Research On The Coordination Of PSS And HVDC Modulation Damping Low Frequency Oscillation

By 2005, a nation-wide interconnected power system has been basically formed. TheNortheast China, North China and Mid China are connected by AC ties, while East China,South China, and Northwest China by HVDC ties. The interconnected grid has loweroscillation frequency, more oscillation modes and wider influenced areas. In addition,when HVDC transmission lines are embedded into synchronized AC system, a newAC/DC hybrid grid structure is developed and the South China Power Grid is itsrepresentation. All of the above trends bring new issues into power system stability andcontrol especially for low frequency oscillation.The power system stabilizer (PSS) has been proved effective for damping both thelocal mode oscillations and inter-area mode ones. Recently, the role that HVDCmodulation plays in suppressing inter-area mode oscillation is emphasized. In this thesis,borrow-damping principle of both PSS and HVDC modulation is investigated, andcoordination of PSS and HVDC modulation in an AC/DC hybrid grid is focused on. Themain research work is summarized as follows:1) The theory of small-signal stability analysis is generalized, and based on theborrow-damping principle of PSS, borrow-damping principle of HVDCmodulation is proposed, that variation of gain of HVDC modulation doesn'tinfluence the general systemic damping.2) By comparing PSS and HVDC modulation, the two methods for enhancingdamping, conclusion that borrow-damping principle unifies them is given. Bothof them increase damping of modes strongly related to them, meanwhile,decreasing damping of other modes.3) The coordinating strategy and effect of PSS and HVDC modulation are discussedin detail. 2DC system and the South Grid are used as test systems. SSAP andBPA developed by China Electric Power Research Institution (CEPRI) are usedas tools for frequency-domain analysis and time-domain simulation.4) The principle of HVDC modulation is further explained from the view of energydistribution. HVDC modulation can suppress active power oscillation of parallelAC ties by increasing its own active power oscillation and that of AC lines onwhich active power is flowing to the modulated DC line.