Application Of Modal Series Method In HVDC/AC Systems

With the development of the technology of power systems and the economy of our country, the power systems will be interconnected, which will inevitably result in frequent occurance of severe transient instability. Therefore, there will be great need for studies of fast estimate of transient stability and emergent control of power systems. Power systems are inherently nonlinear systems, especially those with HVDC lines. When a power system is subjected to a disturbance, it exhibits complex dynamic behavior. The complexity of this behavior depends on the power system loading, type and location of the disturbance. Two approaches are commonly used to study the power system dynamic behavior. One is nonlinear simulation and the other is linear modal analysis. It is difficult to understand and gain a good feeling for the physical nature of the complex dynamic behavior by nonlinear simulation. On the other hand, the validity of linear modal analysis is restricted to a small neighborhood of the operating point and therefore, when a system is subjected to large disturbances and its states are driven away from operating point, the similarity between real time responses and linear modal simulation is lost.Recently, the technique of normal forms of vector fields has been used to analyze the complex behavior of the power systems. A drawback to this method is its need for state variables that do not directly correspond to physical or modal phenomena, nonlinear transformation and solving nonlinear algebraic equations. Also in general it fails to obtain a closed form solution, when there is a second or higher order resonance condition.Modal Series Method is a newly used method to analyze the nonlinear dynamic behavior of power systems. This method can represent the nonlinear system responseand obtain an approximating closed form expression for the zero input response of the nonlinear system, without using nonlinear transformation. This method extends the linear system theory concepts to facilitate the understanding and analysis of nonlinear systems.In this paper the Modal Series Method will be applied in HVDC/AC system. The result will show the effect of nonlinear mode to the system dynamic behavior. And moreover the author extends the method to HVDC/AC power systems with DC auxiliary control strategy. Here DC auxiliary control of power modulation is considered. The Modal Series Method will be used to analyze the transient stability of the studied system, the types of oscillation modes, and the interactions of oscillation modes. It will be also analyzed whether the auxiliary control of power modulation can enhance system stability.