Summated Overcurrent Protection

Hello friends,

Can somebody explain me the principle and working of summated overcurrent protection used as an cheap alternative for bus-zone differential protection. It's like if to a 2-section bus bar 2 transformers are feeding with bus coupler always closed, the summation of one transformer L.T end C.T and c.t on bus bar second section to which other trafo is feeding (although bus coupler is closed, so both transformer feed both section, sharing is there) is taken for protection. Please explain, I can't find anything over internet.

Thanks,

Sohan