ABB RER620 Technical Manual page 98

Section 4
Protection functions
92
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 practically zero. Also, the effects of switching
inrush currents on the setting values can be reduced by using the relay logic, which
recognizes the transformer energizing inrush current and blocks the operation or multiplies
the current pickup 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 General function
block features section 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 12 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 51P operates in IDMT-mode and the two higher stages 50P-1/2
and 50P-3 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 pickup value multiplying input of the instantaneous stage. In this way, the
pickup value is multiplied with a predefined setting during the inrush situation and
nuisance tripping can be avoided.
1MAC309294-MB F
RER620
Technical Manual