Advances in protective device interlocking for improved protection and selectivity

The protection engineer´s challenge is balancing protection and system reliability (selectivity). Engineers may bias towards protection at the cost of system reliability or towards 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, and 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 low 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, developed by the authors, which 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.