Worse still, the most common protection techniques for overhead distribution lines, overcurrent and earth fault protection, cannot detect this scenario. This leaves detection up to eyewitness account or complaints of downstream brownout - a risky proposition.

However, there is a protection technique that addresses this safety risk in a cost effective and simple manner. This is negative phase sequence protection.

Essentially, Negative Phase Sequence Current is a single value measure of imbalance. And unlike Earth Fault Protection that requires a return path through earth, negative phase sequence looks at the imbalance across the three phases and determines whether a line is broken, or in technical jargon, “has a discontinuity”.

This gives Negative Phase Sequence protection the unique advantage of load indifference. No matter how much, or how little, current is flowing through three phases, NPS is measuring the imbalance rather than the absolute values. This allows NPS to detect broken conductors across the range of normal feeder operation, irrespective of load demands.

Implementing NPS Protection

Three phase protection devices on the overhead lines that are capable of vector calculations, such as pole mounted Auto-Reclosers, can read the currents on all three phases and derive the negative sequence current.

By adding Negative Phase Sequence protection to the conventional Overcurrent and Earth Fault protection implemented on most feeders, we can greatly reduce the protection blind spot (jargon: non-detection zone) of broken conductors.

Around the world, many utilities have implemented NPS protection on their distribution lines to mitigate this safety issue. A detailed paper on the calculations of these values is available for you. Visit www.nojapower.com.au