In a differential protection scheme for a power transformer, why must the relay include a slope characteristic rather than relying on a fixed current difference to avoid tripping during a through-fault?

A differential relay works like a scale that checks if the electricity going into a transformer matches the electricity coming out. If the two sides don't match, the relay thinks there is a leak, or fault, inside and cuts the power. The problem is that electrical equipment is never perfect, so the measurements on either si....