The goal of the High Voltage Direct Current (HVDC) Transmission project is to assess and demonstrate the technical and financial feasibility of low-cost small-scale HVDC interties for rural Alaska. The objective is to demonstrate that small-scale HVDC interties are technically viable and can achieve significant cost savings compared to the three-phase AC interties proposed between Alaskan villages. Because these AC interties are very costly to construct and maintain, very few have been built in Alaska. As a result, most villages remain electrically isolated from one another, which duplicates energy infrastructure and thereby contributes to the very high cost of electricity in most villages. HVDC technology has the potential to significantly reduce the cost of remote Alaskan interties, reducing the costs to interconnect remote villages and/or develop local energy resources.
Energy costs in rural Alaska are very high, and are making life in many villages unsustainable. The widely dispersed nature of villages and their small populations result in electrical power plants that have high capital costs and low efficiency. Diesel-electric plants that require expensive fuel are currently the best solution. Fuel transportation costs are extreme for many remote areas.
An economical means of transporting electrical energy from low to high cost locations can reduce overall energy costs in rural Alaska. The current costs for constructing conventional alternating current (AC) transmission systems are very high, ranging from $140,000 to over $300,000 per mile. Most rural interties are not cost effective at this price, and as a result only a few have been built.
Polarconsult Alaska, Inc. proposes a potential solution to this problem. The solution employs a novel systems approach and recent technological breakthroughs to develop a HVDC system specifically designed to use equipment and personnel commonly available in rural Alaska for construction and maintenance. Construction cost estimates for this HVDC system are estimated to be as low as one half the cost of an AC system.
The potential of HVDC for power transmission in rural Alaska has long been recognized, but the cost of the converters to change AC power to DC and back have always made HVDC systems too expensive. This new HVDC technology combines breakthroughs in converter technology and innovative transmission hardware to make HVDC economical.
In addition to its lower costs, this HVDC system also has several substantial technical advantages over AC transmission. Because HVDC is asynchronous, matching phase and frequency between existing village systems will not be an issue. HVDC can also use underwater and underground cables for long distances, which can not be easily done with conventional AC systems. DC cables are also less expensive than AC cables.
With this new system, it will become economical to develop local resources such as wind, hydropower, tidal power, geothermal, small gas fields, methane from coal, or coal fields to help power villages. Package nuclear plants or efficient large diesel plants can be built and used to power multiple villages using HVDC power interties.
A large portion or rural Alaska can directly benefit from this technology. A 1997 study for the Alaska Division of Energy identified 172 rural interties that would lower village electricity costs. With this HVDC system, many of these interties would be economical, and numerous villages could enjoy permanently lower electricity costs. Government programs that fund bulk fuel facilities, power plants, social programs, education, and the Power Cost Equalization program would all benefit from the lower rural energy costs made possibly by this project.
HVDC stands for High Voltage Direct Current. Direct current is the type of electrical current provided by a battery, such as that used for a flashlight or to operate the radio and headlights on a car.
The world's first electrical utility in the 19th century used direct current. This system was started by Thomas Edison in New York City and used buried cables to move direct current from his power plant to nearby customers. Edison could not boost the voltage of his system, so he had to use large cables to carry enough current to meet his customers' energy demand. Edison's system became limited by the expense of the massive cables needed to move power through the city. Alternating current (AC) technology, promoted by George Westinghouse, presented an elegant solution to the problem of boosting voltage, enabling Westinghouse to use smaller cables and more economically move power to customers. The AC system promoted by Westinghouse became the dominant electrical system that we use today.
By the mid 20th century, the technical challenge of changing DC voltages was solved, and high voltage direct current (HVDC) was introduced as a superior means of electrical transmission in certain applications. Because of the massive equipment needed to increase the DC voltage for transmission, early HVDC applications were limited to moving a lot of power over long distances.
Subsequent technological advances have further improved HVDC technology, enabling it to be a viable and superior option for ever shorter and smaller power transmission projects. Unfortunately, even today's commercially available HVDC technology is still unable to make HVDC an affordable alternative to AC transmission in places like rural Alaska.
Current HVDC Systems
There are many HVDC transmission systems in current use throughout the U.S. and worldwide. One well-known system is the HVDC line that transmits power from the Columbia River region in Washington and Oregon to southern California. There is also an HVDC system using undersea cables from Washington to Vancouver Island. Power from China's new Three Gorges Dam on the Yangtze River is transported to Shanghai with an HVDC transmission system. A new HVDC system has been proposed that would transmit 1500 MW from Washington to northern California via an offshore route.
HVDC transmission systems also exist under the North Sea in Europe, in South America, and elsewhere. HVDC is asynchronous (it has no frequency or phase), so HVDC transmission is often used to connect different power grids together. Texas utilizes HVDC connections between its domestic AC power grid and the AC power grids of adjacent states. Quebec Hydro used HVDC to connect with Canada's Maritime Provinces, and also to connect with the northeastern U.S. power grid.
HVDC Transmission Properties
HVDC's unique properties in optimizing transmission include:
Superior Electrical Properties
- Suitable for ground return
- No reactance losses, no need for voltage compensation
- No charging current problems
- Corona losses are diminished
- Less cable insulation is required at a given voltage
- Asynchronous connection of isolated AC systems simplifies intertie operation
- Asynchronous connection is more robust to system perturbations
Superior Mechanical Properties
- Fewer wires so structures and foundations can be lighter
- Cross arms are eliminated, and hardware is reduced
- No need to maintain phase-to-phase separations
- Higher voltage is used than for a typical rural AC system, so a smaller wire can be used.
- Fewer wires, stronger wire, and reduced possibility of conductor clashing allows longer spans
- Longer spans means fewer poles, reducing material costs, shipping costs, installation time, and environmental disturbance
- Lower cost and fewer foundations required
For HVDC systems that can use single wire ground return, and with the proposed high strength conductor, it is practical to use light, tall structures to span long distances. On straight sections of line, the proposed system can have spans of about 1,000 feet, similar to the spans along Turnagain Arm north of Girdwood. A single pole without cross arms can be used to support the conductor. The weight and lateral wind forces on the single conductor are reduced by a factor of up to three as compared to an AC system. Phase-to-phase clearance problems are eliminated, and phase-to-ground clearances are reduced. The number of insulators and hardware required is similarly reduced.
Because of the higher voltage, ground wire can be used for most of the conductors. Such ground wire is used to shield large transmission lines in lightning prone areas and is comprised of Alumoweld wire. This wire has been used with a fiber optic core for information transmission, and is extremely strong and more robust than the commonly used ACSR conductors.
The HVDC poles’ foundation will carry the relatively modest vertical loads and can be supported by small-diameter micro piles. Lateral loads such as from wind or angles in the line will be carried by guy wires. The guy wires are connected to screw anchors, grouted soil anchors, or freeze-back pin anchors depending on the soils.
For comparison, the equivalent conventional 75-foot tall AC pole installed in good soils would be an 85-foot long pole buried about 10 feet in the ground. This pole is very heavy, awkward and expensive to transport to rural Alaska. In the poor soils or permafrost common in rural Alaska, the conventional solution is to drive a steel pile 20 to 40 feet into the ground and bolt a wooden pole to the pile. This approach requires mobilizing a pile driver to the remote project site, and then moving the pile driver around in an area with poor soils. This is expensive, and can cause a significant disturbance to the local environment. Also, the connection between the pole and pile is typically the weakest point of the system, but is about where the most strength is needed.
Construction of a mile of straight HVDC transmission line would require only 6 small foundations, 18 guy anchors and connections, plus six insulators, stringing blocks, clamps and armor rod units. Only one conductor is pulled and set in place. The construction calls for climbing the poles or using powered ascenders only six times per mile to tie in the single conductor. All other work is done on the ground. Each pole assembly can be lifted by the small helicopters readily available in Alaska. Each pole assembly is sufficiently light to be erected from the ground using a small crew with a bipod and a small winch.
Land and Submarine Cable
HVDC is far superior for cable applications than AC. Because it is not constantly alternating voltage and current flow, HVDC does not induce rapidly changing fields and currents in the ground nor in the cable shields. This significantly reduces the high capacitive value associated with AC cables. Because of these capacitive values and reactive power losses, the length of AC cable systems is generally limited. HVDC cable lengths are not subject to these limitations.
Because of different voltage stress distributions and lower peak voltages, a DC cable requires less insulation than an AC cable operating at an equal voltage. As an example, a 35 kV AC cable operated at 20 kV to ground would use the same amount of insulation as a 60 kV DC cable operating at 60 kV to ground. This is one factor that will lower costs for DC cables compared to AC cables. The result is that one DC cable will carry the power of three AC cables of similar size.
These electrical advantages of DC cables also result in mechanical advantages. A DC submarine cable of given capacity will be smaller than an AC cable. This is because an AC cable will typically be a bundle of three cables, with thicker insulation, and also with more armor and shielding to protect the cable from physical damage. The lighter DC cable can be laid with smaller equipment, and will be easier to handle. Once in place on the seabed, the lighter DC cable is less likely to be damaged because it is lighter and can make tighter bends. If the cable needs to be repaired, the lighter DC cable is easier to raise to the surface than a heavier AC cable.
A converter is a device which changes alternating current (AC) electricity to direct current, or direct current to AC. Converters have been built around several different technologies over time. The earlier HVDC converters used mercury arc valves, and were massive systems that took up significant space, similar to a large electrical substation or switchyard. Modern converters generally use various solid-state switches to function. Most designs require extensive and expensive filter banks to remove electrical noise from the output. Unfiltered, this noise wastes power, and it can also harm sensitive appliances and electronics.
Solid state switching technologies used in converters include silicon controlled rectifiers (SCRs) and insulated gate bipolar transistors (IGBTs). Each technology has its merits and challenges, both in terms of function, reliability, and cost.
The major criteria needed for a converter to be viable for power transmission in rural Alaska are:
- Affordable life-cycle costs (combined capital, maintenance, repair and operations costs).
- Low power losses in power conversion (both AC-DC and DC-AC).
- Compact, robust hardware that can withstand shipping and conditions in rural Alaska.
- Relatively small power capacity, 1 MW and 5 MW units are envisioned.
- Highly automated operation, requiring minimal specialized operator knowledge for routine operations.
- Modular design, allowing for quick field replacement of failed components.
There is commercially available technology that meets some of the criteria listed above, but none can meet all of these criteria. The AC-LinkTM system developed by Princeton Power Systems (PPS) is the first commercially available technology that is both adaptable to HVDC service and capable of meeting all of the above criteria. PPS' AC-LinkTM system is already commercially deployed for a variety of applications, including in military marine propulsion systems, wind turbine power conversion applications, and others. The AC-LinkTM technology has not been commercially adapted for use as HVDC converters, but the technology is capable of functioning in this capacity.
A one MW AC-LinkTM converter is expected to be the size of two refrigerators. It is expected to have losses of about 1.5%. This efficiency and size is comparable to a conventional AC transformer of similar capacity. The solid state switching technology employed in an AC-LinkTM converter, silicon controlled rectifiers (SCR), is a low cost, robust, and proven technology. The converters would be built to allow SCRs to fail closed, so a single switch failure would not impair the function of the converter. The converters would be constructed in a rack configuration, allowing most components, such as the SCR modules and central capacitor to be quickly replaced in the event of failure. Critical components could be stocked onsite to improve reliability.
The AC-LinkTM converters would sense power demand on the HVDC line and automatically control power flow between villages. Power would be able to flow in either direction along the HVDC line.
All connected AC power systems are synchronized, meaning that they are coordinated so their alternating voltage and current fluctuate at the same phase and frequency. When separate AC systems are connected, they must be synchronized, which can be difficult to do.
Because HVDC does not have a frequency or phase, it is asynchronous. In rural Alaska, asynchronous connections between villages will significantly simplify power interconnections. On a simple two-village intertie, one village will generally be the generator and the other the consumer of electricity, and the asynchronous property of HVDC will not usually be employed. On the more complex multi-village grids that would likely radiate out from Bethel, Naknek, Dillingham, Kotzebue, and Galena, an asynchronous HVDC grid would have significant advantages as one or more power plants combine to meet the electrical demand of the grid.
Larger HVDC projects, such as the proposed Bradfield Connection in Southeast Alaska would have significant advantages over AC. Not only is an asynchronous HVDC intertie technically preferable to an AC intertie, but it may have significant regulatory advantages as well. FERC rules do not apply to HVDC transmission across state lines, but they do apply to AC transmission. By utilizing HVDC on the Bradfield Intertie, Alaska might avoid having all power systems in the state fall under FERC jurisdiction.
All electrical systems require a path for current to flow from the generator to a load, and a return path back to the generator. Without a return path, the electrical system will not function. Normally, the return path is a second wire, such as the white 'common' wire in a house. On three-phase AC transmission systems, the three phase-separated transmission wires can serve as return paths for each other. On single phase AC systems, there is usually a second wire for the return. Very few DC or HVDC systems also utilize a second ground wire for the return path. 'Ground Return' means that the earth is used for this return path instead of a second wire.
Most large HVDC systems are bipolar, meaning that the systems have two main wires energized to positive and negative voltages to complete the electrical circuit. Relative to ground, these two main wires will be energized at a voltage such as minus 60,000 volts and plus 60,000 volts. The positive and negative halves of the HVDC system both move ½ of the power of the total system. These large systems will sometimes have a third ground connection that is used only for emergency operations.
This two-conductor bipolar HVDC configuration is not practical or necessary for a small rural Alaskan HVDC system. A bipolar HVDC system requires two converters at each village (one plus and one minus), and also requires two high voltage wires. This would approximately double the cost and capacity of a rural intertie, eliminating most of the cost advantages of this proposed system and providing additional capacity that is not needed.
The proposed system is a monopolar HVDC line that will use ground return, often called Single Wire Ground Return (SWGR). Ground return is a technique where the second wire on a single phase AC or a DC system is eliminated and the ground, or earth, is used instead to complete the electrical circuit. In developed areas, SWGR is rarely used because it induces modest ground currents and voltages that can rapidly corrode some buried metals, such as utilities. Ground return is best suited to rural areas with limited or no buried metallic utilities. For the proposed HVDC system transferring 1 MW of power, the widely dispersed ground return current would be less than 17 amperes, about equal to that used by a hand held circular saw. SWGR is advantageous because it eliminates the cost and complication of a second wire on the transmission system. In rural Alaska, there are few buried utilities for a ground return system to corrode. The line between Bethel and Napakiak utilized a single phase AC ground return, and this ground return system has functioned very well. Australia, New Zealand, and Africa have successfully installed 1,000's of miles of single phase AC circuits with ground returns. These systems are used in rural areas with loads of up to about one MW.
The technical suitability of ground return will be individually evaluated for each intertie project, but the technique will lower costs for many of the potential rural interties in the state.
Evaluate the technical feasibility of the HVDC converter technology through a program of design, modeling, prototyping, and testing. Evaluate the technical and economic feasibility of the overall system and estimate the potential savings compared to an AC intertie.
The Commission previously funded Phase I of this project which included feasibility analysis of the proposed HVDC system and construction and testing of a prototype 250 kW 12.5 kV HVDC converter to confirm that the technology meets key performance benchmarks. Phase I was completed in 2009.
Complete full-scale prototyping, construction, and testing of the HVDC converters and transmission system hardware to finalize system designs, construction techniques, and construction costs.
Phase II of the HVDC Transmission System project includes design, fabrication and testing of fully functional prototypes of the converter system and transmission system elements. This effort will validate the design and functionality of these systems, and will also validate the efficiencies and feasibility necessary to make HVDC systems successful in remote Alaska intertie applications. The information gained in Phase II shall be used to further develop construction cost estimates and refine the economic analysis of the technology developed in Phase I.
Design, build, and operate a functional HVDC intertie to demonstrate that the technology can be successfully deployed in rural Alaska and achieve the expected functionality and cost savings.
Funding and Partnerships
The HVDC project is solely funded by the Denali Commission, and in 2009 was rolled into the Denali Commission EETG program, as the funding goal of the project closely resembled that of the EETG program; namely, the development of an emerging energy technology for deployment in Alaska focusing on the collection and analysis of project data, economic evaluation, and public dissemination of results.
Phase II of this project is currently being managed by the Alaska Center for Energy and Power (ACEP). Phase I of this project was managed by the Alaska Village Electric Cooperative (AVEC).
Stakeholder Advisory Group
The Stakeholder Advisory Group (SAG) is a professional advisory council designed to provide independent comments, feedback, review, and recommendations to the HVDC project. The SAG is to play a key role in this project, as there are many challenges to implementing HVDC technology in rural Alaska; the input and involvement of Alaska’s rural electric utility industry and related professionals is invaluable. Feedback from the SAG may also be used in project scoping. Some topics that will be put to the SAG include:
- Examination of current Alaska electrical codes. Given the potential of HVDC technology and the unique conditions in Alaska, do they merit modification? What information, testing, or demonstration would be needed to justify code modifications?
- What are the envisioned regional interties or local energy resources that lower-cost HVDC interties might enable? How much power would these HVDC interties move?
- What lessons do existing interties and utility experience offer for design, logistics, and construction of HVDC interties?
- What are potential sites for possible Phase III demonstration activities?
- Feedback on transmission system and component design and demonstration.
- Feedback on project scoping questions, such as appropriate size for converters.
The SAG is chaired by the Denali Commission, and consist of up to 30 members. The SAG will meet formally 3 times over the course of the project, tentatively scheduled for around 4/29/2010, 12/1/2010, and 7/15/2011, to discuss project issues and provide formal feedback to the project. These formal meetings will take place in Fairbanks, Anchorage, or at another convenient location, and also be teleconferenced. Outside of these formal meetings, the SAG membership will receive periodic communications, project updates, questions, or requests for feedback via an electronic mailing list.
The following organizations have been invited to participate:
Alaska Energy Authority, Central Council of Tlingit/Haida Tribes of Alaska, Alaska Village Electric Cooperative, Inside Passage Electric Cooperative, Alaska Power and Telephone, North Slope Borough, Naknek Electric Association, Copper Valley Electric Association, Homer Electric Association, USDA-RUS, Nome Joint Utilities, Kodiak Electric Association, Kotzebue Electric Association, Matanuska Electric Association, Golden Valley Electric Association, Nome Chamber Of Commerce, Bering Straits Native Corp, Bethel Electric Utility, Alaska Department of Labor, Alaska Power Association, Institute of Northern Engineering (UAF), Cooperative Research Network (NRECA), and the Northwest Arctic Borough.
Further organizations may be invited, per the recommendation of the SAG or invitation from the Denali Commission.
The Alaska Village Electric Cooperative (AVEC) was the project manager for Phase 1 of the project, and continues to serve in a technical advisory role. The National Renewable Energy Laboratory (NREL) serves on the Energy Advisory Committee for the Denali Commission, and provides technical review for this project. The Institute of Social and Economic Research (ISER) is providing economic data collection, analysis, and review for project reporting. Princeton Power Systems is a subcontractor to Polarconsult, and is the primary developer of the HVDC converter technology.
Project Links, Resources, and Documents
SAG Meeting Material
- SAG Policies
- SAG Synopsis
- SAG Meeting #1 Agenda
- SAG Meeting #1 Presentation
- SAG Meeting #1 Transcript
- SAG Meeting #2 Agenda
- SAG Meeting #2 Presentation
- SAG Meeting #2 Site Selection White Paper
- SAG Meeting #3 Agenda
- SAG Meeting #3 Presentation 1
- SAG Meeting #3 Presentation 2
- SAG Meeting #3 Transcript
ACEP Review, Analysis, and Comments
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Princeton Power Systems Site Visit, May 2010
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