Voltage sags, swells and interruptions: the IEEE 1159 categories
How IEEE 1159 names short-duration voltage variations by magnitude and duration, how a sag is measured from a waveform record, and how SagSwell reports one.
IEEE 1159, the IEEE recommended practice for monitoring electric power quality, gives names to voltage disturbances by how deep they are and how long they last. Using its names means that “a momentary sag” means the same thing in a utility’s report, a customer’s complaint and an equipment maker’s data sheet.
Sag, swell, interruption
- Sag (called a dip in IEC documents): the RMS voltage falls to between 0.1 and 0.9 pu of nominal for half a cycle to a minute.
- Swell: the RMS voltage rises above 1.1 pu for half a cycle to a minute.
- Interruption: the voltage falls below 0.1 pu.
A sag is described by its retained voltage, the lowest RMS value during the event, not by how far the voltage dropped. A sag to 0.7 pu is a 70% sag in IEEE usage: 70% of the voltage remained. Many people say “a 30% sag” for the same event, so check which convention a report uses before comparing numbers.
The short-duration categories
| Category | Type | Typical duration | Typical magnitude |
|---|---|---|---|
| Instantaneous | Sag | 0.5 – 30 cycles | 0.1 – 0.9 pu |
| Instantaneous | Swell | 0.5 – 30 cycles | 1.1 – 1.8 pu |
| Momentary | Interruption | 0.5 cycles – 3 s | < 0.1 pu |
| Momentary | Sag | 30 cycles – 3 s | 0.1 – 0.9 pu |
| Momentary | Swell | 30 cycles – 3 s | 1.1 – 1.4 pu |
| Temporary | Interruption | 3 s – 1 min | < 0.1 pu |
| Temporary | Sag | 3 s – 1 min | 0.1 – 0.9 pu |
| Temporary | Swell | 3 s – 1 min | 1.1 – 1.2 pu |
Beyond one minute the same excursions are long-duration variations: a sustained interruption, an undervoltage (0.8 – 0.9 pu) or an overvoltage (1.1 – 1.2 pu). A fault record from a relay or recorder is seldom long enough to show those; they belong to a power-quality meter’s trend logs.
How a sag is measured from a waveform
A COMTRADE record holds the waveform, not the RMS value, so the RMS has to be calculated. The usual method, and the one SagSwell uses, is a one-cycle RMS: the root-mean-square of the samples in a window one cycle long, moved along the record. SagSwell moves the window an eighth of a cycle at a time, so the start and end of an event are placed to within a fraction of a cycle.
- An event starts when any phase leaves the 0.9 – 1.1 pu band and ends when all phases are back inside it.
- Its magnitude is the retained voltage of the worst phase for a sag, or the highest RMS for a swell.
- An event whose lowest phase falls below 0.1 pu is reported as an interruption.
- The phases involved are every phase that left the band during the event.
The nominal voltage matters as much as the measurement. If a recorder declares its channels in secondary volts, or a site runs deliberately above nominal, every per-unit value moves with it. Before comparing per-unit numbers from two tools, check which nominal each one used.
What the category does not tell you
The category names an event; it does not say whether equipment rode through it. That depends on the equipment, which is what tolerance curves such as ITIC (CBEMA) and SEMI F47 describe. Nor does a single record say where the fault was: the same upstream fault shows up as different sags on different buses, which is why comparing several recorders’ records of the same moment is worth doing.
In SagSwell, the RMS & disturbances tab lists every sag, swell and interruption in a record with its duration in milliseconds and cycles, its retained voltage and the phases involved, free on as many records as you open.
References
- IEEE Std 1159-2019, IEEE Recommended Practice for Monitoring Electric Power Quality
- Information Technology Industry Council, ITI (CBEMA) Curve Application Note