Consider standard 11 KV over head line feeder protection having one module consisting over current protection for two phases and an earth fault protection. The earth fault peotection is taken from the current flowing through the cable connecting the CT neutral and relay neutral which can be termed as a residual current. Considering slightly different actual ratios for the three numbers CTs, it is standard practice to set a minimum pick up for the earth fault relay.
The CTs used have a metering core, a normal protection core. Some CTs have an additional differential protection core which will have only lesser error. Metering CTs will saturate at double the full load current.
Restricted earth fault(REF) relay of any transformer is preferable to be taken from a differential protection core of a CT. The standard settings used in Kerala State Electricity Board(KSEB) are for EF relay or REF relay taken from normal protection core is 20% of full load setting. If REF is relay taken from standard differential core, if available is 5% only.
Around thirty years ago, on an experimental basis the normal earth fault relay settings were reduced to 10% of full load current for selected feeders and kept unaltered for one year. There were much more tripping compared to previous year for a particular season. Hence it was decided to revert to previous settings.
When making purchase of instrument transformers, INSIST a graph for maximum percentage error versus times of full load current. For metering cores this must be from zero load to full load. For normal protection CTs it must be from two times to twenty times full load because these CTs are supposed to saturate above twenty times full load. In case of differential protection CTs the curve must be available from double full load to eighty times full load.
Any differential relay must withstand eighty times full load for one second and any normal protecion relay must withstand twenty times full load for three seconds. These are not based on any BS but the values used for ENGLISH ELECTRIC (NOW GEC ALSTHOM) RELAYS manufactured in India.
In above posted case find the maximum possible error from graphs supplied by the CT manufactures for a zero impedence EF. As a thumb rule give double of such setting arrived.
Maximum fault current can easily be found in case of secondary of a substation transformer by assuming the higher voltage bus is an INFINETE BUS.Actual fault current will be less than this value, but definetely will not go below 80% 0f this value.
Once I had commissioned a 11 KV panel fed by a 110KV/11KV transformer. The panel had relays from another new Indian manufacturer. During lightning the differention relay of the transformer FAILED with good amount of smoke.Two times the relay was replaced by a relay of same manufacturer and again previous result. Then on my recommendation, the differential relay was replaced by an ENHLISH ELECTRIC differential relay and the problem never occured again.
N P NAIR,C Eng,MIET,FIE(I),SrMIEEE.