| Low Frequency Oscillation(LFO) is an important problem which threatens to the safe and stable operating of large-scale interconnected electric power grids. Entire or part dominant model analysis method based on whole system operating information and models of dynamical component will cause “dimension curse” and unnecessary analysis on irrelevant power units. Equivalent circuit parameters of the generator armature reaction, control system and operating condition contribute to the damping characteristic, while inter-area LFO is mainly caused by weak or even negative damped generators in sending and receiving area grids which are related to tie-lines. If local information of generators which are relevant to oscillating tie-lines is used, it is possible to improve the accuracy of LFO analysis and specific of LFO restraining strategy, at the same time solving all above the problems.Based on local operating information of units related to tie-lines, which is a result of electrical dissection, this paper did an in-depth on how electrical dissection for generators works and how to analyze relevant flow paths’ damping characteristic, how to distinguish inter-area LFO modes using tie-lines power signal, how to restrain LFO by relevant units power re-dispatching and some other problems, then got some valuable research advance.With electrical dissection for grids, according to local operating conditions outside the generators, this paper derived the result of electrical dissection inside the generators for equivalent circuit parameters of the generator armature reaction, combined both operating conditions inside and outside the generator, then got an entire local flow path which started from the inner node of sub source, went via a sub path chain, and ended at a sub load. Based on this sub path chain and electromechanical characteristic equations for sub operation condition, this paper derived Philips-Herffron model of sub sources, then got equivalent damping coefficient of sub sources and generators, making contributions to the further identification of inter-area LFO related weak damping units or flow paths and power re-dispatching for LFO restraining according to difference of related units’ damping characteristics.For accurate and efficient online identifying LFO modes in tie-lines, this paper introduced a new signal processing method for non-stationary signals, ESMD method, and then improved this method. This paper solved the low resolution by moving the signal in frequency spectrum, and proposed a calculating method for frequency shift amount according to time-frequency characteristics of original signal. In the end, with features of inter-area LFO signals, this paper designed a method to identify negative or weak damping LFO modes using improved process, and examined its effect by a case.To restrain inter-area LFO, this paper discussed the strategy of related units’ power re-dispatching based on above theories research results. Applying electrical dissection for grids, this paper would rapidly capture a few generators related to tie-line oscillation in both of sending side and receiving side, then formed subset to mainly check. Then this paper downsized the subset and put negative and weak damping units by threshold shifting into highly suspected subset. For units in this subset, this paper defined an equivalent damping coefficient per power unit index, and designed a method which adjusted power of related units. A case showed this method would get obvious effect of improving inter-area damping characteristic.This research work is supported by National Natural 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 State Grid Company of China Major Project on Planning and Operation Control of Large Scale Grid under Grant SGCC-MPLG-023-2012. |
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