Research On Subsynchronous Oscillation Damping Characteristics Of TCSC

With more and more large stream turbine generator units being put into operation, series compensation techniques are required in most long-distance power transmission cases, in order to solve problems on transfer capacity. However, this trend also highlights related risk of shaft torsional vibration. Moreover, operation scenarios are more diversified with the increase of applications of Thyristor Controlled Series Capacitor (TCSC), and it further brings about more challenges. On the other hand, although many researches are contributed to the effect of TCSC on Subsynchronous Oscillation (SSO), controversies still exist on many related topics. Therefore, it is highly necessary to do some further research on the capability of TCSC for SSO damping which is selected as the study object of this paper, in order to fully understand characteristics of TCSC and provide theoretical guidance for its effective operation.From this viewpoint, aiming at problems in existing researches, this paper focuses on subsynchronous frequency impedance characteristics of TCSC, TCSC modeling for SSO analysis, methods for torsional vibration analysis of systems with TCSC, factors influencing TCSC's capability for damping SSO, and supplementary damping control of TCSC for SSO. The main contents include the follows.1. In the light of problems in existing researches on subsynchronous frequency impedance characteristics of TCSC, an independent analytic method is proposed for this characteristic analysis, based on an appropriate source excitation, considering different synchronization methodologies for firing control and the influence of subsynchronous frequency component on turn-on and turn-off of thyristors. Based on this method, reasons of subsynchronous frequency impedance characteristics of TCSC different from Fixed Series Compensation (FSC) is analyzed in detail, and also the influence of synchronization methodology for firing control, firing angle, subsynchronous frequency, and main circuit parameters.2. Applying the above analytic means of TCSC's subsynchronous frequency impedance in frequency scan method, based on mutual relations among system subsynchronous frequency equivalent impedance, system electrical damping coefficient, decrement factor of torsional modes and shaft modal parameters, a methodology for preliminary evaluation on shaft torsional vibration stability of systems with TCSC via frequency scan is further proposed. Therefore, the problem that traditional frequency scan method cannot be utilized in systems with TCSC is solved. Figures are used to elaborate how to analyze the extent of torsional vibration (un)stability according to subsynchronous frequency equivalent impedance of the system.3. By means of dynamic phasor modeling method and with some hypotheses, four types of dynamic phasor models of TCSC are established respectively based on two control methodologies and two synchronization methodologies for firing control including open-loop and close-loop constant impedance control and line current zero-crossing based and equidistant interval based synchronization methodology, which expands utilization of dynamic phasor modeling method. These models are developed in the form of injected current in PSASP by its user defined(UD) modeling function, for the convenience of model's interconnection with the system. Moreover, the results of eigenvalue analysis based on these models are compared with those of EMTP time-domain simulation, and the correctness of these models is verified.4. Frequency scan and eigenvalue analysis are applied respectively in IEEE First Benchmark Model system, to compare stability of torsional modes among with FSC only, TCSC only and both FSC and TCSC applied. The effects on SSO damping of firing control methodology, firing angle, control parameters, main circuit parameters of TCSC, and compensation ratio of TCSC to FSC when they are used together are also observed. The results show that TCSC has a certain capability to damp SSO, but the damping of torsional modes with TCSC is not entirely better than with FSC in all circumstances. Different firing control methodologies, control parameters, main circuit parameters of TCSC, and compensation ratios of TCSC to FSC make different impact on damping in different compensation level areas, and therefore selection of these methodologies and parameters needs to be well balanced.5. A supplementary damping control strategy of TCSC for SSO considering both damping effect and control reliability is proposed, in which the damping control based on remote shaft speed deviation as the main and the one based on local electrical quantity as the standby. When there is a pause or excessive delay fluctuation in remote signal transmission, main control will be switched to standby control promptly, and therefore the problem of lacking reliability or having weak control effect when relying only on the remote signal or local signal is solved. Effectiveness of this main/standby control scheme is verified by time-domain simulation.