A
CKNOWLEDGEMENTS
This proceedings document was compiled, written, and edited by industry, national laboratory, Federal government, and university workshop participants:
Advanced Energy Analysis William Hassenzahl
American Superconductor
Bruce Gamble (facilitator and lead author for Chapter 3), Steve Fleshler, Jack McCall, Narend Reddy (facilitator and lead author for Chapter 2), and John Ulliman
Electric Power Research Institute Steve Eckroad
Energetics Incorporated
Brian Marchionini and Tenley Dalstrom Oak Ridge National Laboratory
Jonathan Demko (facilitator and lead author for Chapter 5) and Mike Gouge University of Houston, Texas Center for Superconductivity
Venkat Selvamanickam (facilitator and lead author for Chapter 4) and Sue Butler U.S. Department of Energy
Debbie Haught SuperPower, Inc. Drew Hazelton
The participants listed in Appendix B provided technical content for the workshop. This workshop was co-sponsored by the Department of Energy; Los Alamos National
Laboratory; Oak Ridge National Laboratory; Electric Power Research Institute (EPRI); Texas Center for Superconductivity at the University of Houston (TcSUH); Coalition for the
Commercial Application of Superconductors (CCAS); American Superconductor Corporation; and SuperPower, Inc.
E
XECUTIVE
S
UMMARY
On January 21 and 22, 2010 more than 60 representatives of the electric utility industry, grid operators, equipment manufacturers, Federal government agencies, interest groups, universities, and National Laboratories attended the Workshop on Superconducting Direct Current Electricity Transmission to discuss superconducting direct current (dc) electricity transmission. The
workshop sought to:
• Assess the readiness for deployment of superconducting dc cables
• Identify and elaborate on key barriers
• Lay the groundwork for further exploration of research and development activities Superconducting dc cable systems are inherently suited to long-distance, high-power, bulk energy transfer. Superconducting dc cable offers many advantages when compared to
conventional high-voltage alternating current (ac) and conventional dc systems, including lower losses, greater reliability and security, a smaller right-of-way, fewer siting restrictions, the ability to interconnect to the grid at multiple locations, and the ability to be terminated at distribution voltages in or near load centers. Superconducting dc cables are capable of transporting large amounts of electric energy long distances and among the three U.S. Interconnections, while simultaneously reducing the transmission system’s carbon footprint. A recently published report by the Electric Power Research Institute conceives of a superconducting dc cable system capable of moving 10 gigawatts or more of electricity between regions, judged to be practical and ready for the next steps in commercial development using today’s technology.1
As a starting philosophy for the workshop, two intermediate goals on the path to a grid interconnected by superconducting dc cables were established prior to the workshop:
1. A near-term (5 years) goal of constructing a short (<150 kilometers) high power (up to 5 GW) superconductor dc cable.
2. A longer-term (15 years) goal of installing multiple hundreds, to several thousands, of kilometers of superconductor dc cable (10+ GW) to interconnect the nation’s grid and help bring renewable energy to our cities.
Major Findings from the Workshop
• The near-term goal is technically achievable within lower power limits by leveraging existing ac superconductor technology and commercially available, multi-terminal voltage-source converters, but additional research and development may be required.
• The longer-term goal is also achievable, but will require significant design optimization and equipment development.
• Research efforts to date on dc superconducting cables have not gone beyond cable currents of 20 kA. High power (GW-level) systems that employ higher-current cables
(compared to higher voltage) will require additional studies on current distribution among many layers of high temperature superconducting (HTS) wire and the integration of high current and high voltages in cable splices and terminations.
• The viability of commercial superconductor dc cables greatly depends on HTS and cryogenic dielectric material cost and performance. Key attributes for these materials include critical current, annual production volume, piece length, price, joints,
stabilization, and mechanical integrity of the HTS material and insulation high voltage withstand, and aging (lifetime) performance. Today’s HTS wire meets the short dc cable mechanical properties and stabilizer performance requirements. However, production volume, critical current performance, and price need to be improved.
• Achieving both near- and long-term goals demands investigating the most practical and cost effective converter/cable topologies to ensure optimal system characteristics while minimizing cable manufacturing complexity, new related product development, and installation costs.
• Cryostat costs must be reduced while ensuring its reliability. The cost is associated with the choice of materials used and the manufacturing processes. Reliability issues are concerned with maintaining consistent thermal performance of the cryostat, containment of the heat transfer fluid, and mechanical protection of the HTS dc cable over the life of the cable.
• Cryogenic, vacuum, and refrigeration systems capable of meeting the capacity
requirements of long dc cables are available today. However, the necessary reliability can only be achieved by installing redundant components. A trade study of various thermal insulation and cryogenic systems is needed to determine directions for future
development.
• A series of prototype cable systems of various lengths (10s, 100s, and 1000s meters) and of increasing levels of dc current and dc voltage need to be built and tested on an
aggressive schedule. Longer (km) cable lengths will allow for testing and evaluation of cryogenic cooling, pumping, and thermal insulation schemes. The qualification of the cable and ancillaries requires development of dedicated test facilities with the required voltage and current ratings, and refrigeration equipment.
• Reliability must be addressed for all system components as the design evolves. Failure modes and effects analysis are required to identify single mode and undetectable failure paths to enable mitigation by design, redundancy, or operational constraints.
• Several trade studies were determined to be critical to the selection of cable design parameters. These include: 1) comparing life-cycle cost and overall performance for different combinations of current and voltage; 2) comparing long term performance and costs of various thermal insulations schemes; 3) comparing various forms of electrical insulation; and 4) exploring cost and performance of operating temperature and the use of different coolants.
A
CRONYMS
µm Micron
2G Second Generation
A Ampere
ac Alternating Current
AMSC American Superconductor Corporation
Atm Atmosphere
BSCCO Bismuth strontium calcium copper oxide CAPEX Capital Expenditure
CCAS Coalition for the Commercial Application of Superconductors CSC Current Source Converter
CTE Coefficient of Thermal Expansion dc Direct Current
DHS Department of Homeland Security DOE Department of Energy
EE Energy Efficiency
EHV Extra High Voltage
EMTP Electro Magneti
EPRI Electric Power Research Institute ERCOT Electric Reliability Council of Texas
FSU CAPS Florida State University Center for Advanced Power Systems GW Gigawatt
HTS High Temperature Superconductivity HVDC High Voltage Direct Current
Ic Critical Current
IGBT Insulated Gate Bipolar Transistor ISO Independent Service Operator
ITER International Thermonuclear Experimental Reactor J c Critical Current Density
K Kelvin
kA Kiloampere
kA-m Kiloampere-meter
km Kilometer
kV Kilovolt
LANL Los Alamos National Laboratory LHC Large Hadron Collider
LIPA Long Island Power Authority LN2 Liquid Nitrogen
mm Millimeter
MLI Multi Layer Insulation MPa Megapascals
MW Megawatt
NASA National Aeronautics and Space Administration NIST National Institute of Standards and Technology
NREL National Renewable Energy Laboratory
OE Office of Electricity Delivery and Energy Reliability ORNL Oak Ridge National Laboratory
PET Polyethylene-Terephthalate PPLP Polypropylene Laminated Paper
PSLF Power System Load Flow (computer model)
PSSE Power System Simulation for Engineers (computer model) PWM Pulse Width Modulation
R&D Research and Development SF6 Sulfur Hexafluoride
SMES Superconducting Magnetic Energy Storage
SS Solid State
SSTI Sub Synchronous Torsional Interactions
SP SuperPower, Inc.
TcSUH Texas Center for Superconductivity at the University of Houston
V Volts
VSC Voltage Source Converter
W Watts
W/W Watts of Electric Power Input Per Watt of Refrigeration Produced XLPE Cross-linked polyethylene (solid extruded dielectric)
Table of Contents
Acknowledgements ... i
Executive Summary ... ii
Acronyms ... iv
Chapter 1. Introduction ... 1
Chapter 2. AC System Integration ... 5
Chapter 3. Cable Subsystem ... 19
Chapter 4. HTS Materials and Insulation ... 32
Chapter 5. Cryogenics, Vacuum, and Thermal Insulation ... 47 Appendix A. Workshop Agenda ... A-1 Appendix B. List of Participants ... B-1 Appendix C. Key Studies on HTS DC Cables ... C-1 Appendix D. Disclaimer ... D-1
C
HAPTER
1.
I
NTRODUCTION
The Workshop on Superconducting Direct Current Electricity Transmission, held January 21–22, 2010 in Houston, TX, was co-sponsored by the U.S. Department of Energy (DOE); Los Alamos National Laboratory (LANL); Oak Ridge National Laboratory (ORNL); Electric Power Research Institute (EPRI); Texas Center for Superconductivity at the University of Houston (TcSUH); Coalition for the Commercial Application of Superconductors (CCAS); American
Superconductor Corporation; and SuperPower, Inc. The workshop was designed to:
• Assess the readiness for deployment of superconducting dc cables
• Identify and elaborate on key barriers
• Lay the groundwork for further exploration of research and development activities More than 60 experts were in attendance (see Appendix B for the list of participants) to discuss superconducting direct current electricity transmission. The opening plenary session included a series of presentations to set the stage for the workshop, including presentations on:
• DC Superconducting Cable Opportunities and Future, Jack McCall, AMSC
• DC Superconducting Cable System Overview, Steven Eckroad, EPRI
• Power System Integration, Availability, Dale Osborne, Midwest ISO The subsequent chapters in this
proceedings document provide a summary of four parallel breakout group discussions on the
following topics: AC System Integration (Chapter 2), including converter, dc cable interactions, disconnects, dc breakers, fault
current behavior and management, transients on cable, converter topology, converter control; Cable Design (Chapter 3), including insulation type, quench conductors, shielding, splices, terminations, and manufacturing and factory testing; Superconducting Materials & Electrical Insulation (Chapter 4); and Cryogenics, Thermal Insulation, & Vacuum Systems (Chapter 5). Presentations from the plenary session and the summary presentations from each of the breakout groups are available for download at: Electric Power Research Institute has published several reports on the topic of superconducting dc transmission. These reports are available free to the public and can be downloaded at the links found in Appendix C. These reports provided a baseline for the workshop discussion.
Draft 5- and 15-Year Vision for Superconducting DC Electricity Transmission
A draft 5- and 15-year vision (Figure 1.1), developed prior to the workshop and discussed by the four breakout groups, developed a process to transform the existing power grid to one that has continent-wide interconnections based on superconducting dc cables. The vision presents two intermediate stages which will lead to an enhanced, unobtrusive, U.S. electrical grid with a reduced carbon footprint, providing renewable, reliable, and resilient power to the nation. These first steps require the superconductor and high voltage direct current communities to develop the necessary technologies required by a superconducting dc electricity transmission system and for the utility industry and regulatory entities to deploy those technologies.Background of HVDC Transmission
Direct current power transmission technology has been successfully integrated into transmission grids for decades, primarily on a point-to-point delivery basis where power must be transferred very long distances (>600 kilometers). The dc converter stations primarily utilize line
commutated current source technology employing mercury arc valves and, more recently,
thyristors. In the past, dc transmission was typically utilized at voltages between 250 and 600 kV pole to ground voltages, often referred to as high voltage dc (HVDC).
The more recently developed voltage source converter (VSC) technology cost effectively achieves voltages of about 200 kV, with at least one manufacturer having voltage source
converters available with ratings of up to 320 kV and 1850 A. These rating limits are forecast to increase overtime. Some HVDC converter stations employ VSC technology and operate at voltages as high as 200 kV, with the voltage level of dc cables that have been utilized in
conjunction with VSCs being the primary limitation. VSCs offer significant advantages over the line commutated technology, including flexibility of control, ability to have multiple terminals (i.e. not restricted to point-to-point configuration only), and ability to provide reactive support to the ac system with the installed converters.
With about a dozen installations already in operation and a decade of operating experience, this technology can be considered to be ready for large-scale deployment to compliment the
development needs of the next generation of the transmission grid. Table 1.1 (courtesy American Superconductor) lists all VSC-based dc cable transmission systems planned or operating around the world; all but one (as noted) utilize underground or submarine cables of conventional design.
TABLE 1.1.LIST OF VSC-BASED DCCABLE TRANSMISSION SYSTEMS
2
Overhead transmission
3
Siemens HVDC Plus Installation—all others are ABB HVDC Light
Name Location Voltage
(kV) Transmission (km) Power Rating (MW) Year Installed/Planned Gotland Sweden 80 70 50 1999 Directlink Australia 84 65 180 2000 Tjaereborg Denmark 10 4.5 8 2000 Murraylink Australia 150 180 220 2002
Cross Sound United States 150 40 330 2002
Troll A Norway 80 68 80 2004
Estlink Estonia/Finland 150 105 350 2006
BorWin1 Offshore Germany 150 203 400 2009
Caprivi Link
Interconnector Namibia 350 970 300 20092
Valhall Norway 150 292 78 2010
Trans Bay San Francisco 200 55 400 20103
East West
Adaptation of Today’s HVDC Technology for Superconducting DC Cables
Superconducting dc cable systems will utilize and adapt conventional HVDC converter technology. However, an important distinction must be made between commercial HVDC systems (whether overhead or underground) and the emergent superconducting dc cablesystems—one that will require a change in existing trends in design philosophy and commercial practice for converters.
To keep the current (and hence the losses) low, both ac and traditional dc power transmission systems increase voltage to achieve higher power levels. Similarly, the design development and commercial practice in HVDC technology today are to go to higher and higher voltages in order to achieve greater power throughput. Superconducting dc transmission allows for inherently higher currents rather than higher voltages, achieving with low losses very high power levels at ultimately sub-transmission and distribution voltages—the “end-use” voltages inside cities and suburban areas—with obvious advantages.
Thus, long-term commercialization of the superconducting dc cable requires a different
philosophy in converter design, one focused more on higher currents than higher voltages. In the near term, driven as it is today by the need for higher voltages, the converter marketplace will offer solutions that result in higher voltage than the long term optimization will likely require.
C
HAPTER
2.
AC
S
YSTEM
I
NTEGRATION
The integration of ac systems is an important consideration for the design of superconducting dc transmission cables. System integration considerations span several components, including ac to dc and dc to ac converters; converter topologies, control systems, and their impacts on cable design and interactions; dc switchgear, including breakers and disconnects; the impact of fault currents on system performance and design; fault management; transients in the cable; and impact on overall power system operations and stability.
Major dc system components are generally available and ready to be deployed to meet the short and long-term vision. For instance, voltage source converter (VSC) technology has been in operation since 1999 and is considered to be mature enough for full-scale deployment.
Furthermore, a VSC-based HVDC conversion is a very versatile and flexible approach to achieve the key attributes of the superconductor based HVDC transmission system compared to the conventional current source technology. Motorized disconnects are available for EHV
applications in the ac system and disconnects for high voltage dc transmission have already been deployed. Extensive system studies and analysis have been undertaken to determine the expected performance and impact of deploying large scale HVDC transmission system.
Adequately rated dc circuit breakers to interrupt kA level currents are not available
commercially. This should not inhibit the development of short-length near-term projects, but may need to be addressed for the deployment of long (100s of kilometers) lines depending on the converter and cable topology chosen. The development of dc breakers becomes more important at higher current and power levels for the longer transmission systems.
The lack of interoperability of equipment among different manufacturers is another potential barrier. Different manufacturers employ different approaches—multi-level, pulse width
modulation (PWM), combined PWM/multilevel—to achieve the conversion requirements with proprietary control as needed. As the dc transmission spans hundreds of kilometers, the number of components on the system will increase resulting in several different manufacturers’ products being part of the same system.
B arriers to Achieving the Vis ion
• Lack of interoperability between equipment from different manufacturers
• Lack of dc circuit breaker technology with appropriate steady state current ratings that may preclude certain network configurations
• Lack of suitable simulation models for widespread use and education • Lack of actual project(s) where these dc lines could be deployed
• Lack of definition of system protection schemes where identification of fault locations on dc network is simpler
Several needs must be addressed to achieve the vision. A top priority need is to investigate optimal converter and the dc cable topologies that are applicable for high power and longer distance transmission. The action plan for addressing this need is to (1) select a team of converter and cable experts; (2) define practical system ratings, constraints, operational practices and redundancy requirements for the recommended design; and (3) identify the most desirable converter and cable system topologies for the project. This is a short-term need that should occur within the next 5 years which needs to be led by industry, with support from DOE.
Investigation of converter and cable topology could be followed by a live grid point-to-point project. The cable should be designed so that it complies with the specific system requirements for the region in which it will be installed. It is likely that as the dc transmission system develops the cable is designed and installed in a looped configuration instead of point to point, in which case it can be run open in some sections. This arrangement may be necessary to avoid potential control and stability problems that could arise in a closed loop system with near zero resistance cables.
Another top priority need identified was the development of stability and transient analysis models for both superconductor cables and VSC converters. The action plan is to (1) develop PSSE, PSLF, and short circuit models that are made available to the industry for general application studies; (2) develop an EMTDC (PSCAD) model for VSC converters that is able to simulate multi-terminal operation of the system; and (3) develop an EMTDC model of different HTS cable topologies for optimization studies and complete system design requirements. This is a short-term need that should occur immediately and should be led by EPRI/DOE with support from industry.
Another top priority need is to develop high current rated dc circuit breakers. This effort could extend to dc disconnects and cable charging equipment, if such equipment is identified as needed. The action plan for addressing this need includes (1) selecting a team of appropriate industry experts; (2) develop equipment specification; and (3) assess need and timing for development. These activities should occur immediately and needs to be led by industry with support from DOE. After the needs assessment it is necessary to initiate development programs (for different components), which will be led by DOE.
Top R &D Needs for AC S ys tem Integration • Definition of optimum superconductor cable and converter topologies • DC circuit breakers rated up to 25 kA steady state operation
• Stability and transient multi-terminal simulation models for cables and converters • DC network topologies that are practical
• Ability to address high levels of fault currents within the dc network
• Use of standard industrial power system practices for this transmission system • Educational outreach for Industry
TABLE 2.1. AC SYSTEM INTEGRATION BREAKOUT GROUP PARTICIPANTS
Name Organization
Tom Baldwin FSU, Center for Advanced Power Systems Michael Bahrman ABB
John Chan EPRI
Jolly Hayden NextEra Energy Resources Dominic Lee Oak Ridge National Laboratory Sara Mahmood Department of Homeland Security Brian Marchionini (scribe) Energetics Incorporated
Jack McCall American Superconductor Larry Neal CenterPoint Energy Stig Nilsson Exponent
Dale Osborn Midwest ISO Wan Ki Pak LS Cable
Brett Pearlman Vector Consultants
Narend Reddy (facilitator) American Superconductor John Schmall ERCOT
Richard Thome General Atomics
Status of the Technology Today
2.1 HVDC Equipment
Major pieces of equipment in this sub-system include:
• Voltage source converter (or current source if needed for one way power transfer) based terminal stations which typically include power transformer(s), ac harmonic filters, ac phase reactors, dc capacitors and IGBT based converters
• DC substation equipment
• DC harmonic filters, where necessary
• DC isolation and protection equipment including breakers, motorized disconnects, ground switches, and surge arrestors
The following graphic is a simplified one line diagram of a VSC based dc terminal station showing the various components required to connect to the ac grid. In actual practice, many more disconnects and surge arrestors would be utilized on both the ac as well as the dc side of the substation.
Function
The key function of this subsystem is to allow:
• Connection of the dc grid at a particular node to the ac transmission grid
• Transfer of power from ac to dc and dc to ac system as needed, at any instance, at each terminal, independently
• Operation of the system as designed under both normal and contingency conditions taking into consideration safety of personnel and equipment
• Regulation and post fault voltage recovery of the ac system utilizing the reactive power injection capability of the converter stations
Status
Voltage source converter based HVDC technology has been in operation since 1999, primarily in point-to-point transmission applications with power ratings on the order of tens of MW to
hundreds of MW (400 MW). A few projects are already in line to begin commercial operation at the 500 MW level. It is expected that deployment of this technology for multi-terminal
applications will not require any major development work on the converter systems themselves. However, a substantial amount of focus would be needed to develop appropriate high steady state current rated dc breakers and other protection detection and mitigation techniques to
facilitate flexible design and operation configurations similar to those applied in the ac grid. Due to lack of applications requiring such rated equipment so far, these types of breakers and
protection equipment are currently not available commercially. 2.1.1 Voltage Source Converters (and Topology)
Function
This equipment converts power from ac to dc (converters) for transmission through dc cables and vice versa (inverters). Different manufacturers employ different design approaches (multi-level,
PWM, combined PWM/multilevel) to achieve the conversion requirements with proprietary control as needed. While it is imperative that innovation in this technology will assist in large scale deployment with reduced costs and operational benefits, the present technology is more than adequate to meet the short term and long term vision of this program outlined earlier.
Status
Voltage source converter based dc terminals are considered to be very versatile and flexible compared to the conventional current source technology to achieve some of the key attributes of the superconducting dc transmission system. VSC technology has been in operation since 1999 and should be considered to be mature enough for large scale deployment. There are a total of 13 installations in operation with ABB being the largest supplier with twelve systems and Siemens with one). Presently, AREVA does not have any installations at the transmission level; however, they are actively involved in the process of development and testing of VSC valves for this current and voltage levels. It is anticipated that they would be in a position to offer a product for such applications sometime towards the end of 2010.
2.1.2 DC Network Topology
Function
Configuration of the dc cable transmission system that allows the scheduled power transfer capabilities with required levels of redundancy and reliability per industry standard practices.
Status
The most practical and cost effective cable topologies for an identified project will need to be investigated once a project is selected. Depending on the current ratings and availability of dc circuit breakers, there are various options available for implementing long transmission lines. This includes installation of spare pole cable and converter station(s) as well as innovative switchover schemes that allow for adequate transfer of power to healthy parts and installed spare capacity, once a faulted element within the system is identified.
2.1.3 DC Substation Design
Function
Configuration of the dc substation design that lends itself to meeting the objective of integrating the most appropriate dc superconducting cable and overall network topologies to ensure that the short term and long term vision is achieved.
Status
The optimum design of the dc substation presently does not exist and will need to be investigated along with the various high level dc network topologies as well as superconducting cable
topologies.
2.1.4 Fault Detection and Mitigation
Function
• Detect occurrence of any type of faults in the dc transmission system including superconducting cables and converter station
• Make decisions to isolate the faulted component without adversely affecting the operation of the healthy parts of the system
• Clear a fault on one pole while keeping the other pole in operation and undertaking appropriate remedial measures that ensures continuity of supply, for example such as automatically reducing power flow on the system if necessary
This fault detection system would need to take into consideration the extremely low impedance of the superconducting cable sections.
Status
Protection schemes have been developed and deployed for existing VSC based HVDC systems. However, these have been for point-to-point systems where the primary approach has been to de-energize the complete dc system through operation of breakers on the ac connections. For multi-terminal dc transmission systems, advanced dc fault detection systems will be needed to ensure required levels of reliability and availability.
2.1.5 DC Circuit Breakers
Function
To provide a means of breaking the cable circuit while energized allowing isolation of cable sections during fault conditions. The need for dc circuit breakers depends on the cable topology adoption and may not be needed for point to point and other system topologies as chosen for an application.
Status
• High voltage dc circuit breakers suitable for transmission system level applications are presently not commercially available.
• ABB is in the process of developing a solid state breaker with steady state current capability in the range of 2 to 2.5 kA. This product is being designed to match the rating of their highest converter current rating. With lack of market definition and appropriate funding, coupled with increased risks associated with higher current levels, there are no plans currently to develop higher current rated breakers. DC breaker programs with the other major suppliers (Siemens and AREVA) are not known.
• A dc breaker employing puffer type SF6 gas interrupter rated for +250 kV and 8 kA was developed and tested by Hitachi Corporation.4
• The use of explosive assisted fuse as one alternative could be explored further.
This development and test work was not followed through and based on feedback from Hitachi; this program is not active anymore.
• DC circuit breakers are currently available for low current and low voltage applications only.
4
Development and Interrupting Tests on 250 kV 8 kA HVDC Circuit Breaker, Tokuyama et al. IEEE Transactions on Power Apparatus and Systems, Vol. PAS-104, No. 9, September 1985
2.1.6 Motorized DC Disconnects
Function
To isolate dc cable sections for maintenance purposes and system reconfiguration requirements under both energized or de-energized states. Steady state current ratings for converter level applications are available. However, the high current rated disconnects needed for dc superconducting cables may need to be developed.
Status
Motorized disconnects are available for ac system applications up to EHV levels. Disconnects for high voltage dc transmission applications are deployed at conventional dc installations. 2.1.7 Surge Arrestors
Function
To provide means to limit transient voltage surges as a result of such events as lightning or switching activities.
Status
Surge arrestors are very common ac substation equipment. It needs to be investigated whether the current design of surge arrestors can be applied for very long superconducting dc
transmission lines, with the amount of energy discharge required. 2.1.8 DC Charging Equipment
Function
To provide a controlled means of energizing long sections of superconducting dc cables during system start-up.
Status
While the VSC converter controls may have the ability to provide this functionality, more studies are required to identify if any special charging equipment is needed and what design
characteristics are needed.
2.2 Controls and Interfacing
Function
Synchronize operation of individual converters system power transfer and dispatch requirements.
Status
Sophisticated controls are already available for individual station operation in either converter or inverter mode to facilitate flow of power flow between sending and receiving end stations for point-to-point systems. Multi-terminal dc system requires a coordinated control between multiple terminals on a common transmission line and these types of controls are not presently available.
2.2.1 Voltage Source Converter Based HVDC Controls
Function
VSC based HVDC controls regulate the flow of power between the superconducting dc cable system and the ac grid in a bidirectional mode. The controls would also automatically shut down or limit power transfers at particular converter stations to address contingency situations as a protection measure. Since the VSC converters are primarily a STATCOM, they also provide ac grid voltage support function through available reactive power capability during periods of system instability. The controls also provide power oscillation damping functionality as needed.
Status
Primary converter control functions for operation both as a rectifier and as an inverter are already available, primarily to meet point-to-point power transfer requirements. More flexible and
modularized control options are needed to facilitate multi-terminal operation of the
superconducting dc transmission system. The VSC based HVDC controls provide fast rerouting of power during a disturbance event which results in reduced available capacity of the dc system. 2.2.2 Power Dispatch and Scheduling
Function
To facilitate synchronization and operation with the ac transmission system power dispatch and scheduling requirements.
Status
Sophisticated controls are already available for the ac grid operation. Power dispatch and scheduling could be integrated with the ac grid EMS where possible.
2.3 System Studies and Impact Analysis
Function
Detailed system analysis has to be undertaken to determine:
• The impact of deployment of dc superconducting cables on the ac transmission system
• Appropriate dc network configuration (closed loop, open loop, etc.)
• Appropriate reliability and redundancy requirements (feeds back into minimum cable design requirements) to meet national standards
• Need for development of appropriate standards and codes specifically for superconducting dc transmission systems
• Components of superconductor cable and VSC based dc systems that need further continued development
Status
Extensive research work at the conceptual level has been done through EPRI sponsored studies on the subject of impact of high capacity dc superconducting transmission lines in the U.S. Electric Grid. Some of these studies include:
• EPRI 1020330—Study on the Integration of High Temperature Superconducting DC Cables within the Eastern and Western North American Power Grids—November 2009
• EPRI 1020339—Transient Response of a Superconducting DC Long Length Cable System Using Voltage Source Converters—December 2009
• Various regional studies
However, moving forward, specific studies for identified projects will need to be undertaken as the structure of superconducting dc transmission system develops.
2.3.1 System Stability Studies
Function
Undertake detailed simulations to determine:
• Impact of connection of high power capacity dc cable system on ac grid stability
• Maximum power transfer capabilities between regions and interconnections
• Optimum, VSC converter locations and capacities
• AC system upgrades required for interconnection
• Sub-synchronous torsional interactions with ac system components
Status
Work already undertaken includes:
• EPRI 1020330 - Study on the Integration of High Temperature Superconducting DC Cables within the Eastern and Western North American Power Grids—November 2009
• EPRI 1020339 - Transient Response of a Superconducting DC Long Length Cable System Using Voltage Source Converters—December 2009
• Eastern Wind Integration and Transmission Study—January 2010
• 80% renewable Energy 2050—DOE, EE, NREL—May 2010
So far the studies have shown that the large scale deployment of high capacity superconductor cable based HVDC systems will not cause any major stability related issues on the ac system. 2.3.2 Undertake Fault Analysis and Determine Transients in the Cable
Function
Study the impact of the following on Converter station behavior and design, dc system transients:
• AC system balanced and unbalanced faults
• DC system faults: Pole to pole fault, pole to ground faults Study the impact of transients in superconducting dc cable due to:
• Converter normal operation and malfunction
• System switching functions, etc
Study the impact of different system fault currents levels and X/R ratio on
• VSC control and stability requirements
• VSC topology and design requirements
Based on the results of the analysis provide input to the cable design requirements. It is noted that the dc system energy will not feed into ac side fault currents provided that the converter valves are blocked when the ac side fault is applied.
Status
Work already undertaken includes:
• EPRI 1020330—Study on the Integration of High Temperature Superconducting DC Cables within the Eastern and Western North American Power Grids—November 2009
• EPRI 1020339—Transient Response of a Superconducting DC Long Length Cable System Using Voltage Source Converters—December 2009
2.3.3 Simulation models
Function
To provide with reasonable accuracies, the predictive behavior of key components of the dc transmission system and to facilitate the development of required technologies. Development of EMTP and other dynamic simulation (PSSE and PSLF) models for:
• VSC converter stations
• Superconducting dc cable
• Integrated control systems
• Protection systems
• Solid state dc breakers These activities will allow:
• Detailed studies to be undertaken to study impact on ac grid
• Design and/or size dc system components (cables, breakers, disconnects)
• Determine protection system characteristics
• New and innovative protection systems to be developed
• Undertake appropriate reliability and other analysis as required, etc.
Status
Abstract (simplified) models of cables and converter stations have been developed and used for above mentioned studies. More detailed models are needed for the different cable designs and multi-terminal converter stations.
TABLE 2.2.BARRIERS TO ACHIEVING THE VISION Short Term
(0–5 years)
Long Term (5–15 years) HVDC Equipment and Controls
Voltage Source Converters and Topology
• No major barriers identified
• Converter technology can be matched to cable design
• Current limited but can be configured to provide required power (GW)
• Higher current ratings
• Expandability, reliability
• Interoperability between equipment from multiple suppliers
DC Network Topology • Availability of a project provides a significant barrier
• Optimum re-dispatch of power for a backbone system for a contingency situation
• The right fit in current ac system
• Meshed system may not be optimum—need to investigate the current distribution in a long dc transmission system through system studies
DC Substation Design • Clear definition of equipment ratings
• Design needs to match station reliability
• Clear definition of equipment ratings
• (Bus design, apparatus, etc.)
• Design needs to match station reliability
Fault Detection and Mitigation
• No major barriers identified
• Fault location identification
• Monitoring of cable performance
• Contingency plans
• No major barriers identified
• Fault location
• Monitoring of cable performance
• Contingency plans DC Circuit Breakers • Major barrier is availability of dc
breakers
• Need is based on topology and power ratings
• Ability to handle peak fault currents
• Use of conventional breakers/equipment can be considered
• Coordination of energy
dissipation capability with cable design and operating schemes is required
• Major barrier is availability
• Higher ratings (up to 25kA) required, to protect cable
• Coordinate energy dissipation capability with cable design and operating schemes
Motorized DC Disconnects
• No major barriers identified
• Current and voltage ratings need to match load currents and short circuit currents
• Current and voltage ratings need to match load currents and short circuit currents
Surge Arrestors • No major barriers identified
• Energy dissipation, insulation coordination requirement
• No major barriers identified
• Energy dissipation, insulation coordination requirements DC Cable Charging
Equipment
• Lack of knowledge on dc superconducting cable charging needs
• Need to study cable charging equipment needs
• Charging needs for longer length cables need to be identified
Short Term (0–5 years)
Long Term (5–15 years) Controls and Interfacing
Voltage Source Converter Based HVDC Controls
• No major barriers identified but issues such as the following remain:
o Stable controls, transients
o Impact of ac system unbalance conditions—2nd harmonic mitigation due to high levels ac system unbalance
• Practical system topology that allows easy isolation and control
• Interoperability between different manufacturers
System Studies and Impact Analysis System Stability
Studies
• No major barriers identified, however, studies need to consider the following issues:
o Multi in-feed and multi out-feed and operation during faults
o Cable response during energization
• No major barriers identified, however, studies need to consider the following issues:
o Cable energization—use of CSC converters
o Maintaining required charge in cables
Faults Analysis and Transients in the Cable
• No major barriers identified, however, studies need to consider the following issues:
o System unbalance and generation of 2nd harmonic
o Impact on cable design
• No major barriers identified, however studies need to consider the following issues:
o System unbalance and generation of 2nd harmonic
o Impact on cable design Simulation Models • Availability of suitable models
• Education of Engineers/industry need to be considered in order to spur adoption
• Proprietary nature of equipment models that need to be shared
• No major barriers identified
• Impact of different converter suppliers and interaction.
• Standardization of controls
TABLE 2.3.TECHNOLOGY NEEDS TO OVERCOME THE BARRIERS (NUMBERS IN PARENTHESIS INDICATE THE PRIORITY OF THE NEED)
Short Term (0–5 years)
Long Term (5–15 years) HVDC Equipment and Controls
Voltage Source Converters and Topology
• No needs identified • No needs identified DC Network Topology • Investigate optimal converter/cable
topology (#1)
• Identify recommended practices for design and operation (#7)
• No needs identified
DC Substation • Standard practices and technology transfer from specialized industries (SMES, Arc furnaces, etc) (#7)
• Bus design required to
accommodate high levels of dc current
transfer from specialized industries (SMES, Arc furnaces, etc)
Fault Detection and Mitigation
• Explore use of fault current limiting property of HTS – even for short sections (#5)
• Use of standalone fault current limiter/solid state breaker (#6)
• High current measuring devices
• Use of standalone fault current limiter/solid state breaker (#6)
DC Circuit Breakers • Definition of cable fault withstand capability for short(er) applications (#2)
• 25 kA steady state current, 400 kV dc circuit breakers
Motorized DC Disconnects
• Develop high current (#4) (withstand, 12.5kA and short circuit rating
• 25kA steady state current, 400 kV dc circuit breakers
Surge Arrestors • No needs identified Cable Charging
Equipment
• Development of special cable charging equipment if required by study (#3)
• Development of special cable charging equipment if required by study
Controls and Interfacing Voltage Source
Converter Based HVDC Controls
• Control damping needs as defined by study (#3)
• (Multi terminal control can be adapted from existing controls)
• Development of power dispatch Scheduling Program and EMS
System Studies and Impact Analysis System Stability
Studies
• Development of educational material for industry (It could take up to 3–6 yrs for adoption of new technology) (#8)
• Need for investigation of Sub Synchronous Torsional Interactions (SSTI)
Faults Analysis and Transients in the Cable
• Need to identify dc cable transients and their impact on cable design
• Need to identify control loop stability issues for small signal and large signal disturbances (#3). Development of
appropriate models for various
components
• Need to develop PSSE, PSLF and EMTDC (PSCAD) models for general industry usage and for design studies Short circuit models also need to be developed and made available (#3)
TABLE 2.4.ACTION PLAN FOR TOP PRIORITY GAPS Key Activities for the top
priority gap Timeframe of the activity (Years) Partnership strategies Resource requirements (low, medium, high) Top Priority Gap # 1 - Investigate optimal converter/cable topologies
Select a team of converter, cable and study expert
0–5 Lead by industry with support from DOE
Low
Define practical system ratings, constraints, operational
practices and redundancy requirements for recommended design
0–5 Lead by industry with support from DOE
Medium
Identify most desirable converter /cable topologies
0–5 Lead by industry with support from DOE
medium
Top P riority G ap # 2 - Develop High C urrent R ated B reakers , Dis connects (and cable charging equipment if needed)
Select team of industry experts Now Industry with support from DOE
Low Develop Equipment
Specification
Now Industry with support from DOE
Low Assess need and timing for
development
Now Industry and DOE Low Initiate development programs
(for different components)
After needs assessment
DOE funded Medium
Top P riority G ap # 3 - Develop and make available s tability and trans ient models for cables and converters
PSSE and PSLF and short circuit Model development for utility
Now EPRI/DOE lead with support from Industry
Medium
EMTDC (PSCAD) Converter model for cable and complete system design
Now EPRI/DOE lead with support from Industry
Medium
EMTDC HTS cable model Now EPRI/DOE lead with support from Industry
Medium
PSSE and PSLF and short circuit Model development for utility
Now EPRI/DOE lead with support from Industry
Medium
Completion of the action plans for the rest of the gaps was not completed due to time constraints; however, these can be developed along the same lines as for the above to ensure that the short term and long term visions are achieved.
C
HAPTER
3.
C
ABLE
S
UBSYSTEM
Cable subsystem components include the cable core, insulation, superconductor, shielding, and the process for fabricating the subsystem. Ancillary features include joints, cable splices, terminations, and the equipment/processes associated with the pulling of the cable into the cryostat. The cable subsystem implicitly includes the selection of the cable topology, concentric or separate poles, and the selection of voltage for a given rating including the selection of plus and minus poles or a single polarity pole with a neutral return cable. Demonstrations of significant scale of the cable subsystem for ac superconducting cables have been completed, including one in the United States at 138 kV ac, 2,400 A5, and 600 m (3 phases); and one in Japan at 200 kV, 1000 A and 500 m (1 phase).
Cabling methods, insulation approaches, splices, and terminations for ac superconducting cables provide a technology upon which the dc cable can be developed. Development of conventional HVDC cables (oil-filled and solid dielectric) also provides a basis for an HTS variant. A dc cable operating at higher voltage will require redesign or modification and qualification of most
components. Most high voltage superconducting ac cable systems have been designed with separate electrical phases in separate cryostats. Although coaxial cables have the advantage of no external magnetic field, separate poles in separate cryostats can minimize the probability of a pole to pole fault and, hence, mitigate the need for a high voltage, high current dc breaker. Cost optimization for long distance dc cable applications requires a trade-off study of the topologies. Characteristics of the site selected for initial implementation will impact the optimization of the near term cable system. A relatively short cable system is expected; some of the cable subsystem participants agreed that a near term goal of up to 10 kilometers of up to 5 GW cable in 2–5 years is reasonable. The cable system must be designed for reliable operation with a well planned level of redundancy, possibly demanding extra poles in separate cryostats, and the cost would be dominated by the converter stations.. The cost of the requisite converter station equipment is very high (as much as the cable) and requires cost reductions to ensure the viability of the overall system. There was no consensus from the group whether achieving this goal would require significant institutional and private investment resources.
The selection of the voltage for the long range application depends on future pricing of the converter stations and the superconducting wire and the characteristics of the insulation system—the higher the voltage, the lower the cost of the superconductor for a given power rating. Converter manufacturers predict decreased converter cost per unit of power with increasing voltage. The insulation risks are lower for a low voltage system but costs based on realistic future superconducting wire pricing may favor lower current and higher voltage.
Terminations in an ac cable system have been qualified to 138 kV ac and a few kA. Qualification to 5 kA or higher and dc voltage requires component analysis, possible redesign, and testing. A dielectric system will perform differently for dc. The high voltage impulse test requirements for
superconducting dc cables and terminations will require review of existing and pending standards for dc cables.6
The qualification of the cable and ancillaries requires development of dedicated test facilities with the required voltage and current ratings, and refrigeration equipment. Comprehensive factory testing of the cable subsystem is desired, and cable design must be selected consistent with testing requirements and capabilities. If the dielectric requires impregnation with subcooled liquid nitrogen, cold factory testing must be accommodated. Otherwise, alternate insulation or testing methods will be required. This could include either solid dielectric development for superconducting cables, or methods for warm testing of laminated insulation in high pressure gas as an indicator of cable performance when cold.
The key R&D needs required to address the cable subsystem begin with realistic forecasts of future pricing for converters and superconductor wire. Cable subsystem specification, including voltage and topology, can be the subject of design optimization for specific near term and long term sites. Ancillary subsystems, including terminations and splices, must be designed and tested for dc voltage and the higher currents to be expected in future long distance high power
superconducting cable systems. The large number of splices in a long distance dc transmission cable will require demonstration of a high level of individual splice reliability for the design lifetime.
A key part of the design optimization is specification of the insulation subsystem, including considerations of factory testing requirements. Development of a new insulation system or a practical factory test method for existing laminated/liquid nitrogen impregnated insulation systems may be required. A mature conceptual design and detailed specification for a superconducting dc cable test facility is required to determine the costs and schedule for this important part of the cable development needs.
6
“Recommendations for testing dc extruded cable systems for power transmission at a rated voltage up to 250kV”, CIGRE 219, Working Group 21.01 February 2003 (currently being updated to 500kV by CIGRE Working Group B1.32 and planned for release in 2011).
B arriers to Achieving the Vis ion
• Incomplete information on near and long range requirements including a lack of detailed grid requirements for selected sites and agreed upon converter, breaker and wire characteristics • Terminations and splices have not been developed or qualified above 5 kA
• 100% factory testing of cable may be required which may impact insulation selection • No facility exists for fully testing cable systems at these voltage and power levels
TABLE 3.1.CABLE SYSTEM BREAKOUT GROUP PARTICIPANTS
Name Organization
Steve Ashworth Los Alamos National Laboratory
Paul Chu TCSUCH/UH
Steven Eckroad EPRI Bruce Gamble (facilitator) AMSC
Michael Gouge Oak Ridge National Laboratory Drew Hazelton SuperPower, Inc.
David Knoll Southwire Sukil Lee LS Cable Ltd
Ben McConnell Oak Ridge National Laboratory Joseph Minervini (scribe) PSFC/MIT
Wan Ki Park LS Cable
Frank Schmidt Nexans Deutschland GmbH
Status of the Technology Today
3.1 Cable
The cable subsystem is comprised of the insulated assembly which carries rated current, fault currents, and harmonic currents. It has a power rating corresponding to its design dc voltage and dc current.
3.1.1 Topology
Function
The function of the cable is to carry current in steady state and transient conditions. It must be able to be manufactured, tested, installed and repaired. The key topology trades include:
• Separate poles vs. coaxial cable
• Warm vs. cold dielectric
• Cold wet (LN2) vs. cold dry dielectric (conduction cooled). • Coolant flow path outside and/or inside the dielectric
Top R &D Needs for the C able S ubs ys tem
• Optimized design based on updated detailed requirements including cost and reliability • Terminations and splices qualified for selected DC voltages and currents
• Quality assurance plan for selected design (may include 100% factory testing of cable) • Definition of test facility needs
Status
As shown in the data compiled in EPRI 1017792, higher voltage ac superconducting cables tend to favor separate poles. The following two designs were discussed:
• EPRI advanced design
− 10 GW, 0/100 kV, 100 kA, coaxial
− Coaxial cables limit external magnetic field
− 100 kA yields ~ 2400 km wire/mile (90 A wire) ~ 212,000 kA-m wire/mile, which would require a significantly lower wire price than available today
− $15–20/kA-m would be needed to be competitive with 765 kV ac lines for a 1000 mile (1600 km) system (EPRI)7
− $5/kA-m to be competitive with HVDC (conventional) for a 1000 mile system (1600 km) (EPRI)8
• Higher voltage design
− 5 GW, +/-200 kV, 12.5 kA
7 NOTE: These conductor cost goals determined by EPRI are for a 5 GW line @ 0/100kV, not for a 10 GW line.
That is, these costs assume 50 kA per pole current, which 4 times instead of 8 times greater than the “higher voltage” design. Thus, comparisons between the two designs in this chapter and in Chapter 4 must take into account this factor of 2 difference in power—hence, current—levels.
− +/-200 kV is current state of the art for delivered VSC’s by multiple suppliers, +/-320 kV is available from a single source
− Separate poles limit the probability of pole to pole fault
− The 5 gauss line (pacemaker limit) is less than 10 feet from a pair of poles spaced by 5 feet at 12,500 A
− 12.5 kA yields ~ 300 km wire/mile (90 A wire) ~ 27,000 kA-m/mile
− At $50/kA-m wire pricing, a system at this voltage would be competitive with 765 kV ac for a 1000 mile (1600 km) system (AMSC)
Dielectric selection was also discussed. It was noted that there are very few warm dielectric cable designs. PPLP and Cryoflex are the predominate choices for cold dielectric tape. EPRI proposed solid (extruded) conduction-cooled dielectric with the objective of allowing in-factory testing. It was noted that testing standards are not well developed for superconducting HVDC cables. Pole fault and transients need to be considered in selecting the design. It was noted that traveling waves following a lightning strike can cause high voltage gradients across the cryostat vacuum space that need to be considered.
3.1.2 Core (and quench conductor)
Function
The core provides mechanical support and limits superconductor strain when it is pulled through the cryostat. It also limits the temperature rise of the cable during transient over-current with the time required being determined by the characteristics of cable protection equipment.
Status
AC cable design practice is to design such that a fault will not produce a bubble in the cryogen before the breaker can clear the fault. The design requirements for the quench conductor in a superconducting HVDC are not as well understood. EPRI designed9 for 2 pu current for 0.5 seconds with the objective of being able to continue operation after the fault.
Solid state breakers are very fast but only exist up to 10’s of kV. Mechanical breakers require a few cycles to open and only exist up to 6 kA at transmission voltages. Protection systems developed for fusion magnet systems should be considered.
3.1.3 Cabling
Function
This operation includes the application of the cable elements over the core:
• Superconductor • Binder • Shielding • Insulation 9 EPRI 1020458 pg 2-9
• Quench conductor
The function of the cabling equipment is the application of the cable elements. It must be efficient and cost effective. Sufficient superconductor must be included to carry steady state currents with margin. It is required that the superconductor Ic variation along the cable length be minimized. The binder is used to balance tensional forces resulting from the superconductor pitch. Shielding limits ac losses in the cable assembly and carries images of harmonic currents in the cable. Insulation must withstand the pole to pole or pole to ground steady state and transient voltages. The quench conductor function was discussed in the previous section.
Status
For ac superconducting cables, conventional cabling equipment is used to apply superconductor and binder over the mandrel and insulation. The processes for handling superconducting tape during cabling are well established including periodic splicing during cabling. Up to 100 tapes per pass have been achieved. Multi-pass cabling has been successfully done with BSCCO. For example a 6 layer, 397 tape 47.5 kA, stainless steel laminated BSCCO cable was fabricated by North China Electric Machine University.10
Shielding for superconductor cables might be copper and applied in a similar manner, but alternate shield materials should be considered. The shields must be designed for transients following fault.11
3.1.4 Insulation application
Function
This operation includes the application of sufficient insulation for operation under steady state and transient voltage conditions.
Status
• Wet Insulation: LN2 impregnated laminated insulation has been used in most cables and has been applied with standard taping heads in a humidity-controlled environment. PPLP testing at ORNL indicates that dc capability corresponds to the peak value for ac.
Methods for factory testing wet insulation in volume production would need to be
developed. Factory testing of the superconductor will be needed for the foreseeable future and cold dielectric testing could be combined with this test.
• Dry Insulation: Solid extruded dielectrics (XLPE for example) are applied with heaters and extruding heads. Thermal contraction must be managed in cable and termination design. The design needs to accommodate different dielectric permitivities of the
dielectric system including liquid nitrogen and PPLP/Cryoflex for cold “wet” designs and air/vacuum for cold “dry” designs like EPRI. It may be that dry warm tests could be a reliable indicator of cryogenic performance. The extrusion temperature must be compatible with the selected conductor. Study is required to determine if an economic
10
Superconductor Technology Vol. 22 2009 pg1.
insulation material can be identified which has adequate cryogenic mechanical and dielectric properties.
Type and production test requirements need to be established for superconducting dc cables. In general transient and fault requirements are not defined for dc cables.
3.2 Cable Shipping and Installation
3.2.1 Shipping
Function
The cable and its drum must be designed consistent with shipping constraints including max dimensions for tunnels, bridges and interstate transport regulations. A long length is desired to limit splices during installation. The dielectric must be kept clean and dry in shipping.
Status
Shipping limits are well understood by the conventional cable industry. For the Long Island Power Authority (LIPA) program at 138 kV, single phase cable shipping constraints were limited to about 600 m per spool. This cable was shipped with a 143 mm diameter cryostat in place. The “Pipeline” approach may permit longer lengths on a spool since the cable can be shipped
independent of the cryostat. The EPRI design assumed 1 km on a spool and a 137 mm diameter cable.
3.2.2 Cable Installation
Function
The cable must be installed in conduit or cryostat by a process that does not strain the superconductor and which keeps the insulation dry and clean.
Status
Cables are designed with vaults at splice points with the cable pulled through the connecting section. Some cables are assembled into the cryostat at the factory and are pulled as an assembly through conduit. Pulling forces may determine the spacing between vaults.
3.3 Splice System
Function
An electrical connection between cable sections must be made with a resistance low enough that the splice losses are small compared to other system losses. The insulation must be continued through the splice region meeting as a minimum the voltage requirements of the balance of the cable. Two types of joint are required: one with a fluid connection to the refrigerator or pumping station and one without. A key requirement is that splice testing be done in the field.
Status
3.4 Terminations
Function
The termination connects the cable to the ambient temperature transmission line or buswork. It must be designed for low thermal losses (approaching 100 watts per 1000 A per termination pair). The heat transport fluid, usually nitrogen, must be connected to the cable.
Status
In ac superconducting cable programs terminations have been qualified to 138 kV ac. These cables have not been qualified for dc operation. The temperature dependence of the dielectric performance in the termination may require increased size to handle dc applications. AC cables have been qualified to only a few thousand amperes. DC cables may require 10–100 kA. The design must consider these higher currents and the associated forces.
Space charge issues must be addressed in the design of the termination for a dc application. Terminations developed for ac applications will certainly require requalification and may require redesign if used for dc cables. Skin effect is important in ac terminations but will not be as much of an issue for dc designs.
Fluid connections have generally been at the termination but in long dc cables, fluid connections must occur periodically over the length of the cable and should be part of the splice assemblies.
TABLE 3.2.BARRIERS TO ACHIEVING THE VISION Short Term (0–5 years) Long Term (5–15 years) Cable
Topology • A fully developed design is needed to quantify the issues and validate costs for specific applications.
• Wire production must be ramped up to match needs for the initial application (~300 km of wire per mile of 5GW +200/-200 kV cable). The transients for potential near term applications have not been studied. Shield and superconductor harmonic and transient losses must be modeled. The method for accommodating thermal contraction must be integrated with the cryostat, splice, joint and termination designs.
• The key long term issues are cost and reliability. The level of required redundancy needs to be established as part of detailed design studies. Voltage and rating need to be selected consistent with overall cost optimization.
• Review of geomagnetic currents for the selected converters will be required.
Core (and quench conductor)
• Fault currents and duration need to be defined before the core and quench conductor can be defined. A key challenge in defining the fault is the lack of an existing dc breaker at the cable voltage and current
• Similar comments apply to the long term although the
development of appropriate breakers is clearly feasible. Approaches for sectionalizing are needed to take faulted or failed sections out of service.
Cabling • There are no serious cabling issues • In the long run additional capacity will be needed to address the volume needed in long cable projects.
Insulation application
• For the near term application, wet insulation must be qualified for dc use. Material testing may be required for cable dielectric
components to enable analysis and design to satisfy long term requirements and possible polarity reversal.
• A factory testing method is desirable for either wet or dry insulation. If dry insulation were selected than it would need to be developed since there is no solid dielectric qualified for cold operation.
Cable Shipping and Installation
Shipping • No major issues are expected at 1km per spool.
• No major issues are expected at 1km per spool.
Cable Installation
• No major issues are expected but methods for water removal must be reviewed. The limits on cable pulling force need to be studied.
• Methods for keeping the cable clean during installation need to be considered but are not a major barrier.
Splice System
• Resistive contacts for the required current would need to be qualified. The splice insulation processes used for ac
superconducting cables need to be adapted and qualified for use with dc at required voltage levels.
• If solid dielectric were selected, then methods for splicing would need to be developed.