About Superconducting Cables…
The need for modernization of the electric power grid is well documented. This effort will entail not only construction of new cross-country transmission lines, but also increasing the reliability and supporting the growth of more concentrated urban loads. Both will require a robust and stable transmission and distribution infrastructure. American Superconductor Corporation (AMSC) introduced a unique technology in 2007 as a secure, system-level superconductor cabling solution that increases both the capacity of T&D infrastructure and the fault current handling capability of dense urban circuits.

High-capacity, very low impedance superconductor cables that offer significant power density advantage over traditional copper-based cables have been well demonstrated at electric utilities and are now being deployed in the grid. Three of these cables have been energized in the United States over the past two years. Stand-alone fault current limiters based on superconducting materials also offer a new vista in grid security and technical control of system operating parameters.

AMSC’s Secure Super Grids (SSG) technology combines the benefits of both superconductor technologies. This proprietary, system-level, ‘intelligent grid’ solution utilizes customized superconductor power cables and ancillary controls to deliver up to ten times more power than conventional copper cables while at the same time suppressing power surges - or fault currents - that can disrupt service. This unique technology allows for the construction of multiple paths for electricity flow in metropolitan power grids to ensure system redundancy when individual circuits are disrupted due to severe weather, traffic accidents or willful destruction. As such, this technology provides electric utilities with a powerful and secure means to simultaneously address rising electricity demands and steadily increasing fault current levels.

Providing Power to Congested Urban Grids
High temperature superconductor (HTS) cable systems are an ideal retrofit in existing urban utility infrastructure where space is at a premium. Very low impedance HTS cables can be located in proximity to other infrastructure without disturbance because they emit no magnetic fields and are unaffected by the thermal considerations that affect traditional cables. Moreover, HTS cables can be retrofitted into existing ducts or placed in narrow trenches.

While placement concerns are vital considerations, the central benefit of HTS cables – and the main reason why global industries invested over 20 years of R&D commercializing them – is their ability to carry up to ten times the power density of conventional cables. Keeping up with power requirements in areas such as Manhattan, and countless other metropolitan centers, demands a power-dense solution. Today, HTS cables by Southwire carry up to 3,000 Arms at 13.2 kV in the grid that American Electric Power manages in Columbus, Ohio. Long Island Power Authority (LIPA) has installed a Nexans 138kV HTS cable system running nearly a half-mile in length. Rated at 574 megawatts (MW), this system is able to serve 300,000 residents and businesses in New York’s Nassau and Suffolk counties.

Managing Fault Current Magnitudes
SSG’s technology takes the high power handling ability of HTS cables and adds inherent fault current limiting capabilities, further increasing the application options in the power network. In many urban areas, fault currents now approach the limits of conventional equipment and, given the increasing demand for electricity, such currents will likely continue to rise.

All of the new transmission and distribution equipment commonly installed to meet load growth and connect to new sources of generation contributes to increasing fault current levels. Fault currents now exceed 60,000 A in some transmission substations and reach 40,000 A in certain distribution substations. These values approach the limit of today’s circuit breaker ratings. Given the unrelenting expansion of grids, a new solution is necessary.

While higher capacity fault-handling equipment is available, the economics of implementing a large-scale upgrade are not favorable. SSG technology takes advantage of a feature inherent to AMSC’s second generation HTS wire that permits the design of a cable that is able to carry massive amounts of power one minute and then automatically turn into a resistor – a fault current limiter - when an over-current occurs on the system (i.e., beyond a pre-determined level).

This fault current limiting ability hinges on a fundamental property of HTS materials: above a critical current, their superconductivity is quenched and their electron transport characteristics become resistive. It is for this reason that high temperature superconductors have been termed “smart materials,” switching rapidly whenever a fault current exceeds the superconductor’s critical current magnitude. The result is a new tool to enhance the capacity, reliability and security of the power grid.

Applications of cable-based or stand-alone superconductor fault current limiting equipment are diverse and range from enabling normally closed bus-ties, interconnecting substation secondaries to improve reliability, enabling IPP interconnections, and making possible the construction of a more tightly meshed grid to relieve congestion.

The slate of economic benefits is broad and includes: avoiding equipment damage, deferring or eliminating the need for equipment replacement, enabling the use of lower fault-rated equipment, eliminating the losses of series reactors, achieving higher system reliability, facilitating use of lower impedance transformers and enhancing grid stability. Given these compelling advantages, the United States Department of Energy has estimated a potential U.S. market for fault current limiting devices to be on the order of several billion dollars over the next 15 years.

Modeling Illustrates Sizable Fault Current Reductions
The following schematic shows configuration of an existing utility grid used to model behavior of a SSG installation (green line) between two transmission substations.

In this example, a single transmission circuit (i.e., 115kV/138kV cable) and a fairly extensive 69kV “sub-transmission” network connect two major transmission substations. The 115kV/138kV cable has the capacity to transfer up to 230 MVA from one transmission substation to the other, but the 69kV sub-transmission system primarily exists to serve the various distribution substations in the area, and it is not designed as a path to transfer bulk power.

When a significant power source near transmission substation #2 is installed due to ongoing load growth, the utility will have a significant financial incentive to increase the amount of power that can be transferred in the direction of transmission substation #2, which can provide power to neighboring regions or sold to a neighboring utility. In this example, approximately doubling the amount of transfer capability would meet the system owner’s goals.

One potential solution to increase the power transfer capability is to add a second 115kV/138kV conventional cable that is electrically identical to the first between the two transmission substations. This approach would effectively meet the goal of doubling transmission capability. However, the existing fault current levels at both transmission substations are already at a very high level, and the addition of a new conventional circuit will result in even higher fault current levels. Any significant increase in fault current above initial levels would likely require a tremendous investment to replace circuit breakers, transformers and other fault current sensitive substation equipment.

An alternate solution would be to install a 138 kV Secure Super Grid cable system. With a single SSG cable circuit, the power transfer level can be raised to meet the increased power transfer goal. For the purposes of this study, a 2000A, 478 MVA cable was considered. This SSG cable has more than twice the power carrying capability of the existing 230 MVA conventional circuits.

Because the SSG cable alone can supply the desired level of transfer capability, the system owner would have the option of opening the existing conventional 138kV cable, leaving the SSG cable as the only in-service transmission path. This approach would result in fault current reductions of over 27% at transmission substation #1 and over 6% at transmission substation #2 from initial levels. Comparing the SSG solution against the conventional solution option, the reductions in fault current are over 36% and 9% at transmission substations #1 and #2, respectively, with similar increases in power transfer capability.

The impact these various scenarios have on fault currents is summarized in Table 1.

These data affirm the conclusion that Secure Super Grid technology can be applied in a manner that significantly increases power transfer capability while lowering fault current levels.

This solution offers a major value proposition for electric utilities worldwide in dealing with the need to provide increased power capacity in a rapidly growing economic environment. This new system is easily installed and brings increased underground capacity, fault current protection, reliability and security to densely populated urban and metropolitan area power grids.

Consolidated Edison to Deploy SSG Solution
Consolidated Edison, one of the nation’s largest investor-owned energy companies, provides electric service to nearly all of New York City, serving the island of Manhattan over a distribution system organized as shown in Figure 1.

The utility operates individual load islands of 100 MW to 300 MW that it serves at 13 kV, using multiple underground feeds from an area substation. Typically, a substation consists of five 65 MVA 138-13 kV transformers, serving about 150 MW of load. Consolidated Edison’s security standards mandate N-2 contingency capabilities; that is, no loss of load-serving capacity, even after two substation transformers go down.

The utility has publicly discussed its desire to overhaul New York City’s power grid over the next few decades to provide greater reliability and security.

As shown in Figure 2, the utility’s concept is to connect area substations to provide power redundancy while also breaking distribution networks down into smaller compact networks, thereby minimizing the affect of blackouts. The space constraints under the streets of New York and the fault current levels that would result from realizing this vision, however, necessitate the use of SSG technology.

Under a program funded in part by the U.S. Department of Homeland Security, Consolidated Edison is installing a first substation-to-substation link in Manhattan. Since the 13 kV interties have the potential to increase fault currents beyond the interrupting capability of existing substation equipment, superconductor cables themselves will be relied upon to manage fault currents.

Normally a certain minimum length of SSG cable is required to achieve the desired fault current limiting effect. This installation also demonstrates how even a very short SSG cable, placed in parallel with a small shunt conventional cable, can lower fault current levels.

The system operates as follows: Under normal operating conditions, the impedance of the superconductor cable is of order 1/6 or less compared to that of the shunt conventional cable (and optionally a series reactor), so that the dominant portion of the current flows through the high capacity superconductor cable and there is no voltage sag from the conventional cable or its series reactor. When a fault occurs, the superconductor cable switches immediately to a resistive state, limiting the fault current.

The superconductor cable with its HTS wire is designed in such a way that the resistance is large compared to the impedance of the conventional cable, so that the remaining fault current is diverted to the conventional cable (and its series inductor) and is finally limited by the total shunt impedance. After four cycles, a fast switch opens, allowing the superconductor cable to recover to its superconducting state, which occurs in only minutes.

During this time, if the fault has cleared, the conventional cable carries the power based on its overload rating. After a few minutes, the recovered superconductor cable is reconnected to the circuit by closing the fast switch and it again picks up the majority of the power flow. If the fault does not clear during this time, the system circuit breaker opens to initiate the utilities’ standard protection procedures.

Alternatively, the system can be designed to allow two full faults of up to four cycles, so that a first re-closure of the fast switch can be carried out within seconds, compatible with standard utility protection schemes. The parallel conventional copper cable is not necessary in all situations. For instance, longer cable runs could provide enough superconductor wire to absorb the fault energy of the full fault hold time without overheating until existing circuit breakers open.

In shorter runs such as ConEd’s deployment, the parallel cable or an existing parallel connection is necessary to allow the fast switch to open and still maintain current flow to mesh with the existing utility protection procedures. In many cases, parallel cable connections already exist in the meshed utility network. In the special situations where a new conventional cable is required, it is important to note that this cable does not need to be rated for full continuous-duty capacity, but only to be able to carry the larger fault power flow under overload conditions for short durations.

About the Author
Jack McCall is the Director of Business Development T&D Systems for American Superconductor with responsibility for superconductor cable systems, static VAR compensators and related FACTS (Flexible AC Transmission Systems) solutions.  He has more than 25 years experience in the utility T&D business, holding a variety of product engineering, product management, system engineering, business development, marketing, and strategic planning roles.

McCall holds a Master degree in Electric Power Engineering from Rensselaer Polytechnic Institute and a Bachelor of Science degree in Electrical Engineering from Gannon University.  He is a member of the Institute of Electrical and Electronics Engineers (IEEE) and the International Council on Large Electric Systems (CIGRE).

For further information, contact: Jack McCall, American Superconductor Corp. (www.amsc.com), 15775 W. Schaefer Court New Berlin, WI 53151 Tel.: 262-901-6016
Email: jmccall@amsc.com.