Booster Used for Voltage Regulation

A booster is also considered to be the first stage of a multi-stage rocket. There's also the booster that takes the partial vacuum pressure from an internal combustion engine to amplify the mechanical force in braking the vehicle. There are many different engineering disciplines that use the term booster. But in AC power transmission, applying a DC current to a transmission line will not assist the AC component as you surmised. I'm certain that you are mixing techniques.

What you maybe confusing is a synchronous generator/motor that the stator windings are connected to the grid. The spinning rotor magnetic field magnitude comes from a DC current source. This will allow a system to either convert mechanical power into the grid or to take electric power from the grid. Now the prime mover that spins this generator/motor shaft can be another electric motor that takes a difficult to match electric power, a gas/steam turbine etc.

The description sounds rather like an Automatic tap changer with the small shunt wound DC motor driving the tap change switch mechanism on the supply transformer to compensate for supply variations on the HV supply or Large load changes. ??

No, It does not refer to tap changing transformers. We all know the below methods are used to improve voltage profile in a transmission line 1. Booster 2. Auto-transformer 3. Capacitors

Well then, I'll just agree with the other posters that injecting DC into the AC transmission circuit will not produce any worthwhile result, in fact it would be detrimental.

One item that can be used for power factor correction / voltage stabilisation is the "Synchronous condenser" which is a Synchronous motor running unloaded and the excitation to the field coils manipulated to control the PF (overexcitation caused a leading PF). I read a while back about clutches being installed in the drive of peak load alternator sets (in Londo from memory) to allow the alternators to run as synchronous condensers (turbines stationary) when not needed as peak load sets. A good sized flywheel helps even out frequency bumps too.

Also missing from your list is the use of reactors (inductors or chokes) connected to long lightly loaded transmission lines to prevent excessive lead due to line capacitance, especially at night.

I am no expert, now I have heard that an AC power source will render a AC effect to a dc source and your "booster" may be correct? was this/another "Booster" in operation there previously? check Capacitors? relays? fuses? switchs? Any experienced power distrubition tech can help you.

I think the OP means capacitors.