Whole system requirements must be considered in the move towards a low carbon power system
Grant Spence, senior principal consultant
Several European power facilities – particularly coal-fired power plants – face costly upgrades or closure due to new EU emissions limits imposed in May 2017. However, the UK government already aims to close all existing coal-fired power stations by 2025 and to restrict their use by 2023, meaning that the new regulations are likely to have more impact elsewhere in Europe. In April 2017, the UK recorded its first 24-hour period without coal generation since the start of the Industrial Revolution, while in June 2017 power from wind, solar, hydro and wood pellet burning supplied 50.7% of the UK’s energy – more than coal and gas combined. And the electricity from low carbon sources increases to 72.1% when nuclear power is taken into account.
However, the ability to maintain such progress is qualified by the National Grid’s system operability framework (SOF) document published in late 2016. The SOF indicates that progressing to a fully low carbon electricity system will involve much greater challenges than we have faced to date.
It is overly simplistic to think that we can replace existing synchronous machines with renewable generation and/or power sources connected to the GB transmission grid via high voltage direct current (HVDC) interconnectors as the new technologies don’t entirely replicate the operating characteristics of the original plant.
Intermittency and storage are not the only issues
Progress on the future power system’s requirements has focused on addressing the intermittent nature of renewable generation as well as energy trading (MWh) and balancing power (MW) flows. This has resulted in increased development of energy storage and interconnector projects and the development of energy balancing market mechanisms in the UK to include capacity and enhanced frequency response markets.
The SOF shows that resolving such intermittency issues alone is not enough. Other aspects such as system inertia and short circuit levels must also be considered. Lack of system operability can potentially impact the whole system so requires ‘whole system’ thinking and a ‘whole system’ approach.
To date, underlying system requirements are assumed to be included at little or no added cost when generating electricity. Services are paid for via additional contracts, such as ancillary service contracts, which have historically been priced on the premise that our old depreciated power stations supply it at marginal cost. But by migrating to a greener power system, we have eroded system capability to the point where the National Grid sometimes has trouble maintaining system operability without occasionally constraining on conventional power generation.
Let’s facilitate the low carbon transition without bias or constraint
National Grid has proposed a series of future energy scenarios representing four outcomes: (i) two degrees; (ii) slow progression; (iii) steady state; and (iv) consumer power. Only the first is forecast to provide an outcome which complies with the UK’s legislation on decarbonisation. In practice, there are numerous future energy system permutations – some potentially involving technologies we are not even aware of yet. Many emerging options comprise smart systems with the potential to provide services at a low incremental cost. It is important that future services frameworks do not act as a barrier to such solutions and that they provide all system requirements at lowest cost. A holistic systems approach avoids the risk that we procure the easiest services cheaply, potentially making the cost of procuring residual services expensive, resulting in a sub-optimal technical and cost solution.
The topic is complicated and there isn’t necessarily a single solution for each issue. Historically, almost all generation units were synchronous machines with similar behaviour under fault conditions. Present design and operation of the transmission system works on the assumption that the connected generation will behave in the same way. However, this is no longer the case for the UK transmission system which comprises increased levels of wind and solar generation and interconnectors supplying power into the system.
If it’s important, assign a monetary value
Maintaining safe operation of the transmission system in future requires that new generation technologies are designed to behave more like synchronous machines, and/or that the existing transmission system is redesigned to reflect revised (reduced) short circuit in-feeds. Different solutions will incur different costs which can be attributed to different parties. We may need to implement a series of solutions over time as we transition to a low carbon economy.
To facilitate progress towards a low carbon power system, we need to place a value on all system requirements, including system inertia and short circuit levels. To avoid predefining outcomes, these requirements should be determined in terms of functional or service requirements and not by capability or performance of specific types of equipment.
Developing a functional and service-based approach will enable greater opportunities to develop system based solutions with innovative, lower cost technologies and techniques, boosting progress towards a low carbon electricity network.