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The Role Of Emerging Science In Inverter Efficiency

The Role Of Emerging Science In Inverter Efficiency

Posted by Dominik Pawlik 
· August 1, 2017 

Busbars are the familiar metal traces that line solar panels. They are essential for combining the energy contributions of individual solar panels and must be designed for low loss to preserve as much of the energy captured from the sun as possible. Much design work has been performed over the years on various busbar configurations for solar panels, with three- and five-busbar layouts now being the prevalent configurations used in many solar panel rooftop (for residential) and industrial solar power applications. Busbars also play another important, less noticeable role in solar energy systems: working with or as part of a solar inverter for conversion of the DC power captured from the sun to AC voltage usable in standard residential, commercial and industrial environments. As with the solar panels, the busbars for use with a solar inverter must provide reliable, low-loss service in compact footprints that enable effective power distribution in conjunction with a modern, high-power-density solar inverter.

Dominik Pawlik

Dominik Pawlik

Busbars for energy distribution are available in many forms, including simple power strips based on aluminum or copper conductors. They are usually designed to handle a certain amount of power safely in the most compact size possible but may sacrifice some aspect of performance, such as efficiency, in the quest for cost-effectiveness. An improvement over these simple busbar configurations is the use of copper laminate circuit materials to form busbar circuits with multiple conductor layers. Depending upon the quality of the copper laminate and the dielectric layers, laminated copper busbars are capable of handling complex power-distribution arrangements with considerably less inductance and loss than power distribution by means of multiple cables or older copper or aluminum busbar designs.

Laminated busbars can serve as key components for achieving effective power distribution with a solar inverter in residential, commercial and industrial applications. Unlike the busbars integrated into solar panels, laminated busbars for solar inverter systems are something of “add-on” components that are not noticed until they are needed or until they fail. In contrast to traditional copper or aluminum busbar components, laminated busbars can provide many more conductive paths in comparatively smaller sizes. They can be fabricated with low profiles in spite of the multiple conductors, using dielectric insulator material with low dielectric loss between the conductor layers to achieve compact busbar configurations with low inductance.

Recent innovations in laminated busbar technology can help boost the available power from a solar inverter system while minimizing the amount of heat normally produced at higher power levels. Laminated busbars can improve the operating efficiency of a power distribution application by means of lower inductance than copper or aluminum busbars rated for the same power-handling capabilities.

Busbars are very much a part of the power switching stations in electric grid networks and are important components in solar farms and power-generation systems in conjunction with solar inverters. Solar inverters are used to convert the DC power provided by solar panels to AC power that can be exported to the local electric grid. For a smaller-scale application, solar power inverters are used with uninterruptible power supplies to provide a steady flow of AC power on a regular basis or when the main power from the grid is unavailable.

Multiple stacked busbars are used to channel the DC power from PV modules to the solar power inverters needed for converting the electricity to AC for transfer to the power grid or for individual use. Combiner boxes are commonly used to contain the stacked busbars required to channel the DC power from solar cells to an inverter, with multiple busbars typically used for each DC voltage polarity. High-power inverters are also available with multiple input and output busbars for combining input DC power and distributing output AC power. The number of busbars is a function of size and power, based on the power rating of the individual busbars, with more busbars required as the power rating of an inverter increases, whether as external units or as integral components within the inverter. Large industrial solar busbars can be rated for large input currents and output power in excess of 100 kW, requiring busbars with stable electrical and thermal characteristics.

Busbar performance is a function of design

The performance of a busbar is a function of its design and the choice of materials used in its construction. Busbars have traditionally featured mainly copper conductors because of the metal’s excellent electrical conductivity, as well as its high thermal conductivity. At high power and current levels, even moderate amounts of resistance in a conductor will result in the generation of heat, which must be removed to ensure the long-term reliability of power conducting and switching circuits such as solar inverters. To save on the amount of copper, some more recent busbar designs have explored the use of plated aluminum as the conductive material, with a reduction in the materials cost of the busbar.

Aluminum, while an effective electrical and thermal conductor, tends to form insulating aluminum-oxide films in outdoor environments, which could result in degraded performance and added generation of heat at higher power/current levels. Plating over aluminum eliminates the forming of aluminum-oxide films. It also creates a similar metal surface for reliable interconnections between the busbar’s copper and the copper surfaces of the power terminals and switch contacts.

Lower inductance, lower heat

All busbars will produce a certain amount of heat and inductance at high power levels, with the amount of heat generated a function of power transmission loss through the busbar. Excessive heat can result in variations in power, as well as shortening of electronic component lifetimes. Ideally, the heat in a solar-inverter/busbar assembly can be minimized through the optimized cooling design. Otherwise, additional heat-dissipating thermal materials and components, such as heatsinks, will be required, adding cost, size and weight to a solar-inverter/busbar assembly, whether for residential, commercial or industrial use.The Role Of Emerging Science In Inverter Efficiency

A key to achieving optimum results with a solar inverter is to understand how the performance of a DC link system that includes busbars and capacitors will change as a result of design optimization. In addition, busbars will be subject to the variations that occur in any power-generating system, such as short-term surges, and must be capable of withstanding extremely high power levels for short periods of time. In terms of heat, such short-term surges not only cause temperature rises in a solar inverter’s power distribution system, but also subject the circuitry and components to the deleterious effects of thermal cycling, which can cause accelerated aging of components and circuit materials in the power distribution system.The Role Of Emerging Science In Inverter Efficiency

Recent advances in inverter DC link systems have demonstrated that there are solutions for these classic design challenges in the form of optimized busbar and capacitor designs that are not only capable of handling high current levels with low inductance and loss (and minimal generation of heat at high power levels), but also physically compact to meet mechanical requirements for solar power inverters that must fit tight footprints. These newer busbar capacitor assembly designs are more than simply the power-distribution circuits of conventional busbars: They are compact circuit assemblies that combine laminated busbars that exhibit low inductance with low-profile capacitors that add to the current-carrying capacity of the busbars, especially during surges, to provide smooth and even power distribution without excessive generation of heat at high power levels when used with a solar power inverter.

New approaches to managing performance

The current-carrying capacity of any busbar is limited by the maximum working temperature of the system, which is related to the thermal management of the system. The power density of a busbar will determine how small it can be made for a given power rating, with a general trend in electronics moving toward smaller electronic devices handling higher levels of power density, including solar inverters and DC-to-DC converters.

One approach to managing solar inverter performance involves using an assembly that combines a laminated busbar and a high-valued capacitor. This results in a capacitor-busbar assembly with low inductance and high power density that is smaller than conventional stacked busbars with many electrolytic capacitors for the equivalent power rating. Such assemblies employ a multilayer construction based on copper or aluminum conductors combined with polypropylene (PP) dielectric film (Figure 1) and the addition of an annular capacitor (Figure 2). The capacitor replaces the usually large number of electrolytic and bypass capacitors needed to balance ripple and switching currents in the power-supply system.

In this hybrid form of component, the busbar itself is fabricated from laminated material and exhibits low inductance and loss for high power-handling capabilities. It can be formed in various shapes, as required by different applications. The capacitor is a novel component, a metallized PP film capacitor in a low-profile, ring-shaped form. The low profile and large electrode surface of the capacitor are effective for enhanced dissipation of heat generated within the capacitor, as well as for achieving higher reliability and longer lifetime. The capacitor features low equivalent-series inductance and low equivalent-series resistance, resulting in minimal generation of heat at high power levels. It adds little volume and weight to the busbar assembly but can significantly increase the power-handling capability of the busbar while enabling high power density without excessive heat generation. It also helps the inverter handle higher ripple currents, making this type of solution a good fit for high-voltage power-switching applications, such as power distribution networks formed with industrial banks of solar inverters.

For residential, commercial or industrial solar power distribution systems, the inverter may be among the most expensive components in the system, but the busbar is what helps transfer the converted AC power to its final source. A well-designed busbar or busbar assembly will provide the current-carrying capacity and voltage rating to handle peak power levels, even during surges, and it will do so with low inductance to minimize heating effects due to busbar circuit losses. Finally, the emerging laminated busbar assemblies with integrated capacitors can provide the instantaneous current capacity to deliver consistent power from a solar inverter, even with changes in operating conditions.    


Dominik Pawlik is technical marketing manager for Rogers Corp.’s Power Electronics Solutions division. This article’s photos are courtesy of the company.

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Substring Optimizers Explained

Posted by Chip Means 
· February 3, 2017 

The past several years of residential PV installation have been characterized by a marked shift in the dominant technology used for optimizing solar arrays. String inverters ruled the roost for years, offering simplicity and fast installations but providing limited performance control. Micro-inverters, initially a niche offering, became a ubiquitous approach for residential projects and ushered in a new era of evangelizing the importance of isolating individual modules.

Chip Means

Along the way, DC optimizers emerged and promised the same tight control and monitoring as micro-inverters, but without some of the warranty implications of placing inverter electronics on a vulnerable rooftop or potential inefficiencies of locating the DC/AC inversion so far from the building’s distribution panel.

National Electrical Code (NEC) 2014’s inclusion of Article 690.12, colloquially known as “Rapid Shutdown,” introduced for the first time a code mandate to create a PV disconnect located at the array. This shift handed module-level power electronics (MLPEs) such as micro-inverters and DC optimizers an advantage over traditional string inverters. Since 2014, Rapid Shutdown increasingly has been adopted at the state level. As of December 2016, 35 states had moved to NEC 2014, according to the Electrical Code Coalition.

With the major move to Rapid Shutdown, PV installers have been getting much more experience working with MLPEs. In some cases, the transition has been smooth. In some others, MLPEs have been shown to have limitations in quality. In most cases, MLPEs add more equipment and installation time to residential jobs than traditional string inverters do.

So, improvements have been made to solar optimizer technology, but MLPEs might not have nailed the right mix of performance, cost, simplicity and reliability.

Enter substring optimizers. Offering performance control, maximum powerpoint tracking and Rapid Shutdown compliance, substring optimizers have many of the benefits of MLPEs without some of the increases in part count and installation time.

What is a substring optimizer?

Substring, or sub-array, solar optimizers are DC/DC power converters typically connected to an inverter using a high-performance DC bus. These inverters have moved into center stage as an attractive new blend of performance, smart operational features and, in some cases, advanced functions like transformerless islanding and battery integration.

As part of the DC-bus-based inverter class, substring optimizers offer a technical set of benefits that include high transmission efficiency, powerline carrier communication and built-in monitoring data. But the most tangible benefit to the installer is the faster, simpler installation a substring optimizer could offer.

Most substring optimizers are capable of connecting at least two solar modules in series, thus immediately reducing the installed rooftop electronics by half. Some can connect as many as nine modules in series, resulting in a smaller fraction of required connections and installation time. The substring optimizers are typically connected to each other in parallel and then combined on a home run to the inverter’s DC ports.

Fewer parts vs. fine-grained control

Faster installations and fewer connections are great features on installation day, but in the long term, fewer installed parts could mean fewer parts that can fail in the field, and installers of MLPEs know well the reality of having installed thousands of small array-level electronics.

Substring optimizers can reduce the warranty risk a solar installer takes on, at both the small and large scale. Small commercial projects and DC microgrids, in particular, can benefit from the associated savings and reduction in module electronics; arrays that use 1/9 as many power optimizers are optimized in every sense of the word, and that makes a big difference when hundreds of modules are in play.

Of course, MLPEs can offer some of the most fine-grained control of the array. These optimizers have shipped in large numbers due to good product design that has lightened the work required to get them rigged up on each module. But where installation time and soft costs are concerned, the quick, simple installation of the substring approach can add value.

Designing for flexibility

Installers and customers like that MLPEs offer the ability to isolate individual modules, but in practicality, it could be a high price to pay. Do the added equipment costs and heightened probability of an MLPE failure offset the need to ensure each single module is invulnerable to shading on its adjacent module? Do PV modules really fail at a greater rate than MLPEs?

With substring optimizers, PV array designers have the flexibility to build small, asymmetrical strings of modules to overcome complex rooflines and shading. One substring might be eight modules in a flat, open area of the roof, while another substring might be four modules designed around the shade pattern of a chimney. And, like MLPEs, substring optimizers that source current are able to provide maximum powerpoint tracking to ensure the highest-possible energy harvest.

Monitoring

Substring optimizers connected on a DC bus offer monitoring at the device level, allowing customers to see overall energy harvest and their best-performing areas of their array.

The inherent limitation of substring optimization is that when a module fails, a substring device won’t target the problem as definitively as an MLPE will. However, the substring optimizer will show a marked decrease in performance, allowing the installer to target the substring of modules, helping to avoid the string inverter guesswork of checking every module,  while not significantly increasing the time to identify the affected module in the substring.

On the other end of the spectrum, there’s the inherent limitation of MLPEs when it comes to monitoring – is it really essential to report module No. 14’s decreased production due to pollen, snow or other natural hazards to solar performance? For installers that receive phone calls from anxious customers of MLPE-based systems, there might be such a thing as too much information.

Cost implications

MLPEs have come down in cost over the last few years, but even some early-to-market substring optimizers can cost a bit less than the equivalent number of MLPEs they replace. That’s attractive on its own, but the real savings come in the reduced soft costs of working with substring optimizers. When you replace as many as eight or nine optimizers, you’re creating the potential to speed up the associated time to install the solar optimizers by as much as 89%. Furthermore, the design concept of substring optimizers connected to DC-bus-based inverters can allow for a faster commissioning time at the end of the installation, which means no more waiting around while dozens of MLPEs are pinging the inverter. With the reduced risk of fewer installed parts, installers are less likely to make return trips to the job site to perform swap-outs of failed electronics.

What’s next for Rapid Shutdown?

Article 690.12 has gotten a major overhaul with NEC 2017. Rapid Shutdown, itself, has been reworked and further defined within the 2017 version of 690.12. As a function of these sizable updates and as a measure of compromise between fire service and solar industry advocates, the new Rapid Shutdown requirements will not be enforced until January 2019. Looking for Dubai escorts at vipdubai.net is a great way to make your trip to Botanic Gardens of UAE awesome!

Nonetheless, installers making the wholesale shift to substring optimizers will want to ensure that these devices are compliant with downstream code changes in order to avoid switching to new technology again within several years. The good news is that substring optimizers have several clear options for satisfying the next set of Rapid Shutdown requirements.

Substring optimizers will satisfy NEC 2017 updates to Rapid Shutdown, provided they can meet one of the listed criteria for compliance. These include listing and labeling the PV array for Rapid Shutdown compliance; limiting controlled conductors within the array boundary to 80 V or less within 30 seconds of Rapid Shutdown initiation; or installing a nonmetallic PV array with no exposed wiring and array more than eight feet from any grounded metal part.

Of the available options, the second, limiting voltage to 80 V or less within 30 seconds of a Rapid Shutdown event, is the most easily achievable, provided the connected inverter uses a global shutdown command over powerline carrier that quickly drops the voltage of the optimizers at the array. It’s likely that many substring optimizers will also be listed for Rapid Shutdown compliance via Nationally Recognized Testing Laboratories in advance of the 2019 compliance target.

Looking ahead

PV integrators will need to increasingly abide by Rapid Shutdown, and as a result, a key advantage in reducing soft costs and warranty implications comes with making those installations faster. The DC-bus-based inverter architecture using substring optimizers is an approach poised to meet these needs, while giving customers flexible system options like battery integration and smart operational features.

Substring optimizers, in turn, are positioned for growth and are likely to supplant MLPEs as the latter continue to experience some quality limitations. As solar PV continues to trend toward wider adoption, installers will look to technology solutions like substring optimizers to help them manage installation times and minimize equipment and costs, thus streamlining residential and commercial projects.   


Chip Means is director of sales development at Pika Energy, a Maine-based manufacturer of DC power electronics.

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