Relay protection time setting determines how quickly a relay operates in response to a fault, using Time Setting Multiplier (TSM) and Plug Setting Multiplier (PSM) to achieve selective and coordinated tripping.

Key Concepts

1. Plug Setting Multiplier (PSM): PSM represents the ratio of the fault current to the relay's pickup current. It indicates how many times the actual current exceeds the relay's set pickup current and is critical for determining the relay's operating time on inverse time characteristics. A higher PSM results in faster relay operation, while a lower PSM slows the trip, allowing coordination with downstream relays ( ). 2. Time Setting Multiplier (TSM): TSM scales the base operating time obtained from the relay's characteristic curve for a given PSM. By adjusting TSM, engineers can control the relay's trip time to ensure proper coordination between upstream and downstream relays. Lower TSM values produce faster tripping, while higher TSM values delay operation to provide backup protection ( ). 3. Actual Operating Time Calculation: The actual relay operating time is calculated as: Operating Time = Base Time from PSM Curve × TSM This ensures that the relay trips at the correct time for the expected fault current, maintaining selectivity and minimizing unnecessary outages ( ).

Relay Coordination Principles

• Time Grading: Relays are set with incremental operating times along a feeder so that the relay closest to the fault operates first. This prevents upstream relays from tripping unnecessarily ( ).
• Inverse Time Characteristics: For overcurrent relays, the operating time decreases as fault current increases, which is particularly useful in radial networks where fault currents vary significantly ( ).
• Definite-Time Relays: These relays operate after a fixed time delay, independent of fault current magnitude, and are often used in combination with inverse-time relays for selective protection ( ).

Practical Considerations

• When replacing current transformers (CTs), the PSM may need adjustment to maintain the same pickup current without changing TSM ( ).
• Coordination studies require knowledge of maximum and minimum fault currents, load currents, transformer inrush, and motor starting characteristics to set TSM and PSM correctly ( ).
• Proper relay time settings ensure that only the faulty section is isolated, minimizing disruption to the rest of the network ( ). By carefully selecting PSM and TSM, engineers can achieve reliable, selective, and coordinated protection, ensuring that relays operate efficiently under fault conditions while maintaining system stability.