Page 9-4
Effective 10/02For more information visit: www.cutler-hammer.eaton.comIL17562BH04
9.1.9 Unbalance Protection
Unbalanced or negative-sequence currents are usually caused by
unbalanced supply voltage. Certain harmonics, such as 5
th
and 11
th
,
also cause the same undesirable effects in the motor even though the
harmonics are balanced.
The thermal algorithm incorporates the accentuated heating effect of
negative sequence currents or these certain harmonics, and trips more
rapidly for unbalanced conditions than would be expected from the
cold-start balanced protection curve.
In addition to the thermal algorithm, the MP-3000 includes a percent-
unbalance function which alarms and/or trips based on direct
measurement of unbalance. This can be used to speed up tripping
and avoid motor heating for a gross problem such as depressed
voltage on one phase, loss of a phase, or an uncleared power-system
fault external to the protected motor.
The unbalance is the ratio of negative sequence to positive sequence
current. If the motor can be started in either direction (P1L8 = REV), the
MP-3000 takes the larger of the two sequence currents at the time of
the start as the positive sequence current.
The sequence currents are accurately calculated according to the
definition, which relates directly to heating. Note that significant
negative sequence current is present if the phase angles are not
symmetrical, even though the three phase current magnitudes are
equal.
The unbalance trip and alarm functions can each be set from 4% to
40%, or OFF. A common start delay is provided (UBSD, P3L13), plus
separate trip (UBTR, P3L14) and alarm (UBAR, P4L8) run delays. Use
the run delays to ride through external-system unbalanced faults,
which could last for several seconds – check time curve settings of
feeder relays or fuses. Keep in mind that high-resistance or arcing
faults, which may take a longer time to clear, have less tendency to
severely depress voltage than a solid fault.
9.1.10 Ground Fault Protection Application
Use this fault-protection function with a flux-canceling ground fault Ct.
This Ct has a large primary window through which all three phase
conductors can pass. The most common ground fault Cts have a ratio
of 50:5 or 50:1.
The MP-3000 is recognized to UL 1053, Ground Fault Protective Device
standard. This may eliminate the need for a separate ground fault
protector in many applications that formerly required it.
Note that the ground fault trip and alarm current settings GFT, P3L1
and GFA, P4L1 are based on percentage of ground Ct rated primary
current, not on FLA or the phase ct ratio. For example, setting 10%
gives a trip or alarm for an actual ground leakage current of 5 A with a
50:5 Ct (GCT, P1L6 = 50).
Obviously, this function is only useful for a grounded power system—
the ground return is normally made from the neutral of the secondary
wire winding of the supply power transformer. Resistance grounding is
acceptable, as long as the resulting fault current is at a level the relay
can be set to detect.
The ground Ct, which provides sensitive protection for high-resistance
ground faults, may saturate for a robust heavy-current ground fault in a
solidly grounded system. Minimize the saturation problem by
minimizing the burden - use the shortest and heaviest leads possible
between the ground Ct and the relay. The MP-3000 itself has very low
burden, usually much lower than the connecting wiring. Calculate the
current magnitude which saturates the ground Ct, considering the Ct
secondary voltage capability; and the total burden of the Ct secondary
winding itself, the connecting wires, and the relay. Make sure this
saturation current is well above the minimum sensitivity of the phase
IOC function and/or the motor fuses.
Residual connection – wired summation of the phase Ct circuits
through the ground Ct input – requires a much higher GFT setting to
avoid false tripping. Thus sensitivity is not nearly as good as with a
separate flux-canceling Ct. If the relay is installed where a residual
connection is used, GCT should be set to the same value as PCT. The
user must then set the ground fault trip level at a high value to avoid
nuisance tripping from Ct ratio errors, third harmonic and certain higher
harmonics, or other measurement errors producing false residual
currents. Monitor the metered ground current during various loading
conditions to insure good margin between these error currents and the
ground fault trip current setting GFT, P3L1. Also, watch out for phase
Cts which saturate during motor starting - the saturation will produce a
large residual current and a ground fault trip. This may be a problem if
the Cts have low voltage capability (e.g. C5 or C10), or if they have
long wiring runs or are otherwise heavily burdened.
9.2 Motor Cycle Monitoring
This refers to the MP-3000 functions that monitor the motor during
periods of normal operation. Normal operation includes the start cycle,
run cycle, and stop cycle. Trips may occur at any time. The MP-3000
time-tags many critical changes of state and stores them with
supporting data in log books and history records.
The primary function of the MP-3000 is to alarm, and to trip and block
the motor contactor for faults and abnormal or dangerous operating
conditions. It can also exercise some active control of a normally
functioning motor and/or its load. Active control functions include
transition control to full running voltage for a reduced-voltage starter as
explained next; and process load shedding to reduce overload as
explained in 9.1.5 above. Others can be programmed by assigning a
particular internal MP-3000 measurement to a contact output with
output relay settings.
9.2.1 Start Cycle and Transition Tripping
Figure 9.6 shows an example of how the MP-3000 reacts to a normal
operating-cycle current profile. Initially, the motor is stopped and the
current is zero. As long as the MP-3000 is not in a trip state, it will
permit contactor energization by closing its trip contact in series with
the contactor. The contactor is energized by the operator or process
control system through a normal two-wire or three-wire motor control
scheme, external to the MP-3000. The MP-3000 declares a motor start
when it sees motor current exceeding 30 percent of the FLA setting
(P1L1). The message START is displayed and the transition timer
(TRNT, P5L6) begins to run. Also, the MP-3000 watches the large
starting current, noting when the current falls below the transition level
TRNC, P5L5.
Using the TRN XXX setting P5L7, the user can select one of four
transition behaviors:
TRN TIME—Transition to RUN after time setting (P5L6) only. Ignore
current.
TRN I—Transition when starting current drops below setting (P5L5)
only. If the time set in TRNT P5L6 expires before the current transition,
trip the motor.
TRN T+C—Transition on time or current, whichever comes first.
TRN T/C—Transition on time and current. Both must occur, and the
current must drop below the setting before the time delay expires. If the
timer expires before the current falls below the set transition level, trip the
motor.
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