Inrush Detector (81Hbl2); Broken Conductor / Load Imbalance (46Bc) - Siemens 7SR10 Argus Series Manual

5.5

Inrush Detector (81HBL2)

This element detects the presence of high levels of 2nd Harmonic current which is indicative of transformer Inrush
current at switch-on. These currents may be above the operate level of the overcurrent elements for a short
duration and it is important that the relay does not issue an incorrect trip command for this transient network
condition.
If a magnetic inrush condition is detected operation of the overcurrent elements can be blocked.
Calculation of the magnetising inrush current level is complex. However a ratio of 20% 2
Fundamental current will meet most applications without compromising the integrity of the Overcurrent protection.
There are 3 methods of detection and blocking during the passage of magnetising inrush current.
Phase Blocking only occurs in those phases where Inrush is detected.
Large, Single Phase Transformers – Auto-transformers.
Cross All 3-phases are blocked if Inrush is detected in any phase.
Traditional application for most Transformers but can give delayed operation for Switch-
on to Earth Fault conditions.
Sum
Composite 2nd Harmonic content derived for all 3-phases and then compared to
Fundamental current for each individual phase.
Provides good compromise between Inrush stability and fast fault detection.
5.6

Broken Conductor / Load Imbalance (46BC)

Used to detect an open circuit condition when a conductor breaks or a mal-operation occurs in phase segregated
switchgear.
There will be little or no fault current and so overcurrent elements will not detect the condition. However the
condition can be detected because there will be a high content of NPS (unbalance) current present.
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.
© 2018 Siemens Protection Devices Limited
Table 5-4 Magnetic Inrush Bias
7SR10, 7SR11, 7SR12 Applications Guide
nd
Chapter 7 Page 40 of 41
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