CCorporate researCh Centers, universities, power ... - IEEE Xplore

Ship to Grid

C

Corporate research centers, u niversities, power equipment vendors, end users, and other market participants around the world are beginning to explore and consider the use of dc in future transmission and distribution system applications. Recent developments and trends in electric power consumption indicate an increasing use of dc-based power and constantpower loads. In addition, growth in renewable energy resources requires dc interfaces for optimal integration. A strong case is being made for intermeshed ac and dc networks, with new concepts emerging at the medium-voltage (MV) level for MVdc infrastructure developments. At the turn of the 20th century, the use of ac transmission, as opposed to dc, was a justifiable decision for many reasons. Looking toward the future of transmission and distribution, this decision must be reevaluated in light of the changes taking place throughout the U.S. electric grid at all levels: generation, transmission, distribution, and end use. In 2006, Electric Power Research Institute (EPRI) presented a number of valid arguments that strengthened the case for dc infrastructure in the 21st century. Modern advances in the transportation industry have often come

Digital Object Identifier 10.1109/MPE.2012.2212613 Date of publication: 18 October 2012

70

ieee power & energy magazine

1540-7977/12/$31.00©2012IEEE

november/december 2012

through the application of electronics (electric vehicles and magnetic levitation trains, for example). These innovations utilize dc power, requiring an ac-to-dc conversion within the current grid infrastructure. The microgrid, composed of distributed generation (such as photovoltaic systems) and energy storage systems that produce dc power, is a key emerging technology. Finally, utilities are trying to develop solutions for integrating renewable energy with storage devices to deliver dc power. These advances will make utility-scale energy storage more economical and efficient. Power electronics technology is an efficient, powerful, and reliable solution for integrating large amounts of renewable generation into existing grid infrastructure. Increased renewable integration with aggressive growth targets is a mandate set forth by the U.S. government, with a deadline in 2030. Developments in the area of power electronics, including the application of novel semiconductor devices and materials, have unlocked the potential for higher-capacity, faster-switching, lower-loss conversion, inversion, and rectification devices. In recent years, the advent of silicon carbide solid-state electronic devices, which have lower switching and conduction losses compared with silicon devices, has made dc-ac conversion more promising in the near-term time frame. Virtually all voltage and current ratings are possible by utilizing series and parallel combinations of discrete semiconductors. All of these factors are combining to form an opportunity for the development and further deployment of dc technology throughout the electric grid at all levels.

Medium-Voltage DC Concepts in Theory and Practice

Pioneering DC Development: HVdc Technology High-voltage dc (HVdc) has proven its merit over high-voltage alternating current (HVac) transmission for long-distance power delivery applications in many cases. HVdc’s advantages include reduced right-of-way clearance, improved control over power flows, power factor correction, less infrastructure, and reduced losses. Underwater and underground transmission of dc avoids excessive capacitance and dielectric heating effects in the cables. HVdc also provides an asynchronous link for strong but instability-prone ac power systems and offers advantages for renewable energy integration, and advanced HVdc systems can provide black-start capability. The cost of solidstate electronics has decreased while, at the same time, other parameters like voltage rating, current capacity, and efficiency are increasing over the years along with continued improvements in overall system design and construction. These factors, other attributes of HVdc technology, and the

© artville

By Gregory F. Reed, Brandon M. Grainger, Adam R. Sparacino, and Zhi-Hong Mao


ieee power & energy magazine 71

Auxiliary ac Generator

Auxiliary ac Generator

ac Breaker

ac Breaker

Port Propulsion Motor

dc Disconnect

dc Disconnect

Inverter Drive

MVdc Port Bus dc–dc Converters

dc–dc Converters

dc–dc Converters

dc Cable

Vital Load

dc Cable

Vital Load

Vital Load

ac Load

ac Load

dc–dc Converters

dc–dc Converters

dc–dc Converters

MVdc Starboard Bus dc

Disconnect

ac Load

Starboard Propulsion Motor

dc Disconnect

Inverter Drive ac Breaker Auxiliary ac Generator

ac Breaker Auxiliary ac Generator

figure 1. Standard electric ship architecture.

emerging applications of dc resources and supply establish the potential for a paradigm shift in the future development of transmission and distribution systems toward a larger overall dc infrastructure.

Initial Evolution of DC-Based Power Systems: Electric Ship Architecture The concept of an integrated power system (IPS) is a familiar one for the Office of Naval Research and research groups within universities. A generic layout of an electric ship using a dc backbone is provided in Figure 1. The idea of an IPS 72


is to enable all the energy generated to be used by all the ship’s systems. Work on earlier IPS concepts showed that the energy within the ship propulsion system was indeed sufficient to supply traditional ship operation needs so that service generators could be eliminated, yielding significant fuel savings. Efficient use of generated energy is important, but another critical factor in ship design is space savings. Traditionally, every marine load or load circuit was interfaced with a transformer, strictly for galvanic isolation. Transformers have two main functions in addition to providing galvanic isolation: november/december 2012

microseconds. Motor controllers handle faults, maintain the dc link between the rectifier and inverter in various circumstances, and monitor over- and undervoltage while using appropriate protection schemes for a specific occurrence. If the technology and knowledge within the motor drives industry can be mapped to future dc architectures, it is expected that the same benefits in controller performance and protection will be available in these systems.

impedance matching and voltage scaling. Although vital and a workhorse of past and current electric power systems, transformers rated for 60 Hz are large and heavy. For these reasons, research organizations like the Advanced Research Projects Agency–Energy (ARPA-E) are providing funds for investigating new materials for transformer cores so these units can operate at much higher frequencies. The size and weight of the transformer is inversely proportional to the operating frequency. Historically, dc has had protection issues limiting dc distribution systems to a maximum rating of 1,000 V on naval ships. Traditional electromechanical switchgear is slow, and dc voltage collapse can be much faster than ac collapse. Among power engineering professionals, it is well known that ac circuit breakers make use of zero-crossing points to clear faults, but in dc systems there are no zero crossings. Today, solid-state devices and power converters are making dc a more attractive and feasible option. The rectifier and inverter sets found in today’s modern motor drives have the capability of identifying faults in

Medium-Voltage DC Infrastructure: Bridging the Gap Between the Transmission and Distribution Sectors The medium-voltage dc (MVdc) concept is ultimately a collection platform designed to help integrate renewables, serve emerging dc-based and constant-power loads, interconnect energy storage, and address future needs in the general area of electric power conversion, all in a more optimized manner. The need for MVdc technology development has been driven by the liberalization of the energy market, which has led to installations of large-scale wind

ac Transmission Supply

Existing ac Infrastructure

Fuel Cells

Photovoltaic Generation

dc dc

dc dc

Nonsynchronous Generation (Wind)

Future HVdc Intertie

dc dc

HVdc System

ac dc

ac dc

MVdc dc dc

dc dc

dc dc

dc dc

Distribution dc Load Circuits

dc ac Electric Vehicle Future dc Industrial Facility

Future dc Data Centers

Motor Variable Frequency Drives

dc dc

dc ac

dc ac Electronic and ac Loads

Distribution-Level Sensitive Storage Load

Control Algorithm

figure 2. General MVdc architecture layout. november/december 2012


MVdc + -

5-MW Supply

PMSG Based

ac–dc Three-Level Multilevel Rectifier (NPC) MPPT Enabled

Breakers dc Cable

Bidirectional dc–dc Converter

dc Cable

dc–ac Three-Level Multilevel Inverter (NPC)

5-MW Supply

PMSG Based

ac–dc Three-Level Multilevel Rectifier (NPC) MPPT Enabled

PMSG Based

PMSG Based

#1

#2

Bidirectional dc–dc Converter Preliminary Electric-Vehicle Charging Station

dc Cable


5-MW Supply ac–dc Three-Level Multilevel Rectifier (NPC) MPPT Enabled

3.3 kV/480 V

Future Industrial Facility

5-MW Supply ac–dc Three-Level Multilevel Rectifier (NPC) MPPT Enabled

Battery Storage or Generic Load

dc Cable

Voltage Regulator

3.3 kV/33 kV #1

#2

Utility

1-MW Supply + dc–dc Converter PV Plant

MPPT Enabled

figure 3. Current model layout of MVdc architecture.

and solar farms at the transmission and distribution level. The penetration of these renewable energy technologies, both onshore and offshore in the United States, is rapidly increasing and will ultimately require a dc integration link to realize efficiency and optimization gains. The same is true of the various end-use consumer loads, in particular, electric vehicles, as well as sensitive power electronics– based loads, many of which are operating at low-voltage dc levels. A general setup for a proposed MVdc architecture with representative energy supply, distributed resources, and loads is displayed in Figure 2. It is important to note that MVdc is not simply a technological comparative of HVdc or a simple scaling of voltage level from HVdc but rather a new development concept with various and diverse applications. 74


There are certainly potential increases in efficiency that can be realized by employing an MVdc network. MVdc systems will help reduce the number of conversion stages required for integrating a lower-voltage output of a renewable generation resource to the electric grid, which operates at a much higher voltage. These same efficiencies are realized at end-use applications for supplying increasing penetrations of dc-based loads in electrical networks. Hence, the MVdc platform can serve as an additional layer of infrastructure in the electric grid between the transmission and distribution levels, as well as a means of supplying consumers of all types. Of course, an overall increase in complexity may be one consequence of these development efforts. Thorough discussion of technical requirements on the part of engineers and scientists and the development of standards focused on dc operations november/december 2012

will be necessary in order to handle such systems properly. Pursuing these avenues will require significant research and development in the coming years.

MVdc Network Model Development The Electric Power Initiative’s research team in the Swanson School of Engineering at the University of Pittsburgh (Pitt) has undertaken a large initiative in dc research, with a specific focus on MVdc development in the transmission and distribution arenas. At this point, the overall MVdc concept can be found in technical literature, as we have seen thus far with electric ship design; it has been presented in various general forms similar to the diagram shown in Figure 2. To date, however, there exist no complete detailed models or simulations that account for the entire dc network in a high-voltage and high-power environment. A holistic MVdc framework requires more thorough development to address the interactions of the different types of converters with the electric machines found on the dc bus. This is just one area of much-needed research, among many others that must be addressed related to the development of dc infrastructures. The research group at Pitt has added many layers of sophistication to the model shown in Figure 2. The current model has been developed in the Power Systems Computer Aided Design/Electromagnetic Transients including dc (PSCAD/EMTDC) simulation environment and is portrayed in Figure 3. To give an appreciation of the detail of the model, 64-page modules (sublayers within the model) were used

to create the model, seven machines are being simulated at once, and all switching characteristics of the power converters are being captured within a matter of minutes for each simulation. The model includes a custom-developed wind turbine based on a Type 4 configuration. A Type 4 wind turbine includes power electronic converters arranged in a backto-back configuration interfacing an electric machine. Each Type 4 wind turbine is capable of supplying 5 MW to the system. Existing literature is focused on permanent magnet machine–based wind turbines, due to their higher efficiency and the elimination of the field circuit. For this reason, the wind turbines in the model are based on the permanent magnet machine. The topology chosen for the rectifiers (ac-dc) and inverters (dc-ac) in Figure 3 is the three-level neutral-pointclamped (NPC) configuration. The NPC multilevel inverter can generally be configured as a three-, four-, or five-level topology. There are three multilevel topologies, in general, that have found widespread adoption: the NPC configuration, the flying-capacitor configuration, and the cascaded H-bridge topology. With regards to converter control, a maximum power point– tracking (MPPT) algorithm was configured within the rectifier controller, as well as third harmonic injection principles in order to achieve the desired dc bus voltage. The controller for the bottom inverter, connected to the infinite bus via transformer as found in Figure 3, regulates the dc bus voltage and reactive power flowing toward the source. The other main source of generation in the model is a photovoltaic plant capable of supplying 1 MW of power, which can be easily adjusted if desired. Like the wind turbines, the solar

LVdc + –

Synchronous Buck dc dc Constant Power Control

Electric Vehicle (Battery)

Synchronous Buck 20-kW PV

Boost Converter

dc

dc dc 20-kW PV

dc

Constant Power Control

Electric Vehicle (Battery)

Constant Power Control Bidirectional dc–dc Converter

MVdc Network

Voltage Regulator

figure 4. An EV charging station design. november/december 2012


Central Controller

Local Controllers

MVdc Grid

figure 5. A two-layer hierarchical control architecture for the MVdc system.

array’s dc-dc converter includes an MPPT algorithm used to maximize and control output power. The four major load entities on the network are data centers, industrial facilities, an energy storage system, and an electric vehicle (EV) charging station infrastructure. Current EV charging station installations and simulations are focused on ac chargers, which require rectifiers to convert the ac power to dc to charge the battery of the EV. These systems are typically tied to an ac grid and have the ability to connect on-site generation in the form of solar photovoltaic panels, which are often mounted on the rooftops of the charging stations. Having the EV charging station directly tied into an MVdc architecture is expected to improve system efficiency due to the fact that solar power is generated at dc and the batteries in an EV are a dc load; thus, the number of power conversion stages in the overall system is minimized. The energy storage system modeled is based on sodium-sulfur battery technology characteristics.

figure 6. Semisubmersible drilling rig. (Source: “High power clean dc bus generation using ac-link ac to dc power voltage conversion, dc regulation, and galvanic isolation” from the Electric Ship Technologies Symposium, 2009.) 76


To take one example of a load entity in the MVdc model, the EV charging station employs Level 3 dc fast chargers, which can fully charge an EV battery in 15 minutes or less. With these fast chargers, one can assume there will be periods of time in which the electric vehicles are not using the charging station. During these times, the solar panels on the roof of the EV charging station may be generating solar power, and the charging station can be used as a generation source to supply power back to the grid via the bidirectional dc-dc converter. A design for the EV charging station is shown in Figure 4. Note that this diagram represents the subsystem found within the EV charging station block found in Figure 3. The PSCAD/EMTDC model consists of a few photovoltaic arrays, two EV Level 3 dc fast chargers, and bidirectional power flow capability to and from the dc grid. Each component of the system is connected via a dc-to-dc power converter. The battery component of the EV charging model simulates the charging of a single EV. The technological challenges engineers and scientists face today require multidisciplinary action. Within the electric power program at Pitt, individuals concentrating in power systems, power electronics, controls, and high-power semiconductor devices collaborate extensively. MVdc development will need to address all of these angles, from the semiconductor device up through the power system level and including advanced control techniques, to show its merits. Wide-band-gap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), will foster reductions in switching loss that will lead to improved efficiencies in the power converters and in overall network operations. As shown in Figure 2, an overall control architecture surrounds the parallel-connected power converters. The hierarchical control architecture has two layers, as illustrated in Figure 5. The first layer is a decentralized-control layer. This layer consists of distributed local controllers for the MVdc grid. These local controllers are able to connect and cooperate with each other through advanced communications. The connection structure or topology of these controllers is a part of the MVdc design and needs to be determined by solving a structural optimization problem. The cost function of this optimization problem includes a term that penalizes the communication complexity of the interconnection in the MVdc system and is therefore control topology–dependent. The second layer of the hierarchical architecture is a single central controller. The central controller in the hierarchical architecture does not control the local actions directly. It supervises and coordinates the operations of the local controllers in the first layer and adaptively optimizes the control and communication topology in real time. The central controller therefore has greatly reduced responsibilities in control. This is another example of an area of required development for dc design that has great potential for new control algorithms and new means for optimized network communications and interfaces. november/december 2012

Applications Being Considered for MVdc Infrastructure

the use of passive LC filters, multipulse drives, active filters, and active front-end rectifiers. But the additional equipment requires additional space, often where space is at a premium, and increases the weight of the platforms. A host of disadvantages for each of the mitigation methods has been discussed in the literature. With the substantial investigations being conducted by research and consulting teams, it is envisioned that large wind farm deployments along the eastern seaboard (Atlantic Ocean) and West Coast (Pacific Ocean) in the United States, as well as potential developments in the Gulf of Mexico and the Great Lakes, will become more attractive to the investment community. Today, domestic offshore oil drilling takes place in the Gulf of Mexico and Northern Alaska, as shown in Figure 7. The eastern seaboards, from North Carolina up through Maine, are the locations with the greatest potential for offshore oil and gas drilling. One major hurdle preventing large-scale deployment of offshore wind generation along the East Coast is the lack of an electrical collection system to serve as a link between generation and transmission to shore. The MVdc architecture can serve as this link. The colocation of offshore drilling platforms and wind generation will allow for the use of an MVdc collection system and provide an environmentally responsible, economical, and potentially optimal method of powering offshore oil platforms. This innovative idea of connecting the two major themes in energy policy, offshore wind generation and oil reserves, has the potential to open up new business markets

Many government-sponsored projects have been focusing on various means of integrating renewable generation resources, specifically wind and solar, into the grid. There is a better understanding of onshore wind behavior, which has occupied the attention of many engineers in the United States in the last several years, than of offshore behavior. The U.S. Department of Energy (DOE), through FOA-414, has found great interest in exploring the integration of offshore wind potential and has funded a team of organizations to explore wind speed behaviors at sea and determine the optimal locations for placing large wind turbines around the perimeter of the United States. In addition to optimal wind turbine placement, this program is actively investigating ways of integrating the power produced offshore into the grid. The MVdc architecture developed and previously described contains four 5-MW permanent magnet synchronous generator–based turbines. These large wind turbines will likely find their optimal application out at sea, due to the larger wind gusts, more consistent wind patterns, and greater overall wind potential necessary to generate enough power to rotate their large radial blades. Offshore oil-drilling operations, such as those shown in Figure 6, which are heavy industrial environments, rely primarily on variable-frequency ac drives for applications such as propulsion, station keeping, drilling, and pumping product to the surface. In drilling rigs, drill hips, and offshore production platforms, the nonlinear variable-speed drive load makes up 85% of the installed kW. The typical installed drive power for a drilling package is 5,000–12,000 hp (3.7–9 MW). Because the platforms are composed of large electric drive units and other machinery, operations such as those listed suffer from various power quality issues. These power quality issues arise from voltage sags due to starting large motors and energizing magnetic components like transformers. Drilling rigs and oil platforms place considerable harLegend monic strain on generators Areas with Greatest Resources—for 2012–2017, and degrade the quality of the Lease Sale Must Occur Here voltage supplies. The power Areas to Conduct Lease Sales quality is compromised due to Areas Available for Drilling harmonic currents produced Areas Withdrawn from Drilling Until 2022 by Law during the conversion from ac National Marine Sanctuary to dc for variable-frequency drives (VFDs). Harmonic mitigation methods include figure 7. Location of oil-drilling opportunities, 2012–2017. november/december 2012


Wind Generation (Offshore) 5-MW Supply

MVdc Collection Platform (Offshore)

ac–dc Breakers Three-Level Multilevel dc Cable Rectifier (NPC) PMSG Based MPPT Enabled 5-MW Supply ac–dc Three-Level Multilevel dc Cable Rectifier (NPC) PMSG Based MPPT Enabled 5-MW Supply ac–dc Three-Level Multilevel dc Cable Rectifier (NPC) PMSG Based MPPT Enabled 5-MW Supply ac–dc Three-Level Multilevel dc Cable Rectifier (NPC) PMSG Based MPPT Enabled

+

Offshore Oil-Drilling Platform

–

+

dc Cable

–

Bidirectional dc–dc Converter

0.6–2 MW Motors

Voltage Regulator


#1 #2 ac Cable Utility

Voltage Regulator

figure 8. MVdc architecture utilized in an offshore oil-drilling application with a colocated wind farm.

directed toward large oil companies, which take part in the drilling operations, as well as traditional power industry participants, including utilities. Furthermore, the efforts that many manufacturers are making to explore offshore technologies for harnessing and transmitting electric power provide further encouragement that this proposed direction is viable and worthy of further investigation. A diagram providing a system-level interconnection of local wind power generation with an offshore oil-drilling platform is provided in Figure 8. Notice that the “utility” in this figure would represent an onshore connection. Many current studies by the authors are in progress with this configuration, including transient evaluation through a PSCAD/EMTDC simulation environment and loss evaluation between an ac- and dc-based backbone on the platform using software suites such as ANSYS Simplorer.

Practicing Standards in MVac Technology, and Future Needs for DC There are definite boundaries that engineers have developed for classifying low and high voltage levels and products. 78


Determining the range for medium voltage, though, depends on the particular applications that are being investigated. The terms medium voltage and high voltage are often used interchangeably, depending on the perspective of the speaker. Naturally, the word transmission triggers the idea of high voltage in the engineer’s mind, especially in the utility industry, with common transmission-level voltages of 138–765 kV and higher; without question, electric grid planners would state that these values are purely high-voltage ac quantities. A facilities engineer, however, might call 13.8 kV high voltage, although to the utility engineer it would clearly be a distribution-level voltage. IEEE Standard 1585-2002, a guide for the functional specification of medium-voltage electronic series devices used mainly for compensation of voltage fluctuation, specifies that medium voltage ranges from 1–35 kVac. IEEE Standard 1623-2002, a standard meant for specifying shunt-connected devices to compensate for voltage fluctuation, provides the same voltage range. But NECA 600-2003, a standard for installing and maintaining medium-voltage cables, defines the medium-voltage range as voltages between 600 V and 69 kV. november/december 2012

Prior to IEEE Standard 1709-2010, there have been no existing standards specifically for MVdc power distribution systems above 3 kV. This latter standard is the first attempt to describe recommended practices for 1–35-kVdc power systems on ships. Another thorough and extensive standard with regards to electric ships is IEEE 1662-2008, a guide for the design and application of power electronics in electrical power systems on ships. From this brief discussion of some of the ac and dc standards that exist in MVdc applications, reviewers may identify two present issues, as well as several future needs. The first is that there is still some inconsistency from standard to standard when it comes to defining the range for MVac applications. Second, there are a limited number of dc standards at the MVdc power distribution level. Further, a critical parameter that requires much more attention is selecting an appropriate dc bus voltage for grid-level applications. An optimal voltage level needs to be defined that is suitable for integrating renewable generation to the bus and transmitting power at a high enough voltage to maintain system efficiency. Other standards that are clearly necessary include those for dc protection, dc switchgear, and grounding, as well as a new set of standards related to the various phenomena that can occur in a dc environment.

Conclusions The purpose of this article is to describe the advancements taking place in developing and pioneering MVdc technology. dc system design, research, applications, and investment are all growing. Pioneering work in dc at the transmission level started with HVdc technology. With a host of advantages realized from several decades of HVdc applications, it now makes sense for the industry to begin to consider dc in other environments, at other system levels, and for a more diverse set of applications. The MVdc concept, first explored in ship design, is now being investigated in electric grid applications. There is a lot of wide-ranging and substantive R&D that has yet to be evaluated, and guides for evaluating dc collection systems at the electric grid level must be developed. Understandably, dc standards at this level simply don’t exist at present, as there is still so much we still need to learn first about dc systems themselves. At the turn of the 20th century, dc-based systems competed with ac designs, and obviously ac won out for the right reasons, given the historical conditions. But given today’s advancements in semiconductor devices, the penetration of dc-based renewable generation, and the increased use of dc loads, among other factors, reevaluating dc-based systems for a new era is necessary. Electric power professionals have a wide range of experiences with ac-based systems, making a shift toward dc uncomfortable for many. Initially, resistance to these concepts is to be expected—change can be unsettling at times. But as we look ahead to the evolution of future electric power systems, thinking in terms of an


infrastructure that has a larger role for dc technologies is becoming more obvious. Partisan thinking could prevent advantages or disadvantages of the other approach from revealing themselves. A dc interface may not be simple, but its technological benefits are being proven by much current work. Perhaps the optimal solution will take a hybrid approach, in which the ac network will be utilized along with intermeshed and integrated pockets of dc networks where these prove to be best applicable.

Acknowledgments The electric power research group at Pitt would like to extend special thanks to the Commonwealth of Pennsylvania’s Ben Franklin Technology Development Authority (BFTDA) and the ABB Corporate Research Center for their support of the work being pursued in MVdc technology development.

For Further Reading G. F. Reed, B. M. Grainger, A. R. Sparacino, R. J. Kerestes, and M. J. Korytowski, “Advancements in medium voltage DC architecture development with applications for powering electric vehicle charging stations,” in Proc. IEEE Energytech, May 2012, pp. 1–8. T. Ericsen, “The ship power electronic revolution: Issues and answers,” in Proc. 55th IEEE Petroleum and Chemical Industry Technical Conference (PCIC), Sept. 2008, pp. 1–11. I.-Y. Chung, W. Liu, K. Schoder, and D. A. Cartes, “Integration of a bi-directional DC-DC converter model into a real-time system simulation of a shipboard medium voltage DC system,” Electr. Power Syst. Res., vol. 81, no. 4, pp. 1051–1059, Apr. 2011. H.-M. Chou, F. A. Ituzaro, and K. L. Butler-Purry, “A PC-based test bed for NG IPS for ships in PSCAD,” in Proc. IEEE Electric Ship Technologies Symp. (ESTS), Apr. 2011, pp. 135–142. I. C. Evans and R. Limpaecher, “High power clean DC bus generation using AC-link AC to DC power voltage conversion, DC regulation, and galvanic isolation,” in Proc. IEEE Electric Ship Technologies Symp. (ESTS), Apr. 2009, pp. 290–301. Natural Resources Committee. (2011, Mar. 29). Reversing President Obama’s Offshore Moratorium Act (H.R. 1231) [Online]. Available: http://naturalresources.house.gov/

Biographies Gregory F. Reed is with the Swanson School of Engineering at the University of Pittsburgh, Pennsylvania. Brandon M. Grainger is with the Swanson School of Engineering at the University of Pittsburgh, Pennsylvania. Adam R. Sparacino is with the Swanson School of Engineering at the University of Pittsburgh, Pennsylvania. Zhi-Hong Mao is with the Swanson School of Engineerp&e ing at the University of Pittsburgh, Pennsylvania.
