Power Electronic Components and System Installation for Plug-and-Play Residential Solar PV Md Tanvir Arafat Khan
Iqbal Husain
David Lubkeman
Electrical and Computer Engineering Department North Carolina State University Raleigh, NC, USA
[email protected] Abstract—Photovoltaic (PV) system installations in both residential and commercial sectors have increased rapidly throughout the world in the last decade primarily because of the technical advancements and hardware cost reduction. However, challenges remain in the installation and permitting process and the cost is not yet competitive enough for mass adoption. The focus has shifted in recent years to reduce the complexities of PV system installation and streamlining the permitting process. Hardware cost reduction has likely reached limits and only way to reduce cost further is through reduction in soft costs such as installation, permitting and inspection. This paper discusses the barriers in making the PV system Plug-and-Play (PnP) for residential applications, and then, presents a smart interface concept that includes a hardware between the grid and the PV panel and a web portal to bring together all the stake holders for simpler installation and commissioning. A study to evaluate how the commercially available inverters match with the proposed concept is also presented.
I.
INTRODUCTION
Photovoltaic energy offers significant benefits to the society due to its being a zero-emission energy source. However, the high cost of PV systems has been the primary barrier until the last decade. Because of the ongoing improvements in PV technology, the cost and performance of PV systems have reached the point where grid-tied PV system is becoming economically competitive. The United States (U.S.) presently has an installed PV capacity of 7.2 GW [1] and PV usage worldwide has grown about 15-40% for each of the past 10 years [2]. The PV industries around the world are mostly focused on the development of power electronics converters, PV modules and other components required for implementing the PV system and there is less concern regarding the non-technical issues. PV installations are facing multiple challenges because of the utility interconnection permitting processes across the U.S., and the vast dissimilarities regarding system inspection by various jurisdictions. The report by Damian Pitt of Network of New Energy Choices has identified many of the issues regarding PV installation [3]; the report also presents
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recommendations to overcome the situation. Methods to remove the complexities among the jurisdictions are also suggested in the report by Solar America Board for Codes and Standards (Solar ABCs) [4]. The report proposed an expedited permitting process such that the jurisdiction goes through a more consistent procedure to review the permit applications. The expedited permitting process eases the PV installation procedure, although the time required for the whole procedure still remains a big challenge. Local governments of some areas have stepped up to make the process easier by introducing simpler permitting procedures [5]. The city of Oceanside, CA has introduced friendly permitting process for residential PV system in recent days although time delay in installation still exists [6]. The complexity and delay in the permitting and installation process and relatively complicated system have deterred many U.S. households from installing the PV system. A consumer friendly PV package coupling the technical aspects with the permitting process is essential for large penetration of residential PV systems. This paper discusses the barriers in widespread adoption of the solar PV system for residential applications, and then, presents a smart interface concept for residential PnP installations of PV systems using a smart inverter and a web portal. The objective is to simplify the overall installation and commissioning through technical innovations in the inverter and its interface with the grid. The smart interface includes a hardware between the grid and the PV panel and a web portal to bring together all the stake holders. The PV inverter, termed as a smart inverter, incorporates the technical innovations of both grid friendly features and diagnostic functions which it communicates to the smart interface for PnP residential installations. The diagnostic features integrated into the smart inverter and smart interface will expedite the system approval process. The following sections discuss the current permitting requirements, smart inverter, smart interface and web portal. A study to evaluate how the commercial inverters match with the proposed concept is also presented.
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II.
CURRENT PERMITTING REQUIREMENTS
The residential PV installation permitting steps can be categorized into three major aspects of electrical, structural and utility interconnection as shown in the Fig. 1. Fig. 2 shows the steps currently followed to satisfy the residential PV installation requirements. Generally, the consumer contacts the vendor and the vendor completes the initial system sizing before selling their product. Later, the consumer has to schedule an appointment with a certified installer for the PV system installation. The installer follows article 690 of the National Electric Code (NEC) that specifies the codes related to obtaining the electrical permit for the PV system [7]. PV rating, protection circuit, AC and DC disconnects, ground fault protection, system grounding, wiring methods, and equipment markings are the major items checked based on the article NEC 690. Besides NEC 690, the electrical components of the PV system must have UL certification according to the UL 1741 standard [8]. Building codes from International Code Council (ICC) are followed to satisfy the structural permit of PV systems [4]. Roof information, mounting system structure, weight of the PV system, dead load and wind load of the roof are the key factors evaluated through structural permit. Utility currently follows the interconnection standard from IEEE 1547 which is a standard for interconnecting distributed resources with electric power systems [9]. NEC 705 also provides directions regarding the basic utility integration [7]. Power factor (pf), harmonic injection, grid synchronization, over current and under voltage disconnects, and anti-islanding are the technical aspects related to the utility permit. Once the PV installer completes the system installation per the codes and requirements, a permit application is submitted for the site visit by the field inspectors and authority having jurisdiction (AHJ). As AHJ approves the system installation, another application is to be submitted for utility integration. The final approval for system operation comes from the utility.
Figure 1. Residential PV Permitting.
Figure 2. Current permitting process of PV system.
This entire procedure increases the time and cost while requiring many to have in-depth technical understanding about PV systems. III.
GRID FRIENDLY PLUG-AND-PLAY SMART INVERTER
Inverters are the most critical component of a grid-tied PV system. The inverter technology has come through significant changes and upgrades over the years. There are many types of inverters which are employed with PV systems, namely centralized inverter, string inverter and the microinverter [10]. Microinverter provides a suitable approach for the PnP system since it can accommodate many of the utility interconnection requirements such as higher pf, lower harmonic injection, and accurate maximum power point tracking (MPPT). The block diagram of a present residential PV system with microinverters is shown in Fig. 3. An inverter can be designed with two specialty features to be a grid friendly inverter: First, the inverter should have the basic functionalities to satisfy the code requirements which will expedite the installation process, and second, the inverter may include advanced grid-friendly features. To meet the utility interconnection requirements, the inverter must have the capability of sensing the grid voltage and frequency, DC voltage and current, and ground fault related insulation resistance. Grid voltage is sensed from the utility line, and the acceptable range for the 240VAC system is between 211 to 264 volts [9]. Ground fault is monitored at the DC side of the inverter by measuring the insulation resistance. For the advanced features, the inverter is desired to have high pf (close to unity), MPPT with an accuracy of more than 95%, lower harmonic distortion and reactive power control. The inverter incorporates all the advanced features along with the diagnostic functionalities for PnP to make it a smart inverter. The smart inverter can communicate with the smart interface that is described in the next section through serial communication.
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Figure 3. Block diagram of residential PV system.
The inverter can also pass the measured data from the PV string and the grid to the interface for diagnostic purposes which makes this inverter different from the other inverters available. The designed inverter consists of a two stage converter to connect the PV module to the grid, and a controller. The controller consists of an inverter controls segment to maintain MPPT and a grid side controls segment to regulate the power flow to the grid. The designed inverter is of 500 watts with an output voltage of 240VAC; the block diagram of the smart inverter developed in Simulink is shown in Fig. 4. Simulation results of grid voltage and current through phase locked loop (PLL) are shown in Fig. 5. The inverter is supplied from two 250 watts PV panel in the simulation. The installation and advanced functionalities of the inverter are implemented in its controller. The unique feature of this inverter is the interfacing mechanism with smart interface where the inverter will not operate without the signal from the interface.
The smart interface will only send signal to the inverter once it has the utility permitting code from the web portal based permitting system and will replace all the combiner box and connectors shown in Fig. 3. IV.
SMART INTERFACE
The smart interface is the heart of the proposed system that links the microinverter with the web portal for the permitting process. The conceptual work flow and the algorithm are shown in Fig. 6. Fig. 7 shows the smart interface that includes the hardware components (relay, breakers, serial communication transmitter and receiver, ground fault measuring device and power supply). The algorithm that initiates the diagnostics of the inverter before sending any signal to the utility also resides in the smart interface. The interface is disabled until the authorization code from the utility is received to run the self-diagnostics of the inverter and the mechanical installation. Once the code is received, the smart interface unit checks the PV connection by checking the DC side voltage of the inverter through serial communication. If the PV connection requirements are satisfactory, then the interface checks for the PV side grounding to satisfy the system grounding.
Figure 5. Grid Voltage and Current.
Figure 4. Smart micro inverter.
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Start
Wait for 2 hours
Permitting Code from utility, UC=1?
N
Y Inverter self diagnostic
Troubleshoot N
Send signals to microinverters
Report grid friendly advanced features
N N Vgrid, Frequency Check
DC values check
Ground fault Check
Y Y Wait for 5 minutes
PF, MPPT, VAR,THD
Y Self diagnostic complete?
N
Y Notify Utility for Final Approval
Figure 6. Algorithm for smart interface.
DC side voltage and current measurements [15]. The interface will also send system initiation information along with performance data to the utility server. V.
WEB PORTAL FOR PERMITTING PROCESS
Figure 7. Smart interface hardware components.
As the smart interface described in earlier section completes all the safety checking and permitting requirement for the PnP system, connecting the smart interface with a web portal will simplify the permitting process as the documentation flow and approval process can be streamlined. The steps in the process to complete the installation without any site visit by the authorities are shown in Fig. 8. The steps in the permitting, approval, and installation process are discussed briefly in the following.
Different methods proposed in the literature have been evaluated to develop a suitable ground fault detection method for the proposed PnP system. Ground fault detection methods are based on the on-site measurement of temperature, irradiance, power loss, operating voltage and current [11], [12]. A minimum covariance determinant estimator based method is described in [13] to find series arc and ground faults using a resistor across a single module. In [14], a PV ground-fault detection technique using spread spectrum domain reflectometry (SSTDR) method has been presented. No sensors are required using the SSTDR to check the ground faults and can be used in the absence of the solar irradiation which is suitable for the PnP concept. In this method, the ground fault is measured by comparing the autocorrelation values generated using SSTDR. Once the PV connection and ground fault measurement results are received, the smart interface initiates system diagnosis check through the smart inverter, which includes ground fault status identification, grid voltage and frequency sensing, and
Step 1: Vendors upload all related product information along with certification information beforehand into the web portal. AHJ and utility verify whether equipment satisfying code requirements are being listed for use by the customer. This step does not involve the customers. Step 2: Consumer searches for authorized vendor from the web portal and orders the product. Step 3: Consumer contacts the certified installer to install the PV system. Step 4: Installer completes installation according to the guidelines, ensures code compliance, and uploads all the relevant documents to the portal with his unique identity. This document includes check list of codes, key installation pictures and short video of the installed system. Step 5: Local AHJs are notified through the portal after submission of the complete installation package. Step 6: AHJ reviews and approves the installation through the web portal.
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Vendor
6
Transferred rating data of power, current and Voltage
AHJ
1
5
2
Web Portal Consumer
3
Resulted binary data of system verification
7 4
Utility
Installer
Figure 9. Serial data communication for information transfer.
8
Figure 8. Web portal for PV permitting process.
Step 7: Utility is contacted through the web portal upon approval from the AHJ. Step 8: Utility reviews and provides the approval code to the portal which is then assigned with the installed PV system. This enables the smart interface to initiate the system selfdiagnostics. Step 9: The smart interface of the PV system receives the utility code and will send signal to the microinverters for diagnosis before supplying the grid. This step is not shown in Fig. 8. In the proposed process, there is no additional task for the consumer other than buying the system from the vendor and contacting the installer; the process is automated through the web portal. The whole process can be completed within hours to several days depending on the system size and installer installation time whereas it takes two weeks to months for getting the installation approval at present. VI.
SERIAL COMMUNICATION EXPERIMENT
Serial communication is tested, proposed system has been verified experimentally for the web portal based concept. A microcontroller is being used as the controller for the smart interface as well as for the communication interface to the web portal. A desktop computer is used as the cloud interface for testing the communication and the overall process. Fig. 9 shows data communication results obtained from the serial communication. The voltage, current and power rating of the equipment to be installed are passed to the smart interface to verify the system specifications. Once the sizing is verified, the smart interface returns a positive command to the web portal to initiate the next step.
The first plot in Fig. 8 shows the data passed from the user interface and the bottom plot presents the returned value of (1/0) from the controller. Representative values of 30.64V for voltage, 12.31A for current, and 98.68W for power are received in the smart interface for the results shown in Fig. 9. VII. MARKET EVALUATION Three commercially available microinverters in the market are tested for their features and performance to evaluate their compatibility with the proposed PnP process. The tested microinverters are from three different vendors and their features are listed in Table I. The microinverters are tested under different weather conditions to evaluate their performance. The temperature and irradiance are varied with the help of solar simulators and the inverter output power data is collected for the experiments performed. The solar simulators act as the PV source and the simulator’s irradiance level is varied to simulate different environmental conditions. PV string data, MPPT, pf, harmonic distortion, California energy commission (CEC) efficiency and overall efficiency are measured using a controlled power flow experimental set up. The power flow is changed from 10% to 100% of the power level so that the performance over a wide range can be monitored. In Fig. 10 and Fig. 11, sunny day, cloudy day and heavily cloudy day denote the different weather conditions for this experimental setup. 800, 500 and 300 watts per square meter are considered as the irradiance level for sunny, cloudy and heavily cloudy day, respectively. Efficiency and the CEC efficiency of the different microinverters are shown in the Fig. 10 and Fig. 11, respectively. Modification of the diagnostics algorithm will be required in these commercial inverters to make them compatible with the proposed approach of this paper.
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Table I. CO OMPARISON OF COMMERCIAL MICROINVERTERS
Requirements
Commercial Microinverter1 9 9 9
IEEE 1547 UL 1741 Communication Gateway Startup sequence Input DC side protection Utility Interconnection Smart Interface
S Sensing DC, AC voltage and Ground Fault Checking 9 ×
Commercial Microinverter2 9 9 9
Commercial Microinverter3 9 9 ×
Sensing DC and AC voltage 9 ×
Sensing S DC and AC voltage 9 ×
VIII. CONCLUSIONS AND D FUTURE WORK
Figure 10. Efficiency comparison of three commercial micro-inverters.
The existing requirements for the t permitting process of solar PV systems are discussed to evaluate e the scope of the PnP system for residential units. A smart interface with web portal based permitting process alo ong with a smart inverter is proposed that is expected to increase the numbers of c enabled residential PV installation. The communications smart interface linking with the inverter with a web portal will greatly simplify the inspection requirements for permitting. The safety issues of the t PV system are also considered in the proposed system m so that the NEC codes can be followed. The web portal concept is expected to eliminate the physical site-visitt and paperwork. An evaluation of three commercial inveerters is also presented to provide information on the featurees currently available in these inverters. ACKNOWLEDG GMENT
CEC EFFICIENC CY M-1
M-2
M-3
ES REFERENCE
0.96 0.955 0.95 0.945 0.94 0.935 0.93 0.925 Sunny Day
This project was supported by y an award from the Department of Energy DE-EE00060 036.
Cloudy day
H Heavy Cloudy day
Figure 11. CEC Efficiency comparison of three coommercial microinverters.
[1] Renewables 2013 Global status report, REN21 Secretariat, Paris, 2013. hotovoltaic Technology [2] Robert M. Margolis, “Ph Experience Curves and Mark kets,” NCPV and Solar Program Review Meeting, Denver, D Colorado, Mar. 2003. [3] Pitt. D. (2013, Dec 20). Takiing the Red Tape Out of Green Power: How to Overco ome Permitting Obstacles to Small-Scale Distributed d Renewable Energy [Online].Available:http://www w.gracelinks.org/media/p df/red_tape_report_online.pdff [4] Brook, B., "Expedited permit process for PV systems," Solar America board for codees and standards, Rev 2, Jul. 2012. 1 San Francisco Solar [5] SF Environment (2013, Jan 10). Photovoltaic Permitting Guid de [Online]. Available:
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http://www.sfenvironment.org/sites/default/files/fliers/ files/sfe_re_sf_solar_pv_permitting_guide.pdf [6] Building Division, City of Oceanside (2010, Feb 21). Photovoltaic Permitting [Online]. Available: http://www.ci.oceanside.ca.us/civica/filebank/blobdloa d.asp?BlobID=22451 [7] National Electrical Code 2011, National Fire Protection Association, 2011. [8] Inverters, Converters, and Controllers for Use in Independent Power Systems, UL 1741, 2001. [9] Standard for Distributed Resources Interconnected with Electric Power Systems, IEEE Standard 1547, 2003. [10] Kjaer, S.B.; Pedersen, J.K.; Blaabjerg, F., "A review of single-phase grid-connected inverters for photovoltaic modules," Industry Applications, IEEE Transactions on, vol.41, no.5, pp.1292, 1306, Sept.-Oct. 2005. [11] Zhihua Li, Yuanzhang Wang, Diqing Zhou, and Chunhua Wu, “An Intelligent Method for Fault Diagnosis in Photovoltaic Array”, ICSC 2012, Part II, CCIS 327, pp. 10, 16, 2012. [12] Ancuta, F.; Cepisca, C., "Fault analysis possibilities for PV panels," Energetics (IYCE), Proceedings of the 2011 3rd International Youth Conference on, pp.1, 5, 7-9 July 2011. [13] Braun, H.; Buddha, S. T.; Krishnan, V.; Spanias, A.; Tepedelenlioglu, C.; Yeider, T.; Takehara, T., "Signal processing for fault detection in photovoltaic arrays," Acoustics, Speech and Signal Processing (ICASSP), 2012 IEEE International Conference on, pp.1681, 1684, 25-30 March 2012. [14] Alam, M.K.; Khan, F.; Johnson, J.; Flicker, J., "PV ground-fault detection using spread spectrum time domain reflectometry (SSTDR)," Energy Conversion Congress and Exposition (ECCE), 2013 IEEE , pp.1015,1020, 15-19 Sept. 2013. [15] Bin Gu; Dominic, J.; Jingyao Zhang; Lanhua Zhang; Baifeng Chen; Jih-Sheng Lai, "Control of electrolytefree microinverter with improved MPPT performance and grid current quality," Applied Power Electronics Conference and Exposition (APEC), 2014 TwentyNinth Annual IEEE , pp.1788, 1792, 16-20 March 2014.
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