Date Code 20020129
Differential, Restricted Earth Fault,
3-47
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
feeders. You may use inverse-time overcurrent elements to prevent transformer damage due to
excessive through currents caused by slow clearing external faults. Thermal and mechanical
damage curves should be available from the transformer manufacturer for specific transformer
designs. You can consult several references, including the IEEE C37.91,
Guide for Protective
Relay Applications to Power Transformers
, that provide generic through-current limitations for
various classes of transformers.
Set the SEL-387E Relay instantaneous overcurrent elements to detect high-current faults within
the transformer differential protection zone. Use definite-time and time-overcurrent elements to
detect lower current faults inside and outside the transformer differential protection zone. Use
appropriate delays to coordinate with upstream and downstream 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 and fourth-harmonic current and often contains a significant dc component.
Unlike conventional electromechanical overcurrent elements, the SEL-387E Relay overcurrent
elements ignore all but the fundamental frequency current, making them insensitive to the off-
fundamental-frequency content of the magnetizing inrush current. The SEL-387E Relay
instantaneous, definite-time, and time-overcurrent elements need only be set with regard to
expected load and fault conditions.
Where the SEL-387E Relay 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-387E Relay may see consists primarily of 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. The 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 their 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.
Overcurrent Element Operating Quantities
The SEL-387E Relay phase overcurrent elements respond to the maximum phase current
magnitude, Ip, where Ip is the largest value of |Ia|, |Ib|, and |Ic|. Set phase overcurrent element
pickup settings above the highest expected load current to avoid tripping on normal load current.
You may set the pickup lower if you use torque control.
Since 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.
