The Research On Security Evaluation And Analysis Of Unit-Grid Dynamics In Power System

The generating unit damage or instability of power grid resulting from the interaction of generating unit and power grid often occurs in domestic and overseas power system. The long-distance high-power transmission of generating unit through power grid has become a prevalent mode as the implementation of "West Electricity East Transmission" policy in China. As a result, one problem is low frequency oscillation of inexplicit mechanism often occurs, the other is subsynchronous oscillations (SSO) stand out as the application of series capacity compensation and HVDC transmission line. The generating unit and power grid belong to different power group after the innovation of power system in China. So the inconsistency between protecting generating unit from damage and ensuing power grid stability desiderates to be solved. Analyzing thoroughly and evaluating the Unit-Grid dynamic process of power system is the foundation of ensuing the security and stability of large generating unit and power gird.The Unit-Grid dynamic process is analyzed from the rigid body motion stability point of view in this paper. Firstly, according to the theory of single-rigid body motion stability, the effect of governor system disturbance on power system small signal stability was analyzed through putting up the Unit-Grid coupling model and digital simulation platform. Simulation results indicate that the constant amplitude low frequency oscillation will occur when the frequency of disturbance from governor system is near to the natural frequency of power system low frequency oscillation. Whereafter, according to the theory of multi-rigid body motion stability, the subsynchronous oscillation was analyzed by complex torque coefficient method. The subsynchronous damping controller (SSDC) was designed and the validation of SSDC was proved by simulation. The stability boundary of multi-rigid body motion was researched and the turbine-generator shaft model was improved. Shaft damage of Zhangze power plant was analyzed using Complementary Cluster Energy Base Criteria (CCEBC). Simulation result indicates that CCEBC is an effective method on stability analysis of heteronomy shaft torsional vibration because it can consider the Unit-Grid energy exchange after fault.For the inaccurateness problem of shaft parameter, the uncertainty shaft parameter model was adopted to describe turbine generator shaft. The shaft parameter sensitivity for eigenvalue was calculated. The change of shaft parameter with large sensitivity has distinct influence on simulating subsynchronous oscillation. The Probabilistic Collocation Method (PCM) was adopted to simplify the relationship between the uncertainty in shaft parameter and the oscillations of shaft system. Shaft parameter uncertainty will result in high uncertain degree and the SSDC designed in this paper can effectively damp SSO even if the shaft parameter is uncertain.The mechanism of shaft damage was analyzed basing on material mechanics theory. The evaluation method about the effect of SSO on turbine generator shaft was put forward. If the cut stress exceeds intensity limit, the shaft will be destroyed. If the cut stress is between intensity and fatigue limit, the fatigue damnification should be calculated. By applying rainflow algorithm, the SSO of variational amplitude was decomposed into a series of costant amplitude SSO. And then, the fatigue damnification was calculated according to the shaft S-N curve. If the fatigue comulation coefficient is equal tol, the 100 percent life of shaft will be used up. Consequently, the shaft will be destroyed due to fatigue invalidation. The validity of the evaluation method about the effect of SSO on turbine generator shaft was shown through the subsynchronous resonance of IEEE first benchmark model and subsynchronous oscillation induced by HVDC examples.