TRANSFORMER PROTECTION OVER CURRENT

CONSIDER YNyn0 TRANSFORMER

HV side Over current protection

1.Directional over current HV side (67HV)
2.Directional over current HV side (Neutral current) (67NHV)
3.Non directional Time delayed over current HV Neutral side (51GHV)

LV side Over current protection

(Incase Transformer to LV side Bus tie normally close ( NC ) )

1. Directional over current LVside (67LV)
2. Directional over current LVside (Neutral current) (67NLV)
3. Non directional Time delayed over current LV Neutral side (51GLV)

LV side Over current protection

(Incase Transformer to LV side Bus tie normally open ( NO ) )

1. Non directional over current LV side (50/51 LV)
2. Non directional over current LV side (Neutral current) (50/51 NLV)
3. Non directional Time delayed over current LV Neutral side (51GLV)


TV side Over current protection

1. Non directional Time delayed over current LV side (51TW HS &LS)

Why 67HV instant of 50/51HV on HV side?

The Transformer HV side protection should be directional over current because it should operate only for fault in the transformer HV side. If fault on HV bus then the Bas bar protection should operate.

For the above fault Bus bar protection will operate because 67HV relay is directional over current with curve setting but bus bar protection setting is instantaneous.

FAULT IN TRANSFORMER HV SIDE

Now 67HV transformer protection will operate and BB protection will not operate because BB protection CT will cover this fault.

In this cause first HV side relay will trip and open the HV side CB and full fault current will flow through the LV side and LV side relay will trip and open the LV side breaker.



FAULT IN TRANSFORMER LV SIDE

Consider a fault in transformer LV side and the current flow through LV side CT is more than current flow through HV side CT. Therfore first LV side relay will operate and open the LV side CB. Then all fault current flow through the HV side CT and 67 HV will operate and open the the HV side CB. In case LV side Tie open condition then there is no source to feed the fault from LV side. So for this condition 67LV will not operate and 67HV operate.

LV SIDE BUS TIE NORMALLY OPEN CONDITION

If LV side tie breaker normally open then 67LV is no need . Then non directional relay can use for protect the power cable between transformer LV side to LV SWG. And CT connection can take from the LV CT bushing CT.

CHANGE THE ANY ONE TAP POSITION OF TRANSFORMER AND MAKE THE CIRCULATING CURRENT TO TEST THE DIRECTIONAL OVER CURRENT FUNCTION


What is the meaning of 67NHV?

This is Negative Sequence Overcurrent Protection (NPS).

This scheme worked based on residual current produced during earth fault and phase to phase faults. In normal the vectorial sum of all three phase current at neutral point should be zero.
But any earth or phase to phase fault accure a residual current generate due to unbalance.So a leakage current flow through the CT neutral path.This current sence by the NPS element and initiat the trip or alarm.

Negative Phase sequence overcurrent elements give more sensitive to resistive P-T-P faults, where phase over current elements may not be operate.

In certain application residual current may not be detected by an earth fault relay due to the system configuration. Example :- an earth fault relay applied on the Delta Side of a Delta - Star transformer is unable to detect earth faults on on the star side. However negative sequence current will be present on the both sides of the transformer for any fault condition , irrrespective of the transformer configuration. Therefore, a negative phase sequence overcurrent element may be employed to provide time-delayed protection for any any uncleared asymmentrical faults.