Page 53 - Energize September 2021 HR

TECHNICAL

Figure 6: Simplified equivalent circuit of Figure 5

negative sequence impedance, making the ratio non-zero, but in
the absence of load impedance information it is better to empirically
evaluate field data before applying the 46BC feature. Prior service
suggests that a 20% ratio is sufficient on four-wire networks
for sensitivity but is not immune to spurious tripping in highly
unbalanced networks.

Conclusion
Broken conductor detection in the distribution grid is a scenario
which can be suitably detected using the negative phase sequence
current. When using NPS alone, knowledge of the negative
sequence impedance of a load is necessary for appropriate
Figure 5: Equivalent circuit for a broken conductor on a four-wire three- calculation. A special case exists for three-wire networks, where
phase network the positive and negative sequence load impedances cancel out
in a phase discontinuity scenario, allowing utilities with three-
The key difference between the four-wire and three-wire systems is wire distribution networks to use ANSI 46 broken conductor
the inclusion of the zero sequence in the resultant calculation. protection in the absence of load impedance data.
Calculating for i 1: In four-wire systems, the calculation becomes marginally
more complex, but can be approximated by empirical data, giving
engineers visibility of the potentially hazardous broken conductor
scenario.
NOJA Power’s OSM Recloser system is equipped with a full
And assuming that i 2 is negative and acknowledging the topology as suite of negative phase sequence and broken conductor protection
a current divider: elements, allowing utilities to capitalize on the benefits of their
existing asset base and provide sensitivity to this dangerous
category of fault for a negligible expenditure. n

Acknowledgement
This article was first published in Australasian Power Technologies
Publications’ 2020 Purchasing Directory and is republished here with
permission.
To calculate the minimum broken conductor ratio in a four-wire
system, we need to know both the zero and negative sequence load Contact Martin Van Der Linde, NOJA Power,
impedance. As a rule, there is more zero sequence impedance than martinv@nojapower.com.au, www.nojapower.com.au

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