DECS-100 Functional Description
3-5
Underfrequency
When generator frequency drops below the selected knee frequency setpoint, the voltage setpoint is
automatically adjusted by the DECS-100 so that generator voltage follows the selected PU (per unit) V/Hz
curve. When operating on the selected PU V/Hz curve, the UNDERFREQUENCY ACTIVE indicator lights
on the front panel and in BESTCOMS. Underfrequency control is disabled below 12 hertz. The knee
frequency is adjustable from 40 to 65 hertz in 0.1 hertz increments and the PU V/Hz curve may be set at a
slope of 0 to 3 in 0.01 steps through BESTCOMS software. A slope of 0 effectively disables the
underfrequency function. The DECS-100 has a minimum regulation point of approximately 30 percent of the
nominal setpoint.
Protection
The DECS-100 includes four protective functions: overexcitation shutdown, generator overvoltage shutdown,
loss of generator sensing shutdown or transfer, and overexcitation limiting. Each function has a
corresponding front panel indicator that lights when the function is active. An active function is also
annunciated through BESTCOMS software.
Overexcitation Shutdown (Field Voltage)
This function is enabled or disabled through BESTCOMS software. When enabled, if field voltage exceeds
the adjustable field overvoltage setpoint for 10 seconds, the OVEREXCITATION SHUTDOWN indicator on
the front panel and in BESTCOMS lights, the relay output closes, and the DECS-100 shuts down. When the
DECS-100 is powered up following an overexcitation shutdown, the OVEREXCITATION SHUTDOWN
indicator will light for 5 seconds. The Field Overvoltage setpoint is adjustable from zero to 250 Vdc in integer
steps.
Generator Overvoltage Shutdown
The DECS-100 monitors the sensed generator output voltage. If it exceeds the adjustable Generator
Overvoltage setpoint of the nominal setpoint for 0.75 seconds, the GENERATOR OVERVOLTAGE indicator
lights on the front panel HMI and in BESTCOMS, the relay output closes, and the DECS-100 shuts down.
When the DECS-100 is powered up following a generator overvoltage shutdown, the GENERATOR
OVERVOLTAGE indicator will light for 5 seconds. The Generator Overvoltage setpoint is adjustable from 100
to 120 percent in integer steps.
Loss of Generator Sensing Shutdown or Transfer
The DECS-100 monitors the sensed generator output voltage and takes protective action if a loss of sensing
voltage is detected. A loss of sensing voltage is detected during the following conditions.
The sensed voltage is less than 50 percent of the rated voltage (one-phase or three-phase sensing).
A total loss of any phase occurs (three-phase sensing).
The voltage difference between any phase (line-to-line) and the three-phase average exceeds 20
percent of nominal (three-phase sensing).
A time delay of zero to 25 seconds is adjustable through BESTCOMS software. This delays the protective
action in order to allow field forcing in applications that do not sense B-phase generator current. The default
time delay setting is 10 seconds.
BESTCOMS software allows the selection of one of two protective actions for a loss of sensing. Either a
complete shutdown or a transfer to Manual mode may be selected.
If shutdown is selected and a loss of sensing occurs, the LOSS OF GENERATOR SENSING indicator on
the front panel and in BESTCOMS lights, the relay output closes, and the DECS-100 shuts down after the
adjustable time delay expires. When the DECS-100 is powered up following a loss of generator sensing
shutdown, the LOSS OF GENERATOR SENSING indicator will light for 5 seconds.
If transfer to manual is selected and a loss of sensing occurs, the relay output closes, and the DECS-100
transfers to the Manual mode of operation after the adjustable time delay expires. The DECS-100 will remain
in this mode of operation until power is cycled.
This function is disabled when the frequency decreases below 12 hertz or when a generator short circuit
condition is detected. A generator short-circuit is determined when the B-phase CT current exceeds three
times the per unit value. Loss of sensing shutdown or transfer is not active during the soft-start time.