While the most common form of voltage instability is the progressive drop of bus voltages,the risk of overvoltage insta-

While the most common form of voltage instability is the progressive drop of bus voltages,the risk of overvoltage insta- bility also exists and has been experienced at least on one system [19].It is caused by a capacitive behavior of the network (EHV transmission lines operating below surge impedance loading) as well as by underexcitation limiters preventing generators and/or synchronous compensators from absorbing the excess reactive power.In this case,the instability is associated with the in- ability of the combined generation and transmission system to operate below some load level.In their attempt to restore this load power,transformer tap changers cause long-term voltage instability.