BE1-951
Protection and Control
4-15
Figure 4-9. Time-Overcurrent Elements
Retrieving Logic Output Status Information
The status of each logic variable can be determined through the ASCII command interface using the
RG-STAT (report general-status) command. See Section 6,
Reporting and Alarm Functions, General Status
Reporting for more information.
51 Time-Overcurrent Element
BE1-951 relays have one element for phase (51P), two elements for neutral (51N and 151N), and one
element for negative sequence (51Q) inverse time-overcurrent protection. Figure 4-9 shows the 51 elements.
The 51N, 151N, and 51Q elements are identical in configuration. Each element has two outputs: Pickup (PU)
and Trip (T). A Block logic input is provided to disable the function. When this expression is TRUE, the
element is disabled by forcing the outputs to logic 0 and resetting the timers to zero. This feature operates
in a similar manner to the torque control contact of an electromechanical relay.
Each inverse time-overcurrent function has a mode, pickup, time dial, and curve setting. See Appendix
A,
Time Current Characteristics for details on each of the curves available. To make the protective
element use integrated reset and emulate an electromechanical induction disk reset characteristic, the
user can append an R to the selected time current characteristic curve designation. An available
programmable curve can be used to create a custom curve by selecting coefficients in the inverse time-
characteristic equation.
When the measured current is above the pickup threshold, the pickup logic output is TRUE and inverse
timing is started according to the selected characteristic. If the current stays above pickup until the element
times out, the trip logic output becomes TRUE. If the current falls below the dropout ratio, which is 95
percent, the function will either reset instantaneously or begin timing to reset depending on the user's setting.
The phase overcurrent protective functions use the highest of the three measured phase currents. If the
current is above the pickup setting for any one phase, the pickup logic output is asserted. If the trip condition
is true, the trip logic output is asserted.
If the target is enabled for an element, the target reporting function will record a target for all phases that are
above pickup when the protective function trip output is TRUE and the fault recording function trip logic
expression is TRUE. See Section 6,
Reporting and Alarm Functions, Fault Reporting Functions for more
details on the target reporting function. The time overcurrent elements have adaptable targets. If one is set
for directional control, it will report a 67T target (67TA, B, C, N or G). If one is set for non-directional control,
it will report a 51 target.
BESTlogic Settings for Time-Overcurrent Elements
The logic settings for the 51 inverse time-overcurrent functions are provided in Table 4-7. These settings
enable an element by attaching it to the CT input circuits and provide blocking control as determined by the
logic expression assigned to the block input.
