On 9 August at 4.52 p.m. in Great Britain there was a partial blackout caused by almost simultaneous (in the space of a few seconds) disconnections of two large units from the network: 

• Little Barford gas power plant, 
• Hornsea offshore wind farm.

A total of 1.38 GW of power was suddenly lost from the British system. As a result, there was a sharp drop in frequency to 48.8 Hz (the desired range is 50.5-49.5 Hz) and cascading events in the power system. The activated back-up power was not sufficient to stop the frequency drop, so about one million customers, i.e. 5% of the power system, were automatically disconnected from the power grid. Thanks to this, the network was stabilized, which protected the rest of the British from a major disaster. After less than an hour, the electricity was restored. 

What has been known so far?  

• This was the biggest failure of the British energy system in 10 years. The National Grid reports that the first in a sequence of events was a lightning strike to the grid in the north of London. These are very frequent situations and normally do not pose a major threat. This time, however, a few seconds later, there were parallel technical problems in two generating units - wind and gas. These were the direct causes of the blackout. 
• According to the National Grid, when the failure occurred, the level of back-up capacity was 1 GW. It is calculated taking into account the scenario in which a single, large unit fails. In the UK, it is either the Sizewell B nuclear power plant (1.26 GW) or the Cross-Channel DC interconnector (1 GW). Despite the activation of available back-up, including as much as 475 MW of battery storage capacity, 350 MW of demand reduction and only 200 MW of generation, the scale of the losses did not allow to overcome the problems. 
• On 9 August in the UK, 30% of electricity was produced from wind power, 30% from gas, 20% from nuclear power, 10% from imports and the rest from hydropower, biomass and coal. Such a mix is becoming the norm in the UK. It is the consequence of a dynamic increase in installed renewable capacity and a sharp decline in the share of coal-fired units. In the last decade, the share of RES has quadrupled to more than 20 GW, accounting for 25% of installed capacity. Offshore wind farms already provide 8.5 GW.

What are the findings?

• Power cuts are undesirable, but unfortunately, they do happen. They may result both from problems in the transmission grid (e.g. as in 2003) and from the failure of generation units. The last major blackout took place in Great Britain in 2008. At that time, the reason was also the simultaneous shutdown of two large power plants - the Logannet coal-fired power plant and the Sizewell B nuclear power plant.[1] 
• The repeated failure of the two large system units will provoke a discussion about the level of backup capacity and whether the power system will be able to cope with every event in the grid. Maintaining a larger backup is expensive. The British incident also shows that what counts is the quality and speed of response of back-up capacity - its flexibility is crucial. 
• Contrary to the opinions voiced at the beginning of the debate, it was not the unstable nature of RES production that caused the blackout. As the National Grid emphasises, weather conditions and generation were in line with forecasts and the problems in power plants were of a technical nature. However, the integration and balancing of variable sources requires dealing with new challenges: daily and seasonal variability, the need to improve forecasting or inertia in the system.  

British experience shows that increasing renewable penetration is a challenge for the power system. This does not mean, however, that RES are the cause of all problems. Many countries are already coping with this challenge, and a number of others, including Poland, will sooner or later face it. If we "arm" our system with a mechanism of flexible backup capacity and introduce changes in the functioning of the energy market, there is a chance that we will protect ourselves against undesirable events. 

 1. https://www.theguardian.com/business/2008/may/28/power.cuts