While we await the National Grid report on the matter, we do know that it all started with an incident that is not that uncommon, the tripping of a large power generator. The generator in question was the combined-cycle gas turbine plant at Little Barford in Bedfordshire, close to the Cambridgeshire border.
In the seconds that followed, several gas-fired power stations automatically increased load and it appears that two units at the Dinorwig pumped storage power station immediately ramped up from 300MW to 600MW and the system regained balance.
However, a couple of minutes later, the Hornsea off-shore wind farm tripped, shedding around 650MW of generation and suddenly there was insufficient generation to meet demand and, more importantly, there was insufficient back-up generation (known as ‘spinning reserve’) remaining to immediately meet this demand. Within a fraction of a second the grid frequency had fallen to under 49 Hz and this triggered the national grid’s secondary protection systems.
In a system with no protection, an uncontrolled fall in grid frequency leads to disaster. When frequency falls, gas turbines (operating at high loads) produce less power and, if the frequency falls below around 47.5Hz, most such units will trip leading to further imbalance between supply and demand. Steam turbines used in coal and nuclear power plants will generally trip at around 47Hz. Ultimately, a frequency fall below 47Hz will cause a total blackout of the UK grid.
To recover from such a blackout would take a lot more than the 46 minutes needed to recover most of the network last Friday. A full system recovery could take days. Fortunately, the grid is designed to respond to such doomsday scenarios by a series of actions.
Stage One Protection
The grid is protected against sudden increases in demand, with the classic example being surges in demand when several popular TV programmes end at the same time. For these events, the grid has several seconds notice and can match the rise in demand by asking generators to increase load and for pumped storage stations, such as Dinorwig (a.k.a. ‘Electric Mountain’) in Snowdonia. There is even time to start-up turbines.
For unforeseen events, such as the tripping of a power station, the grid must ensure that there are enough flexible power stations operating to allow the loss of generation to be replaced by the remaining plants. Flexible means that the generators can increase generation almost immediately in direct response to a drop in grid frequency. Fossil-fired and pumped storage plants are best at this, however, renewable generators normally offer little or no support.
For the turbines that were already on-load at Dinorwig, the output could be increased almost instantaneously to cover lost generation from Little Barford, but to bring on additional turbines takes over 10 seconds. This may seem fast, but it’s too slow to catch the rate of decrease in frequency that was experienced when the Hornsea wind farm tripped.
Stage Two Protection
If an increase in demand or loss of generation exceeds the expected maximum (as happened on Friday), then the grid must ensure that the frequency does not drop anywhere near the 48Hz or less danger area.
To do this, balance between generation and demand can only be achieved by reducing demand. This is achieved by selective tripping (known as ‘load shedding’) of substations around the country via local automated frequency trip switches. Each substation has different set-points and the lower the frequency falls, the more substations disconnect themselves from the grid.
The settings on each substation are adjusted from time to time, so that it becomes a lottery as to who will be tripped first. Normally, vulnerable customers are protected from getting disrupted supply and the grid will be looking at the list of affected customers as part of their review, particularly for Ipswich Hospital where the loss of grid power was exacerbated by the failure of a back-up generator to kick in.
Will this happen again?
The risk of imbalance between supply and demand is greatest when there is a higher ratio of inflexible generation (wind, solar and nuclear) compared with flexible generation (gas and coal plants and pumped storage). On Friday, we had low demand from the transmission network (circa 29,000MW) since we are in mid-summer during a holiday period, but had high generation from wind turbines due to high wind speeds (circa 8,000MW) – so flexible generation was at a premium. Fossil-fired and pumped storage accounted for only 30% of our generation. National Grid is correct to say that this is an unusual event and that the double failure of two power plants pushed us into the load shedding.
However, the forecasts for the next decade are that our demand will remain at close to current levels, yet the amount of wind generation will continue to increase.
The failed Hornsea off-shore wind farm will be 1,200MW in capacity when phase 1 is complete. However, phase 2 will add another 1,400MW and, ultimately, the installation will achieve around 6,000MW. Total off-shore wind generation off the UK coast is eventually expected to reach around 36,000MW.
This means that, unless measures are taken, the probability of a repeat of Friday’s incident is more likely. Fortunately, there are measures that can be taken:
So, what does this all mean?
As we move towards increasing levels of renewable generation, the risks of similar incidents will increase.
We should also note that Little Barford has a rated power output of around 700MW, yet the Hinkley Point ‘C’ nuclear power plant that is under construction has a rated output of 1,600MW – the impact on the grid if that tripped could be extreme.
So, the need for ‘additional measures’ to balance the grid is increasing year on year and this means that demand-side response, fast frequency response and spinning reserve will all need to grow in the coming decade, all of which will ultimately add to the cost of the electricity delivered to your business.
To better understand the possible impact on your business and how to mitigate the risks, contact us on 0800 408 1499 or email us for more information.