Loughborough University Institutional Repository Accurate determination of distribution network losses

Distribution losses can be difficult to measure when the loss power has a similar proportion to monitoring equipment sensor tolerances. Measurements for the Western Power Distribution Losses Investigation project have addressed this problem by using high resolution demand measurements combined with loss calculations with an IR method. This is combined with additional measurements to verify accuracy of the network database used to calculate losses in each network branch. INTRODUCTION Distribution operators are interested in reducing losses due to their impact on customer bills. Distribution losses in the UK have been estimated at between 5.8% and 6.6% of electricity delivered [1] but operators have limited knowledge of the contribution from each voltage layer or from specific feeders. Further investigation is required so that measures to reduce losses can be effectively targeted. The Western Power Distribution (WPD) Losses Investigation project, undertaken in collaboration with Manx Utilities Authority, aims to make measurements of technical losses on a selected set of HV and LV feeders [2]. The measurements will be used to develop methods to provide accurate estimates of losses for a wide range of network feeders. This differs from previous work in which losses have been estimated on average over multiple feeders, or for average demand profiles. These estimates have limited data on the power factor, unbalance, or stochastic time variation of real customer loads. Recent developments in logging instrumentation and metering have made more detailed measurements feasible. A practical benefit of making measurements on individual feeders is that the number of nodes needing instrumentation is manageable. However, the percentage losses are then relatively small and measurement tolerances become a critical concern. This paper compares loss calculations using a power difference method with calculations based on power-flow analysis. The uncertainty of each method is investigated, together with techniques that can be used to validate the network database on which the calculations depend. LOSS MEASUREMENTS Measurement setup Monitoring equipment has been installed on a number of HV and LV feeders. In each case, the current and voltage are recorded at all of the nodes at which power enters or leaves the feeder networks, as shown in Fig. 1 and Fig. 2. Fig. 1 HV feeder measurement configuration Fig. 2 LV feeder measurement configuration On the HV test feeders, the current and voltage are recorded by a GridKey or Sub.net logger at the primary substation. Measurements at the primary substation or at HV customer connections use existing voltage transformer (VT) and current transformer (CT) circuits. The use of the existing CT wiring has been necessary as it is not straight forward to fit sensors directly onto the feeder cable cores without also including currents in the sheaths. The current and voltage are also measured at each distribution substation using GridKey loggers fitted to the LV cabinet. The HV test feeders are located in the WPD operating area in Milton Keynes, UK but the LV test feeders are located in the Isle of Man where the vertically integrated authority can install appropriate metering at customer connections. Measurements are recorded with a 1-minute resolution and the clocks of the logging instruments can be synchronised to within a few seconds. The logging data provides limited data on distortion and so the analysis has assumed operation at 50 Hz. It is recognised that the actual losses will be slightly higher due to current harmonics and due to demand variation within the 1 minute intervals [3]. However, the initial measurements suggest that these are minor effects. Current 33 kV 11 kV Logger Primary substation Distribution substation Logger LV feeders HV customer Voltage Current 400 V Open point Selected HV feeder