Explain the Synchronous Compensators
These are synchronous motors running without mechanical loads (or with load if required). Similar to the synchronous generators, they can absorb or generate reactive power depending on the level of their excitation. When over-excited, they generate VArs; i.e. they behave like capacitors. When under-excited, they absorb VArs; i.e. they behave like reactors. Their output can be automatically and smoothly controlled and is independent of the system voltage. That is they can maintain (or even increase) their output at reduced voltages.
Synchronous compensators have high capital cost, slow response and require regular maintenance.