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Flicker In A Nutshell

This page introduces shortly to the problems of flicker and shows a few simple flicker calculation possibilities.

Origin of Flicker

	Source impedance (reference network) acc. [1] (click to enlarge)

The voltage of public power networks is changing over time. These changes originate in the voltage drop generated over the source impedance of the grid by the changing load current of an equipment or facility.

The fluctuations of the load in time generate flicker, i.e. changes in the luminous flux of a lamp. The effects of flicker can range from disturbance to epileptic attacks of photosensitive persons.

Flicker Model

The requirements of a flicker measurement equipment are defined in the standard [1] IEC 61000-4-15. The flicker meter is subdivided in several function blocks which simulate a 230 V/60 W incandescent lamp (reference lamp) an the human perception system (eye-brain model).

From the resulting momentary value of flicker the short term flicker value Pst is generated in the last block according to a statistic process over a predefined observation interval. Long term flicker Plt is calculated as the cubic average of several Pst values.

In the standard [2] IEC 61000-3-3 the observation intervalls and the limiting values for Pst and Plt are specified:

Flicker meter block diagram acc. [2] (click to enlarge)

Opearing Condition of the EUT

The flicker standard states that the EUT (Equipment Under Test) has to be operated during the test in a way which is the worst case state with respect to flicker.

If the EUT is operated in a (relatively) constant fashion during the whole test, Plt = Pst will result. If this state is is feasible and realistic this means Pst has to fulfill the limits for Plt (which are lower!)

Estimation

A purely analytical calculation of Pst is almost impossible. In the standard [2] there are formulas which allow the estimation of the Pst values to be expected:

n = number of load changes in the observation interval

Tp = duration of the observation interval in seconds

F = shape factor (1 step-wise voltage changes)

d = relative voltage change relative dU / U

In [2] there are also shape factors for different curve forms.

Flicker Facts

• Flicker is generated by load changes. Only the amplitude of the load change is relevant, not the absolute value.
• A reduction in Pst can only be attained through:
  - less load changes n
  - smaller load changes dP
  - soft (F < 1) instead of hard (F = 1) load changes (e.g. by the help of power electronics)
• The relationship between amplitude of load changes and Pst is linear, i.e. halving the switched load results in half the Pst
• The relationship between number of load changes per time (n/ Tp) and Pst is non-linear. A halving of load changes reduces Pst by only about 20%. In order to have half the Pst, the number of load changes must be reduced by a factor of 9!

References