The Holmgren connection is generally used for standard overcurrent-earth fault protection, but also applies to distance protection. Differential protection can use Holmgren connection where the relay itself is doing the differential "calculation" rather than the direct circulating current connection between the multiple CT groups as the Merz-Price connection 
Note for OC-EF applications it is possible to have any 2 out of the three A-B-C elements as noted here: Holmgren Connection.pdf |