Studies On The Static Stability Situation Assessment Of Power System Under The Wide-area Environment

With the continuous expansion of power system interconnected range, the continuous development of power industry market reformation, and the rapid growth trend in large-scale renewable resources of power generation including wind energy, optical energy, etc, the power system running environment faces increasingly complex situation and the operation safety faces serious challenges. The maturity of wide-area measurement system (WAMS) technology and the development of smart power grid lay a foundation for on-line stability situation assessment and preventive control in power system. Currently, the on-line stability situation assessment, which bases on measured information, focuses mainly on two aspects:first, the static voltage stability situation assessment based on Thevenin equivalent method; second, the transient angle stability assessment based on measured power angle trajectory. Although the static stability of power system indeed involves two sides, voltage and power angle, the two are a dialectical unity under certain conditions. Consequently, facing the new situation, carrying out the study of on-line static stability situation assessment for power system under the wide-area environment, has important theoretical significance and engineering application value.Thus, the power transmission rules and the static stability mechanism is the fundamental to realize the "on-line assessment and real-time decision-making" power system monitoring framework. Under the wide-area environment, this thesis used Thevenin equivalent method as the means, and from the static stability point of view, the electromagnetic active power as a link to contact voltage and angle as a dialectical unity. Thus, the power system stability analysis theory is enriched, which contributes to the on-line stability monitoring implementation. The main research work and innovation of this thesis are shown as follows:(1) Formulating the identification of Thevenin equivalent parameters as an optimization problem, an iterative optimized identification method of Thevenin equivalent parameters based on trajectory sensitivity is proposed. Its objective is to minimize the difference between the square of calculated voltage and the one of the measured voltage, and its constraint is the physical limitations of identified parameters, which is simple and reliable to compute. The initial value of the identification parameter is adaptively step-changing adjusted based on the trajectory sensitivity of the equivalent parameters to the node voltage, which improves the identification precision. The simulation demonstrates the effectiveness and practicality of the proposed method, which has the prospect of application in real-time power system.(2) Inheriting the traditional static angle stability and voltage stability assumptions and concepts, a static stability region is constructed in the space of phase angle and power-factor angle (?-?). Based on this, a fast calculation method for quantitative load shedding is proposed, in which the node stability index is taken as an object. Based on local measurement information, the Thevenin equivalent parameters of each node can be acquired, so the stability patterns and the relative stability margin can be directly demonstrated. When the stability margin is at low degree, the approximate load-shedding can be quickly calculated for dispatcher to refer to. The method can indirectly avoid the calculation of power flow and specific critical value, and compensate for the defects of traditional method in the aspects of system scale, network parameters dependence, convergence and speed of calculation. Simulation results show the validity of this method, which has explicit physical meaning, requires less computation work and has the prospect of application in real-time stability assessment and preventive control of power system operating condition.(3) Based on Thevenin equivalent parameters, the analytical expression formulas of active power loss and static voltage stability related to power-factor angle under certain active power model are derived in this paper. According to these formulas, a real-time region reflecting the relationship between active power loss and static voltage stability margin is constructed in a two-dimensional coordinate system. Three new reference indexes are proposed, based on which node reactive power compensation can be quickly estimated and the objective of optimal reactive power flow can be chosen. Real-time tracking the Thevenin equivalence parameters, the reactive power compensation and optimizing direction of node can be confirmed clearly. The method provides a new idea for localized reactive power compensation, control and equalization planning under the wide-area environment.(4) The concepts of static angle stability and static voltage stability are inconsistent, which brings inconvenience to stability analysis and control. From the dynamics views, this thesis argues that the essence of the power system static stability is the balance during the source-network-load power transformation process, while voltage is just a physical quantity which supports the conversion process. And the value of voltage has not necessary internal relationship with system static stability, and it is just boundedness as a non-linear power system output variable. Therefore, this thesis deduces the maximum system active power transmission capability, and its semi-circular boundary in R-X coordinate system. Based on comprehensive physical parameter sensitivities, stability criterion of generalized synchronous torque coefficient, which expanded the static stability theory, is also presented in this paper. In addition, the voltage support level index is also given to measure system voltage support capability. Theory analysis and various simulation examples show the validity and rationality of the newly developed criterion, and it is expected to realize on-line stability monitoring in power system based on wide-area measurement system.(5) This thesis method has been embedded in Power Grid AVC system to realize on-line static stability situation evaluation. Combined the small single analysis and the stability index to analyze the stability performance of wind power to Yantai Power Grid. In addition, with continuation power flow and with the Thevenin equivalent, the static voltage stability and reactive power compensation analysis and research of variety Shandong Power Grid operation ways in the year of 2010 winter actual projections data is presented, the research results can give useful reference for Shandong Power Grid operation and planning. Finally, facing the dispatch center monitoring, It has introduced the frame and design concept of the "static stability of power system monitoring and early warning system", and has completed the corresponding function module development that will further expand the monitoring function of the intelligent scheduling support system.In summary, based on the power system measured or simulation information, this thesis has completed comprehensive research and exploration in the Thevenin equivalent parameters identification, the static stability margin, the quantitative load-shedding, the localized reactive power compensation, generalized static stability criterion and so on. And put forward the corresponding models and methods, the availability is effective verified by numerical simulation and practical power system applications. Of course, there are several details that need further research in practical applications, so that the theory and practice can be further enriched, developed and improved.