3.15
Date Code 20020903
SEL-587Z Instruction Manual
Relay Elements
Overcurrent Protection
Conventional instantaneous overcurrent elements must be set sufficiently high
to avoid tripping on transformer magnetizing inrush current, where peak
currents may be many times the transformer full-load current. Transformer
magnetizing inrush current contains substantial second-harmonic current and
often contains a significant dc component. Unlike conventional
electromechanical overcurrent elements, the SEL-587Z overcurrent elements
ignore all but the fundamental frequency current, making the relay insensitive
to the off-fundamental-frequency content of the magnetizing inrush current.
The SEL-587Z instantaneous and inverse-time overcurrent elements need only
be set for expected load and fault conditions.
Distribution Transformer Applications
When the SEL-587Z is applied to a distribution substation transformer serving
load centers, expected load conditions include steady-state load as well as
transient conditions caused by hot and cold load pickup.
Hot load pickup inrush occurs when a distribution circuit is energized shortly
after being de-energized, such as in a feeder trip-reclose cycle. Hot load
pickup inrush current that the SEL-587Z may see consists primarily of motor
starting current from motor loads, incandescent and fluorescent lighting load
inrush, and resistive heating element inrush. The overall effect is an inrush
current several times the normal load current that may last for several seconds.
Cold load pickup inrush occurs when a distribution circuit is energized after
being de-energized for a relatively long period of time. Cold load pickup
includes many of the same inrush characteristics as hot load pickup, but is
usually more severe and longer lasting because more thermostatically
controlled systems need to satisfy heating or cooling requirements after the
prolonged outage.
For these reasons, overcurrent protection must be tailored to meet the
protection requirements for the specific transformer, avoid tripping for various
types of nonfault transient conditions, and coordinate with upstream and
downstream protection devices. These factors constrain the selection of
settings and characteristics for the applied overcurrent protection.
Phase Overcurrent Protection
Set phase overcurrent element pickup settings above the highest expected load
current to avoid tripping on normal load current.
Because you can use the negative-sequence overcurrent elements to detect
phase-to-phase faults, you can set the phase overcurrent elements for three-
phase fault detection only. This setting selection improves the ratio of the
minimum phase fault current to maximum load current required for secure
phase overcurrent relay application.
Negative-Sequence Overcurrent Protection
The negative-sequence elements respond to |3I
2
| current, where 3I
2
= Ia + Ib •
(1
)
240) + Ic • (1
)
120). The negative-sequence overcurrent elements are
uniquely suited to detect phase-to-phase faults and are not sensitive to
balanced load.
The negative-sequence element is 1.73 times more sensitive to phase-to-phase
faults than a phase overcurrent element with the same pickup setting.
Содержание SEL-587Z
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