I don't know how it works in the UK, but in the US, there are three major interconnections within the country, and each interconnection has system operators. Th

I don't know how it works in the UK, but in the US, there are three major interconnections within the country, and each interconnection has system operators. The system operators are responsible for managing the grid, including the frequency. I've been in several US-based power control rooms, and the current system frequency always gets pride of place on the instrument panels (along with the current system load and sometimes the grid time). System operators are also very concerned about maintaining the balance between their generation and their load. The grid time is itself an interesting concept. Because the grid runs at a known frequency, it's possible to use it as a time base for a clock. The grid time is considered to be the current time for such a clock. It basically integrates the grid frequency error over time. Regarding generators that lag the grid, it's not quite as simple as generators turning into motors. The generators all have real time governors that adjust their throttles based on their current operating speed. (The operating speed is directly linked to their operating frequency, for the vast majority of units.) In the case of a generator operating too slowly, the governor immediately requests more input power to compensate. Because this happens grid-wide, this gives the entire grid the ability to quickly respond to short-term fluctuations in operating frequency. Longer term frequency response is handled by dispatching individual units up and down to balance power and load. In the event that a generator cannot adequately respond, it will trip (shut down) and disconnect from the grid entirely. This is a protective measure that keeps the generator from skipping cycles. (You can think of this as the electrical equivalent of gears skipping teeth, and can be hugely destructive to the equipment.)