Distribution tariff design considering electric vehicle loads

Abstract

As part of reducing the emissions from personal transportation EV adoption is expected to rise dramatically in the coming decade. Whilst this has a positive effect on climate targets issues the current distribution networks for electricity were not built to accommodate the large rise in EV. This problem can be tackled in a number of different ways, such as by expanding network capacity, or including of storage at the distribution level. This thesis, however looks at price based incentives to mitigate the adverse effects of EV charging on distribution grids. In particular a restructuring of distribution grid tariffs was considered. Distribution grid tariffs are the tariffs paid by customers to distribution system operators (DSOs) for use and maintenance of the grid. Currently most customers connected to the low voltage distribution grid pay a flat rate for the grid tariffs. However, by restructuring these grid tariffs incentives can be provided in order to make sure EVs use the flexibility which exists in the charging sessions in order to limit congestion issues. In particular this study will look at public charging, that is charging points (CPs) connected to the distribution network and operated by a charging point operator (CPO). Two particular proposals for grid tariff redesign were assessed. In the capacity subscription plus model (CAP+) the customer chooses a subscribed capacity, that is a power up to which the customer can freely use the grid. This subscribed capacity has options at a few different capacity sizes with associated costs. When the customer exceeds the subscribed capacity an exceedance fee has to be paid for each exceeded kilowatt-hour. The other considered option is a particular case of dynamic grid tariffs where the CPO pays differing prices for power used at particular times. How much power can be used at each time at the differing price levels is determined the day ahead. This tariff design bundles all CPs connected to the same transformer, thus the total power is what matters rather than the individual power of the CPs. A perfect information model was constructed to find the optimal CPO behavior on a cost-wise basis under the different tariff designs (current, dynamic and CAP+). It is evident from the results that if the tariff design is left unchanged, and no alternative measures are taken to address the issue of transformer overloading due to EV charging at public charging points, problems are likely to occur. Introducing the CAP+ tariff design can mitigate part of this problem. But as the CAP+ tariff design focusses on individual usage peaks rather than the collective network peak which causes this transformer overloading it is not as effective at reducing transformer overloading as the Dynamic tariffs can be. For the implementation of a tariff design more factors need to be considered, however. The regulatory authority (ACM) is responsible for accrediting a tariff design and considers factors such as non-discrimination, simplicity, transparency and more. In this regard the dynamic tariffs are more controversial as it requires technological capabilities and has a fairly complicated mechanic involving predicted transformer loads. Whether the advantages of the dynamic tariff design in terms of efficient network use, and thus overall costs reductions, outweigh the problems with current legislation and these regulatory principles is, in the end, a decision to be made by the regulator and is left outside the scope of this research.