Dynamic Modeling Of Thyristor Controlled Apparatus And Subsynchronous Damping Control

With the development of power systems in China, it is desired to utilize thetransmission line corridors effectively and increase the transmission capacity. Sincethe series compensation may shorten the electrical distance of transmission line, it iswidely used in transmission systems from the coal-power bases. High Voltage DirectCurrent (HVDC) transmission lines are also widely used in China with itsadvantages in transferring heavy load over long distance. However, seriescompensation system may result in Subsynchronous Resonance (SSR) and causeharmful damage to the shafts of turbine generators. Studies showed that there areunstable SSR in Yimin, Tuoketuo, Shangdu and Jinjie power plants. The analysis onGuizhou-Guangdong Ⅱ HVDC transmission system indicated thatSubsynchronous Oscillation (SSO) may occur during islanded rectifier operation.SSO in Gaoling back-to-back HVDC project is under investigation now.Although SSR is different with SSO in mechanism, the methods of analysis andcontrol strategies are quite similar. The thesis focuses on precision modeling ofthyristor controlled apparatus as HVDC and SVC, the mechanism of oscillationtransferring and the torsional interaction of SSR/SSO in AC/DC systems,furthermore, the subsynchronous damping control stratgies. The main contributionsof this thesis are as follows:(1) An improved sampled-data model of12-pulse HVDC converter is presentedin this thesis, in which the igniting delay under small perturbations is considered forthe first time. Based on the equidistant firing control scheme and Phase LockLoop(PLL) control both of which are commonly used in modern power systems, themodel of HVDC system is represented. Under small perturbations, the output offiring controller can not affect the converter directly because of the ignition delay. The process of igniting is considered in detail, therefore, the sampled-data model isimproved. The model is compared with PSCAD simulaiton by eigenvalue analysisand complex torque coefficient method. The validation shows that the improvedsampled-data model is more accurate and effective in studying SSO of the HVDCsystems.(2) Based on separate modal design principle and phase compensation method,the SubSynchronous Damping Controller (SSDC) is proposed. Since SSDC withconstant parameters is not robust enough, an adaptive SSDC is proposed based onBP neural network to mitigate the SSO in Gui-Guang Ⅱ HVDC system. As thegenerator speed must be acquired far from the rectifier station, the frequency of ACbus is taken as an alternative input signal of the SSDC. Comparision shows that bothtypes of SSDC are equally effective at the designated operating point.(3) The mechanism of the SSO transferring and electrical damping is illustratedin this thesis, furthermore, UIFfwand UIFbware specially defined for the first time,which represent the “Forward Unit Interaction Factor” and “Backward UnitInteraction Factor”, respectively. By means of analyzing the impact of AC systemson electrical damping compensated by the SSDC, the frequency of rectifier AC busresponding to generator rotor oscillation can be defined as UIFfw, and the generatorelectrical damping compensated by SSDC can be defined as UIFbw. The product ofUIFfwand UIFbwequals to UIF, which is used to denote the electrical distance of theunit from the HVDC system before.(4) A sampled-data model of SVC is proposed for the first time in this thesis.Comparing the sampled-data model and quasi-steady state model with the detailedmodel in PSCAD, the results show that the sampled-data model is more precise thanthe quasi-steady state model. The quasi-steady model of SVC over-estimates theeffect of subsynchronous damping controller in SVC. Based on the sampled-datamodel, a supplementary subsynchronous damping controller in SVC is designed,which is validated by mitigating the SSR in Jinjie series compensation transmissionsystem. (5) The torsional interacton among turbine-generator units in complicatedmulti-machine systems is analyzed in this thesis. Between two paralleled units withidentical parameters and loads, both approximate in-modes and anti-modes exist intorsional oscillations. As to the in-mode oscillations, the oscillation amplitudes oftwo generator shafts are almost the same, but two phases are much different. As tothe aiti-mode oscillations, the oscillation amplitude of the generator with light-loadare larger than the generator with heavy-load. The torsional interaction between twogenerator shafts with different inherent frequencies in two different power plants isvery weak. When the parameters of two generator units in different power plants areidentical, the torsional interaction between them is quite notable.(6) The unified complex torque coefficient method is applied into themulti-machine power systems. It is proved that each torsional mode has its uniquecorresponding expression for complex torque coefficient. The traditional complextorque coefficient methods are the simplified ones of unified multi-machine version.Though the precisely described complex torque coefficient method is a little bitweak in practice, however, it is very significant in verifying the simplifiedapproaches of the complicated power systems.