Why are high transmission system voltage problems more likely in the initial stages of system restoration?

High transmission system voltage problems are more likely in the initial stages of system restoration primarily because of the excessive MVAR (Mega Volt Ampere Reactive) supply from the energization of transmission lines. When transmission lines are energized, they can introduce a significant amount of reactive power into the system, particularly if these lines are long or if they are connected without adequate balancing of reactive power resources.

During the restoration process, the system is often transitioning from a state of outages to an operational condition, and the sudden energization of these lines can lead to overvoltage conditions. This is especially true when there is an abundance of reactive power being supplied without a corresponding load to absorb it. A well-balanced system requires both sufficient real power (MW) and reactive power (MVAR) to maintain voltage levels within acceptable limits. In the early stages of restoration, if reactive load is low or if there aren't enough reactive power resources to mitigate the surge from newly energized lines, voltage levels can spike.

In summary, the primary concern during the initial phases of system restoration is how the reactive power dynamics can fluctuate dramatically due to the energization of transmission lines, leading to potential overvoltage issues.