7SR18 Applications Guide
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Figure 2.3-2 Blocking Scheme Using Instantaneous Over-current Elements
Typically a time delay as low as 50 ms on the incomer 50-1 element will ensure that the incomer is not tripped for
outgoing circuit faults. However, to include for both equipment tolerances and a safety margin a minimum time
delay of 100 ms is recommended.
This type of scheme is very cost effective and provides a compromise between back-up over-current busbar
protection and dedicated schemes of busbar protection.
2.4 Negative Phase Sequence Over-current (46NPS)
The presence of Negative Phase Sequence (NPS) current indicates an unbalance in the phase currents, either
due to a fault or unbalanced load.
NPS current presents a major problem for 3-phase rotating plant. It produces a reaction magnetic field which
rotates in the opposite direction, and at twice the frequency, to the main field created by the DC excitation system.
This induces double-frequency currents into the rotor which cause very large eddy currents in the rotor body. The
resulting heating of the rotor can be severe and is proportional to (I
2
)
2
t.
Generators and Motors are designed, manufactured and tested to be capable of withstanding unbalanced current
for specified limits. Their withstand capability is specified in two parts; continuous capability based on a figure of
I
2
, and short time capability based on a constant, K, where K = (I
2
)
2
t. NPS over-current protection is therefore
configured to match these two plant characteristics.
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