DC Networks with DC/DC Converters for Integration of Large Renewable Sources

Lead Research Organisation: University of Aberdeen
Department Name: Engineering

Abstract

This project studies various aspects of integration of large renewable power parks with DC networks which include DC/DC converters.
UK and China alike have enormous wind power potential which theoretically can exceed total national energy demand. Much of this energy is located offshore or in remote sites like North Scotland and North West China which have no electrical grid or have very weak grid infrastructure. These factors together with wind energy intermittency cause integration challenges, demand new approaches in developing transmission/collection grids and call for substantial use of power electronics.
There is large number of point to point High Voltage Direct Current (HVDC) links worldwide and HVDC has proven beneficial for interconnecting wind energy. Nevertheless DC transmission has not evolved into widespread DC grids because of a range of technical challenges such as difficulties with DC voltage stepping and DC fault isolation. It is expected that DC grids will have same security level as AC systems, but all grid functions, topologies, operation and control will be different and need substantial further studies.
The DC networks may not adopt AC grid topologies because of high costs and losses associated with DC/DC voltage stepping and DC fault isolation components. However unlike AC transformers, the DC/DC converters will be highly controllable and flexible since they will be based on power electronics. This controllability enables DC/DC electronic units to take numerous other functions like controllable voltage stepping, circuit breaker and a power regulator in a single component. In this project we study the development of DC grids exploring the power electronics DC/DC components. This project will investigate development of DC grids by considering the essential requirements (AC systems experience) like security, stability, reserve, fault responses loss minimization. To meet these requirements we explore in depth power electronics DC/DC components and consider also semiconductor-based DC CB and mechanical DC CB.
The main aims of this project are:
1. To study integration technologies and control strategies of large scale renewable power parks with DC networks incorporating DC/DC converters,
2. To study key technologies required for DC grids including a multifunctional and multiterminal electronic DC/DC substation (power electronics unit connecting multiple DC lines),
3. To investigate meshed and hybrid DC grid topologies, their security and interface with national AC grids,
4. To develop new wind generator topologies, converters and controls suitable for connecting to DC grids,
5. To strengthen existing and build new lasting collaborative links between UK and China institutions.
The offshore DC grid is recognized as being highly important for UK energy sector. The interconnections with EU grid will increase security of electricity supply, reduce the spinning reserve concerns with wind energy and provide access points for offshore renewable sources. The offshore wind power in round 3 program has the potential to supply significant portion of the UK energy needs and thus to reduce greenhouse emissions and avoid building of new Nuclear power plants. It is projected that energy for transport and heating will be more dependent on the electricity grid, demanding strengthening of transmission grid and only offshore DC grid can meet these requirements in short-medium time frame. In China, there is now large number of LCC (Line Commutated Converter) HVDC lines operating solely as point-to-point links and new lines are planned for connecting large wind resources in north regions and offshore. There would be significant performance/operational and cost benefit if multiple wind parks could be tapped along these DC lines or if DC lines could be interconnected on the DC side
The main beneficiaries of this research include high power converter manufacturers, grid operators and renewable energy developers.

Planned Impact

Knowledge
This project deals with some of the most challenging topics in the electrical power industry: its impact will be both deep and widespread. When the technical aims are achieved, the research community will gain understanding of the principles of building and operating DC transmission grids and integrating large renewable power plants with DC grids. It is known that a number of research organizations in UK, China and worldwide are studying the feasibility of DC grids, but most studies consider simpler multiterminal HVDC with circuit breakers. CIGRE currently has 5 working groups on DC grid topic.

Economy
The UK manufacturers (ALSTOM and GE Energy) have a strong research program on DC transmission/Distribution grids and wind energy. The research on MW-size DC/DC converters is in early stages but a DC fault tolerant DC/DC converter may become facilitator of DC grids and game changing technology. The multifunctional electronic DC substation can become pivotal point for staged building DC grids.

All the UK grid operators support the North Sea grid initiative. Scottish and Southern Energy is building the Moray firth HVDC hub which will be the first offshore DC substation where several DC lines will meet. The National Grid and other EU TSOs will benefit from the results on DC grid interface with national AC systems and with wind farms. The National Grid jointly with SSE is planning a multiterminal eastern HVDC ineterconnector which serves dual purpose: to interconnect Scotland with England and to provide access points for offshore wind energy in Firth of Forth.

The impact of DC grid technology will be wider than renewable energy industry. The oil industry companies are eagerly anticipating technologies for connecting to North Sea DC cables in order to reduce fossil fuel use on platforms, to improve security of power supply to platforms and to reduce costs of exploring remote oil/gas fields. The technologies proposed in this project can aid in integrating other large MW DC sources like solid-oxide fuel cells or storage systems.

The project has potential for significant impact on the academic and professional community considering the wide interest in DC grids.

Since the project develops technologies that facilitate large scale renewable energy integration, the potential impact on economy is significant. Large scale wind generation is at the heart of energy policy in the UK, China and many other countries. Currently, energy policy makers are considering very important decisions on the future of energy supply. The technical feasibility and cost-effectiveness of large-scale renewables is the key argument which will influence future balance between nuclear and renewable energy.

People and society

The Academic partners' track records of research excellence and the leading edge research nature of the proposed project will create a world-class research base to accelerate industrial innovation, to help develop and drive industry, and to greatly enhance student skills, opportunities for sponsorship, internship and employment. This will be of directly benefit to society.

In addition to the investigators, 3 researchers in the UK and 5 in China will work on this project. This team will work on the most advanced smart grid concepts and the project will significantly advance their skills. Indirectly UK and China societies will benefit from the advancements in the knowledge pool in the power systems/electronics field which has been recognized as crucially important for UK industry.

The development of new and strengthening of existing UK-China collaborations in the field of Smart Grids will benefit both UK and China. The two countries are pooling their knowledge base and staff resources to jointly address the very important engineering challenges relating to electrical energy. It is planned to continue collaborative links between UK and China institutions beyond the NSF-EPSRC project.

Publications

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Description A new converter has been developed which is capable of transferring power between DC networks of different voltage levels. This converter would take role of transformer in conventional AC transmission systems. The converter also achieves the function of traditional circuit breakers and is capable of interrupting DC fault currents. A multiport version (DC hub) is also developed which may be employed to interconnect more than two DC systems. The project has also developed lab-scale hardware demonstration converters.
Exploitation Route Two patents have been generated during the project, in order to protect IP for new converter designs. We are looking for commercialization opportunities for these deigns.
Sectors Energy

 
Description The impact is demonstrated through engagement in two CIGRE B4 working groups: 1.) B4.58 "Control Methodologies For Direct Voltage and Power Flow in a Meshed HVDC Grid" (active in 2011-2017) with prof Jovcic as one of UK representatives and chapter 5 author. 2.) B4.76: "DC-DC converters in HVDC Grids and for connections to HVDC systems" (active in 2017-2020) with Prof Jovcic as chairman. CIGRE technical brochures are very influential documents in industry and commonly act as standards.
First Year Of Impact 2017
Sector Energy
Impact Types Policy & public services