Advances in Protective Device Interlocking for Improved Protection and Selectivity

The challenge for protection engineers is to balance protection and system reliability (selectivity). Engineers may bias toward protection at the cost of system reliability or toward desired (or code required) selectivity at the cost of protection. The need for compromise is often accepted as an unalterable reality. Simultaneously, the need for greater personnel safety has added a desire for space protection in addition to the usual bus protection imperative. The industry has provided devices, techniques, and schemes to minimize, or even eliminate, this compromise. Tools include smarter instantaneous sensing algorithms, differential protection, zone-selective interlocking, and arc-flash relays. Zone-selective interlocking is a cost-effective solution for low-voltage (LV) and medium-voltage (MV) applications. Today, zone-selective interlocking spans different voltages between inverse protection characteristics, such as short-time slopes and fast instantaneous protection, in both LV and MV systems. However, traditional schemes must still nest pickup current thresholds to accommodate sensing tolerances. Arc-flash relays have emerged as excellent space protectors but present concerns in LV applications. This paper reviews recent improvements in zone-selective interlocking capabilities and presents schemes, which are developed by the authors, that eliminate the need for nesting pickup levels of multiple devices and for successfully mixing arc-flash and current sensing devices into one system to simultaneously provide fast, reliable, and sensitive protection.