Inverters with intelligent MPPT in partial shading situations

This article shows that photovoltaic systems based on inverters with MPPT (Maximum power point monitoring –

Intelligent maximum power point tracking) can perform well even in situations of moderate partial shading, an advantage that is normally attributed exclusively to systems based on power optimizers on the market.

In fact, power optimizers are an excellent strategy for photovoltaic systems with modules operating under different conditions, including partial shadows.

However, in systems with modules installed in the same orientation, there may be advantages in using conventional inverters, as long as they are equipped with intelligent MPPT algorithms, which are capable of mitigating the influence of shadows on the photovoltaic modules.

Smart MPPT is a feature that equips some photovoltaic inverters available on the market. These MPPT algorithms, more sophisticated than traditional algorithms, are capable of identifying shading situations and searching for the global power peak when the IV and PV curves present multiple points of maximum power.

This article was inspired by a white paper produced by SMA, manufacturer of inverters, in which the function ShadeFix of intelligent MPPT, which is claimed to be able to reduce the impact of shadows on photovoltaic systems.

Although the white paper is produced by the manufacturer itself and is more similar to an advertising piece than a technical article, the document presents some interesting comparisons between conventional inverters with intelligent MPPT (as is the case with the ShadeFix feature) and power optimizers.

Among other things, the white paper from SMA presents results of a study carried out at the University of Southern Denmark, which compares photovoltaic systems based on two different technologies:

1. inverter string with smart MPPT and;
2. Power optimizers.

The main focus of the case study presented in this article is the evaluation of inverter performance with ShadeFix compared to the performance of systems based on power optimizers. Comparisons were made across a variety of no-shade and partial-shade conditions.

The document consulted does not reveal which power optimizers, nor from which manufacturers, were used in the comparison. The main results obtained in experimental comparisons provide conclusions about energy production, system reliability, ease of maintenance and fire safety and the preservation of the installer's life.

Read too

• Understanding the effect of partial shadows on photovoltaic systems

• Dynamic peak manager: a solution for PV systems with shading

• Understanding the IV and PV curves of photovoltaic modules

Optimization of energy generation

For most photovoltaic system owners, solar energy represents a significant investment with expected financial returns. These returns depend on key factors including instantaneous energy production (performance) and production over the lifetime of the system.

Manufacturers around the world have been looking for solutions to provide solutions that can maximize energy generation and improve the return on investment in photovoltaic systems. While most photovoltaic systems are naturally designed to receive unobstructed, shadow-free light, shading can occur in some situations.

Several innovative solutions available on the market can reduce the undesirable effects of shading in photovoltaic systems. Shade mitigation strategies vary around the world and can depend on several factors.

The most used approach in the residential photovoltaic market involves trying to optimize energy production in each photovoltaic module using a larger set of components – this is the case with optimizers or microinverters.

Although this model has shown advantages over the old inverter technology, strings, it can be shown that the sophistication of MPPT algorithms in inverters strings can provide similar results, with a drastically reduced number of components and reduced system complexity.

Current market assumptions

A perception currently in the market is that placing small electronic devices under each photovoltaic module is the only way to obtain maximum performance from photovoltaic systems in situations of partial module overload. These components are commonly known as power optimizers.

They are also included in the MLPE device group (module level power electronics – power electronics at module level). Systems with optimizers individually convert the energy of each photovoltaic module, ensuring that each module can always operate at its maximum power point.

The modules are treated individually, which naturally makes the system immune to shadows of any kind, as one module does not affect the other. This undoubtedly improves energy production, especially under certain conditions, such as when photovoltaic modules are partially shaded.

However, according to inverter manufacturers, strings (and rightly so), this is not free: there is additional economic cost and complexity cost in this solution. MLPE solutions employ a large number of components, as it is necessary to employ a power converter (power optimizer or microinverter) for each photovoltaic module.

It is possible that in inhospitable locations, with aggressive climatic conditions, this is not the best solution – while inverters strings They can be stored in appropriate locations, free from dust, heat and humidity. It is not the purpose of this article to make a comparison between conventional inverters and MLPE, and we will leave this (very controversial) subject for another article.

More power with fewer components

SMA has incorporated into its inverters strings the ShadeFix system. As we said previously, this is a maximum power tracking solution for photovoltaic modules known generically as intelligent MPPT or dynamic MPPT. Other manufacturers use other trade names for this type of solution. ShadeFix (or smart MPPT, generally speaking) mitigates the effects of partial shading and increases energy production.

The manufacturer claims that its inverter with the ShadeFix feature is capable of providing results similar to those obtained with power optimizers, with the advantage of a smaller number of components – which supposedly can reduce the chance of failures and increase the system's useful life. photovoltaic, among other advantages.

Case study

An experiment was carried out at the University of Southern Denmark. The researchers tested three different systems. Two consisted of core module-level optimization solutions and the third used a power inverter. string with ShadeFix MPPT technology.

The conclusions of the study were as follows:

• In conditions without shadows, the optimizer technology coupled to the modules provided a lower performance than the inverter with ShadeFix;
• In conditions without shadows, but with cloudy skies, the performance of systems with optimizers was also lower than that of the inverter with ShadeFix;
• In conditions with moderate shadows (tree branches, chimneys, etc.) the performance of systems with optimizers was also lower than that of the inverter with ShadeFix;
• Only in conditions of severe shading did the optimizer solution outperform the inverter. string with ShadeFix.
• The results of this type of study may vary depending on the location. In sunnier locations, systems with optimizers may present superior performance.

In fact, some studies carried out in our research group at UNICAMP show that this can occur, since optimizers can reduce the power incompatibility of modules in a string.

The main information resulting from this study is that we can state that systems based on power optimizers and with an inverter equipped with an intelligent MPPT system (such as ShadeFix) can produce very close results.

It is concluded, after all, that maximizing generation in shaded photovoltaic systems is not exclusive to systems with power optimizers and can also be achieved with inverters. string traditional, as long as they are equipped with more sophisticated or intelligent MPPT strategies.

Maximizing generation in shaded photovoltaic systems is not exclusive to systems with power optimizers and can also be achieved with inverters. string traditional.

Some considerations about system reliability and useful life

Although it is not the purpose of this article to make an exhaustive comparison between conventional inverters and optimizers, the white paper from SMA presents some interesting considerations about the advantages of inverters strings about optimizers – remembering that advantages and disadvantages of one or another type of system depend on the type of application and a multitude of variables that must be evaluated by the photovoltaic system designer.

It is not a simple task to point out advantages or defects of conventional inverters and MLPE systems. We have already seen, according to the case study results presented previously, that energy production in moderate shading situations, typically found in most rooftop photovoltaic systems, can be very good with inverters. string equipped with intelligent MPPT systems.

The results can be as good (or better, as this specific study pointed out) than those obtained with power optimizers installed directly on the modules. With the premise that the generation results are similar, what would be the big difference in one or another type of photovoltaic system?

In a typical 50 kW commercial installation, a string A common battery can have around 2,000 electronic components. They are all housed in a weather-protected cabinet and are easy to maintain and replaceable.

In comparison, the same system using optimizers can have more than 60,000 electronic components. And most of these electronic components are found in the form of small boxes housed beneath photovoltaic modules, exposed to humidity and intense thermal cycles.

Given the large number of components and aggressive operating conditions, a higher failure rate is to be expected, as would occur in any type of electronic system.

This problem can be minimized with high-level engineering and an intense testing cycle before the product is placed on the market, in addition to a warranty period compatible with the expected useful life of the photovoltaic system (typically around 25 years).

There are good optimizer manufacturers that offer excellent levels of reliability. Therefore, the greater number of components and operation in aggressive conditions can be a problem that can be overcome depending on the quality of the products.

As much as we can trust optimizers and equate their level of reliability to that of power inverters, strings, there is still a very complicated issue when we talk about systems installed on roofs.

In case of failure of a single component, it will be necessary to carry out an intervention on the roof, involving the removal of modules to replace the defective equipment. This can be costly and a headache.

In relation to what was mentioned in the previous paragraph, the inverters strings they are actually more practical from an installation and maintenance point of view.

If we can imagine (hypothetically) that the components will never fail, it can be said that inverters string and optimizers are equivalent and using one or the other is a simple choice or a matter of taste in some cases (but not in all situations).

Given the difficulty of carrying out maintenance on roofs, conventional inverters (preferably with dynamic MPPT) are undoubtedly more practical – but there will also be situations in which optimizers will perform better, even with known maintenance difficulties.

Finally, let's talk about the safe operation and maintenance of the system. In some countries it is already mandatory to use individual shutdown systems on photovoltaic modules. This is required so that at no time, in any part of the circuit, can dangerous voltage levels be required.

In case of interruption in the operation of the photovoltaic system, for any reason, each of the photovoltaic modules must have its own switch or shutdown system. This has not yet been discussed and is not required in Brazil, but it will soon happen, we hope.

Power optimizers have the advantage of inherently being able to perform shutdown per module. But also inverters string conventional devices already have this feature with the use of quick shutdown devices that can be coupled to the modules.

Even with the use of shutdown devices per module, the number of components in systems with conventional inverters is extremely lower than that found in systems with optimizers.

Disconnectors are very simple devices, with few components. Its function is to cut off the output of the photovoltaic module when the equipment receives a shutdown command signal, normally coming from the inverter itself.

Conclusion

The main objective of this article was to show that inverters string equipped with an intelligent MPPT system (such as SMA's ShadeFix) can be as good as systems with optimizers in moderate shading situations.

The ability to reduce the impact of partial shadows (when only a portion of a system's modules have shadows) was exclusive to photovoltaic systems with per-module power optimizers, before the appearance of dynamic MPPT algorithms in conventional inverters.

The central theme of the article was to highlight the effectiveness of the ShadeFix system, but some comparisons were also made between the use of a conventional inverter string and the use of power optimizers, highlighting the smaller number of components and the lower risk of defects provided by inverters. strings, which supposedly puts them at an advantage over power optimizers coupled to modules.