Momentum is a word that crops up in every day journalism and is probably a concept that most are familiar with on some level. Momentum is often thought of in an abstract sense, of an object or entity gaining unstoppable impetus. Political campaigns gain momentum. An avalanche roaring down a mountainside grows in momentum. The same principle is true, and vitally important, in the generation of electricity.
The term used in electricity generation is a little different and perhaps less well known: “inertia”. But inertia is simply momentum in a rotational direction. The heavier and faster an object rotates, the more inertia it has and the stronger it's ability to keep spinning. It's a bit like the bigger the ship, the slower it turns. Why is it important? Fundamentally, because inertia is vital to help the lights stay on.
Electricity generation at a national level all happens in alternating current at a frequency of 50Hz in the UK. In other words, every electricity generator in the UK spins at the exact same speed of 50 rotations a second, or 3000rpm. The combined inertia of all of these generators is extremely useful, as even a relatively small deviation from 50Hz could destroy electrical equipment or result in wholesale blackouts and total grid failure. National Grid, responsible for maintaining electricity supply have a licensed requirement not to let frequency move by more than +/- 1Hz, save in exceptional circumstances.
The loss of a large power station can mean an instant imbalance between electricity generation and electricity demand of the magnitude of gigawatts. The most common reason for this is a generation fault that automatically trips a power plant offline for safety reasons, but equally could apply to losses due to terrorist act or natural disaster. In 2008, two significant large plant trips within minutes of eachother forced countrywide blackouts in the UK.
The loss of generation during a trip is immediate and substantial, and the first line of defence against grid failure is system inertia. All the rotating generators have the momentum to keep going at 50Hz and will all work a bit harder to make up for the loss for valuable seconds, giving National Grid the time to respond to the imbalance (in 2008, balance was restored by choosing to temporarily shut down parts of the grid). Without that immediate response, the system would collapse and need a full reboot (known as a black start), taking days to restore power nationally. Imagine a power cut at a national level; no electricity at home or work. No internet, street lights, transport logistics. No heating. Huge economic damage.
Astute readers will have taken exception to my above statement that every electricity generator rotates at 3000rpm – what about wind turbines? - and rightly so. A wind turbine rotating at 3000rpm would make one hell of a sight. A more refined statement is that conventional “synchronous” generators (think nuclear, coal, gas and oil) provide inertia while “asynchronous” generators, where the speed of rotation is dependent on the strength of the resource, like the wind or tidal generation, do not. Yet other sources of electricity, like solar power or imported electricity from undersea high voltage direct current cables have no physical rotation at all and do not provide traditional inertia.
This introduces some obvious challenges to the deployment of renewable energy. The more non-synchronous generation you have, the less inertia you have to respond to a fault like a large power plant tripping. This is becoming a problem particularly in places with advanced renewables penetration; Ireland (Northern Ireland and Republic of Ireland combined) has a commendable target of 40% renewable electricity by 2020 and is on course to achieve it, but currently concerns over inertia provision are capping non-synchronous electricity generation to 50% of demand. If there is more wind generation than 50% of demand, currently that wind (the primary deployed renewable technology) cannot be used and has to be held back or switched off.
This solution is clearly untenable in the long run. It is inefficient from a cost and carbon perspective to hold back zero carbon, zero cost electricity for the sake of running expense and carbon intensive fossil fuel plant. Politically, it's embarrassing and requires considerable more capital investment to reach the 40% target. Fortunately there is plenty that can be done to raise the non-synchronous generation cap, and some countries are taking the lead on this.
It is vital for system operators to recognise that non-synchronous generators can all provide inertia through clever use of power electronics. Although not ‘traditional’ in the sense of conventional power plants with a large rotating mass, this synthetic inertia does the job. A wind turbine for example, contains a rotating mass a physical rotating mass and technical control of the pitch and blades can provide an inertial response if required. The problem is simply one of capital expenditure for the generator, as the extra control systems cost money. However, the savings to the system of raising the system non-synchronous penetration limit certainly outweigh this if you look at the whole picture. In Germany, new wind farms are now mandated to provide synthetic inertia to help with renewable integration.
Another option, if policymakers are wary of burdening the fragile growth of renewables which are becoming directly cost competitive, is energy storage. A lithium ion battery for example can provide immediate inertial response in milliseconds of a trip, and unlike a conventional fossil fuel generator, it can do so without having to be synchronised and generating. Battery storage therefore is a hugely promising technology for lifting the non-synchronous penetration cap, reducing constraint costs and facilitating wind integration.
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 synthetic inertia services can provide them same level of system security without requiring inefficient constraints on renewable generation.
