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Abstract--High-Voltage Direct-Current (HVDC) power

transmission is an increasingly important adjunct to conventional alternating current (ac) transmission of electric energy. The modern era dates from the middle 1950s when a 20 MW, 100 kV undersea cable system fed power to Gotland Island in the Baltic Sea off the coast of Sweden. Currently in China there are systems approaching 7,000 MW operating at +/- 800 kV. This presentation addresses the historic applications of HVDC transmission and leads to the modern era where significant growth in North America is being realized. This paper is a brief summary for a panel on HVDC transmission fundamentals. More detail will be available in the presentation.

Index Terms—High voltage DC transmission, Power Electronic Equipment

I. INTRODUCTION NTEREST in HVDC transmission has increased in the United States with the rapid development of large wind farms. Maps of available wind energy show huge

concentrations in the upper Midwest, along with areas where offshore wind energy looks to be significant. Additionally, large solar energy farms are piquing interest in the Southwest. Ironically, as would be expected, these regions typically do not have concentrations of load areas and so transmission is an essential element of any implementation plan. The attributes of HVDC match well with the variability of energy availability from these resources. But there is of course the problem of moving that energy from source to load. In this presentation we discuss the characteristics of HVDC transmission, give examples of present applications, and discuss the direction that the technology is heading. We will conclude with an overview of the stages for implementing a HVDC transmission project.

II. CHARACTERISTICS AND APPLICATIONS HVDC transmission characteristics can be categorized as follows:

• Controllability – in normal operation the current/power order is set and observed. They can be quickly changed in response to a major disturbance.

Willis F. Long is with the University of Wisconsin-Madison, Madison, WI

USA (e-mail: willis@epd.engr.wisc.edu). Wayne Litzenberger is with POWER Engineers, Portland, OR USA (e-

mail: wayne.litzenberger@powereng.com).

• Routing - current follows the transmission line/cable rather than wandering about the parallel or underlying circuits. Congested circuits can be bypassed, there is no inadvertent flow.

• Integration of remote diverse resources – for example wind turbines in the North Sea or along the Eastern coast of the US.

• Higher power with fewer lines, and no intermediate substations needed – consider the Itaipu system in southern Brazil with two sets of dc towers and three sets of ac towers.

• In the same vein, narrower rights-of-way are possible compared with equivalent ac transmission.

• The argument has been made that bipolar dc transmission is equivalent to a double circuit ac system.

• In a long-distance dc system the losses are less than with an equivalent ac system.

• There is no stability distance limitation. • Reactive power demand is needed at the terminals but

not along the line. • DC can be used to interconnect asynchronous ac

systems, or even ac systems of different frequencies. • DC has been proposed to interconnect “islanded” ac

systems to limit cascading outages. • There is no length limit with underground or

submarine cables.

III. AC/DC AND DC/AC CONVERSION There are two fundamental conversion principles. Line-commutated converters (LCC) have incorporated the traditional Graetz bridge for the interface. Earlier systems used mercury arc valves for the switching elements in the bridge; since the mid-1970s arrays of solid-state thyristors perform this function. The very large systems mentioned earlier use this technology. The bridges are sometimes referred to as current source converters. Both the rectifier (ac-dc converter) and the opposite-end inverter (dc-ac converter) have identical equipment installed in the switchyard. They are controlled somewhat differently. The alternative converter arrangement is called a voltage-sourced converter (VSC). This is a relatively new arrival on the scene and uses turn-off power semiconductors (e.g., insulated-gate bipolar transistors or IGBTs). The VSC has enhanced capabilities compared to the LCC (e.g., four quadrant real and reactive power control) but lower voltage

Fundamental Concepts in High-Voltage Direct-Current Power Transmission W.F. Long, Fellow, IEEE, and W. Litzenberger, Fellow, IEEE

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978-1-4673-1935-5/12/$31.00 ©2012 IEEE

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and power ratings. The converter losses are a bit higher. The VSC characteristics make it an ideal companion for off-shore wind farms.

IV. WHERE IS THE TECHNOLOGY HEADING? As the ratings of conventional LCC systems increase it is clear that this mature technology is looked at favorably for very large and/or remote renewable resources (including hydro in that definition). As ratings of VSC systems increase and as losses continue to reduce, applications for multi-terminal and even grid systems in Northern Europe will become a reality. The debate over the need for very fast dc circuit breakers will heat up. The search for improved power semiconductor materials (silicon carbide? diamond?) will continue. And some of our professional colleagues will continue to state that Edison was wrong and Westinghouse was right, alternating current is the only way to deliver electrical energy.

V. HVDC PROJECT IMPLEMENTATION The process of purchasing, installing and commissioning an HVDC system is an activity that will extend over several years. Obtaining permits can easily occupy 2 years by itself. Once a contract is signed with a supplier, manufacturing and installation can take an additional 2 to 3 years. This panel session will assist a utility and its project manager in organizing the staff needed to complete the project. Although most of the design work, manufacturing and construction will likely be carried out by suppliers or contractors, the purchaser will still need dedicated staff to develop the specification, analyze bids, execute the contract(s), monitor equipment testing, inspect construction, work with the supplier on commissioning, and ultimately to operate the system.

Design and construction of an HVDC system will necessarily rely heavily on one of only a few world-wide suppliers of such systems. As a utility begins the lengthy and expensive process of purchasing an HVDC system, this presentation will assist in developing plans for the execution of the project from development of the specification to the completion of commissioning.

A. Pre-Award Activities • Planning Studies

• Cost Estimates

• Permitting

• Converter Station Specification

• Bidding and Bid Analysis

B. Post-Award Activities • Manufacturing

• Testing

• Construction and Installation

• Commissioning