Converters for HVDC power transmission

Today, low carbon and renewable/sustainable energy systems are among the fundamental research subjects both in academic and industrial fields. According to the European Union’s Renewable Energy Directive [1], at least 20% and 27% of final energy consumption in the union shall be supplied from renewables by 2020 and 2030, respectively. Fulfilling this objective requires enabling technologies for the large scale integration of renewable energy sources into the electric power grid. Such integration includes the transfer of energy over long distances and exchanging energy between remote power grids.

Renewable integration by transferring energy over long distances can be fulfilled by high-voltage direct current (HVDC) transmission links, which can transfer energy from locations where renewable resources are abundant, i.e. offshore or deserts, to locations where electricity is mostly needed, i.e. large cities. Such connections are termed point-to-point connections and are not limited to integration of renewables, but can be employed to interconnect power grids of different regions or even countries for enabling the exchange of energy. The already existing and the proposed point-to-point HVDC links in Europe are illustrated in Figure 1 (a).

Furthermore, renewable integration by exchanging energy between remote power grids that span an entire continent – or even several continents – can be realized by interconnecting multiple HVDC links, so that eventually an HVDC power grid can be formed. Such a grid is advantageous in many ways as it 1) enables the transmission of renewable energy from remote locations; 2) collects the energy from a vast amount of renewable installations and allows it to flow freely. Energy can be thus transported to locations where it is mostly needed; 3) offers alternative routes of transferring energy. Therefore, energy losses can be reduced and energy availability remains high in cases that some HVDC links are inoperable; 4) allows trading of energy between countries, or even continents, so that the available energy is utilized extremely efficiently.

Both point-to-point HVDC links and HVDC grids rely on power electronic converters that control the energy flow and convert energy from DC to AC, AC to DC or DC to DC. Such converters are required to handle enormous amounts of energy and hence must be very carefully designed and tested. As a result, the cost of such converters is high, which may hinder their adoption. Another important requirement for HVDC converters is availability, i.e. the amount of time that the converter is operating properly. The availability of an HVDC converter depends on its ability to handle faults without being destroyed (fault tolerance) as well as to automatically return to normal operation after the fault is cleared. Fault-tolerant converters are important for point-to-point HVDC links but they are vital for HVDC grids since one single faulty converter may bring down the operation of a large part of such a grid. Therefore, research that aims at more cost-efficient and fault-tolerant HVDC converters is of paramount importance for the wide adoption of the technology.

                  (a)                                                                         (b)

Figure 1 HVDC transmission in Europe: (a) Point-to-point links [2] and (b) Proposed grid via interconnection of multiple links [3].

Another crucial component for enabling the reliable operation of HVDC grids is the HVDC breaker. This component can be used in combination with an HVDC converter for enhancing or enabling the fault-tolerant operation of the latter. But more importantly, HVDC breakers enable the disconnection of the part of the HVDC grid that experienced faulty conditions and allow the rest of the HVDC grid to operate undisturbed. Such HVDC breakers are required to handle huge currents and thus, their design is not trivial. It should be noted that the HVDC breaker technology emerged only recently and is still in its infancy. Therefore, research that aims at the development of the HVDC breaker technology is extremely important for the construction of HVDC grids that will allow efficient harvesting, distribution and utilization of renewable energy.

igure 2 Technology breakdown for achieving large-scale integration of renewables. This goal is a driver for HVDC link, HVDC grid applications, which require the converter and breaker technologies: the arrows point at the technology that is required for each case.

The connection between renewables, point-to-point HVDC links, HVDC grids, converters and breakers is illustrated in Figure 2. It can be seen that large-scale integration of renewables acts as a driver for the application of HVDC links and grids which rely on the base technologies of converters and breakers. Research within the high-power electronics group at KTH aims at evolving these technologies by the execution of several projects that focus on different aspects of these technologies. These projects are outlined and briefly described as follows.

  • Power Electronic Converters for Ultra High Voltage Direct Current Grids (UHVDC): this project focuses on new power converters for UHVDC grids with the ultimate aim of developing solutions that can make UHVDC grids more efficient, less costly and more reliable.
  • Hybrid HVDC converters: this project aims at combining two existing technologies of HVDC converters for creating converters with higher power-transfer capability, fault-tolerance and efficiency.
  • Wireless Control of Autonomous Submodules in Modular Multilevel Converters: the project aims to control the autonomous submodules of the MMC wirelessly, thus decreasing time and cost on the installation and maintenance of the converter.
  • HVDC breakers: the objective of this project is to conceive, analyze and test new HVDC breaker concepts.

Links and references

  1. EP. (2009). Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009 on the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC. Official Journal of the European Union, 140(16), 16–62.
  2. Wikipedia-HVDC links in Europe: https://en.wikipedia.org/wiki/European_super_grid
  3. European SuperGrid: http://www.solarfeeds.com/the-european-super-grid/

Power Electronic Converters for Ultra High Voltage Direct Current Grids

Increased use of uncontrollable renewable energy sources causes an ever increasing need for long-distance and sub-sea power transmission. For these applications frequently high-voltage direct current (HVDC) transmission is preferable to alternating current (AC) transmission. So far most HVDC installations are point-to-point connections. However, an increased use of HVDC connections will eventually make it economically favorable to connect these into an HVDC grid. Large, continental HVDC networks will be necessary to balance renewable energy sources on a large scale, but today’s converters are not adapted to such large systems. The project aims to develop more cost-effective converters that will function as part of future HVDC networks, whereas particular attention is given to converters that can handle ultra-high voltage as this will be required for large, continental networks, e.g. in China and Brazil, and will have high significance for their energy system. Moreover, being able to link HVDC point-to-point connections to a DC network can provide several benefits. Most notably, the cost of equipment can be radically reduced since fewer converter stations are needed and the reliability in the system can be increased.

In an HVDC grid the short-circuit fault handling methodology will be very different from that of an AC grid since the short-circuit current is only limited by the resistance. It will therefore be necessary to limit the current flowing between the AC and DC grids within a very short timeframe (few milliseconds) in case of a DC-side fault. This will call for new solutions both in terms of converter topologies and control algorithms. The modular multilevel converter (MMC) with half bridge cells has rapidly been established as the dominant choice of converter topology for commercial HVDC installations using voltage source converters (VSC). However, the half-bridge cell MMC loses control of the dc-side and ac-side currents whenever a short-circuit fault occurs on the dc side. Thus, other converter topologies will be required for DC grids. This is currently the subject of intense research and development, in industry as well as in academia.

Aims

The project should develop and analyze new technical solutions with regard to power electronic converters suitable for future HVDC grids operating at UHVDC. Of particular interest in this regard is to identify new circuit topologies with better properties and lower cost than those already in use.

Methods

The methodology will involve analytical studies, numerical calculations and simulations. The project should determine the requirements on power electronic converters compatible with UHVDC grids. It will therefore entail a rather in-depth study of the state-of-the-art in terms of fault detection and protection in bipolar UHVDC grids. This will result in tentative target specifications of AC/DC converters for UHVDC grids. Based on the findings from the first part the project will aim at conceiving and investigating new power electronic solutions applicable for UHVDC grids with particular emphasis on bipolar grid configurations. The results will be new concepts and designs. For at least one of the studied concepts, identified to the most promising and most interesting for further study, an experimental verification will take place.

Research Group

Project leader: PhD Student Stefanie Heinig(1).

Supervisors: Professor Hans-Peter Nee(1), Associate Professor Staffan Norrga(1), Kalle Ilves(2)

Affiliations: (1)KTH Royal Institute of Technology, Stockholm, Sweden, (2)ABB Corporate Research, Västerås, Sweden.

Links and References

S. Heinig, K. Ilves, S. Norrga, and H.-P. Nee, “On Energy Storage Requirements in Alternate Arm Converters and Modular Multilevel Converters,” in 18th European Conference on Power Electronics and Applications (EPE), 2016.

D. P. Sadik, S. Heinig, K. Jacobs, D. Johannesson, J. K. Lim, M. Nawaz, F. Dijkhuizen, M. Bakowski, S. Norrga, and H. P. Nee, “Investigation of the surge current capability of the body diode of SiC MOSFETs for HVDC applications,” in 18th European Conference on Power Electronics and Applications (EPE), 2016.

http://www.energiforsk.se/program/elektra/projekt/36275-omvandlare-for-dc-nat/

Additional funding (apart from StandUp for Energy)

This project is funded by the Elektra program of the Swedish Energy Research Centre (URL: http://www.energiforsk.se/program/elektra/)