Local Propagation Mechanism And Modeling Method Of Subsynchronous Resonance

Large-scale long-distance transmission lines with series compensation is the most economical way so far and is widely applied in china. But the subsynchronous oscillation results from the interaction of large-scale thermal power units and series compensated transmission line,is a serious threat to the security and stability of power system operation, so it is an urgent problem that must be resolved in suppressing subsynchronous resonance(SSR). So far the traditional modeling method(TMM) is the way to obtain the practical SSR model for an actual system, and it is the basis for the SSR analysis and suppression-technology study. However, the details of the SSR local propagation mechanism(LPM) remain unknown so that they cannot theoretically support the TMM. Thus, the unreasonable partition of modeling zones limits result in inaccurate results for SSR analysis and suppression-technology study. In order to solve this problem, this paper studies the LPM and modeling method of SSR.(1) This paper deduces the formula for two components of electromagnetic torque respectively, which is produced by subsynchronous frequency resistive current and inductive current, and it can be found that the subsynchronous frequency resistive current provides negative damping torque, while the subsynchronous frequency inductive current provides synchronous torque. Thus, it reveals the fundamental reason for the weak damping or the negative damping of the torsional mode of the generator shaft is the excess high ratio of the sub-synchronous frequency equivalent resistance and equivalent reactance. The partial propagation mechanism of subsynchronous resonance(SSR) caused by interaction between turbine-generators and series compensated line is proposed on the base of the induced mechanism of SSR. According to partial propagation mechanism of SSR, it is clarified that the electric resonance constrains the propagation of SSR.(2) According to the partial propagation mechanism, we deduce the formula relating the propagated subsynchronous power and the electrical negative damping coefficient. By using the fomula, then a new method of identifying SSR modeling boundaries was developed. The method is: The research unit can be equivalent to the superposition of synchronous frequency voltage source and the subsynchronous and supsynchronous voltage source. By using the simulation measurement of consumption of subsynchronous and supsynchronous active power and reactive power of each branch, setting the corresponding threshold value, keeping all the branches whichis greater then the threshold value as the study zones of SSR and deducing, This method provide the valuable reference for practical engineering research on SSR.