Open Source and Low Cost Remote Monitoring Prototype for Grid-Connected Photovoltaic Generation Systems

Transcription

1 1 Open Source and Low Cost Remote Monitoring Prototype for Grid-Connected Photovoltaic Generation Systems Gabriel de Andrade Torelli, André Mendes Martins, Rômulo Mello Alves, Tuliana Pinto Martins, Ivani Rodrigues dos Anjos, Rui Manuel Ribeiro Freire e Carlos Augusto Guimarães Medeiros Abstract This paper presents the development of a prototype of a supervisory system to monitor (locally or remotely) a low voltage and grid-connected photovoltaic generator. Its main features are: simplicity, low cost and easy customization. The system is composed by a modular open-source electronics prototyping platform and by an open-source software named "STELLA FV". With these characteristics, this work aims to spread out an important, renewable, distributed energy source, since it implements a friendly, adaptable supervising system that does not increase the final cost of the whole project substantially. Index Terms Distributed power generation, photovoltaic systems, remote monitoring. P I. INTRODUCTION OWER generation from renewable sources is continuously expanding around the world and in Brazil as well. Among many, solar photovoltaic (PV) energy is a very attractive alternative to Brazil due to our remarkable annual medium solar irradiation (compare it with Germany, for example), with a particularly good incidence in the state of Goiás [1]. This source has many inherent advantages: direct conversion of solar energy to electricity without movable elements; no emission of carbonic gas while operating; low maintenance cost, good useful life; and modular expansion. In addition, the technological advances and the growing of photovoltaic industry and market, are year after year making it overcome its drawbacks, namely relative low efficiency and high costs. Another interesting aspect of this resource is that it can be used to supply electrical installations isolated from the main electrical grid, such as rural communities, public phones, satellites and so on. But, in this case, rechargeable batteries are commonly used to store energy; and since their useful life is relatively short, the residues they generate need to be recycled periodically. G. A. Torelli, A. M. Martins, R. M. Alves, T. P. Martins and I. R. Anjos ( s: [email protected], [email protected], [email protected], [email protected] and [email protected], respectively), are with Pontifícia Universidade Católica de Goiás (PUC-GO) as Electrical Engineering students. R. M. R. Freire ( [email protected]) is with PUC-GO as Computer Engineering student. C. A. G. Medeiros ( [email protected]) is with PUC-GO, Goiânia-GO, , Brazil, as Electrical Engineering and Control and Automation Engineering ( Mechatronics ) professor. There is also the possibility of feeding the main electrical grid through a dc/ac inverter. Here, the installations range from small size systems, connected at low voltages, to large scale photovoltaic systems, composed by thousands of modules and by a substation to connect it to the electrical grid. In fact, systems connected to the grid are not new and constitute most of installations in the world; they are present in in Germany, Japan, US, Spain, to cite only a few. Brazil has some photovoltaic installations, but they are mainly for research. An important plant that deserves to mentioned is the one located on Tauá-CE, which has started up in 2011 with 1.0 MWp. Summing up, the amount of power generated from photovoltaic systems in Brazil is not yet relevant. There are many studies, discussions and projects on the way. Although there are some specific standards for photovoltaic (see "Associação Brasileira de Normas Técnicas - ABNT"), so far there is no national code to regulate the connection to the grid, a situation that is expected to be reverted soon. II. PV SYSTEMS CONNECTED TO THE GRID A PV module is composed by many photovoltaic cells, usually made of silicon. Two or more modules are connected to make a PV panel. However, these PV modules provide electricity in dc signals. In order to supply ac loads to the electrical grid, it is necessary to use a dc/ac inverter of grid-tie type. This configuration is shown in Fig. 1. Fig. 1. Simplified view of the photovoltaic generator connected at utility's bus by dc/ca power inverter. In Fig. 1 the local load supply is done after the inverter. It can be complemented by the electrical grid. Alternatively, the PV generation can be exported to the electrical system. That is the context of this work, described in detail below.

2 2 III. GENERAL DESCRIPTION OF THE SYSTEM The prototype has been developed having research and teaching as its main purposes, but it is useful to anyone interested in the practical aspects of PV systems, such as installation, operation, maintenance and its general behavior. It is composed by a solar panel, a grid-tie inverter connected to low voltage bar utility, a hardware that acquire and send data through an USB port to a personal computer (running as a server), an UPS (uninterruptible power supply or no-break), local load and, finally, by a conventional, bidirectional, electronic kwh meter, as shown in Fig. 2. The nominal power of the photovoltaic panel is 450 Wp. The modules were grouped in pairs and connected in series (34 V), making out three groups connected in parallel giving a nominal current of 13.2 A. The inverter is grid-tie, designed to operate in synchronously with low voltage, 220 V/60 Hz bus. Other data are: input voltage range 24 to 52 V (dc); nominal power 450 W; assembled by Mass Power; model SUN-500G. It is important to emphasize that this inverter has an internal islanding protection, meaning that it automatically turns off if the voltage of the electrical grid is interrupted. Moreover, this inverter operates only within the presence of grid voltage, so that it does not supply loads or injects power to the grid before the synchronism, even if there is input of dc voltage from the photovoltaic panel. According to the manufacturer's specifications, this inverter offers a total harmonic distortion (THD) less than 5%. This value satisfy the IEEE standards [2], which establish the value of 5% for voltage level less than 69 kv. B. Local Load and Electrical Installations Taking the solar panel as previously specified, the next step were determine the load. The idea was to supply the load of a simple, popular house. The project has been taken and adapted from a popular house project entitled "Casa Própria" on CREA-GO magazine, year I, number 1, August/2006. The scheme was drawn in a reduced scale on a MDF sheet (Medium Density Fiberboard), where the consumer points were located, see Fig. 4. The electrical installations have only two single-phase circuits, 220 V/60 Hz, one for illumination and the other for general power supply (sockets). Fig. 2. Prototype diagram. The solar panel has been placed on terrace of "E" building, located at Campus I, Area III, PUC-Goiás, Goiânia-GO. This place has been chosen in order to make the access to the PV generator easier. The remaining equipments are located at laboratory of alternative and renewable energy, at room 101F. A. Photovoltaic Generator and Inverter The photovoltaic panel is composed by six modules of monocrystalline silicon cells (SIEMENS SP75). Each module: has the following nominal characteristics: power 75 Wp, 17 V, 4.4 A. They are attached on a metallic base known as "metalon", as shown in Fig. 3. Fig. 4. Local load representing a popular house. Thus, the electrical load was adjusted according to the power supply available from PV, but, of course, its supply can be complemented by electrical grid when necessary. For the sizing of electrical wires and protections it has been adopted the usual procedures and calculations for low voltage installations [4]-[10]. However, some particular cares have been taken into account due the presence of the dc circuit and a lightning protection system. Table I summarizes the electrical protections traditionally used in small low voltage PV systems and those effectively used in this prototype. Fig. 3. Photovoltaic panel composed by six modules.

3 3 TABLE I CONSIDERED AND EFFECTIVELY USED ELECTRICAL PROTECTIONS. Type: Overcurrent dc side. Overcurrent ac side. Residual differential circuit breaker (DR). Grounding of metallic mass. External lightning protection system. dc. ac. meter side. Utilization and commentaries: Yes, circuit breaker. Yes, circuit breakers. Including bipolar circuit breaker for the connection inverter/grid. No. Electrical installation inside laboratory, a place dry and controlled. Yes. "Metalon" base (where are fixed the FV modules) connected to the grounding conductor. Yes. Yes. One for each pole (positive and negative), type II, located before inverter. No. Prototype already inside laboratory with own electrical installations. No, for the same reason as stated in the previous item. As remarked in Table I (see the second item), the connection between the inverter output (ac) and the electrical grid is done specially by a bipolar circuit breaker (besides additional protection, it is important to be able to isolate the inverter totally from remaining circuit when necessary; e.g. for maintenance). Note that according to the connection point, all energy not consumed by the local load will be exported to the electrical grid; the bidirectional kwh meter as exhibited in Fig. 2 takes into account only the net energy. Also, in this kind of installation is common the use of a kwh meter after the inverter. However, in this work the measurement is done exclusively by the supervisory system, described as follows. IV. SUPERVISORY SYSTEM All the conception, project, hardware arrangement and software needed for the monitoring system (supervisory) have been developed in this research. A LAMP server configuration, a combination of Linux, Apache, MySQL and PHP, has been choosen. A. Considerations About Hardware and LAMP Server The server's hardware is composed by a personal computer with modest requisites (Intel Pentium 4, 2.0 GHz clock). For the acquisition data (electrical voltages and currents), an Arduino MEGA with the ATmega1280 microcontroller has been used. It has a 10 bits A/D converter, with analog input voltages between 0 and 5 V. The LAMP server is composed by: - operational system: Linux Slackware 13.37; - HTTP server: Apache ; - database: MySQL ; - interface software written in PHP. Voltage and current sensors were positioned at three points (see Fig. 2): (a) photovoltaic panel output; (b) inverter output; (c) same electrical place of kwh meter. For the purpose of monitoring PV output (voltage and current dc signals) it has been used Hall effect sensors. For the ac signals it has been chosen small current transformers (like clamps) for the electric current, and a voltage transformer for the electric voltage (a kind of commercial 220/9 V ac power adapter). The output waveforms from these sensors have positive and negative values, but the A/D converter of the microcontroller can not read the negative ones. Thus, the signal conditioning stage converts each ac signal to a waveform that has a positive peak less than 5 V, and a minimum value above 0 V. This task has been done by common circuits based on voltage division principle. These circuits scale down the waveform and add an offset in order to eliminate the negative component. After that, electrical signals are ready for A/D conversion. Besides data acquisition, the Arduino MEGA was programmed to compute and send to the LAMP server (through USB port) the electrical quantities: rms voltages and currents, real and apparent powers and power factor. Arduino is an open-source electronics prototyping platform, with open source program, whose hardware and software are relativity simple to use. To program the microcontroller one uses the Arduino Programming Language, whose syntax is similar to the C language. The program of Arduino used CSV standard (commaseparated values) for data output. B. The STELLA FV program STELLA FV is the name of the software developed to manipulate the database and to provide a friendly user interface. Although other possibilities were considered, it has been decided to write a tailored program, because an alternative with all the characteristics needed could not be found: easy and direct customization, remote data access, open-source and freeware program. With STELLA FV it is possible monitor online the electrical quantities stated above as well the energy (kwh). The program generates graphs and logs; energy values are also recorded per day. The latter feature was included to provide a log of the history of photovoltaic production of the place and measure the seasonal influence as well. It has been written in PHP and communicates with MySQL, the database manager. MySQL is a well known open-source program that integrates seamlessly with PHP and it has excellent effectiveness and stability, even on low performance computers. STELLA FV runs both locally and remotely through a web navigator and it is independent of operational system or additional software. Fig. 5 shows an example of log generation (the original scripts are in Portuguese).

4 4 from PV panel, electrical grid and inverter. The local load was adjusted to 300 W approximately. Fig. 5. Log generation on STELLA FV - original scripts in Portuguese. The CSV generated by the Arduino software is collected through a PHP script, which establishes a permanent streaming with the device. The data are then recorded in the database. Other PHP script is responsible to select the last values and to send to the screen application, as depicted in Fig. 6. Fig. 8. Real power, load 300 W approximately. A test with a load of about 50 W only has also been performed, as shown in Fig. 9. In this case the exceeding (generated) power has been exported to the electrical grid (positive signal). Fig. 6. Data management. Some results from STELLA FV are presented below. V. TEST RESULTS The results shown in this section have been taken in sunny days, with clear skies. First off all, the islanding protection of the inverter was tested. The curves of rms voltage from electrical grid and inverter were superimposed. In this test, the ac voltage from utility bar has been interrupted by opening intentionally a circuit breaker. It can be seen in Fig. 7 that in this situation the inverter output voltage falls off to zero even with dc voltage from photovoltaic panel at its input. Fig. 9. Real power, load 50 W approximately. These graphs present the typical behavior of a PV system generation. The dc power curves have been shown for research purposes. In practice, users or final consumers are more interested in how much energy their system has produced (at inverter output), and mainly how much they must pay (final net energy). Naturally it is possible to monitor (and even to control) remotely and simultaneously, many PV generators located in different places. To this end, each LAMP server with a STELLA FV program installed receives a unique IP address. Thus, it is enough to access the IP associated with a given PV system, as suggested by Fig. 10. Fig. 7. Islanding protection test. Fig. 8 shows the results of the interaction between power Fig. 10. Central monitor of many PV generations.

5 5 Finally, it is possible to centralize all the information (like a kind of virtual power station). In addition, it is also possible to monitor the generators virtually anywhere by using different devices such as smartphones, tablets, netbooks, etc. VI. CONCLUSIONS The main advantages of this prototype are its low cost and the use of modular, open hardware and software. The STELLA FV is an example of an efficient solution that can be customized. Remote monitoring through internet is an interesting characteristic that does not require the installation of any extra software to access the STELLA FV. As an improvement, one can use a wireless signal transmission between the Arduino board and the server computer. This can be accomplished by the known APC 220 module, which can transmit data signals within the range of 1.0 kilometer; it does the serial conversion (so it behaves as if Arduino were plugged on an USB port of the server, without need for additional configuration or software). Further facilities may also be incorporated, as for example: monetary gain, an option to check the amortization of the initial investment of the entire system, comparative carbonic gas emission avoided, monitoring PV panel's temperature and so on. VII. ACKNOWLEDGMENT The authors thank electricians of electrical engineering laboratories and DSG (General Service Division) of PUCGoiás for technical support. VIII. REFERENCES [1] TIBA, C., "Atlas Solarimétrico do Brasil: Banco de Dados Solarimétricos.", UFPE/CHESF, MME - ELETROBRÁS CEPEL/CRESESB, Brazil, [Online]. (In Portuguese). [2] IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Standard , April, [3] CARVALHO, D. V. P., CARVALHO, F. W. S., CAIXETA, L. G. P., NEVES, M. L., "Estudo e Implementação de um Sistema Solar Fotovoltaico para a Geração de Energia Elétrica", Trabalho de Conclusão de Curso de Graduação em Engenharia Elétrica, PUC-GO, (In Portuguese). [4] FILHO, J. M., Instalações Elétricas Industriais, 6ª edição, LTC: Rio de Janeiro, (In Portuguese). [5] CRESESB, Manual de Engenharia para Sistemas Fotovoltaicos, Grupo de Trabalho de Energia Solar GTES, CEPEL-CRESESB, Edição Especial, PRC-PRODEEM, (In Portuguese). [6] ABNT Norma Brasileira - Instalações Elétricas de Baixa Tensão, NBR 5410, 2a ed., (In Portuguese). [7] JÚNIOR, L. O., "Sistemas Fotovoltaicos Conectados à Rede: Estudo de Caso 3 kwp Instalados no Estacionamento do IEE-USP", Dissertação de Mestrado, IEE/USP, São Paulo, (In Portuguese). [8] FELIX A. FARRET, F. A., SIMÕES, M. G., Integration of Alternative Sources of Energy, John Wiley & Sons, [9] SCHULTZ, B., WETTINGFELD, J., "Proteção contra Descargas Atmosféricas e Surtos em Instalações Fotovoltaicas", artigo original da Alemanha, revista Eletricidade Moderna, Aranda Editora, ano 39, n. 450, setembro de 2011, pp (In Portuguese). [10] ABNT Norma Brasileira - Proteção de Estruturas Contra Descargas Atmosféricas, NBR 5419, 2a ed., (In Portuguese). IX. BIOGRAPHIES Gabriel de Andrade Torelli was born in Goiânia-GO, Brazil. He is an electrical engineering student at "Pontifícia Universidade Católica de Goiás" (PUC-GO). He is currently involved with remote monitor applied to PV generation. His main areas of interest are signal processing, telecommunication, automation and open-source software. André Mendes Martins was born in Barra do Garças-MT, Brazil. He is graduated in Electromechanical Technology (Industrial Production specialty), by actual IFG. He is electrical engineering student at PUC-GO, where works on research about PV generation. He has practical experience with electromechanical assembly of "PCHs" and mining companies. His main areas of interest are electrical power systems and renewable energy sources. Rômulo Mello Alves was born in Anicuns-GO, Brazil. He is graduated in Eletrotechnic by actual IFG. He is electrical engineering student at PUC-GO, where had worked on research about renewable sources focusing PV generation. His main areas of interest are renewable energy sources, PV generation, distributed generation and power system operation. Tuliana Pinto Martins was born in Itumbiara-GO, Brazil. She is concluding electrical engineering course at PUC-GO, where works on research about PV generation connected to the electrical grid. Her main areas of interest are renewable energy sources and power electronics. At present works as trainee in design and engineering development. Ivani Rodrigues dos Anjos was born in Goiânia-GO, Brazil. She is graduated in Eletrotechnic (1988) by actual IFG. At present she works in the CELG (utility company of Goiás state), at power distribution sector specifically in the "Centro de Operação da Distribuição (COD)". She is electrical engineering student at PUC-GO, where had researched about PV generation. Her main areas of interest are electrical power systems, protection and renewable energy sources. Rui Manuel Ribeiro Freire was born in Goiânia-GO, Brazil. He is computer engineering student at PUC-GO. At present he works with management, development and security in telecommunication servers. He had expertise in VoIP servers and projects to optimize WAN traffic. His main areas of interest are development and security in telecommunication, distributed systems, Unix and Kernel Development. Carlos Augusto Guimarães Medeiros was born in Brasilia DF, Brazil. He received the BSc and MSc degrees in electrical engineering in 1995 and 1998, from Federal University of Goiás, and Federal University of Uberlandia, respectively. He is currently teaching at PUC-GO in the Electrical Engineering and Control and Automation Engineering ( Mechatronics ) courses. His main areas of interest are photovoltaic and other renewable sources, energy conservation and power quality.