Selecting Undervoltage Relays for Refineries and LNG

After the undervoltage relay is applied correctly, a refinery or LNG electrical system disconnects vulnerable equipment or sheds selected loads before sustained low voltage causes motor overheating, stalled drives, unstable control power, or a wider bus collapse. The relay detects the condition; the protection design must define which equipment trips, which loads shed, and which processes remain energized. Treat the relay as one element of a coordinated voltage-protection scheme, not as a universal trip device.

Symptom interpretation

An undervoltage relay, identified by device function 27, operates when its measured voltage falls below a configured threshold for the configured delay. The term here means voltage below the acceptable operating range of the connected equipment, not merely a momentary numerical dip.

Measure voltage at both the protected bus and the equipment terminals. A healthy upstream bus with low motor-terminal voltage points toward the feeder, switching device, or connection rather than a system-wide undervoltage event.

Voltage-response mechanism

Induction-motor torque capability decreases sharply as terminal voltage falls. A mechanically loaded motor can therefore slow while attempting to produce the required torque. Slip and current may increase, raising motor heating even though the supply voltage is lower. If available torque falls below load torque, the motor can stall.

Small motors commonly disappear from the bus because their contactors release when control voltage falls below the coil's holding capability. That incidental dropout may protect the motor, but it is not a coordinated undervoltage scheme. Large motors, latched switching devices, variable-speed drives, and control systems may react differently, so one bus-voltage threshold cannot be assumed to protect every load.

Undervoltage load shedding addresses a second mechanism: too much connected load can prevent a weak or disturbed source from restoring voltage. Removing selected loads reduces demand and can allow the remaining electrical system to recover. The scheme must distinguish between a short, tolerable dip and a sustained condition that requires action; pickup threshold and time delay provide that discrimination.

Refinery and LNG protection objectives

In refinery and LNG service, the consequence of a voltage disturbance depends on the process duty. Loss of a nonessential motor is different from loss of lubrication, cooling, ventilation, control power, or equipment needed for an orderly shutdown. Classify loads before assigning relay outputs.

The design decision is therefore not simply whether to install a 27 relay. Decide what low-voltage condition constitutes unacceptable operation, how long each load can tolerate it, and what system state must follow relay operation.

Relay application procedure

1. Define the protected zone. Identify the bus, feeder, motor, or control-power circuit represented by the relay voltage input. Confirm that the sensing point remains representative during every permitted switching configuration.
2. Inventory load responses. Record which contactors release, which drives inhibit or trip, which large motors remain connected, and which loads can be shed without defeating essential process functions.
3. Obtain voltage-time limits. Read the allowable operating and ride-through limits from the equipment documentation. Use the contactor coil data, motor protection requirements, drive settings, and control-power limits applicable to the installed equipment.
4. Select the action. Assign direct motor trip, staged load shedding, alarm, or another defined control action. Keep essential and nonessential loads in separate priority groups.
5. Coordinate threshold and delay. Set the relay from the approved electrical study so normal voltage variation does not cause operation, while a damaging or nonrecoverable condition does. Coordinate the delay with upstream fault clearing and equipment ride-through capability.
6. Define reset and restoration. Require stable recovered voltage before reset. Sequence motor restarts so simultaneous acceleration does not create another voltage depression.
7. Document dependencies. Record breaker status, bus configuration, permissives, interlocks, bypasses, and process shutdown signals that can enable, block, or alter the undervoltage action.

Commissioning verification

1. Check 1: voltage input. Expect the relay indication to agree with a calibrated measurement at the defined sensing point, including the correct phase quantities used by the scheme.
2. Check 2: threshold response. Lower the test voltage through the configured pickup value. Expect 27 pickup at the approved setting within the relay's stated accuracy.
3. Check 3: timing. Hold the test voltage below pickup. Expect the output to operate after the configured delay, not during the intentional ride-through interval.
4. Check 4: output path. Prove the complete circuit from relay output through logic, interposing devices, trip coils, and load-shed commands. Expect only the assigned equipment to change state.
5. Check 5: reset behavior. Restore voltage above the configured reset level. Expect the relay to reset according to the selected reset mode without initiating an uncontrolled restart.
6. Check 6: operating sequence. Test the permitted bus configurations and staged actions. Expect voltage recovery logic, alarms, event records, interlocks, and restart sequencing to match the approved cause-and-effect documentation.

Recurring application pitfalls

Wrong practice starts with treating every low-voltage indication as a reason to trip everything. Broad simultaneous tripping can remove essential auxiliaries, complicate process recovery, and create a large coincident restart demand.

Another recurring error is relying on contactor dropout as motor protection. Coil behavior varies with the actual control voltage and device characteristics; it also provides no deliberate load priority or coordinated delay. Conversely, delaying a motor trip without checking its voltage-time capability can leave a heavily loaded motor stalled and heating.

Potential-transformer circuit problems can imitate a genuine bus undervoltage. A lost fuse, open conductor, incorrect ratio configuration, or phase-selection error may make the relay operate while the power bus remains healthy. Use phase-resolved measurements and supervision logic appropriate to the measurement arrangement.

Finally, test restoration as carefully as shedding. A bus that recovers after loads drop can collapse again if all motors restart together. Use permissives and a defined restart sequence based on source capacity and process priority.

Frequently asked questions