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Rob Barker, Director of Power Quality Expert, discusses voltage flicker, sags and swells in power quality measurements.
The term power quality is generally used as an all-encompassing, catch-all description for power measurements. We often talk about power factor, real power and reactive power being associated with the quality of the power being used, but these terms are more correctly associated with energy usage and energy efficiency.
Power quality is more related to voltage, and standard BS EN 50160 defines the voltage characteristics that the supply needs to comply with. The standard covers various parameters, including the measurement most people associate with power quality – harmonics. Although harmonics contribute to power quality, there are a number of other measurable quantities of the voltage that may be having an effect on the electrical installation and equipment.
Flicker
Flicker refers to the perceived fluctuation in light intensity that occurs when voltage fluctuations impact lighting systems. It is often caused by rapidly changing loads in the electrical network and can cause visual discomfort along with affecting the performance and lifespan of sensitive equipment, especially in industries that rely heavily on precise lighting conditions or use sensitive equipment.
What is flicker?
Flicker is characterised by voltage fluctuations occurring multiple times over a specific period, resulting in perceptible changes in light output. These variations can happen at irregular intervals and may differ in their duration and intensity. The frequency of the voltage fluctuations can lead to more pronounced flicker, which can be noticed in lighting systems where rapid changes in brightness can cause a visual annoyance.
| Flicker effects | Source of flicker | Frequency |
| Headaches/visual effects | Low frequency fluorescent | 100Hz |
| Neurological effects | Amplitude moderated flickering light | 20-75Hz |
| Seizures in photosensitive epileptics | Various | Various |
| Unperceived neurological effects | Light-emitting diode (LED) | Up to 200Hz |
| Unperceived retinal effects | Various | 76-162Hz |
| Visual effects | Computer monitor | 70-110Hz |
Table of flicker effects
Voltage flicker measurement
The method for measuring flicker is defined in standard IEC 61000-4-15, which provides a framework for the evaluation of voltage flicker.
The assessment of flicker involves calculating values which are used to measure for both the Perceptibility Short Term (PST) flicker over a standardised 10-minute period, and the Perceptibility Long Term (PLT) flicker over a standardised two-hour period. A short-term flicker value of 1.0 or more means that 50% of individuals will perceive the flicker to be both noticeable and irritating.
Calculating flicker
The calculations for PLT and PST both use the voltage variation and the number of switching events to measure how they impact the level of flicker:
Voltage flicker
Both the voltage variation and the number of switching events contribute to the overall flicker level and flicker characteristics. While the voltage variation determines the magnitude of the fluctuations, the number of events influences the frequency and pattern of the flicker. It is important to consider both aspects when evaluating and mitigating flicker issues in lighting systems to ensure a comfortable and visually stable environment for people.
Voltage surges and dips
Voltage dips (also known as voltage sags) and surges (or swells) are the most common types of power quality disturbances.
All electrical systems and equipment are designed to work within a specific voltage level, and changes in the voltage below or above the defined low and high limit could damage the equipment, cause outages and other power quality issues.
What are voltage surges (swells)?
Voltage surges are defined as a momentary increase in the RMS voltage to 110% or more above equipment recommended nominal voltage range for a period of 10ms (1/2 cycle) to 1 minute, as defined in the IEC 61000-4-30 standard.
Voltage surges are less common than voltage dips (sags) and they are usually related to a system fault. Surges can occur as a result of a single line to ground fault, which will briefly raise the voltage level of the unfaulted phases as shown in the example. These swells are more likely to happen in ungrounded or floating ground delta systems and can also occur when a large load is turned off.
Voltage RMS 1/2 cycle and waveform charts during a voltage surge (swell)
What are voltage dips (sags)?
A voltage dip (sag) is a brief reduction in the RMS voltage to 90% or less below the nominal voltage for a period of 10ms (1/2 cycle) to 1 minute. The cause of the dip is usually a sudden large change of load through the source impedance, such as in a motor startup or a short circuit event.
Voltage dips (sags) are a common power quality event that can occur several times per year at a typical industrial site’s PCC (Point of Common Coupling) and even more at the equipment connections. Modern standards now require that electrical equipment has the ability to ‘ride through’ brief voltage dip events with the requirement depending on the dip characteristics and the application, like those found in renewables generation.
Voltage RMS 1/2 cycle and waveform charts during a voltage dip (sag)
Fault ride through
The ability to ‘ride through’ network disturbances is a critical consideration for renewable energy connections.
During a voltage dip caused by an external event, such as a network fault, both wind and solar generators must maintain their connection to the grid. This requirement, known as Fault Ride Through (FRT) or Voltage Ride Through (VRT), supports the grid by injecting reactive current to support the voltage when a dip is detected.
Although the initial fault event can typically last less than 200ms, the grid can take up to 1.5 seconds to return to 90% of the nominal voltage. It is essential that renewable generators remain connected during this entire recovery time so that normal energy production and active power delivery can resume immediately once the grid stabilises.
Overcoming power disturbances
Using advanced power quality meters and power quality analysers with a waveform record functionality allows you to capture almost any event in your system. These meters can record voltage dips and swells and can be set with triggers and thresholds to capture additional power disturbance events for later review in the meter’s software.
Using the information provided from the analysers can help when selecting the correct power quality solution. Devices such as voltage stabilisers can be installed to provide real-time compensation to stabilise voltage, enhance power factor, help mitigate harmonics and reduce voltage flicker and fluctuations in real-time.
The IEEE 1668 standard, Recommended Practice for Voltage Sag and Short Interruption Ride-Through Testing for End-Use Electrical Equipment Rated Less than 1000V, can help with guidance and ways to manage these types of voltage event.
This article appeared in Electrotechnical News July/August 2026 edition – read it here
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