f97.beERIK SCHMIDT’S WORKSHOP
← All terms

Glossary · Energy industry & energy transition

Flicker

flicker voltage fluctuation power quality supply quality grid operation load change grid connection

Flicker refers to repeated or rapid fluctuations of the supply voltage, which people perceive mainly as flickering or as fluctuating brightness of lamps. Technically, flicker is part of voltage quality. It does not mean the flickering of a single defective lamp. It means a property of the voltage at a point in the grid that affects all devices connected there together. The supply remains in place, and the RMS value of the voltage may well lie within the permissible band on average. Even so, the constant unsteadiness is disturbing.

Causes: load changes meet grid impedance

Physically, flicker arises when the current drawn or fed in by an installation changes rapidly. Every current produces a voltage drop across lines and transformers. If the current changes abruptly or periodically, the voltage at the connection point also fluctuates. How strongly depends on the grid impedance, that is, inversely on the short-circuit power at the point of common coupling. In simplified terms: the relative voltage change is roughly as large as the ratio of the power change to the short-circuit power. The angle of the grid impedance determines whether changes in active or reactive power have a stronger effect. In a weak grid, the short-circuit power is small relative to the installation. There, the same load change causes significantly larger voltage fluctuations. A weak grid is therefore not unreliable, but electrically sensitive to individual installations.

Classic sources of flicker are arc furnaces, welding equipment, motor starts, presses, frame saws, cranes, lifts and compressors with cyclic operation. Generation installations also come into question, for example wind energy installations in gusty wind or poorly controlled converters. In low-voltage grids, many smaller devices can in addition jointly produce noticeable values if they switch in clusters. Flicker also propagates: fluctuations in the medium-voltage grid pass largely through the local distribution transformer into the underlying low-voltage grid. For this, the D-A-CH-CZ rules use a usual transfer coefficient of 1.0.

Measurement: flickermeter, Pst and Plt

Flicker is not judged by personal impression but with a flickermeter according to IEC 61000-4-15. In several functional blocks, the device simulates how a reference lamp reacts to voltage fluctuations and how the human eye-brain system perceives this fluctuation in light. The reference is historically a 60 W incandescent lamp. The assessment covers modulation frequencies from fractions of a hertz up to about 33 to 35 Hz. It is most sensitive at about 8.8 Hz. Even very small fluctuations of well under one percent of the voltage can be disturbing there.

Two dimensionless indicators are derived from the measurement signal. The short-term flicker severity Pst is determined statistically over ten minutes each. Pst = 1 is defined such that about half of observers find the flickering of the reference lamp disturbing. For the long-term flicker severity Plt, twelve consecutive Pst values over two hours are averaged cubically. Individual strong phases therefore count more heavily than with a simple mean. Pst and Plt are neither volts nor kilowatts. They combine the depth, frequency of occurrence, shape and frequency of the voltage changes into a single figure for the disturbing effect.

The method has a limitation: depending on their driver electronics, modern LED lamps react differently from incandescent lamps. Many are less sensitive, but some with simple drivers can also be more sensitive. The voltage-based flickermeter therefore describes the voltage, not necessarily the actual light flicker of every luminaire. There are separate assessment approaches for luminaires, for example the light flickermeter according to IEC TR 61547-1. The grid-side rules continue to rely on Pst and Plt.

Standards and limits in Europe

Several sets of rules at different levels apply to flicker. They are standards and technical rules, not laws. They become binding only through product law, technical connection conditions or contracts.

  • Characteristic of the grid voltage: the European standard EN 50160 (current edition 2022) describes what voltage public grids can be expected to deliver under normal operating conditions. According to it, Plt should not exceed 1 for 95 % of the time in each week. Industrial grids are excluded. The standard itself points out that perception is subjective: complaints can occur even below Plt = 1.

  • Compatibility level: for public low-voltage grids, a compatibility level of Plt = 0.8 according to EN 61000-2-2 is specified. The D-A-CH-CZ rules require that this value is not exceeded in any case.

  • Individual devices up to 16 A per phase: IEC/EN 61000-3-3 limits the emission, measured at a reference impedance. The limits are Pst ≤ 1.0, Plt ≤ 0.65 and a steady-state relative voltage change dc ≤ 3.3 %. The maximum relative voltage change dmax is 4 % in the basic case, higher under certain conditions. For devices between 16 A and 75 A, IEC 61000-3-11 (connection subject to conditions) applies; for higher currents, further parts of the series apply.

  • Customer installations on the grid: for Germany, Austria, Switzerland and the Czech Republic, the D-A-CH-CZ rules for the assessment of grid disturbances (3rd edition, 2021/2023) describe how grid operators set emission limits for individual installations. For low voltage, they specify as the jointly permissible emission of all grid users Pst = 0.8 and Plt = 0.6. This share is allocated to the individual installations by means of an allocation key. 95 % values over one week are evaluated. No single Pst value may exceed 1.3 times the permissible value.

  • Delimitation by repetition rate: according to the D-A-CH-CZ rules, in low voltage a single installation may change the voltage by no more than 3 % in the case of frequent events (from 0.01 per minute). From a repetition rate of 0.1 per minute, a flicker assessment is additionally necessary.

The distinction between the grid side and the installation side is important: EN 50160 describes the voltage quality to be expected in the grid. Device and connection rules, by contrast, limit what a single installation may contribute. Because the contributions of many installations superimpose, the limits for individual installations are stricter than the values for the grid as a whole. With increasing short-circuit power, the permissible individual values in public grids are even supposed to decrease, because at a strong node usually more grid users are affected.

Delimitation from other disturbances

In a voltage interruption, the supply fails. A voltage dip is a significant, usually brief reduction, for example due to a grid fault. Rare individual events such as a motor start once a day are assessed as rapid voltage changes, not as flicker. Harmonics distort the waveform. Frequency deviations concern the grid frequency of the entire interconnected system. Flicker, by contrast, describes a local unsteadiness of the voltage magnitude. It says nothing about whether the power system as a whole is stable or whether sufficient generation is available.

Reactive power is not the same as flicker, but is closely linked to it. Particularly in grids with predominantly inductive impedance, rapidly changing reactive power strongly alters the voltage magnitude, for example with arc furnaces. Fast compensation systems such as SVC or STATCOM are therefore used specifically against flicker.

Significance for grid connection

All users of a grid section share the same voltage. An installation that generates flicker therefore also disturbs neighbours who have nothing to do with it. Before larger or fluctuating installations are connected, the grid operator therefore checks which flicker emission is permissible at the specific grid connection point. The short-circuit power, the magnitude and frequency of the power change, the existing background level and the number of other grid users all count here. The same machine can be unproblematic at a strong industrial node and unacceptable at a rural end of the grid.

If the values are not sufficient, there are several ways forward: soft start or frequency converter instead of direct-on-line starting, slower power ramps, less frequent switching operations, storage to smooth the power, a compensation system, or a connection at a higher voltage level or grid reinforcement. The options differ in cost, responsibility and effect. Flicker is therefore also a criterion for who bears which connection measures.

Typical misunderstandings

  • Flicker is not only a comfort issue. Visible flickering is the yardstick of the assessment, but strong voltage fluctuations can also disturb sensitive controls and processes.

  • The amount of energy consumed says little. A device with low electricity consumption can cause a lot of flicker through abrupt cycling. A large, steadily running load, by contrast, hardly any. What matters is the behaviour of the power over time, not the annual energy.

  • A Pst value below 1 does not guarantee freedom from complaints, and a value above it does not necessarily mean visible flickering with every luminaire. The indicator is based on a standardised reference lamp and an average observer.

Electrification and power electronics

With heat pumps, charging points, battery storage and PV inverters, the situation in distribution grids is changing. Well-controlled power electronics can reduce flicker: it ramps power up and down smoothly, avoids inrush current peaks and supports the voltage with reactive power. Poorly parameterised controls, devices reacting simultaneously, or oscillations between a converter and a weak grid can, however, also amplify voltage fluctuations. Connection rules therefore limit not only the maximum power but also ramps, switching-on operations and emissions. Whether an installation is climate-friendly or economical does not yet answer the question of whether it operates in a grid-compatible way at the specific connection point.

Sources

Related glossary entries

Edited by eCMS on 12.11.2026 05:05

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.