Throughout the five-thousand-year history of civilization in China, the emphasis is on pursuing perfection on the basis of balanced development. The breakthrough of any new technology for photovoltaic inverters should be based on the principle of moderate advance, balanced development as the goal, starting from reality, adopting a combination of new and old technologies, and maximizing the benefits in the life cycle from the perspective of users. It is the golden mean of inverter design selection. Choosing the necessary knowledge series of medium-power string inverters, after introducing "moderately ahead" and "balanced development", this article will explain the golden mean of inverter design - safety first.
During the operation of the PV system, due to the quality of components, improper external environment or improper installation and construction operations, components damage and circuit failures may occur. Photovoltaic power plants are generally installed in wilderness areas or on rooftops, and their natural environment is harsh. Natural disasters, such as natural disasters such as typhoons, snowstorms, and dust, are inevitably encountered. Or, they are cut off by artificial cables, and snapped by animals such as rats. Because of other damages such as cables, the safety of the power station is very important.
String monitoring: Monitor the voltage and current of each component of the PV input. During the operation of the PV system, due to the quality of the components, external environmental impact, or improper installation and construction operations, component damage and circuit failures may occur.
If there is no string monitoring function, some minor problems will not be detected in the early stage, which will eventually cause major problems. Some problems require specialized personnel to conduct on-site inspections, which takes a long time and causes large losses in power generation.
The PID effect (Potential Induced Degradation) is called potential-induced attenuation, and PID direct hazard means that a large amount of charge accumulates on the surface of the battery sheet, resulting in the passivation of the battery sheet surface, making the power of the module attenuate, reducing the power generation, and reducing the power station revenue of the solar power station.
By integrating the PID protection module in the inverter, the PID phenomenon of components can be effectively avoided, and the power generation loss of the power plant can be reduced. At the same time, the PID module has a repair function, which can repair the components that have PID problems and restore the parameters of the components to normal.
DC arc detection: Fire is the most costly loss of photovoltaic power plant. If it is installed on the roof of a factory building or a residential building, it is easy to endanger personal safety. Once a fire occurs in a photovoltaic power station, it cannot be used directly to extinguish the fire with water. The power must be cut off at the fastest speed. There are many factors of fire accidents in photovoltaic power stations. DC arcing is the main reason.
Gruitt launched a circuit protection device, AFCI, whose main function is to prevent arcing caused by a fault. It has the ability to detect and distinguish between the normal arc and the fault arc generated when the inverter is started or stopped or switched, and the circuit is cut off in a timely manner after a fault arc is found.
DC fuses are indispensable: due to improper device selection, incorrect installation methods, or fuse quality problems, in a certain period of time, the DC-side fuse fault accidents of photovoltaic power plants are frequently caused, causing some losses to customers. In fact, these problems are also There is no problem with the fuse itself, and there are now solutions. However, some manufacturers of inverters have therefore eliminated DC-side fuse protection, instead adopting only two parallel strings of MPPTs in each path, and use anti-reflection diodes as overcurrent protection. This practice itself is a greater harm.
As an over-current protection device, the fuse can cut off the fault circuit at the fastest speed in the event of a short-circuit fault in the system and avoid greater losses. Photovoltaic power plants are generally installed in the wilderness or on the roof. The natural environment of these places is bad and the power station will inevitably be damaged. Therefore, the protection function of the DC fuse in the photovoltaic system is indispensable. It is an irresponsible practice to completely cancel the DC fuse protection.
The inverter system consists of multiple MPPT input loops. Each MPPT is connected to two strings. Each PV string is connected in parallel through the Boost boost circuit. The Boost boosting circuit in the previous stage is generally connected in parallel with the bypass element. The purpose is to When the voltage rises to a certain value, the Boost boost circuit is bypassed to improve the efficiency of the system. This is equivalent to only one MPPT. All the loops of the front stage are connected together. In this case, a short circuit occurs on one path and the voltage will drop. The currents of other strings will flow to this path, causing the short circuit current to expand several times. If there is no fuse protection, it will cause fire.
Inverter fuse design selection and installation should pay attention to the following points, can effectively reduce the impact of fuse fuses without failure.
(1) The fuse should be selected from the qualified products of the regular manufacturer. The rated current is appropriate. The current is too small to be easily misjudged. If the current is too large, no protection is provided.
(2) Where the fuse is installed in the air inlet, where the temperature is low,
(3) The fuse is not exposed and there is an external arc shield to prevent the arc of the fuse from catching fire.
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