Damp Characteristics And Suppressive Countermeasures Of Low Frequency Osicillation Based On Electrical Dissection Network

The main reason of low frequency oscillation is the existence of weak damp generation units in power system. The damp characteristic of generation units depends on the inside structures and parameters of units as well as the outside operation condition of network. It is the premise and the essential condition to draw up the proper measures to suppress low frequency oscillation that how to correctly grasp the situations of damp level and the reasons of lightly damp of the units which run under the different network conditions. In the dissertation, considering of operation situations of the outside network of the units, intensive study has been performed on how to analyze the equivalent damp-coefficients of units, how to locate the critical power supply paths which affect the units damp characteristics, how to improve the damp of power system from the view of units-network coordinating and how to enhance system's ability to restrain low-frequency oscillation through appropriate operation dispatch strategies.For a real power system, although the scale of outside network that the units connected to may be large, the damp characteristics only depend on the units themselves and a part of network the units supplying. The high-dimensional damp characteristic analysis of a large-scale network can be decomposed to low-dimensional of single units, if correctly know the information of local source-flow paths of the units from the partial network. Based on this idea, electrical dissection is performed in power transmission paths from the studied units applying the theory of the electrical dissection of the source-flow path, to obtain the sub-network which reflects the local source-flow path situations of the units. From the sub-network obtained, calculation formula of the equivalent damp coefficients is deduced considering the local source-flow paths situations of the units, which provides the theoretical foundation for the further researches on the damp problems with thinking of network-side condition.Due to the local source-flow paths operation condition of the units composing of multiple sub-operation conditions of electrical dissection sub-path chains when calculate the equivalent damp coefficients, the damp influence weight index of electrical dissection sub-path chains is defined in order to evaluate the influence degree of different sub-path chains and sensitivity of its parameters on equivalent damp coefficients, and at the same time, sensitivity of equivalent damp coefficients about the reactance of electrical dissection sub-path chains and the relative phase angle is also given. Information above contributes to correctly grasp the key reasons of the outside network that lead to weak damp of the units, which will be helpful to improve the damp characteristic of the units through network-side regulation and control.One of the effective measures to suppress low frequency oscillation is damp coordinated controlling in units-network sides. Weak damp units can be found using local source-flow dissection theory and main factors that weak the damp of the units can be obtained. Coordinated control measures in units-network sides are taken accordingly. Assessment function of damp satisfactory degree is introduced, which is beneficial to combine the engineering practice and theory analysis. The results are satisfied with the cost of optimization reduced.With the continuous expansion of network scale and the proportion increasing of wind power and other uncertainty sources, the risk of dangerous situations causing low frequency oscillation may increase. Aiming at the dangerous situations such as the ultra-prediction growth of load in the peak period, outage and maintenance of key lines,supernormal increasing of the wind power in the valley- load period, the disorder of primary and secondary frequency regulation, the measures to suppress low frequency oscillation in the peak load period and in the valley- load period are proposed from the view of operation by analyzing the damp characteristic changing trends of the key units.This research works are supported by the National Nature Science Foundation of China "Theory of electrical dissecting paths between sources and flows, and its application on dispatching of power systems" under Grant 50477008 and "Optimization theory research on green energy efficiency of large power systems including intermittent power supply" under Grant 50877014.