Research On The Small-signal Stability Of The Hybrid Cascaded HVDC System

The characteristics of the reverse distribution of energy resources and load centers in China determines the necessarity of high voltage direct current(HVDC)transmission technology suitable for large-capacity power transmission over long distances.Due to the advantages of low cost and bulk power transmission capability,the line commutated converter-based HVDC(LCC-HVDC)is acknowledged to be one of the most mature solutions.However,with two or more LCC-HVDC links are fed into a regional power grid,resulting in a multi-infeed HVDC system.The multi-infeed HVDC system is vulnerable to the continuous commutation failure,putting a severe threat to the safe and stable operation of the receiving AC grid.To address this challenge,the modular multilevel converter(MMC)with self-commutation capability is adopted at the inverter-side.Therefore,hybrid HVDC system combining the merits of both LCC and MMC are developed rapidly.The hybrid HVDC system can be classified into three types: 1)hybrid multi-infeed HVDC;2)hybrid multi-terminal HVDC;3)hybrid cascaded HVDC.The small-signal stability of the hybrid HVDC system is very important.As for the small-signal stability of the hybrid multi-infeed and the hybrid multi-terminal HVDC systems,many scholars have conducted related researches.However,there is no relevant research on the small-signal stability of the hybrid cascaded HVDC system.This paper proposes a small-signal modelling method based on the interfaced equivalent circuits of LCC and MMC,which is suitable for the hybrid cascaded HVDC system.The smallsignal stability of the hybrid cascaded HVDC system is analysed in detail based on the established small-signal model.The main work of this paper is as follows:1)An improved dynami phasor model of LCC is proposed.It considers the actual sinusoidal variation of the valve current in the commutation process instead of linear approximation to that based on the quasi-steady-state equations in the existing literature,and is then employed in the small-signal modelling of the hybrid cascaded HVDC system.2)The interfaced equivalent circuit of LCC and the interfaced equivalent circuit of MMC considering the internal dynamics are derived,and a small-signal modelling method based those for the hybrid cascaded HVDC system is proposed.3)The small-signal model of the LCC-MMCB hybrid cascaded HVDC system is established based on the proposed small-signal modelling method.The accuracy of the smallsignal model is verified against the PSCAD/EMTDC time-domain simulation.The influences of two combinations of active power control objectives of MMCB,the master-slave control and the uniform control,are analysed based on the established small-signal model.