ABB RELION REX640 Technical Manual page 700

Section 4
Protection functions
694
The secondary circuit voltage can easily exceed the isolation voltage of the CTs,
connection wires and the protection relay because of the stabilizing resistance and CT
saturation. A voltage dependent resistor (VDR, R
shown in Figure 381.
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 386 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.
1MRS759142 C
) is used to limit the voltage as
u
Technical Manual
REX640