10-8 9440300990
Protection DECS-250
Motor Protection
The DECS-250 compares the real power (kW) flowing into the motor with the reactive power (kvar) being
supplied. Operation of synchronous motors drawing reactive power from the system can result in
overheating in parts of the rotor that do not normally carry current. The 40Q pickup response is shown in
Figure 10-10.
Figure 10-10. Generator Capability Curve vs. 40Q Response
Pickup and Trip
A loss of excitation condition exists when the level of absorbed vars exceeds the loss of excitation (40Q)
threshold for the duration of the 40Q time delay. A time delay setting of zero makes the loss of excitation
element instantaneous with no intentional time delay. If the pickup condition subsides before the time
delay expires, the timer and pickup are reset, no corrective action is taken, and the element is rearmed
for any further occurrences of loss of excitation.
The 40Q threshold is expressed as a percentage of the rated var flow for the machine. Loss of excitation
protection can be enabled and disabled without altering the pickup and time delay settings.
BESTCOMSPlus loss of excitation settings are illustrated in Figure 10-11.
Figure 10-11. Loss of Excitation Protection Settings
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