Enhanced Renewable Integration through Flexible Transmission Options (ERIFT)

Lead Research Organisation: Imperial College London
Department Name: Electrical and Electronic Engineering

Abstract

China is installing wind farms faster than any other nation and the UK is leading deployment of offshore wind farms. Both nations will face challenges in connecting renewable sources in remote areas over (electrically) long cable or overhead DC routes and also challenges in controlling a system with a high proportion of asynchronous generation. This proposal identifies areas of common technical challenge and lays out a joint programme to analyse the issues and assess possible solutions.

Fully exploiting the potential transfer capacity is vital; in China this is from western/central hydro resources to eastern/costal cities and in the UK from Scottish and North Sea wind resource to southern load centres. The realisable transfer capacity from a set of AC and DC routes depends on exactly how the control of the system is arranged under differ loading conditions (which affects stability constraints) and on the risk profile of the configuration (which affects the de-loading for redundancy and security reasons). We will investigate coordinated control of AC, DC and FACTS elements to optimise transfer capacity and will develop tools to support operator decision-making including risk analysis. A national transmission system run with a high proportion of asynchronous generation has little inertia and so explicit frequency response needs to be synthesised to maintain the system frequency within limits. Two specific aspects will be investigated: the ability to provide frequency response services across a DC link (from kinetic energy from wind turbines at the remote end) without explicit communication of frequency data and the headroom required in power converters (turbine and HVDC) to accommodate this service. The protection schemes that identify and isolated faulty equipment are also impacted by use of asynchronous generation since their power converter interfaces offer much lower short-circuit currents that are difficult to discriminate. The problem will be analysed in detail and re-engineering of the protection schemes proposed.

A series of technical challenges that arise from widespread use of HVDC to connect remote renewable resources will also be studied. First is the operation and protection of multi-terminal HVDC that would allow meshing of DC routes to form more secure networks providing that the power flow and fault management issues can be addressed. Second is the reassessment of wind farm collection networks to eliminate potentially wasteful energy conversion stages. Third is innovation in HVDC converter design to make cost-effective the supply (or in-feed) of small fractions of power at intermediate points.

The main programme of work will be conducted by post-doctoral research associates in China and the UK each of whom will have a 3-month placement in the other country to deepen the interaction between the research teams. In China, PhD projects will also be arranged around the key research themes. In the UK, the work will be allied to the existing PhD cohorts in the universities. A steering committee that meets alternately in China and the UK will manage the programme.

The key outputs will be the analysis methods that are developed to assess the problems identified (and which are useful to grid system operators); proposals for engineering solutions (useful to system operators and equipment vendors) and verification of these proposals through scaled laboratory test systems and real-time simulation.

Planned Impact

The proposed research programme will provide significant advances in technologies for wind energy collection, connection and integration such that renewable resources can support grid system operation not just provide energy to displace fossil fuels. By playing a full role in support the system (through, e.g., frequency regulation and damping provision) and by lowering potential barriers (e.g. re-engineering protection around low fault current) renewable sources become assets not problems and meeting governmental targets becomes less technically burdensome.
The need to transport renewable energy from remote areas to load centres requires transmission system expansion which places an economic burden on a country. Achieving the increased transmission capacity with a minimum of capital assets is important to citizens and governments. HVDC links are cost effective against AC links above certain distances but the greater prize is to increase asset utilization by coordinated control of DC networks and mixed DC and AC systems. Providing analysis and verifying control schemes that demonstrate increased transfer capability after accounting for reliability constraints leads to more effective system planning and lower cost provision of energy.
The benefits to society as a whole are realised through enhanced performance by grid system operators such as The State Grid Corporation of China, National Grid (GB), Scottish Power and Scottish & Southern Energy. The consortium includes SGCC as full research partner and NG and SSE as industrial partners with whom the UK universities have worked closely for many years.
HVDC systems are supplied by three major European headquartered companies (Siemens, ABB and Alstom Grid) and an emerging Chinese industrial base. There has been much recent innovation around multi-level converters. Imperial has performed detailed studies of converter topologies for Alstom leading to 3 joint patent filings and Cardiff have worked with them on system control issues. This is an established relationship through which converter IP can be transferred and exploited.
A feature of the proposal is the bringing together of detailed multi-level converter models and system stability models. This is not industrial practice yet and progress here would benefit a system operator's understanding of subsystem interactions and an equipment vendor's understanding of transient loadings on their equipment.
Providing software tools to support wind farm design is a specialist activity undertaken by the likes of Garrad Hassan. Indeed, their "Bladed" software will be used in the project. There is potential to expand the options covered through of DC collector networks in terms of losses and protection.
Like other system operators, National Grid is considering the preparation it needs to put in place if multi-terminal DC networks become a reality. This includes understanding planning, operational, protection and reliability issues. Birmingham and Imperial have already assisted in workshops with National Grid to map out which issues known and unknown. This project will added analysis and simulation of cases studies to progress the understanding.
A societal issue for several parts of the world is how small communities on the routes of major transmission corridors can accelerate their economic development through tapping into some of that energy. This could apply to rural central China or North Africa. A cost effective means to tap HVDC routes would allow development to be widely stimulated.
The cooperation among the three strong UK academic teams and three top Chinese teams in power systems and power electronics, all with excellent experimental facilities, will for the first time provide an exceptional basis for extensive international collaboration in this key area. This will stimulate UK and Chinese academia and industry to attract more value from international activity.

Publications


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Junyent-Ferre A (2015) Blending HVDC-Link Energy Storage and Offshore Wind Turbine Inertia for Fast Frequency Response in IEEE Transactions on Sustainable Energy
Lakshmanan P (2015) Assessment of collection systems for HVDC connected offshore wind farms in Electric Power Systems Research
 
Description This collaborative project has made several advances in the use of High Voltage DC transmission technology of relevance both to UK in connecting to offshore wind farms and China in connection of remote onshore wind and hydro generation. Specific advances have been in power electronics for voltage conversion; the means to control power flow and mitigate the impact of faults in DC networks and methods by which HVDC can contribute to the frequency control of AC networks.
Exploitation Route Findings are being communicated to transmission grid operators in China (SGCC) and UK (National Grid) and to equipment manufacturers such as Alstom Grid.
Sectors Energy