Ultra-high voltage (>30KV) power devices through superior materials for HVDC transmission

Lead Research Organisation: University of Warwick
Department Name: Sch of Engineering

Abstract

In October 2014 the UK energy surplus during winter months dropped to below 5% overcapacity. In the future, this emergency overcapacity may be further diminished and actually become devastatingly insufficient, necessitating the national grid to divert large power demands to areas at opposite ends of the country or face serious and harmful disruption to energy supply. A viable solution to this supply problem is to build new national high voltage DC (HVDC) energy network connections in addition to more international connections to the super grid. To implement HVDC effectively, companies are considering two options: 1) to implement mature Si 300 MW HVDC technologies (circa 2009) requiring large overhead, land requirements, maintenance costs and cooling systems by scaling with the number of converters per line. Or 2) to invest in technologies which upscale the blocking voltage and the current capacity of individual power devices in a converter where fewer line converters and greater efficiency can be achieved for 2 GW MMC HVDC. Even by reducing the series chain effect in power transmission across the UK and conversion a 3% saving can equate to three 500 MW coal power stations from the current UK power usage of approximately 37 GW.

This fellowship seeks to develop revolutionary Silicon Carbide (SiC) material for ultra-high voltage (UHV) >30 kV power devices with large current ratings, up to 150A, with the intention of pushing the current rating as far as possible. The current rating of UHV vertical devices depends on availability of large surface areas (> 1 cm2) and is presently limited due to defects from excess material deposits forming on the wafer during the material growth. This is a problem which I believe will be a critical roadblock to such technology and receives little attention as the proposed power ratings are currently off the 5-10 year power electronics roadmap. Problematically, many in the field trust it will be solved at that point, however, no major research drive is currently underway to solve this essential problem.

Chemical vapour deposition (CVD) is the industry gold-standard technique for creating the semiconductor materials used in these UHV devices due to its excellent uniformity, scalability and reproducibility and so must be developed for quick uptake of any power device technology. For UHV devices the material choice and quality is key, where a defect free, thick (~100 um) layer with a large surface area is needed. Chlorinated chemistry is a recent development in SiC CVD and helps push growth rates up to 100 um/hr for thick layers and can be used to better achieve low background doping densities which are both required for high power technology. Here, thick high quality material will be achieved by state of the art epitaxial growth in the UK's only industrial SiC CVD at Warwick. In tandem to improving the material, its superiority will be shown by fabrication of vertical UHV devices: Schottky Diodes, PiN Diodes and MOSFETS, whilst developing IGBT processing, to show their potential for future modular multi-level converter (MMC) HVDC networks.

Bipolar devices such as Si thyristors and Si PiN diodes will be used in rail traction and grid level HVDC applications due to the high quality of Si material which allows large current ratings. In 2014, Yole predicted that these sectors would boost the >3.3 kV SiC market and in 2015 Mitsubishi showed an all SiC 3.3 kV traction inverter system. I further predict that SiC devices will only completely replace Si IGBTs in the > 10 kV range when the current-limiting surface defects are minimised and device reliability due to minor material defects is better understood. Only then will large current ratings be achieved, which will allow the technology to surpass current HVDC technology. This fellowship directly studies these limiting mechanisms and will develop the material and associated technology to underpin this step change in power technology

Planned Impact

The Silicon Carbide market is currently of the order of $200 million p.a. and is rising exponentially, set to reach $1 billion p.a. by 2022, predicting that Silicon Carbide will start to replace Silicon in 2022 and be a much sought after future technology. The UK can gain a foothold in this research area from developing this material ahead of the expected roadmap power ratings, the support statements accompanying this proposal echoing this intention. This would allow both intellectual property and manufacturing to be developed and in turn permit new industry to flourish in the UK. Society more generally will benefit from the continued prosperity of indigenous businesses and will also enjoy sustainability benefits arising from the increased, and more efficient, use of renewable energy sources. It is expected that some of these benefits will start to feed into the industrial base after 10 years of the programme start, with increasing levels of uptake beyond that date.

This fellowship seeks to deal with the critical trilemma of power transmission: rising costs of technology, reduction of CO2 emissions and energy security. The electricity generation and distribution sector will be the major benefactor from efficient and secure energy transmission across the UK - in turn allowing energy providers to supply cheap, reliable power across the UK. An intention to interweave sustainable energy into the network underlies this requirement and is implicitly thought to possible though renewable energy. The uptake for weather dependent renewable energy sources, such as wind or solar, is slow due in part to unreliable generation all year and imparts a major constraint on nationwide implementation: energy resilience versus power generation uptake from green sources. For serious uptake of renewable energy, energy transmission cross-country needs be improved to mitigate this unreliability and reduce energy security risk, currently this is not possible and inevitably increases consumption from risk-free non-renewable sources. However, if the requirement for extra energy availability is lower due to efficient nationwide transmission, this constraint is relaxed and allows a greater percentage of green energy sources to be incorporated into the national grid. Not only will this will enable the UK to meet the CO2 emissions targets set by the G8 summit and the climate change act 2008, it will allow surplus energy to be sold on a common market to the rest of the EU, possibly worldwide, and leave a legacy of sustainable energy where the UK will take leadership by foresight into the technology. Directly, the partners from the Energy Systems Catapult (ESC) and the Energy Research Accelerator (ERA), centrally located in the Midlands, (e.g. Highview and JLR) would both benefit from the extended foresight into power technology which this fellowship will deliver - allowing informed decisions for the UK's future.

With projected population growth and the necessary housing expansion the property sector would also indirectly benefit from the devices proposed in this project. They will allow significant reduction of the area needed for the supporting power transmission and conversion infrastructure, preserving greenland and areas of natural beauty and hence increase the density of housing closer to major cities and towns. Other sectors such as Aeronautics and Defence (Rolls-Royce, BAE, Halliburton, TT electronics, etc.) would benefit from high purity Silicon Carbide material through its use in harsh-environments where its temperature resilience and radiation hardness would lead to robust devices. Such applications of these robust devices could be in downhole drilling for oil extraction, high-frequency data transmission, low-temperature high radiation space applications and would lead to reduced volume of devices to reduce the load taken by power electronics on aeroplanes.

Publications

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