High-Set Overcurrent Element Hoc - Toshiba GRT100 Series Instruction Manual

CT saturation during through faults.

2.11.2 High-set Overcurrent Element HOC

High-set overcurrent element HOC is an instantaneous overcurrent characteristic, and is applied
in the differential circuit. The characteristic is expressed by the following equation:
I d  kh
Id is defined as follows for three-winding transformer.
I = |kct1I + kct2I
d 1
where,
kct1, kct2, kct3: CT ratio matching settings of primary, secondary and tertiary winding
HOC is an un-restrained current differential element which can protect a transformer against
damage due to a heavy internal fault, because it has a simple operation principle and high-speed
operation. Note that HOC is not immune to transformer inrush currents and therefore cnnot be
applied with a sensitive setting.
2.11.3 Restricted Earth Fault Element REF
The restricted earth fault element REF has dual percentage restraining characteristics. Figure
2.11.2 shows the characteristics on the differential current (Id) and restraining current (Ir) plane.
Id is a differential current between the residual current of each winding and the neutral current
and Ir is a restraining current which is the larger of the residual current and the neutral current.
Characteristic DF1 is expressed by the following equation:
 p1I + (1-p1) ikkct
I
d r
where,
p1: slope of DF1 (fixed to 10%)
ik: minimum operating current
kct: CT ratio matching of line CT to neutral CT (when plural line CTs are applied, maximum
kct is employed.)
For the 1REF element, I
circuit with one neutral CT and three line CTs. (For the REF element application, see Appendix
+ kct3I |
2 3
I d
DF1
kctik
kp
Figure 2.11.2 Restricted Earth Fault Element
and I are calculated by the following equations when applied to a
d r
 39 
I d = I r (one-end infeed)
DF2
I r
6 F 2 S 0 7 8 9