Mitigating Voltage Sags & Interruptions with Dynamic Voltage Restorers (DVR)

1. Voltage Sags: The Silent Threat to Automated Production Lines

A voltage sag is a short-duration reduction in voltage below its nominal value, lasting from half a cycle to a few seconds. Most sags originate from faults on the utility transmission or distribution network, such as lightning strikes, tree contact, or the starting of large motors nearby. Although brief, a sag is enough to drop out contactors, reset PLCs, or trip motor drives on undervoltage protection.

For continuous-process industries such as semiconductors, plastic film, synthetic fiber, or precision forming, even a few seconds of line stoppage can scrap an entire batch and require hours of recovery. A Dynamic Voltage Restorer (DVR) is designed to inject the missing voltage instantaneously, before sensitive loads ever perceive the disturbance.

2. Classifying Sag Severity per Standards

IEC 61000-4-30 defines how voltage sag events are measured, characterizing each event by residual voltage and duration. Equipment ride-through capability is commonly assessed against the ITIC curve and the SEMI F47 standard, which is widely used in the semiconductor industry. These curves specify how deep and how long a sag the equipment should tolerate while continuing to operate.

At the network level, EN 50160 describes the voltage characteristics customers can expect from public distribution systems, including the reality that sags are a natural phenomenon of any grid. Customers with sensitive loads are therefore responsible for hardening their own installations. Comparing actual site event statistics against equipment tolerance curves is the rational basis for any mitigation investment decision.

• IEC 61000-4-30 — measurement methods and sag/swell event definitions
• ITIC curve — IT equipment tolerance to voltage disturbances
• SEMI F47 — sag ride-through requirements for semiconductor tools
• EN 50160 — voltage characteristics of public distribution networks

3. DVR Operating Principle and Sizing

A DVR is connected in series between the source and the load through an injection transformer. When a sag is detected, its inverter draws energy from internal capacitors or an energy storage bank, synthesizes the missing voltage component, and injects it immediately, typically responding in under a quarter cycle so the downstream load sees near-normal voltage throughout the event.

DVR sizing depends on three main factors: the power rating of the protected load, the sag depth to be compensated (for example, down to 50 percent residual voltage), and the maximum compensation duration determined by the energy storage size. Event statistics from several months of continuous monitoring allow the specification to match the site's actual risk profile without paying for excess capability.

4. Alternatives Comparison and Application Cautions

A DVR is not the only answer to sag problems. Alternatives include online UPS systems, which also cover full outages but incur continuous conversion losses; ride-through power supplies for control circuits; and drive parameter tuning such as kinetic buffering to improve sag tolerance. The choice depends on the protection scope: control circuits alone may justify only low-cost measures, while protecting an entire production line makes a DVR or large UPS worthwhile.

Because a DVR sits in series with the load, a reliable bypass must be engineered so an internal fault does not itself interrupt the load. Coordination with downstream overcurrent protection is also required, and commissioning should include real scenario testing, such as sag injection with a test set, to verify response time and compensation depth against the design.

• Compare options: DVR, online UPS, ride-through supplies, drive tuning
• Engineer a reliable bypass for any series-connected device
• Coordinate with downstream overcurrent protection
• Perform real sag-injection testing during commissioning