Addition of four-quadrant power controllability to multi-level VSC HVDC transmission

In self-commutating multi-level voltage source conversion (VSC) transmission, the reactive power contributions of the two converter terminals are interdependent. To overcome this problem, a new control scheme is proposed, applicable to high voltage direct current (HVDC) interconnections of large power ratings with more than one 12-pulse group per converter. The basis of the proposed strategy is the use of a controllable phase shift between the firings of the individual converter groups. The theory is verified by extensive EMTDC simulation.