The voltage stability problem would be more conspicuous in the free energy trading market because power flows on transmission lines could be increased to the thermal limits. This limit is normally higher than surge impedance loading of the line, which leads to degradation of system voltage characteristics. In urban area systems, it would become more serious as there are fewer lines, each designed with large thermal capacity to overcome the difficulty of constructing new overhead transmission lines in these areas. In Japan, large-scale power failure occurred in the Tokyo metropolitan area in 1987 (about an 8-GW loss) because of voltage instability. In Tokyo, a large amount of capacitance by a 275-kV underground cable system created potentially adverse effects on voltage-stability characteristics, making voltage stability one of the most important issues regarding system security. Proper reactive power control with adequate capacity is essential. The control can be achieved through coordination of generator reactive power (or voltage) control, shunt capacitor and transformer tap-changer control, and dynamic reactive power suppliers such as synchronous condenser or Static Var Compensator (SVC). The access requirements for the new generators-especially with relatively large capacity-should be properly specified in such terms as power factor, voltage control and step-up transformer tap changer setting.
For transmission system facilities, adequate addition of shunt capacitors for maintaining voltage characteristics in lower voltage systems should be properly planned while considering control schemes. In the event that an additional shunt capacitor fails to fulfill the system reliability or operational criteria alone, synchronous condensers or SVC become alternatives. Some utilities use detailed simulation analysis with quasi-dynamic time frames to determine the most economical and reasonable solution. In the free market environment, a variety of operations can be expected, increasing the time given to studying voltage stability. Screening methods to identify the critical cases from a huge number of power flow conditions also are being studied.