Relay protection undervoltage setting

Undervoltage protection relays (ANSI 27) safeguard electrical equipment by tripping circuits when voltage drops below preset thresholds, typically using two-stage settings at 90% and 85% of rated voltage.

Purpose of Undervoltage Protection

Undervoltage protection prevents damage to motors, generators, transformers, and other industrial loads caused by low-voltage conditions. When voltage drops, equipment draws higher currents, leading to overheating, insulation damage, and reduced operational life. The undervoltage relay detects these conditions and trips the circuit breaker or triggers an alarm to protect the system.

Working Principle

Undervoltage relays monitor phase-to-phase or phase-to-neutral voltages using potential transformers (PTs). When the measured voltage falls below a preset threshold for a defined time, the relay issues a trip command. Two common types of undervoltage protection are: ANSI 27-1: Monitors each phase independently; trips if any phase drops below the threshold.

ANS

I 27-2: Monitors all three phases together; trips only if all phases fall below the threshold.

Typical Settings

Undervoltage relays are usually configured with two-stage tripping: Stage 1: Trip at 90% of rated voltage, typically for grid circuit breakers. Stage 2: Trip at 85% of rated voltage, typically for generator circuit breakers. Time delay is applied to avoid nuisance tripping due to transient voltage dips.

Typical time delays range from 0. 1 to 10 seconds depending on system requirements. Hysteresis is often fixed at around 98% to prevent relay chatter.

For motors, the voltage setting should be based on the level at which the motor would stall or draw excessive current. A common practical setting is 0. 8 Vn with a 2-second delay for sensitive applications, balancing protection and operational continuity.

Coordination Considerations

Ensure relay settings do not cause unnecessary plant shutdowns. Stage 1 should protect against temporary grid voltage dips, while Stage 2 protects the generator or critical equipment. Verify settings with generator excitation systems, as undervoltage can also result from excitation failure.

Adjust settings according to bus voltage levels (e.g., 400V, 6. 6kV, 33kV) and the presence of backup generators.

Summary

Undervoltage relays (ANSI 27) are essential for protecting electrical equipment from low-voltage damage. Proper configuration involves: Selecting voltage thresholds (typically 90% and 85% of rated voltage). Setting appropriate time delays to avoid false trips.

Coordinating with other protection devices and system voltage levels. Considering equipment-specific requirements, such as motor stall voltage or generator excitation conditions. These settings ensure reliable protection while minimizing unnecessary downtime and equipment stress.