Electrical matrimony

“Without cables and connectors, power plants are just dead silicon, glass and aluminum,” Guido Ege, of Lapp Group’s product management, candidly tells pv magazine . Yes, the fraction held by cables and connectors of the total system cost is small. Nevertheless, it is not a reason to dismiss them as after-thoughts because they play a crucial role in transporting the precious power module manufacturers try their hardest to generate. The cables have to cover all possibilities of power ranges to be transported and the connectors complete the daisy chaining.
Cables and connectors have to endure the term that a PV module has to fulfill on the field and on rooftops, that is 25 years. However, the products are different, with different materials and characteristics. “They need to be designed to withstand warming and UV radiation over the long term, or else resistance will increase over time, which leads to an overall power loss,” explains Ege. Now, from a practical point of view, the installers are the crew who handle the connection of the cables to one another in the daisy-chain process.Most cables, these days, come pre-assembled, meaning that male and female connectors are already attached to the cables and there is no need for the installer to do this attachment process on site. However, this pre-assembly makes sense most of the time for ground-mounted installations because the modules are usually aligned in a fixed manner, with a fixed distance from one another. Thus there is not necessarily a need for the person in the field to adjust cable lengths from module to module. Now, rooftop installations are another story. Here the distance between the modules may not necessarily be constant, thus needing individual cable calculations. Pre-assembled cables and connectors may also not work if the installer has to run a cable through a wall or a structure or a narrow crevice; situations that can arise with rooftop installations. In this case, the installer will have to embark on the cable-connector attachment process on site. Moreover, the height of the connector matters as well when putting the cables and connectors through narrow crevices. Lower heights, that is flatter models, can be squeezed through smaller openings easily when the need arises. Flatter models like Lapp Group’s (twelve millimeter height) come in handy in such cases. Lapp Group’s Product Manager for photovoltaics, Stefan Koch, explains, “Flat connectors are also needed for thin film modules and when they need to pack these modules, it is better to get thinner connectors which won’t take up more space.” The product data for connectors actually states that the cables or other connectors that can be used together. Mostly, connector manufacturers also produce their own PV cables. After all, it makes sense to provide the product in a double-pack. Lapp Group for example supplies pre-assembled cables and connectors due to their patented welding process that is done in the factories. “The Epic Solar 4 Thin connector is designed for module manufacturers. And our system is mateable with the MC4,” Koch explains. He adds that at the end of the year Lapp Group will be introducing their own new field mounted connectors for the installers. Nevertheless, there are manufacturers that offer connectors that are compatible with other manufacturers’ cables. Lumberg’s connectors are suited for Leoni Studer’s cables, like they are offered on Schott Solar’s ASI modules. Tyco’s Solarlok is suited for HUBER+SUHNER and Draka’s cables as TRITEC’s website states.
According to Daniel Lenel, Technical Leader Solar Products, Amphenol Industrial, the company’s connectors can be mounted on any PV and USE-2 rated cables, provided the jacket diameters are between 4.5 and eight millimeters. The connector bodies and cables glands are identical for the entire cable range but the appropriate contacts have to be selected according to the conductor size. As long as high quality PV cables are matched to connectors with appropriate contact size and cable gland size, the manufacturer does not have to be the same.

Standardization

Lenel says, “Connectors specifically designed for PV applications are fairly new. Most currently standardized (non-PV) connectors started out at some point as private, proprietary designs that were copied by others and eventually standardized. The driving force for standardization is usually exerted by the OEMs that require several sources but also intermateability. However, this doesn’t always end up in a standard. There are many industry-wide used connector designs that were never formally standardized. As you may be aware, there is a de-facto industry standard, the Multi-Contact MC4 connector. Some manufacturers, including Amphenol, try to match all significant characteristics obtained by analyzing the MC4 and therefore are intermateable for all practical purposes.” But at the end of the day, the MC4 is a private design and Multi-Contact has the control over the master design details.
The lack of standardization in the connector world leads to independent manufacturers explicitly stating that their crimping tool ought to be used with their connector to avoid complications. FPE Fischer’s Managing Director Thomas Hoffmeister says that it would be nice to have standardization in connectors but the process would be difficult. Hoffmeister says that when the entire system is connected, the same connectors ought be used preferably. “Everybody tries to use the Multi-Contact crimping tool. Some others like Phoenix have clips and thus you do not need a crimping tool. This is easier for the installer. It would be worse if the poor guy has to bring a few crimping tools onto the roof. In such a sense, it would be nice to have some standardization,” explains Hoffmeister.
So, there is no standard design or contact standard, thus making interchangeability and compatibility an issue. HUBER+SUHNER designed their connector families to ensure proper interconnection to themselves. Having in mind that it can be potentially highly dangerous to interconnect connectors from different sources, that look alike at the first glance and might be mated together, but are not verified or designed to be interconnected.Phoenix Contact’s alloys for one are corrosion reducing to allow long lifetimes. This, however, means that their connectors are not compatible to other plugs. Beck reasons, “If there are unknown alloys in foreign plugs that do react with ours, how could we grant a long lifetime? This is a very big problem of compability.” Industry experts like Lenel believe that pressure from OEMs and electrical authorities will eventually drive a connector standard.

Electrical standards

What has been established are electrical standards such as the National Electric Code or NEC, a codified requirement for safe electrical installations and the Underwriters Laboratories (UL) in the U.S. UL categorizes connectors under critical components of PV modules and subjects them to UL 1703, 1977 and 746C. The word ‘National’ might imply a nationwide adopted federal code but this is not true as Lenel highlights. “In the U.S., there is no federal code for electrical installations. It is up to every state to adopt their own standard. Most states just use the NEC, but some states make changes or have additional requirements. In the 2008 revision, the NEC addressed the PV cable and PV connector issues and required that locking connectors are used,” he adds. IEC 60364 is the base for most national electrical codes. However, most countries have their own adaptation of IEC 60364 published as a national code. TÜV Rheinland applies the EN 50521. Lapp Group’s Koch says perhaps the pin, the diameter of the pin, the sealing and locking systems need to be defined. These are the standards the market needs. “TÜV, our laboratories and others all test according to these EN standards. TÜV marketed their certification very well and so now their certification is preferred by the market,” says Koch.
Mani Tamizhmani, Professor at the Department of Engineering Technology at Arizona State University states that understanding PV connectors are important due to electrical safety issues and contact resistance. Tamizhmani says that power losses and lifetime reduction of connectors can occur due to contact resistance. In small installations, the power loss may not be that significant but it can add up in utility scale installations. Low contact resistance is crucial for the operations reliability for a long period at high currents.Lenels adds, “We specify the contact resistance of our H4 connector with solid crimp contacts at 0.25 m? (milliohm). However, measured contact resistance is typically between 0.1 and 0.15 m?.” PV cables and connectors exist in hostile environments, with high ambient temperatures. A few degrees higher contact- or crimp temperatures can already push the material limits. This can cause material failure like cracks or corrosion. Often the effects of high temperature are compounding; a marginal contact deteriorates with prolonged operation at excessive temperatures, resulting in increased resistance which in turn increases the temperature. The contact resistance is controlled by cable-contact connection, the material used for the contact and even the type of crimping used to connect.

Crimp, click or weld

HUBER+SUHNER’s Product Marketing Manager Peter Müller shows pv magazine the cross section of what a H+S crimp looks like. “It is important that the effective contact area between cable and connector is as big as possible. Also the typical barrel type crimp shows advantages in long-term reliability due to the full protection of the crimp in the enclosed barrel. The better the contact the lower the losses,” explains Müller. Lenel says that barrel type (tube type) crimps show the best performance, electrically and mechanically. B-type crimps (open crimp) have typically a slightly lower performance. “Amphenol’s high performance contacts use barrel crimps but we also offer stamped contacts with B-type crimp for high volume applications.” Apparently, B-type crimps are better suited for automatic high volume assembly machines. Do companies that weld like Lapp Group or have click-contacts like Phoenix Contact fair better with contact resistance? Phoenix Contact’s Andreas Beck says, “We have a flat contact surface where the inner litz wires also are contacted-so we reach a larger contact area. Our resistance shows no difference to crimp contacts.” Beck adds that some connections from Phoenix Contact use crimping too. The issue he sees with crimping is that the usage of wrong crimping tools may result damages in the crimp holder itself and that can affect the lifetime. Those problems can be avoided by using the spring technology.
Lenel has another take. “Crimping provides a fast, high performance termination that can be equally well done in the field as well as on specialized automated equipment for high volume applications.” He explains that welded terminations have good performances but can only be made on special, industrial equipment and require a specially formed contact. These welded contacts are for high volumes and factory assembly according to Lenel. Lapp Group’s Epic Solar 4 Thin boasts a zero transition resistance. This resistance advantage is assured by the patented welding technique. Koch explains that the welding process works with very high temperatures. The stripped wires from the cable and the connector contacts melt into one unit in this process. “With crimping you press the contact around the wires of the cable and over time the resistance increases. Higher resistance produces high temperatures. And this high temperature (80 to 90 degree Celsius) ages the plastic material faster,” Koch explains.
Resistance of cables is also an important thing to note. “Resistance in cables is determined by diameter – the larger, the lower is resistance. For optimum profitability of a PV plant, you have to calculate the break-even point. At some point, the cost for more copper that results from a larger diameter becomes higher than the gain that decreased resistance delivers,” Ege says.
Contact materials differ as well from pure copper, bronze, beryllium-copper-tin alloys and so on. “Contact material as well as plating material affects the contact resistance. Best performance show copper and copper alloys. Pure copper is a very good conductor, however it is quite soft,” Lenel elaborates. Many connector designs require a spring-like material to ensure a good connection between the mating contacts. The design and material for these inserts is crucial for the performance of the connection as Lenel explains highlighting the company’s Radsok technology connector. Special copper alloys are usually used for such inserts.

Avoiding accidents

Installers are careful people, so benefit of the doubt can be granted. Nevertheless, not everyone else can be granted this benefit. What happens if someone pulls the connectors apart, regardless of on a rooftop or on the field? Or what if somebody trips over a cable accidentally? Active PV systems can operate up to 1,000 Volt DC. An absolutely lethal voltage. DC current, running through the cables and connectors in a PV installation, is more dangerous than AC, because DC is smooth, unvaried and continuous unlike AC. If a connector is unmated under load, arcs may develop that can burn through insulation. Even traces of moisture in such cases can conduct enough current to electrocute someone. So, loose connectors and easily disconnected cables can be lead to misfortune. “The NEC 2008 requirement for locking connectors was long overdue and an absolute must for PV systems safety,” adds Lenel. IEC 60512 requires a minimum separation force of 50 Newtons (N) for non-locking and 80N for locking connectors. A MC4 connector, for example, fulfills the requirement under the above-mentioned NEC code, which calls for the connectors to lock once put together and requires a tool to separate them. These locking connectors have relatively low insertion forces compared to other types. The non-locking versions are held together by static friction between them. Most connectors have a lock-system like connectors from Amphenol Industrial, Lapp, HUBER+SUHNER and Lumberg. Müller explains the difference between the company’s Radox Solar Connector push-pull and the Solar Connector with integrated Twist Lock. The Solar Connector push-pull has no locking systems like the Solar Twist Lock. “The push-pull connectors can be separated, using a certain pull force. The Twist Lock connector has an additional mechanical lock using the principal of a thread. To open it a twist and a pull of the connector couple is necessary, but no tool. Thus an unintentional separation will be avoided. For the American market, even the Twist Lock function is usually not enough. In the U.S., an additional tool to open the connection is required. Therefore we provide the NEC lock, a plastic cage, in additional which, after closing, can be opened only with a screwdriver.” Others provide a similar click system or rely purely on the wonders of static friction. As an example, Tyco Electronics introduced the Solarlok latching style of PV coupler housing for the market. The installer is able to get a tactile feel and hears an audible click when he connects.
The next factor is the force required to rip the cable from the connector. Ege adds “Obviously, the cable needs to be fastened securely and permanently, and resistance should be as low as possible.” IEC 60512 states the minimum cable gland retention force as 60N or 80N depending on the cable diameter. If the force required to pull the connectors apart is significantly high (for example, 100N), then any really strong tugging might rip the cable from the connector first, if the cable gland retention is lower. This then exposes live wires. Lenel agrees, “This indeed is a real danger that is often overlooked. Amphenol designed the connector specifically so that the locking mechanism opens reliably before the cable is pulled out under an overload condition.” He says installers and operators have to ensure they don’t overlook this aspect.

Teething problems

‘Marten paralyses solar system’ was one headline that caught pv magazine ’s attention. Animals bit through the cables of an installation in Wülfrath in Germany. The local newspaper, the Rheinische Post, reports that the efficiency of the modules on Marianne and Klaus Alders’ roof fell rapidly, reaching only 40 percent of the previous levels. A roofer discovered a whole nest of animals on the roof and the animals had been gnawing away on the connectors and cables. What was interesting was to hear Alders say that they were not covered by building insurance for such a case. The Alders suffered 5,000 Euros worth of damage. To hinder unnecessary interruptions and losses during the energy generation phase, it is important for system owners to invest in the right cables and connectors. These ‘protected’ cables maybe more expensive than the normal ones but in the long run, they prevent damages that can happen through animals. Companies like Lapp Group offer cables with steel outer sheathings to keep away animals which take a shining to cables. Cables have to therefore not just endure the atmospheric conditions but also be protected against these toothy creatures. Jens Quednow, of Solarpraxis AG’s engineering department says that with open installation and flat roof systems, especially in rural areas in Germany for example, it is often the case that martens nibble DC cables. He advises running cables in protective conduits and for AC cables, running them underground and also in protective tubes. Christian Dürschner from the same department adds that he is aware of one case where a raccoon had chewed on a DC cable, triggering an arc which eventually led to a fire.
For underground cables, the horror of heat, humidity and bacteria are problems. These can damage cables over time as well as expose the live wires. Insulation damage can cause arcs, leading to dangerous situations as mentioned before. “Cross-linked material that is extremely sturdy helps keep insulation intact over long terms, 25 years. We have our own lab where we test all products for those requirements in the development phase, and the maintenance of our quality in production are being controlled all the time,” explains Ege. Essential also for cables is to be halogen-free which many cable companies like HUBER+SUHNER, Draka, Sibert, Prysmian, Tyco and Lapp Group among others offer. Cables with halogen content tend to emit dangerous fumes. The corrosive element of these gases can potentially damage electronics wherever the smoke travels and the toxic elements can be hazardous to people.
With the drive towards complete automation and the solutions for seamless handling of cables and connectors already in place, it will only be a matter of time before standards (for connectors) emerge in this fraction of the photovoltaic chain. Safety aspects should not be overseen or dismissed as secondary when investing in these components. The pressure is increasing to ensure that photovoltaic installations are as uncomplicated and safe as possible in order to compete with the conventionals. It is just not enough to have the best modules and inverters present. One ought not be a scrooge about the additional components, as these little connections can make or break the entire system.