ABB RELION 620 Series Technical Manual page 480

Section 4
Protection functions
474
Busbar protection scheme
The basic concept for any bus differential protection relay is a direct use of Kirchoff's first
law that the sum of all currents connected to one differential protection zone is zero. If the
sum is not zero, an internal fault has occurred. In other words, as seen by the busbar
differential protection, the sum of all currents that flow into the protection zone, that is,
currents with positive value, must be equal to currents that flow out of the protection zone,
that is, currents with negative value, at any instant of time.
Figure 223 shows an example of a phase segregated single busbar protection employing
high-impedance differential protection. The example system consists of a single incoming
busbar feeder and two outgoing busbar feeders. The CTs from both the outgoing busbar
feeders and the incoming busbar feeders are connected in parallel with the polarity.
During normal load conditions, the total instantaneous incoming current is equal to the
total instantaneous outgoing current and the difference current is negligible. A fault in the
busbar results in an imbalance between the incoming and the outgoing current. The
difference current flows through the protection relay, which generates a trip signal.
1MAC504801-IB E
620 series ANSI
Technical Manual