Acting Director of Communications
Posted on 09/10/2017 by Nick Sharpe
Rising bills, blown fuses and blackouts – all have been threatened recently as consequences of the rise of at-home EV charging.
For National Grid, changes in the electricity network are part of the day job.
But what does it take to keep our electricity network running smoothly?
A recent consultation from National Grid Electricity Transmission sets out five key system ‘needs’ which keep the electrons flowing and the lights on.
And while it’s not bedtime reading, for those of us concerned in the supply and transmission of electricity it’s information that’s useful to know.
Overall, National Grid sets out five key system needs:
- System Inertia and Rate of Change of Frequency (RoCoF)
- Frequency Response
- Reactive Power / Voltage Support
- Black start
In this first of a two-part blog we’ll look at 1 and 2, setting out the issues which require change and how that change could affect renewables generators.
The second part of this blog, which will be published next week, will assess the implications of changes to 3, 4 and 5 above.
ONE: System Inertia and Rate of Change of Frequency
Inertia determines the speed at which system frequency will change with any imbalance between supply and demand.
RoCoF is the Rate of Change of Frequency:
- A system with low inertia will change frequency very quickly.
- Higher inertia makes the change slower, and therefore more manageable.
What is changing?
Just a few years ago the electricity system could be thought of as being a little like a large supertanker: try to accelerate or stop, and the change in speed of the vessel is not instantly noticeable.
This sort of high inertia system has its benefits: giving National Grid’s control room time to manage changes in frequency is definitely one of them.
The trouble is that the system is beginning to behave more like a Formula 1 car.
With fewer synchronous generators (like coal and gas) providing inertia to the system, and more distributed renewables coming on line, a foot on (or off) the accelerator has instant consequences.
A reduction in inertia increases the Rate of Change of Frequency, and when the power drops or the unexpected happens adjustments must be made much more quickly.
What is the solution?
National Grid already acts to deal with this sort of problem: last year’s auction for ‘enhanced frequency response’ was designed for this very reason.
However, some inertia will still be required to hold the frequency long enough to allow frequency products to react (no matter how fast they are).
Moving forward, National Grid (through its current consultation and other ongoing work), is exploring additional options like taking the value of inertia into account in new frequency response products and even exploring new technologies like new ‘hybrid synchronous compensators’.
What role will renewables play
There is a growing body of evidence which suggests wind farms have the ability to provide ‘synthetic inertia’.
While National Grid is currently exploring this opportunity, it is important to ensure that any new products delivered through this consultation will allow renewables to compete with other providers of inertia in the future.
There are also some wider technical issues where changes to standards and regulations for renewables may help avoid unnecessary complications.
For example, many distributed generators have an inbuilt protection mechanism known as RoCoF Loss of Mains (LoM), which will automatically disconnect the generator where the RoCoF levels increase above a certain point.
While this sounds useful, it may exacerbate the issue. For example, during a frequency event this protection mechanism may cause distributed generators to automatically trip off the system, which in turn would cause frequency levels to fall faster and further.
Ofgem is currently exploring whether it makes sense to change the standards by which RoCoF LoM levels are set.
The purpose of the RoCoF LoM protection is to ensure that a distributed generator connected to a part of the network which becomes ‘islanded’ (cut off from the main system) but remains energised it isn’t exposed to out-of-phase re-closure*.
TWO: Frequency response
Frequency response is directly related to levels of system inertia, and to the size of the generation or demand that could come off the system.
Two types of frequency response are already in use by National Grid: Dynamic and Static.
Dynamic response is used to continually follow and control minor deviations in frequency due to minor imbalances in supply and demand.
Static response is only activated when a fixed frequency limit is breached.
What is changing?
National Grid currently procures an expected level of ‘firm frequency response’ ahead of time, which is expected to remain stable.
Its challenge lies in finding suitable levels of response that can be available and act quickly.
Although a trial of enhanced frequency response last year highlighted the availability of such products, there is currently no route to market for sub-10-second response provision.
National Grid believes that faster response should be incorporated into the wider products, rather than awarding new contracts through a second tender.
What role will renewables play?
Renewables are resource-led: when ‘fuel’ (wind, sunlight, water etc) is available they can provide response to frequency events.
In fact, wind farms can provide both low and high frequency response (under the right conditions)
For example, when generators are not deployed at full output they could turn up to increase frequency if required and vice versa.
With the possibility of having very close to real-time markets with secondary trading, a wind farm can take advantage of changes on its forecast to bid that extra volume into the ancillary services market as balancing, frequency response and even fast reserve
As with other ancillary services it is in our view important that renewables can compete with other technologies to provide support to the system operator, and National Grid should aim to ensure that is the case in future.
* Out of phase re-closure occurs when a live electrical island is reconnected to the main system, and the phase angle and frequency of the waveform in the island is different to that of the main system. Imagine running a 10k and turning a corner to find you’re now taking part in a marathon: trying to keep up is likely to result in some damage.