## Reactive power charges and voltage control

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Equation (10.8) also shows that it is desirable to operate as near to unity power factor as possible (i.e., q = 0) if network losses are to be minimized. This preference to reduce reactive power flow is reflected in tariffs applied by many distribution companies to charge customers either for reactive energy (kVArh) or for the peak reactive power (kVAr) drawn during a period. Such tariffs are usefully applied to loads where both real and reactive power flows are in the same direction and so any reduction in reactive flows leads to smaller voltage variations on the network (voltage drop in the case of loads). However, they can have perverse consequences when applied to generation where it may be desirable for network voltage control to draw some reactive power and so limit voltage rise. In this case the generator wishes to draw reactive power to control the voltage rise caused by the real power export but will be charged for this reactive demand.

It is anticipated that, in time, reactive power/energy tariffs will cease to be applied to embedded wind generation and will be replaced by the concept of embedded generation taking part actively in the voltage control of distribution circuits perhaps through the commercial mechanism of an ancillary services market.

The design of distribution networks tends to be driven by voltage considerations rather than by thermal limits on plant. Therefore the connection of embedded wind farms is often limited by considerations of voltage variation. At present it is usual to evaluate these voltage limits by considering: (1) the maximum output of the wind farm in conjunction with the minimum network load, and (2) zero output of the wind farm with maximum network load. This rather simplistic approach can lead to a connection for a wind farm being rejected for conditions which only persist for a few hours per year. Thus, there is a move towards probabilistic assessment of voltage using so-called probabilistic load flows which allow calculation of the duration of the times in which voltage limits are violated. Probabilistic load flows may be based either on analytical calculation techniques or use some form of Monte Carlo simulation. The input is a probabilistic description of the network loads and generation and the output is a similar representation of network voltages and flows. Results of such studies may be used to predict the cost of lost revenue if generation is curtailed during periods of low network load or the increased charges for drawing higher levels of reactive power.