9440300990 11-1
DECS-250 Limiters
11 • Limiters
DECS-250 limiters ensure that the controlled machine does not exceed its capabilities. Overexcitation,
underexcitation, stator current, and reactive power are limited by the DECS-250. It also limits the voltage
during underfrequency conditions.
Overexcitation Limiter
BESTCOMSPlus Navigation Path: Settings Explorer, Operating Settings, Limiters, OEL
HMI Navigation Path: Settings, Operating Settings, Limiters, OEL
Operating in the overexcited region of a generator’s capability curve can cause excessive field current
and field winding heating. The overexcitation limiter (OEL) monitors the level of field current supplied by
the DECS-250 and limits it to prevent field overheating.
The OEL can be enabled in all regulation modes. OEL behavior in manual mode can be configured to
limit excitation or issue an alarm. This behavior is configured in BESTlogic™Plus.
Two styles of overexcitation limiting are available in the DECS-250: summing point or takeover. OEL
settings are illustrated in Figure 11-3, Figure 11-4, and Figure 11-6.
Summing Point OEL
Summing point overexcitation limiting compensates for field overcurrent conditions while the machine is
offline or online. Offline and online OEL behavior is dictated by two separate groups of settings. Primary
and secondary setting groups (selectable in configurable logic) provide additional control for two distinct
machine operating conditions.
Offline Operation
For offline operation, there are two levels of summing-point overexcitation limiting: low and high. Figure
11-1 illustrates the relationship of the high-level and low-level OEL thresholds.
Figure 11-1. Summing Point, Offline, Overexcitation Limiting
The Offline, Low-Level OEL threshold is determined by the Low-Level setting. When the OEL becomes
inactive, the High Current Timer counts down from either the high time, if the High Current Timer has
expired, or from the amount of time spent at high level, if the High Current Timer has not expired. The
generator is permitted to operate indefinitely with this level of excitation.
The Offline, High-Level OEL threshold is determined by the High Level and High Time settings. When the
excitation level exceeds the High Level setting, the DECS-250 acts to limit the excitation to the value of
the High-Level setting and a High Level Timer is initiated. If this level of excitation persists until this timer
reaches the High Time setting, the DECS-250 acts to limit the excitation to the value of the Low-Level
setting.
FIELD CURRENT
TIME IN SECONDS
High
Current
Time
CONTINUOUS
P0063-09
0-10sec
Low
Current
Level
High
Current
Level
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