For photovoltaic modules used in grid-connected systems, a complete interconnection cable is typically provided, including a sealed junction box and plug-in connectors at the end of each section of the cable. Adjacent components are connected in series (the positive pole is connected to the negative pole, or the negative pole is connected to the positive pole) to form a component string. After the required number of modules are connected in series to form a module string, the circuit needs to be connected to a centralized location, usually a photovoltaic combiner box, where it is connected in parallel with other module strings. The fuses of all module strings will also be installed in the photovoltaic combiner box. The DC link is a very important component of the photovoltaic system, and many key factors must be considered during the design and installation.
①Cable path and required length
After determining the installation location of all equipment, the cable path must be determined. Figure 1 shows the main cable path. As with any system, the installer should look for the path that minimizes the length of the cable. The cable length and cross-sectional area determine the voltage and power loss it can withstand.
In addition, when planning the cable path, the wiring path of the photovoltaic array should minimize all conductive loops. Reducing the conductive loop will reduce the risk of overvoltage caused by lightning strikes in the photovoltaic array, and reduce the interference to AM/FM radio signals.
The wiring path from the PV array to the inverter (through the PV combiner box) uses DC cables, and cables with appropriate DC voltage and current should be used. According to local regulations and specific national standards, the current-carrying capacity and rated voltage of the cables used can be determined. Copper wires are generally better than aluminum wires, and there are single-core and multi-core products. All cables need to be insulated to protect the lines from damage to the surrounding environment and to protect the safety of people and equipment. Depending on temperature, sunshine, oil or water resistance, and site conditions (dry or wet), cable insulation is diverse. The color of the insulation shows the polarity of the wire and generally depends on the national electrical code/standard. The standards of cable insulation colors vary greatly from country to country. In many parts of the world, brown insulation indicates a positive wire, and blue or gray insulation indicates a negative wire.
However, red (positive) and black (negative) are also very common color combinations. In a typical case, the green and yellow insulation indicates the ground wire, but this is not true everywhere. During the installation process, the coloring rules stipulated in the local wiring code should be followed.
When selecting a cable for a system (that is, determining the cable specification), the current carrying capacity and expected voltage drop must be considered, that is, how much current the wire can carry, and how much power loss is caused by the internal resistance of the wire.
Cables can be divided into many types according to their current carrying capacity at a certain operating temperature. The current carrying capacity characterizes the amount of current that a wire or cable can pass before it is damaged. The current-carrying capacity depends on the wire type (copper or aluminum), wire size/specification, insulation class (insulation class for wet conditions outdoors), maximum insulation temperature and site (air-cooled, ducted or landfilled). As the temperature increases, the current-carrying capacity will decrease.
Voltage drop is a key issue in photovoltaic systems. As the current increases and the size of the wire decreases (that is, the cross-sectional area of the cable or wire is smaller, the internal resistance of the wire is higher, and the voltage drop is larger), the voltage drop will increase. The size and resistance of terminals, fuses, and disconnecting/isolating devices will also affect the voltage drop. For grid-connected photovoltaic systems, the usual approach is to appropriately select the cable specifications so that the voltage drop across the DC wire or AC wire does not exceed 1%. The regulations of most countries specify the maximum allowable voltage drop and should be carefully referred to. For example, the German standard stipulates that the maximum voltage drop is 1%, but currently a 5% voltage drop is allowed in Australia. Since the voltage drop represents a reduction in the output power of the system, it is recommended to reduce the voltage drop in the cable as much as possible. Generally, it is cheaper to buy a large size cable than to increase the number of components to compensate for the output power loss. If the voltage drop is not considered reasonably, it may affect the operation of the inverter detection circuit.
Photovoltaic array cables should also be marked with ratings for use under humid conditions, have UV stability, have appropriate protection against various elements, and be rated at the maximum system output voltage and current according to local electrical standards. Now, many companies put a “solar cable” label on the cables they produce, indicating that they are dedicated to photovoltaic systems. The design of solar cables should ensure safety in outdoor environments, with high resistance to ultraviolet rays, and double insulation (also called double-sheathed cables). Solar cables are used to carry DC current and voltage, and many local standards require them to be marked, so that they can be easily distinguished from other cables. Solar cables come in a variety of sizes and specifications, and system designers can choose flexibly to ensure that the voltage drop is minimized. Solar cables are usually flexible, so they are easier to install. It is a good practice to install and fasten the cable to avoid direct sunlight as much as possible, so that the cable is not directly fixed on the roof and does not move in the wind. Mechanical protection greatly reduces the risk of cable damage, ground faults, and even fire. The cable should be secured, and it is possible to use a conduit to protect the cable. In addition to the deterioration of the cable under severe weather conditions, there is also the risk of the cable being attacked by opossums or rodents. Placing the cable in the conduit can solve these protection problems.
③ Photovoltaic combiner box
The photovoltaic combiner box is used to combine multiple photovoltaic branches into a few parallel circuits, thereby reducing the number of wiring. The size of the PV combiner box must be correct, so that there is enough space to place the specified number of cables, and there is no risk of breaking the cables. The cables inside the combiner box should be appropriately colored and marked, and the ratings of the combiner box should be suitable for the environment in which it is installed.