Our newest blog is from James Wylie a PhD student in the Department of Electrical and Electronic Engineering here at Imperial College London. He has written a piece on his work on building reliable systems to transmit electricity between countries and across Europe.

Around the world there are growing numbers of renewable generation connecting to power networks, which is helping reduce the amount of CO₂ entering our atmosphere from fossil-fuelled power stations. This has made a positive effect regarding global warming, and now we are seeing increased interconnection between countries, especially within Europe, to help further the penetration of renewables and make our networks more robust against unplanned outages due to the unpredictability of the weather.

The weather across the UK and Europe varies a lot, with it predominantly being windier in the north, and sunnier in the south. This means as we increase wind capacity in the UK and Ireland, and solar generation in Spain and Italy, we are beginning to reach thermal and power constraints on local networks due to there being too much generation compared to demand in localised areas.

Taking Scotland as an example, the increasing levels of wind generation can result in wind turbines having to be curtailed during peak generation and low demand, and may also result in difficulties in maintaining power supplied to customers during times of there being no generation from wind.

An interconnector between the UK and Norway, which is known as the North Sea Link (NSL), is currently under construction to help prevent the scenarios of loss of load or curtailment from happening. Norway has a lot of pumped-hydro stations, which can be used as a storage mechanism for excess wind generated in Scotland. When there is too much power being generated within Scotland, power can be transmitted to Norway and used to pump water back to the top of reservoirs instead of curtailing wind turbines in Scotland. This also means the pumped-hydro stations in Norway can support Scotland by transmitting power back to the UK at times of no wind generation.

The NSL is using High Voltage DC (HVDC) with Voltage Source Converters (VSC) as the technology for the link as it is more economically feasible to use DC transmission than AC, due to the length of the sub-sea cable.

My research focuses on VSCs, which are used in the conversion of AC to DC, with a VSC station located at both ends of the transmission link. I investigate their reliability and the different ways of making their availability as high as possible. The most popular VSC to date is the Modular Multi-level Converter (MMC). The MMC is the preferred VSC due to the reduced need of filtering of the AC sine wave, resulting in a smaller converter footprint when compared with other VSCs. This is achieved by inserting and bypassing many identical sub-modules in series, creating a near perfect AC sine wave. They are, however, relatively new when compared with other kinds of VSCs. This means there is not a lot of public data for their reliability, so I am gathering information and data to get a better understanding of their life expectancy, and how often to perform maintenance. An MMC has a high component count with implications for reliability, as more components means there are more things to break or fail. The MMC, however, provides opportunity to introduce redundancy within the converter due to its modular design, by inserting extra sub-modules within the converter to allow a few module failures between maintenance periods.

Including redundancy within the MMC increases the overall availability of the converter, by being tolerant to a small number of module failures between maintenance periods, but the increase in modules lowers the overall efficiency of the converter.

This leads to the three trade-offs that I am investigating:

1. The rate of maintenance.
   How expensive is maintenance, and how much of an effect is there if the maintenance interval is increased, especially for offshore converter stations where maintenance is more costly when compared to onshore.
2. The availability.
   What is the penalty of loss of load, and how much does it cost for unplanned maintenance, especially offshore when specialized repair vessels are in high demand.
3. The number of redundant modules.
   Increasing the number of redundant modules within the converter has the effect of increasing the availability, but lowering the efficiency of the converter, which will become costly over the lifetime of the converter.

My future work will investigate how to operate the MMC in a degraded mode, so power can still be transmitted even if part of the converter has failed. Operating in this mode will enable renewable generation to continue to connect to transmission grids, where currently wind turbines may be curtailed if there is a failure with the converter.

Understanding these kinds of factors in the use and operation of VSCs will make it easier for long term planning for international power connections. These interconnects will be key to building an electricity-focussed, low-carbon energy system.

Biography

James Wylie is a PhD student in the Future Power Networks and Smart Grids CDT, a collaboration between the University of Strathclyde and Imperial College London.

He is working on the reliability of Voltage Sourced Converters in conjunction with his industry partner GE’s Grid Solutions. Currently based in the Control and Power Group in the Department of Electrical and Electronic Engineering he has an MRes from the University of Strathclyde, as part of his PhD studies, and a BEng from Northumbria University in Electrical and Electronic Engineering.