Hybrid Renewable Energy and Microgrid Research ... - IEEE Xplore

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Abstract— The National Renewable Energy Laboratory. (NREL) conducts a variety of research in the area of hybrid power system integration and microgrids.
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Hybrid Renewable Energy and Microgrid Research Work at NREL Benjamin Kroposki, Senior Member, IEEE, and Gregory Martin, Member, IEEE

Abstract— The National Renewable Energy Laboratory (NREL) conducts a variety of research in the area of hybrid power system integration and microgrids. NREL is studying hybrid (stand-alone) power systems incorporating photovoltaic power in the United States and assessing the characteristics and status of government-installed hybrid systems greater than 2 kW over the last 30 years. This research provides information to the community on hybrid power systems, presents lessons learned from operational experience, and provides analysis of challenges and success of the assessed systems. NREL also supports the development of IEEE P1547.4 Draft Guide for Design, Operation, and Integration of Distributed Resource Island Systems with Electric Power System as part of the further development of IEEE 1547-2003 Standard for Interconnecting Distributed Resources with Electric Power Systems. This standard is summarized here, providing alternative approaches and good practices for the design, operation, and integration of microgrids. Index Terms— Standards, interconnected power systems, power distribution, solar power generation, photovoltaic power systems, hybrid, microgrid.

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I. INTRODUCTION

FF-GRID photovoltaic (PV) hybrid power systems supply energy to loads in autonomous, islanded power systems when utility grid connection is not available. Microgrids are sections of an electric power system (EPS) that can operate connected to or disconnected from the utility system (islanded). Microgrid systems have a design that allows repeated, automatic connection and disconnection to and from the utility system. NREL is supporting development of interconnect standards for microgrids (IEEE P1547.4 and IEEE 1547-2003). In addition to utility operators and industrial users, these standards are also potentially important to stand-alone hybrid power systems that may connect to a utility EPS, or may one day do so. Adoption of standards, technologies, and components common to both types of systems can reduce cost and leverage more standardized components across two types of power system installations. In parallel to microgrid standards development, NREL is researching hybrid PV power systems to document and analyze information about existing systems. Solar hybrid systems are typically found in the United States in remote areas that have no grid power connection, have adequate sunshine, and are difficult to supply with fuel. Hybrid PV systems often offer a cost-competitive method of reducing expensive fuel deliveries and generally undesirable emissions. Many PV hybrid systems can operate with or without grid 978-1-4244-6551-4/10/$26.00 ©2010 IEEE

power, making them a versatile and reliable source of electrical power. In the United States, most of these systems are government-run installations. The National Park Service and the Armed Forces own the vast majority of U.S. PV hybrid systems. II. HYBRID SOLAR POWER SYSTEMS An assessment of solar PV hybrid electric power systems in the U.S. is underway at NREL. Typical applications of these systems include remote park stations and visitor centers, military base power, data collection sites, off-grid homesteads, telecommunications sites and portable power. It is important that experience and knowledge gained from over 30 years of PV hybrid system operational experience is used to guide future developments and designs. Nearly all PV hybrid systems include the following components: • Solar photovoltaic electric power source • Conventional generator (diesel or propane fueled) • Battery energy storage • Inverter, including system control and management • AC load demand These components are investigated within a variety of U.S. PV hybrid systems having ratings greater than 2 kW. A listing of such PV hybrid systems is presented below, followed by analysis of technical challenges and lessons learned from several of the systems. An in-depth case study of the PV hybrid system at Dangling Rope Marina, Utah is presented. More detailed data and maintenance experiences are given to provide a more complete picture of a successful system. A. Listing of PV Hybrid Systems in the US The list of PV hybrid systems given here includes all offgrid PV hybrid installations in the U.S. greater than 2 kW. This list is given in Table 1, along with brief site information. Results show that 68% of systems are operated by The National Park Service (this includes the Fish and Wildlife Service (FWS)), representing 25% of installed PV power. The US Armed Forces operate 40% of the systems, though these comprise 70% of the installed PV power. Information was gathered by contacting people and organizations and holding interviews with experienced and informed affiliates of each system. The quantity and quality of the available data varies greatly among systems in the list. The standard data collected for each installation includes: • Owner • Designer / installer company

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• Install date • Peak PV power • Generator power and type • Inverter power and topology • Battery energy • Load energy and power • Location • Major maintenance and replacements Whenever available, additional maintenance information was recorded and technical obstacles and challenges were discussed and recorded. TABLE I LIST OF US PV HYBRID SYSTEMS Ow ner

Site

DOD DOD DOD NPS DOD DOD DOD DOD DOD NPS FWS NPS TEL NPS FWS NPS NPS NPS FWS FWS NPS NPS NPS FWS NPS PRIV NPS FWS DOD NPS NPS NPS NPS NPS NPS MREO NPS

China Lake - Superior Valley CA China Lake - Kim Site CA China Lake - Nato Site CA UT Dangling Rope Marina China Lake - Junction Ranch CA Channel Islands - Santa Cruz CA San Clemente Island CA Grasm ere Point, Mountain Hom e ID China Lake EOD Darw in Site CA Natural Bridges National Monument UT Farallon NWR CA Channel Islands - Santa Barbara CA Carol Springs Mountain AZ Mojave NP - Joshua Tree CA Missisquoi NWR VT Dry Tortugas NP FL Channel Islands - Santa Cruz CA Roger’s Peak CA Brazoria NWR TX Bosque del Apache NWR NM Channel Islands - Santa Rosa CA North Manitou Island MI Mojave National Preserve - Hole-InCA Imperial NWR AZ North Cascades NP WA North Dumpling Island CT Pinnacles CA Haw aiian, and Pacific Islands NWR/ Te HI Savannah (Off-shore) GA Hozomeen WA Channel Islands - Anacapa CA AuSable Lake Station MI Channel Islands - San Miguel CA Grand Canyon - Parashant AZ Sullivan's Cabin Pictured rocks MI Manzanita CA Volcanoes National Park HI

US State

PV (kWp) 344 239 162 160 126 120 94 77 50 50 45 29.5 25 21 15 14.4 13.6 12.8 12.5 12 11.4 11 10.8 10 10 10 9.6 9 7.5 7 5.8 5 4 3 2 1.2 0.9

B. Technical Challenges and Lessons Learned Although comprehensive records of system shut downs, service calls, trip events and failures are not attainable in many cases, certain maintenance themes emerge from the research. There are essentially four components requiring maintenance; the PV array, the generator, the inverter and the batteries. Generator maintenance was a concern in many of the interviews. Typical, regular diesel engine or propane engine maintenance is, of course, necessary for all systems. Several of the interviews reported additional problems with down time of generators. This creates an issue with keeping the system running from PV and battery alone during generator

maintenance; most systems are not sized for this. The inverter is a complicated component of the system. Not only is it responsible for electricity conversion between DC and AC, but it controls and manages the overall system, making decisions to charge the battery and when to run the generator. The inverter contains the system control electronics, stores system parameters and settings, and provides the operator and programming interfaces. Unplanned service actions associated with the inverters were found to be common and a source of worry for system operators. Much of this worry stems from the difficulty in getting manufacturer technicians on-site for service requests. Further, there are a small number of hybrid inverter manufacturers in existence, and this number has dwindled in recent years. Typical inverter failures involve the main circuit board, semiconductor switches (IGBTs), and capacitors. Sometimes parameter settings in the inverter must be changed or tuned, a task difficult for most site maintenance managers. Battery replacements are an expensive maintenance undertaking which occur at least once or twice during the service lifetime of the system. Replacement intervals range from 3 to 10+ years, depending on battery type, battery utilization, and environmental conditions. All assessed systems used lead-based batteries, due to cost and availability. A lead-based battery that lasts 10 years under normal, regular use is generally considered to have met or exceeded its lifetime expectancy. When batteries are oversized above their expected throughput and power, they tend to last longer. Jostling during handling of batteries in transport and installation can affect battery lifetime in some products. The photovoltaic array itself requires almost no maintenance or regular replacement. All PV panels experience degradation, but typically remain in usable service for over 20 years. In one case, an aging PV array was replaced with new panels having twice the efficiency of the originals. The installer had only to install half the original number of panels to realize the same power output. Given the large number of aging systems in this study, upgrading the panels, when the time comes, can be a major improvement to the power system, greatly reducing fuel consumption. C. A Case Study: Dangling Rope Marina, Utah Dangling Rope Marina is a very remote site on the central bank of Lake Powell in southern Utah. The marina exists to refuel boats using Lake Powell and to provide a park visitor’s center and refreshment stop. The marina boasts a PV hybrid system with a 135 kW (peak) solar array, a 250 kVA Kenetech (Trace) hybrid inverter, two 250 kVA propane fueled generators, one 230 kVA diesel fueled generator, and a 2900 kWh lead acid battery bank. This power system provides electricity to the fueling docks, the visitor center, crew housing, and to refrigeration barges. The Dangling Rope power system has operated successfully since its installation in 1996 [1], and continues supplying power today. However, there have been some technical hurdles and maintenance required along the way. The operations manager at Dangling Rope provided valuable information from his experience

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operating and troubleshooting the system. The Dangling Rope system typically experiences a monthly load energy demand of 30 MWh, with loads peaking around 100 kW. Generally, the PV array produces more energy than the generators, though in some months the generators produce nearly the same amount of energy as the solar array. One important finding is the fact that the two 250 kW propane-fueled generators cannot supply large transient loads on the system. The design and control of these generators causes them to trip off-line when subjected to the transient loads caused by refrigeration barges at Dangling Rope. For times when demand is high and the batteries are low, a third 230 kW diesel-fueled generator can be manually dispatched to meet the load demand. The diesel-fueled generator is capable of supplying the transient load without shutting down. The original lead-acid batteries installed with the Dangling Rope system were replaced in 2003. Battery utilization has generally been lower than the rated battery system energy (2900 kWh) during the off season (November – March), though the system load demand has increased since the initial installation. Batteries are exercised closer to their rated capacity during the peak months to meet peak load demands, mainly the large transient loads from the refrigeration barges. It is possible that periodic underutilization led to a longer lifetime of the batteries. Figure 2 shows an image of the newly-installed battery system at Dangling Rope.

service, however he has difficulty performing some of the more complicated tasks to diagnose and repair the inverter. The PV array at Dangling Rope has been functional since its installation in 1996. Although its output power capacity continues to degrade, sufficient power is produced to charge the batteries. Dangling Rope operators are considering replacing the PV array in the next few years.

Fig 4. Photographic image of the Dangling Rope solar array (2009).

The hybrid PV power system at Dangling Rope has been a successfully operating system since its installation 13 years ago. Operation of the system to meet transient and seasonal loads results in over 50% of its energy coming from PV power. The major maintenance and operational issues of note include propane fueled generators incapable of supplying load transients, an aging inverter causing occasional “bugs”, and a newly replaced battery. III. MICROGRIDS

Fig. 2. Photographic image of the new Dangling Rope battery system (2009).

The hybrid inverter system that converts power and manages power flow in the system is a Kenetech HY-250. The inverter has been in service since 1996, but has experienced several unplanned service events to reset components, adjust parameters, and replace capacitors. There have been no major or catastrophic failures of the unit, though it produces more unplanned service events than any of the other equipment. Service is complicated by the fact that the inverter manufacturing company responsible for the inverter design has reorganized two times since the original installation. Kenetech Windpower changed to Trace Inverters, which is now Xantrex. The maintenance manager at Dangling Rope commented that Xantrex provides good phone support for

A. Technical Overview Microgrids are electrical systems capable of intentional islanding (operating autonomously), or operating connected to a utility electric power system. They can be formed either at a customer facility or location that includes parts of the local utility distribution system, and have at least one distributed energy resource (DER) and associated loads. DER can be either distributed generation (DG) or distributed storage (DS) and are often both used to provide energy within the microgrid. In a microgrid, the loads and energy sources can be disconnected from and reconnected to the utility with minimal disruption to the local loads. Microgrids provide many potential benefits to customers including improving reliability by providing power to the islanded portion of the electric power system (EPS) during a utility outage and resolving power-quality issues by reducing total harmonic distortion at the loads. There are also utility benefits such as resolving overload problems by removing load from the EPS by allowing a part of the EPS to intentionally island. This facilitates maintenance on the utility system while intentionally islanded customers still remain powered.

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Any time a microgrid is implemented in an electrical distribution system, it must be well-planned to avoid problems. For a microgrid to function properly, a switch must open and the DER must be able to carry the load on the islanded section. This includes maintaining suitable voltage and frequency levels for all islanded loads. Depending on the switch technology, momentary interruptions may occur during transfer from grid-parallel to islanded mode. If power is lost, the DER assigned to provide power to the intentional island should be able to restart and pick up the island load after the switch has opened. Power flow analysis of island scenarios should be performed to ensure that proper voltage regulation is maintained and establish that the DER can handle inrush currents from large loads. The DER must be able to loadfollow during islanded operation and sense if a fault current has occurred downstream of the switch location. When power is restored on the utility side, the switch must not close unless the utility and islanded portions are in synchronization. B. Microgrid Standards IEEE Standards Coordinating Committee 21 (SCC21) is developing IEEE P1547.4 Draft Guide for Design, Operation, and Integration of Distributed Resource Island Systems with Electric Power System as part of the further development of IEEE 1547-2003 Standard for Interconnecting Distributed Resources with Electric Power Systems. Currently in final draft form, this document will cover microgrids and intentional islands that contain DER connected with utility electric power systems. It provides alternative approaches and good practices for the design, operation, and integration of the microgrids and covers the ability to separate from and reconnect to part of the utility while providing power to the islanded electric power system. The guide covers the DER, interconnection systems, and participating EPSs. It is intended to be used by EPS designers, operators, system integrators, and equipment manufacturers when planning and operating microgrids. Its implementation will expand the benefits of DER by enabling improved EPS reliability and building on the requirements of IEEE 1547-2003. IEEE 1547.4 covers key considerations for planning, designing and operating microgrids. The topics include: • impacts of voltage • frequency • power quality • inclusion of single point of common coupling (PCC) • multiple PCC • protection schemes and modifications • monitoring • information exchange and control • understanding load requirements of the customer • knowing the characteristics of the DER • identifying steady state and transient conditions • understanding interactions between machines • reserve margins • load shedding • demand response

• cold load pickup • additional equipment requirements • additional functionality associated with inverters Clear standards that have been developed and approved by stakeholders are important to define the role of DER in microgrids and utility systems, and to define microgrid behavior. These standards provide a common platform or template for planners, designers, installers, and operators to work from. In addition, standards developed for microgrids are likely to be applicable to other types of systems such as commercial or residential hybrid systems that have the capability to connect to the utility grid. IV. ACKNOWLEDGMENT The authors would like to acknowledge the IEEE 1547.4 Working Group, and the maintenance manager at Dangling Rope Marina, Gary Heinze. V. REFERENCES [1]

A. L. Rosenthal, “Dangling Rope Marina: A Photovoltaic Hybrid Power System”, Quarterly Highlights of Sandia’s Photovoltaic Program, Vol 1, April 1998.

VI. BIOGRAPHIES Benjamin Kroposki (S’90, M’93, SM’00) received his BS and MS in electrical engineering from Virginia Tech, Blacksburg, VA and PhD from the Colorado School of Mines in Golden, CO. Dr. Kroposki manages the Distributed Energy Systems Integration group at NREL and serves as chairman for IEEE P1547.4 Guide for Design, Operation, and Integration of Distributed Resource Island Systems with Electric Power Systems. Gregory Martin (S’00, M’07) received his BS in electrical engineering from the University of Wisconsin, Madison, and his MS from the University of Colorado, Boulder in 2007. Greg works as a research and design engineer at the NREL.