Jobson Carvalho

During negotiations for the acquisition of a photovoltaic system, it is common for the customer to create great expectations regarding their new acquisition, especially considering the promise of financial savings from “energy from the Sun”. expectations due to the low performance of the photovoltaic solar generator, that is, the amount of energy that was “forecast” or promised to the customer during the commercial negotiation stage will not be generated. It is known that to achieve success in a photovoltaic project it is necessary a detailed study of the customer's consumption profile, in addition to all technical care in the design and execution of the photovoltaic system. In summary, the final performance of the photovoltaic system can be affected by one of the following phases that make up a project: Dimensioning: During During the negotiation phase with the customer and preparation of the commercial proposal, sizing must be carried out with correct assumptions, through a thorough analysis of the customer's energy bills and with the help of sizing and simulation tools for photovoltaic systems (such as the PVSyst).Project: The technical design of the photovoltaic solar generator must be carried out in accordance with good photovoltaic engineering practices, respecting the relevant technical standards. Errors in the structure of electrical cabling, the organization of strings and arrays and other aspects can ruin everything. Typically, the lack of training of companies in the solar sector is what most affects this project phase.Installation: It is not enough to have a well-designed project – it needs to be well executed. Poorly organized cables, poorly made connectors and installers who step on modules are just some of the problems encountered in solar installations. Here too, the training of the supplier company is fundamental to the success of the project. Sizing the photovoltaic system At this stage, the size (power) of the system that will be installed and the resources to be used are defined – inverters, microinverters, optimizers, type and quantity of modules, etc. At this stage, it is necessary to analyze the consumer's average monthly consumption based on at least the previous 12 months, in order to design an appropriate system so that the customer pays the lowest amount possible on their next energy bills. Very calm at this time! Many solar companies, eager to close the deal, promise customers the world. Some customers will close the deal without question, but the headache may come later, if the promise made does not come true. The best thing to do is to analyze the customer's historical consumption and use tools (such as PVSyst) to sizing the system correctly. Even so, it is important to inform the client (and preferably establish this in the contract) that, despite the best engineering practices used in sizing the solar system, the result may vary depending on the local climatic conditions. Absolute certainty of generation, even, never it will be possible. The important thing is that the system is correctly dimensioned, with correct technical foundations, and that the customer receives information with the maximum transparency possible. Figure 1: It is important to analyze the customer's consumption history to correctly size the photovoltaic solar generator. Technical project After the commercial negotiation phase, the technical project stage will begin. At this point, it is necessary to pay attention to the characteristics of the installation site such as available area, inclination and azimuth, possible interference with the potential for shadow projection, in addition to the solarimetric information of the region where the system will be installed. When analyzing all areas available for installation, you must- identify which regions of the roof have the best generation results, something that can be verified with sizing and simulation tools such as PVSyst. The best way to interconnect the modules must also be taken into account in order to maximize the generation of the photovoltaic system. A very common mistake made by solar energy companies, due to lack of technical knowledge, is the interconnection of modules located in different parts of the roof within the same string (set of modules in series). This practice, in addition to not complying with photovoltaic system design standards, compromises energy generation and also the safety of photovoltaic systems. Having the maximum amount of information and paying attention to details, with the support of design tools, it is It is possible to design a system capable of meeting the customer's consumption demand with a good margin of success, with the best cost-benefit for the customer. Figure 2: Example of an image obtained with software for studying photovoltaic systems. The closer to the yellow color, the better the generation. Darker colors indicate the incidence of shadows and other factors (such as the slope of the roof towards the South) that reduce the collection of energy from solar radiation. Installation of the photovoltaic solar generator The installation of the system must strictly comply with the technical project. A poorly designed project (or the total absence of a project, which is not rare) will compromise the quality and safety of the installed system. A very common error during installation, even when there is a detailed project, is inverting the polarity of modules. in series. How is this possible? – we might ask ourselves. The figure below illustrates a very common situation in photovoltaic projects on residential roofs, which can lead to this error. Figure 3: Modules from the same string (connected in series) physically distanced on a residential roof. The image above illustrates a situation in which 11 modules must be connected in series, forming a string, but 3 modules will be far from the others. In this case there is a need to make an extension cable to connect one group to the other. If the connection scheme is of the leapfrog type (see what leapfrog is in this article), it will be necessary to make two extension cables. So that the size of the extension is as small as possible, but enough to meet the need, and also because the Projects (when they exist) are generally not sufficiently detailed, the crimping of the connectors usually happens after the cables have been passed during the assembly of the work. That's precisely why it's possible to crimp identical connectors at both ends, that is, male-to-male or female-to-female. This will cause the polarity of one of the sets to be reversed when the modules are connected. In addition to reducing the energy generated, exposing the reversed module to reverse current is potentially dangerous and can cause fires. Fortunately, module inversion can be easily identified by measuring the open circuit voltage of the string after assembly. In other words: this could be easily noticed if the professionals involved in photovoltaic assemblies at least knew what an open circuit voltage is and were instructed to measure it in the commissioning stage, which is the testing phase carried out after the assemblies have been carried out. . Often the need for commissioning has simply been neglected by professionals and installation companies.System commissioningA good photovoltaic project must go through the final commissioning phase, during which at least two things must be measured:VOC – ISC string open circuit voltage – String short circuit currentSee this article to understand the parameters of the IV curve of a photovoltaic module or string. Two of these parameters (VOC and ISC) can easily be measured with a common low-cost multimeter. Any installer of photovoltaic systems with a minimum of instruction should be able to perform these two basic tests. For example, when considering a module with an open circuit voltage of 40 V, in a string with 11 modules in series it is estimated that the VOC voltage will be close to 440 V. Even with the variation in sunlight intensity, the output voltage of the modules undergoes little (practically no) change, which makes the result of the open circuit voltage test very reliable. The figure below illustrates the result of measuring voltage and of the power generated in a photovoltaic system already in operation. The green graph shows the operating voltage of a photovoltaic string, which remains constant for most of the day, suffering reductions or variations only at the beginning or very end of the day, when the intensity of solar radiation is very low. The best time to perform commissioning tests on photovoltaic systems is always around midday, when the open-circuit voltage (or operating voltage, which is shown in the graph in the figure below) remains approximately constant regardless of variations in solar irradiance. Figure 4: Result of monitoring the operating voltage and power of a real photovoltaic system. While the power (in blue) fluctuates throughout the day, the output voltage of the strings (in green) remains in an approximately constant range. The green graph shows the operating voltage of the modules, but a similar profile (practically constant) applies to the open circuit voltage. Divergent orientations in string parallelism When installing the system, it is also possible to experience problems related to the parallelism of strings with orientation or number divergent modules, which will negatively impact generation and increase the possibility of fire. In reality, rigorously analyzing these facts, this type of situation should be avoided already in the design phase. Mixing modules with different orientations and inclinations in the same string reveals the lack of preparation of many photovoltaic “designers” operating in the Brazilian market. Regarding the parallel association of strings with different numbers of modules, this, in addition to a lack of preparation, reveals a great lack of responsibility . Any minimally prepared professional in the solar sector knows that it is prohibited to parallel strings with different numbers of modules. What can happen if this rule is not followed? See in this article the result of this “explosive” mixture of different strings connected to the same input of an inverter. Figure 5: Monitoring the operating voltage of 4 strings in a photovoltaic system, with the strings organized in 2 groups with 2 strings in parallel. In Figure 5 we see the result of monitoring the voltage of a solar system with 4 strings of modules positioned in different orientations. The data presented was obtained on the day corrective maintenance was carried out on the system. From early morning until 14 pm there were two arrays (with 2 strings each). Each array (with 2 parallel strings) was connected to an inverter. The strings in this project had 11, 19, 9 and 18 modules. In short, the (totally absurd) situation was this: System with 2 inverters with 2 MPPT inputs on each one. In total there were 4 MPPT inputs available. The strings were grouped into arrays of 2. Each array was connected to just one input of an inverter. This situation is illustrated in Figure 6 below. Figure 6: PV system connected incorrectly, with strings of different sizes connected in parallel and using only one MPPT input from each inverter. Let's see the absurdity of the situation mentioned above. The inverters had 2 MPPT inputs each. It would therefore be possible to perfectly accommodate 4 different strings in this project. Due to the designer's (and also the installer's) lack of knowledge, this error went unnoticed during the design and installation phases. There certainly was no commissioning stage. In the graph in Figure 5, we observed a green and a purple line, each corresponding to a group of 2 parallel strings. From 14 pm onwards the strings were separated and we noticed on the graph the recording of 4 different voltages, now for each string individually. The voltages of the individual strings are much higher than the voltages obtained before 14 pm. This clearly shows that it is not a good idea to parallel strings of modules that operate under different conditions (tilt, orientation, shading or number of modules in series). After corrective action, the system acquired this configuration: System with 2 inverters with 2 MPPT inputs on each. Each of the 4 strings was individually connected to one of the 4 available MPPT inputs. This situation is illustrated in Figure 7 below. Figure 7: Corrected PV system, with strings of different sizes connected independently to one of the 4 available MPPT inputs. The result of the corrective action of a photovoltaic system, with the separation of different strings that were previously in parallel, can be observed in the graphs in the following figures. Figure 8: Monitoring the energy generated from the solar photovoltaic system before corrective action (when different strings were connected in parallel). The system has two inverters. Each color (blue and green) corresponds to the generation of one inverter. Figure 9: Monitoring of energy generated by the photovoltaic solar system after corrective action (when strings were left independent). The system has two inverters. Each color (blue and green) corresponds to the generation of an inverter. Contributors to this article: Bruno Henrique Kikumoto de Paula - Electrical Engineer (UDESC), Master's in Electrical Engineering (UNICAMP). Specialist in project management, inspection and commissioning of photovoltaic systems. More than 10 years of experience in the photovoltaic industry. Instructor in solar energy courses at UNICAMP. See other articles by this author. Marcelo Gradella Vilallva - Specialist in photovoltaic systems. Doctor (PhD), Master's and Bachelor's degree in Electrical Engineering. Lecturer and researcher on the permanent staff of the Faculty of Electrical and Computer Engineering (FEEC) at UNICAMP - State University of Campinas. Director of LESF - Laboratory of Energy and Photovoltaic Systems at UNICAMP. Author of over 200 internationally acclaimed technical articles in the fields of power electronics and photovoltaic systems. Author of the book "Photovoltaic Solar Energy - Concepts and Applications". Pioneer in training in photovoltaic systems in Brazil. He coordinates the Photovoltaic Solar Energy Extension Program at UNICAMP (http://cursosolar.com.br), where he teaches courses on Introduction to Photovoltaic Solar Energy, Design and Sizing of Systems with PVSyst, and Installation and Integration of Systems Connected to the Electrical Grid. See other articles by this author. Become a partner ofCanal

Jobson Carvalho

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