MLPE and power optimizers for photovoltaic modules

Learn more about MLPE technology and the advantages and disadvantages of power optimizers

19 minute(s) of reading

In this article, we will address the advantages offered by power optimizers as a solution to the problem caused by shadows in photovoltaic systems.

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IV and PV curve of photovoltaic modules and arrays

Photovoltaic modules have an electrical behavior that is described by their current and voltage curve (IV). The product of voltage and current gives rise to the power and voltage (PV) curve.

If you have a module, a string (modules connected in series) or an arrangement (several strings connected in parallel), there will always be equivalent IV and PV curves, as shown in the illustrations below.

Figura 1: Curvas I-V e P-V de um módulo, um string e um arranjo fotovoltaico — Figure 1: IV and PV curves of a module, a string and a photovoltaic array

The IV and PV curves of the modules, strings or photovoltaic arrays would always have the same format if the modules of an installation were always subject to the same light intensity. We know this is not always true.

In solar plants this can happen, but in rooftop installations, especially in urban regions, the situation is very different. Only one string or a single shaded module can knock out the power generation of an entire system.

Figura 2: Curvas I-V e P-V de strings com sombreamentos uniforme e parcial — Figure 2: IV and PV curves of strings with uniform and partial shading

In Figure 2 we see what happens when just a few modules of a string receive shade. This is what we call partial shading.

The same situation can be found in systems with several strings in parallel, even if only one of the strings has a small part of the modules shaded.

The problem highlighted in Figure 2 is the presence of several points of maximum power in the IV and PV curves, a phenomenon that occurs in situations of partial shading of the photovoltaic modules of a system.

In the IV curve, the point of maximum power is located at the knee of the curve, while in the PV curve, the point of maximum power is located at the peak of the graph.

But what is the problem caused by the presence of multiple maximum power points? To understand this, it is necessary to understand how the photovoltaic inverter works.

There is a control system in every photovoltaic inverter called MPPT (maximum power point tracker – maximum power point tracker).

What does MPPT do?

When we connect a set of modules to the inverter input, respecting the characteristics of the equipment, it does not know how many or which modules are connected.

The inverter has no knowledge of the module model, power, number of connected modules and series. The only thing the inverter knows is that it must extract energy from the modules and convert it into alternating current.

The photovoltaic module has an optimal operating point which is its maximum power point, located at the knee of the IV curve or at the peak of the PV curve, as we have already mentioned. One of the functions of the photovoltaic inverter is to adjust the operating point of the modules, making them operate as close as possible to their maximum power point.

Since the inverter does not know the characteristics of the modules that are connected to it, there must be some way of “tracking” the point of maximum power. This is the function of the MPPT system found in all solar inverters. grid tie, used in photovoltaic systems connected to the electrical grid.

The MPPT system allows searching for the maximum power point of the photovoltaic module (or set of modules) through small disturbances. The inverter increases the operating voltage of the modules a little and observes what happened.

If the power has increased, it continues to increase even more. If the power decreased in the first disturbance, then the algorithm instructs the inverter to move in the opposite direction, decreasing the voltage to observe whether the power will increase or decrease.

This simple method, called disturbance and observation, equips almost all solar inverters grid tie. The idea of the algorithm is to cause slight disturbances in the voltage of the modules and observe what happens as a result. This way, the inverter tracks the IV and PV curves in search of the maximum power point.

Figura 3: Funcionamento do algoritmo de MPPT de “perturbação e observação” com uma curva P-V normal (sem sombras ou com sombreamento uniforme) — Figure 3: Operation of the “perturbation and observation” MPPT algorithm with a normal PV curve (without shadows or with uniform shading)

Figura 4: Algoritmo de MPPT preso a um máximo local em situação de sombreamento parcial dos módulos fotovoltaicos — Figure 4: MPPT algorithm tied to a local maximum in a situation of partial shading of photovoltaic modules

The perturbation and observation algorithm works very well when everything is perfect, that is, when all modules have the same light intensity. In this case the inverter will be able to find the maximum point of the PV curve.

However, in situations of partial shading, most inverters will not be able to find the maximum power point of the system due to the fact that there are several maximums, as illustrated in Figure 4. In this example, there is a global maximum (which would be the optimal power point). system operation) and a low power local maximum.

In cases like the one illustrated in Figure 4, which occur in situations of partial shading, the MPPT mechanism will almost always be stuck at some point of local maximum, being unable to see the existence of the global maximum. What does this mean in practice?

Translating into simple language: you can have a 10 kW system that will only provide 300W because of a simple shadow existing in just 1 installation module.

There must be a way to protect ourselves from partial shadows, which are a very common situation in photovoltaic systems. What can we do?

There are several strategies, which include separating modules into smaller sets, adopting inverter and optimizer technologies, or using inverters with intelligent MPPT algorithms.

What is MLPE?

The electronics embedded in the photovoltaic module are a good translation for the expression MLPE (module level power electronics), which has become well known in the literature.

Having the electronics embedded in the modules means that we bring the electronics close to the modules, instead of taking the electrical connections from the modules through long cables to traditional inverters.

A very well-known example of MLPE are microinvestors, those small inverters that are installed on roofs, next to the modules, eliminating the use of power inverters. strings. In this article, I want to introduce readers to power optimizers, which are the younger brothers of microinverters, but are a little different.

Figura 5: Conceito de MLPE: a eletrônica é levada para perto dos módulos fotovoltaicos. Exemplos de soluções MLPE para sistemas fotovoltaicos são os microinversores e os otimizadores de potência — Figure 5: MLPE concept: electronics are brought close to the photovoltaic modules. Examples of MLPE solutions for photovoltaic systems are microinverters and power optimizers

The electronics embedded in the module are an alternative to the inverter grid tie traditional. By traditional we mean those inverters that receive at their inputs one or more strings with a certain number of modules connected in series.

That's what we call reverse string (inverter strings) or central inverter, the name we give to high-power inverters used in solar plants.

Power optimizers: the latest technology in MLPE

Power optimizers for photovoltaic modules are small electronic converters that can be installed directly on the photovoltaic module terminals. They can be affixed underneath the modules, on the same mounting rails, or they can be glued directly to the back surfaces or attached to the module frames.

The optimizer is essentially a DC-DC power converter, with direct current input and output. Its function is to serve as an intermediate stage of energy conversion between the photovoltaic modules and the inverter. Instead of strings of modules connected to the inverter, as in traditional solutions, we find strings of modules with optimizers.

In photovoltaic systems with optimizers, the modules are not directly connected to each other, as we see in the figure below.

Each module is linked to its own optimizer. The optimizers are connected without series, forming strings which are then connected to the inverters. The difference between a string traditional and a string with optimizers is shown in the following figures.

Figura 6: String tradicional e string com otimizadores de potência — Figure 6: Traditional string and string with power optimizers

What is the advantage of optimizers? The best answer is that each photovoltaic module is monitored and optimized by its own optimizer.

The main function of the optimizer is to search for the maximum power point of each module individually. This search is carried out in a dedicated manner, decoupling each of the modules of a photovoltaic system.

The practical effect of this is that the influence of partial shadows is practically eliminated on the photovoltaic system as a whole. Only the shaded modules are influenced by the shadow, with a reduction in generation due to the lower intensity of light received.

However, the presence of shadows in some modules does not affect other modules in the system. The non-shaded modules continue to operate normally, each with its own IV curve and PV curve, without the existence of local maxima and without any impact on the rest of the system caused by the shaded modules.

Figura 7: O efeito das sombras parciais é eliminado com o uso de otimizadores, pois cada módulo passa a ter o seu MPPT individual. No caso do string tradicional o MPPT atua sobre o conjunto todo, correndo o risco de ficar preso a um máximo de potência local — Figure 7: The effect of partial shadows is eliminated with the use of optimizers, as each module now has its own individual MPPT. In the case of the traditional string, the MPPT acts on the entire assembly, running the risk of being stuck at a maximum local power.

Figura 8: Sistema fotovoltaico com otimizadores da Solar Edge. Além da principal vantagem dos otimizadores (imunidade a sombras) é possível monitorar individualmente a produção de dos módulos, o que possibilita uma visão detalhada do desempenho do sistema. Fonte: SolarEdge — Figure 8: Photovoltaic system with Solar Edge optimizers. In addition to the main advantage of optimizers (shadow immunity), it is possible to individually monitor the production of modules, which allows a detailed view of the system's performance. Source: SolarEdge

There are few worldwide manufacturers of photovoltaic optimizers. This is a relatively new technology on the market and existing solutions are covered by a large number of patents. SolarEdge, Tigo and Maxim are examples of companies that develop optimizer technologies for photovoltaic modules.

SolarEdge has a large share of the global market and its solutions are the best known. In Brazil, there are already several systems in operation with this solution.

Optimizers are becoming very popular in Brazil. The installation method is similar to that of microinverters. The equipment is positioned underneath the modules, as shown in the following figures.

Figura 9: Instalação genérica: otimizadores de potência são instalados de forma semelhante aos microinversores, permanecendo sob os módulos fotovoltaicos e normalmente presos aos mesmos trilhos de suporte. Fonte: Divulgação — Figure 9: Generic installation: power optimizers are installed in a similar way to microinverters, remaining under the photovoltaic modules and normally attached to the same support rails. Source: Disclosure

Figura 10: Instalação em telhado: otimizadores de potência são instalados de forma semelhante aos microinversores, permanecendo sob os módulos fotovoltaicos e normalmente presos aos mesmos trilhos de suporte. Fonte: Divulgação — Figure 10: Rooftop installation: power optimizers are installed in a similar way to microinverters, remaining under the photovoltaic modules and normally attached to the same support rails. Source: Disclosure

Figura 11: Instalação em usinas: otimizadores de potência são instalados de forma semelhante aos microinversores, permanecendo sob os módulos fotovoltaicos e normalmente presos aos mesmos trilhos de suporte. Fonte: Divulgação — Figure 11: Installation in power plants: power optimizers are installed in a similar way to microinverters, remaining under the photovoltaic modules and normally attached to the same support rails. Source: Disclosure

Figura 12: Instalação genérica: otimizadores de potência podem ser fixados diretamente à moldura dos módulos fotovoltaicos. Fonte: Divulgação — Figure 12: Generic installation: power optimizers can be attached directly to the frame of the photovoltaic modules. Source: Disclosure

Advantages of optimizers

Shadow immunity

Shadow immunity is perhaps the main advantage of optimizers. There are divergent opinions about the main benefit of optimizers, given the countless advantages they offer. Some experts claim that reducing losses due to mismatch of power is the main advantage.

Are you going to install a photovoltaic system on a roof with chimneys, trees, buildings or any obstacle that could cause shadows? Don't even think twice: using optimizers can maximize the efficiency of the photovoltaic system and provide a result that is much better than what would be obtained with conventional inverters.

Installation of modules in different conditions and characteristics

Shadows are not the only problems that can be solved by optimizers. Optimizers make the designer's life a lot easier and can make very complex projects viable in which there are modules distributed over several roof pitches, with different inclinations and azimuth orientations (azimuth orientation is the direction in which the panel is oriented: North, South, East and West).

With traditional inverters it is recommended never to mix in the same string or in the same MPPT input modules with different characteristics (power, inclination, orientation, etc.), whereas in projects with optimizers this is not a problem.|

Figura 13: Otimizadores permitem a instalação de strings com módulos em diferentes condições operacionais. O caso mais comum são projetos com águas de telhados com inclinações e orientações distintas. Fontes: SolarEdge e PVSol — Figure 13: Optimizers allow the installation of strings with modules in different operating conditions. The most common case is projects with roof waters with different slopes and orientations. Sources: SolarEdge and PVSol

Reduction of losses due to power mismatch

This is a little known but no less important advantage of optimizers. In traditional photovoltaic systems, modules are connected in series. Even taking care to use modules from the same manufacturer and with the same power, we know that one module is never the same as another.

Small differences or mismatches of power between the modules of a string cause a reduction in the overall efficiency of the set. Even if the inverter searches for the maximum power point of the string, individually the modules will not operate exactly at their maximum power point.

This is a problem ignored in projects with traditional inverters. strings. Optimizers, in addition to the advantages we mentioned previously, can also solve this problem, providing an additional increase in the efficiency of the photovoltaic system. In other words, we are able to produce more energy, always extracting maximum power from each module in the system.

Final considerations

So far we have talked about the advantages, but it is always good to also talk about the negative points and the decision criteria for adopting a certain technology. No technology is perfect.

When people ask me what the best converter technology is for photovoltaic systems, I always disagree or respond with a simple “I don’t know”. The best answer is that each case must be analyzed individually.

Are you building a large capacity solar plant? I would not recommend using optimizers in this case. On a large scale, the cost of optimizers makes their application unfeasible.

Large solar plants must be built with inverters strings or large power central inverters. There is no better solution than these for large plants.

Are you building a micro or mini generation system with power up to a few hundred kilowatts? Are you in doubt about using inverters? strings conventional or optimizers?

Your decision may be based on some of these factors: security, cost, optimization against shadows, installation complexity or others.

Security: Optimizers and microinverters (MLPE solutions in general) are technologies well known for their safety. The possibility of a fire caused by an electric arc is practically zero in systems with this type of technology. This does not mean, on the other hand, that a system with a traditional inverter is not safe. Any well-made, well-designed and well-executed photovoltaic installation offers minimal risk of fire or failure. But when it comes to MLPE solutions, this risk does not exist. In the case of microinverters (which are not the focus of this article, but nothing prevents us from mentioning them here) the risk of electrical arcing does not exist as the systems work with low voltages, as the modules are not connected in series. In systems with optimizers, the modules are indirectly connected in series, but the optimizers have electric arc detection mechanisms that interrupt operation in the event of a fire risk. Furthermore, another advantage (from a safety perspective) of optimizers is the absence of open circuit voltage in the string when the system is not in operation. We will talk about these security features in other articles.
Cost: on a large scale, systems with optimizers will cost more than a system with a conventional inverter. For micro generation systems (most residential systems) the cost of optimizers will be imperceptible and the solution may even be advantageous. As power increases, the cost of systems with optimizers may become prohibitive, but their application is still viable in systems with up to a few hundred kilowatts. The increase in efficiency provided by optimizers must also be taken into account. Even in systems where optimizers can add a cost of up to around 10% to the value of the inverters, a study must be carried out on the additional energy generation that this technology provides. In the long term, considering the useful life of the photovoltaic system, optimizers can prove to be advantageous due to the reduction of mismatch power of the modules, even if at first glance they appear to be a slightly more expensive solution.
Shadow immunity: There is not much to say about this item. If the photovoltaic system is subject to a lot of shadows, the best solution is to use MLPE. Whether the solution will be to use a microinverter or optimizer is up to the designer. In general, optimizers are economically advantageous compared to microinverters, but both technologies are equally capable of immunizing photovoltaic systems against partial shade.
Installation complexity: on roofs with a lot of water and little contiguous space for the installation of strings, MLPE may be the only or best possible technical solution. Mix in the same string modules with different characteristics or operating conditions has never been a good idea. But this is not a problem when employing optimizers. The designer and installer gain a lot of freedom with the reduction in complexity provided by optimizers. Modules can be installed in any location, without any concern about incompatibility between them, either due to the characteristics or the installation method of each of the photovoltaic system modules.

Connection mode

Building a photovoltaic system with optimizers is not very different from that with traditional inverters. strings. The modules will be connected in series (through the optimizers) and then the connection of the string will be taken to the inverter (this is illustrated in Figure 8).

As in the case of microinverters, there are optimizers on the market for one or two modules, as illustrated in the following figures.

Figura 14: Otimizador simples (para um módulo fotovoltaico) e duplo (para dois módulos). Fonte: SolarEdge — Figure 14: Single (for one photovoltaic module) and double (for two modules) optimizer. Source: SolarEdge

Figura 15: Modo de ligação de um string com otimizadores simples. Fonte: SolarEdge — Figure 15: How to connect a string with simple optimizers. Source: SolarEdge

Conclusions

Optimizers offer several advantages for photovoltaic systems. In this article, we extensively explore the issue of immunizing systems against the effect of partial shadows, but we can mention other benefits of optimizers:

reduction of power losses due to module incompatibility (with consequent increase in energy generated)
increased safety of installations (by eliminating the risk of electrical arcs and reducing open circuit voltages of strings);
ease of installation of modules with different installation or operational characteristics – most common cases: formation of strings with modules located in different roof areas or subject to different shading conditions.

If we can mention any disadvantages of optimizers, perhaps we can talk about the cost that this equipment adds to photovoltaic systems. On a large scale (high power systems) optimizers tend to become unfeasible. It would be unthinkable (although technically possible) to build a multi-megawatt solar plant with optimizers.

In micro and minigeneration systems, however, optimizers can be technically and economically attractive given the benefit of increasing the operating efficiency of the photovoltaic system.

Optimizers are available on the Brazilian market and their use has been growing significantly. The convenience of using them must be evaluated by the designer together with the consumer who will make the investment in purchasing the photovoltaic system, taking into account the advantages presented and the additional cost that the technology presents compared to traditional systems.

Technical material

You can find catalogs, datasheets and other information about Solar Edge optimizers in the manufacturer area on Canal Solar: Solar Edge in the Solar Channel – Manufacturer Area

Marcelo Villalva

Specialist in photovoltaic systems. Professor and researcher at the Faculty of Electrical and Computer Engineering (FEEC) at UNICAMP. Coordinator of LESF - Energy and Photovoltaic Systems Laboratory at UNICAMP. Author of the book "Photovoltaic Solar Energy - Concepts and Applications".