The World-Wide Development of High Voltage DC Systems

A report submitted to the Department of Electrical and Computer Engineering, McMaster University, Hamilton, Ontario, Canada, as part completion of course

Electrical Power Systems

February 2016

by Zeeshan Ashraff

Candidate for Bachelor of Engineering, Electrical

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Contents Acknowledgements........................................................................................................................................... Abstract............................................................................................................................................................. HVDC.................................................................................................................................................................. Power Electronic Devices in HVDC.................................................................................................................... History................................................................................................................................................... Recent Applications.............................................................................................................................. Future Applications............................................................................................................................... Circuit Arrangements........................................................................................................................................ SCR and VSC Topologies........................................................................................................................ Footprint............................................................................................................................................... Interconnecting Circuits........................................................................................................................ Future HVDC Systems – Voltage, Current and Power........................................................................... Large Systems World-Wide............................................................................................................................... Global Presence.................................................................................................................................... Research, Development and Application.............................................................................................. Personal Viewpoint........................................................................................................................................... Development of HVDC Systems............................................................................................................ Value of Discussions.............................................................................................................................. References......................................................................................................................................................... List of Images.....................................................................................................................................................

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Acknowledgements First and foremost, I would like to thank my supervisor, Dr. Nigel Schofield, for being a great mentor. Dr. Schofield’s endless support in the form of supplying a diverse collection of resources made the successful completion of this report possible. Moreover, I must extend my gratitude towards the Teaching Assistants for their tireless efforts in passing on their knowledge of Electrical Power Systems. Abstract This report focuses on the world-wide development of high voltage DC systems for electrical power transmission and distribution. The history of power electronic devices in HVDC systems is examined, along with current technologies being utilized. Power electronic devices being investigated and proposed for future HVDC converters are also investigated. The footprint of a typical high power HVDC converter station, its voltage, current and power ratings are mentioned. The integration of HVDC into existing electrical power utility grids is also covered. The presence of HVDC systems and the countries prominent in the field of HVDC research, development and application are examined. Finally, the author’s personal viewpoints are presented.

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HVDC It is not uncommon to employ HVDC systems when designing future global transmission grids. Although it has been around for 60 years, HVDC was less prevalent during its first 30 years, acting as more of a nice technology. HVDC systems either classically use thyristors for conversion or VSC (voltage source converters). A typical HVDC transmission characteristically has a power rating of several hundreds of megawatts (MW), although some are rated over 1000 MW. They use a combination of overhead lines and undersea/underground cables, depending on which of the two is needed. [1][2][7] It is safe to assume the major equipment in HVDC stations will provide over 30 years of service before being replaced. The HVDC link could alternatively be revamped to increase its lifetime through the use of newer technology out on the market. HVDC systems require fewer transmission lines when compared to their alternating current (AC) counterparts, making HVDC systems more economical in terms of land and money. This is due to the fact that HVDC systems are capable of transmitting more electrical power over longer distances than alternating current (AC) systems. [7]

These advantages combined allow HVDC systems to connect remote sources of electrical generation to load centres often thousands of kilometres away. In situations where AC power systems are not viable, HVDC systems enable the use of renewable energy sources. The HVDC market continues to rise after establishing itself as a mainstay in many transmission grids. [1][7] Power Electronic Devices in HVDC History

The origins of DC transmission can be traced back to the 1800s, when the German cities Miesbach and Munich were connected via a 50-km-long 2-kV DC transmission line. At this point in time, the only way to convert between consumer voltages and DC transmission voltages was through the usage of rotating DC machines. This was in stark contrast to AC systems, where conversion is achieved simply by means of an AC transformer. Additionally, AC devices require little maintenance, further accelerating their incorporation at a very early stage in the development of electrical power systems. [2][9]

The disadvantages that the AC power systems present are the limit on transmission capacity, transmission distance, incompatibility between AC systems of differing frequencies, system instability and undesirable power flow scenarios. These limitations necessitated the development of DC transmissions as a supplement to the AC transmissions. [2] In the nineteen-thirties, the invention of mercury arc rectifiers facilitated the design of line-commutated current sourced converters. Germany once again acted as a trailblazer, signing the first contract for a commercial HVDC system in 1941 [8]. The city of Berlin was to be the recipient of a 60 MW underground cable of 115 km length. By 1945, the system with +/-200 kV and 150 A was ready for energizing. [11] The next major breakthrough came in the form of the replacement of mercury arc valves by thyristors in the late nineteen-seventies. Parallel/series connections of oil-immersed thyristors and an electromagnetic firing system were used for the design of the outdoor valves for Cahora Bassa, Africa’s fourth-largest artificial lake. The designs were further augmented in the form of air-insulated air-cooled valves and air-insulated water-cooled designs. The latter is still considered to be the state of the art in HVDC valve design. The arrival of

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thyristors with higher current and voltage ratings reduced the number of series-connected thyristors per valve and entirely purged the need for parallel connections. [7] Recent Applications

VSC (voltage source converters) technology, called HVDC Light, was launched in 1997 by ABB. This innovation employed insulated-gate bipolar transistors (IGBT’s) for conversion. The VSC method is an adaptation of the traditional HVDC approach, and is useful for transmitting lower power ranges (100 to 1200 MW) over short and medium long distances. This technology permits the usage of overhead lines as well as environmentally friendly oil-free underground and underwater cables. ABB has continued enhancing HVDC systems, with its most recent development being the

ultra-high voltage direct current systems (UHVDC). These systems possess rated voltages of up to 800 kilovolts (kV) and are the biggest leap in transmission capacity and efficiency in over 20 years. [2][7][12] Semiconductor devices with turn-off capability are needed for voltage source converters to function. Insulated Gate Bipolar Transistors (IGBT) with high voltage ratings have further jettisoned the development of voltage source converters for HVDC applications in the lower power range. The voltage source converters are compact in design, have a four-quadrant operation capability and high losses. Siemens has been involved in the development of self-commutated voltage source converters with ratings up to 250 MW, named HVDC Plus Power Link Universal Systems.

Similarly, Alstom offers voltage source converters in the form of their Alstom HVDC MaxSine. [12] Future Applications

Thyristor valves convert AC into DC and are a necessary component of any HVDC converter station. The switches themselves are found in the form of thyristors, which allow for the valve to become controllable. An exciting field in the world of power electronics has been the development of light-triggered thyristors (LTT), which can be turned on by injecting photons into the gate rather than electrons. The use of light triggered thyristors can reduce the number of components in the thyristor valve by 80%, which has increased the reliability and availability of the transmission system. [3] A fibre optic cable is used to transmit the gating light pulse through the thyristor housing directly to the thyristor wafer. The required gate power is a significant improvement over electrically triggered thyristors (ETT) coming in at around 40 mW. Light triggered thyristors also possess unlimited black start capability and are able to operate during system undervoltage or system faults. In the electrically triggered thyristors, this is not possible unless enough firing energy is stored long enough on the thyristor electronics. [3][4]

Figure 1: Siemens’ HVDC Plus IGBT converter modules

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Circuit Arrangements SCR and VSC Topologies

All HVDC systems were Current Source Systems (thyristor based) in their early days and have been available since the nineteen-fifties. A state of the art Current Source System can be rated up to 6300 megawatts and UHVDC (+/-800 kilovolts). Voltage Source Converter HVDC has been available since 1997. A state of the art Voltage Source Converter HVDC can be rated up to 800 megawatt and (+/320 kilovolts). Voltage Source Converters hold several advantages over their Current Source System counterparts, including a much smaller footprint, more accessibility, controllable reactive power and fewer harmonics. [5] Footprint

The footprint of HVDC converter stations varies greatly by the amount power. ABB’s HVDC Classic is capable of handling 8000 MW of power and is their most economical way of transmitting power over long distances. This design requires long submarine cable connections, and a typical footprint (e.g. 600 MW) is around 200x120x22 meters. ABB’s HVDC Light, which is capable of handling power from 50 MW to 2500 MW, is structured so that each

terminal is an HVDC converter plus an SVC. Suitable for both submarine and land cable connections, the HVDC Light’s footprint (e.g. 1200 MW) is 100 x 150 x 20 meters. [14] Since HVDC Light uses more advanced components, it is natural that it is capable of handling more power than its Classic counterpart, despite having a smaller footprint. This trend is followed by other manufacturers as well, with the classic current source HVDC systems requiring more space than their VSC counterparts. [7] In AC transmission, a double circuit is at around a price of 250,000 USD each, and AC substations and series compensation (above 600 km) are around 80 million USD. The cost of an HVDC system depends on the power capacity to be transmitted, type of transmission medium, environmental conditions and safety requirements. HVDC transmission can be estimated to around a price of 250,000 USD/km, with converter stations costing about 250 million USD. [7][14][15] Interconnecting Circuits

HVDC transmission has reached a point where it can develop into a grid of its own. Lightning fast HVDC breakers ensure that the meshed HVDC is perpetually available. The AC systems already in place use less costly AC breakers to transform

power between different voltage levels. The interconnection of HVDC systems to existing electrical power utilities can serve a number of purposes. HVDC systems can reinforce the AC grid by address the AC systems’ four main constraints comprising power balance, thermal overloading, synchronicity and availability of reactive power. [8] There are differing ways in which AC and DC systems are interconnected. The HVDC system can either be embedded in the AC system or be used to interconnect two separated AC systems. The embedding of the HVDC system is possible only when the AC frequency is the same in all HVDC stations. Despite this, the HVDC maintains the ability to mitigate existing bottlenecks on the AC side. When used to interconnect two separated AC grids, the AC frequency differs between the HVDC stations. This method is useful for alleviating power imbalances in one of the AC grids by applying proper HVDC control. [6][8] The Caprivi Link, which is the first HVDC Light project, interconnector is an example of an interconnecting circuit between converter stations. The Zambezi station, in the Caprivi Strip in Namibia, is interconnected to the Gerus converter station via a 950 km long bipolar DC overhead line. The conductors of both the negative and positive polarities are mounted on the same pole. There is a double-circuit electrode lines 25

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km long. To reduce line losses, the link is operated with parallel DC lines and earth return in the monopole stage. The link is operated as a balanced bipole with zero ground current in the bipole stage. [6]

Future HVDC Systems – Voltage, Current and Power

The energy consumption of the world is constantly rising. To meet these demands and to integrate renewable energy sources, the transmission grids must be strengthened to improve controllability. Several gigawatts (GW) of bulk electricity must be transmitted and bottlenecks must be removed, all while maintaining a favourable public opinion. Newer HVDC technologies, voltage source

converters in particular, have the capability to double or triple power capacity. The challenge lies in converting existing predominant alternating current lines to direct current ones. [7][13] The Itaipu interconnection in Brazil introduced long-distance HVDC hydroelectric overland transmission rated at +/- 600 kV in 1984. China took this a step further by increasing the voltage rating for HVDC classic current source converters to 800 kV and currently transmits more than 7000 megawatts (MW) over long distances. Converting a 400 kV line or a double circuit 220 kV line from AC to DC can increase transmitted power by more than 1000 MW. Similarly, converting a double circuit 400 kV line to a triple 500 kV line can increase the amount of power transmitted by 5000 MW. [13] It is often feasible to increase the power-carrying capacity of existing transmission lines rather than constructing new routes. China is examining the possible usage of 1,100 kV for power transmissions of 10 to 13 GW per line for distances over 2000 km. They are also considering an increase of the rated current of 800 kV systems. 800 kV DC transmission systems have been in operation recently. When the transmitted power is pushed beyond the 10 GW mark the 800 kV lines become unfeasible due to the necessity of an additional line. Ultra High Voltage DC systems are

designed to transmit power over 10 GW and must respond to a rise in requirements on availability and reliability. [14] The first step in the conversion is identifying a transmission line and substation suitable for AC/DC conversion. Next, possible voltage and current values are calculated, and the type and rating of the DC converter is chosen. Once established the HVDC system increases power transfer capability, investment cost, time, power flow control and reliability. The proposed conversions, if executed smoothly, can be accomplished in less than one month. The newly placed HVDC systems are suitable for connecting increased remote generation and de-bottlenecking congested areas. [8] Large Systems World-Wide Global Presence

There are over 140 HVDC projects world-wide, and half of them have been delivered by ABB. A rapidly growing sector, the HVDC transmission is expected to reach $9.62 billion by 2018. Power consumption demands continue to rise across the globe. The major economies of China and India are working towards harnessing energy

Figure 2: Valves with Light Triggered Thyristors

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from renewable sources to satisfy their power consumption needs. This factor added to the lower level of transmission losses incurred by HVDC transmission makes HVDC a very inviting prospect. [7][9][10] The European Union has set a target of meeting 20% of their electricity needs via renewable energy sources, further justifying the need for HVDC systems. Europe and Asia-Pacific are the fastest growing markets, although the American market also claims a substantial share in the HVDC market. Research, Development and Application

In the late nineteen-nineties, ABB commercialized advanced and compact converters built from IGBTs. Siemens commercialized a more advanced IGBT converter, clearing several hurdles the previous versions presented. In 2013, Alstom demonstrated a converter incorporating full-bridge submodules that extinguished DC currents up to 40 times as fast as an AC breaker. Seeing as how Siemens, ABB, and Alstom originate in Germany, Switzerland and France, respectively, it is natural that one of the three countries would spearhead HVDC research, development and application. [8] Of late, Germany has taken the lead in HVDC. The underlying cause of

this is Germany’s shift from the use of nuclear power and fossil fuels to renewable resources. Germany’s grid is at capacity and it is considering an aggressive transfer of HVDC from a supporting act to the main feature. This move is natural as the installation of HVDC seems cheaper than modifying AC grids. If these proposed projects come into fruition it would lead to the formation of a “supergrid” of inter connected DC lines capable of transporting electricity on a continental scale. [8][10] The means to move AC electricity down particular lines does exist, however, the expensive to put into operation. The power electronics in HVDC provide an ideal solution to this problem by pulling in electricity from close to its point of generation and injecting it into the AC grid hundreds or even thousands of kilometres away.

Personal Viewpoint Development of HVDC Systems

The development of HVDC technology has been staggering of late. In spite of this, there are areas which can be enhanced further enhanced. Increased power ratings, lower transfer losses, and further

reductions in HVDC station sizes are examples of the improvements. The designs of ultra-high voltage and rating above 10GW hold great promise. The interconnection of weak AC networks will be simplified through the use of new VSC technologies. New subsea transmission cable projects are proving to be vital in the expansion of the global power network. Clean electricity from HVDC systems can be used for powering offshore oil and gas installations. The societal impact of technology is always an important issue. HVDC can help alleviate the cost for the user during its entire life span. The initial investment, equipment and plant size, and engineering, commission efforts and processes can significantly reduce costs. Additionally, a key issue of total electrical system losses is also minimized by the usage of HVDC. The demand for HVDC is driving performance improvements in the components, such as thyristors and IGBTs, used within the systems. [10] Renewable energy sources such as hydro, wind and solar in remote areas can only be made accessible through the usage of robust electrical transmission. HVDC transmission systems play this part perfectly by being the best technical and most economical long distance transmission solution. Supplementing this with the superior ability of control, HVDC

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remains a force to be reckoned with as far as future energy systems based on renewable energy sources are concerned. [12] Value of Discussions

The discussions on HVDC systems’ impact on society and the environment are absolutely worthwhile. As has been mentioned in section 5.1, there has been growing concerns about the usage of “unclean” renewable energy sources, such as fossil fuels, and their detrimental effects on society.

HVDC systems make otherwise inaccessible clean renewable energy sources available. Using clean energy is a step in the right direction as far as the continued existence of the human race is concerned. The smaller footprint left behind by HVDC systems, combined with the aforementioned environmental benefits will lead to greater public approval. An area of discussion which has this many benefits are unquestionably worth investigating. Sustainable engineering is the art designing systems capable of using

resources in a sustainable manner. HVDC systems certainly play their part in sustainable engineering. By making inaccessible renewable energy sources more accessible, HVDC systems are promoting a more sustainable planet. They are more reliable and need less maintenance than their AC counterparts, resulting in more sustainable systems. The idea of designing and operating systems which lead to a more sustainable planet is of utmost importance to an engineer.

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References [1] Glover, J. D., Sarma, S. M. and Overbye, T. J.: “Power System Analysis and Design”, Cengage Learning, ISBN-10: 1-111-42577-9 [2] Website: http://www04.abb.com/global/seitp/seitp202.nsf/c71c66c1f02e6575c125711f004660e6/d8e7ec7508118cf7c1257c670040069e/$FILE/Introducing+HVDC.pdf, last visited on 30/01/2016 [3] Website: http://www.siemens.com/about/sustainability/en/environmental-portfolio/products-solutions/power-transmission-distribution/high-voltage-direct-current-transmission.htm, last visited on 29/01/2016 [4] Website: https://library.e.abb.com/public/33dfbb0011b4d43cc12579f800493921/POW0077_ACDC_Conversion.pdf, last visited on 29/01/2016 [5] Website: http://www.ece.mcmaster.ca/seminars%20and%20events/July292013_seminar_Mike_Barnes_slides.pdf, last visited 28/01/2016 [6] Website: https://www02.abb.com/global/abbzh/abbzh250.nsf/0/21c74c0215238eafc1257949005701d0/$file/ABB_Factsheet_Caprivi-Link-Interconnector.pdf, last visited on: 30/01/2016 [7] Website: https://library.e.abb.com/public/aff841e25d8986b5c1257d380045703f/140818%20ABB%20SR%2060%20years%20of%20HVDC_72dpi.pdf, last visited on: 28/01/2016 [8] Website: http://spectrum.ieee.org/energy/renewables/germany-takes-the-lead-in-hvdc, last visited on 30/01/2016 [9] Website: http://www.prnewswire.com/news-releases/global-hvdc-transmission-market-to-see-1697-cagr-to-2018-273302701.html, last visited on 29/01/2016 [10] Website: http://new.abb.com/docs/default-source/ewea-doc/hvdc-light.pdf?sfvrsn=2, last visited on 29/01/2016 [11] Website: http://electrical-engineering-portal.com/analysing-the-costs-of-high-voltage-direct-current-hvdc-transmission#2, last visited on 30/01/2016 [12] Website: http://www.energy.siemens.com/ru/pool/hq/power-transmission/HVDC/HVDC-Classic/HVDC-Classic_Transmission_References_en.pdf, last visited on 30/01/2016 [13] Website: http://www.gegridsolutions.com/alstomenergy/grid/Global/Grid/Resources/Documents/Effective%20HVDC%20solutions%20up%20to%20800%20kV-epslanguage=en-GB.pdf, last visited on 31/01/2016 [14] Website: http://www.sgcc.com.cn/big5/ywlm/mediacenter/corporatenews/05/247219.shtml, last visited on 29/01/2016 [15] Website: http://www.gegridsolutions.com/alstomenergy/grid/Global/Grid/Resources/Documents/Voltage%20source%20converter%20Switchyards-epslanguage=en-GB.pdf, last visited on 31/01/2016

List of Images [1] Cover Page: http://abbcloud.blob.core.windows.net/public/images/5efce144-9ce8-473b-83aa-270ba491bdcf/presentation.jpg [2] Figure 1: http://www.siemens.com/press/en/presspicture/?press=/en/presspicture/2015/energymanagement/2015-04-Inelfe/im2015040626emen.htm&content[]=EM [3] Figure 2: http://www.siemens.com/press/pool/de/pp_cc/2005/04_apr/sc_upload_file_sosep200501_32_ptd_072dpi_1264107.jpg [4] Figure on Page 6: https://handlemanpost.files.wordpress.com/2013/08/offshore-wind-crop.jpg