Well... strictly speaking, it's your utility company might use a capacitor bank to minimize a drop in line voltage, but that's only if you are having problems with low power factor and that is generally not a problem with residential service unless you live in a factory district. In most cases, it's a power company problem and there are few, if any residences that switch in capacitance to correct for low power factor.
A brownout is an extended-time drop in voltage to your service usually caused by heavy loads (like summer-time peak air conditioning). There is not much you can do about them unless you have a line conditioner, which might also be described as a variable buck-boost transformer that effectively changes taps on-the-fly. Some power company substation transformers will do this for you. Not sure what you mean by buffering capacitance unless you are speaking in a very, very general sense, as in the stored energy of a ferro-resonant transformer or a mechanical flywheel. Beachcomber