ABB RELION 620 Series Technical Manual page 257

1MRS757644 F
620 series
Technical Manual
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 using the IED 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 maintaining selectivity and also for
the protection to operate without additional time delays. Additional information about
available measuring modes and current transformer requirements can be found in the
General function block features
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.
Often the above requirements can be best fulfilled using multiple-stage overcurrent
units.Figure 113 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 PH3LPTOC operates in the IDMT-mode and the
two higher stages, PH3HPTOC and PH3IPTOC, in the DT-mode. Also the thermal
withstand of the line types along the feeder and the 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 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. This way, the start value is multiplied with a
predefined setting during the inrush situation, and nuisance tripping can be avoided.
section in this manual.
Section 4
Protection functions
251