Selecting Wire & Cable For Solar PV Applications

Cables are extensively used in photovoltaic systems and their management can often be critical for effective functioning. Various standards for cables used for the purpose are also being discussed. - Dr L Ashok Kumar

The renewable energy market is growing rapidly. This growth applies to wind energy as well as to solar energy. Due to public interest, a number of governments have decided to support the renewable energy economics with large amounts of subsidies. Thus, even a further increase in this market can be expected. Cable management is one of the most important aspects of the safety and longevity of nearly every photovoltaic (PV) system. This is primarily due to the extensive use of exposed cables in the DC PV array. Since the equipment is installed outdoors on rooftops and in open fields, the electrical conductors must be rated for sunlight resistance and be supported and secured properly. Most electricians are referred to as ‘indoor wiremen’ and are familiar with installing conductors in conduit, but have little, if any, experience working with exposed cables in power systems. These exposed cable power systems are common in the utility and petroleum industries and there are decades of experience with these types of systems. Since the National Electrical Code (NEC) does not regulate either of these industries, there is little in the NEC to guide these types of installations.

Photovoltaic refers to the direct generation of electricity by solar irradiation. Photovoltaic cells use special semiconductor materials, the most common being silicon, to harness solar energy. When light strikes the cell, some of the solar irradiation is absorbed within the semiconductor material. The energy of the light is now transferred to the semiconductor, knocking electrons loose and allowing them to flow freely and release energy. CSP technologies use mirrors to reflect and concentrate sunlight onto receivers that then collect the solar energy before converting it to heat. The resulting thermal energy is then used to produce electricity through a steam turbine or heat engine that drives a generator.
Currently, three common types of CSP technologies exist: a parabolic trough, a solar power tower, and a solar engine. A parabolic trough is shaped like a half-pipe and is covered with mirrors that are aligned north-south and pivot to follow the sun during daylight. These mirrors concentrate the sun’s rays onto heat transfer fluid pipes at 30 to 100 times their normal intensity. The pipes are then used to produce steam to spin a turbine to power a generator. Solar power towers, on the other hand, generally use thousands of flat sun tracking mirrors called ‘heliostats’ to concentrate the sun’s radiation onto a single tower-mounted receiver, at which point same process as the parabolic trough is applied. The last type of aforementioned CSP technologies is the solar engine, which contains both a solar concentrator and a power conversion unit. The most popular example of a solar engine is the Stirling engine, which uses a sun-tracking mirrored parabolic dish to direct captured heat to a hydrogen gas-filled piston. The piston then drives the engine to produce electricity. Over a twenty-year period, a Stirling engine system can generate over 850 MW of electricity.

Although the sun is an excellent energy source during the day, the method in which solar energy is stored is critical due to the lack of continuous supply. An effective and low-cost way of storing solar power is employing the use of molten salts. Salts have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems. Off-grid PV systems traditionally store excess energy with rechargeable batteries. With grid-tied systems, the excess electricity can be sent to the transmission grid and kept track of using net metering programs. Net metering programs give such systems a credit for each megawatt of electricity delivered to the grid. These credits offset electricity provided from the grid when the system cannot meet demand, effectively using the grid as a storage mechanism.

The photovoltaic market is a very specific market. Various national regulations are to be taken into account. The system size can vary from a small home power supply with a nominal power of few kilowatts up to large centralised solar plants in the Gigawatt range. The components used in these systems must be suitable for these specific applications. One common determining factor for all photovoltaic power systems is the outdoor use, which brings along high temperatures and, of course, high UV radiation. Weathering and humidity need to be taken into account, as well. Furthermore, safety and reliability aspects are very important.

Cable Requirements

Despite being a critical component, wiring for solar panels is rarely discussed. In home solar power systems, there are four components to connect together: the solar panels, the charge controller, the batteries, and the inverter. The charge controller is used to prevent the batteries from overloading; the wires that connect the panel to the charge controlled should be correctly sized to minimize transmission power loss.

Correspondingly, the further away the panels are, the larger the wire gauge should be. The inverter is used to convert the DC power collected by the panels into AC power, which is the most popular form of electricity accepted by appliances. These systems are typically outdoors, so any cable used for this type of application needs to be ultraviolet radiation resistant and suitable for wet locations. For solar tracking panels, the cables used need to be flexible as the panels will be moving along with the sun.

CSP systems have a lot of requirements similar to solar panel. However, in addition to the water and UV resistance, the cables used need to be able to withstand high temperatures.

Standards for Solar Power Cables

Both the United States-based agency Underwriters Laboratory (UL) and the German-based agency Technischer Uberwachungs-Verein (TUV) have approvals specifically for wires used in photovoltaic applications. UL has two types of approvals for photovoltaic applications: USE-2 and type PV. Originally, the standard approval for photovoltaic applications was USE-2. However, once there was a greater demand for wire for solar power, UL designed the UL type PV approval to meet the needs of such applications better. USE-2 and type PV approvals are similar; however, there are a few differences. PV wire can be used in both grounded and ungrounded PV arrays and is rated +90°C wet and +150°C dry with approvals for 600V, 1000V, and 2000V. Although USE-2 is only suitable for grounded PV arrays per NEC, it has a +90°C wet and dry rating and is only rated for 600 V. In addition to the better temperature and voltage range, type PV wires have better sunlight resistance, low-temperature flexibility, flame resistance, and thicker insulations for additional element protection.

UL 4703 Standard

In 2005 the American Underwriters Laboratories (UL) published the UL subject 4703 ‘Photovoltaic Wire.’ It covers single-conductor, insulated and integrally or non-integrally jacketed, sunlight resistant, photovoltaic wire in several temperature and voltage ratings for interconnection wiring of grounded and ungrounded photovoltaic power systems.

The standard UL 4703 is based on the service entry cords USE-2 and specifies some additional requirements for photovoltaic cables. The UL standard leaves more freedom for the manufacturer as far as the cable construction and the selection of materials is concerned. One or two insulation layers can be chosen optionally as well as a ‘skinned’ single layer construction. For two layer construction, as specified in the DKE-document, several combinations of wall thicknesses are allowed in UL 4703. But in each case the total sum of wall thickness will be higher than specified in the German specification (see table 1). Thus, the diameter for the same conductor cross section will be higher that reduces the suitability for several connectors. And the increased need of insulation compound will add to the production cost.

The main focus of UL 4703 is the fire performance. Tests to be performed are the Vertical Flame Test acc. to UL 1581, Section 1060 or optional the flame test VW-1 acc. to UL 1581, Section 1080. Based on RHW-2 the flame test FT-2 acc. to UL 1581, Section 1100 is also required. Therefore, another cable construction will be necessary to meet the UL-requirements.

The new developed solar cable is a single core cable built with a tinned stranded conductor. All materials are halogen free, flame resistant and fire retardant. No corrosive gases will be released in case of fire and the smoke density is low. The insulation and jacket materials are extremely resistant to weathering, UV-radiation and abrasion. The wide temperature range from -50°C to +150°C (fixed installation) enables the use of this cable in extreme weather conditions. Additionally, it is salt water resistant and resistant to acids and alkaline solutions.

The cable is flexible and designed for high mechanical loads. So it is suitable for fixed installation as well as for moving applications without tensile load. It is especially designed for outdoor use, which means direct sun radiation and air humidity, but due to the halogen free flame retardant cross-linked jacket material the cable can also be installed in dry and humid conditions indoors.

Fire Performance

Although the danger of fire propagation in outdoor applications such as solar plants is not a great risk for the safety of people, a good fire performance is required to protect the technical equipment. The document specifies a flame propagation test on the completed cable according to IEC 60332.1.

Absence of Halogens

In the case of fire, acids caused by the smoke of halogenated materials are a serious danger for people’s health as well as for the function of electric and electronic devices. In former times halogen free cables were required in public areas such as hospitals, airports and other similar structures. But due to the increasing importance of electronics in all areas of everyday life, this quality is increasingly required in industrial premises, too.

As far as for solar cables, this characteristic is especially important for solar power devices on residential buildings. Several tests have to be performed to prove the absence of halogens in solar cables. Electrical conductivity and pH value of the smoke are to be quantified according to European standard EN 50267-2-2. The content of chlorine and bromine is determined according to EN 50267-2-1 and a special test is developed for the content of fluorine in Annex C of the discussed specification.

There are different standards for solar cables in different countries. The requirements are very high, but they differ due to the various national philosophies in respect to safety and reliability issues as well as market and subvention aspects. The determining factors in this application are external conditions, which do not depend on national laws and can depend only on few local aspects. Therefore, a standard in the future will be established by international market acceptance. Due to the different requirements and philosophies in each individual standard, one cable cannot satisfy both standards at the same time. In order to meet these standards and not to compromise the performance of the cable, one type of cable is needed to satisfy the requirements of the specification and another cable type is needed to satisfy the UL requirements.

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