Relay protection inverse time limit value

The inverse time limit value of a relay defines the minimum current at which the relay begins to operate and the operating time decreases as the fault current increases, typically starting at 1.1–1.3 times the pickup current depending on the relay characteristic.

Overview of Inverse Time Overcurrent Relays

Inverse time overcurrent relays (IDMT relays) are designed to operate faster for higher fault currents and slower for moderate overloads, providing coordination with upstream and downstream protective devices. The operating time is inversely proportional to the magnitude of the current exceeding the relay's pickup value, which is calculated as the product of the current transformer (CT) secondary rating and the relay current setting ( ).

Pickup Current and Time Dial

• Pickup Current (I_pickup): The minimum current at which the relay starts to operate. Proper selection ensures the relay does not trip during normal load but is sensitive enough to detect faults ( ).
• Time Dial Setting (TDS): Adjusts the operating time along the relay's inverse time characteristic curve. Higher dial values increase the operating time, allowing coordination with other relays ( ).

Inverse Time Limit Values

• For very or extremely inverse relays, the relay typically starts operating when the current exceeds 1.3 times the set pickup current.
• For long-time inverse relays, the relay starts at 1.1 times the set pickup current ( ).
• The operating time decreases as the fault current increases, following the relay's characteristic curve, which can be standard IEEE (U5, SI, EI) or IEC (C1–C5) curves ( ).

Practical Calculation

1. Determine the fault current at the relay location.
2. Calculate the multiple of pickup current: $M=\frac{I_{fault}}{I_{pickup}}$.
3. Use the relay's inverse time equation or standard curve to find the operating time $t$ for the given multiple $M$ and time dial setting.
4. Ensure coordination with upstream relays by adding a margin time to the operating time of downstream relays ( ).

Standards and References

• IEC 60255-151 defines standard inverse time overcurrent curves and the point where the curve transitions to a definite time characteristic ( ).
• IEEE standards provide equations for electromechanical and microprocessor relays to ensure proper coordination under varying current conditions ( ).

Summary

The inverse time limit value is a critical parameter for relay protection, ensuring that the relay operates only when the current exceeds a threshold slightly above the pickup current (1.1–1.3× I_pickup) and that the operating time decreases with increasing fault current. Proper selection of pickup current, time dial, and coordination with other relays ensures selective and reliable protection of the electrical network ( ).