Refinery Voltage-Sag Ride-Through: Coordinating VFDs, Contactors and Process Permissives

1. Treat ride-through as a chain

A VFD that remains powered does not guarantee that a refinery pump continues running. Its input contactor, control supply, PLC output, instrument permissive or downstream starter can change state before the drive reaches its own undervoltage limit. Define success at the process boundary: acceptable flow interruption, retained motor torque, and an orderly recovery without an unintended restart. A measured terminal-voltage event and a time-aligned sequence of events are more useful than a generic percentage on a purchase specification.

Schneider identifies supply dips and undervoltage-management parameters as relevant to ATV320 undervoltage faults. This supports checking the actual drive manual, not transferring one family's thresholds to another. OEM fault guidance.

2. Gather the event and device envelopes

Request the residual voltage by phase, duration, phase-angle jump and point of measurement. An upstream recorder may miss the extra drop across the transformer and loaded feeder. Separate an external network fault from a local motor start; the remedies and recovery currents differ.

For every device, collect guaranteed hold-up or dropout data at its actual loading and temperature: DC-link capacitance, DC undervoltage threshold, kinetic ride-through behavior, auxiliary-supply autonomy, contactor coil characteristics and restart logic. Identify whether the run command is maintained, latched or pulse-based. Include process shutdown inputs and remote I/O. Emergency shutdown actions remain authoritative; ride-through logic must not mask a valid demand.

3. An illustrative energy check

Assume, solely for screening, a 90 kW drive with 8 mF effective DC capacitance, a pre-event bus of 650 V, and a usable lower boundary of 450 V. Ignore rectifier input during the sag and assume 60 kW constant DC demand. These are hypothetical values, not an Altivar guarantee.

For a hypothetical 150 ms event, stored capacitor energy alone is plainly insufficient. Residual input power or controlled recovery of mechanical energy could alter the result; so could losses and torque demand. The calculation rejects an unsupported capacitor-only claim. It does not predict the actual trip time or authorize changing the lower voltage boundary.

Now suppose a selected control supply has a documented 200 ms hold-up at the installed load, while an AC coil has no guaranteed retention for this event. Keeping the PLC alive cannot resolve that contactor uncertainty. Obtain the coil envelope or a supported control architecture before declaring the chain adequate.

4. Choose the intervention at the weak link

Do not add delays everywhere. A delay can hide loss of supply, prolong an unsafe condition or create conflicting restart permissions. A transformer change helps only if source impedance is a significant cause of the measured event; it cannot remove an upstream sag with an unchanged residual voltage.

5. Recovery is part of the study

Check simultaneous reacceleration, transformer loading, rectifier recharge and motor flying-start behavior. Several healthy drives restarting together can deepen the recovery dip. Assign restart groups using process priorities and supplier capability, then confirm that isolation and shutdown functions still act correctly.

If a contactor is on the drive output, use the manufacturer's approved coordination. Schneider's output-disconnect guidance describes stopping output before the power contacts open. An input contactor and an output contactor have different duties. Output-disconnect guidance.

6. Evidence for acceptance

The procurement package should contain the tested event envelope, drive model and firmware, supported parameter set, contactor and auxiliary-supply data, and a cause-and-effect matrix. Use an agreed controlled test method with the OEM and site team; do not reproduce faults on an operating process bus. Record motor speed, bus voltage, contactor state, run demand and shutdown signals on a common time base. Close each unexplained reset before acceptance.

Relate local starts to the LNG motor-starting study, and assess instrument supply behavior using the UPS selectivity guide.

7. Frequently asked questions and references

Can a UPS on the PLC make the pump ride through? It can preserve controls, but does not supply motor energy or guarantee contactor retention.

Should the undervoltage delay always increase? No. Use the OEM envelope and process risk decision; delay alone cannot create energy.

Does a retained run command mean automatic restart is acceptable? No. The process owner must approve the recovery sequence and permissives.

Primary references: Schneider undervoltage guidance; Schneider output-disconnect guidance.