Basler Electric DECS-100 - Manual (Page 30)

3-6 DECS-100 Functional Description 9287500991 Rev M
Limiters
DECS-100 limiters consist of an overexcitation limiter (OEL) triggered by an increase in field current and
an underexcitation limiter (UEL) triggered by excessive leading vars.
Overexcitation Limiting
The DECS-100 provides two types of overexcitation limiting: summing point and takeover.
Summing Point OEL. When the level of field current increases above the adjustable current level setpoint,
the front panel and BESTCOMS Overexcitation Limiting indicators light. If the overexcitation condition
persists for the duration of the user-adjustable alarm time delay, the relay output at terminals AL1 and
AL2 closes.
The advantage of a summing-point type of OEL is that it can provide a smooth transition into and out of
the limit. Its drawback is that it does not control field current directly, but has to work through the normal
voltage regulator, and may be influenced by changes in terminal voltage.
Takeover OEL. When takeover-style overexcitation limiting is used, the level of field current at which
limiting occurs is determined by an adjustable current level setpoint. If the field current increases above
the adjustable current level setpoint, the front panel and BESTCOMS Overexcitation Limiting indicators
light and the field current is limited and forced to follow the characteristic time curve. See Figure 3-2.
The advantage of a takeover type of OEL is that it provides direct control of the exciter field current
without relying on the normal voltage regulator’s control action. Its drawback is that it may not provide a
smooth transition into and out of the limit.
If hardware shutdown is enabled, the DECS-100 will be disabled when the time delay expires. When the
DECS-100 is powered up following a shutdown triggered by overexcitation limiting, the Overexcitation
Limiting indicator will light for five seconds.
The current level setpoint is adjustable from 0 to 15 Adc. The alarm time delay is adjustable from 0 to 10
seconds.
Figure 3-2. Time Characteristic Curve for Takeover OEL
Underexcitation Limiting
When the level of leading vars increases above the adjustable var level setpoint, the front panel and
BESTCOMS Underexcitation Limiting indicators light. If the underexcitation condition persists for the
duration of the adjustable alarm time delay, the relay output at terminals AL1 and AL2 closes.
If hardware shutdown is enabled, the DECS-100 will be disabled when the time delay expires. When the
DECS-100 is powered up following a shutdown triggered by underexcitation limiting, the Underexcitation
Limiting indicator will light for five seconds.
The var level setpoint is adjustable from 0 to 100% of the rated vars. The rated var level is determined by
the following equation:
3 level var rated
× × =
B AVG
I V
Where IB is the DECS-100 nominal current sensing rating (1 Aac or 5 Aac)