Flicker compensation

    Flicker describes the subjective impression of light density fluctuations, caused by fluctuations in the supply voltage.

    There may be many reasons for these voltage dips so the individual consumers must be reviewed separately taking the particular grid situation into consideration. In order to assess the cause of the fault and the impact of remedial measures, consideration needs to be given to the loads, timing and grid parameters in the least favorable operating case.
    The amount of flicker is directly dependent on the level, form and repeat rate of the change in voltage. For its part, the change in voltage depends on the grid impedance and the power characteristics of the consumers.
    In accordance with EN 50160, the long-term flicker level Plt on the low voltage side must not exceed 1 for 95% of the measurement period. The measurement must be taken over a week with an averaging time of 10 min.

    Cause:

    •  Activating and deactivating large loads
    •  Reciprocating saws, stone crushers, elevators
    •  Starting engines, eccentric drives
    •  Electric arc furnaces
    •  Pulsed power levels (oscillation package control)
    •  Wind turbines, magnetic resonance tomography
    •  Welding machines and systems, inverters

    Effect:

    • Light density fluctuations and as a result impacts on the human body ranging from lack of concentration and general feelings of discomfort to epileptic fits
    • Reduction in quality (e.g. of welded connections)

    Flicker compensation is fundamentally different from standard reactive power compensation. The aim is not a particular target power factor but to keep the voltage constant during rapid changes in load. The drop in voltage during a change in load can be split into two components; a "drop in actual voltage" and a "drop in reactive voltage". The influence of these components is stated using the ratio of active resistance to reactive resistance of the grid impedance R/X. Capacitive reactive power results in increases in voltage and inductive reactive power results in decreases in voltage (i.e. Kapp's triangle). The total drop in voltage is calculated as follows:

    ∆U=(R*∆P)/U+(X*∆Q)/U

    The goal of flicker compensation is to compensate for the mostly inductive ΔQ such that ΔV=0. This requires slight overcompensation to eliminate the summand of active power ΔP from the equation (making ΔQcomp a negative value).

    0=R*∆P+X*(∆Q+∆Qkomp)

    Depending on how dynamic the load fluctuations are, the compensation power can either be fed in via dynamic compensation systems and/or active mains power filters. Alternatively changes to the operating behavior of the load or increases in the short-circuit capacity can also reduce the flicker value. The short-term load behavior needs measuring in each case in order to configure flicker compensation.