Systematic modelling and analysis of TAPP voltage control schemes

It is the responsibility of the distribution network operator (DNO) to maintain voltage profiles within statutory limits across a range of conditions such as varying load or intermittent output of distributed generation. An effective device for voltage control in distribution networks is the on-load transformer tap changer equipped with automatic voltage control (AVC) relays. In order to increase security of supply, it is common practise for the DNO to parallel transformers. A number of factors that can influence the optimum tap position are primary voltage level, load level, power factor, distributed generation and circulating current. This paper presents, firstly, an overview of automatic voltage control methods for paralleled transformers and related standard DNO practice. Secondly, a detailed description of the established transformer automatic paralleling package (TAPP) schemes is presented. Finally, modelling and analysis of the TAPP scheme is performed for a range of distribution network case studies in order to determine and evaluate the effect of the above factors on optimum tap position. Results from the modelling and analysis of the TAPP scheme and conclusions are also presented.