The relay protection trip value is greater than the set value

A relay may trip at a current higher than its set pickup value due to the combined effects of plug setting multiplier (PSM), time multiplier setting (TMS), and system coordination requirements.Understanding Relay Pickup and SettingsThe pickup current of a relay is the minimum current at which the relay begins to operate. It is determined by the number of active turns in the relay coil and is adjustable via the plug setting. The Plug Setting Multiplier (PSM) represents how many times the actual current exceeds the relay's pickup current and is used to calculate the operating time based on the relay's inverse definite minimum time (IDMT) curve . The Time Multiplier Setting (TMS) adjusts the relay's operating time. A higher TMS increases the delay before tripping, while a lower TMS reduces it. The actual tripping time is calculated as the product of the operating time from the IDMT curve and the TMS .Reasons for Tripping Above Set ValueSystem Coordination: Relays are often set with a margin above normal load to prevent nuisance tripping. For example, the pickup may be set above the maximum load current, so the relay only trips during actual faults .PSM Influence: If the fault current is only slightly above the pickup, the relay may take longer to trip. The relay's IDMT characteristic means that the higher the fault current relative to the pickup, the faster it operates. Therefore, a relay may appear to trip at a higher current than the nominal pickup due to the time-current relationship .CT Accuracy and Ratio: The current transformer (CT) secondary current may differ slightly from the actual primary current due to CT ratio errors or saturation, causing the relay to see a slightly higher or lower current than expected .Mechanical and Electrical Delays: Electromechanical relays have inherent mechanical delays, and numerical relays may include intentional time delays for selectivity. These delays can result in the relay tripping after the current exceeds the pickup value .Load and Inrush Currents: In transmission lines or transformers, inrush or charging currents may temporarily exceed the pickup without indicating a fault. Relays are set to ignore these conditions, effectively raising the apparent trip threshold .Practical ImplicationsEnsure PSM and TMS are correctly coordinated with upstream and downstream relays to maintain selectivity.Verify CT ratios and accuracy to ensure the relay sees the correct current.Consider load margins and inrush currents when setting pickup values to avoid nuisance trips.Use IDMT curves to predict actual operating times and currents for different fault levels. By understanding these factors, engineers can explain why a relay trips above its nominal pickup and adjust settings to balance protection dependability and system security.