Optimised Reactive Power Supports using Transformer Tap Stagger in Distribution Networks

In distribution networks, the operation of two parallel transformers in small different tap positions, i.e. known as tap stagger, can provide a means of absorbing reactive power. The aggregated VAr absorption from many pairs of parallel transformers could help mitigate the high voltages in the upstream transmission system during periods of low demand. Considering distribution networks as reactive power support providers, this paper presents an optimal control strategy for the tap stagger to minimise the introduced network losses as well as the tap switching operations. A genetic algorithm based solution procedure is developed to find the optimal dispatch for the transformer tap positions, in order to achieve the VAr absorption required by the transmission system. The proposed method is compared with two alternative control approaches based on the rule-based method and the branch-and-bound algorithm, respectively. Two practical UK distribution networks are modelled from real data and used to demonstrate the effectiveness of the proposed control strategy under different operating conditions.