ABB RELION REX640 Technical Manual page 335

Protection and control

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1MRS759142 F

Table 366: Proposed functionality of numerical transformer and busbar overcur-

rent protection. DT = definite time, IDMT = inverse definite minimum time

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

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

Technical Manual

Operating char.

Selectivity mode

time selective

blockable/time

current selective

time selective

Protection functions

Operation speed