| HVDC has 40 years of development history. It has many characteristics such as large transmission power, highly controllable, quickly adjusting, flexible operating and so on. HVDC has been widely used in modern power system because of its advantage in long-distance transmission capacity and regional power system connection. However, HVDC system has the possibilities to cause subsynchronous torsional vibration interaction with turbine generator shaft mechanical system. Today, its principles are quiet clear. It is intrinsically linked with HVDC control system characteristics. Due to large gain and phase lag in low torsional vibration frequency, HVDC system will adversely cause torsional vibration interaction. This paper mainly focuses on the research of subsysnchronous oscillation (SSO) caused by HVDC transmission system, its main contents include the following aspects:1) Combined with small signal model of power system with HVDC and classic control theory, a kind of design method for HVDC supplementary subsynchronous damping controller (SSDC) is proposed in this paper based on modal separation design method and optimal phase compensation. Then, suitable SSDC for Xing-An HVDC system is designed and the SSO suppressant effect of SSDC is also indicated from two aspects including Eigen value analysis and electrical damping curve. Meanwhile, the influence on control effect while changing SSDC parameters is also analyzed. Finally, the following designed SSDC was added into PSCAD/EMTDC model of Xing-An HVDC system and its effect was further proved by time domain simulation. 2) A visualized software for SSO/SSR small disturbance analysis in power system is developed. Object-Oriented technology and modularization was the main two technologies used in small signal modeling of this program. Modularized modeling method was used not only in electric power elements modeling such as generator and HVDC system, but also in network modeling and power system modeling. A general method for electric network modeling based on electromagnetic transient model is also put forward so that the program can be suitable for AC/DC power system with any kind of network structure or operation mode. Finally, key technology used in programming, structure, function and characteristics of MANSDAP which was developed by using the above technique were introduced in detail.3) MANSDAP is used for SSO/SSR small disturbance analysis in practical AC/DC power system. Combined with several example system, first combined with benchmark HVDC system in EPRI report, this paper indicated the influence of DC transmission power, firing angle at rectifier, synchronizer characteristics and parallel AC system strength on electrical damping of rectifier side generators. Then, combined with Xing-An HVDC system with several kinds of operating mode(island operating mode, strong operating mode, weak operating mode) and a series compensated system in Northern China, from two aspects including Eigen value analysis and complex torque coefficients analysis, this paper proved that the calculating speed and scale of MANSDAP is suitable for general scale of AC/DC power system and its calculating result is also accurate and convincible.
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