Page 116 - Practical Power System and Protective Relays Commissioning

114 Practical Power System and Protective Relays Commissioning

FIGURE 10.9 Linear coupler current transformer.

FIGURE 10.10 Linear coupler current transformer (CT) versus normal CT.
TPY Class
These are CTs with antiremanence air gaps (remanence , 10%). Otherwise
they behave the same as the TPX class.

TPZ Class
TPZ are CTs with a linear core (remanence may be neglected). The specified
transformation accuracy only applies to the AC component while exponen-
tially decaying DC is radically reduced.
In summary:
The IEC classification for TP of CTs as follows:
TPS: Class X in British Standards
TPX: Class P, no remanence limit
TPY: Remanence less than 10% small air gap in core
TPZ: Very low remanence and can be neglected

A linear coupler air gap core with linear characteristics is shown in
Fig. 10.9 and is discussed in the following section.
In linear coupler V and I primary has a linear characteristic and are used in
voltage balance differential relays used in BB protection.