What is “Reactive Supply” and “Voltage Control” and what if there is insufficient supply or control?

The FCRPS (energy) is operated in such a way as to provide reactive supply or “tuning” for all the energy flowing across the FCRTS (transmission). 

Depending on the line loading, the FCRPS is operated to produce (or absorb) reactive power in order to maintain (control) transmission voltage within acceptable limits.