Transformer Differential protection problem.

Hello Team,

We have one transformer which have KBCH Differential relay installed to protect T/F from internal fault or fault within protection zone. We have observed when ever fault occur out side the differential zone then also Differential operate but normally it should not operate. 

Following below are the Settings,

Configuration - HV+LV

HV CT Ratio - 200.0:1

LV CT Ratio - 400.0:1
HV Ratio correction - 1.46

HV Vactor correction - Yy0(0 deg.)
LV1 Ratio correction - 1.46

LV1 Vactor correction - Yy0(0deg)

Id> 01. p.u

Id>> 10.0 p.u

Io> HV - 0.100 p.u

Io> LV - 0.100 p.u

IoF - 50%

t0F - 10.00 S

CT ratio correction factor for T/F primary & secondary winding seems not correct. That's why the relay respond to outside fault. Kindly share your comments.