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252
www.GEMultilin.com
252
Bus Pr
ot
ection
Bus Protection
www.GEMultilin.com
www.GEMultilin.com
Introduction
Busbars in power systems are the location where transmission lines,
generation sources, and distribution loads converge. Because of
this convergence, short circuits located on or near the busbar tend
to have very high magnitude currents. The high magnitude fault
currents require high-speed operation of the busbar protection to
limit equipment damage. However, this high-speed clearing must
be balanced against the need for security. Tripping incorrectly for
an external fault may cause large outages, and jeopardize power
system stability. The high fault magnitudes increase the possibility
of CT saturation during external faults close to the busbar, and CT
saturation increases the possibility of an incorrect operation of the
busbar protection.
Protection of the busbar may be complicated and varies with
the topology of the bus. Many busbars connect all circuits to one
common segment of busbar. The complication for these buses is
simply the number of connected circuits. However, a specific busbar
may have multiple bus segments, with individual circuits that
connect to different bus segments depending on operating needs.
For such complex buses, busbar protection must be able to protect
each bus segment individually, and dynamically keep track of the
circuits connected to a specific bus segment.
Busbar Protection Techniques
The choice of protection technique used for a specific busbar
depends on the protection requirements for speed and security,
balanced against the cost of implementing a specific solution,
and the operating requirements for a specific bus. Common
methods of protecting busbars include overcurrent-based
interlocking schemes, overcurrent-based differential protection,
high-impedance differential protection, and percentage differential
protection. Interlocking and overcurrent differential protection can
be implemented with any suitable overcurrent relay from GE Multilin,
and performance has to be balanced in terms of speed and security
against the reduced cost of protection. These types of protection
are typically applied on distribution busbars, where fault current
magnitudes are lower and speed is generally less critical than with
transmission busbars.
Differential protection provides high speed fault-clearing necessary
for critical busbars such as transmission busbars, or distribution
busbars where arc flash hazards are a concern. High-impedance
differential protection or percentage differential protection may be
the correct choice depending on the bus configuration and specifics
of application. Both methods address loss of security for external
faults due to CT saturation.
Distribution Busbar Protection
Distribution busbars typically have a single incoming source
supplying multiple radial distribution feeders. For these applications,
the chief concerns for protecting the bus are normally meeting
operating requirements and cost of protection. High speed clearing
to maintain system stability is not normally necessary. Security
is maintained by simple time coordination, or via hardwired
communications in reverse interlocking protection schemes.
The most significant factor in terms of operating speed is arc-
flash hazards. The slow operating time of overcurrent-based bus
protection can result in a larger arc-flash zone or in more restrictive
hazard protection requirements. In these instances, high speed
differential protection is appropriate.
Transmission Busbar Protection
The predominant requirements for protecting transmission
busbars is the speed and security of the protection scheme. These
requirements are built around the need to minimize equipment
damage and maintain system stability during fault events. If these
are the only two considerations for transmission busbar protection,
then high-impedance differential protection may be appropriate.
High-impedance voltage differential protection is a solution to
the challenge of CT saturation during external faults, as the high
impedance of the relay forces the error current due to the saturated
CT back through the CTs instead of the relay operating coil. The relay
uses a setpoint to differentiate between the maximum error voltage
due to CT saturation, and the full voltage of an internal fault.
When also considering the requirements for operating a specific bus,
and the cost of installation, high-impedance differential protection
schemes have some limitations. The major limitation is the strict
requirements on the CT circuits necessary for the high-impedance
scheme. All CTs used in the scheme must have the identical
performance class and turns ratio, must be tapped at full ratio, and
must be dedicated to the bus protection scheme. Additionally, the
secondary lead burden from the each CT to the relay should also
be identical. These requirements are necessary to keep the level of
error voltage as low as possible to prevent maloperation of the relay.
Making modifications to an existing bus protection scheme, such as
adding an additional circuit, may be very challenging in engineering
and installation.
52
51
51N
52
51
51N
52
51
51N
52
51
51N
52
50 50N
51 51N
Inverse Time Overcurrent
or Reverse Interlocked
Overcurrent
Incoming Source
Radial Feeders
Overcurrent based interlocking schemes for simple Bus protection
Bus Protection