Research On The Countermeasures To Subsynchronous Oscillation Of Power Systems

With the extended implementation of west-to-east power transmission project in China, long distance and high power transmission technique is imperative. In order to improve power system stability and enhance the level of power transfer capability of existing transmission lines, series capacity compensations and HVDC transmission systems will be widely used in our country. However, subsynchronous oscillations arising from the series compensations and HVDC become a non-ignord problem which will threaten the safety of the power systems. So it is of distinct value of engineering application and pratical significance to research the Countermeasures to subsynchronous oscillations of the power systems. This dissertation focuses on several typical damping solutions with new ideas and new design methods. By means of electrical damping analysis and the simulation validations, the damping effects of each scheme are respectively studied. The main works are organized as follows:(1) In order to describe the strength of the torsional interactions of the turbine-generator units, torsional interaction factor(TIF) is defined and is taken as an index to estimate the existence of torsional interactions of units. By time domain simulation method, the torsional interactions of turbine-generator units are analyzed. The simulation results validate some concepts on the torsional interactions of the units and provide the basis for the reduction principles of multi-machine power systems for SSO study.(2) The realization and the application field of the test signal method which is taken as the analysis method of this dissertation is introduced. Based on a practical project in Tuoketuo power plant, an integrated approach combining the calculation of electrical damping versus frequency and time domain simulation is applied in analyzing the SSR problem of Tuoketuo Power Plant caused by its series compensation transmission scheme. The final results can be used as guidance to design the series compensation transmission scheme for Tuoketuo Power Plant phase four. And by this example it shows that the test signal method is easy to implement and suitable for engineering application.(3) The solution to mitigate the SSO based on the excitation system is studied in detail. The influences of several typical excitation systems and PSS without any additional damping control on damping characteristics of unit's subsynchronous resonance are analyzed. Based on the analyses, a method for damping SSO by designing the phase compensation block of PSS is studied. The electrical damping calculation results show that this scheme can mitigate the SSO to some extent, but the damping capability is not absolute. Further a damping scheme using a supplementary excitation damping controller is analyzed and a design mehod based on phase compensation is presented. The effectiveness of the SEDC for damping the SSO is validated by both the electrical damping analysis and the time domain simulation.(4) The simulation model of TCSC considering the switching circuit is built by PSCAD/EMTDC. The study system is modified from the first IEEE SSO benchmark model by changing a part of the fixed series capacitor to TCSC. The open-loop control and three types of close-loop control including constant current, constant power and power oscillation damping control are respectively considered. The impacts of the thyristor conduting angles and the control parameters of the TCSC on the electrical damping are studied. The conclusions provide the references for the design and the opration of the TCSC to avoid the SSO.(5) A comprehensive research on SVC for damping the SSO is developed. The basic operation principle of the SVC to damp the SSO,the various control strategies and the method of estimating the size of the SVC are all introduced in detail. Then a simulation model of SVC is built and its proportional controller is designed. The effectiveness of the SVC for damping the SSO is validated by both the electrical damping analysis and the time domain simulation. Further a detailed practical project is studied to exhibit the effctiveness of the SVC for damping the SSO.(6) The mathematic model of SSSC in dq reference frame is founded and a double loop control including an inner loop adopting current control and an outer loop adopting voltage control is adopted to control the inverter of the SSSC. A detailed model of the SSSC including its switching circuit and its controller is built. The effectiveness of the control system is validated by the simulations. Then the IEEE first benchmark model with a SSSC as a part of the total compensation is considered and the influences of the SSSC on the subsynchronous damping characteristics are investigated.(7) Aiming at the SSO problem arising from HVDC, its mechanism is analyzed and the principle of damping the subsynchronous oscillations is expounded. Then a method of designing the SSDC for an HVDC transmission system based on the phase compensation is presented. This method is of clear physical concept and can be implemented easily. The electrical damping analysis and the simulations results show that the designed SSDC can effectively damp the SSO arising from HVDC.