The Improvement Of Small Signal Stability Analysis Software And Its Application In Ac/dc Parallel Transmission Power System

Modern power systems become large even ultra-large ones. Small signal stability becomes one of the main factors which threaten the operational safety and restsict the transmission capacity of the system. In order to study the characteristic of AC/DC large-scale power system, Shanghai Jiaotong University developed the small signal stability analysis package (SSAP). In this paper, according to the practical requirements in the small signal stability analysis of the large interconnected AC/DC parallel power system, modifications and improvements of SSAP are made in the eigenvectors calculation method, regional eigenvalue search, eigenvalue identification and HVDC quasi-steady model verification, meanwhile the small signal stability of Southern Power Grid is analyzed by SSAP. The contents are as follows:(1) Eigenvector sparse method in the small signal stability analysis is studied. The sparse inverse iteration method for augmented state matrix is deduced and the implementation of this method is illustracted in details. Results show that the sparse inverse iteration method is superior to sparse Newton's iteration method both in efficiency and convergence speed.(2) A new regional eigenvalue search strategy is proposed based on user-defined search damping ratio. The strategy solves incomplete calculation problems caused by improper parameter settings and strengthens the additional search strategy. By the comparison of two strategies, It shows that absolute search area is larger, and the computation is more reliable in new search strategy. It is the first time to propose the eigenvalue identification, to explain the reason of extraneous root and propose the judging criteria.(3) Small signal quasi-steady-state model of High Voltage Direct Current (HVDC) is introduced in this paper. A detailed verification of HVDC linear quasi-steady-state model for the low frequency small signal analysis is made by the comparison of EMTDC and MATLAB time-domain simulation. Further verification is made by comparing with the calculation results of SSAT software developed by PowerTech company of Canada in a large-scale power system. Results show that the HVDC quasi-steady-state model in SSAP is valid.(4) The small signal stability of Southern Power Grid in 2010 is analyzed by SSAP. HVDC dual frequency modulation, power modulation andγ modulation are added to suppress the interarea low frequency oscillations. Eigenvalue results show that: using HVDC rapid modulations can improve the system damping effectively. Factors which affect small signal stability of Southern Power Grid are summarized, including the HVDC model, HVDC transmission line capacity, HVDC modulation and small hydropower unit modeling.