Application of FACTS in the Portuguese transmission system: investigation on the use of phase-shift transformers

In keeping with European Union Directives for promoting renewable based energy, a significant increase in installed wind and small-hydro power is expected in Portugal in ten years. A significant part of the identified renewable potential is located in the Northwest region, where a power generation surplus already exists. Therefore, expansion of the existing transmission grid is required, in order to carry the excess power to the load centres. In this paper three possible expansion solutions are investigated: 220 kV expansion, 400 kV expansion equipped with a standard auto-transformer or with a phase-shift transformer (PST). These three alternatives have been evaluated in what concerns each one´s contribution to power loss reduction and its performance both in steady-state normal operation and in contingency status. The studies performed allow the conclusion that the installation of a PST, which would be the first unit in Portugal, is an added-value to the transmission grid. Given its ability to control active power, the PST ensures that the 400 kV subsystem is the preferred route for power transportation between the generation and load centres. Therefore, a significant reduction in power losses is achieved and a positive contribution is given to lower the power flowing through the existing 220 kV subsystem, both in normal steady-state and contingency modes of operation