SecoGear Medium-voltage Switchgear Application and Technical Guide
DET-882
System and Equipment Protection
28
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BASIC SYSTEM PROTECTION
Phase Overcurrent Protection
Recommended phase overcurrent protection consists of
one time and instantaneous phase overcurrent element
(50/51) in each phase operated from a current transformer
in each phase. This arrangement provides complete phase-
overcurrent protection for the circuit, even when one phase
element is removed from the circuit for testing; it also
provides local backup if one of the three phase elements is
inoperative.
Overcurrent relays are available with standard ANSI
inverse, very inverse, or extremely inverse and definite time
characteristics. Many microprocessor-based relays also
offer these ANSI curve shape characteristics. In the
absence of additional system information, for a single
characteristic device the very inverse characteristic is most
likely to provide optimum circuit protection and selectivity
with other system protective devices.
This characteristic is intended for application where the
magnitude of fault current is primarily determined by the
distance from the source to the fault. The extremely
inverse characteristic is well suited for applications in
which selectivity with fuses or reclosers is a requirement.
The inverse characteristic is useful in those rare
applications in which selectivity with other inverse or
definite time relays is a concern. It is also useful on
systems that have a multiplicity of local generators at the
distribution voltage and where the magnitude of fault
current is determined primarily by how many generators
are in service at the same time. Most microprocessor
based relays have the entire above characteristics field
selectable which would allow the specifier to select a relay
with minimal information and select a characteristic when
more complete information is available.
Incoming Lines
Incoming line phase-over-current protection is typically
time delay only (51), furnished without instantaneous
attachments (50), or on digital multi-function relays, the
instantaneous is disabled, but the function stays available.
This allows the relay to be selective with feeder relays
having instantaneous attachments (50/51).
Feeders
Instantaneous phase-overcurrent relay (50) settings for
radial utility distribution feeders are usually set as low as
possible considering, among other things, “cold-load”
pickup and other circuit requirements. Instantaneous
phase-overcurrent relays for industrial or commercial
building radial circuits are usually set high enough (but well
below the available short-circuit current) to prevent false
tripping for faults at the lower-voltage terminals of large
transformer banks and to provide selectivity with groups of
large motor starters. Instantaneous settings should be low
enough so that the combination of time and instantaneous
settings provides protection below the conductor short-
circuit heating limit.
GE Multilin 350, or 850 digital multi-function relays provide
complete incoming line or feeder protection and
monitoring.
Feeder Ties
For feeder-tie (cable connected) circuits to downstream
distribution circuit-breaker lineups, selectivity is enhanced
by disconnecting, disabling, or delaying the instantaneous
element (50) of the phase-overcurrent relays and setting
the time-overcurrent (51) element to trip at less than the
short-circuit heating limit of the conductors.
Bus Ties
Bus-tie circuits, within the same lineup of switchgear
including two incoming lines, are frequently specified
without overcurrent-protection. When overcurrent
protection is provided for this type of circuit, relays are
connected in what is termed a “current summation”
connection. The use of this connection provides the
opportunity for selectivity between main or tie breakers
and feeder breakers minimizing relay operating time delay.
Transformers
Transformer-overcurrent protection criteria are specified In
Section 450 of the 2005 National Electrical Code.
Permissible short-circuit capabilities for transformers are
specified in ANSI Standard C57.12. Selection of
transformer-overcurrent protection is governed by these
criteria.
NEC requirements determine the pickup of the time-
overcurrent phase protective relays. The time dial setting is
determined by ANSI requirements and the connection of
the transformer. The inrush and short-circuit current
magnitudes determine the instantaneous setting of the
phase-overcurrent protective relays. The GE Multilin SR345,
or SR845 relay provide options for transformer protection,
metering, and monitoring.
Generators
Overcurrent relays, applied on generator circuits, are used
for feeder backup rather than overload protection. These
overcurrent relays are typically voltage-restrained
overcurrent relays (51V). They operate faster and are more
sensitive for faults close to the generator than for faults
remote from the generator. The GE Multilin SR889 relay
provides complete generator protection, metering and
monitoring.
