How to choose the right solar cables for your PV system?

TABLE OF CONTENTS

• Selection of DC Solar cables
• Selection of AC solar cables
• What is the difference between a single-core cable and a multi-core cable?
• Wiring tips for photovoltaic systems

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In recent years, the technology of the photovoltaic system is developing faster and faster, the power of single modules is getting bigger and bigger, the current of the string is also getting bigger, the current of high power modules has reached more than 17A.

In terms of system design, the use of high power PV modules and reasonable over-provisioning can reduce the initial investment cost and the cost of electricity.

The cost of AC/DC Solar cables in the system is not low, How should the design be selected to reduce costs?

Selection of DC Solar cables

DC Solar cables are installed outdoors underneath, it is generally recommended to select the irradiated cross-linked photovoltaic cables, after irradiation by high-energy electron beam, the molecular structure of the cable insulation layer material from linear to three-dimensional mesh molecular structure, temperature resistance level from non-cross-linked 70°C to 90°C, 105°C, 125°C, 135°C, even 150°C, than the same specification of the cable's load capacity increased by 15- 50%, The solar cable can withstand severe temperature changes and chemical erosion, and also it can be used outdoors for more than 25 years.

DC solar cables need to be selected from regular manufacturers with relevant certifications in order to ensure the long-term outdoor use of the demand.

Video illustration of the temperature resistance of irradiated and ordinary cables

PV DC solar cables are currently more commonly used as PV1-F 1*4 4-square cables, but with the increase in PV module current and the increase in power of the stand-alone inverter, the length of DC cables is also increasing and the use of 6-square DC solar cables is also increasing.

According to the relevant specifications, it is generally recommended that the loss of PV DC should not exceed 2%, we use this standard to design how to choose DC solar cables, PV1-F 1*4mm² DC cable has a line resistance of 4.6mΩ/m, PV 6mm² DC cable has a line resistance of 3.1mΩ/m, assuming that the DC PV array working voltage is 600V, 2% voltage drop loss is 12V, assuming that the module current is Assuming a module current of 13A and using a 4mm² DC cable, it is recommended that the distance between the furthest end of the module and the inverter should not exceed 120m (single string, without positive and negative poles).

▲ PV cable line loss calculation

Selection of AC solar cables

In order to reduce PV system costs, the solar modules and Solar inverters of PV power stations are now rarely configured according to a 1:1 ratio, but a certain amount of over-matching is designed according to light conditions, project needs, etc.

For example, if a 100KW inverter is used for a 110KW module, the maximum AC output current will be 158A according to the 1.1 times over-provisioning on the AC side of the inverter, and the AC solar cable will be selected according to the maximum output current of the inverter. This is because no matter how much the modules are over-matched, the inverter AC input current will never exceed the maximum output current of the inverter.

▲Inverter AC output parameters

The main reason for this is that the inverter's maximum output current will always exceed the inverter's maximum output current. *25mm 0.6/1kV, indicating a 25 square cable. Multi-core stranded branch cable specifications are expressed in terms of the number of cables in the same circuit * nominal cross-section, e.g. 3*50+2*25mm 0.6/1KV, which means three 50 sq. ft. fire wires, one 25 sq. ft. zero wire and one 25 sq. ft. earth wire.

What is the difference between a single-core cable and a multi-core cable?

A single-core cable is one that has only one conductor in one insulation layer.

A multi-core cable is a cable with more than one core of insulation.

In terms of insulation performance, both single-core and multi-core cables need to meet national standards.

The difference between multi-core cables and single-core cables is that single-core cables are directly earthed at both ends, and the metal shield of the cable may also produce a loop current, forming a loss;

multi-core lines are generally three-core lines, because in cable operation, the total current flowing through the three cores is zero, and there is basically no induced voltage at the ends of the metal shield of the cable.

From the circuit capacity, single-core and multi-core cable, the same cross-section, single-core cable rated load capacity is greater than the rated load capacity of three-core cable;

single-core cable heat dissipation performance than multi-core cable heat dissipation performance, in the same load or short-circuit situation, single-core cable heat less than multi-core cable, more secure;.

From the cable laying, multi-core cable laying more simple and convenient.the cable has an inner layer and multi-layer double protection more secure;

single-core cable laying bending easier, but the long distance laying difficulty single-core cable is greater than multi-core cable.

From the installation of the cable head, the single-core cable head is easier to install and convenient to divide the line. In terms of price, multi-core cables are slightly more expensive than single-core cables.

Wiring tips for photovoltaic systems

The wiring of a photovoltaic system is divided into a DC section and an AC section.

These two sections of wiring need to be wired separately, with the DC section connected to the modules and the AC section to be connected to the grid.

There are more DC cables in medium and large power stations. To facilitate future maintenance, the wire numbers of each cable should be posted firmly. Strong and weak wires are separated, and if there are signal lines, such as 485 communication, they have to be routed separately to avoid interference. The alignment should be prepared for threading pipes and bridges, try not to let the lines exposed, and the alignment will look better when it is horizontal and vertical.

Try not to have cable glands in the threading tubes and bridges, as maintenance is not convenient. If there are scenarios where aluminium wires replace copper wires, reliable copper and aluminium adapter terminals must be used.

Cables are a very important component of the entire PV system and represent an increasingly high proportion of the cost in the system. When we do the design of a power station, we need to save as much as possible on the system costs while ensuring reliable operation of the power station. Therefore, the design and selection of AC and DC cables for photovoltaic systems is particularly important.