Choose the Correct Solar DC Cable for Solar PV System
The DC trunk line is the transmission line from the photovoltaic module system to the inverter after being converged by the combiner box. If the inverter is the heart of the entire square array system, then the DC trunk line system is the aorta. Because the DC trunk line system adopts an ungrounded solution, if the cable has a ground fault, it will cause much greater damage to the system and even the equipment than AC. Therefore, PV system engineers are more cautious about DC trunk cables than other electrical engineers.
Choosing the correct DC solar cable for the photovoltaic system installed in your home or office is critical to performance and safety. Powerful solar cables are designed to transfer solar energy from one component of the system to another for conversion into electrical energy. Your everyday copper wire will do the job correctly and you will probably end up with a system failure.
Comprehensive analysis of various cable accidents, we conclude that cable ground faults account for 90-95% of the entire cable fault. There are three main causes of ground faults. First, the cable manufacturing defects are non-qualified products; second, the operating environment is harsh, natural aging, and damaged by external forces; third, the installation is not standardized and the wiring is rough.
There is only one root cause of the ground fault¡ª¡ªthe insulation material of the cable. The operating environment of the DC trunk line of photovoltaic power plants is relatively harsh. Large-scale ground power stations are generally desert, saline-alkali land, with large temperature differences during the day, and very humid environments. For buried cables, the requirements for filling and digging of cable trenches are relatively high; and the operating environment of distributed power station cables is not better than that on the ground. The cables will withstand very high temperatures, and the roof temperature can even reach 100-110¡æ. The fire-proof and flame-retardant requirements of the cable, and the high temperature have a great influence on the insulation breakdown voltage of the cable.
Therefore, before installing and running the system, you need to ensure that the size of the solar cable installed is proportional to the current and voltage of the system. Here are some features, which should be checked before turning on the system;
1. Ensure that the rated voltage of the pv dc cable is equal to or greater than the rated voltage of the system.
2. Ensure that the current-carrying capacity of the solar cable is equal to or greater than the current carrying capacity of the system.
3. Make sure that the cables are thick and protected enough to withstand the environmental conditions in your area.
4. Check the voltage drop to ensure safety. £¨The voltage drop should not exceed 2%.£©
5. The withstand voltage of the photovoltaic DC cable should be greater than the maximum voltage of the system.
In addition, the selection and design of PV DC trunk cables for photovoltaic power stations should also consider: the insulation performance of the cable; the moisture-proof, cold-proof and weather resistance of the cable; the heat-resistant and flame-retardant performance of the cable; the laying method of the cable; the conductor material of the cable ( Copper core, aluminum alloy core, aluminum core) and the cross-section specifications of the cable.
Most of the PV DC cables are laid outdoors and need to be protected from moisture, sun, cold, and ultraviolet. Therefore, the DC cables in distributed photovoltaic systems generally choose photovoltaic-certified special cables, taking into account the output current of the DC connectors and photovoltaic modules. At present, the commonly used photovoltaic DC cables are PV1-F 1*4mm specifications.
You can ensure that the correct solar cable is selected for the system from the following aspects£º
The thickness of the solar cable you choose for the system depends on the voltage of the system. The higher the system voltage, the thinner the cable, because the DC current will drop. Choose a large inverter to increase the system voltage.
The voltage loss in a photovoltaic system can be characterized as: voltage loss = passing current * cable length * voltage factor. It can be seen from the formula that the voltage loss is proportional to the length of the cable. Therefore, the principle of array to inverter and inverter to parallel point should be followed when exploring on site. In general, the DC line loss between photovoltaic array and inverter shall not exceed 5% of the output voltage of the array, and the AC line loss between the inverter and parallel point shall not exceed 2% of the output voltage of the inverter. The empirical formula can be used in the process of engineering application: ¡÷U=(I*L*2)/(r*S)
Among them ¡÷U: cable voltage drop -V
I: The cable needs to withstand the maximum cable-A
L: Length of cable laying -m
S: the cross-sectional area of the cable-mm²
r: Conductivity of conductor-m/(¦¸*mm²), r copper=57, r aluminum=34
Before buying, please check the current rating of the solar cable. For the connection of the inverter, the selected pv dc cable rated current is 1.25 times the maximum continuous current in the calculated cable. While for the connection between the inside of the photovoltaic array and between the array, the selected pv dc cable rated current is 1.56 times the maximum continuous current in the calculated cable. Every manufacturer, such as Slocable, has published a table listing the current ratings of cables manufactured according to their size and type. Make sure to choose the correct size cable, because a wire that is too small can quickly overheat and also suffer a significant voltage drop, which will cause power loss.
The length of the cable is also an important factor to consider when choosing the correct cable for a solar system. In most cases, the longer the wire, the better the current transmission. But it is best to use simple rules of thumb to calculate the required wire length based on the current capacity of the system.
Current / 3 = cable size (mm2)
Using this formula, you can easily obtain the most accurate and suitable system cable size and avoid any accidents or system failures.
The insulating (sheath) layer of qualified products is soft, flexible and flexible, and the surface layer is tight, smooth, without roughness, and has pure gloss. The surface of the insulating (sheath) layer should be clear and scratch-resistant Mark, products made of informal insulating materials, the insulating layer feels transparent, brittle, and non-tough.
Regular cables will be marked with photovoltaic cables. Mark the special cables for photovoltaics, and the outer skins of the cables are marked with PV1-F1*4mm.
The national standard has clear data on the thinnest point of the uniformity of the wire insulation layer and the average thickness. The thickness of the regular wire insulation is uniform, not eccentric, and tightly squeezed on the conductor.
It is a wire core produced from pure copper raw materials and subjected to strict wire drawing, annealing (softening), and stranding. Its surface should be bright, smooth, free of burrs, and the stranding tightness is flat, soft and tough, and not easy to break. The ordinary cable core is purple-red copper wire. The core of the photovoltaic cable is silver, and the cross-section of the core is still copper wire purple.
The conductor is shiny, and the conductor structure size meets the standard requirements. Wire and cable products that meet the requirements of the standard, whether they are aluminum or copper conductors, are relatively bright and free of oil, so the DC resistance of the conductor meets the standard, has good conductivity and high performance.
The standard product certificate should indicate the name of the manufacturer, address, after-sales service telephone, model, specification structure, nominal section (usually 2.5 square, 4 square wire, etc.), rated voltage (single-core wire 450 /750V, two-core protective sheath cable 300/500V), length (the national standard stipulates that the length is 100M¡À0.5M), inspection staff number, manufacturing date, and the product¡¯s national standard number or certification mark. In particular, the model of the single-core copper core plastic wire marked on the regular product is 227 IEC01 (BV), not BV. Please pay attention to the purchaser.
As a product that affects people and property, cables have always been listed as the focus of government supervision and inspection. Regular manufacturers are subject to inspection by the supervision department on a periodic basis. Therefore, the seller should be able to provide the inspection report of the quality inspection department, otherwise, the quality of the wire and cable products lacks basis.
In addition, to determine whether it is a flame-retardant cable and an irradiated cable, a better way is to cut off a section and ignite it. If it ignites and burns spontaneously soon, it is obviously not a flame-retardant cable. If it takes a long time to ignite, once it leaves the fire source, it will extinguish itself, and there is no pungent smell, indicating that it is a flame-retardant cable (flame-retardant cable is not completely unignitable, it is difficult to ignite). When it burns for a long time, the irradiated cable will have a small popping sound, while the unirradiated cable does not. If it burns for a long time, the insulating surface sheath will fall off seriously, and the diameter has not increased significantly, indicating that the radiation cross-linking treatment has not been carried out.
And put the cable core in 90 degree hot water, the insulation resistance of the truly irradiated cable will not drop rapidly under normal conditions, and it will remain above 0.1 megohm/km. If the resistance drops rapidly or even lower than 0.009 megohm per kilometer, the cable has not been cross-linked and irradiated.
Finally, the influence of temperature on the performance of photovoltaic dc cables should also be considered. The higher the temperature, the lower the current-carrying capacity of the cable. The cable should be installed in a ventilated place as far as possible.
So choosing the right wire sizes for your Solar system is important for both performance and safety reasons. If the wires are undersized, there will be a significant voltage drop in the wires resulting in excess power loss. In addition, if the wires are undersized, there is a risk that the wires may heat up to the point in which leads to the catching of fire.
The current generated from the solar panels should reach the Battery with minimum loss. Each cable has its own Ohmic resistance. The voltage drop due to this resistance is according to Ohm¡¯s law:
V = I x R (Here V is the voltage drop across the cable, R is the resistance and I is the current).
The resistance ( R ) of the cable depends on three parameters:
1. Cable Length: Longer the cable, more is the more resistance
2. Cable Cross-section Area: Larger the area, the smaller is the resistance
3. The material used: Copper or Aluminum. Copper has lesser resistance compared to Aluminium
In this application, copper cable is preferable. Copper wires are sized using the gauge scale: American Wire Gauge (AWG). The lower the gauge number, the less resistance the wire has and therefore the higher current it can handle safely.
Supplement: Insulation characteristics of PV DC cables
1. The field strength and stress distribution of AC cables are balanced. The cable insulation material focuses on the dielectric constant, which is not affected by temperature; while the stress distribution of DC cables is the maximum insulation layer of the cable, which is affected by the resistance of the cable insulation material. The influence of the coefficient, the insulation material has a negative temperature coefficient phenomenon, that is, the temperature increases and the resistance decreases;
When the cable is in operation, the core loss will increase the temperature, and the electrical resistivity of the insulating material of the cable will change accordingly, which will also cause the electric field stress of the insulating layer to change accordingly. In other words, the insulating layer of the same thickness will change due to the temperature. As it increases, its breakdown voltage decreases accordingly. For the DC trunk lines of some distributed power stations, due to the change of the ambient temperature, the insulation material of the cable ages much faster than the cables laid in the ground. This point should be paid special attention to.
2. During the production process of the cable insulation layer, some impurities will inevitably be dissolved. They have relatively small insulation resistivity, and their distribution along the radial direction of the insulation layer is uneven, which will also cause different volume resistivities in different parts. Under the DC voltage, the electric field of the cable insulation layer will also be different. In this way, the insulation volume resistivity will age faster and become the first hidden danger point of breakdown.
The AC cable does not have this phenomenon. Generally, the stress and impact of the AC cable material are balanced as a whole, while the insulation stress of the DC trunk cable is always the most impacted at the weakest point. Therefore, the AC and DC cables in the cable manufacturing process should have different management and standards.
3. Cross-linked polyethylene insulated cables have been widely used in AC cables. They have very good dielectric properties and physical properties, and are very cost-effective. However, as DC cables, they have a space charge problem that is difficult to solve. It is highly valued in high voltage DC cables.
When the polymer is used for DC cable insulation, there are a large number of local traps in the insulation layer, resulting in the accumulation of space charge inside the insulation. The influence of space charge on the insulating material is mainly reflected in two aspects of electric field distortion effect and non-electric field distortion effect. The impact is very harmful to insulating materials.
The so-called space charge refers to the part of the charge that exceeds the neutrality of a structural unit of a macroscopic substance. In a solid, the positive or negative space charge is bound to a certain local energy level and provided in the form of bound polaron states. Polarization effect. The so-called space charge polarization is the process of accumulating negative ions on the interface on the positive electrode side and positive ions on the interface on the negative electrode side due to ion movement when free ions are contained in the dielectric.¡¡
In an AC electric field, the migration of positive and negative charges of the material cannot keep up with the rapid changes in the power frequency electric field, so space charge effects will not occur; while in a DC electric field, the electric field is distributed according to the resistivity, which will form space charges and affect the electric field distribution. There are a large number of local states in polyethylene insulation, and the space charge effect is particularly serious. The cross-linked polyethylene insulation layer is chemically cross-linked and is an integral cross-linked structure. It is a non-polar polymer. From the perspective of the entire structure of the cable, the cable itself is like a larger capacitor. After the DC transmission is stopped, It is equivalent to the completion of charging a capacitor. Although the conductor core is grounded, it cannot be effectively discharged. A large amount of DC power still exists in the cable, which is the so-called space charge. These space charges are not like AC power. The cable is consumed with the dielectric loss, but is enriched at the cable defect; the cross-linked polyethylene insulated cable, with the extension of use time or frequent interruptions and changes in current strength, it will accumulate more and more space charges. Speed ??up the aging speed of the insulating layer, thereby affecting the service life. Therefore, the insulation performance of the DC trunk cable is still very different from that of the AC cable.