SecoGear Medium-voltage Switchgear Application and Technical Guide DET-882
System and Equipment Protection
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Residual-ground relaying (51N or 50/51N) is suitable for
feeders on solidly grounded systems or resistance
grounded systems with available ground-fault currents
greater than about twice the maximum current
transformer rating. It is also required for feeders, which
must be selective with other downstream feeders having
residual-ground overcurrent relaying.
Transformers and Generators
Ground-overcurrent relaying for wye-connected
transformers, wye-connected generators and neutral-
deriving transformers usually employs neutral-ground
relaying, as discussed previously under “Incoming Lines.”
This provides system backup ground relaying. Settings,
however, are normally too high to provide good ground-
fault protection for the apparatus. Ground-fault protection
is better obtained by using a scheme of differential
relaying, as described later in this section.
Directional Phase Overcurrent Protection
Directional phase-overcurrent relays (67) operate for
current flow in only one predetermined direction. Incoming
lines, operating in parallel from separate sources, require
directional phase-overcurrent relay protection to provide
sensitive operation and to assure selectivity between
incoming-line breakers for phase faults on the source side
of one of the breakers.
This directional phase- overcurrent protection is furnished
by using relays, polarized to operate on current flowing
toward the source. The directional-overcurrent relay
without instantaneous function is appropriate for most
applications. The pickup of this relay should be set at a
value slightly below full-load current. The time delay
function can be set to permit selectivity with upstream
feeder breaker or line instantaneous relays.
Occasionally a directional-overcurrent relay (67) with
directional instantaneous function is applied to incoming
lines fed by long “dedicated” service lines; the
instantaneous directional unit is set to operate for faults
located approximately 80 % to 90 % of the distance from
the incoming line to the source.
For large local transformers, the instantaneous unit on a
high side directional overcurrent relay is set slightly above
the low-voltage symmetrical rms amperes contributed
through the transformer to a fault on the higher voltage
side of the transformer.
Directional phase-overcurrent relays can be voltage-
polarized from bus VTs connected in open-delta, delta-
delta or wye-wye. Polarization is necessary to establish the
current phase relationships between voltage and current
to determine the direction of current flow.
While earlier electromechanical directional-overcurrent
relays usually had only one time-current characteristic,
digital multifunction versions are available in three-phase
(and ground, if desired) packages with inverse, very inverse,
and extremely inverse (and other) characteristics that are
field-selectable.
Directional Ground Overcurrent Protection
Incoming lines operated in parallel from separate
grounded sources require directional ground-overcurrent
relays (67N) to assure selectivity between incoming-line
breakers for ground faults on the source side of each of the
incoming-line breakers. For solidly grounded systems and
many impedance-grounded systems, a multifunctional
digital relay usually is appropriate. This relay is set at a low
pickup to permit selectivity with the other incoming-line
non-directional ground-overcurrent relaying.
All directional-ground relays must be polarized. For
systems with local, grounded supply transformers, the
current transformer located in the transformer neutral-
ground connection may be used for polarizing. For systems
with remote-supply transformers, a set of local wye-
broken delta connected voltage transformers (or wye-wye
VTs with wye-broken delta auxiliary transformers) may be
used for polarization. On occasion, dual polarization may
be desirable.
As mentioned in the phase units above, digital versions are
available in packages with inverse, very inverse, and
extremely inverse (and possibly other) characteristics field-
selectable. This function (67N) may also be packaged in
with all three phases of directional phase overcurrent.
High Impedance Ground Fault Detection
Many distribution system ground faults do not generate
enough current to be detected by traditional overcurrent
protection. These faults frequently result from a broken
conductor falling in contact with a poor conducting surface
or an object having relatively high impedance (e.g., tree
branches, dry ground). A high percentage of arcing
downed conductors may be detected by new digital relays
with “high Z” capabilities, specifically designed for this
purpose, such as GE Multilin F60 Feeder Protection Relay.
Differential Protection
Differential protection is a method of equipment protection
in which an internal fault is identified by comparing
electrical conditions at all incoming and outgoing terminals
of the equipment. By virtue of the connection and settings,
this protection operates only for faults in the apparatus
being protected, or “in the zone of protection.” Hence,
differential protection does not need to coordinate with
devices protecting other downstream conductors and
equipment. Differential protection considerations for
specific equipment is discussed in later sections.
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