BECKWITH ELECTRIC M-3425A, Инструкция

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4–115
System Setup and Setpoints – 4
87 Phase Differential
The Phase Differential function (87) is a percentage
differential with an adjustable slope of 1–100%.
Although this protection is used to protect the
machine from all internal winding faults, single-
phase to ground faults in machines with high
impedance grounding may have currents less than
the sensitivity of the differential relay (typically
between 3 and 30 primary amps). Ranges and
increments are presented in Figure 4-87.
Turn-to-turn faults are not detected by differential
relays because the current into the generator
equals the current out (see functions 50DT and 59X
for turn-to-turn fault protection.) Even though the
percentage differential relay is more tolerant of CT
errors, all CTs should have the same characteristics
and accuracies.
A typical setting is 0.3 amps.
A typical setting is 10%.
A typical setting is one cycle. Typical settings given above
assume matched current transformer performance, and that
transformer inrush of the unit transformer does not cause
dc saturation of the generator CTs. If there is a significant
difference in current transformer ratings (C800 vs C200, for
example), or if saturation of the generator CTs is expected
during energizing of the step up transformer, more appropri-
ate settings might be 0.5 A pick up, 20% slope, and a delay
of 5 to 8 cycles.
If line side and neutral side CTs do not have the same ratio,
the ratio error can be corrected (the line side measured current
is multiplied by the phase CT correction settings.)
Line Side CTR
Phase CT Correction =
Neutral Side CTR
To provide restraint for CT saturation at high offset
currents, the slope is automatically adjusted (at
a restraining current equal to two times nominal
current) to four times the slope setting, see Figure
4-86.
For very high currents in large generators, the
proximity of CTs and leads in different phases
can cause unbalanced currents to flow in the
secondaries. These currents must be less than the
minimum sensitivity of the relay.
There are two elements in this function. Element #2
is intended to provide phase differential protection
for SFC (Static Frequency Converter) starting gas
turbine generator applications. Element #1 should
be disabled with a contact blocking input during a
converter start operation (generator off-line), since
the current is carried by only neutral side CTs and
the resulting differential current may mis-operate
87#1 function. The 87#2 element, which is set with
a higher current pickup, will still provide protection
for this condition.
87 #1 PICKUP
Amps
87 #1 SLOPE
%
87 #1 DELAY
Cycles
87 #2 PICKUP
Amps
87 #2 SLOPE
%
87 #2 DELAY
Cycles
87 PHASE CT CORRECTION