LCC Bipole with one station earthed via a large capacitor : Theory and simulations

If one of the two poles will trip for some reasons in a regular bipolar HVDC transmission without a return conductor, it is necessary to block the other pole temporarily during the transition to a monopolar operation with metallic return, this procedure will take 1-2 seconds and will affect the connected AC networks by extra disturbances. The object of this survey has been to answer the question if it is possible to have a balanced bipolar transmission without a return conductor and during a trip of one pole control it as an HVDC transmission with series connected converters by adding the by-pass switches and the logic for a series connected HVDC converter system which means that a shutdown of the transferred power during the transition phases is not necessary. The survey has been performed with the software PSCAD where an already existing series connected converter circuit had been modified to a bipolar system and the control logic for the transition phases has been created. There were some problems with various control signals between the software HiDraw and PSCAD, this was solved by different methods and the result shows that it is possible to have a transition between bipolar operation and monopolar operation with metallic return without the need to totally shut down the power transfer completely. But this requires a large capacitor