Restricted earth fault protection is unit protection. ie it responds to earth faults within a defined zone. The boundaries are the measuring CT's. For example, if you applied REF to a star connected winding you would summate the outputs of the CT's in each phase and also the neutral, and the protection would operate for faults within the zone bounded by the CT's. Balanced earth fault protection is not unit protection. It will operate for earth faults downstream of the CT's. BEF works by summating the currents from all 3 phases. If you were protecting a delta transformer winding you could use BEF, and broadly the protection would only respond to earth faults in the winding, not beyond. If you applied BEF to a HV feeder, it would respond to all earth faults downstream of the CT's.
Regards
Marmite
Hi Mokwadi.
I recommend to you will start from link:
faq238-1287
in additional some small tips:
Theory and considerations
When the sum (residual) connection of the phase CT's is used instead of core balanced CT (known as ring or toroidal CT) the small dissimilarities of the CT's and especially partial saturation will cause not-true apparent current in the secondary circuit. This might cause false relay operations.
Normally the apparent earth-fault current seen by the relay because of the CT errors is relative small. In case of a phase-fault, the phase current can easily be 5..20 times the nominal load current. Assuming, for example, that because of the CT errors 1% of the phase fault current will be seen by the earth-fault relay this becomes 5..20 x 1% = 5..20% of the rated secondary current.
Partial saturation of CT's is also very possible with high fault currents and also with lower currents because of DC component (motor start-ups, transformer inrush situations and cold load pick-ups). Saturation will increase the error of CT's rapidly causing even higher apparent E/F-current in the secondary circuit. There is therefore a potential danger that the earth-fault relay can activate at phase-faults etc. when actually no earth current is flowing in primary circuit.
(When the protection has stability in such way that earth-fault protection will not trip (faster) in case of phase-faults is known as phase-fault stability.)
Recommendations
The recommended minimum setting is 10% of the rated load current. Settings especially below 5% has been found problematic.
Serial resistance (stabilising resistor) might also be required for the measuring circuit in order to limit effects of the partial CT saturation.
REF it' stype of differntial protection, as saied Marmite and Ralph.
I will found linl abot REF and will send to you.
Best Regards.
Slava
