Example Problem

I am having a very difficult time understanding the logic here: I know this is a delta to why transformer but the CTs are measuring line currents on both high and low sides. My high side current calculation are correct and match the solution, but on low side the solution includes root(3) multiplied by Isec. where'd this root(3) come from? 

Any help would be appreciated. 

what book is this from?

The problem states that it is an ELECTROMECHANICAL differential relay. 

As you know, delta-wye transformers have a 30 deg phase shift between primary and secondary. When doing differential protection, we need to compensate for this phase shift (otherwise, currents going into the differential relay will be out of phase). So, the 3 CTs at the delta side of the transformer have to be connected in wye, and the 3 CTs at the wye side of the transformer have to be connected in delta. This will ensure that the currents going into the differential relay are in phase.

Therefore, the CTs at the primary side of the transformer are connected in wye, and the CTs at the secondary side of the transformer are connected in delta. Since the CTs at the secondary side of the transformer are connected in delta, you need to multiply the CT output current by the SQRT(3) factor in order to get the line current.

This method to compensate the 30 deg phase shift is only used with electromechanical relays. With microprocessor relays we use a compensation matrix in the relay to correct for the phase shift.