After the installation of PV system, the user is most concerned about the power generation, because it is directly related to the return on investment of the user. There are many factors that affect the power generation, such as components, inverters, cable quality, installation orientation Angle, tilt Angle, dust, shadow occlusion, component and inverter matching system scheme, line design, construction, grid voltage and other factors. This series of articles will explore each of these factors based on real cases. This paper mainly discusses the influence of solar panel cable selection error on power generation.
1 Case scene 1
This is a project in Wuhan, Hubei Province, where the customer installed a 20kW project in August 2014. After more than three years of operation, the electricity generation has gradually decreased, from the initial average of more than 70 degrees per day to the current average of more than 60 degrees per day. The main performance is at the peak of power generation at noon, the grid-connected switch often jumps open, and sometimes the scene will smell burning odor.
Field investigation: the components have no shadow occlusion, the surface is clean and dust-free, the components have the same color and no color difference, and the inverter has no leakage current alarm, grid voltage alarm and other information. The author disconnects the inverter, disconnects the AC switch wiring and the inverter wiring terminal, found that the cable front wiring terminal has been partially oxidized, black; it's too hot to put your hands on. The sealing ring of the AC waterproof connector of the inverter is not installed, the AC terminal is partially oxidized, and there is some water vapor.
After analysis, the customer's AC cable is aluminum wire, which is the main reason for the oxidation of terminal parts and the non-waterproof joints.
2 Theoretical Analysis
(1) Copper-aluminum joints are vulnerable to electrochemical corrosion, and the direct connection of copper and aluminum can form a chemical battery. This is because aluminum is easy to lose electrons to become the negative electrode, while copper is difficult to lose electrons to become the positive electrode. Therefore, a 1.69V electromotive force is formed between the positive and negative electrodes, and a small current passes through to corrode the aluminum wire, which is called electrochemical corrosion. This will cause poor contact between copper and aluminum, and contact resistance increases. When there is a current through, the temperature of the joint will rise, and the temperature rise will accelerate the corrosion of the joint, increasing the contact resistance, resulting in a vicious cycle, until burned.
(2) The surface of aluminum wire is easily oxidized in air. Membrane resistance exists more or less on the surface of all conductors. If the membrane resistance causes the connection to overheat, the overheating will increase the membrane resistance, the worse the conductive situation, and this kind of overheating in the aluminum wire connection is particularly serious. This IS BECAUSE EVEN if THE surface OF the aluminum wire is scraped clean, it only needs to be exposed to the air for a few seconds to oxidize and immediately form a film of alumina. Its thickness is only a few microns, but it has a very high resistivity, thus presenting a large film resistance. Therefore, when the aluminum wire is connected, it should be immediately coated with conductive paste after scraping the surface of the aluminum wire to cut off the contact between the surface of the aluminum wire and the air, otherwise the contact resistance will be increased.
(3) The output waterproof connector of the inverter is designed according to the wire diameter of copper wire. If aluminum wire is used, a large size wire is required. For example, 30KW inverter, the design output uses 10 square copper wire, with aluminum wire needs 16 square, the cable area increases, and the waterproof terminal area is limited, may not accommodate, can only remove the waterproof sealing ring.
3 Case Scene 2
The customer installed a 40kW inverter. Shortly after it was running, smoke appeared at the AC end, and the AC switch tripped. When the junction box is opened, it is found that the AC cable is loose and a short circuit occurs.
After inspection, the customer's AC line is a hard copper wire, which turns a bend when it comes out, causing the stress to increase, and the AC wiring screw slowly becomes loose. After a long time, it will be loose and cause a short circuit.
When selecting an AC cable, it is best to choose flexible copper wire. On the one hand, it can avoid the chemical corrosion of copper and aluminum. On the other hand, it can reduce the pressure contact force and avoid loosening.