| For a long time, the focus of experts in power system stability is the power angle stability, but seventies in the 20th century, there were several power utilities related to voltage instability in the world, which resulted in large economic loss and long time blackout. After that, the study of voltage stability has gradually become experts and scholars'concerns, but compared to the study of power angle stability, the study of voltage stability is still not enough depth.With the rapid development of the power industry, the structure of power grid is becoming more and more complex. The proportion of HVDC in power system is becoming larger and larger, But one of the problems of HVDC is converters'spending lot's of reactive power, which is 40%-60%of the DC active power, and this makes the voltage stability problem of DC system side connecting to AC system becomes more serious, Especially for AC systems whose receiving end is weak, small disturbances or failures can easily lead to collapse of AC system bus voltage of converter, so the study of voltage stability of AC/DC systems is necessary.Because of the diversity of DC system operation and control, the analysis of AC/ DC system voltage stability is very complex, many voltage stability analysis methods used for AC/DC system are resulting from AC system, but the difficulty is as the increase of load, the voltage of converter AC bus declines, so the variable of DC system and adjustments of the operation mode need to be constantly consideredThe AC/DC power system can be divided into AC system and DC system two parts and analyzed separately, since the control of DC system is established, the injection power into a converter is decided only by the AC bus voltage magnitude of the converter and the rest of converters in DC system, then the DC system is equivalent to a AC system, so in this way put the flow calculation of an AC/DC system is simplified into a pure AC system flow calculation.Because conventional continuation power flow algorithms are carried out using standard N-R power flow calculation, the calculation speed is very slow, so this paper proposed an improved scheme, we used fast decoupled power flow calculation in the initial stage, which can greatly accelerate the calculation speed, and taking into account the similarly quadratic characteristic of theλ-V curve, we used Lagrange quadratic interpolation in the prediction step, and local parameter method in the correct step, and in this way the step can vary automatically, which solves the difficult of selecting steps. We can quickly and accurately obtain the static stable limit andλ-V curves of nodes through the improved CPF. And the result of test system verified the feasibility and effectiveness of the improvements.Finally we put the improved CPF into use through a software named VSAS developed by Visual C++. We used VSAS to analyze the voltage stability of Shandong Power Grid and got the static stable limit under heavy load. The results show that, Jinan, Weifang and Qingdao are weak areas of voltage stability in Shandong power grid. Although the Ningdong HVDC project eases the energy shortage in eastern Shandong power grid, the north power transmission path of Jinan-Zibo-Weifang-Laoshan is still very weak, so based on the principle of the power being laid out balanced and reasonably, Shandong power grid still needs large-scale power transmission from west to east. |
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