Voltage sag and swell
Voltage sag (or dip) is a momentary reduction in the root mean square (RMS) voltage, typically between 10% and 90% of the nominal voltage, with a duration ranging from 0.5 cycles to 1 minute. Its counterpart, voltage swell, is a temporary increase above 110% of the nominal voltage, usually in the range of 110% to 180%, with similar durations. Both disturbances constitute the most frequent power quality events in industrial and commercial electrical systems.
Definition and Characteristic Parameters
Section titled “Definition and Characteristic Parameters”The magnitude of a voltage sag lies between 10% and 90% of the nominal voltage, while its minimum duration is 0.5 cycles (10 ms at 60 Hz / 8.33 ms at 50 Hz). The following table summarizes the typical values of both phenomena.
| Parameter | Voltage Sag (Dip) | Voltage Swell |
|---|---|---|
| Magnitude (% of Vnominal) | 10% – 90% | 110% – 180% (1.10 p.u. – 1.80 p.u.) |
| Duration | 0.5 cycles (10 ms @ 60 Hz / 8.33 ms @ 50 Hz) to 1 minute (60 s / 3,600 cycles @ 60 Hz) | Same range as sag |
| Typical frequency of occurrence | Can represent up to 70% of all power quality disturbances in an installation | Less frequent, often associated with load rejection |
Classification by Duration (IEEE 1159)
Section titled “Classification by Duration (IEEE 1159)”IEEE 1159 establishes three temporal categories for short-duration events: instantaneous (0.5–30 cycles), momentary (30 cycles – 3 s), and temporary (3 s – 1 min). The following table details the limits.
| Category | Duration (cycles) | Duration at 50 Hz (ms / s) | Duration at 60 Hz (ms / s) |
|---|---|---|---|
| Instantaneous | 0.5 – 30 cycles | 10 ms – 600 ms (0.6 s) | 8.33 ms – 500 ms (0.5 s) |
| Momentary | 30 cycles – 3 s | 600 ms (0.6 s) – 3 s | 500 ms (0.5 s) – 3 s |
| Temporary | 3 s – 1 min | 3 s – 60 s | 3 s – 60 s |
Main Causes
Section titled “Main Causes”Single-phase-to-ground faults in the distribution system are the most common cause, as they can cause voltage sags of 30% to 60% in adjacent circuits. In addition to faults, there are other relevant causes:
- Starting of large motors: the inrush current can reach 5 to 7 times the rated current, causing a momentary sag.
- Switching operations of power transformers: sudden energization originates inrush currents that can exceed 10 times the rated current, dropping the voltage at the point of common coupling.
- Switching of large loads or sudden demand variations.
- Atmospheric phenomena (lightning) that cause transient line-to-ground faults.
- Voltage swells typically appear when disconnecting large loads or when operating switches in the presence of capacitor banks.
Consequences on Equipment
Section titled “Consequences on Equipment”Voltage sags of only 20% of the nominal voltage with a duration of 2 to 3 cycles (34 ms–50 ms @ 60 Hz) can cause contactor opening and stoppage of automated processes. The main adverse effects are:
| Equipment / System | Effect of Voltage Sag |
|---|---|
| Contactors and electromechanical relays | Disconnection with voltages below 80% (0.80 p.u.) for more than 1–2 cycles. |
| Variable speed drives (VSD) | Trip due to undervoltage on the DC bus when the mains voltage drops below 70%–85% (0.70 p.u.–0.85 p.u.). |
| PLCs and control systems | Reset or failure if the power supply does not ride through the sag; typical sensitivity from sags of 30%. |
| Discharge lamps (HID) | Shutdown and subsequent re-ignition time of several minutes (3 min – 15 min). |
| Computers and IT equipment | Reset or data loss with sags greater than 30% and duration longer than 8 ms (0.5 cycles @ 60 Hz). |
Mitigation Methods
Section titled “Mitigation Methods”Dynamic voltage restorers (DVR) can compensate for sags of up to 50% of the nominal voltage in less than 2 ms, keeping the load within operating limits. The most common mitigation technologies are:
| Method | Description | Typical Compensation Range |
|---|---|---|
| Dynamic Voltage Restorer (DVR) | Injects voltage in series via a power converter to compensate for the sag. | Sags of up to 50% (0.50 p.u.), response < 2 ms. |
| UPS (Uninterruptible Power Supply) | Provides backup with batteries; online mode completely isolates the load. | Covers sags of 100% (interruptions) depending on autonomy (minutes to hours). |
| Static Synchronous Compensator (STATCOM) | Compensates deep sags via fast reactive power exchange. | Sags of up to 30%–40% (0.30 p.u.–0.40 p.u.). |
| Fast-switched capacitors | Raise the voltage at the connection point during the event. | Sag attenuation of 10%–20% (0.10 p.u.–0.20 p.u.). |
| Automatic reclosers and protection coordination | Reduce the fault duration and therefore the voltage sag duration. | Depends on network topology. |
Applicable Standards
Section titled “Applicable Standards”IEEE 1159 classifies short-duration voltage variations into three temporal categories — instantaneous, momentary, and temporary — and provides the fundamental definitions of sag and swell. Other reference standards are:
- IEC 61000‑4‑30 / EN 61000‑4‑30: power quality measurement methods, including detection of dips and temporary overvoltages.
- EN 50160: characteristics of voltage supplied by distribution networks; establishes statistical limits for the frequency of voltage dips.
- SEMI F47: specification for semiconductor manufacturing equipment defining voltage sag tolerance limits.
- ITIC (CBEMA): susceptibility curve for electronic equipment, widely used to evaluate immunity against sag/swell.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the difference between a voltage sag and a blackout?
Section titled “What is the difference between a voltage sag and a blackout?”A voltage sag is a partial reduction in voltage (10%‑90%) lasting between 0.5 cycles and 1 minute; a blackout or interruption corresponds to a total loss of voltage (≥90% reduction) that can last for longer periods.
Which equipment is most susceptible to voltage sags?
Section titled “Which equipment is most susceptible to voltage sags?”Electromechanical contactors, variable speed drives, PLCs, and discharge lamps are particularly sensitive; a sag of only 20% for 2‑3 cycles can stop a production line.
How is a voltage sag measured according to standards?
Section titled “How is a voltage sag measured according to standards?”A power quality analyzer that records the RMS voltage every half-cycle according to IEC 61000‑4‑30 is used; the event is characterized by its residual magnitude, duration, and time stamps.
Are voltage swells harmful to equipment?
Section titled “Are voltage swells harmful to equipment?”Yes. Voltages above 110% of nominal can damage power supplies, reduce the lifespan of LED luminaires, and cause overvoltages in electronic equipment if not protected.
How can I mitigate voltage sags in an industrial plant?
Section titled “How can I mitigate voltage sags in an industrial plant?”The most effective solution is to install a DVR or an online UPS for critical loads; it is also possible to improve protection coordination and use soft starters to limit inrush currents.
What duration must a voltage sag have to be considered according to IEEE 1159?
Section titled “What duration must a voltage sag have to be considered according to IEEE 1159?”An event is classified as a voltage sag if its duration is greater than 0.5 cycles and less than or equal to 1 minute; shorter durations are considered transients, and longer ones are cataloged as sustained undervoltages.