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ABSTRACT

In the UK, there is concern over the increasing amounts of reactive power (Q) flows. The Transmission System Operator has noticed increasing amount of Q supplied to the distribution network by the transmission system. Allowing distributed generators (DG) to import Q, or to operate at unity power factor to reduce voltage and allow more connections at low costs is likely to make the situation worse. In the long term it will increase losses and costs. In many situations it is beneficial to use DG to supply the Q demand as it is usually located closer to network loads than conventional generation. This paper considers the carbon emissions of a modelled segment of typical distribution network, for a highly loaded winter scenario and a lightly loaded summer scenario. The DG provides less than 10% of the winter load and can operate importing or exporting Q. The network losses are computed for a range of power factors for the DG and from these the carbon emissions of the network are calculated. Analysis of these results shows that the optimum winter power factor of 0.97 exporting Q (which matches the load power factor), whilst the optimum summer setting is unity. In the summer scenario, most of the Q required by the loads is generated by the lightly loaded lines at higher voltage levels. In winter these lines are highly loaded, absorbing Q and hence the export of Q from DG is beneficial. Whilst the export of Q is beneficial in winter, it is recognised that the power factor setting of DG are often constrained by voltage limits. Importing Q allows DG to connect at minimum cost. This research shows that such cheap connections compromise the ability of the network to operate efficiently in terms of carbon emissions, network design and energy delivery.