Co-ordination of Fast Numerical Relays and Current Transformers – Overdimensioning Factors and Influencing Parameters

This paper reports on some of the results of the working group WG 34.02, established by the Study Committee 34 to study and investigate co-ordination of relays and conventional current transformers (CT). The objective of the WG 34.02 work is to suggest common recommendations on how the manufacturers should specify the CT requirements and also suggest a guideline for co-ordination of relays and current transformers.

The paper will show and discuss a more realistic and correct method of calculating the overdimensioning factor of current transformers when co-ordinating fast numerical relays and current transformers, than what has normally been the common practice.