One of the principal influences on
protective relay settings is load; hence, maximum load current level,
in turn, may influence fault detection sensitivity.
Phase overcurrent relays
Phase overcurrent relays must be set to
avoid operation on all of those “normal” conditions to which they
may be subjected, such as transformer inrush, motor starting current,
maximum emergency load conditions, and maximum recoverable swing
conditions.
This usually entails a time overcurrent
pickup setting above a maximum load current level and/or a
coordinated instantaneous pickup setting to ensure security of the
relays against misoperation. The sensitivity achievable is,
therefore, somewhat coarse, but many applications in which they are
used do not require extreme sensitivity.
Ground overcurrent relays
Ground overcurrent relays have the
advantage of utilizing a current source that supplies little or no
normal current to the relays. The sensitivity achievable is
substantially better than that afforded by phase overcurrent relays.
Only unbalanced load current and normal system unbalance affect the
setting of these devices.
Directional overcurrent relays
Directional overcurrent relays have the
same restrictions as phase and ground overcurrent relays for load
flow in their tripping direction. Properly selected directional
elements block tripping for load flow and faults in the nontripping
direction.
Phase distance relays
Phase distance relays have a relatively
fixed reach; they operate most sensitively when fault currents are
present and less sensitively when only load current exists. Fault
currents typically lag voltage by 60° or greater. Load current
typically leads or lags voltage by 30° or less. Although less
sensitive in the load angle region, phase distance relays may require
a setting for adequate fault coverage that may limit line loading.
Ground distance relays
Ground distance relays may also be
susceptible to the error associated with ground fault resistance and
out of-phase sources. Further, they may have overreach and underreach
characteristics for the “leading” and “lagging” phases in
responding to phase-to-phase-to-ground faults, unless provision is
included to compensate for these factors.
Many ground distance relays operate on
phase current and voltage inputs, making them susceptible to operate
under heavy load conditions. For this reason, ground distance relays
are usually supervised by ground overcurrent elements, which must be
set to avoid operation for heavy unbalanced loads. Load current will
also influence the “reach” of these devices where fault
resistance is involved (Giuliante, McConnell, and Turner).
Pilot systems—two terminal
The influence of load on pilot systems
is highly dependent on the nature of the protective relaying scheme.
Those systems using overreaching distance measurement, such as
directional comparison blocking, permissive overreaching transfer
trip (POTT), and directional comparison unblocking, have the
advantage of limited load angle sensitivity and have the absence of a
critical reach due to the nature of the relaying system.
These relays have very little influence
from load except in very long line applications, and this is often
accommodated by blinders that prevent operation of the protective
relaying system under balanced, three-phase load conditions. For
direct underreaching transfer trip (DUTT) schemes, the pilot distance
relays are set short of the remote line terminal.
This setting makes the scheme less
susceptible to tripping under heavy loading conditions. However, the
reach variation of the distance relay as a result of prefault load
current is much more critical than for the overreaching schemes.
It is imperative that load or fault
current, or any combination of the two, never be able to cause
operation of the Zone 1 relay for any condition other than a fault on
the protected line. Phase comparison and current differential schemes
are not normally susceptible to operation under load condition
because of their inherent nature of comparing current into the line
at one terminal with current out of the line at the other.
However, load does influence the
setting of fault detectors in phase comparison blocking schemes and
in current differential schemes when operation following channel
failure is allowed. Current differential schemes may be sensitive to
tapped loads, and settings should be chosen accordingly.
Also, high levels of through load
current may reduce the fault detection sensitivity of both phase
comparison and current differential schemes. When transmission cables
are used, special considerations may be required for the fault
detector settings because of the capacitance of the cables.
Pilot systems—three terminal
Load in a three-terminal line
application may represent an outfeed condition for an internal fault.
Depending on the particular type of relaying system, this may produce
an undesired blocking effect.
Three-terminal applications generally
have at least one weak source and, consequently, care must be
exercised to ensure that either the contribution to an internal fault
exceeds this load current outfeed, or the relaying system bases its
response on the total internal fault current.
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