A machine or transformer placed in series with a power line to inject a supplementary voltage, compensating for voltage drop over long distances. In historic DC systems, this was a motor-generator set, while AC networks use specialized booster transformers or tap-changing setups to keep the line voltage within statutory limits at the far end of a feeder.
Heard on the job
The voltage at the end of the rural feeder is dropping below 210 volts, so we need to install a booster to keep the customers in spec.