Selectivity Study_Electrical Power Protection IEC60947-364

Understanding Selectivity in Power Systems

Selectivity is a critical concept in power system protection. Achieving complete selectivity ensures that protective devices minimize the impact of faults like short circuits or other undesirable events. The goal is to isolate and shut down only the absolute minimum portion of the power system necessary.

The definitions of total selectivity and partial selectivity are detailed in Part 2 of the IEC 60947-2 standard ("Low voltage Equipment – Part 2: Circuit-breakers"). Let's break them down:

• Total Selectivity: (2.17.2) This refers to overcurrent selectivity where the load-side protection device operates and clears the fault without causing the upstream protection device to trip. This is the ideal scenario for minimizing disruption.
• Partial Selectivity: (2.17.3) Also related to overcurrent selectivity, partial selectivity allows the load-side device to operate up to a specific overcurrent level without tripping the upstream device. This provides a degree of coordination but may involve a small, controlled outage.

Overload Zone vs. Short-Circuit Zone

Protective devices are designed to operate within specific zones based on the type of fault:

• Overload Zone: This is the range where thermal protection (for thermal-magnetic trip units) and Class L protection (for electronic trip units) are typically activated. These devices protect against sustained overloads.
• Short-Circuit Zone: This zone is where magnetic protection (for thermal-magnetic trip units) and Class S, D, and I protection (for solid-state trip units) are normally triggered. These devices respond rapidly to high-current short circuits.

Selectivity Techniques & Operational Considerations

Various selectivity techniques are employed to achieve the desired protection coordination. Understanding these techniques from an operational point of view is crucial for effective power system design and maintenance. These techniques ensure that the correct protective device operates for a given fault, minimizing unnecessary outages and maximizing system stability.