I presume the interconnection is between one grid and another...

I presume the interconnection is between one grid and another grid, when you have two different power supplies joining together there are a number of things that have to match, i.e. voltage, Phase relationship, Q factor, etc.

For instance Phase relationship is important, if the two phases connecting together for instance were 180 degrees out of phase they would basically cancel each other out, it would also place great stress on the two generators.

That is my very basic understanding of it, I am sure other will have more detail.