Long-term voltage control is aimed at maintaining the voltage magnitude of buses at the transmission and distribution levels within a given range and properly share the reactive power among available resources. Traditionally, longterm
controllers are the voltage control of underload tap changer (ULTC) transformers that interface transmission and distribution networks and, in some countries, the secondary voltage regulation (SVR) that coordinates the reactive
power supply of conventional synchronous machines. This chapter discusses these two controllers but with an unconventional focus. In fact, the interaction of ULTCs with and the coordination of an SVR of renewable energy sources
(RESs) is presented and illustrated through several examples.
