An improved methodology for determining MV to LV voltage unbalance transfer coefficient

The International Electrotechnical Commission (IEC) has recently released a technical report (IEC/TR 61000-3-13, Ed. 1.0, 2008) in relation to the assessment of voltage unbalance emission by installations connected to MV, HV and EHV power systems. As in the cases of harmonics and flicker, this requires a quantitative measure of propagation of voltage unbalance from upstream (higher voltage) to downstream (lower voltage) systems in terms of transfer coefficients. Naturally, these transfer coefficients depend on the downstream load composition. The existing method for determining MV to LV voltage unbalance transfer coefficient suggests a value of unity in relation to passive loads in general. However, this paper reports that MV to LV transfer coefficient of unity is conservative in the presence of commonly prevailing constant power loads. Further, the paper reports on an improved methodology for estimating MV to LV voltage unbalance transfer coefficient taking system and load characteristics into account.