Whole-line continuity

Industrial Voltage Sag Protection

Industrial voltage sag protection keeps a critical load operating when supply voltage falls below its acceptable window for a short time.

Problem boundaryVoltage remains present but drops below equipment tolerance.
Protection scopeSingle drive, grouped VFDs, or a whole critical feeder.
First decisionMeasure depth, duration, phase behavior, and actual trip sequence.

How it works

Connect the disturbance to the protected boundary.

A voltage sag is a reduction in RMS voltage for a limited duration. Equipment does not respond only to the lowest voltage: phase involvement, duration, DC-link energy, control power, protection thresholds, and process interlocks all influence the outcome.

Whole-line protection commonly injects the missing voltage in series with the supply. If the sag becomes very deep or approaches zero volts, the system must also supply real energy for the required ride-through interval.

Application fit

When it fits—and when it does not.

When it fits

  • VFD-driven motors trip during short grid disturbances.
  • A mixed-load production line must remain energized as a system.
  • A feeder supplies drives, controls, and auxiliary equipment with different tolerance curves.
  • Recurring events justify a measured engineering assessment rather than repeated restarts.

When it does not fit

  • Do not select a sag compensator before confirming the event that causes the trip.
  • Long backup times normally require a battery UPS, Battery Energy Storage System (BESS), generator, or hybrid architecture.
  • A pure reactive-power issue or motor-starting problem may call for a STATCOM, SVC, soft starter, network change, or process redesign.
  • No architecture should be described as 100% protection without a defined voltage-duration envelope and tested load behavior.

Architecture selection

Choose the Protection Boundary

WS-DVR — Whole-Line AC Protection

Protect a critical feeder when several types of sensitive loads must remain energized together.

Explore WS-DVR →

VFD DC-Bus Support — Drive-Level Protection

Protect VFD-driven equipment directly at the DC link when the production risk is concentrated in accessible drives.

Explore VFD DC-Bus Support →

Power Quality Assessment

Measure the disturbance first when sag depth, duration or the actual trip mechanism is unclear.

Describe the Event →

Engineering inputs required

Define the project with measured information.

  • Single-line diagram and critical feeder boundary
  • Nominal voltage and time-aligned load profile
  • Measured sag depth, duration, and phase behavior
  • Trip sequence and required ride-through window

Site-Fit Engineering

Cabinet configuration can be adapted to the available installation envelope. This low-profile system was configured horizontally for a pharmaceutical plant compressor room with limited ceiling height.

Air compressor ride-through →
Horizontal low-profile power-continuity cabinet installed in a pharmaceutical plant compressor room with limited ceiling height.
Low-profile power-continuity cabinet installed in a pharmaceutical plant compressor room.

Next engineering step

Wilson Load-Layered Architecture →Air compressor ride-through →Verified Fujian textile deployment →Protect drives at the DC bus →
Sources / References
  1. IEC 61000-4-30: Power quality measurement methods. Measurement and interpretation methods for voltage dips, interruptions, harmonics, transients, and other power-quality parameters. Accessed 2026-08-24.
  2. IEEE 1159-2019: Monitoring Electric Power Quality. Recommended practice for describing, measuring, and interpreting power-quality phenomena. Accessed 2026-08-24.

Preliminary diagnosis

Tell Us What Happened

If you only know that equipment stopped during a short power disturbance, that is enough to start.

No waveform or power-quality record is required. Just tell us what stopped and what you observed.

Describe the Event

Engineering review

Tell Us What Happened

If you only know that equipment stopped during a short power disturbance, that is enough to start.

Describe the EventEmail an Engineer