Progress and Status of a 2G HTS Power Cable to be Installed in the Long Island Power Authority (LIPA) Grid

Progress and Status of a 2G HTS Power Cable to be installed in the Long Island Power Authority (LIPA) Grid

Abstract

Underground high temperature Superconductor (HTS) power cables have attracted extensive interest in recent years due to their potential for high power density. With funding support from the United States Department of Energy (DOE), the world’s first transmission voltage level HTS power cable has been designed, fabricated and permanently installed in Long Island Power Authority (LIPA) grid. The HTS cable was successfully commissioned on April 22, 2008. In 2007, a new DOE Superconductor Power Equipment (SPE) program to address the outstanding issues for integrating HTS cables into the utility grid was awarded to the current project team (LIPA II). The goal of the LIPA II is to develop and install a replacement phase conductor manufactured using AMSC’s second generation wire. In addition to the replacement of the phase conductor, the team will also address the outstanding components development necessary for full scale integration into a power grid including integral management of thermal shrinkage of the cable conductor, optimization of the cryostat design to mitigate the implications of potential cable damage, and the development and demonstration of a field splice in the operating utility grid and modular higher efficiency refrigeration system. This paper will report on the progress and status of LIPA II program. In addition, in-grid operation experience of existing 1G HTS Power cable is presented.

Index Terms—HTS cable, 2G HTS power cable, LIPA and transmission level voltage.

Introduction

All over the world, there is growing need for a reliable and sustainable electric power infrastructure. High capacity underground HTS power cables have long been considered a possible solution that help relieve the congestion that aging power grids worldwide are placed under. Power cables using HTS wires have been developed to increase the power capacity in utility power networks while maintaining a relatively small footprint. Over the past decade, several HTS cable designs have been developed and demonstrated successfully. All HTS cables have a much higher power density than copper-based cables at the similar voltage levels. Moreover, because they are actively cooled and thermally independent of the surrounding environment, they can fit into much more compact installations than conventional copper cables, without concern for spacing or special backfill materials to assure dissipation of heat. This advantage reduces environmental impacts and enables the installation of compact cable systems with three to five times more capacity than conventional circuits at the same or lower voltage. In addition, HTS cables exhibit much lower resistive losses than conventional copper or aluminum conductors.

With funding support from the United States Department of Energy (DOE), the world’s first transmission voltage level HTS power cable using first generation HTS wires has been designed, fabricated and permanently installed in Long Island Power Authority (LIPA) grid. The HTS cable was successfully commissioned on April 22, 2008. In 2007, a new DOE Superconductor Power Equipment (SPE) program to address the outstanding issues for integrating HTS cables into the utility grid was awarded to the current project team (LIPA II). The goal of the LIPA II is to develop and install a replacement phase conductor manufactured using AMSC’s second generation wire. This paper will report on the progress and status of LIPA II program. In addition, in-grid operation experience of existing 1G HTS Power cable is presented. The cable system performance is compared before and after more than two years of operation.

Product Description

The LIPA I project is a Superconductivity Partnership Initiative (SPI) between the United States Department of Energy (DOE) and industry to develop a long length transmission voltage high temperature superconductor power cable. In 2007, a new DOE Superconductor Power Equipment (SPE) program to address the outstanding issues for integrating HTS cables into the utility grid was awarded to the current project team (LIPA II). American Superconductor Corporation is the prime contractor as well as the manufacturer of the high temperature superconducting wires. Nexans is providing the development and manufacturing of the cable, terminations and cryostat as well as site and installation support, and Air Liquide is providing the cryogenic refrigeration expertise, equipment, installation as well as operations monitoring and support. The host utility, LIPA, has provided the site, civil work, controls and protection, transmission planning and the operation of the HTS cable.
The cable system was designed to meet the following specifications:
Voltage: 138 kV
Current: 2,400 Arms
Total Power Carrying Capacity: 574 MVA
Length: 600 meters
Design Fault Current: 51,000 Arms

LIPA II Project

General description

In 2007 a program to address the outstanding issues for integrating HTS cables into the utility grid was awarded to the current project team. This project will address the following issues:
Field Joint
Field Repairable Cryostat
Modular High Efficiency Refrigeration System
Thermal Contraction Compensation within the Cable

While the LIPA I project was designed to demonstrate the technology, the goal of LIPA II is to demonstrate a commercial system. Within the LIPA II project, one of the three existing phases of the 600 m long LIPA I installation will be replaced by a LIPA II phase that exhibits the characteristics described above.

In the framework of the LIPA I program, the ability of a superconducting cable to safely handle fault currents had been demonstrated, the specification for the LIPA I cable being a short circuit current of 51 000 A. The short circuit current must not cause the cable temperature to rise above the boiling point of liquid nitrogen at the operating pressure of the cable, as such a rise in temperature would lead to bubble formation in the liquid nitrogen. If these bubbles penetrate the liquid nitrogen impregnated insulation, a high voltage failure may occur.

It is one of the goals of the LIPA II project to not only demonstrate that superconducting cables can be designed to survive fault current events (the so-called fault current tolerant design), but to also demonstrate the ability of a superconducting cable to suppress fault currents, that is to demonstrate a fault current limiting design. Since only one phase of the existing LIPA I cable will be replaced with a LIPA II phase, it is however necessary to manufacture and install a 600 m long fault current tolerant phase, and demonstrate the fault current limiting functionality in the lab only.

The design work of the LIPA II project has included the detailed analysis and design of both fault current limiting and fault current tolerant designs. The YBCO conductor used in the LIPA II project allows the design of both fault current tolerant and fault current limiting designs. This is in contrast to the BSCCO material used in the LIPA I project, which, by virtue of the silver matrix, exhibits a lower normal state resistance and is therefore not suited for the manufacture of a fault current limiting superconducting power cable. Concurrent to the design work, dummy cables have been manufactured at the Nexans plant in Halden, Norway. These dummy cables were used to establish the design of a 30 m long fault current tolerant and a 10 m long fault current limiting cable prototype. Both the fault current tolerant and the fault current limiting prototypes have been manufactured and are currently awaiting testing in the Hanover test field of the Superconducting Cable Systems division of Nexans. Successful testing of the 30 m long prototype will validate the design for the 600 m long replacement phase to be manufactured within the LIPA II project.

Field Repairable Cryostat

A field repairable cryostat is one of the items being developed in the LIPA II project that demonstrate the readiness of superconducting power cables for application in the grid. Damage to the cable cryostat may occur for a variety of reasons (for example through civil works), and a speedy repair process is necessary in a commercial cable system.

While the LIPA I cable cryostats have only one vacuum space over their entire 600 m length, the LIPA II cable cryostat has been developed to include a multitude of suitably spaced vacuum barriers inside the cryostat, subdividing the vacuum space. The spacing of the barriers has been chosen so that each vacuum space can be evacuated via a single pumping port. This ensures that a vacuum failure anywhere along the length of the cryostat can be repaired by having access to the location of the failure point only.

However, being able to evacuate from one pumping port alone is only one of the requirements for the demonstration of a field repairable cryostat. Of equal importance is the demonstration that a cryostat that has suffered severe water damage can be evacuated again to a level that ensures acceptable thermal losses. As the cable cryostat contains multiple layers of superinsulation, one obvious concern is that the superinsulation is rendered useless by water damage. Another concern is the entry of contaminants that cannot be evacuated to an acceptable vacuum level.

To demonstrate that a damaged cable cryostat can indeed be repaired, Nexans has performed two tests designed to replicate the water damage that is expected in the field. In one test, the vacuum space of a 10 m test cryostat was exposed to high humidity, while in the second test, the vacuum space of a 40 m test cryostat was completely flooded with contaminated ground water; the contaminants being a cable pulling lubricant that is commonly present in the pipes into which power cables are pulled, as well as regular dirt that is found on the streets. The 10 m cable cryostat that was intentionally damaged by high humidity only was evacuated and then the thermal loss measured again, the after-repair thermal loss was the same as the loss before the repair of the cryostat. The 40 m cryostat was damaged by cutting the hole seen in Fig 1 while the thereby completely filling the vacuum evacuated cryostat was contaminated water, immersed in the space of the cryostat. At the beginning of this test,the cryostat was filled with liquid nitrogen to represent the on site state of the cryostat during such events. The cryostat was then repaired by providing a drainage hole to drain as much of the water as possible, and then re-welding and closing the damaged section. Finally, the repaired cryostat was evacuated again. The after-repair measurement of the repaired cryostat showed thermal losses acceptable for cable operation, albeit a factor of 2.6 higher than the original cryostat thermal losses. Together, these two tests have confirmed that a cable cryostat can be repaired even in very severe instances of damage and contamination.

Installation of a Field Joint

The successful testing and operation of a field joint is of equally vital importance for the demonstration of a commercial cable system, as it enables (in principle) unlimited cable lengths by joining cable sections. The length of the cable sections to be joined is commonly determined by transport constraints of the cable. Furthermore, in the case of very severe cable damage (i.e. damage to both the cable cryostat and the cable core), field joints are required to repair the cable system and put it back into service.

As the joint connects superconducting cable lengths carrying high currents, a small joint resistance is necessary to guarantee safe operation of the joint (the paper lapped dielectric insulation is not only a good high voltage insulator, but also a rather good thermal one). The electrical loss at nominal current is < 3W for the phase layer joint connection, and several Watts for the screen joint connection.

Thermal Contraction Compensation

The composite cable contracts by ~ 0.3 % when cooled from room temperature to 77 K. For a cable of 600 m length, such as is the case in the LIPA I and LIPA II installations, this equates to a contraction of 1.8 m. While it is possible to manage such a contraction in the terminations of the cable (as was done in LIPA I); for longer length installations it is necessary to develop a method to compensate the thermal contraction of the cable that is independent of the cable length. The obvious approach is to ensure over-length is introduced during cable manufacture and cable laying, so that the contraction of 0.3 % can be taken up by the previously introduced over-length.

The mechanical and contraction behavior of the various constituents of the cable is currently being evaluated to determine the appropriate amount of over-length to introduce to ensure that the cable contraction is handled independently of the terminations in a manner that is suitable for long lengths of cable.

To be able to test the mechanical and contraction behavior of the cable constituents, a test cryostat schematically was built. In addition to the evaluation of thermo-mechanical properties, this 10 m long test cryostat can also be used to measure ac loss in 10 m long samples. It will also be used to measure the 10 m long fault current limiting prototype, as described below.

Fault Current

As discussed above, the fault current behavior of superconducting power cables is commonly classified as either fault current tolerant or fault current limiting. While it was originally envisioned to install one fault current limiting LIPA II phase as a replacement for the existing fault current tolerant LIPA I phase, a closer analysis of the cable system showed that such an installation would be electrically imbalanced. Consequently, the framework of the project now calls for the installation of a fault current tolerant 600 m long replacement phase in the LIPA grid, as well as the laboratory demonstration of a 10 m long fault current limiting prototype.

The short circuit behavior of both the fault current tolerant and the fault current limiting cable have been calculated. The current wave form (prospective and limited) and the temperature rise in various layers of the fault current limiting design during a fault current event are shown.

Modular High Efficiency Refrigeration System

A 22000 W @ 72K cryogenic refrigerator dedicated to long-length High-Temperature-Superconductor cable is under development. The main objectives are: high reliability, maintenance free, high efficiency, low foot print and stand alone packaged system.

A preliminary design studies were performed to develop a new cryogenic refrigeration technology. This Innovating Process is based on a Reverse-Turbo-Brayton-Cycle. The essential innovation concerns the assembly of all active elements on a single shaft: centrifugal compressor, high-speed synchronous motor operating on magnetic bearings and cryogenic turbo expander. This preliminary design includes the following:
A thermodynamic analysis of the Turbo-Brayton cycle;
A selection of the most promising process architectures;
The development of refrigerator numerical models;
The performance optimization result of each architecture;
The refrigerator architecture choice;
The refrigerator preliminary design (drawings and plans);
The refrigerator off design performances calculation using a dedicated computer code.

The cryogenic refrigerator preliminary design meets the program goals and achieves high thermodynamic performances: 28% of Carnot efficiency. It is mainly composed of a 200 kW Two-Stages-Moto-Compressor, a 80 kW Moto-Turbo-Compressor (under manufacture) and a Cold Box.

In the Reverse-Turbo-Brayton Refrigerator, the Moto-Turbo-Compressor represents the critical component. Therefore, this component has been study in detail.

A particular attention was paid to the design of the cold head. This crucial piece of the refrigerator has several roles:
Minimize the thermal losses;
Enable a temperature gradient (60 to 300 K);
Ensure a mechanical aspect of the system during cool-down.

Calculations have been made in order to simulate the rotation of the machine (centrifugal stresses) and the cool-down of the cold part. These calculations pointed out the validity of the design. The Moto-Turbo-Compressor manufacture is in progress and its delivery is planed in February 2011.

After this critical component manufacture, entire Turbo-Brayton Refrigerator including the 2nd active machine (Two-stages-moto-compressor) could be manufactured and tested within 2 years.

LIPA I Cable In-Grid Operating Experience

In April, 2008 The LIPA I cable using AMSC first generation (1G) BSCCO 2223 HTS wire was successfully commissioned. Once the cable was connected to the grid, normal operation commenced. During operation on the grid, all related measurements are gathered by the data acquisition system which has remote monitoring capability. Fig 9 illustrates a typical ten day temperature and pressure history of the LN2 supply and return to and from the cable system. The 2K temperature cycling as seen in FIG 9 was mainly due to the add-on cryo-thermo on and off.

This is much lower than the cable design specification of 2400 Arms which was designed for a different site, but consistent with the projections provided by LIPA for this part of the grid. During the initial in grid operation, there were several conditions requiring the cable to be removed from the grid. Some of the issues were related to unexpected site conditions while others were related to HTS cable protection scheme. None of the trips were initiated by a failure of any of the mechanical equipment. For example during an extremely hot day, one trip occurred when the temperature inside the refrigeration building exceeded the normal operating limits of the equipment. This issue was resolved by implementing better ventilation and temperature controls in the building.

As the root cause of issues operating a superconducting transmission cable in the utility environment are discovered, corrective actions supported by appropriate testing are performed, ensuring the situation will not be repeated, then the cable system is reconnected to the grid. One lesson learned during in grid operation was how to effectively integrate the HTS cable protection scheme with existing utility protection schemes. Over-protection of the HTS cable resulted in several, unnecessary nuisance trips. Several cable system trips during the initial in grid operation were based on over constraining the operational parameters. For example, based on the original HTS cable protection scheme, any flow rate dip greater than 10% initiated a cable trip. However, subsequent thermal and hydraulic simulation indicated a 10% flow dip for a short time period would not damage the cable system even if a bolted fault hit the HTS cable at the same time. After carefully studying the operating parameters, the protection scheme was modified to minimize unnecessary trips, but still provides enough protection for the HTS cable and termination hardware.

During past two years, the cable has been taken off line for several times due to a normal relay trip testing initiated by LIPA. This provided a chance for the team to study the cable thermal and electrical performance. During these events, even the cable was off-line, the refrigeration system was in normal operation mode. TABLE 1 presents the two test results of this study over more than two years period. The first one was tested in May 2009 while the second was taken in June 2010. As shown, the cable cryostat loss is almost same as when the cable was commissioned in 2008. The slightly loss difference is in the range of measurement error. The test results indicated that the integrity of the cable cryostat has been maintained. Due to low operating currents, the cable electric loss is very low as expected.

Future Work

Over the next year, AMSC and the partners will remove one of the existing 3 phase conductors and replace it with one fabricated from 2G conductor and return the cable to operation in the LIPA grid. Once installed it will continue to be monitored and evaluated by the project team.

Conclusion

A 138 kV, 3 phase transmission cable system has been successfully fabricated and integrated into the grid on Long Island NY. It is the first demonstration of an HTS cable at transmission voltage on the grid. It has being designed to operate reliably and to withstand fault currents up to 51,000 amps RMS. The cable will be operated for a least a full year until the LIPA 2 project is ready to integrate into the site.

Acknowledgment

The authors wish to thank the U.S. Department of Energy for their financial support and the readiness review team for their technical input and guidance.

References

S.Mukoyama et al, “Manufacturing and Installation of the World’s Longest HTS Cable in the Super-ACE Project” IEEE Transactions on Applied Superconductivity, VOL 15, NO 2, June 2005

H.Yumura, et al “Phase II of the albany HTS Cable Project “IEEE Transactions on Applied Superconductivity, VOL19, NO 3, June 2009

T.Masuda et al, “Design and Experimental Results for Albany HTS Cable”, IEEE Transactions on Applied Superconductivity, VOL 15, NO 2, June 2005