Voltage Stability In Multi-infeed Hvdc Transmission Systems

With the development of society and economy, it has a much higher demand on power. The distribution of energy source and the unbalance of regional economic development make optimizing energy distributing configuration necessary. Owing to the fact that High Voltage Direct Current transmission has advantages in large capacity and long distance, HVDC has developed rapidly. As electricity transmission from West to East and nationwide power network interconnection has been developed, multi-infeed HVDC (MIDC) connected to one AC system has emerged in South China Power Grid and East China Power Gird. It has important meaning of safety evaluating for MIDC system.The thesis introduces advantages and disadvantages of HVDC transmission, and makes a survey of HVDC transmission projects both in domestic and abroad. By referring to lots of documents, the thesis classifies and analyzes methods of voltage stability for AC/DC system, especially for multi-infeed HVDC system. The interaction of AC and DC depends on the capacity of DC compared with AC, namely short current ratio (SCR). The voltage stability analysis based on SCR is widely used in academic circles and engineering circles. It can provide reference basis for system planning. Because traditional SCR can't take interaction of DCs into account, it is extremely urgent to propose a new evaluating way for MIDC system. At present, there are not unified standards for multi-infeed SCR. The thesis defines multi-infeed SCR by formula derivation and theoretical analysis, and proves two calculation methods . Further more, the multi-infeed critical SCR is derived, and the thesis proposes the judgment standard of weak MIDC system. For weak MIDC system, it advises measures to improve system stability. In addition, the thesis discusses the relationships between multi-infeed maximal available power and multi-infeed voltage sensitivity factor. It is proved that MSCR can indicate the voltage stability of multi-infeed HVDC system. Further more, the thesis uses MSCR to analyze weak MIDC operation problems, for example, transient over-voltage, harmonic resonance, voltage stability. By simulations, it can prove indices correctly, and can make contribution to system planning.