Ancillary Service for Transmission Systems by Tap Stagger Operation in Distribution Networks

With the integration of intermittent renewable generation, transmission voltage problems have become more dynamic. The deployment and tuning of numbers of VAr compensators at various locations may not be cost-effective. This paper presents an alternative method, which utilizes existing parallel transformers in distribution networks, to provide reactive power absorption services for transmission systems with low demands. The operation of parallel transformers in small different tap positions, that is, with staggered taps, can provide a means of absorbing reactive power. The aggregated VAr absorption from many pairs of parallel transformers could be used to provide voltage support to the upstream transmission network. This paper focuses on the economic and technical analyses of using the tap staggering method. For the economic analysis, the associated costs of applying the tap staggering technique are investigated through static load-flow studies. The IEEE Reliability Test System is used to carry out the studies and the results are compared with the installation of shunt reactors. In the technical studies, the dynamic impacts of tap staggering or reactor switching on transmission voltages are analyzed. From the results, the tap staggering technique has more economic advantages than reactors and can reduce the voltage damping and overshoots during the transient states.