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Traditional and Electronic Current Transformers Theory Testing Chapter | 10 113

10.5.5 Transient Performance Current Transformers
IEC 60044-6 classifies these into types TPX, TPY, and TPZ. TPX is the
closed iron-core CT while TPY and TPZ are CTs with air gaps in the core.

10.5.5.1 TPY Class
An antiremanence CT has a small gap in the core magnetic circuit, thus
reducing the possible remanent flux from approximately 90% of saturation
value to approximately 10%. This gap is quite small, for example 0.12 mm
total, and so the excitation characteristic is not significantly changed by their
presence. However, the resulting decrease in possible remanent core flux
confines any subsequent DC flux excursion, resulting from primary current
asymmetry, within the core saturation limits. Errors in current transformation
are therefore significantly reduced by these CTs.

10.5.5.2 TPZ Class
Linear CTs constitute an even more radical departure from the normal solid
core CT. It incorporates a large air gap, for example, 7.5 10 mm. As its
name implies, the magnetic behavior tends to be linear by the inclusion of
this gap in the magnetic circuit. However, the purpose of introducing more
reluctance into the magnetic circuit is to reduce the value of the magnetizing
reactance. This in turn reduces the secondary time-constant of the CT,
thereby reducing the overdimensioning factor necessary for faithful
transformation.
The standard IEC 60044-6 classifies CTs according to transient perfor-
mance (TP) when short circuit current has exponentially decaying DC offset.
This standard differentiates four TP classes depending on the layout of the
CT core.

TPS Class
TPS class is a closed iron-core CT with small leakage flux. Behavior is
defined by the magnetization curve (knee point voltage, magnetizing current)
and the secondary winding resistance. It corresponds to Class X in British
Standard BS3938-1973 and PL class in Australian Standard AS1675-1986. It
is designed for application in differential protection schemes.

TPX Class
TPX is a closed iron-core CT without limitation of the remanence (no limit
for remanent flux).
This construction corresponds to Class P in IEC60044-1 but TPX addi-
tionally specifies the TP.