Measuring Impedance: Towards an Efficient Smart Grid

September 7, 2016 | 00:00
Measuring Impedance: Towards an Efficient Smart Grid
Measuring Impedance: Towards an Efficient Smart Grid
Over the last fifteen years, the UK's energy landscape has changed dramatically. In 2000, over 90% of its electricity came from large scale coal, gas and nuclear power plants, with only 3% of installed capacity in the UK network provided by renewables. As we become ever more aware of the impact of fossil fuels on our climate, governmental policies, environmental legislation and societal pressure have significantly changed how we source our electricity. Renewable generation now accounts for around 25% of electricity generated in the UK and the generation mix is not only becoming increasingly diversified, but the UK’s energy generation as a whole is shifting towards a decentralised power network. Increased electricity generation from renewable sources is crucial if we are to achieve the UK’s climate change targets and gain higher energy security but this comes with its own problems. National and international energy grids need to move from manually balanced national systems to more autonomous and interconnected systems to be able to cope with the increased variability of energy supply from renewables and to allow it to cope with resulting power quality issues. The current infrastructure is simply not designed to deal with this as reported by the UK Government's Energy and Climate Change Committee (ECCC), which, in a report from the 21st June, highlighted the importance of managing this greater variation in supply and demand.

That same report highlighted the importance of continuing to develop a 'smart grid', an ICT enabled power grid that can manage supply and demand much more efficiently. To see this come to fruition, there are some key measurement challenges that must be dealt with: measuring and accounting for network impedance, measuring more accurately the fluctuation in power supply across a network as it happens (including network disturbances in the form of power quality issues) and, finally, where to use this measurement technology across the grid.

Impedance is the effective resistance to electricity flow and is a key component of managing a network. It can affect electricity flow and the capacity of the grid on any particular day but it also has an impact on power quality. We need to better understand this in order to manage grid stability effectively. Temperature heavily influences network impedance and as a result how much electricity can be put through the system. We don't currently monitor the real time fluctuations in impedance that can stem from a number of environmental factors including temperature variations. Currently energy networks have to operate under the assumption that capacity is lower than it may be in reality to avoid overloading the network. This means there is a significant amount of capacity not being used and our energy networks are operating inefficiently as a result.

This issue of inefficient operation is compounded by the increasingly divergent sources of energy we are using. As wind speed increases and decreases, and levels of sunlight vary, the power being delivered across the network varies significantly. Again, this variation is not being measured comprehensively, meaning that we currently have limited understanding of power quality issues propagating through the system, resulting in limitations in our ability to effectively manage grid stability.

Projects across Europe are working to develop, test and ultimately implement the technology that will help us measure impedance and power supply across a network far more than we currently can. Bornholm, a Danish island, is already reliant on renewable energy for the majority of its power and is working on becoming 100% renewable by 2025. The UK’s National Physical Laboratory is testing the technologies that will provide the necessary measurements to determine the impacts renewables have on the network and will ultimately be a crucial part of a smart electricity grid in the future.

As an example, Phasor Measurement Units (PMUs) measure various electrical parameters across a network, including phase. Phase, a measure of the relative position of the peak of a waveform cycle, is a key indicator for grid stability: if the phase between locations on a network is changing rapidly, this is an indication of an unstable network at risk of failure. PMUs are GPS enabled and can be set up as a synchronized network taking time stamped measurements that can help us identify such risks and act as a kind of alarm system. Using an array of PMUs to understand how phase changes propagate across a network allows us to understand the impact of demand and generation on the grid and also enable improved grid management systems to ensure stable electricity supply to users.

Real world tests like Bornholm allow us to understand how best to use this equipment. Monitoring a countrywide network is expensive and complex, and so demonstrating the technology in a smaller closed system is essential before scaling up to national level. But more importantly, the ultimate end goal is for these monitoring devices to provide the data for autonomous management of the network; if the data being collected across the grid is not providing a true-to-life picture of electricity delivery and system capacity, the network will never be capable of running as efficiently as it could. With capacity margins (network capacity minus our total electricity demand) shrinking significantly in the UK, ensuring network efficiency is essential.

There are still a lot of unanswered questions before we achieve a truly ‘smart’ grid. What is the best configuration for PMU placement? How do variations in impedance affect the efficiency and stability of a network? What power quality issues will arise from smart interventions on the system? These questions and more need to be addressed by not only national entities but on a pan European and ultimately international basis.

Without an intelligent network that can manage our diverse energy sources more effectively, it is going to be difficult to reach our ambitious and legally binding climate change targets. As the ECCC states in the very first sentence of its report, "networks are at the very heart of our low carbon ambitions". By developing smart grid technology and implementing it intelligently, the UK and Europe can ensure stable, efficient and interconnected networks based on localised, renewable-led energy; a movement that will help achieve sustainable, secure and affordable electricity for future generations.

Marieke Beckmann is Research Lead at the National Physical Laboratory's Centre for Carbon Measurement.
Loading comments...
related items