ABB RELION 611 Series Technical Manual page 104

Section 4
Protection functions
98
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 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 the Measurement modes
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 42
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. In this way the start value is multiplied with a
predefined setting during the inrush situation and nuisance tripping can be avoided.
chapter in this manual.
shows an example of this. A brief coordination study has
1MRS757454 D
611 series
Technical Manual