Open Circuit (46Bc); Negative Phase Sequence Overcurrent (46Nps); Undercurrent (37) - Siemens 7SR242 Duobias Technical Manual

2.5

Open Circuit (46BC)

Used to detect an open circuit condition e.g. an OLTC failure.
There will be little or no fault current and so the differential protection elements will not detect the condition.
However the condition can be detected because there will be a high content of NPS (unbalance) current present.
An NPS / PPS ratio > 50% will result from an open circuit condition.
A time delay can be applied to prevent operation for transitory effects.
2.6

Negative Phase Sequence Overcurrent (46NPS)

The Negative Phase Sequence (NPS) over current is intended to be used to detect uncleared system faults and
conditions such as broken primary connections that may produce significant NPS current.
This unbalance may cause rotating plant such as generators or motors to overheat and fail.
This may also be used to monitor the state of the tap changer and alarm for faults with diverter resistors or
switches.
Typical Settings are 5 to 10% for Tap Changer alarm and 10 to 15% for system fault or broken conductor.
2.7

Undercurrent (37)

Where current decreases beneath defined levels this can indicate low load or CB open conditions, it can also be
used to indicate that no current is flowing.
The undercurrent function is used:
As a fault current check i.e. that no fault current continues to flow and that an auto-isolation sequence
may safely be initiated.
As a check that a CB has opened – this can be used in addition to or in place of CB auxiliary switch
indications.
©2015 Siemens Protection Devices Limited Chapter 7 Page 32 of 64