Is it possible to provide a relay with a summated output of two current transformers?


Yes, this is possible. For this task, we will have to make changes in the relay model itself. This means we will have to add a second current transformer (CT) device, measurement block and to add additional logic block where mathematical addition will be performed.


In the attached pfd file you will find two relays.

1. WithoutAddition - this is a standard generic F50_F51 relay with only one input CT.

2. WithAddition - this is the modified F50_F51 relay type so that it contains two CTs. Output current values of both CTs are being fed into an internal logic block that performs the mathematical addition of phase current values that are later being distributed to all available overcurrent blocks.