In an electric power grid, what happens to unused current?
The quick answer is that there is no ‘unused current’. When you switch a light bulb you connect it to a power source – effectively the powerstation that is converting the prime power source – energy in coal or nuclea fision – which gets converted to electricity. The power station is committed to generate electrical power at a standard frequency and voltage. As more people switch on light bulbs and all manner of other elctrically powered applicances, the demand on the power station increases which in almost all powerstations means that the steam throttle is opened to provide more enegy to the turbines that drive the electrical generators. All this happens automatically and smoothly. Listen to the engine of a small gasoline powered stand-by generator. When you start it up it hums along at a constant speed. Connect a load such as a power tool or flood light and you will hear the engine load up to maintain the same speed. If you look close at the carburettor as the load is switched on you
The electric power grid is an example of something called a “servomechanism” where there is feedback that keeps the fuel intake of the generators (and therefore the amount of power they put out) closely related to demand for power. In the morning when millions of people get up, flip on lights, put on the coffee and so forth, it sucks electricity out of the system. Automatic equipment therefore pumps more fuel to the generators so that the amount of power going INTO the system remains fairly stable. It’s not perfectly stable, of course, which is why really sensitive equipment like computers requires surge protectors and either an uninterruptable power supply or at least an automatic shutdown. Too bad most refrigerators don’t have that also, since a brownout can burn out the compressor motor.
There actually is always some unused electricity in the system – reactive power (measured in Volt-Amps Reactive (VAR). All alternating-current systems have this issue. All lines, transformers, motors, and other devices have properties of capacitance and inductance, both of which store or delay current or voltage for a brief period. The result is that you have voltages and currents that are out of phase with each other. The amount of phase difference is expressed as “power factor.” Though these out-of-phase components don’t do any useful work, they circulate through the system and can affect system voltage, among other things. To counteract the negative effects of reactive power, utilities (and, often, large customers) use banks of capacitors and inductors at strategic locations. They can either be switched by system operators or electronic devices (one automatic device is called a Static VAR Compensator.) Controlling generator excitation and using special synchronous motors called “con
