From the perspective of the different functions undertaken, the cables in the photovoltaic system can be mainly divided into two types: DC cables and AC cables.
1, DC cable
(1) A series cable between components and components.
(2) Parallel cables between strings and their strings to DC distribution boxes (confluence boxes).
(3) The cable between the DC distribution box and the inverter.
The above cables are all DC cables. They are laid outdoors in large quantities and need to be protected from moisture, sunlight, cold, heat, and UV rays. In some special circumstances, they must also be protected against acids and alkalis.
2, AC cable
(1) Connecting cable from inverter to step-up transformer.
(2) Connecting cable for step-up transformer to power distribution unit.
(3) The connection cable from the power distribution unit to the grid or user.
This part of the cable is an AC load cable, and there are many indoor installations. It can be selected according to the general power cable selection requirements.
Photovoltaic special cableA large number of DC cables in photovoltaic power plants need to be laid outdoors, and the environmental conditions are harsh. The cable materials should be determined according to UV resistance, ozone, severe temperature changes, and chemical erosion. The long-term use of cables with common materials in this type of environment will result in fragile cable jackets and even breakdown of cable insulation. These conditions can directly damage the cable system, and at the same time increase the risk of short-circuiting the cable. In the medium and long term, the possibility of fire or personal injury is also higher, which greatly affects the service life of the system. Therefore, it is very necessary to use photovoltaic dedicated cables and components in photovoltaic power plants. Photovoltaic-dedicated cables and components not only have the best weather resistance, UV and ozone resistance, but also withstand a wide range of temperature changes.
(1) The withstand voltage of the cable must be greater than the maximum voltage of the system. Such as 380V output AC cable, to elect the 450/750V cable.
(2) The connection between the inside of the photovoltaic array and the square array, the selected cable rated current is 1.56 times the maximum continuous current in the calculated cable.
(3) AC load connection. The rated current of the selected cable is 1.25 times the maximum continuous current in the calculated cable.
(4) Connection of the inverter, the rated current of the selected cable is 1.25 times the maximum continuous current in the calculated cable.
(5) Consider the effect of temperature on the performance of the cable. The higher the temperature, the smaller the cable's current-carrying capacity, and the cables should be installed wherever possible for ventilation and heat dissipation.
(6) Consider that the voltage drop should not exceed 2%.
In operation, the DC loop is often affected by various unfavorable factors and causes grounding, making the system unable to operate normally. Such as extrusion, poor cable manufacturing, insulant material failure, low insulation performance, insulation aging of the DC system, or the presence of certain damage defects can cause grounding or become a grounding hazard. In addition outdoor environment, small animals invade or bite can also cause DC ground fault. Therefore, in this case, armored, cabled with a rodent protection functional sheath is generally used.
Distributed photovoltaic common inverter cable selection:
Note: The cable in the table is set according to the standard of the national standard copper wire.
If the cable length is greater than 50 meters, refer to the larger size cable.
How to select the most economical inverter output AC cableIn the photovoltaic system, the temperature of the AC cable is also different due to the different environment in which the line is installed. The distance between the inverter and the grid point is different, resulting in different voltage drop on the cable. Both temperature and pressure drop will affect the loss of the system. Therefore, the line diameter of the output current of the inverter must be reasonably designed. Considering all factors, it is necessary to reduce the initial investment of the photovoltaic power plant and reduce the system's line loss.
In the cable design selection, the technical considerations mainly include the rated ampacity, voltage, temperature, and other technical parameters of the cable. The cable diameter, bending radius, and fire resistance are also considered during installation. The cable price must be taken into consideration when calculating the cost.
1. The output current of the inverter and the cable current carrying capacity must be the sameInverter output current is determined by the power, single-phase inverter current = power / 230, three-phase inverter current = power / (400 * 1.732), and some inverters can also be 1.1 times overload, refer to the next Picture:
The cable current capacity is determined by the material, wire diameter, and temperature. There are two kinds of copper and aluminum wires in the cable. Each is useful. From the viewpoint of safety, it is recommended that the inverter output the copper wire for the AC cable. The BVR is generally selected as the single-phase cable. Wire, PVC insulation, copper wire (soft) cloth wire voltage rating of 300/500V, three-phase selection of 450/750 voltage (or 0.6kV/1kV) grade YJV, YJLV irradiation cross-linked polyethylene insulation PVC For the sheathed power cable, the relationship between the cutoff of the wire and the temperature, if the ambient temperature is higher than 35°C, the allowable current should be reduced by about 10% for every 5°C increase; if the ambient temperature is lower than 35°C, the temperature The permissible current can be increased by about 10% for each 5°C decrease. Under normal circumstances, if the cable is installed in a ventilated place, the maximum ambient temperature is below 40°C. If the cable is installed in a place where it is exposed to sunlight, the maximum ambient temperature may be To 50 degrees.
Such as a 20kW inverter, the output current is 32A, the reference table, 4mm cable at 40 degrees, the carrying capacity is 32A6mm cable at 50 degrees, the carrying capacity is 33A, so 20kW inverter uses 4mm cable or 6mm Cables can meet the requirements.
2, cable economic designIn some places, the inverter is far away from the grid connection point. Although the cable can meet the requirements of the current-carrying capacity, due to the long cable length, the line loss is relatively large. In this case, consider using large cables to reduce the loss because the cable The greater the resistance, the less internal resistance. But also consider the price of the cable, the AC output of the inverter seals the outside diameter of the terminal block.
For example, for a 20 kW power station, the total length of three cables from the inverter to the grid connection point is 100 meters. The average daily sunshine time is 4 hours. The output current is 32 A. The output cables can be 4 mm, 6 mm, 10 mm, and 16 mm. 4 square cable total internal resistance 0.46 ohms, market price is 3.5 yuan/meter; 6 square cable total internal resistance 0.31 ohm, market price is 4.8 yuan/meter; 10 square cable total internal resistance 0.18 ohm, market price is 9.5 yuan/meter 16 square cable total resistance 0.115 ohms, the market price is 13.5 yuan/meter.
Use 4mm cable, one-day loss is I2Rt = (302 * 0.46 * 4) / 1000 = 1.656 degrees, about 600 degrees a year, according to 0.75 yuan per kilowatt-hour calculation, loss of 450 yuan a year;
6mm cable is used, the loss of one day is I2Rt=(302*0.31*4)/1000=1.116 degrees, about 400 kWh per year, 0.75 yuan per kWh, and 300 yuan per year;
Select 10mm cable, the loss of one day is I2Rt = (302 * 0.18 * 4) / 1000 = 0.65 degrees, about 240 degrees of electricity a year, calculated at 0.75 yuan per kilowatt-hour, a year loss of 180 yuan;
Use 16mm cable, one-day loss is I2Rt = (302 * 0.115 * 4) / 1000 = 0.414 degrees, 150 electricity a year, according to 0.75 yuan per kilowatt-hour calculation, a year loss of 110 yuan;
Can be seen from the table, although 4mm cable can meet the output requirements of 20kW inverter, but the cable power loss is relatively large, select 6mm cable to make up for the cable price difference a year, so the AC output cable try to use more Rough cables, from an economic point of view, it is better to choose 10mm for 20kW.
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