Restricted Earth Fault Protection Application

REF is a special application of high impedance, Merz-Price Circulating Current differential protection applied to typically transformer or generator windings.

In all differential applications, the prime objective is to ensure that the scheme is stable for through current flow such as load current or external faults.

This is achieved by making sure the CTs are all matched to have the same dynamic performance across the entire current range.

In order to achieve this it is necessary to use class X CTs since these have the characteristics of:

• Winding Turns Ratio (N) is specified, not current ratio, in order that the manufacturer doesn't add or remove turns to obtain a certain current accuracy at a particular current which may be different for each of the CTs in the group - i.e. one CT may have 1 turn added/subtracted to achieve certain accuracy whilst by chance another CT in the group has had 2 or more turns added or subtracted.  What we need is that the ampere-turns balance from primary to secondary of each CT is exactly the same, hence the turns must be the same.
• Kneepoint voltage (Vkp) is specified, not rated CT terminal voltage.  This is because we need the internal voltage of the CT to be sufficient to drive the secondary current around the entire loop even if one of the other CTs is saturated.  Hence it has to include the entire, worst-case loop impedance to the other CT and the CT winding resistance of each CT.
• Winding resistance (Rct) is specified in order that the kneepoint voltage requirement can be checked
• Excitation current (Ie) is specified since the effective fault sensitivity (Ieff) for a given relay operating current (Is) needs to include the 'loss' associated with all CTs in parallel - i.e. Ieff = Is + (n x Ie) where n is the number of CTs in parallel

The final consideration is then to make sure that the calculations of minimum Vkp suits all scenarios of ph-e and ph-ph faults both internal and external to the zone.

1. Star-side T/F-fed Internal EF
   In this application only the earth CT is in the loop and operation must be ensured.
   
   
    
2. Star-side T/F-fed External EF
   In this application the earth leg CT AND a line CT are involved and the REF scheme must be stable - i.e. the CTs must not produce differential current above the relay operating current.
   Other protection is required for this fault condition. 
   
3. Star-side T/F-fed Internal Ph-Ph
   In this application, there are no CTs on the star side involved and the REF must be stable. 
   Other protection is required for this fault condition. 
   
4. Star-side T/F-fed External Ph-Ph
   In this application, there two line CTs on the star side are involved and the REF must be stable - i.e. the CTs must not produce differential current above the relay operating current. 
   Other protection is required for this fault condition. 
   
5. Delta-side grid-fed Internal Ph-E
   Assuming the grid has an earth point "somewhere" a winding fault to earth will result in current flow to the earth point from one, possibly two phases and hence REF will operate.
   
   
   
   
6. Delta-side grid-fed Internal Ph-Ph
   regardless of the grid earth point "a phase to phase winding fault will not cause the REF to operate.
   

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