How to properly select cables for photovoltaic systems?
In recent years, the technology of the photovoltaic industry has developed faster and faster. The power of a single module is getting bigger and bigger, and the current of the string is getting bigger and bigger. The current of high-power modules has reached more than 17A. In terms of system design, the use of high-power components and reasonable reserved space can reduce the initial investment cost and kilowatt-hour cost of the system. The cost of AC and DC cables in the system is not low. How should we design and select to reduce costs?
1. Selection of DC cables
DC cables are installed outdoors. It is generally recommended to select irradiated and cross-linked photovoltaic cables. After high-energy electron beam irradiation, the molecular structure of the cable insulation material changes from a linear type to a three-dimensional mesh molecular structure, and the temperature resistance level increases from 70℃ for non-cross-linked cables to 90℃, 105℃, 125℃, 135℃, and even 150℃, which is 15-50% higher than the current carrying capacity of cables of the same specification. It can withstand drastic temperature changes and chemical erosion and can be used outdoors for more than 25 years. When choosing a DC cable, you should choose a product with relevant certification from a regular manufacturer to ensure long-term outdoor use. The most commonly used photovoltaic DC cable is the 4-square cable of PV1-F1*4, but with the increase in the current of photovoltaic modules and the increase in the power of a single inverter, the length of the DC cable is also increasing, and the application of 6 square meters of DC cables is also increasing.
According to relevant specifications, it is generally recommended that the loss of photovoltaic DC should not exceed 2%. We use this standard to design how to choose DC cables. The line resistance of PV1-F1*4mm² DC cable is 4.6mΩ/meter, and the line resistance of PV6mm² DC cable is 3.1mΩ/meter. Assuming the working voltage of the DC component is 600V, the 2% voltage drop loss is 12V. Assuming the component current is 13A, using 4mm² DC cable, the distance between the farthest end of the component and the inverter is recommended not to exceed 120 meters (single string, excluding positive and negative poles). If it is greater than this distance, it is recommended to select a 6mm² DC cable, but it is recommended that the distance between the farthest end of the component and the inverter should not exceed 170 meters.
2. Calculation of photovoltaic cable line loss
To reduce system costs, the components and inverters of photovoltaic power stations are rarely configured in a 1:1 ratio but are designed with a certain over-matching according to lighting conditions, project needs, etc. For example, for a 110KW module, a 100KW inverter is selected. According to the calculation of 1.1 times the over-matching of the inverter AC side, the maximum AC output current is about 158A. The AC cable can be selected according to the maximum output current of the inverter. Because no matter how many components are configured, the AC input current of the inverter will never exceed the maximum output current of the inverter.
3. Inverter AC output parameters
Commonly used photovoltaic system AC copper cables include BVR and YJV. BVR means copper core polyvinyl chloride insulated soft wire, YJV cross-linked polyethylene insulated power cable. When selecting, pay attention to the voltage level and temperature level of the cable. Flame-retardant type should be selected. Cable specifications are expressed by the number of cores, nominal cross-section, and voltage level: single-core branch cable specification expression method, 1*nominal cross-section, such as 1*25mm 0.6/1kV, indicating a 25 square cable. Multi-core twisted branch cable specification expression method, the number of cables in the same loop*nominal cross-section, such as 3*50+2*25mm 0.6/1KV, indicating 3 *50 square live wires, 1* 25 square neutral wire, and 1* 25 square ground wire.