Whitepaper about flicker

Flicker

What exactly are Flicker?

Flicker refers to the subjective impression of light density changes or an impression of unsteadiness of visual perceptions, caused by luminous stimuli with temporal fluctuations of the light density or the spectral distribution. From a technical perspective, voltage variations cause light density changes in lamps, which can result in visual perceptions referred to as flicker. From a certain threshold value the appearance of flicker can be disturbing. The disturbing effect of voltage variations depends here on the extent of the repetition rate and the curve form of the change in voltage. The short-term flicker strength and long-term flicker strength are defined measures of the disturbing effect.

Voltage variations, caused by individual devices (on the low voltage network), are permissible if the resultant flicker disturbance factor is not greater than1. The long-term flicker disturbance factor averaged from twelve values must not exceed a value of 0.65.The most simple method for evaluating the value is the = 1 p.u. curve. P.u. stands here for the "unit of perception" and is the maximum tolerance level for the interference sensitivity of the human eye with regards to its perception of light fluctuations. It is also not permissible to exceed the value = 1 p.u. in combination with all interferers.

Fig.: Development over time of short-term flicker (PST)
Fig.: Development over time of short-term flicker (PST)
Fig.: Practical example for flicker: Gravel quarry
Fig.: Practical example for flicker: Gravel quarry

But why measure flickers?

Very high flicker levels cause an increase in maintenance effort and faults on electronic equipment, up to and including destruction (mains power supplies).

Moreover, the flicker causes employees to become tired more quickly, to become irritable and to lose concentration, particularly at workstations in office buildings. The continual adjustment of the optic nerve to the changing lighting conditions causes tiredness quickly and finally transfers to the person‘s overall sensitivity. Therefore, the power quality standard EN 50160 sets out threshold values which must be adhered to, in order to help prevent the negative effects of the flicker.

Flicker measurement

Since the flicker manifests itself in very quick and small voltage variations, it can only be captured using very high quality measurement instrumentation. This is described in DIN EN 61000-4-15. The instantaneous value of the flicker is of less interest here, as longer-term effects are to be considered. Therefore, the short term flicker PST and long term flicker PLT values have been defined. These are mean values taken over 10 or 120 minutes. 

The UMG 512, UMG 511 and UMG 605 measurement devices from the Janitza electronics GmbH product range measure all three values and therefore comply with the DIN EN 61000-2-4 standard. 

However, due to the empirical nature of the flicker (see above), this is not a simple, linear measured value such as the voltage. A flicker meter must therefore capture the empirically-determined test level of square wave signals and sinusoidal voltage variations for certain frequencies correctly in accordance with the specified accuracy, in order to be able to be separated into the appropriate flicker meter classes. The UMG 512 and UMG 511 are in class F1 and the UMG 605 is in class F3 for flicker meters. 

However, in order to receive meaningful values, the values obtained in PST and PLT must be „graded“. Since these are mean values that were obtained at different frequencies, a higher level for one single frequency would otherwise carry very little weight; i.e. the values must be weighted. As a result, higher values are considered more in the final value than a very small level. 

The power quality standard EN 50160 standardizes a permitted flicker level of 1 in the mains power supply and therefore provides a guideline for its measurement. If the standard is violated continuously, measures must be taken to stabilize the voltage (higher transformer powers, dynamic compensation systems, UPS, etc.). For manufacturers of non-linear electrical loads, the flicker emission is standardised to PST=1 and PLT=0.65 (EN 61000-3-2). The most simple method for evaluating the value is the = 1 p.u. curve. P.u. stands for the „unit of perception“ and is the maximum tolerance level for the interference sensitivity of the human eye with regards to its perception of light fluctuations. It is also not permissible to exceed the value = 1 p.u. in combination with all interferers. 

Fig.: Development of flicker
Fig.: Development of flicker
Fig.: Effective power development dependent on the volume and consistency of material
Fig.: Effective power development dependent on the volume and consistency of material

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