ABB RELION REX640, Техническое руководство, Страница: 336 / 1960

example, a grading margin of 150 ms in the DT mode of operation can be used,
provided that the circuit breaker interrupting time is shorter than 60 ms.
The sensitivity and speed of the current-selective stages become as good as
possible due to the fact that the transient overreach is very low. Also, the effects
of switching inrush currents on the setting values can be reduced by using the
protection relay's logic, which recognizes the transformer energizing inrush current
and blocks the operation or multiplies the current start value setting of the selected
overcurrent stage with a predefined multiplier setting.
Finally, a dependable trip of the overcurrent protection is secured by both a proper
selection of the settings and an adequate ability of the measuring transformers
to reproduce the fault current. This is important in order to maintain selectivity
and also for the protection to operate without additional time delays. For
additional information about available measuring modes and current transformer
requirements, see
Chapter 11.6 Measurement modes
in this manual.
Radial outgoing feeder overcurrent protection
The basic requirements for feeder overcurrent protection are adequate sensitivity
and operation speed taking into account the minimum and maximum fault current
levels along the protected line, selectivity requirements, inrush currents and the
thermal and mechanical withstand of the lines to be protected.
In many cases the above requirements can be best fulfilled by using multiple-stage
overcurrent units.
Figure 203
shows an example of this. A brief coordination study
has been carried out between the incoming and outgoing feeders.
The protection scheme is implemented with three-stage numerical overcurrent
protection, where the low-set stage PHLPTOC operates in IDMT-mode and the two
higher stages PHHPTOC and PHIPTOC in DT-mode. Also the thermal withstand
of the line types along the feeder and maximum expected inrush currents of the
feeders are shown. Faults occurring near the station where the fault current levels
are the highest are cleared rapidly by the instantaneous stage in order to minimize
the effects of severe short circuit faults. The influence of the inrush current is
taken into consideration by connecting the inrush current detector to the start
value multiplying input of the instantaneous stage. By this way the start value
is multiplied with a predefined setting during the inrush situation and nuisance
tripping can be avoided.
Protection functions 1MRS759142 F
336 REX640
Technical Manual