| Under certain conditions, compared with traditional AC transmission systems, high- voltage direct current transmission(HVDC) has a great advantage, which has asynchronous capabilities, line capacity, small net loss and the power is easy to be controlled. Since 1954, the world’s first HVDC project(Gotland and the Swedish mainland HVDC project)was put into operation in Sweden, the operation of the HVDC system has a history of nearly 60 years. C hina has a vast distribution of energy reserves and uneven economic development, which led to a large number of western power delivered to the eastern coastal areas. For long-distance and high-capacity transmission, there are more advantages to use HVDC transmission system.subsynchronous oscillations may be caused by HVDC controller in power system, the fundamental reason is that the rapid control produces negative dampings on turbine generator shafts.Subsynchronous Oscillation Damping controller(SSDC) was first put forward in EPRI EL 2708, the American electric science institute’s report, to restrain the subsynchronous oscillations in HVDC system. The fundmental function is to provide the appropriate positive damping for steam turbine generator in the subsynchronous frequency range, thereby effectively suppressing the SSO. At present, the design principles of SSDC are proposed in engineering fields, but none of synthetical and specific design methods are given.It is focused on the stability analysis of subsynchronous oscillation in HVDC system, and a detailed introduction on SSDC design methods are discussed in this paper, including the following aspects.(1)The Hopf bifucation phynomenons of subsynchronous oscillation in HVDC systems was researched in this paper. Based on PSCAD/EMTDC software, it builds the HVDC simulation model to observe how the constant current controller’s parameter variations influence on subsynchronous oscillations, and simulate the Hopf bifucation phynomenons. By using the sofeware MATLAB, it can process subsynchronous oscillation waveform datas and make the critical stability parameters automatically come true.(2)A new algebraic stability criterion which considers the synchronous torque coefficient and Electrical damping torque coefficient in both AC and DC systems was proposed in this paper. It is a necessary and sufficient condition to determine the stability of the system, which based on the equations of the shaft motion and the complex torque coefficients concept.this criterion verify and expand the the theory of complex torque coefficient methods by compared with Canay’s complex torque coefficient method. This paper gives both AC and DC system examples to use the criterion respectively, the calculation results are identical to the simulation results.(3)The SSDC design principles commonly used for domestic projects was deeply analyzed and verificated. There is intersection angle between angular velocity and the electromagnetic torque, the relationships between electrical damping and intersection angle are analyzed according to the SSDC mechanism. There are some conclusions made by simulations that the system can be retained stable when the angles are in the range of(-90,90)degrees, whether the electrical synchronizing torque coefficients are positive or not. In addition, it is believed that 45 degrees margin in projects can meet the system stability requirements when there are small disturbances in power system.(4)A synthetical design method about subsynchronous oscillation damping controller based on wide frequency broadband is proposed in this paper. It is refered to the classic automatic control theory and synthetical methods. The first step is to put forward design indexes according to the phase charactors in order to determine a certain quantity o f the lag-lead links and the band-pass filter parameters. Second the SSDC augments are calculated according to formulas. It is suitable for practical engineering for the SSO frequencies in a range can be restrained. |
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