Table 366: Proposed functionality of numerical transformer and busbar overcur-
rent protection. DT = definite time, IDMT = inverse definite minimum time
O/C-stage
Operating char.
Selectivity mode Operation speed
Sensitivity
HV/3I>
DT/IDMT
time selective
low
very high
HV/3I>>
DT
blockable/time
selective
high/low
high
HV/3I>>>
DT
current selective
very high
low
LV/3I>
DT/IDMT
time selective
low
very high
LV/3I>>
DT
time selective
low
high
LV/3I>>>
DT
blockable
high
high
In case the bus-tie breaker is open, the operating time of the blockable overcurrent
protection is approximately 100 ms (relaying time). When the bus-tie breaker is
closed, that is, the fault current flows to the faulted section of the busbar from two
directions, the operation time becomes as follows: first the bus-tie relay unit trips
the tie breaker in the above 100 ms, which reduces the fault current to a half. After
this the incoming feeder relay unit of the faulted bus section trips the breaker in
approximately 250 ms (relaying time), which becomes the total fault clearing time in
this case.
Figure 202: Numerical overcurrent protection functionality for a typical sub-
transmission/distribution substation (feeder protection not shown). Blocking
output = digital output signal from the start of a protection stage, Blocking in =
digital input signal to block the operation of a protection stage
The operating times of the time selective stages are very short, because the grading
margins between successive protection stages can be kept short. This is mainly due
to the advanced measuring principle allowing a certain degree of CT saturation,
good operating accuracy and short retardation times of the numerical units. So, for
1MRS759142 F
Protection functions
REX640
Technical Manual
335