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.
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.
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.
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.