EirGrid and Soni, the two electricity transmission system operators (TSOs) on the island of Ireland, have been recognised with a prestigious international award for their delivery of an innovative technology solution which has facilitated a significant increase in the amount of renewable energy which can be carried on the grid.
The two organisations took home the award for Best Technological Innovation and System Integration at the Renewables Grid Initiative Awards in November. The awards celebrate outstanding practices in grid development.
The solution addresses the fundamental differences between renewable power and the electricity generated by conventional power stations. These differences centre around the nature of the power grid. The grid is designed to operate at a frequency of 50hz. Conventional power stations using spinning turbines to generate electricity are directly connected to the grid and provide a high degree of stability to the grid through both the inertia of the heavy turbines in use and their ability to rapidly increase and decrease power output in response to demand changes.
Transmission system operators like EirGrid have to constantly balance supply and demand to maintain a frequency as close to 50hz as possible. Sudden changes to that frequency can result in power outages and problems for consumers. Maintaining that balance has become much more difficult due to the increased amount of renewables on the system. Renewable generation does not offer the inertia or controllability associated with conventional generation.
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“Traditional generation provides a lot more services to the grid,” says Emma Fagan, principal engineer and DS3 programme manager with EirGrid. “It provides inertia and is synchronised with the grid. Renewables are not synchronised.”
“If a large generator goes off the system there is a supply and demand imbalance and the frequency drops,” adds EirGrid head of future operation Eoin Kennedy. “We need inertia and fast frequency response to manage that.”
The traditional response would be to have reserve traditional generating capacity on standby with power stations operating below capacity, for example. That becomes more complicated with a high proportion of renewables on the system.
“If the frequency changes slowly we can handle that,” says Fagan. “If it changes fast we have a problem. It changes quite slowly when you have lots of conventional generation on the system. As you take traditional stations off the grid the frequency can change much faster, and it becomes more difficult to respond. We need to make sure it doesn’t change too fast and ensure that everyone else connected to the network can handle the rates of change.”
Part of the solution is to employ battery storage to provide reserve power but that will not be sufficient.
The EirGrid and Soni solution looked at the rate of change of frequency (Rocof) itself. The standard acceptable level of Rocof for grids around the world is 0.5hz per second. The two system operators have managed to double that to 1hz per second, thereby lowering the level of inertia required to operate the system securely and enabling an increased level of renewables on the system.
The project saw EirGrid and Soni work closely with ESB Networks and NIE Networks to change existing settings on the distribution system, carry out a compliance testing programme with conventional generators, and implement a state-of-the-art dynamic stability assessment tool with look-ahead capabilities to monitor and assess Rocof in real-time in control rooms.
“We are now able to operate the power system with up to 75 per cent renewables,” says Fagan. “The limit was 50 per cent a decade ago. The target set by the government for 2020 was to have an average of 40 per cent renewables on the system. We needed to reach 75 per cent capacity to achieve that average. The latest target for 2030 is 80 per cent renewables on the system. We will need to reach 95 per cent capacity to get to that average. Our next goal is to achieve that and meet the other targets set in our Shaping Our Electricity Future strategy.
“When we came out with the proposal first, it was a shock to the industry. It was seen as transformational. We worked with the generators to put the changes in place and test them and then carried out system trials. We successfully closed out the trial in May of last year.”
The solution was developed in collaboration with a number of partners. “There is an old saying that those who drink the water should remember those who dug the well,” says Kennedy. “A lot of different industry partners contributed to the success of the project. These included ESB Networks, NIE Networks, generators, and the regulatory authorities. Ireland is already at the forefront internationally in terms of operating a synchronous power system with a very high level of variable renewables. Reaching the 95 per cent target by 2030 will be unprecedented worldwide.”