Grid inertia has for many years been provided by rotating equipment at fossil fuel plant. New sources of inertia have to be found as those plants close. Andrew Strong, technology developer & business development manager at Cambridge Consultants, argues that common standards are key.
Frequency control of the grid has for many years been managed principally using the control systems built into large (rotating) generating plants. Increasing the load on the power system causes the system frequency (nominally 50Hz in the UK) to decline. Power station control systems monitor this and make adjustments to the power input to restore operation at the target frequency.
Much has been written about the emerging complexities of achieving the same functionality as the grid evolves towards decentralised and distributed generation. In this new grid configuration, most of the generation will feed the distribution network rather than the transmission network, and renewable technology will not have the same inertia or stability provided by the rotating equipment of the past. Also, gone are the days when the grid was under the control of a single control system (or methodology) and we are moving potentially towards an undesirable (maybe catastrophic) scenario in which multiple (but unsynchronised) control systems with different response characteristics could oscillate in response to changing grid frequency. The result is a growing threat of severe problems in controlling voltage and/or frequency – potentially leading to blackouts.
As system operator, National Grid (which has a statutory responsibility to manage the UK grid’s frequency at 50Hz ± 1%) is running the £6.9 million SMART Frequency Control (SFC) project. This is investigating how to maintain system stability in the light of an increasing renewable contribution without increasing the risk of system failures. The flexibility of modern software-based control systems should make light of the task of synthesising the frequency responses needed to maintain stable operation of the grid, and is likely to be combined with technologies such as highly stable time references (from GPS, for example) and high-speed internet or wireless communications.
Keys to success are likely to be:
- A unified measurement and control algorithm approach between stakeholders (network and renewables operators) so the response to load across the grid is synchronised, and control systems work in harmony rather than independently. There is potential here for a UK or EU/global standard that defines the general approach, algorithms and energy storage (outlined below) used to support grid stability.
- A common standard for communication between all generation nodes. It may be that the system frequency remains the principal reference and communications medium, but others, including internet connectivity, could be considered.
- Effective contingency measures for under or over-frequency. This could include demand-response approaches such as load shedding at times of maximum demand on the grid, shedding excess capacity into rapid-response energy storage – this could, for example, be installed in the infrastructure of each wind turbine – and the use of fast-reacting, programmable solar PV inverters.
The future use of dynamic (real-time) pricing using the next generation of smart electricity meters also promises to help smooth demand at peaks by management of end-user demand.
The development of an alternative approach to frequency control will become increasingly important with the growing prevalence of renewables. Without well-aligned communications and comprehensive standards, we could be heading to a future of poor power stability and more frequent localised interruptions.
This article first appeared in the January 2018 issue of New Power