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�Abstract— A critical aspect of solar photovoltaic (PV) systems

integration is the power electronics interface technologies for DC power conversion. Successful integration of PV systems at all levels requires reliable, low-cost, and efficient interface capabilities. The research work for this paper presents a novel PV architecture and the development of a low voltage full bridge isolated boost converter prototype DC converter design, that is suited for solar PV integration, and provides a foundation for future advances in the areas of operational efficiency, reduced switching losses, and improved conversion ratios that the team plans to investigate. The design topology is established for future development to medium voltage applications, especially with respect to larger PV installations. The prototype converter was successfully designed, constructed, and tested for operation. In addition to the prototype converter development, the work includes a simulation model for design and test verification. The model will be utilized for analysis of future developments aimed at optimization methods and advances to the initial prototype design and to be implemented in a full scale medium voltage PV system.

Index Terms-- Solar Photovoltaics (PV), Power Electronics Conversion, Full Bridge Isolated Boost Converter, Medium Voltage DC (MVDC)

I. INTRODUCTION

n order to realize higher penetrations of renewable energy resources throughout the entire energy chain, from

distributed micro-generation levels up to utility scale, it is imperative to advance the state-of-the-art of integrated solar photovoltaic (PV) systems for increased overall efficiency. One critical aspect of PV systems integration is the power electronics interface technologies for DC power conversion. Successful integration of PV systems at all levels requires reliable, low-cost, and efficient interface capabilities. By developing a low voltage full bridge isolated boost converter prototype for testing of optimization principals related to improved efficiency, reduced switching losses, and improved conversion ratios in the power electronics interface, a basis is established for evaluating improvements in converter design topologies. A review of the trends and current state-of-the-art in PV system converter technologies is provided as a basis and motivation for the research and prototype development, along with an overview of the rapid expansion and importance of

G.F. Reed, L. A. Solomon III, and B.M. Grainger are with the Department of Electrical & Computer Engineering and the Power & Energy Initiative, in the Swanson School of Engineering at the University of Pittsburgh, Pittsburgh, PA 15210 USA (e-mails: reed5@pitt.edu, lss17@pitt.edu,bmg10@pitt.edu).

DC power systems at all levels of the power network and in specific reference to the expansion of PV and other renewable energy integration. Details on the research conducted and the development of the prototype of a full bridge isolated boost converter are presented and discussed, along with applications for medium voltage DC (MVDC) integrations. Future research initiatives will concentrate on continued testing, optimization, and higher capacity designs of the successfully developed prototype.

II. TRENDS IN INVERTER TECHNOLOGY WITH INCREASING PHOTOVOLTAIC SYSTEM INTEREST

The National Renewable Energy Laboratory (NREL) plays a key role in implementing the goals established by the U.S. Department of Energy’s Solar Energy Technologies Program. One major outcome of the program is to reduce the normalized energy cost for photovoltaic (PV) systems to $0.06/kWh by 2020 [1]. To meet the goals of the US government and sustain the PV market over a long term period, government support for inverter research and development is likely to be necessary.

A. Experience with Inverter Technologies With the recently passed energy bill, approximately $400

million worth of solar incentives exist, and which includes a 30% investment tax credit for both residential homeowners and commercial building owners [1]. In the residential and small commercial sector, trends have shown decreases in 3kW installations or less, with increases in 10-30kW installations.

As with any technology, inverters have evolved significantly through manufacturing innovations and technology improvements. During the 1980s, inverters were difficult to install and saw efficiencies within the 85-90% range. The 1990s saw the first large-scale production of PV inverters with the first PV string inverter arriving around 1995. By the late 1990s, transformer-less and high frequency designs reached efficiencies above 95% with reliability improvements allowing warranties to be offered for up to two to even five years [1].

Through the start of the 21st century, inverters have increased in size (larger than 2kW), sophistication (data logging, communications, and diagnostics), and added capability. The master-slave configuration for improved efficiency, discussed briefly in [1], is a concept that the research team plans to investigate further and is discussed later in this work with applications to DC infrastructures. The “master” inverter essentially controls the operation of the remaining inverters, referred to as “slaves”, in such a way that the “slave” inverters only contribute to system demands when

Prototype Development of a Full-Bridge Isolated Boost Converter for Solar Photovoltaic Systems Integration

Gregory F. Reed, Member, IEEE; Luke A. Solomon III, Student Member, IEEE;Brandon M. Grainger, Student Member, IEEE

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the power produced by the modules is sufficient. Transformer-less topology based inverter designs, popular in Germany and Japan, are becoming attractive in the research environment for reasons of reducing the inverter weight and losses.

Inverters generally need to be replaced every five to ten years. The task of designing an inverter to last for more than ten years before being replaced appears very unlikely with the current state of various technologies. If consumers, who are increasingly investing in PV systems, are not aware of this issue and are faced with replacing or repairing an inverter (which is at most 20% of the initial cost investment of a PV system), discontent from this significant expense could turn many away from the market and potentially harm the image that comes with photovoltaic advancements [1].

The reliability and cost of the power electronic units are highly correlated. When asked if twenty year reliability would be achieved, Sustainable Energy Technologies stated “A 20-year lifetime for PV inverters is at least 10 years away”. Mitsubishi commented that “A 20-plus-year life for inverters is impossible. Some parts of the inverters would need to be replaced over such an extended period….” and GE Energy mentioned “A 20-year life would not be practical without a significant impact on cost. A 15-year life is more reasonable…” [1].

B. Challenges for Achieving Future Targets The challenges that PV inverters face are in the areas of

manufacturing, design/technology, regulation, and policy. For this work, the design/technology and regulation of the inverters are addressed.

1) Design / Technology Impacts on PV Inverters

Semiconductor Technology: The switches currently being used to perform the DC to AC conversion contribute to the reliability state of the inverters. The reliability of the switches is inadequate for PV inverters. SiC switches show promise for better switching performance but the technology is currently not cost effective for mass production.

Experimentation with Inverter Topologies: Efficiency, cost, and reliability of the inverter are all dependent on the topology that is used for the power conversion. There are always tradeoffs associated with designs. For example, the tradeoff of switching at higher frequencies is its impact on switching loss magnitudes. Certain topologies are better than others and can improve inverters with respect to efficiency, cost, and reliability. The difficulty from a development perspective is that inverter manufacturers in general do not have the resources to experiment with a wide range of topologies.

Capacitors in PV Systems: Capacitors impact the reliability more severely, as compared to any other component in the inverter due to their temperature sensitivity. Research and development in the capacitor industry is limited and is not a high priority, mainly since capacitors used in PV systems correspond to a small sector of the capacitor market. Inverter manufacturers can normally get around this issue in the design phase, but this route impacts cost.

2) Regulation

The regulations for grid-connected PV inverters vary between the European nations and the United States. For this reason, regional markets exist worldwide because manufacturers cannot develop a worldwide inverter module. In fact, due to these regulatory differences between countries, inverter manufacturers are required to modify components which increases costs (different safety requirements, interconnection strategies, and testing requirements) [1].

The installation cost differs between the United States and European nations. For example, the cost to install a 5 kW system, as stated in [1], is about $800/kW in Germany and $1,400/kW in the United States. This is because the US requires accessible AC disconnects and conduits for DC cabling.

III. A NEW ERA OF DC POWER SYSTEMS

The previous section provided a brief overview of the challenges and expectations of the inverter manufacturers to meet the demands placed on the industry by the United States government. Without question, inverters will play a key role in the future electric grid infrastructure as renewable energy resources penetrate the system at higher percentages because of the AC to DC power conversion that will be required.

Corporate research centers and universities around the country are beginning to 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 in end-user equipment. Computers, televisions, and other appliances use low-voltage DC by means of a single-phase rectifier followed by a DC voltage regulator [2]. Electrical energy production from some renewable energy resources, including solar, is at DC. In order to achieve the desired DC magnitude, multiple conversions may need to take place. As pointed out in [2], the use of DC distribution systems would mitigate the need for multiple conversions, offering savings and higher reliability due to the decreased number of components used in the system. DC systems also have lower losses and voltage drops in lines.

As was the case at the turn of the 20th century, a justification in using AC compared to DC will need to be evaluated as the current electric grid begins to make drastic changes in the United States. In 2006, EPRI published a presentation that established a few points that made the case for DC applications in the 21st century and are summarized below [3]:

� Era of Electronics – equipment is increasingly being operated with DC, hence, requiring an AC to DC conversion

� Era of the Micro-Grid – distributed generation systems (photovoltaic systems for example) produce DC power

� Era of Electronics in Transportation – DC power could help power hybrid vehicles and commercial fleets

� Era of Information Technology – DC power could enhance energy efficiency in data centers

� Integration Ease – storage devices (batteries, capacitors, etc) deliver DC power

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The utilization of power electronics technologies used to integrate renewable energy resources along with the concept of a micro-grid are often found coupled together in the literature. A micro-grid is a small power system consisting of several micro-resources (photovoltaic systems, wind turbines, fuel cells, battery storage units, etc) that provides heat and electricity to local users. The benefit of the micro-grid is that the system can operate while being disconnected from the grid. A practical micro-grid would act as a plug-and-play type assembly allowing renewable energy resources to connect to the system without altering the overall controllability of the resources or protection schemes [4]. A DC micro-grid is shown pictorially in Figure 1.

Fig. 1. Micro-Grid Supported with DC Busbar [3]

IV. DIRECTED MVDC PHOTOVOLTAIC RESEARCH AND PRELIMINARY CONVERTER DESIGN

Traditionally, photovoltaic panels have been integrated into the grid using small scale DC to AC inverters that were applied to either a small group of panels or even a single panel. Figure 2 illustrates the simplified AC collector topology for photovoltaic (PV) panels. There are several advantages for applying a dedicated inverter to each PV panel including the ability to implement local maximum power point tracking (MPPT) control at every panel. This allows each converter to be operated at the most desirable point depending on the real time conditions. However, the traditional topology illustrated in Figure 2 may not allow for an optimal configuration in terms of power electronic converter efficiency. It is this reason that medium voltage DC (MVDC) collector circuit architecture and other architectures should be thoroughly investigated.

Fig. 2. Traditional AC Collector Circuit for Photovoltaic Panels

A. MVDC Architecture Options for Integrating PV Systems For several reasons outlined in previous sections of this

paper, it is becoming more interesting to investigate the application of MVDC collector circuits for larger installations of photovoltaic panels.

Reference [5] provides a couple of configurations used for integrating multiple wind turbines that could possibly be adapted to PV panels since the generation source is the only item that changes. These architectures are provided in Figure 3.

(a) (b)

(c) Fig. 3. DC Collector Architectures [5]

Each of the collector architectures has distinct advantages and disadvantages. Collector (a), in Figure 3, has the advantage of using low voltage turbines but, with more power conversion stages, results in a system of lower efficiency and increased cost. Collector (b) provides the advantage of using fewer converters and, therefore, less power electronic components. Because fewer components at each wind turbine are used with this configuration, maintenance at each wind turbine is minimized and is mainly needed at the collection point. The drawback of this configuration is that medium voltage turbines must be used because rectifiers are only present. Collector (c) is a series connection compared to the parallel collector labeled as (a) [5].

Fig. 4. MVDC Collector Architecture

Figure 4 provides architecture adapted from Figure 3 that could potentially be used for photovoltaic applications. The idea is to move away from an inverter at each panel and put a DC/DC converter at each panel followed by a centralized inverter because of efficiency reasons. Other reasons for

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supporting this architecture, but not limited to the following, include the inherent DC source generation in solar panels and better transmission path performance with DC signals. The voltage can then be inverted back to AC before being injected into the grid.

B. Preliminary DC/DC Converter Design for PV Architecture Solar panels typically produce low voltage DC power. One

method of interfacing the solar with a DC collector circuit is to make use of a DC/DC converter that will boost the low voltage from the solar panel to a higher DC voltage to make it more suitable for DC distribution. Several DC/DC converter topologies exist that perform a step-up function on the input voltage. Some of the more notable DC/DC converter topologies include the boost, buck-boost, Cuk, and SEPIC converters [6]. Ideal models of these converters boast conversion ratios approaching infinity. However, due to the loss mechanisms in the circuit including switching losses and conduction losses associated with the semiconductor devices and resistive losses associated with inductors and capacitors, the conversion ratio of the converters are quite small. Even if a converter is designed with drastically oversized semiconductor components in an attempt to minimize losses associated with the devices, a conversion ratio of 8 is difficult to achieve in a non-isolated design.

In order to achieve a conversion ratio of greater than 10, it is often necessary to make use of a transformer-isolated power electronic converter. The turns ratio of the transformer in the circuit can be exploited to produce nearly any desirable conversion ratio. An isolated design gives several other distinct advantages over non-isolated designs including galvanic isolation between the input and output stage and arbitrary determination of the sign of the output voltage with respect to the input voltage using the transformer polarity.

A full-bridge isolated boost converter was chosen as the converter topology to be used in conjunction with the power electronic converter. Figure 5 illustrates the full bridge isolated converter topology. Note that the only non-ideal component of the converter shown on the figure is the magnetizing inductance of the transformer. Other components of power electronic converters that should be investigated when designing to eliminate switching loss include the substrate capacitances of the power MOSFETS and the leakage inductance associated with the transformer. These components establish resonant networks in the converter and can be the culprit of component failure. Remaining details are beyond the focus of this work.

Fig. 5. Full Bridge Isolated Boost Converter

A full bridge topology was selected because it is well suited for higher power applications due to the four active semiconductor switches in the design. The various ideal

waveforms associated with this topology are shown in Figure 6. One can deduce from the conduction sequence that the four MOSFETs and the two diodes are equally utilized in the converter.

Fig. 6. Full Bridge Isolated Boost Converter Ideal Waveforms [6]

An ideal model of the converter was modeled using the MATLAB/Simulink PLECS library. Figure 7 illustrates the top level hierarchy of the MATLAB/Simulink model of the full bridge isolated converter. The simulation was primarily used for two tasks. First, the ideal modulation scheme was modeled and tested to verify system behavior. Secondly, the available filter capacitor and inductor values were used in a parameter sweep that was conducted using the model.

Fig. 7. Simulink PLECS Model of Isolated DC/DC Converter

A switching frequency of 10kHz was chosen for several reasons. There were several high frequency transformers commercially available, in stock that fit this range, and within budget. Next, a thermal simulation revealed that no additional cooling such as the addition of a heat sink with forced air cooling was required to operate the converter at 10kHz. The 10kHz switching frequency was in the range of the microcontroller chosen as a controller for the system. Lastly, both the voltage and current ripple was less than 1% with reasonable inductor and capacitor values when operating the converter at 10kHz. As one can see from the voltage and

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current waveforms illustrated in Figure 8, the voltage and current ripple are negligible for the purposes of this prototype. An open loop control scheme was initially implemented in the simulation model to build and verify the modulator. A symmetrical conduction sequence was chosen in order to ensure losses were distributed evenly among all semiconductor devices. For the sake of time constraints, an existing sine / triangle modulation scheme was implemented in the PLECS model.

Fig. 8. Voltage (V) and Current (A) versus Time (secs) for a 100W Load

A small scale 100W full bridge converter was designed and built to serve as a test bed for future work. The converter was designed to accept 12VDC from a small solar panel and produce an output voltage of 150VDC under various load conditions. A relatively large filter inductance of 220µH was chosen to minimize current ripple. Similarly, a large filter capacitance of 25µF was chosen to minimize voltage ripple. IRF2903 power MOSFETs, rated for 75A and 30V, were chosen due to their low on resistance and fast switching capabilities. The IRF16YQ150C power diodes are rated for 3A and 150V.

A dsPIC33FJ16GS502 16-bit digital signal controller from Microchip was chosen as the converter controller because of its multiple PWM output channels and ease of programming. A voltage divider network coupled with an OP amp based voltage follower was used to provide output voltage feedback to the controller. A LEM closed loop hall effect sensor was used to provide current feedback to the controller. A printed circuit board was laid out using the EAGLE software package. Gerber files were exported from the PCB model and the files were sent to a board house for fabrication. The PCB board was then populated by hand and tested. A picture of the converter is shown in Figure 9.

Fig. 9. Preliminary Design of Full Bridge Isolated Boost Converter

Figure 10 provides two gate drive signals for two of the four semiconductor devices. The top signal and bottom signal reflect a duty cycle of 75% and 50%, respectively. Although these concepts are general knowledge, this snapshot of the oscilloscope provides evidence that the microcontroller is programmed and is performing as expected for this test condition.

Fig. 10. Two of Four Gate Drive Signals (5V/div on Y-Axis);10kHz switching

Figure 11 displays the output voltage of our converter. Strong agreement exists between the simulation results shown in Figure 8 with the true output voltage of the physical converter. The output current could not be measured because, in the preliminary design, a current sensor was not considered and will need to be considered in future modifications.

Fig. 11. Output Voltage of Converter (40V/div on Y-Axis)

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V. CONCLUSIONS AND FUTURE RESEARCH INITIATIVES

A low voltage full bridge isolated boost converter prototype has been successful designed, constructed and tested. The objectives of the work are in establishing a basis for evaluating optimization principals related to improved efficiency, reduced switching losses, and improved conversion ratios in the power electronics interface of PV systems, and for evaluating improvements in overall converter design topologies. The development of the system architecture, the novel idea behind this work, and prototype converter is focused on applications for medium voltage DC (MVDC) integrations and for advancement on larger scale PV installations. Future research initiatives will concentrate on continued testing, optimization, and higher capacity designs of the successfully developed prototype, which will be utilized in a PV system field demonstration at the medium voltage level.

The objectives of future research initiatives related to this work are to develop the power electronic architecture in several areas. The first area of investigation will be the analysis of multiple DC/DC converters with DC/AC converters connected in parallel. This scenario presents several challenges in terms of control system architecture and system stability in MVDC applications. Traditional architectures include master/ slave relationships between all converter local controllers. More advanced control strategies, including but not limited to, embedded optimal control and game theory algorithms can be applied to the supervisory control problem. The local control strategies for the individual converters need to be designed using a closed loop algorithm to control both the voltage and current outputs, while still having the capability to interface with remaining converter controllers in the PV farm.

The second area of investigation will be to develop and perform a full thermal circuit analysis to appropriately design the thermal management system. This thermal management system is required in order to demonstrate the technology at more relevant power levels.

The third area of investigation will be to evaluate other DC/DC converter topologies other than the full-bridge isolated boost converter with regards to system efficiency. Visions exist for building multiple sets of DC/DC converters and inverters and evaluating various combinations, with the developed control architectures, to determine which converter topologies, when connected in parallel, establish optimal system efficiency for MVDC PV applications.

VI. REFERENCES

[1] P. Denholm and R. Margolis, "Very Large-Scale Deployment of Grid-Connected Solar Photovoltaics in the United States: Challenges and Opportunities," NREL, Conf. Paper. CP-620-39683, April 2006. [Online]. Available: http://www.nrel.gov/pv/pdfs/39683.pdf

[2] D. Nilsson, "DC Distribution Systems," Ph.D. dissertation, Department. of Energy and Environment, Chalmers University of Technology, 2005.

[3] C. Gellings, “Are We at the Threshold of a New Era of DC Power Systems?,” EPRI, Palo Alto, CA, June 2006. [Online]. Available: http://www.totalenergycompany.com/pdf/articles/Gellings.pdf

[4] P.J. Binduhewa, A.C. Renfrew, and M. Barnes, “MicroGrid Power Electronics Interface for Photovoltaics,” in 4th IET Conference on Power Electronics, Machines and Drives, 2008., pp. 260-264.

[5] C. Meyer, “Key Components for Future Offshore DC Grids,” Ph.D. dissertation, Rheinisch-Westfallischen Technischen Hochule Aachen, Germany, September 2007, pp. 94-95.

[6] R.W. Erickson, D. Maksimovic, Fundamentals of Power Electronics, 2nd

ed., Kluwer Academic Publishers, 2001.

VII. BIOGRAPHIES

Gregory F. Reed (M’1985) received his B.S. in Electrical Engineering from Gannon University, Erie PA; his M. Eng. in Electric Power Engineering from Rensselaer Polytechnic Institute, Troy NY; and his Ph.D. in Electrical Engineering from the University of Pittsburgh, Pittsburgh PA. He is the director of the power and energy initiative in the Swanson School of Engineering and associate professor in the Electrical and Computer Engineering Department at the University of

Pittsburgh. He also serves as the IEEE PES Vice President of Membership & Image. He has 25 years of electric power industry experience, including utility, manufacturing, and consulting at Consolidated Edison Co. of NY, Mitsubishi Electric, and KEMA Inc.

Luke A. Solomon III (S’2006) received his B.S. in Electrical Engineering from The Pennsylvania State University, State College PA; and his M.S. in Electrical and Computer Engineering from The Georgia Institute of Technology. Luke has over 6 years of experience focused in real time control systems engineering and power systems analysis. His work includes the implementation of 3-level modulation schemes in a real time control system

and thermal circuit reduction to reduce simulation time. He is currently pursuing a Ph.D. in Electrical Engineering with a concentration in power electronics.

Brandon M. Grainger (S’2006) was born in Pittsburgh, Pennsylvania. Currently, he is finishing his Master’s degree in electrical engineering from the University of Pittsburgh with a concentration in electric power engineering and plans to further pursue his Ph.D. in electrical engineering specializing in high and medium voltage power electronic applications. From April 2008 to April

2009, Brandon interned for Mitsubishi Electric Power Products, Inc and, during the summer of 2010, with ABB Corporate Research Center in Raleigh, NC. Brandon’s research interests are in power electronic technologies including HVDC and FACTS devices, power electronic converter design, and power electronic applications suitable for renewable integration. He is a student member of the IEEE, Power & Energy Society, and Power Electronics Society.