9492600990 8-3
DECS-150 Protection
Loss of sensing 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 (I1) is greater than twice the rated current for a three-phase CT
connection.
Table 8-1 defines loss of sensing trip criteria. All criteria in a single column must be met.
Table 8-1. Loss of Sensing Trip Criteria
Single Phase Sensing Selected
V1 > Balanced Voltage* % of
AVR Setpoint
V1 < Balanced Voltage* %
of AVR Setpoint
AVG
< Balanced Voltage*
% of AVR Setpoint
V2 > Unbalanced Voltage* % of
V1
I1 < 200% I
rated
I1 < 200% I
rated
or
or
* Denotes the value used for the loss of sensing settings.
The Loss of Sensing screen is illustrated in Figure 8-3.
Figure 8-3. Loss of Sensing Screen
Frequency Protection
BESTCOMSPlus Navigation Path: Settings Explorer, Protection, Frequency
The frequency of the machine terminal voltage is monitored for overfrequency and underfrequency
conditions.
Overfrequency
An overfrequency condition occurs when the frequency of the machine voltage exceeds the 81O pickup
threshold for the duration of the 81O Time Delay setting. The Voltage Inhibit setting, expressed as a
percentage of the rated machine voltage, can be implemented to prevent an overfrequency trip from
occurring during startup when the machine voltage is rising toward the nominal level. Overfrequency
protection can be enabled and disabled without altering the Pickup and Time Delay settings.
Overfrequency pickup and trip elements in BESTlogicPlus can be used in a logic scheme to initiate
corrective action in response to the condition.
The Overfrequency screen is illustrated in Figure 8-4.
Shutdown Enable
When checked, Shutdown Enable causes the DECS-150 to stop excitation when the overfrequency
element trips.
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