The frequency is currently 50.034Hz, at some point it will be below 50Hz to keep the average at 50Hz.
When the frequency drops below 50Hz, it suggests that demand is outpacing supply, and vice-versa when the frequency is above 50Hz.
Grid voltage can vary due to your location and all sorts of factors, but frequency is, of course, always synchronised wherever you are.
National Grid pays a lot of money for "frequency response" to ensure the grid remains balanced. Traditionally this was provided largely by coal spinning reserve. But as most coal-fired plants have now closed, it's now provided by diesel farms (yuck!) and increasingly, battery storage.
http://www2.nationalgrid.com/uk/services/balancing-services/...
In the US frequency response is mostly Nat Gas powered. There are new projects that are battery based, but another way to achieve this is on the demand side, by requesting non critical loads to turn off or throttle themselves when the frequency drops too low. I think this is being done at an industrial scale (i.e aluminium smelters) but it is possible on the domestic scale also with EVs electric tank water heaters, etc.
Going forward, given the intermittency of renewable energy, these methods (primarily storage) will be more important as the energy supply becomes more variable.
I'm not sure about other utility companies, but when I was a TSO we never did this. For the most part, AGC worked fine to maintain frequency.
General Topic: http://www.pnnl.gov/main/publications/external/technical_rep... http://www.sciencedirect.com/science/article/pii/S0196890416...
HVAC Demand/Response: http://ieeexplore.ieee.org/document/7407597/?reload=true
Aluminum Smelter Demand Response: http://repository.cmu.edu/cgi/viewcontent.cgi?article=1836&c...
> "Autonomous Demand Response for frequency regulation is real..."
I wasn't saying that ADR wasn't "real," just that it's not something we're currently doing for frequency control in actual practice...and of course that might change as engineers continue researching and pushing the field further. It definitely looks like ADR solutions are on the horizon though.
As a former Transmission System Operator I can tell you the most likely scenario you see this is when generation trips offline (demand exceeds supply for a short time).
I've sat in an Arizona control room, 1700MW of NW hydro would trip offline and we'd see a massive drop in interconnect frequency.
(In most countries you can see a little bit of a frequency dip at times of high demand. It also has the effect of reducing the power draw by synchronous AC motors.)