Generator Trip on Reactive Power (32Q)

Equipment : 5 MW Synchronous Generator

Protection Relay : ABB REG630

Incident : Unexpected tripping during transition from Islanding to Grid Synchronisation.

# The Scenario

The facility operates a 5 MW generator. The electrical system includes a multi-stage Capacitor Bank intended for power factor correction (PFC).

• Islanding Mode : The generator serves the local load. Capacitor banks are in fixed compensation, maintaining a stable PF 0.85.

• Synchronization Mode : The system transitions to the grid. The capacitor banks are supposed to switch to automatic mode, tracking the grid's reactive requirements.

# The Incident

During a nighttime synchronisation sequence, the generator successfully paralleled with the grid. However, shortly after synchronisation, the ABB REG630 relay issued a 32Q trip signal, shutting down the generator.

# Technical Root Cause Analysis

The failure occurred due to a Reactive Power Surplus during the transition.

• The "Nighttime" Variable : At night, plant inductive loads (motors/machinery) usually decrease.

• Capacitor Over-compensation : The capacitor banks remained "ON" at a level higher than the actual demand.

• The Physics : When the generator is tied to a strong grid and there is excess capacitive VARs in the system, the system voltage rises. To counteract this and keep the terminal voltage stable, the Generator’s Automatic Voltage Regulator (AVR) reduces excitation.

• The Trip : If the external capacitors provide more reactive power than the load needs, the generator is forced to sink (absorb) reactive power. Once this absorption exceeded the setpoint in the REG630 (Reverse VARs), the 32Q protection operated to prevent the generator from heating up or losing synchronism (becoming an induction generator).

Parameter                                          Islanding Mode (Fixed Caps)                        Synchronisation (Excess Caps)

Reactive Source                                Generator + Fixed Caps                                        Grid + Over-active Caps

Gen Status                                         Lagging (Providing VARs)                                      Leading (Absorbing VARs)

AVR Action                                         Boosting Excitation                                                Cutting Excitation (Under-excited)

Relay State                                         Normal                                                                   32Q Threshold Violated

# Corrective Actions & Recommendations

To prevent a recurrence, the following technical adjustments are recommended:

• PFC Controller Logic : Modify the Capacitor Bank Controller logic to ensure it remains "OFF" or at a minimum stage before the breaker closes for synchronisation, only ramping up after the grid connection is stable.

• REG630 Settings Review : Verify the 32Q settings. Ensure the time delay is sufficient to ride over momentary switching transients, without risking the machine's stability.

• Interlocking : Implement an electrical interlock that prevents synchronisation if the reactive power compensation is in "Manual/Fixed" mode.

• Nighttime Setpoints : Program the PFC controller with a "Low Load" profile to prevent over-compensation during off-peak hours.

# Summary for the Customer

The trip was not a relay malfunction, but a successful protection operation. The ABB REG630 saved the generator from an unstable under-excited state caused by the capacitor banks providing more reactive power than the grid and load required at that specific time.