For all the huge, hulking buildings of concrete and steel that house coal, gas or nuclear power stations, or towering 100-metre-tall wind turbines, generating electricity is in fact a very precise and quite delicate business.
Just small fluctuations in an electric current can wreak serious damage. Fortunately, the means of generating electricity provides its own protection against this. Unfortunately, the renewable energy technology we will use now and for the future removes this protection – with potentially disastrous results.
A quick physics lesson
Electricity is created by generators – rotating electromagnets that convert mechanical energy into electrical energy. In order to power an entire nation’s electricity grid, they must all be tuned to spin at the same speed. In the UK, this speed is a frequency of 50Hz – in other words, every electricity generator in the UK spins at 50 rotations per second, or 3,000rpm.
Even a relatively small deviation from 50Hz could destroy electrical equipment, result in blackouts, or even total grid failure. National Grid, the operating company responsible for maintaining the UK electricity supply, is obliged to prevent the grid frequency from moving by any more than just 1Hz in either direction, save in exceptional circumstances.
Fortunately the spinning rotors of each generator carry their own inertia – an object’s tendency, after a force has been applied to it, to continue in motion even after the force is no longer applied. Taken together, the combined inertia of all of the nation’s electricity generators is extremely useful for maintaining stability.
Avoiding system reboot
The loss of a large power station can mean an instant imbalance between electricity generation and electricity demand, by a gigawatt or more. The common reason is a fault that automatically trips a power plant offline for safety reasons, but equally the same would apply due to terrorist attack or natural disaster. For example, in 2008 two large UK power plants tripped within minutes of each other, resulting in blackouts.
So when this immediate and major loss of electricity occurs, the first line of defence against a dangerous drop in system frequency that could lead to total grid failure is the inertia of the system. The inertia of all those rotating generators keep the system going at 50Hz, providing valuable seconds with which to respond to the imbalance (in 2008, balance was restored by choosing to temporarily shut down parts of the grid).
Without an immediate response, the system would collapse and need a full reboot (known as a “black start”), which would take days to restore power. Imagine a national power cut: no electricity at home or work. No internet, street lights, or transport logistics. No heating. Huge economic losses.
The untenable solution
Astute readers may disagree with the claim that every electricity generator rotates at 3,000rpm – what about wind turbines? And rightly so – a wind turbine rotating at 3,000rpm would be a sight to behold. A more refined statement is that conventional generators (think nuclear, coal, gas and oil) provide inertia while “asynchronous” generators, where the speed of rotation is dependent on the strength of the wind or tide, do not. In fact there are sources of electricity, like solar power or electricity imported from undersea high voltage cables, with no physical rotation at all.
And so it should become clear that deploying this kind of energy generation poses a challenge. The more asynchronous generation in the system, the less inertia is available to provide valuable time in the event of a fault. This is becoming a problem, particularly in places where renewables make up a considerable proportion of the energy mix.
Northern Ireland and the Republic of Ireland together have a target of 40% renewable electricity by 2020 and are on course to achieve it. But concerns over system inertia have led to a cap of 50% of demand on asynchronous, renewable forms of energy generation. In other words, if wind and solar generate more than half the demand, that excess is dumped and wasted.
This solution is untenable in the long run. It’s inefficient to hold back zero-carbon, zero-fuel-cost electricity for the sake of running expensive and carbon-intensive fossil fuel power plants. Politically, it’s embarrassing, but there are ways around this – and some countries are taking the lead.
Overcoming entrenched attitudes
System operators need to recognise that renewables can still add to system inertia through clever use of power electronics to supply a synthetic inertia that does the same job.
The inertia of rotating generators can be thought of as additional kinetic energy that is transformed into electrical energy to maintain the frequency. A wind turbine already contains a rotating mass – the blades – so if it can be made to generate more power this will have the same frequency stabilising effect. The turbine controller can adjust the pitch of the blades, usually to ensure they remain at the most efficient speed as the wind changes. So, quickly altering blade pitch to generate more power in response to a failure on the grid would provide a temporary boost to counter a dangerous frequency dip.
The problem is simply one of capital expenditure for the generating company, as the extra control systems cost money – but taking the broader view, ensuring that more renewables do not lower system inertia to dangerous levels provides protection against much greater potential losses. In Germany, the framework is already in place that requires new wind farms to provide synthetic inertia.
Another option, if politicians are wary of burdening energy companies with more costs, is energy storage. A lithium-ion battery can provide immediate power within milliseconds of a trip – effectively emulating the effects of conventional inertia, without moving parts. Nor do batteries need to be synchronised with the grid – providing the equivalent service of back-up power stations but without having to inefficiently run power plants at part load.
The solutions to the emerging challenges for electricity generation already exist. The key to unlocking them is to overcome entrenched conservative attitudes in the energy industry, and reassure regulators and system operators that building synthetic inertia into the grid can provide them same level of security, without cramping the deployment of more renewable technologies in the future.