The country of Sweden, where one of the first three-phase transmission circuit was built late in the 19th century, also pioneered the development of Series Compensation of EHV transmission corridors. The first 220 kV installations was built in the early 1950´s, as series Capacitors were introduced in the 1000 km long transmission circuits from Sweden’s hydro resources in the north to the load centers in the southern part of the country. Using extensive series compensation proved to be the superior choice when the transmission levels had to reach above the natural stability limits.
Over the years improvements in the material used and in the production technology of the high voltage capacitors, have increased the energy density and decreased losses and space requirements tremendously. Consequently it has proved extremely cost effective to upgrade the old installations with the state of the art solutions in series compensation technology. In the Swedish 400kV grid there were at the turn of the millennium some 5000 Mvar of Series Capacitors installed.
The problem and the solution
A recent improvement of the grid performance was realised with the installation of the 400kV/100Mvar Thyristor Controlled Series Capacitor (TCSC) in Stode. This installation eliminated the operational concern related to Subsynchronous Resonance (SSR). SSR was experienced with one of the large nuclear generating units located at the receiving end of the transmission, when certain line outages occurred. Before the installation of the TCSC in Stode, the series capacitor had to be bypassed regularly, with possible power transfer restrictions following.
The alternative
The TCSC eliminates the electrical conditions that are necessary for SSR to develop, such that the grid can always be operated with optimum compensation. In this respect the TCSC provides unequalled functionality as the only available alternative for the grid owner was to reduce the amount of series compensation.