Microprocessor-based relay protection operating time

The operating time of a microprocessor-based time-overcurrent relay varies inversely with current magnitude and is adjustable using curve type and time dial settings to ensure proper coordination with other protective devices.

Operating Principles

Microprocessor-based relays, such as the MICRO-51, provide phase and ground overcurrent protection in utility, industrial, and commercial power systems, operating from conventional current transformers without continuous control voltage drain . The time-overcurrent (51) element is used when a deliberate time delay is required to coordinate with downstream or upstream protective devices . The trip time decreases as the fault current increases, following an inverse relationship.

Characteristic Curves and Settings

• Curve Types: Commonly used curves include inverse, very inverse, and extremely inverse, which define how the relay's operating time changes with current magnitude .
• Time Dial: Adjusts the overall time delay of the relay to achieve coordination with other devices. The time dial and curve type are set independently.
• Pickup Setting: Determines the minimum current at which the relay begins timing. Proper selection ensures the relay operates on the most inverse part of the curve for the expected fault range.
• Reset Curves: Microprocessor relays may include timed delay or EM-style resets, which affect subsequent trip times if the relay picks up again before fully resetting .

Coordination with Fuses and Other Devices

For proper protection, the relay must coordinate with fuses and other relays:

• Extremely Inverse Curves: Often used to match the I²t characteristics of fuses, ensuring backup protection over the full current range .
• Minimum TCI (Time Current Interval): A minimum interval of 0.4 seconds is recommended to account for CT saturation, timing errors, and fuse characteristics .
• Primary Pickup Setting: Typically 3–4 times the fuse rating to prevent crossover of relay and fuse curves.

Practical Considerations

Microprocessor relays require initial and periodic testing, though maintenance is minimal due to the lack of moving parts. Self-tests and reporting simplify monitoring, and recommended maintenance intervals are generally every 2 years or as determined by the utility . The operating time can be fine-tuned to balance selectivity, speed, and reliability in the protection scheme. In summary, the operating time of microprocessor-based relays is highly configurable, allowing utilities to achieve precise coordination with other protective devices while maintaining fast and reliable fault clearance.