Controlling harmonic instability of HDVC links connected to weak AC systems

The effectiveness of providing positive damping of harmonic oscillations by either modifying the AC-system filter characteristics or adding to the rectifier control circuitry are examined. Two alternative measures of controlling harmonic instability are investigated: the use of C-type AC filters, and a simple addition of a harmonic damping circuit in the rectifier converter control. The design of a C-type filter is presented and its effectiveness described. Harmonic damping by controls consists of taking the output from a fast-response DC current transducer, passing it through a filter and a gain, and adding the output to the firing-angle order. Both methods result in stable recovery from faults at both rectifier and inverter terminal busbars for a DC system inherently harmonically unstable