BECKWITH ELECTRIC Syncrocloser M-3410A, Instruction Book

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M-3410A Instruction Book
3–28
46 Negative Sequence Overcurrent (Current
Unbalance)
Intertie Protection : The Negative Sequence
Overcurrent function (46) provides protection against
possible damage due to phase-to-phase and ground
faults, as well as unbalanced current conditions
from load or open conductors.
This function has a definite time element and an
inverse time element. The definite time pickup
value and definite operating time are normally
associated with an alarm function. The inverse
time element is usually associated with a trip
function. The inverse time function can be selected
as one of the eight curve families: definite, inverse,
very inverse, extremely inverse, and four IEC curves.
The user selects the pickup and time dial settings.
This protection must not operate for system faults
that will be cleared by feeder/line relaying, therefore,
proper coordination must be assured. This requires
consideration of feeder line protection, bus
differential, and breaker failure backup protections.
Generator Protection : The Negative Sequence
Overcurrent function provides protection against
possible rotor overheating and damage due to
unbalanced faults or other system conditions which
can cause unbalanced three phase currents in the
generator.
This function has a definite time element and an
inverse time element. The definite time pickup
value and definite operating time are normally
associated with an alarm function. The inverse
time element is usually associated with a trip
function and has a pickup and an operating time
defined by an (I
2
) 2 t = K , where K is the Time Dial
Setting and I
2
is the per unit negative sequence
current.
The minimum delay for the inverse time function is
factory set at 12 cycles to avoid nuisance tripping.
A maximum time to trip can be set to reduce the
operating times for modest unbalance. An important
feature that helps protect the generator from damage
due to recurring unbalance is a linear reset
characteristic. When I
2
decreases below the pickup
value, the trip timer takes four minutes to reset
from its 100% trip level. Figure D-9, Negative
Sequence Overcurrent Inverse Time Curves for
Generator Protection, illustrates the inverse time
characteristic of the negative sequence overcurrent
function.
Operating times are lower than what is shown in
Figure D-9, (46) Negative Sequence Overcurrent
Inverse Time Curves for Generator Protection, when
measured current values are greater than 15 A (3 A
for 1 A rated circuit).
The first task of setting this function is to determine
the capabilities of the associated machine. As
established by ANSI standards, the machine limits
are expressed as (I
2
)
2
t = K . The value of K is
established by the machine design and is generally
provided on test sheets of the machine. The relay
can accommodate any generator size because of
the wide range of
K settings from 1 to 95. Typical
values can be found in ANSI C50.13-1977.
The negative sequence pickup range is from 3% to
100% of the Nominal Current value input during
system setup (see Section 3.1,
Relay
Configuration ).
This protection must not operate for system faults
that will be cleared by system relaying. This requires
consideration of line protection, bus differential and
breaker failure backup protections.
46DT PICKUP - The pickup setting is usually quite
low (3–5%) and the output of this function is usually
connected to alarm only.
46DT DELAY - Time delay should be set high
enough to avoid alarms on transients.
46IT PICKUP - The 46 Inverse Time pickup setting
should coincide with the continuous negative
sequence current capability of the generator
operating at full output.
46 IT TIME DIAL - The time dial setting corresponds
to the K provided by the generator manufacturer for
the specific unit being protected. See Appendix D,
Figures D-1 to D-9, for the negative sequence and
overcurrent inverse time curves.
46IT MAX DELAY - The maximum trip time is used
to reduce the longer trip times associated with low
to moderate imbalances to a preset time.