Publish Time: 2023-05-05 Origin: Site
In photovoltaic systems, we need to use three types of cables: photovoltaic cables, AC cables and grounding cables. We all know that photovoltaic cables are usually laid outdoors and need to be protected from moisture, direct sunlight, low temperature and ultraviolet rays. So you must choose photovoltaic professional cables, which cannot be replaced by conventional cables, and TUV-certified photovoltaic cables have UV protection, insulation protection and DC withstand voltage, usually 600V DC cables and 1500V DC, which are better than conventional cables. The more commonly used wiring is PV1-F4 square.
Grounding cables are mainly used for lightning protection grounding of solar energy systems. We just need to determine the ground cable we use to meet the system ground resistance requirements.
AC cables are used to connect the AC output of the inverter to the grid. They are usually installed outdoors, so they also require the same protection characteristics as PV DC cables. The selection of AC cables becomes more complicated due to the different output currents of the inverters. At present, the main basis for selecting AC cables is the relationship between cable diameter and ampacity, but the influence of ambient temperature, voltage loss, and laying method on the ampacity of photovoltaic cables is generally ignored. Then how to correctly choose the correct AC cable in the photovoltaic system.
The following points need to be considered in the selection of photovoltaic cables for solar energy systems:
1. Voltage loss
The voltage loss in a solar photovoltaic system can be expressed as:
Voltage loss = passing current * photovoltaic cable length * voltage coefficient
The voltage loss is proportional to the length of the PV cable.
When designing and installing the system, the principle that the distance from the photovoltaic module array to the inverter and from the inverter to the grid connection point should be as close as possible should be followed.
We need to ensure that the DC voltage loss between the photovoltaic array and the inverter is less than 3% of the output voltage of the array, and the AC voltage loss between the inverter and the grid connection point does not exceed 2% of the output voltage of the inverter.
Calculation formula: 4U=(I*L*2)/(r*S)
Note: 4U: Cable voltage drop -V
I: The maximum current that the cable needs to withstand -A
L: cable laying length - m
S: cable cross-sectional area-mm2
r: Conductivity of conductor -m/(Ω*mm2), r=57 for copper, r=34 for aluminum
When we calculate the ampacity of photovoltaic cables, in addition to referring to the parameters in the ampacity table, we also need to consider the type of wire, installation method, and ambient temperature, and obtain the actual current value through these correction factors.
The current-carrying capacity of photovoltaic cables varies according to the ambient temperature. The data sheet of each manufacturer's photovoltaic cable will have a corresponding temperature correction coefficient table in order to make the correct choice.
3. Parallel laying of multi-core photovoltaic cables
In the actual installation scenario, the AC photovoltaic cable of the photovoltaic system may be laid in parallel with multiple multi-core photovoltaic cables. For example, in a small-capacity three-phase system, the AC photovoltaic cable uses "1 four-core cable" or "1 five-core cable" cable. The single-phase system will use "1 two-core cable" or "1 three-core cable" cable; in the large-capacity three-phase system, the AC wiring uses multiple photovoltaic cables in parallel instead of single-core large-diameter cables. In this case, the current carrying capacity of the actual PV cable will be attenuated. We need to take this attenuation into account at the beginning of the project design,
We take a residential project with a single-phase inverter installed as an example to calculate the PV AC cable. The on-site AC cable is 30 meters away from the grid connection point. We use AC cables with a PVC sheath.
1. Rated output current = 26.0A
2. Maximum output current = 27.3A
3. Cable type: 1 two-core AC cable with PVC protection;
4. Cable part: the maximum AC output current is 27.3A, and the normal rated current of 4 square photovoltaic cables is 39A (in air).
5. When the ambient temperature is 45°C, the temperature correction coefficient is 0.79;
6. The single-phase inverter uses a two-core AC cable, and the correction coefficient is 0.85;
Actual ampacity calculation (coefficient correction):
39A*0.79*0.85=26.2A <27.3A
Voltage loss: 4U= (I*L*2) /(r*S)= (27.3*30*2)/(57*4) =7.18V ;
The grid voltage is 230V, so the voltage loss is greater than 230V*2%=4.6V.
The selected AC cable cannot be used in this example because the maximum current carrying capacity for trouble-free operation is lower than the maximum output current of the inverter used. Example solution:
Use 6 square PV cables
The normal rated current of 6 square photovoltaic cables is 50A (in air).
Actual ampacity calculation (coefficient correction):
50A*0.79*0.85=33.575A >27.3A
Voltage loss: 4U= (I*L*2)/(r*S)= (27.3*30*2) / (57*6) =4.78V; the grid voltage is 230V, so the voltage loss is close to 230*2%= 4.6V.
Therefore, 6 square photovoltaic cables are the best choice.
In conclusion
To avoid considerable voltage loss and avoidable failures in solar PV systems, the correct PV cable must be selected every time. Every system needs to have wiring decisions built in during the design phase to take into account the distance between critical components (modules, inverters, grid connections) and any other external factors that may affect the current carrying capacity of the wiring, such as external ambient temperature. Selecting TUV certified PV cables along with proper system design will ensure you have the safest and most efficient PV cable solution for your next solar PV installation.
Since our establishment in 2013, XSD Cable has been one of the professional manufacturer in the field of wire and cable.