| Ultra high voltage AC transmission system (UHVAC) and ultra high voltage DC transmission system (UHVDC) have been used in China due to the reversed distribution of natural resources and load centers. In order to enhance the transmission capacity, series compensation was used in some UHVAC system, and therefore subsynchronous oscillation (SSO) may be excited. SSO will threaten the stability of the power system. The development of new energy scale is increasing in China. As a representative of new energy, wind power was integraed into the traditional power system based on thermal power, which would challenge the research on SSO. Considering the characteristics of power system in China, the following research has been carried out:When eigenvalue analysis method was used in analyzing subsynchronous damping, damping conservation phenomenon in SSO was discovered. In a series compensation system, the total system damping should be conserved when compensation level is varied. In a HVDC system with supplemental subsynchronous damping controller (SSDC), the total system damping should be conserved when the gain of SSDC is varied. With the increasing of SSDC’s gain, the damping of SSO modes increase, but the damping of some other modes decrease, which means that the damping of SSO modes are borrowed from other modes. In the design of controller, a large gain may weaken the damping of other modes, even make the system unstable.In previous studies, it is considered that speed-based power system stabilizer (PSS) has negative influences on SSO, and power-based PSS has little influence on SSO. In this thesis, it is found that in addition to the type of input signal, phase compensation method of PSS is very important for SSO. The results show that the speed-based PSS with lag compensation and inverting almost has no effect on SSO. This kind of PSS can use the ideal speed deviation as the input signal, and avoid the torsional sensitivity.As an important part of the electrical damping of the generator, damper windings is very important to subsynchronous damping. The frequency of subsynchronous oscillation ranges from a few to a few tens hertz. Under different frequencies, the intensity of skin effect and magnetic saturation differ. Moreover, temperature, the structures of damping system and the errors of classic generator design formula will have impacts on the equivalent parameters of damper windings. Then SSO characteristics will be affected. Based on IEEE SSR First Benchmark Model, effects of damper windings’parameters on subsynchronous damping are analyzed. With the increasing of damper winding’s resistance, the damping of SSO mode 1 to mode 4 increases, and the damping of SSO mode 5 almost unchanged. The effects of damper windings’leakage reactance on SSO have no obvious regularity, but the errors of damper windings’leakage reactance may have great influence on subsynchronous damping capacity evaluation.In new energy generarion system, the drive train model of wind turbine generator is crucial for SSO analysis. Three-mass model is adopted in the thesis. The models of Doubly-Fed Induction Generator (DFIG)-based wind farm, DFIG’s control system and Line Commutated Conveter-HVDC (LCC-HVDC) are established. Eigenvalue, participation factor and time domain simulation are used in analyzing SSO of a DFIG-based wind farm interfaced with LCC-HVDC. The results show that SSO mode is sensitive to RSC’s parameters (KpQs, Kprd and Tird). If the controller parameters are not set properly, SSO with negative damping will appear, and the system will be unstable. As wind speed increases, the damping of SSO mode decreases slightly initially and then increases, yet the frequency increases slightly. If the operating state of HVDC and ac side bus voltage of rectifier station change, the SSO mode will be affected. As ac bus voltage of rectifier station increases, rectifier firing angle (a) increases. In the meanwhile, the damping of SSO mode decreases, and the frequency of SSO mode is almost constant.Inner Mongolia area is rich in coal resources and wind energy resources. Based on Hu-Liao HVDC system with wind farm, a detailed electromagnetic transient simulation model is established, and effects of wind farm integration on SSO characteristics of thermal generators are studied. The results show that in wind-thermal-bundled power system interconnected with HVDC, if the wind turbine generators can maintain stable operation, the subsynchronous damping of thermal generator increases with wind farm integrating. The larger the penetration rate is, the more obvious the increasement is. The closer the wind farm is to the thermal generator, the obvious the increasement is. Although wind farm integrating can reduce SSO of thermal generator, it can not fundamentally solve SSO problem of thermal generator.In order to look for effective damping control method for thermal generator’s SSO, Subsynchronous Oscillation-Dynamic Stabilizer (SSO-DS) based on voltage source converter is adopted in this thesis. The model of SSO-DS with two level is established. The mechanism of mitigating SSO by SSO-DS is studied. The effects of main circuit parameters and controller parameters on SSO damping characteristics are analyzed. When the inductor and capacitor parameters vary in the range of ± 30%due to environment changes, the variation of SSO damping ratio won’t exceed ±10%. With the increasing of damping control module’s gain, SSO damping increase. But a large gain may weaken the damping of other modes due to "borrow damping phenomena", even make the system unstable. In phase compensation block of damping control module, SSO damping weakens when compensation angle deviates from the optimum value.According to SSO in Hulunbeier power plant, SSO-DS with cascading H-bridge is designed, which is more suitable for the actual project. By the real-time digital and physical closed-loop simulation test, the control strategy is verified, and the control parameters are optimizated, also the validity of SSO-DS is verified. The operation records show that SSO-DS can mitigate SSO-DS effectively. |
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