1. Plug Setting Multiplier (PSM) PSM indicates how many times the relay secondary current exceeds the relay pickup current, typically measured at the CT secondary. It is crucial for inverse definite minimum time (IDMT) relays, as it determines the basic operating time according to the relay's characteristic curve. A higher PSM results in faster tripping, while a lower PSM slows the relay operation. PSM settings must comply with IEC 60255-151 standards for overcurrent relays to ensure reliable performance under fault conditions . 2. Time Setting Multiplier (TSM) TSM scales the base operating time derived from the relay's characteristic curve. It is used to achieve proper coordination between upstream and downstream relays, allowing downstream relays to trip faster while upstream relays provide backup. Lower TSM values result in faster tripping, and higher TSM values delay operation to maintain selectivity. TSM selection follows IEC 60255-3, which defines standard inverse, very inverse, and extremely inverse time-current characteristics .
Grading time is the intentional time difference between consecutive protection stages to ensure selectivity. Proper grading prevents unnecessary tripping of upstream relays and limits supply interruption to the smallest possible area. Inverse time relays generally require longer grading times due to measurement inaccuracies and fault current variations. Selectivity diagrams, which plot time-current curves for all relays in a chain, are used to determine appropriate grading times and ensure reliable backup protection .
Relay protection aims to quickly isolate faulty sections while maintaining system stability. Relays must be reliable, sensitive, and fast, discriminating between normal operating conditions and faults. The protection system may include overcurrent, differential, distance, directional, and special relays, each with specific settings and coordination requirements. Numerical relays allow precise calculation of operating times and grading, improving selectivity and reducing post-fault disturbances .
The relay protection setting mechanism combines PSM, TSM, and grading time to achieve speed, selectivity, and coordination. Proper configuration ensures that faults are cleared quickly, minimizes damage, and maintains continuity of supply in the unaffected parts of the network. Standards like IEC 60255 guide the selection and calculation of these settings to ensure safe and reliable operation .
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