Analysis On The Low Frequency Oscillation Phenomenon Of The “Small Grid With Large Generating Unit” System

In order to achieve regional power transmission in large scale and make the most of renewable energy generation, China is building an interconnected ultra-high-voltage AC/DC power system. Besides, large capacity generators have the advantages of high efficiency, low consumption and economy. To increase energy efficiency and alleviate the environmental pollution, the large capacity generators will be dominant in our power system.The large-scale grid with large capacity generators is economic and environmentally friendly, but will face more potential stability problems. For the UHV AC/DC interconnected power system, a special operation mode or some faults or overhaul will make local grid be in weak interconnection with the whole system. Thus the local grid becomes “small grid with large generating unit” structure. Compared with the conventional large-scale power grid, “small grid with large generating unit” structure has its specificities. Its ability of bearing disturbance, transient stability and dynamic stability will have great changes. Besides, the large capacity generators will exert huge impact on the “small grid with large generating unit” structure and the one-machine-to-infinite-bus system is not suitable for this structure. The influence of the potential stability problems in the “small grid with large generating unit” structure cannot be ignored and should be further studied.In the UHV AC/DC interconnected power system, for the existing special low frequency oscillation, comprehensive simulations of HUAZHONG-grid have been carried out with PSASP in this paper.Main researches are as follows.(1) By simulating with the whole HUAZHONG-grid, the special low frequency oscillation phenomenon in the “small grid with large generating unit” structure is discovered. Through the analysis of the eigenvalues and damping of the HUAZHONG-grid, we know it has no weak or negative damping oscillation modes.(2) To have a further study on the special low frequency oscillation phenomenon in the “small grid with large generating unit” system, the definition of the “small grid with large generating unit” system is proposed in this paper; the operation modes and faults are analyzed at the same time.(3) Given that negative or weak damping mechanism cannot be used to explain the special low frequency oscillation phenomenon in the “small grid with large generating unit” system and the one-machine-to-infinite-bus model is not suitable either, based on multi-machine system, the power system energy function with six-order generator model is deduced in this paper. Besides, the low frequency oscillation phenomenon is deeply analyzed with the generator energy function.(4) The changes of large capacity generator parameters as well as its effects on the low frequency oscillation are also studied in this paper.(5) At last, the advices for avoiding the “small grid with large generating unit” system are proposed.