Naval Superconducting Integrated Power System (SIPS)

Transcription

1 P a g e 1 Naval Superconducting Integrated Power System (SIPS) J. Miller, D. Santosusso, M. Uva, K. Woods, and B. Fitzpatrick ABSTRACT A Superconducting Integrated Power System (SIPS) based on high temperature superconductor (HTS) technology is a long-term Naval Surface Warfare Center Carderock Division (NSWCCD) vision that requires a commitment over several decades to achieve the full benefits of the technology. Future ship construction offers opportunities for single system implementation such as HTS propulsion motors, auxiliary or main generators, HTS degaussing, and a superconducting power distribution network for ship services, directed energy weapons, and radar. The Navy is currently funding dozens of cryogenic and superconducting development projects through both SBIR s and core funding, all of which have been pursued based on the promise of weight reduction, cost reduction, higher electrical efficiency, and higher power density. A future naval warfighter equipped with such advanced technology will vastly improve the Navy s capability to project power and dominance over the world s seaways. Components that are evaluated in this document include HTS DC power cables, cryogenic coolers, thermal insulation, energy storage, superconducting power protection approaches, cryogenic power electronics, and cryocooling systems. This paper also summarizes the ongoing work and illuminates needed insertion points for continued development. SHIPBOARD ELECTRICAL ARCHITECTURE STUDY F uture naval ships will require significantly more electrical energy to power the increasing electrical loads 1. The deployment of the electromagnetic rail gun, high power laser, high power radar, and directed energy weapons will require more power and efficient power management to direct the energy to the required systems 2. Future ship hulls are not expected to increase in size beyond the DDG 51 design; therefore, the next generation of naval power systems must include high power-dense components. This increase in electrical demand and the need to reduce operating costs within the constrained hull is necessitating the shift to an integrated power system (IPS) 3,4. Traditionally, shipboard power systems separate propulsion from the electrical system. An IPS combines the generation and loads busses to form a micro-grid that shares power between sources. The architecture is expected to allow more flexibility in power flow management. This paper discusses the state of development of these components and the needed areas of development. IPS is a power architecture design that supplies electrical energy for propulsion, hotel services, and other legacy loads while adding the capability of powering directed energy weapons. This document proposes a medium voltage DC (MVDC) employing high temperature superconductor (HTS) power cables capable of carrying 4kA in a cable with less than a 3 diameter. Superconductors are a viable solution to the increasing power demand needed for electric propulsion and directed energy weapons 5. Resistive energy losses on a distribution line will become enormous, as high capacity power is required by the load. HTS material exploits the very low resistivity nature of the conductor. The HTS DC network proposed in later sections of this paper can operate of relatively low voltages eliminating the need for high voltage insulation and high voltage circuit protection. SUPERCONDUCTING MEDIUM VOLTAGE DC ARCHITECTURE Naval Surface Warfare Center Carderock Division (NSWCCD) proposes an electrical architecture using superconductors featuring an MVDC network. The design uses distribution zones to increase redundancy and survivability in the event of battle damage or catastrophic failure. A conventional copper-based solution to increase power flow capability will apply two major changes: 1. Add more copper cables to handle high current demand adding weight and volume or increasing the system voltage to handle the power demand without increasing current

2 P a g e 2 2. Install larger generators to increase the power available adding weight and volume In order to achieve the required power capability, a conventional design will require high voltages that will increase the size of the electrical insulation and add high voltage power electronics to protect the power system, both of which will increase weight and volume. When comparing the current density of copper (350 Amps/cm 2 ) to HTS wire (30,000 Amps/cm 2 ) 6, the benefit is apparent. The HTS system will be lighter and smaller while exhibiting zero real impedance on the entire DC network. The most prominent source of losses associated with any superconducting power system is found in the cooling system and is discussed in this paper. The amount of copper has a significant impact on the ship design and has a direct impact on the cost of the systems. Additional cabling increases the procurement, installation and life cycle costs and overall system weight, which could reflect a reduction in mission payload or performance. Additionally, once the current levels exceed the limit of the protection system, a higher voltage or a new protection scheme will be required. HTS technology offers an alternative solution to increase the power density of shipboard distribution while providing the additional weight and space for the mission payload at a reduced total cost. INTEGRATED POWER SYSTEM IPS combines ship s power generation, propulsion, service distribution (hotel and mission power supply), and power management into a single integrated electrical power system. Traditional power and propulsion systems implement mechanical generation and propulsion systems separate from the power generation and distribution system. Figure 1 illustrates traditional and integrated power and propulsion systems. Ship Service Integrated Power System Motor Propulsion Ship Service MWs of Electric Power Segregated System Propulsion MRG Ship Service MWs of Total Power 6-9MWs of Electric Power Figure 1: An example of an IPS electrical distribution architecture compared to a traditional segregated System The primary benefits of an integrated system include: Improved fuel economy by running fewer generators Increased power available for high power mission loads Increased survivability by distributing power flow in isolated zones SIPS architecture will incorporate multiple electrical zones with power generation and load distributed throughout the ship to maximize ship s capability through power and survivability. A major challenge associated with today s IPS technology is that compact power converters and distribution equipment required for high power flow cannot yet match the power density of the mechanical systems used in traditional plant line-ups 7,8. In order to obtain IPS benefits over a wide range of platforms, the Navy has indicated that maximizing the power density of Next Generation IPS (NGIPS) is a primary goal 9.

3 P a g e 3 HTS POWER CABLE A key component to any distribution system is the power cable. In this paper, we propose using a superconducting DC architecture. The network is composed of DC cables configured to maximize efficiency and redundancy of the system. DC cables are preferred to AC cables as they will exhibit zero resistance and nearly zero impedance. An AC cable on the other hand, will produce significant losses from hysteresis and eddy currents induced in the surrounding materials 10. By using a centralized DC system, we exploit the best aspects of HTS cables while avoiding some of the biggest engineering challenges associated with carrying AC through HTS cables. HTS power cables have been developed in industry for over a decade; however, most of these projects focus on nitrogen cooled cables. As we discuss in later sections of this paper, the use of liquid nitrogen introduces safety hazards in a shipboard environment. In order to mitigate these risks, the Navy is focused on developing heliumbased components, as helium does not present an asphyxiation hazard to personnel. The thermal management section of this paper develops a comprehensive analysis of the current cryogenic technologies, as they are applicable to naval power systems. While power distribution systems employing high temperature superconductors (HTS) have the potential to meet Navy ship electrical needs, further development is required to improve the performance and increase the technological readiness of key system components. In order to address the IPS power density requirements, we incorporate HTS technology to improve the power density within the IPS architecture. While some technologies such as large HTS propulsion motors and HTS degaussing systems have matured, there are new challenges associated with cryogenic power distribution systems that need to be addressed. There is a clear benefit in terms of weight, volume and efficiency if advances are made to meet these challenges. A traditional IPS system with copper is complicated, and an IPS system using superconductors operating at extremely low temperatures is even more so by adding complex thermal energy management systems. Given these engineering challenges are solved; the resulting system could have a weight savings of 75% and a volume savings of 80% 11. Fig. 2 shows the potential size reduction for equivalent power requirements. Copper (12) 2.2 OD HTS (1) ~2.75 OD Figure 2: Comparison of copper vs. HTS power cables in a relative shipboard cable run In today s market place, conventional power cables retain better cost effectiveness when compared to superconducting cables; however, successful commercialization of HTS technology is driving private industry to improve manufacturing techniques that will continue to reduce the cost of HTS tapes. THERMAL MANAGEMENT With the advent of superconducting advanced magnetic degaussing, the technology to cool the conductor to the appropriate operating temperatures is well underway. The HTS cable must operate at cryogenic temperatures (20K 100K); therefore, cryogenic coolers capable of maintaining this low temperature must be included. Long transmission lines applicable to public utilities (1 100 km) experience the most significant thermal losses through the insulated cable walls. For this reason, long cable designs usually employ liquid nitrogen to keep the conductor operating in its superconducting state. Short distribution cables applicable to a shipboard architecture on the other hand, use cables no longer than 100 meters. Therefore, the most significant thermal loss arises from the current lead terminations. These terminations connect the room-temperature conductors to the cold-temperature HTS conductors. The equation below simplifies the thermal load produced by the cable and its nodes using currently available technology. The first term, Q 1, represents the losses emanating from one pair of current leads under full load. It is dependent on the transport current (I max ) and the real impedance of the material (usually copper). The second term, Q 2, is the thermal load introduced by the heat leak through the cable s insulation and is dependent on the length of cable (L). =

4 P a g e 4 Q 1 (watts) = 0.1(watts/amp) * I max (amps) Q 2 (watts) = 1.5(watts/meter) * L (meters) Q total = (Q 1 * N) + Q 2 ; where N is the number of current lead pairs The variables I and L are determined by the power required by the ship and the size of the ship. Therefore, the total heat load applied to the system is reduced by improving the thermal insulation at the feed throughs and thermal insulation around the length of the cable. CRYOGENIC TERMINATIONS As seen in the above relationship, an overwhelming portion of the losses occurs at the transition between warm and cold environment. Minimizing the heat injected into these transitions will reduce the heat load on the coolers, which will increase the compressor s Carnot factor, and increase the efficiency of the overall system. The effort focuses on the high current leads used to connect a cold HTS cable to a room-temperature generator and load bus. The desire to minimize this heat load has led the Navy to invest in the development of these current leads. For ship-scale current levels and transmission lengths, the thermal loads produced at the current leads dominate the heat load on the cooling system. Careful optimization of both the current lead design and the system cooling capacity will maximize the system s efficiency. Under the Small Business Innovation Research (SBIR) program, the Navy has funded the development of a multi-stage current lead design based on a system-level optimization. The goal of which is to minimize the power required, size, weight, and cost of the HTS power transmission system. Optimized terminations are currently under development, and they represent the biggest engineering challenge to overcome. Terminations used in industry are too large, and therefore, are not feasible for use aboard a Navy ship. Therefore, compact helium-cooled terminations must be developed. These terminations will be compact to fit in small spaces aboard a ship, while injecting minimum heat into the cryogenic environment. CRYOGENIC COOLERS Another challenging aspect of HTS power distribution is the cooling system. In past HTS systems, the heat loads were found mostly in the mechanical, structural, and vacuum subsystems. The predicted losses for a power distribution system are measured in kilowatts (which is at least two orders of magnitude higher than degaussing and the 36MW HTS motor). The Navy is funding two technology development projects for novel cryocoolers that boast higher Carnot efficiency than ever before. The increased heat load adds another layer of challenges to the design, but the technologies to manage these issues are in development. Two small businesses are currently developing novel high efficiency cryogenic coolers to provide between 0.5 and 1 kw of thermal energy removal at cryogenic temperatures. This thermal energy removal potential is approximately 50% higher than any current naval cryogenic cooler systems. The most important results from these studies are the weight and space reduction, increase in energy efficiency, and increase in HTS capabilities resulting from use of these novel systems. POWER CABLE THERMAL INSULATION In order to minimize the heat leak into the cable, the Navy is funding small businesses to develop a robust thermal insulation package for a superconducting cable assembly. A thin, lightweight aerogel serves as the base material for the insulation package, and is interleaved with infrared (IR) reflective material (metallized Mylar ). The benefits that an aerogel-based thermal insulation package will provide in comparison to the current state of the art, multi-layer insulation (MLI), are highlighted: More efficient installation Robustness/durability Space/weight savings: aerogel allows larger inner pipe, smaller outer pipe - No gap required between insulation and outer pipe - No spacer cord required (as is required for MLI to minimize thermal shorts) Low cost for shipyard implementation (no cleanroom necessary) Improved thermal performance compared to MLI (by ~40%) with better reliability (no thermal shorts/rework) Hydrophobic aerogel able to withstand high humidity environments without degradation

5 P a g e 5 Cable insulation is currently being developed by private industry to address thermal penetration into the HTS system, which plagues the operability of HTS wire. Development of novel cable insulation designs using advanced materials allow for an increase in system capabilities due to reductions in heat penetration into the cryogenic system. Due to the finite cryogenic cooler capacity, the reduced thermal penetration rate into the HTS system allows for a larger network of cables and a reduction in the overall system weight and volume by removal of additional cryogenic equipment. ENERGY STORAGE Future electric weapons will employ high power pulses of electrical energy 2. The supporting power system will require some form of energy storage to meet these projected requirements. Advanced compact energy storage technology allowing rapid release of electrical energy is desired. The proposed design includes strategically placed energy storage modules that serve three main functions: 1. Increase power quality during normal ship operations (load leveling) 2. Backup power supply in the event one or multiple generators go offline 3. Additional stored energy for high power loads It is estimated that a 200MJ pulse-forming network is required for the Navy s railgun to achieve the desired muzzle energy of 63MJ 2. Superconducting magnetic energy storage (SMES) is a technology that looks attractive for this application because of its high power and energy densities. SMES is a novel technology that stores magnetic energy in the form of circulating current. Because the conductor has zero resistance, the storage medium is very efficient, and exhibits high energy density. Figure 3: Integration of bidirectional DC-DC converter on an MVDC Ring Bus 7 The power electronics necessary for connecting an energy storage module (ESM) to a DC system are simpler than the interface needed for an AC based network because they can eliminate the AC to DC conversion step when interfacing with batteries and capacitors. The ESMs will operate more efficiently on a DC network with the elimination of the need for additional converters. Furthermore, consolidating ESMs into strategically placed locations that can be used for multiple functions reduces the number of modules needed throughout the ship saving size and weight. The strategic placement of ESM s can also increase survivability if sized properly to the load demands of its respective zone. PROTECTION PHILOSOPHY AND REDUNDANCY The proposed architecture consists of an MVDC ring with a main bus on both the port and starboard sides of the ship with two longitudinal ties, one in the forward zone and one in the aft zone of the distribution system. The architecture is a zonal network; allowing any zone to be fed from either the starboard or the port bus providing the necessary redundancy to vital loads. The Navy will continue to operate with an n-1 generator setup to maximize reliability. With the introduction of HTS cables to the distribution system, the installed cables have the capacity to supply all four zones from the main bus to provide additional survivability. The challenge with this system is choosing the appropriate protection scheme. An MVDC circuit breaker will be needed to protect the superconducting network against faults and other transient events. Currently, MVDC circuit breakers under development have problems handling the high peak current introduced during a fault on a DC network due to the lack of a zero

6 P a g e 6 crossing that occurs in an AC system 12. Given that above cryogenic technologies become available, the complex design challenges can be overcome using the proposed HTS cables by taking advantage of HTS natural current limiting phenomena 13. However, HTS cables have a problem handling high power levels once quenched (when they current limit). Conveniently, this challenge may be overcome with a hybrid MVDC circuit breaker, 14 a technology that is under development. This MVDC circuit breaker could open the path of conduction very quickly if the high peak currents are suppressed. This protection method is one basis for continuing research on HTS cables for an MVDC distribution system. This symbiotic relationship between the MVDC circuit breaker and the HTS cables should be studied further as this may be a way to implement a MVDC system to meet the needs of future war ships. NAVAL SUPERCONDUCTING TECHNOLOGIES The success in the development of HTS advanced magnetic degaussing system and a full-scale propulsion motor have proven HTS technology s viability. In January 2009, NSWCCD successfully completed full-power testing of the world's first 36.5-megawatt (49,000 horsepower) superconducting ship propulsion motor at the U.S. Navy's Integrated Power System Land-Based Test Site in Philadelphia. Earlier in 2008, the Navy successfully installed an HTS degaussing coil aboard the USS HIGGINS (DDG 76), which underwent sea trials until March 2010 with successful operation during deployment with over 9,000 hours of operation and 37,000 nautical miles underway. Both projects showed clear benefits of HTS systems with greater than 50% weight reduction vs. copper equivalents and increased efficiencies. These systems are shown in Figure 44. Figure 4: (left) 36.5MW HTS propulsion motor under test, (right) HTS degaussing system installed in the USS Higgins DDG-76 The HTS motor and degaussing programs have advanced the state of the art in HTS in naval applications and have shown the potential benefits of these systems. These successful programs illustrate that the technology works, and that it can yield great benefits. Power distribution is the next step for HTS technology development. The feasibility of a Superconducting Integrated Power System will depend on the development of necessary technologies that support power distribution such as high power terminations, circuit breakers, cryocoolers, and improved thermal insulation. SHORE POWER A potential stepping-stone to superconducting shipboard power applications is the use of HTS cables for ship to shore power cables. The process to establish shore power connectivity for a naval ship requires the use of numerous copper cables extending from power substations on the pier to receptacles aboard the ship. A feasibility study has been conducted to investigate the use of HTS cables to transmit power from land based power substations to naval ships 15. Since a single HTS cable can replace an array of copper cables, the time required to connect a ship to shore power would decrease, therefore reducing the amount of time the ship must run its generators. The reduction in weight will also improve safety for the personnel handling the cables. Figure 5: Shore power cables connected to a DDG-51 There are many challenges to overcome before an HTS ship-to-shore power cable could be used to replace the conventional copper cables. The first is the ruggedness of an HTS cable. Because the cable would be located near the brow, there is a potential for crew to step on the cable. In addition, the cables are stored in a loose coil on the pier in all weather conditions when not in use. An HTS power cable must be designed to weather the harsh conditions encountered on the pier. Additionally, there is a potential for vehicles to drive over the cables when they are stretched across the pier. Therefore, the cable must be designed to withstand these forces.

7 P a g e 7 HTS cables for shore power applications might not be the best fit for the technology due to the harshness of the operating environment, but the research will provide a solid foundation and significant gains in knowledge pertaining to the operation and ruggedness of this technology. The lessons learned in these efforts will be applied directly to future distribution systems aboard Navy ships. There are many advantages of using an HTS cable for transmitting power from shore power substations to naval ships. When discussing the concept with both ship and shore crews, the most favorable benefit was reducing the man-hours required to handle the heavy cables. From the ship perspective, hard cost savings would be gained from the generator fuel savings. Other benefits to a single cable would include better utilization of man-hours, fewer safety hazards, and improved morale for the crew. There are also many challenges to overcome including ruggedness, cryogenic system maintenance and cool down times, dielectrics, and ease of connection to ship and shore. With further technological advances being made every day in the field of superconductivity, the concept of using an HTS cable to connect a naval ship to a shore power substation is a realizable goal for the future. FUTURE WORK While an MVDC superconducting distribution system eliminates the problem of AC losses, the design introduces new system integration challenges. One such challenge can be found in understanding the steady state and transient power flow in the DC network. A study will be needed to analyze the network s stability when exposed to high transient events. The results will drive further improvements to the circuit protection schemes. The Navy is investing in the development of novel thermal insulators that will improve the efficiency of the cooling system. The development of HTS power is introducing a paradigm shift from long cable runs with one set of current leads found in degaussing and superconducting magnets to short cable runs with two pairs of high current leads found in power applications. Because of this, the Navy is investing in the development of more efficient current leads. The proposed architecture is achievable with today s thermal management methods. With further development, however, the system will get more compact and weigh less. The Navy will continue its development of high current DC power cables, which is a key component in this architecture. Combined with optimized terminations, the testing of this cable will increase our understanding of the applied physics and will validate our predictions for power and thermal performance. The proposed architecture deploys large energy storage modules throughout the ship to manage the power available. Future efforts will include multiple studies to examine the mission requirements as it pertains to the power flow through the distribution network and the optimal location and sizing for these modules. As noted above, further development of the cryogenic MVDC circuit breakers must be emphasized. The ongoing development of HTS cables can be leveraged to develop cryogenic circuit protection technology. CONCLUSION This paper is a response to the realization that increasing shipboard power demand is driving significant power density advancements in power distribution architectures. Current state of the art superconducting technology is assessed for naval applications. This paper discusses the state-of-theart technology while pointing out key areas lacking attention. The development of superconducting components is progressing well and results indicate that the promised weight savings and increased power density are achievable. In addition, this system will take advantage of emerging high power dense components like HTS machines and fault current limiters as they become more mature.

8 P a g e 8 REFERENCES [01] N. Doerry, Next Generation Integrated Power System, Naval Sea System Command, November [02] T. Wolfe, Preliminary Design of a 200 MJ Pulsed Power System for a Naval Railgun Proof of Concept Facility, Pulsed Power Conference, 2005 IEEE, June 2005 [03] So-Yeon Kim, Feasibility study of Integrated Power System with Battery Energy Storage System for naval ships, Vehicle Power and Propulsion Conference (VPPC), 2012 IEEE, Oct, 2012 [04] M. Roa, ABS Rules for Integrated Power Systems (IPS), Electric Ship Technologies Symposium, 2009.IEEE Apr 2009 [05] Huibin Zhao, Advantage of HTS DC power transmission, Applied Superconductivity and Electromagnetic Devices, Sep 2009 [06] Qiang Li, Crossover of thickness dependence of critical current density Jc(T,H) in YBa2Cu3O7~d thick films, Applied Physics Letters, Vol 84, No18, May 2004 [07] Il-Yop Chung, Integration of a Bi-directional DC-DC Converter Model into a Large-scale System Simulation of a Shipboard MVDC Power System, Electric Ship Technologies Symposium, ESTS IEEE, Apr 2009 [08] Ma Weiming, A survey of the second-generation vessel integrated power system, Advanced Power System Automation and Protection (APAP), 2011 International Conference, Oct 2011 [09] N. Doerry, Next Generation Integrated Power System: NGIPS Technology Development Roadmap,, Naval Sea Systems Command Washington DC, 30 Nov 2007 [10] Seokho Kim, AC Loss Analysis of HTS Power CableWith RABiTS Coated Conductor, Applied Superconductivity, IEEE Transactions, Jun 2012 [11] B. Fitzpatrick, High Temperature Superconductor (HTS) Degaussing System Assessment Philadelphia, PA, NSWCCD Technical Report, NSWCCD-98-TR- 2004/030, Oct [12] Hebner, Robert, et al, DC Protection for Ships, ESRDC December 2011 [13] Kalsi, S., HTS Fault Current Limiter Concept, [14] Johnson, B.K., IEEE Transactions on Applied Superconductivity, Vol 7, No 2, 1997, pg 419 [19] Corzine, K., Structure and analysis of the Z-source MVDC breaker, Electric Ship Technologies Symposium ESTS, 2011 [15] P. J. Ferrara,, Applied Superconductivity, IEEE Transactions, Vol 21, no3, 2011, pg