| Dynamic behavior of electric power system is generalized a set of nonlinear differential and algebraic equations. The former presents the course of control and discusses the problem of angle stability. The later presents the controlling course of operating node and discusses the problem for voltage stability. The key to analysis of voltage stability region and margin is how to gain the critical voltage collapse boundary, but it refers to the difficulties in mathematics to solve the high dimension nonlinear equations. It is impossible to gain the accuracy solution of critical voltage collapse node in mathematics method until now. However, being a steady-state operating alternating current circuit (ACC) and a synthetic description about power network, the nonlinear equation satisfies ACC rules. In this paper, the critical voltage collapse boundary has become an important research object. Furthermore, a physical method based on R_p-OR_q. plane is used to discuss the problem of voltage stability region and margin. The main works have been done as follows:1.Starting with the simple ACC branch and doing study by converting the problem into the R_p.OR_q. plane, we use operational zone of a large scale electric power system are consisted up with intersection of stability operating characteristics for each simple ACC branch adequately to evade the difficulties in solving nonlinear algebraic equations. As a result, the boundary for critical voltage collapse has been gained by this way.2. A physical method, which provides a quantitative calculation method for operational zone boundary in large scale power system, about how to determine voltage stability region and margin index is created on R_p.OR_q.plane. By using a flexible node algorithm for equality constrains condition in power system and adjusting load for local node, we obtain the critical voltage collapse node. Based on this, the proposed algorithm has been tested and achieved a very good performance on...
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