Can we increase power transmission capacity faster?
In my previous blog, I explained how best practices, quality assurance and testing can help in delivering essential power transmission expansion “first time right”. But this may still not be fast enough. Expanding the grid in the “classical” way, by building new overhead lines or laying cables, is relatively slow. Finding the right routing, getting the necessary consent and dealing with public resistance – so-called NIMBY (Not In My Back Yard) and CAVE (Citizens Against Virtually Everything) groups – all take a lot of time. It is not unusual for ten years or more to pass between having the initial idea and starting construction. For the upcoming energy transition, with rapid increases in renewable energy sources, that could be too long.
So how can we speed up the increase in transmission capacity? We need to think of alternatives to the classical approach that can be realized faster or intermediate solutions to bridge the gap while new connections are built. That means finding smart ways of using existing power existing power or increasing capacity with a minimum of physical changes and societal impact. The good news is that there are many possible options – if we can remove some obstacles.
What are the options?
Dynamic loading of power transmission systems
Today’s power transmission systems operate mainly based on nominal power rating, which relates to the maximum safe operating temperature in constant power operation. In practice, however, power flows are intermittent and the temperature is time dependent. This offers a possibility to increase loading by tens of percent, given specific limitation to system usage, without exceeding temperature limits.
This principle is already applied in new cable connections to wind farms, but it could also be implemented in existing power systems if they are adequately modelled and monitored during operation. The required modelling and monitoring of existing assets is quite a challenge technically but it can be done.
Uprating HV overhead lines
Transmission capacity can be increased by replacing the insulators and conductors with HTLS (High Transmission Low Sag) conductors. Getting consent to replace conductors is not a major issue, as there is less physical and environmental change involved. Hence capacity gains can be achieved much more quickly than by building new HV lines. This option requires power system analysis and design studies (e.g. system voltage stability and loadability, mechanical stability analysis for reinforcement, corona, EMF, audible noise, insulation coordination, suspension length and maximum allowable sag).
Hybrid AC/DC solutions
Replacing AC circuits in the line with DC circuits can approximately treble transmission capacity. As with the previous option, consent to build is required – this time for converter stations at each end of the connection as well as the line itself. These converters require significant extra investment, but the environmental impact and construction time is still much less than for building new HV lines.
This option also requires more extensive power system analysis and design studies (e.g. system voltage stability and loadability analysis, mechanical stability analysis for reinforcement, corona, audible noise, ion current density, insulation coordination). However, the existing routing and infrastructure can be re-used, thus saving time and effort. DNV GL has performed several feasibility and design studies on hybrid connections, finding that they are feasible in most cases. You can read more about this option in our Hybrid Grids position paper by my colleague Professor Peter Vaessen.
FACTS (Flexible AC Transmission Systems) enhance controllability and increase transfer capacity of power networks. They are generally based on power electronics that provide greater control over the AC transmission system parameters. By adding FACTS to their grids, an operator is better able to control voltage and (loop)flows. FACTS can, to a certain extent, prevent bottlenecks by making better use of available capacity in connections elsewhere in the grid.
HVDC system ancillary services
Modern HVDC technologies are increasingly being deployed in existing HVAC grids. In particular, VSC (Voltage-Source Converter) technology has unique capabilities to deliver ancillary services. For example, it ability to control power flow between its nodes allows power flow in a connected HVAC grid to be optimized as well. This capability is not always used today but has real potential to increase the overall transmission capacity in existing grids.
Connecting energy storage systems such as batteries or power-to-gas facilities to the grid means the connection to and from the storage system will be used more effectively than its normal rating over time. Various energy storage technologies are already being deployed. However, the rate of technology development in the storage sector far outpaces the development of standards. To ensure a safe implementation, DNV GL developed the GRIDSTOR Recommended Practice for grid-connected energy storage. It provides all stakeholders a guideline for safe and sustainable grid-connected energy storage.
Flexible Transmission Systems
This is an option that has perhaps not been so widely considered so far. DNV GL recently delivered a Flexible Transmission System solution in the north of The Netherlands to mitigate a congestion issue. The solution is based on a concept developed by DNV GL, called Emergency Restoration System (ERS). ERS can be used to temporarily realize a new connection or strengthen an existing connection in case of temporary congestion or emergency. The system consists of highly mobile smart towers and is specifically designed for realizing transmission lines quickly, even in difficult conditions. When no longer needed, the towers can be quickly removed and re-used elsewhere.
What are the obstacles?
This option – individually and combined – offer huge potential to increase transmission capacity without the long delays faced in building new lines. But various obstacles need to be addressed before they can by deployed to full effect. For example, in several discussions with the energy sector, we see that even where the benefits to both the operator and society are unequivocal, the market design and regulations create a “virtual bottleneck” that needs to be resolved. Hence, as a first step, market regulation and design need to be adapted and harmonized.
Second, operators need to dare to change their way of operations and look past nominal power ratings. Nobody wants to be held responsible for a power failure due to “going beyond the normal practice of nominal limits”, even though the operator always operated systems within their actual physical limits. Here a power system verification by DNV GL can give comfort.
The examples above show there is a large portfolio of proven technologies and smart solutions that can help to improve transmission capacity rapidly with a minimum of physical and capital-intensive changes. Of course, we still need to aim for first-time-right implementation and, as with building new lines, that means making use if lessons learned, best practices and power system verification. And for innovative products and services, it also requires technology qualification.
This is my second blog to take a closer look at the specific challenges involved in expanding power transmission networks to support renewable energy sources. Please feel free to contact me if there is a specific challenge you would like to hear more about. In the meantime, to find out more about why we need to expand transmission networks and how the global energy landscape may look in 2050, take a look at DNV GL’s Energy Transition Outlook 2017.