9440300990 10-5
DECS-250 Protection
Table 10-1. Loss of Sensing Trip Criteria
Loss of either 1 or 2 phases
(3-phase sensing)
Loss of all 3 phases
(3-phase sensing)
Loss of single-phase
sensing
3-phase, 3-wire sensing selected
3-phase, 3-wire sensing selected
Single-phase sensing selected
BV% of AVR Setpoint > VGEN
rated
OR
OR
V1 = Positive sequence voltage
V2 = Negative sequence voltage
I1 = Positive sequence current
I2 = Negative sequence current
I
rated
= Rated current
BV% = Balanced voltage percent
UV% V1 = Unbalanced voltage percent
VGEN = Average generator voltage
When all criteria in a column are true for the duration of the time delay setting, a LOS trip condition
occurs.
A LOS condition can be used to initiate a transfer to manual (FCR) control mode. It also can be
configured in BESTlogicPlus to initiate other actions. Protection can be enabled and disabled without
altering the individual loss of sensing settings.
LOS protection is automatically disabled when a short circuit exists. A short circuit is detected when the
measured current is greater than twice the rated current for a single-phase CT connection and when the
positive sequence current is greater than twice the rated current for a three-phase CT connection.
Figure 10-6. Loss of Sensing Protection Settings
Frequency Protection
BESTCOMSPlus Navigation Path: Settings Explorer, Protection, Frequency
HMI Navigation Path: Settings, Protection, Frequency Protection 81
The frequency of the generator terminal voltage is monitored for overfrequency and under-frequency
conditions.
Overfrequency
An overfrequency condition occurs when the frequency of the generator voltage exceeds the 81O pickup
threshold for the duration of the 81O time delay setting. Overfrequency protection can be enabled and
disabled without altering the pickup and time delay settings. Overfrequency pickup and trip elements in
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