8/3/2019 Earth Fault Protection Types

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Earth Fault Protection Types

This is the different between types of earth fault protection

Residual earth fault protection: Uses three CTs and the earthfault relay is connected in the residual point of the three CTs.Since a residual current only exists when a fault current flows toearth, you can obtain a relative low setting. We use settings inthe order of 5-10% FLA, but it will depends on the type of CTs,the burden of the relay on the CTs, leakage currents etc. See itmany times with two or three O/C elements in the circuit, andthe E/F relay (residual) connected between the star point of therelay group and the CTs. Need to coordinate with similar relays

downstream. 

Sensitive Earth Fault Protection: Normally used on systemswhere the earth fault current is limited to a very low value(Neutral earthed through resistors, Peterson coil, reactance, etc)Scheme consists out of a ring CT (Core Balance CT) around thethree phase conductors, and the relay connected to thesecondary side of the CT. A very low setting is possible, evensometimes lower then 1% FLA. Need to coordinate with similar relays downstream.

Restricted Earth Fault Protection: Is a type of zone of protection,or a differential scheme. You get two types, low impedance andhigh impedance. Low impedance is normally current operatedand high impedance voltage operated. High impedance REF isthe most commonly used. Relay connected in parallel with threephase CTs and one neutral CT. It will look just for faults inside

the transformer, and need not to coordinate with relaysdownstream (can operate very fast). With this kind of protectionyou can protect over 90% transformer windings. Need to usespecial class CTs, previously called class X CTs. I am not reallyfamiliar with low impedance REF, but have seen it incorporatedinto some the new microprocessor relays.

Standby earth fault protection: Relay connected to the neutralCT. Normally a kind of back-up protection with a relative long

delay or many times a long-time inverse curve. Need tocoordinate with downstream E/F relays.