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Transformer Differential protection problem.

Rohit Sharma
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.
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  • 0
    I'm sure you're right, the interposing CT settings are wrong, so any heavy through current would look like an internal fault. I'd need more information to tell you the right settings, so I'll tell you the principles instead. One way to get the right settings is to set the ratio so that full load current through the transformer gives one amp into the comparator. You also need to get the right vector groups. The manufacturers' manuals are normally good
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  • 0
    I'm sure you're right, the interposing CT settings are wrong, so any heavy through current would look like an internal fault. I'd need more information to tell you the right settings, so I'll tell you the principles instead. One way to get the right settings is to set the ratio so that full load current through the transformer gives one amp into the comparator. You also need to get the right vector groups. The manufacturers' manuals are normally good
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