DC voltage control in stacked converters at low pulse numbers

The increased demand of power from renewably energy sources has driven the development of wind power offshore forward. The power ratio has increased and problems with HVAC cable systems have led the industry to look at other options. One way to handle the increased amount of power is to use HVDC transmission. However the HVDC technique based on thyristors can not be used since it requires a stiff alternating current source at both ends. Instead VSC based HVDC is a possible solution. The VSC based HVDC used today has never been optimised regarding volume which is needed since offshore platforms are very costly. Therefore a new solution has been developed which builds on the idea of connecting a certain number of VSCs in series. In this way harmonic elimination by using phase shifting can be used. This technique leads to lower demand for filter capacity, which saves volume. In this case the phase reactors have been removed and the needed control inductance is obtained from the transformer. In this thesis two different control models for the series connected VSCs has been developed and tested. The first one, control model A, works by seeing the series connected VSCs as a single large VSC. Therefore only one current vector controller and one DC-link voltage controller is needed. The other model is control model B and it uses a current vector controller and a DC-link voltage controller for each VSC. The results from the tests show that control model A works as thought but further tests are needed. This is due to problems with the distribution of the DC voltages for the VSC. Control model B on the other hand does not work as thought. The current vector controllers have substantial problems with step response and the DC-link voltage controllers can not perform a start. This model does also require further tests