Basler Electric BE1-11t - Manual (Page 104)

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Trip
The Trip output becomes true when a restrained pickup condition persists for the duration of the element
Time Delay setting. In BESTlogicPlus, the Trip output can be connected to other logic elements and to a
physical relay output to annunciate the condition and to initiate corrective action. If the target is enabled
for the element, the BE1-11t will record a target when either the Trip or Unrestrained Trip outputs become
true. See the
Fault Reporting chapter for more information about target reporting.
Transient Monitor
A transient monitor detects the effects of CT saturation during a through fault. The 87 element monitors
the change in restraint current versus the change in operate current. For an internal fault, the restraint
current and operate current will experience a step increase at the same time. For an external fault, there
should be no operate current. If CT saturation occurs during a through fault, the operate current will
increase at some time after the restraint current increases. In this case, a two-cycle delay is added to the
restrained differential output to enhance security.
Harmonics
The second and fifth harmonic functions check the ratio of the second and fifth harmonic operate currents
to the fundamental operate current. Traditional harmonic restraint units operate on the ratio of harmonic
current to total operate current versus the ratio to only the fundamental operate current used by the
BE1-11t. For this reason, the BE1-11t will provide greater security for inrush and overexcitation with the
same harmonic inhibit ratio settings used with traditional differential relays. When either of these two
comparators is above the threshold, the percentage-restrained output is blocked from setting the
Restrained Trip logic output. If the second or fifth harmonic inhibit comparators are picked up for any of
the three phases, the Second Harmonic Inhibit and Fifth Harmonic Inhibit logic outputs respectively are
also set.
In many cases, the second harmonic content of the inrush current may show up primarily in only one or
two phases, which can cause one or two phases to not be inhibited. The BE1-11t allows the second
harmonic currents to be shared between the three phases. When second harmonic sharing is enabled,
the magnitude of the second harmonic operating current is summed from all three phases and this
magnitude is used by the second harmonic comparator for each phase instead of the second harmonic
operate current for only that phase. This is superior to other methods of cross blocking since each phase
element operates independently in its comparison of operating current to harmonic current. Thus, security
is enhanced without sacrificing dependability because a faulted phase will not be restrained by inrush on
unfaulted phases as is the case with cross blocking schemes.
Unrestrained Tripping
The 87 element provides high-speed tripping for high-grade faults inside the zone of protection. If the
operate current is above the Unrestrained Tripping threshold for any of the three phases, the
Unrestrained Trip logic output becomes true. The transient monitor function also enhances security for
this function by doubling the pickup threshold when CT saturation is detected. The minimum setting for
the Unrestrained Tripping threshold should be the maximum inrush current with a small margin.
Programmable Alarm
An 87 Alarm condition is indicated when the percentage restrained differential protection is nearing a trip
condition on load. This alarm triggers a diagnostic routine that attempts to determine the source of the
mismatch that is causing the differential unbalance.
The alarm appears on the front-panel display, web page interface, and on the Alarms metering screen in
BESTCOMSPlus. Refer to the
Alarms chapter for information on how to program alarms.
Element Blocking
The Block input provides logic-supervision control of the element. When true, the Block input disables the
element by forcing the Trip and Pickup outputs to logic 0 and resetting the element timer. Connect the
0BPhase Current Differential (87) Protection BE1-11t