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7SR11 & 7SR12 Applications Guide
However if the line is on light load, the negative phase sequence current may be very close to, to less than the full
load steady state unbalance arising from CT errors, load unbalance etc. This means a simple negative phase
sequence element would not operate.
With such faults a measurable amount of zero sequence current will be produced, but even this will not be
sensitive enough.
To detect such a fault it is necessary to evaluate the ratio of negative phase current (NPS) to positive phase
current (PPS), since the ratio is approximately constant with variations in load current and allows a more sensitive
setting to be achieved.
In the case of a single point earthed system, there will be little ZPS current and the ratio of NPS/PPS in the
protected circuit will approach 100%
In the case of a multiple earthed system (assuming equal impedances in each sequence network) an NPS / PPS
ratio of 50% will result from a Broken Conductor condition. This ratio may vary depending upon the location of the
fault and it is therefore recommended to apply a setting as sensitive as possible.
In practice, this minimum setting is governed by the levels of standing NPS current present on the system. This
can be determined from a system study or measured during commissioning making sure it is measured during
maximum system load conditions to ensure all single phase loads are included.
Operation is subject to a time delay to prevent operation for transitory effects, a minimum delay of 50sec may be
recommended.
5.6.1
Broken Conductor example
Information recorded during commissioning:
I full load = 500A
I NPS = 50A
Therefore the ratio is given by 50/500 = 0.1
To allow for tolerances and load variation a setting of 200% of this value is recommended and therefore the ratio
for 46BC setting should be set at 20%.
To allow for adequate time for short circuit fault clearance by time delayed protection the 46BC delay should be
set to 50seconds.
To ensure the broken conductor protection does not operate incorrectly during low load conditions, where the
three phases are less than 10% of normal load, the element should be inhibited by setting the 46BC U/C
Guarded to Yes and selecting a 46BC U/C Guard Setting to 0.1 x In
5.7
Circuit-Breaker Maintenance
The Relay provides Total, Delta and Frequent CB Operation Counters along with an I
amount of wear and tear experienced by a Circuit-Breaker. Alarm can be provided once set levels have been
exceeded.
Typically estimates obtained from previous circuit-breaker maintenance schedules or manufacturers data sheets
are used for setting these alarm levels. The relay instrumentation provides the current values of these counters.
Page 48 of 48
2
t Counter to estimate the
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