Analysis ofModulation Controllers of

Thispaperdescribes partofourrecent worksona project ofChinaSouthern PowerGrid(CSG),whichinvolves analysis anddesign ofwide-area coordinated HVDC modulation controllers todampoutinterarea oscillations forCSG in2008. Thepaperwillputtheemphasis onanalysis issues. Duetothe limitation ofmodeling ofHVDC control loopsinsmallsignal stability analysis, previous workson similar project ofCSG strongly depended ontrial anderrormethod, whichtests eachset ofcontroller parameters withtime-domain simulation toobserve howthecontrollers work.Thepaperinduces a novelHVDC modulation modelcompatible withNEVA.Basedonthenew model,smallsignal stability analysis, together withrootlocus technology, givesdeeperinsight intothemechanismof oscillations andeffect ofmodulation controller. Somemeaningful analysis results ofHVDC modulation controller arepresented in thispaper,including different roleof respective HVDC modulation controller andofwide-area feedback signals. A strongnonlinear modalinteraction phenomenon whichisso called 'modalresonance' isdetected atthesametime, whichis seldomreported inliteratures. Consequently, a schemeof coordination ofmultiple HVDC modulation tomitigate modal resonance isproposed. Finally, some analysis results are validated by mostdetailed HVDC modelandtime-domain simulation.