Study On Early Warning System Of Power Grid’s Low-frequency Oscillation With Multiple Disturbances

With the development of power system and the increase of its conplexity, especially with the progressive development of UHV regional large-scale power grid in recent years, new voltage levels occurred, and the stability of the system and a series of new problems needed to be studied and solved. The expanding of grid scale also meant the increasing of uncertain factors. Multiple disturbances with the system damping worsening were led by continuous changes of system’s operation status caused by a number of disturbance events in time, space and types, which couldn’t be neglected in the stable operation of the system. Therefore, early warning study of multiple perturbations in low-frequency oscillation in power system was of a strong practical value and engineering significance.The paper was based on the actual research project. A comprehensive study on multiple perturbations in the early warning of low-frequency oscillation in power system was studied from the aspects of theory analysis and application. The main work and research results could be summarized as follows:The practical exploration and research were made on the analysis method of multiple perturbations low-frequency oscillation in power system. The energy function was applied to the analysis and study on low-frequency oscillation. The cross section flow analysis model that can be used in actual analysis was established. By selecting the observation section, the analysis of whole network could be transformed into the analysis of each observation section so that distributed decoupling network analysis could be achieved. On the basis of this model, the conventional energy function based on inertia center was simplified and the traditional stability index was improved. The energy margin index, branch potential energy stability index, the dominant mode damping ratio index, interval power oscillation section first swing index were put forward. These indicators were based on tie line observation section. Its calculation was simple and convenient. It could be used to the warning of the low frequency oscillation and the judge of the severity of the disturbance.Indicators and methods of low frequency probable stability analysis applied to interconnected grid were put forward, and probability analysis was made in actual interconnection grid. In the condition of known grid uncertain probability distribution, two-point estimate was used to calculate the size of the system low frequency oscillation probability caused by these uncertainties based on the small perturbation method. Then the risk assessment theory was used to analysis the system low frequency instability caused by multiple disturbances. The control costs of the grid and the risk of low frequency were provided. The compare ways of undertaking risks and improving the transmission capacity for the grid corporation were given by means of the comparison of small disturbance and large disturbance probability analysis, which could be used as a reference available for grid operation personnel weighing the economic and security as well as an important indicator in the long-term warning.According to the energy function stability margin and other three indicators combined with dynamic damping and peak to peak indicators which were used in the engineering practice, a multi-time scale warning indicator system and the grading standard were built. The safety status of the power grid was comprehensively evaluated. To warn the low frequency oscillation, the changes in current status or in the fault state were got by online matching and calculation of the power grid fault information and power flow, combined with multiple disturbance cases and the established offline database.The relevant early warning software was developed according to the established early warning indicator system. Because of the construction of ultra-high voltage grid, the oscillation, lower than0.1Hz, was appeared in the power grid. In that case,10s was set as the unit for length of received data. Six indexes, including the stability margin of energy function, branch potential stability, dominant mode damping ratio, peak value of the power oscillation in transmission section, peak-to-peak value and dynamic damping ratio, were on-line calculated. The warning was issued when any index was in the warning range based on the setting safe grade evaluation of power grid and cask effect.The phenomenon of power grid’s low frequency oscillation with multiple disturbances was studied and analyzed in this paper. An effective early warning indicator system was established base on the actual engineering projects. The relevant early warning software was developed and the online application of the actual power grid was done. The basis of theoretical study and industrial application for the later analysis in early warning of low frequency oscillation were established.