CYG PRS-7367 - Page 212

Protection Functions
204 PRS-7367
3.42.2.5 Operation Characteristic
Ir
64REF_Cur_Str
Tripping zone
Restrained zone
k
64REF_Cur_K
Figure 3.42-2 Operation characteristic of REF
Where:
Id is the differential current, Ir is the restraint current；64REF_Cur_Str is the start of differential
current.
3.42.2.6 CT Transient Characteristic Difference Detection
To prevent the effect of incorrect differential circuit zero-sequence current on DIFF_REF in case of
CT transient characteristic difference and CT saturation induced by external faults, the device
integrates CT saturation criterion with positive-sequence current restraint at each side. When
DIFF_REF protection trips, zero-sequence current at each side must fulfill the following formula.
100
3 IBI 
Where ：
0
3
I
is the zero-sequence current at a side.
1
I
is its corresponding positive-sequence current.
B
0
is a proportional constant and the value is 0.6.
3.42.2.7 CT Saturation Detection
To prevent incorrect tripping of differential protection caused by CT saturation in case of external
fault, CT saturation detection element of the device would judge the saturation of CT and
determine whether to block relevant differential protection.