Busbar for renewable energy and storage

What changes when the current is DC

A renewable energy system collects power at DC and often distributes it at DC before conversion. A solar array combines strings into a DC collection bus. A battery container connects racks to a DC bus and then to a power conversion system. A wind turbine uses a DC link inside its converter. The busbar in each case carries a current that does not reverse, and that changes the design emphasis.

At DC, the current distributes across the conductor section more evenly than at high frequency, so the bar can be sized on resistance and heat rather than skin depth. The larger issue is that the current is continuous for long periods. A solar busbar carries current for the whole daylight period, and a battery busbar can carry current through a full charge or discharge cycle. The temperature rise is steady, and the joint runs at that temperature for hours.

Converters add ripple on top of the DC. The ripple is small compared with the DC component in many designs, but it still contributes to heating and to the electrical noise in the system. The busbar layout should keep the converter loop short, in the same way as an inverter busbar, even when the main current is DC.

Outdoor duty: UV, humidity and salt

An outdoor busbar sees conditions that a panel busbar never sees. Sunlight degrades polymer insulation and coatings. Humidity condenses on cold surfaces and carries dissolved salts into joints. Coastal sites add chloride, which attacks copper and aluminium quickly. Day and night cycling moves the bar and the enclosure at different rates.

The enclosure is the first defence, but it is not a complete one. A sealed enclosure can breathe through a cable gland or a gasket as the internal air expands and contracts, and that breathing pulls in moist air. A breather or a desiccant can help, but the busbar and the joints still need a finish that tolerates the humidity.

Thermal cycling is the slow failure mode. A joint that is correct at 20 degrees can open slightly at 70 degrees and close again when it cools. Over many cycles the clamp load can fall and the contact resistance can rise. A washer that maintains its load, and a joint design that does not rely on a soft gasket to hold the force, are more important outdoors than indoors.

Battery storage: high current and fault energy

A battery storage busbar carries a high current at a low voltage. The current can be thousands of amperes in a container, and the fault energy is limited by the battery and the protection rather than by a transformer. The busbar has to carry the continuous current with a low loss and survive the fault until the protection operates.

The busbar also carries current in both directions, because the battery charges and discharges. The thermal design should consider the worst continuous case, which is usually the discharge or the charge at the maximum rate, and the duty cycle over a day. A bar sized for the average current may exceed its temperature limit during a long high rate cycle.

The mechanical layout matters in a rack. A busbar that runs along a rack has to allow for the rack tolerance, the module insertion and the service access. A bar that is rigidly fixed at every module can be preloaded by the rack, and a bar that is too loose can vibrate. The support should locate the bar while allowing a small amount of movement.

• Size for the worst continuous charge or discharge current, not the average.
• Check the fault current and the protection time at the battery terminals.
• Allow for rack tolerance and module insertion in the support design.
• Keep the converter loop short even when the main current is DC.
• Plan the service access for module replacement.

Copper, aluminium and the coating decision

Copper is the default for high current joints because it has a high conductivity and a stable contact resistance when plated. Aluminium is lighter and cheaper per kilogram, but it has a lower conductivity, so the bar has to be larger for the same current, and it forms an oxide that must be removed before a joint is made. The choice is a balance of weight, cost, joint practice and space.

The coating is where outdoor designs differ. Tin plating protects copper and keeps a solderable, bondable surface. Silver plating gives a lower contact resistance but tarnishes, and the tarnish can increase resistance if the joint is not designed for it. Nickel plating resists wear and some chemicals but has a higher contact resistance than silver. A powder coating or an epoxy coating protects the bar body but must be masked at the joints.

The coating has to suit the joint as well as the environment. A thin tin layer can be displaced under a bolted joint and expose the copper at the edge. A silver layer can migrate or tarnish in a humid, sulphur bearing atmosphere. The specification should state the coating, the thickness, the masked areas and the test.

Joints and dissimilar metals

A joint between copper and aluminium is a corrosion cell waiting for moisture. The two metals have different potentials, and in the presence of an electrolyte the less noble metal corrodes. The usual answer is a bimetallic transition plate or a plated barrier that separates the metals, with a finish that keeps the contact resistance stable.

The joint faces should be prepared immediately before assembly. An aluminium face oxidises within minutes in humid air, so the oxide has to be removed and the joint closed before it reforms. A joint that is prepared in the morning and bolted in the afternoon may already have a film that raises the resistance.

The fastener and the washer are part of the joint. A stainless fastener in an aluminium bar can gall or corrode, and a plain washer that is too small can crush the coating. The drawing should show the fastener material, the washer type and the torque, and the joint should be sealed against moisture where the design allows.

Inspection and condition monitoring

The first article of a renewable energy busbar should include a dimensional check, a plating thickness check, a contact resistance check on a sample joint and an insulation check. If the bar is coated, the coating thickness and the masked areas should be recorded. If the bar is laminated for a converter, the partial discharge check applies.

In service, the useful checks are a thermal image under load and a periodic torque or resistance check on accessible joints. A thermal image taken at a known current gives a baseline. A joint that runs hotter than its neighbours at the same current is a candidate for attention, even if it is still within its absolute limit.

For a remote site, the maintenance interval should match the access. A rooftop array or an offshore platform is expensive to visit, so the design should reduce the number of joints that need checking and make the ones that remain accessible. That is a design decision, not a maintenance plan added at the end.

What to send for a quote

Send the system diagram, the DC current profile, the fault level and the protection time. State the site climate, the enclosure type and the expected service life. Add the rack or structure drawing, the module or string layout and the joint interfaces.

If the site is coastal or industrial, say so on the enquiry. The finish and the joint design change with the environment, and a supplier who is told only the current will price a bar that may not last.

• System diagram and DC current profile
• Voltage class, fault level and protection time
• Site climate and pollutant exposure
• Enclosure type and breathing path
• Rack, structure and module layout
• Joint interfaces and permitted metals
• Service life and maintenance access