In 2005, the famous American photovoltaic manufacturer SUNPOWER proposed a new discovery. This phenomenon is called "surface polarization." Surface polarization occurs when a reverse high voltage is applied to the assembly. The surface of the battery accumulates a negative charge over time. These charges attract positive charges to the surface of the cell to form a recombination center. Conversely, when a negative voltage is applied to the component, the polarization also changes accordingly, in which case the performance of the component will not be affected. This is the most famous phenomenon of PID (PotenTIal Induced DegradaTIon). It has been found through research that the PID phenomenon is generally a reversible surface polarization phenomenon.
What is the danger of PID?
China's traditional installed provinces have shifted from the northwest to the southeast. Compared with the arid and windy sand climate in the northwest, the influence of the hot and humid climate in the southeast on photovoltaic power plants is quite different. The PID problem has become one of the important factors affecting the power generation of photovoltaic power plants. Especially in power plants with high temperature and high humidity, such as distributed roofs and fishing lights, the probability of PID generation increases greatly.
The actual PV power plant field test found that the power of some components dropped sharply after 1 to 2 years of construction, and the power attenuation of some components was as high as 50%. There are many causes of component attenuation, such as photo-attenuation, aging attenuation, cracking, cell rupture, etc. One of the important reasons is the component PID effect. The picture below shows the infrared image of the PID effect, and the battery with serious PID effect is black.
Figure 1: Infrared photo of the PID effect
SMA has carried out relevant experimental verification. The following is the relevant experimental situation. You can find that the component power characteristics have been greatly attenuated after only a few tens of hours.
Figure 2: Experimental simulation diagram
Figure 3: Experimental results
Domestic experimental data shows that 54.4% can be attenuated in 2 years! It can be seen that the PID effect has a great impact on the output power of the module, and it is the "terrorist killer" of the photovoltaic power generation.
PID effect prevention
First of all, component manufacturers have done a lot of work on materials, structures and other aspects and made some progress; for example, anti-PID materials, anti-PID batteries and packaging technologies can be used. The use of non-ethylene-vinyl acetate copolymer packaging materials, the use of frameless components or double glass components, etc., can reduce the PID effect to a certain extent.
This article focuses on how to perform PID prevention from the inverter side. There are roughly three solutions for prevention on the inverter side:
The first one is the grounding of the negative pole. We introduced it in the PID effect. The PID is caused by the negative voltage of the negative pole of the component. The corresponding negative grounding method can be used to reduce this attenuation. The PV module or inverter The negative pole of the device is directly grounded through a resistor or a fuse, so that the voltage of the negative pole of the battery board and the ground metal frame are kept at the same potential, and the negative bias is eliminated. This solution is mostly used for the centralized inverter, and the panel voltage is raised. There is a risk of electric shock, and the string type is rarely used because of factors such as generally without an isolation transformer;
Figure 4: Schematic diagram of the negative grounding scheme
Figure 5: Different inverters have different negative voltages to earth
The second type is the virtual ground of the negative pole. Both the centralized and the string type can use the negative virtual grounding method to prevent and control the PID effect. This scheme will also bring the panel voltage to rise and the risk of electric shock.
Figure 6: Schematic diagram of the negative virtual grounding scheme
The third scheme is the nighttime repair scheme proposed by SMA. We have already proposed in the previous PID effect. The PID effect is reversible. It can be repaired by a certain voltage floating charge of the PV anode (or cathode) to the earth at night. The relevant characteristics of the panel; offset technology does not change the DC side operating voltage, which can effectively repair the panel attenuation; SMA China goes one step further, integrating this technology into the inverter and integrating it into the inverter, which is more economical and efficient.
Figure 7: Night Float Technology
Note: The anti-PID inverter technology with negative grounding or virtual negative grounding has the problem of raising voltage, which brings electric shock hazard and needs to strengthen the safety management of the power station. However, the night floating charging technology does not have this problem, especially integrated into the inverter. The inside of the device is a good solution.
Prevention cases and effects
In the case of inverter PV negative grounding, two PV arrays with the same conditions in the same place in an actual power station are selected. Among them, the centralized inverters used do not have anti-PID function, but also have anti-PID. Features. The test found that the centralized inverter with the PID module installed can effectively prevent component PID attenuation and greatly reduce power generation loss.
Table 1: Concentrated negative electrode grounding comparison results
Case results using nighttime repair inverter technology
Table 2: Results after using SMA night repair technology
(Note: Some experimental data quoted the data of Wang Shujuan, a PV colleague of the Design Institute)
to sum up
As China's PV industry moves from west to east and from north to south, the influence of temperature and humidity on the PID of components is increasing, affecting the life span of PV panels and customer benefits, and affecting the healthy development of the entire industry; The use of anti-PID technology is also receiving more and more attention. More and more photovoltaic power plants require inverters to have anti-PID technology.
SMA China offers a complete PID solution, especially the integrated floating night charging technology inside the inverter, which is cost-effective.
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