GE 889 Instruction Manual page 407

CHAPTER 4: SETPOINTS
NOTE:
NOTE
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
The relay incorporates four programmable FlexCurves™ - FlexCurve A, B, C and D. The
points for these curves are defined by the user in the EnerVista program. User-defined
curves can be used for thermal overload protection in the same way as IEC curves. Each of
the four FlexCurves™ has 120-point settings for entering times to reset and operate; 40
points for reset (from 0 to 0.98 times the Pickup value) and 80 for operate (from 1.03 to 20
times the Pickup value).
The equations shown above (EQN1, 2, 3, 4) are not applicable when a setpoint Overload
Curve is programmed as a FlexCurve.
Once the load current exceeds the overload level (k x I
is, heat accumulation becomes greater than heat dissipation. The thermal overload
element reacts by incrementing the thermal capacity used (TCU) at a rate dependent on
the selected thermal curve and overload level. When the accumulated thermal capacity
reaches 100%, the Thermal OL OP operand is set. This operand remains asserted until it is
reset.
The thermal overload protection element estimates thermal capacity used TCU using the
following equations calculated each power cycle. When current is greater than the pickup
level, I > k x I , element starts increasing the thermal capacity (EQN 5):
b
When current is less than the dropout level, |I < 0.97 x k x Ib|, the element starts decreasing
the thermal capacity (EQN 6):
where:
• TCU = thermal capacity used at index n
n
• TCU = thermal capacity used at index n - 1
n-1
• ∆t = power cycle duration
MONITORING
), it enters an overload phase; that
b
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