and zero-sequence voltage. The detection of a zero-sequence current 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 on the
2
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.
1MAC504801-IB E
620 series ANSI
Technical Manual