033-2231 8646/47
info@e-cube.inPower factor and harmonics get most of the attention in power quality discussions, but a third category of problem — disturbances in the voltage waveform itself — is often the one actually causing production stoppages and equipment damage. Sag, swell and flicker are distinct phenomena with distinct causes, and treating them as interchangeable “voltage problems” usually leads to the wrong fix.
Voltage sag
A sag is a short-duration drop in RMS voltage, typically to 10–90% of nominal, lasting from half a cycle to a few seconds. Common causes include large motor starting (a motor’s inrush current can be 6–8 times its running current, pulling the local bus voltage down momentarily), faults on the utility network or an adjacent feeder, and sudden large load switching. Sags are the single most common cause of unexplained VFD trips, PLC resets and contactor drop-outs — sensitive electronic equipment often has a voltage tolerance window narrower than what mechanical equipment shrugs off without issue.
Voltage swell
The inverse of sag — a short-duration rise in RMS voltage, usually caused by sudden large load disconnection, single-line-to-ground faults on other phases, or switching of large capacitor banks without proper controls. Swells stress insulation and can shorten the life of connected equipment, particularly electronics and lighting.
Flicker
Flicker is rapid, repetitive voltage fluctuation, often too fast to register as a distinct sag or swell event but visible as perceptible light flicker and capable of disturbing sensitive process equipment. It’s typically caused by loads that draw sharply varying current in quick succession — arc furnaces, welding equipment, large reciprocating compressors, and cranes are classic sources.
Why these matter beyond nuisance
- Repeated VFD nuisance tripping from sags disrupts continuous processes and can cause material scrap or rework.
- Sensitive control and instrumentation resets during sag events can cascade into a full line stoppage even when the sag itself lasted under a second.
- Chronic swells shorten electronic and lighting equipment life measurably compared to a stable supply.
- Flicker from a plant’s own equipment can violate utility connection agreements if it propagates back onto the shared grid.
How correction is approached
Unlike PF and harmonics, sag/swell/flicker correction is often about isolation and conditioning rather than filtration: isolation transformers and line conditioners decouple sensitive loads from the disturbances generated elsewhere on the same supply, while properly staged capacitor and motor switching (soft starters, staged contactor sequencing) reduces the disturbance a facility’s own equipment injects into its internal network in the first place.
As with every power quality issue, the starting point is measurement — a power analyzer with event-capture capability logs sag/swell/transient events with timestamps and duration, correlating them against production logs to identify the actual source before specifying a fix.
Related reading: see our isolation line conditioners for protecting sensitive loads from upstream disturbances.