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AbstractSmart grid technologies allow the electric grid to better adapt to the dynamic behavior of renewable energy and distributed generation, helping both consumers and utilities to access these resources and harvest their benefits. Todays grid needs to shift from centralized supply sources to fixed, predictable loads and to accept power from the renewable and distributed energy resources all over the grid. As these resources are intermittent in nature, the grid needs integrated monitoring and control, as well as integration with substation automation, to control differing energy flows and to plan for standby capacity to absorb the intermittent generation.

The major part in a solar photovoltaic (PV) system is the conversion stage and its characteristics dominate the behavior of the solar PV system. Intelligent interfaces to a PV system that allow the grid to handle two-way flows of power and communication are required and would allow the seamless integration of solar technologies in high-penetration scenarios.

In this paper, the smart grid features are reviewed with a focus on renewable energy integration. This is followed by surveying commercially available transformerless inverters for solar PV applications and their features vis-a-vis smart grid. The description and the simulation of a proposed transformerless inverter are included in this paper.

Index TermsSmart grid, smart inverter, transformerless micro inverter, common mode voltage, leakage current.

I. INTRODUCTION

Solar photovoltaic (PV) systems are used for many applications thanks to their low maintenance requirements and zero pollution. Worldwide annual solar PV installed generating capacity has grown at an average rate of 57% over the last 10 years [1]. In 2010, the cumulative worldwide solar PV capacity reached 47 GW and represented about 0.78% of worldwide generating capacity [1]. In 2009, the renewable energy in the European Union (EU) accounted for 11.6% and 18.2% of the gross final energy consumption and the total electricity consumption, respectively [2]. Fig. 1 depicts the EU PV cumulative solar PV capacity addition between 2006 and 2010 and shows a significant capacity growth [2-3]. This rapid growth is mainly due to the decreasing costs and prices, increasing conversion efficiency, improving manufacturing

M. Bouzguenda, A. Gastli and A. H. Al Badi are with the Department of Electrical & Computer Engineering, Sultan Qaboos University, Al-Khoud 123, Oman (e-mails: buzganda@ieee.org, agastli@squ.edu.om, abadi@squ.edu.om). T. Salmi is with the Department of Electrical Engineering, National Engineering School of Sfax, P.O.B: W, 3038, Sfax, Tunisia (email: tarak_sel@yahoo.fr).

technology, economies of scale and national incentives. As shown in Fig. 2, smart inverters accounted for 20% of worldwide market in 2010. It is expected to reach 58% of the 27-GW worldwide inverters market in 2015 [3].

There are three common grid interactive PV systems: the centralized inverter system, the string inverter system and the module integrated inverter system, also known as the micro inverter system. The major part in each system is the conversion stage and its characteristics dominate the behavior of the overall solar PV system. Ongoing research activities are carried out to design intelligent interfaces that would replace the old system that relies on one-way power flow and no communications. In fact, the new interfaces allow two-way flows of power and communication, thereby guaranteeing seamless integration of solar technologies in high penetration scenarios. Following are examples of ongoing research activities:

Device and system self-protection and reliability including hardware and software progress for better balance of system (BOS) reliability and lifetimes.

Improved inverter topologies and system modeling for better hardware design and adaptive controls.

Customer friendly products: plug-and-play system connection.

Improved system communications, sensors and total system control for PV.

Improved integrated advanced concepts for micro inverters, micro grids and infrastructure support for grid stability.

In this paper, the smart grid features are reviewed with a focus on renewable energy integration. This is followed by surveying commercial micro inverters and their features vis-as-vis smart grid.

Fig.1. Cumulative photovoltaic capacity installed in the European Union in 2009 and 2010 (in MWp)

Solar Photovoltaic Inverter Requirements for Smart Grid Applications

M. Bouzguenda (IEEE S. Member), A. Gastli (IEEE S. Member), A. H. Al Badi (IEEE S. Member) and T. Salmi

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Fig.2. World Market for Standard and Smart Inverters (MW shipment) [3]

II. SMART GRID INITIATIVES

In its contemporary form, the smart grid encompasses the facilities, control systems and protocols from the electric generators to the retail customers and their usage patterns. The smart grid delivers real-time energy information and knowledgeempowers smarter energy choices and provides timely and affordable solutions. This is the role for a smart grid, which should enable the integration and optimization of more renewable energy and plug-in electric vehicles, drives significant increases in the efficiency of the network and enables consumers to manage their energy usage and save money with a minor lifestyle adjustment. Table I, shown at the bottom of paper, summarizes the value of smart grid deployment for the different stakeholders [5]. According to this table, renewable and distributed generation systems offer benefits to all stakeholders and a significant integration of such sources requires the smart grid to handle extensive distributed generation and storage resources and dynamically manage all power sources.

Meanwhile, not many electric companies have implemented their smart grid. In fact, as of March 2010, only 8 percent of utilities around the world have done so, while 37 percent have their projects underway [12].

III. SMART GRID CONNECTIONS TO RENEWABLE RESOURCES

As harnessing the natural and renewable energies of the sun, wind, hydro, geothermal, and biomass improves the sustainability of energy production and delivers benefits to the environment, their grid integration is the driver for smart grid which has the following features:

Smart grid technologies and concepts reduce barriers to the integration of renewable resources and allow power grids to support a greater percentage of variable renewable resources.

Enabling smart grid technology, such as distributed storage, demand response, advanced sensing, control software, information infrastructure, and market signals, increases the ability to influence and balance supply and demand.

With smart grid technology, grid operators can better coordinate and control the system in response to grid conditions, thus allowing integration of increasingly greater levels of renewable resources more effectively and at lower cost.

Advanced Metering Instrument (AMI) and internet-based services engage demand response and distributed storage

to accommodate higher penetration and cost-effective integration of renewable energy generation.

Smart grid technologies that support the integration of renewable resources at the distribution level include AMI, distributed storage, demand response, and distribution automation.

Advanced and automated integration systems, such as inverters and converters with communications software interfaces, enable distributed management and application integration for renewable generation.

Worldwide smart grid related research activities are being carried out to develop the right tools and technologies, in particular in the area of solar PV systems.

IV. SOLAR PV DESIGNS FOR SMART GRID INTEGRATION:STATE-OF THE ART

For seamless gird tied PV interconnections, a typical solar PV should provide two-way flows of power and communication between the smart grid and the solar PV system. At the heart of this intelligent system is the inverter. Three solar PV inverters are available which are the string, the central and the newly developed micro inverter, known also as integrated AC module inverter.

A. CENTRAL INVERTERS

The conventional solar PV installations feed DC voltage to a central inverter for conditioning and distribution locally or across the power grid. Furthermore, the DC voltage carried through the array to the central inverter may have significant fire and safety hazards, leading to increased costs for cabling and, in turn, higher costs for installation and maintenance. Therefore, to mitigate individual panel effects, solar PV designers have moved power conversion to each individual string, or set of series-connected panels in a large array.

B. STRING INVERTERS

String converters provide DC-DC conversion to enhance the power delivered to the central inverter by each string. As with string converters, string inverters offer incremental improvement in the overall array efficiency compared to conventional central inverter installations, yet still permit a single degraded panel to have an unduly large impact on overall output. This approach reduces the impact of a single poorly-performing panel to its string rather than the entire array. Therefore, string inverters eliminate the need for a central inverter by providing DC-AC conversion at the output of each string.

By eliminating the central inverter and its potential as a single point of failure, this approach improves system robustness. However, such installations still need to contend with the hazards and costs associated with DC voltage transmission.

C. MICRO INVERTERS

Recent researches focus on micro inverters which take the concept of string inverters to the next level - providing DC-AC conversion from each individual panel rather than an entire string.

3At the higher systems level, micro inverter design calls for real-time execution of a number of precise algorithms for efficient DC-AC conversion, circuit protection and PV panel power optimization through maximum power-point tracking (MPPT). This is essential to maintain the optimum panel loading required to sustain the best possible power output from the panel served by each micro inverter module. This concept is shown in Fig. 3. The processor and control unit is used to control power flow from the PV panel to the grid and executes the MPPT algorithm, fault control, and digital communication routines. Other requirements include high efficiency, low cost, wide DC input voltage range, high quality and standards compliance.

Fig. 3. Intelligent PV system interface concept

However, as the micro inverter is mounted to the back of panel, it calls for specific flexible designs such as plug-and-play technology. The main obstacle in this case is the transformer which is heavy and would reduce installation flexibility. As consequence, new trends focus on transformerless inverter systems. However, the omission of the transformer reduces their safety due to the galvanic connection of the DC source to the grid which generates a leakage current between the PV panel frame and the ground. Besides, the galvanic connection allows the DC current to be injected to the grid which saturates the distributed transformers [4, 6-11]. These issues are treated in the newly developed inverter systems.

To show the validation of the potential of the new inverter architecture, a number of solar suppliers and utilities have developed a wide range of micro inverter systems. Some of these systems are listed in Table II. As depicted in this table, most of these systems are rated to meet individual solar panel wattage.

TABLE II. FEATURES, POWER AND VOLTAGE RATINGS FOR SELECTED COMMERCIAL MICRO INVERTERS

Model Number Company Power (W)

AC Voltage

(V) Features

M215-60-2LL-S22

Enphase Energy

215 208/ 240

Operates with 60 cell solar modules 96% CEC efficiency 25 year limited warranty the Enphase Envoy communications

gateway Web-based monitoring and analysis

M190-72-208-S12/3

Enphase Energy

190 208/240

Pairs with most 60 and 72-cell PV modules

Power line communication 15 year warranty Enlighten software monitoring Compliance UL1741/IEEE1547 | FCC Part

15 Class B

D380-72-2LL-S12/3

D380-72-2LL-S12/3-NA

Enphase Energy

380 208/240

Per-module MPPT and DC-to-AC conversion

Integrated monitoring and power line communications

15 year warranty NEMA 6 enclosure rating Compliance CSA listed per UL

1741/IEE1547, FCC Part 15 Class B

P235LV-240 Solar Bridge Pantheon

225 240

Power line carrier 25 year warranty Compliance: FCC Part 15 Class B UL

1741 CSA 107.1

S215NA2240/ S215NA2240T/

SPARQ Systems, Inc.

215 240

Power line carrier 25 year warranty Compliance: UL1741 IEEE 1547 CSA

C22.2 NO. 107.1, FCC part 15, class B

SMI-D480W-60 Enecsys 480 110/240

Built-in wireless communication system so that performance data for the connected solar modules can be monitored in real-time.

EURO250 Solar Europa

235 195/235

Long life time and high quality power Constant remote monitoring at PV module

level Communication :Power Line CarrierWarranty :15 years

PV AC Module ExelTech 216 120

Microprocessor-controlled Maximum Power Point Tracking (MPPT) operates over entire DC input voltage range.

Anti-islanding protection built into every module (per UL1741 and IEEE 1547 safety standards).

Grid Smart SP201-1

Japer Electronics

200 108/120/132

Individual IP address Grid Smart provide active/reactive power Monitoring: Ethernet

V. A NOVEL TRANSFORMERLESS INVERTER TOPOLOGY FOR SMART GRID EASY INTEGRATION

To alleviate some of the issues raised above, a novel micro inverter design that has the minimum leakage current level within the German DIN VDE 0126-1-1 standard of 30mA is presented. Brief description of the proposed topology and simulation results are given in the following section.

Conventional inverter use pulse width modulated (PWM) as control strategy. In the full-bridge topology of Fig.4, each diagonal switches commutate during one half grid cycle with the same switching order. During this period, when the zero voltage is applied to the load, a leakage current takes place and finds its path through point N to the ground.

Fig.4.Full-bridge transformerless inverter topology

Accordingly, the proposed topology is based on disconnecting the DC source from the grid when the zero voltage takes place. This would allow the power to be kept at the AC side which significantly minimizes the generated leakage current. At the same time, the DC current injected to the grid is negligible.

4Fig.5 shows the proposed transformerless inverter topology. It consists of a conventional full-bridge with an extra bypass at the AC side.

Fig.5.Full-bridge transformerless inverter topology

The AC bypass is to disconnect the DC source from the grid when the zero voltage is realized. The working principle of such topology is summarized in Table III:

Based on this strategy, the proposed transformerless topology was simulated using non-real components. The results, shown in Fig.6, reveal a leakage current level less than 20mA which is within the German Standard, previously mentioned.

TABLE III SWITCHING STRATEGY OF THE PROPOSED TOPOLOGY

Switch Positive half wave Negative half wave

S1 x OFF

S2 OFFy

S3 OFFy

S4 x OFF

S5 x OFF

S6 OFF y

Fig.6. Proposed topology: from top to down, filter inductor voltage, output voltage, common mode voltage, leakage current and grid current.

VI. CONCLUSION

The paper reviewed smart grid features and renewable energy integration requirements. Many ongoing and completed projects prove that renewable energy integration, in particular, solar photovoltaic systems plays a key role in smart grid development. The literature search showed that micro inverters are evolving rapidly and would be used for off-grid and grid connected applications on a much larger scale than the standard inverters. Micro inverter companies are working on improving the conversion efficiency and communications. In the mean time, smart grid infrastructure must provide the ability to forecast PV resources for capacity planning and integration of GIS and weather. The proposed micro inverter topology was presented and simulated and results indicate that such a topology meets leakage current standards.

VII. REFERENCES

[1] State of Solar Union, Present and Future, Shyam Mehta, IEEE Power and Energy Magazine, May/June 2011, pp.92-94.

[2] EurObservER, 2009. [3] http://www.pvmarketresearch.com/press-

release/Smart_Inverter_Shipments_to_Grow_to_27_GW_by_2015_Grid_Integration_the_Key_Driving_Factor/4, accessed September 21, 2011.

[4] M. Calais, J. Myrzik, T. Spooner, V. Agelidis, "Inverters for Single Phase Grid Connected Photovoltaic Systems", the 33rd IEEE Conference on Power Electronics Specialists, Vol. 4, 23-26, June 2002, pp.1995-2000.

[5] R. Gonzalez, J. Lopez, P. Sanchis, and L. Marroyo, Transformerless Inverter for Single-Phase Photovoltaic Systems, IEEE Transactions on Power Electronics, vol. 22, March 2007, pp. 693697.

[6] Smart Grid Enabler of the New Energy Economy, EAC Smart Grid Subcommittee 2008.

[7] T. Kerekes, R. Teodorescu, M. Liserre, Common Mode Voltage in case of Transformerless PV Inverters Connected to the Grid, IEEE International Symposium on Industrial Electronics, June 30th- July 2nd 2008, pp. 2390 2395.

[8] Fritz Schimpf, Lars E. Norum, Grid Connected Converters for Photovoltaic, State of the Art, Ideas for Improvement of Transformerless Inverters, NORPIE/2008, Nordic Workshop on Power and Industrial Electronics, June 9-11, 2008, pp.1-5.

[9] Roberto Gonzalez, Jesus Lopez, Pablo Sanchis, Eugenio Gubia, Alfredo Ursua and Luis Marroy, High-Efficiency Transformeress Single-phase Photovoltaic Inverter, Proceedings of the 12th IEEE Conference on Power Electronics and Motion Control, 30 Sep. 2006, pp.1895-1900.

[10] Rodrguez, Gerardo Vzquez, Emiliano Aldabas, A new high-efficiency single-phase transformerless PV inverter topology, IEEE Transactions on Industrial Electronics, June 2009, pp.1-5.

[11] J. Myrzik, M. Calais, "String and Module Integrated Inverters for Single Phase Grid Connected Photovoltaic Systems- A Review", Proceedings of the IEEE Conference on Power Tech, Bologna, Italy, 23-26 June 2003, Vol.2.

[12] http://www.microsoft.com/presspass/press/2010/mar10/03-11SmartGridPR.mspx.

VIII. BIOGRAPHIES

Mounir Bouzguenda received his B.S. degree in Electrical Engineering the Pennsylvania State University, USA, in 1985. He also received his M.S. and Ph.D. degrees in Electrical Engineering from Virginia Polytechnic Institute and State University, USA in 1988 and 1992, respectively. Dr. Mounir taught in Virginia, Maryland and Washington, DC and Tunisia. He also worked as a consultant in the US,

Tunisia and Oman. Dr. Mounir joined Sultan Qaboos University-Oman as Associate Professor in 2009. His interests include smart grid, renewable

5energy systems, power systems and power electronics. He has authored many technical papers in these areas.

Adel Gastli received the B.Sc. degree in Electrical Engineering from National School of Engineers of Tunis, Tunisia in 1985. He worked two year in the standardization and certification of electric products in Tunisia. He received the M.Sc. and Ph.D. degrees from Nagoya Institute of Technology, Japan in 1990 and 1993 respectively. He joined the R&D Department at Inazawa Works (elevators and escalators) of Mitsubishi Electric Corporation in

Japan from April 1993 to Aug. 1995. He joined Sultan Qaboos University in Aug. 1995. He is currently a Professor of Electrical Engineering at Sultan Qaboos University, Muscat, Oman. He has established, in 2003, the Renewable and Sustainable Energy Research Group (RASERG) at Sultan Qaboos University and served as RASERG coordinator since then. He has authored and co-authored more than 80 papers. His current research interests include electrical machines, power electronics, drives, as well as renewable energy.

Dr. Abdullah H. Al-Badi obtained the degree of B.Sc. in Electrical Engineering from Sultan Qaboos University, Oman, in 1991. He received the degree of M.Sc. and Ph.D. from UMIST, UK, in 1993 and 1998, respectively. In September 1991, he joined Sultan Qaboos University as Demonstrator and in 2008, he became an Associate Professor. He has published over sixty papers in well known international journals and

conferences in the fields of electrical machines, drives, interference and high voltage. He carried out several projects on the effects of AC interference on pipelines. He is a Senior Member of IEEE.

Tarak Salmi was born in Kairouan in Tunisia, on September2, 1975. He graduated from Nasrallah Secondary School, Kairouan, and studied at the University Sfax. His special fields of interest include Power Electronics and Photovoltaic Systems. In 2000, Tarak received the BS degree from Tunis University of

Sciences and the MS degree from Monastir University of Sciences in 2007. Currently, he is pursuing his Ph.D. at the National Engineering School of Sfax (ENIS) in Tunisia.

TABLE I. POTENTIAL AND REAL BENEFITS TO BE REALIZED BY BUILDING AND IMPLEMENTING A SMART GRID

Benefits Stakeholder

UtilityIndependent Generator

Residential Commercial IndustrialFuture

GenerationsPositive Environmental Impact

Grid integration of high levels of renewable resources will require Smart Grid to manage extensive distributed generation and storage resources.

Smart Grid can reduce distribution losses, thus reducing power generation demands. Advanced metering technology can help measure electricity use and calculate the carbon footprint.

A high penetration of PHEV will require Smart Grid to manage grid support of vehicle charging. Potential use of PHEV as Vehicle to Grid will absolutely require Smart Grid technologies.

A Smart Grid enables intelligent appliances to provide feedback through the system, sense grid stress, and reduce their power use during peak demand periods.

System Reliability and Economics

Distributed generation is supported because the grid has the ability to dynamically manage all sources of power on the grid.

Smart grid technologies allow faster diagnosis of distribution outages and automated restoration of undamaged portions of the grid, reducing overall outage times with major economic benefits.

Price-sensitive peak shaving reduces the need for peaking generation capacity investments.

Smart Grid's automated diagnostic and self-healing capability prolongs the life of the electric infrastructure.

Smart Grid technologies allow better use of transmission paths, improving long distance energy transfers.

Price-sensitive peak shaving defers the need for grid expansion and retrofit.

Increased efficiency of power delivery Smart Grid technologies, such as synchrophasors, offer the promise of reducing transmission congestion.

Direct operating costs are reduced through the use of advanced metering technology (AMR/AMI) such as connects/disconnects, vehicle fleet operations and maintenance, meter reads, employee insurance, etc.

Economic Development Standards and protocols supporting interoperability will promote product innovation and business opportunities that support the Smart Grid concept.

Consumer Choice Provide consumers with information on their electric usage so they can make smart energy choices.

Real-time pricing offers consumers a "choice" of cost and convenience trade-offs that are superior to hierarchical demand management programs.

Integration of building automation systems offers efficiency gains, grid expansion deferral, and peak shaving.

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