Damping Analysis Of HVDC System Based On CTC Method In The Frequency Domain

High Voltage Direct Current transmission (HVDC) will cause torsional interaction under certain conditions. Among various factors improper rectifier control has fateful effect. Negative damping is the essence of this phenomenon, which results in fatigue damage in generator shaft system. It goes against system stability. With more HVDC projects brought into operation, large-scale AC/DC parallel transmission system turns into development trend. Torsional interactions between HVDC and nearby generators become more and more obvious. It is quite necessary to study Subsynchronous Oscillation (SSO) casued by HVDC, while network topology structure and technical innovation of HVDC make the theoretical analysis more complex. From the viewpoint of electrical damping, the paper focuses on issues related to SSO caused by HVDC by using Complex Torque Coefficient method (CTC).Firstly, the paper briefly introduces SSO phenomenon, and expounds three explainations about the mechanism of this phenomenon. Research status relates to impact factors, analysis method, and control measures is presented to indicate what need improvement in existing achievements, which provides guidance to this study.Secondly, according to the need of theoretical analysis, the detail models of research objects are built, which includes HVDC primary system model, AC/DC parallel system model, and HVDC control system model. CTC is a common method for analyzing SSO. Combining this method with system model, mathematical expression of electrical damping is derived, that lays a foundation for the following analysis.Chapter 3, 4, and 5 are the main parts of the paper. Based on the model, it analyzes the damping characteristics of HVDC system by using CTC combined with frequency scanning method. According to the mathematical expression, programs are written to calculate and draw the curves of electrical damping. Theoretical studies consist of three aspects: damping analysis of generator in the rectifier side; damping analysis of generator in the inverter side; and damping characteristic of AC/DC system. Researches on rectifier side focus on the influence of Linear Constant Current control (LCC), Linear Constant Power control (LCP) and Nonlinear Constant Current control (NCC) and their parameters. In addition, the parameters of DC line, strength of AC system on inverter side, DC power level, firing delay angle, operation in reducing voltage, reactive power compensation of the converter station, and phase locked loop are also considered as influencing factors. Calculation results demonstrate rectifier control strategy and its parameters have the most remarkable influence on SSO, and NCC has the best positive damping characteristic. Special attention should also be paid to DC reactance, DC power level, firing delay angle, operation in reducing voltage, and reactive power compensation of the converter station. On inverter side, studies are mainly related to inverter control, including Linear Constant Voltage control (LCV), Linear Constant Extinction Angle control (LCEA), and Nonlinear Constant Extinction Angle control (NCEA). Results show that the system has positive damping to SSO when the inveter is under LCV and NCEA control. Only under LCEA control there is small negative damping in low frequency range. Therefore, torsional interaction usually will not happen to the generators nearby the inverter.Studies on SSO of AC/DC system include the infulence of AC transmission line together with its Series Capacitor Compensation (SCC). The results show that torsional interactions can be alleviated to a great extent when HVDC system parallels with an AC transimmsion line without SCC. When the AC transmission line is compensated by SCC, an obvious degeneration trend happens to the damping characteristics, which are mainly determinated by the compensation degree and the resistance of the AC transimission line.Lastly, according to the essential reason of SSO, three different Supplemental Subsynchronous Damping controllers are designed to improve the damping characteristic of linear control strategy on rectifier side. They all improve the system damping in different degree.This paper is supported by National Natural Science Fund of China (50807016) and Guangdong Natural Science Funds Project (9151064101000049).