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This is a really interesting site. Lots of posters that love and know lots about heating systems - including steam. Then there is an advanced discussion about how power systems operate - wow. Here is a slightly different take on what inertia means in a power system.

Before the days of modern power electronics everything that rotates was connected to the grid synchronously and contributed to frequency stability and this includes motors as well as the generators in the power stations. Motors are considerable fraction of the load on most grids, industrial and domestic (fridges/freezers/aircon). There is no doubt than thousands of tons of spinning metal does contain lots of energy.

However, AC power systems are very sensitive to frequency. The transmission system has distributed inductance and capacitance in the form of cables and to maintain stable voltages discrete capacitors and inductors are dynamically switched in and out of circuits and transformer voltage ratios are changed. Because of this frequency sensitivity, AC systems are controlled to very tight frequency limits - 1% or better. In terms of keeping a grid stable you can only use 0.5% of the considerable stored energy.

What really keeps a traditional power grid stable is some generators operating at less than maximum output that can rapidly change power output in response frequency changes (the technical term is spinning reserve). Inertia provides the energy while this ramping up of power output takes place. This balancing must happen within seconds. Things like generators failing or transmission lines failing need to be take into account when deciding how much generation capacity is running as spinning reserve and where it is located in the system. This is what power system operators do all the time and it normally works. When it goes wrong it hits the news.

Power electronics come of age and there are new ways of connecting power sources - PV, wind, DC power links that don't contribute to spinning inertia. On the load side inverter motor drives are very common in the industrial and domestic equipment that don't contribute to spinning inertia. Electronic power supplies for LED lighting and computers take the same amount power regardless of voltage.

With high levels of renewable energy on power grids operators can end up with all their traditional generation acting as spinning reserve. Or worse, having to run generators on no load to maintain inertia and curtailing renewable sources.

The way out of this is to make all sources of power and loads frequency sensitive. You don't need much actual storage from spinning metal or batteries when everything is frequency sensitive. If there was a requirement for PV or wind turbines to operate at 95% output when the grid frequency is 50 or 60Hz there would be an automatic reserve of 5% if the frequency started falling.

Converting all sources and loads to frequency sensitive versions is easy to do technically but virtually impossible to do retrospectively. You could start with the bigger renewable installations.

Renewable sources are intermittent so will always require backup from some source of dispatchable power.

I wonder why the industrial revolution really got going when steam power came along and we no longer had to rely on windmills and waterwheels? There is a working textile mill that is maintained as a museum near Manchester airport (uk) which contains a time clock for workers to clock in and out. It is linked to the waterwheel driving the mill - so lower water levels = longer working hours!

John

It was a matter of when, not if — and we'll see the same sort of thing on this side of the pond.