| Mid-term voltage stability attracts more and more attention from researchers worldwide, due to various voltage collapse incidents which have occurred in recent history. The main issues in this dissertation deal with mechanisms of voltage instability, on-line assessment of voltage stability by a quasi-steady state method, and voltage stability enhancement by reactive power reserve management and tertiary/secondary voltage control.; The mechanisms of under load tap changers (ULTC) and slow dynamic loads in mid-term voltage instability are studied in detail. Operation of ULTCs can improve the maximum power transfer and can thus be beneficial in improving voltage stability margins by matching line-transformer impedance with load impedance. However, ideal maximum power transfer capability cannot be realized because of the possibility of unstable equilibrium point and a reduction in the stability region because of ULTC operation.; A fast time domain simulation method based on the quasi-steady state method is a compromise between efficiency of static methods and accuracy of dynamic simulations and is proposed for on-line stability assessment.; Voltage stability can be improved by proper management of reactive power operation. At the tertiary level of voltage control, a reactive reserve management program based on optimal power flow and the Benders decomposition technique is presented to improve voltage stability. An optimal inter plant voltage control is also proposed to enhance the stability margin. Test results are shown on a small system and a median-sized system. |
