Protection coordination

Protection coordination, also called selectivity or selective coordination, means setting protective devices so that the device closest to a fault clears it and the devices upstream wait. A coordination study takes the device settings, including CT and VT ratios, puts them into a short-circuit model of the system and works out the clearing times for each zone of protection. The results are shown as time-current curves, fuse selectivity ratios or breaker selectivity tables. Some fuse families only need a 2:1 amp-rating ratio between upstream and downstream fuses to be selective. Curves need a time margin between them to allow for timing errors, relay overshoot and CT errors. One relaying seminar gives a typical coordination time of 0.4 s for electromechanical relays and 0.3 s for digital relays. There is a trade-off: keeping upstream breakers selective can make them slow or less sensitive to low arcing currents. Methods such as zone-selective interlocking try to give both fast protection and selectivity. When coordination cannot be complete, the study aims to get as much selectivity as possible while still protecting the equipment. In PV work, a high share of PV on a feeder can change fault current levels, so the utility may need to review existing protection coordination before it approves an interconnection.

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• sentence 1: support check: partial - Passages discuss coordination and fault clearing sequence, but none state the definition that the closest device clears the fault while upstream devices wait, nor the synonyms selectivity.
• sentence 4: support check: partial - The 0.4 s and 0.3 s coordination times and error sources are supported, but the 'relaying seminar' attribution is not in the passages.
• sentence 7: support check: partial - High PV changing fault current and requiring coordination review is supported, but the utility reviewing before approving interconnection is not explicitly stated.
• sentence 1: review: partial - The passages say downstream devices clear first, with the breaker opening before the primary fuse and upstream devices operating only if downstream ones fail, and NETA uses 'selective coordination', but the bare synonym 'selectivity' and the general definition are not…
• sentence 7: review: partial - NREL says high PV penetration can change fault current levels and require a review of protection coordination, but it does not say the review happens before the utility approves an interconnection. -->