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Bus Pr
Bus Protection
In addition, high-impedance differential relays also have some
limitations in terms of normal operations and maintenance. The
relay sees only the voltage from the differential junction point, and
therefore cannot provide any auxiliary protection functions such
as breaker failure, or record the individual currents from each CT
connected to the relay. Data to analyze fault events must therefore
come from additional sources. Another operating limitation of high-
impedance differential is the ability to handle routine bus switching,
such as removing a circuit breaker for maintenance. Typical, the
differential relay must be blocked during such switching operations.
While this type of switching is uncommon with the typical single
segment busbar, it is a routine occurrence with multiple segment
busbars, making high-impedance differential schemes difficult to
apply on such multiple segment busbars.
Percentage differential relays, also known as low-impedance
differential relays, provide similar operating speed, and can provide
a similar level of security, as high-impedance differential relays. In
addition, low-impedance relays are simple to apply, as there are
no special requirements for CT performance class, turns ratio, or
secondary lead burden other than good performance practice. A
microprocessor-based low-impedance differential relay measures
input currents from each set of CT, and therefore can provide
auxiliary functions such as breaker failure for every circuit, and
measure and record all currents during a fault event. In addition,
switching events can be routinely handled in the relay, and low-
impedance differential relays can be specifically designed for
multiple segment busbars.
GE Multilin Busbar Protection
GE Multilin provides protective relays that support all busbar
protection techniques, including overcurrent, high-impedance
differential, and percentage (low-impedance) differential. The
protection techniques for overcurrent and high-impedance
differential protection are well known. GE Multilin low-impedance
differential relays are designed to provide specific performance
advantages on applications for all busbars, from single segment
busbars with up to 24 connected circuits, or large multiple segment
busbar configurations. These include the correct restraint while
facing CT saturation during a fault event, detecting the failure of a
CT secondary circuit connected to the relay, protection of multiple
segment busbars, and providing enough digital inputs and outputs
for proper bus protection.
CT Saturation
Saturation of a CT connected to a low-impedance differential relay
during an external fault produces an incorrect differential current
that may cause the relay to operate. GE Multilin relays use adaptive
trip logic to prevent an operation due to CT saturation during
external faults.
The adaptive trip logic is designed around the differential
characteristic. The restraint current of the differential element
is based on the maximum measured current, as opposed to the
traditional magnitude sum of the currents. This ensures ideal
restraint for the actual fault condition, balancing sensitivity
and security. The differential element uses a dual slope-dual
breakpoint characteristic matching the differential characteristic
to the saturation performance of the CT ensuring security, while
maintaining sensitivity.
The differential characteristic can correctly restrain for many
external faults when CT saturation occurs. However, the differential
element itself is not enough to ensure correct restraint to external
faults when severe CT saturation occurs. The differential protection
is always supervised by a directional element. The directional
element compares the angle of the measured fault currents. If at
least one current is away from the sum of the remaining currents
by an angle greater than 90
, the fault is considered an external
fault. If all fault currents are within 90 of each other, the fault is
considered internal.
During low magnitude fault events, the differential element must
assert and the directional element must declare an internal fault
for the relay to trip. A low magnitude fault event occurs when the
differential current is less than the breakpoint for the second slope
of the element characteristic. During high magnitude faults a CT
saturation detector additionally supervises the differential protection.
During such a fault, the differential protection may operate only if
the differential element asserts while no CT saturation is present,
or when the directional element declares an internal fault when CT
saturation is present. The CT saturation detector simply sets a logic
flag when the restraint current exceeds the setting for the second
breakpoint of the differential characteristic, and the differential
current remains below the first slope of the characteristic.
Differential characteristic region
Adaptive Trip logic