Inertia in electricity systems – Energy Basics

Energy Basics is a series covering fundamental energy concepts.

Perhaps you’ve had critics of the energy transition shout “inertia” at you. Perhaps it’s keeping you up at night. Is our dream of a clean energy future impossible? This article will reassure you that losing inertia is something clean energy technologies can deal with.

Large, fossil fuel and synchronous generators have historically dominated our electricity system. Fossil fuels are burnt to force high temperature and pressure gases through turbines. These turbines rapidly spin shafts connected to alternators that generate AC electricity.

We are transitioning to a very different electricity system. We are building small-scale, clean and asynchronous generators. Wind turbines that spin at variable speeds, much slower than synchronous generators. Photovoltaic solar panels and batteries have no moving parts at all.

A key difference between these two systems is the inertia of the generators. Fossil fuel generators posses a lot of inertia due to the rapidly spinning & heavy turbine connected to the alternator. Once the turbine is spinning it’s hard to get it to stop – in the same way that it’s hard to stop a truck traveling at speed.

The speed at which the shaft & alternator needs to spin at is directly proportional to the desired grid frequency. In fact the grid frequency is the result of the speed that all these synchronous generators spin at. The frequency of electricity generated by a synchronous generator is given by

Poles refer to poles of the alternator.  120 is used to convert minutes to seconds and poles to pairs of poles.

The grid is an interconnected system – changing grid frequency requires changing the speed of every generator connected to the grid. This interrelationship becomes useful during times of supply & demand mismatches. Any imbalance needs to work to change the speed at which every generator on the grid spins. If these generators posses a lot of inertia, then the imbalance needs to work harder to change the grid frequency.

This is the value of inertia to the grid – it buys the grid operator time to take other actions such as load shedding or calling upon backup plant. These other actions are still needed – inertia won’t save the grid, just buy time for other actions to save the grid.

So now we understand that fossil fuel generators have inertia and how it is valuable to the grid (it buys the system operator time during emergency events). What does this mean for our energy transition? Do we need to keep around some fossil fuel generators to provide inertia in case something goes wrong? The answer is no.

Modern wind turbines can draw upon kinetic energy stored in the generator and blades to provide a boost during a grid stress. This ‘synthetic inertia’ has been used successfully in Canada, where wind turbines were able to supply a similar level of inertia to conventional synchronous generators.

Figure 1  –  Conceptual fast frequency response from a wind turbine

Photovoltaic solar and batteries also have a role to play. Both operate with inverters that convert DC into AC electricity. The solid-state nature of the devices means that they operate without any inertia. Yet this solid-state nature allows inverters the ability to quickly change operation in a highly controllable way. Inverters can quickly react to deliver whatever kind of support the grid needs during stress events.

Clean technologies are ready to create a new electricity system. Now we need to make sure we incentivize the technology that our grid needs. Market incentives should support technologies that can supply inertia on our cleaner grid. The level of support could be logically set so that the level of inertia on the grid will remain at the same level as our old fossil fuel based grid. That way, no one can complain.

Thanks for reading!