Sensitive and Restricted Earth Fault

[bob]

Hi Ralph,

 

What are basically the differences btw earth , sensitive earth and restricted earth fault so far transformer protection is concerned.

Thks.

 

Bob

[RalphChristie]

Bob:

 

Residual earth fault protection: Uses three CTs and the earth fault relay is connected in the residual point of the three CTs. Since a residual current only exists when a fault current flows to earth, you can obtain a relative low setting. We use settings in the 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 it many times with two or three O/C elements in the circuit, and the E/F relay (residual) connected between the star point of the relay group and the CTs. Need to coordinate with similar relays downstream.

 

Sensitive Earth Fault Protection Normally used on systems where 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 the three phase conductors, and the relay connected to the secondary side of the CT. A very low setting is possible, even sometimes 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 and high impedance. Low impedance is normally current operated and high impedance voltage operated. High impedance REF is the most commonly used. Relay connected in parallel with three phase CTs and one neutral CT. It will look just for faults inside the transformer, and need not to coordinate with relays downstream (can operate very fast). With this kind of protection you can protect over 90% transformer windings. Need to use special class CTs, previously called class X CTs. I am not really familiar with low impedance REF, but have seen it incorporated into some the new microprocessor relays.

 

Standby earth fault protection Relay connected to the neutral CT. Normally a kind of back-up protection with a relative long delay or many times a long-time inverse curve. Need to coordinate with downstream E/F relays.

 

Hope this helps

 

Regards

Ralph