what is PID and how to prevent and repair PID phenomenon

PID effect is Potential Induced Degradation. The direct harm of PID to the module is that a large amount of charge accumulates on the surface of the cell, which aggravates the passivation effect of the cell surface, resulting in a decrease in the fill factor, open-circuit voltage and short-circuit current of the cell, and the power of the cell module is attenuated, and the attenuation degree can reach 50%.

Regarding the cause of the PID effect, the current consensus in the photovoltaic industry is that with the large-scale application of photovoltaic systems, the system voltage is getting higher and higher. Usually 18-22 solar modules are connected in series to reach the MPPT working voltage of the inverter, which leads to high open circuit voltage and working voltage.

Taking a 450W 72-cell battery module in the STC environment as an example, the open circuit voltage of 20 solar modules is as high as 1000V, and the working voltage is as high as 800V. Since the photovoltaic power station needs to be equipped with lightning protection and grounding works, the aluminum alloy frame of the general module is required to be grounded, and a DC high voltage of nearly 1000V will form between the cell and the aluminum frame, resulting in a voltage bias between the circuit and the metal grounding frame.

Another cause is component encapsulation. The packaging process of photovoltaic modules cannot guarantee 100% insulation. In the long-term use process, it is easy to cause leakage, resulting in more and more electron loss in the PN junction, and the electrical conductivity is getting worse and worse, which eventually leads to the decline of the power generation performance of the solar module.

We will interpret how to prevent and repair the PID phenomenon from both the solar module side and the inverter side.

solar module side
The PID test of photovoltaic modules is carried out before leaving the factory. The PID test standard is based on the combination of IEC62804 photovoltaic module performance test standard, IEC61215, IEC61730 photovoltaic module safety test standard, which can well predict whether the PID effect will occur during the use of photovoltaic modules. . Customers can also ask the manufacturer to provide the corresponding PID test report when purchasing PV modules.

Inverter side
The inverter has the following three solutions to deal with the PID effect:
Option 1: Use the negative grounding method to eliminate the negative voltage from the negative pole of the solar modules to the ground
This solution is suitable for isolated photovoltaic inverters, including high-frequency isolated inverters and power frequency isolated inverters. After the negative electrode is grounded, the negative voltage of the solar panels to the ground is eliminated, and the PID phenomenon can be effectively suppressed. For non-isolated photovoltaic inverters, it is necessary to add an isolation transformer to achieve negative grounding.

Option 2: Use the virtual neutral grounding scheme to eliminate the negative voltage from the negative pole of the solar module to the ground
This solution is suitable for a centralized photovoltaic power station composed of multiple string photovoltaic inverters. By raising the potential of the virtual neutral point, the negative-to-ground voltage of each inverter string is close to zero potential to achieve PID suppression function.

Option 3: Use the forward bias voltage scheme to fix the PID effect
This solution is suitable for distributed photovoltaic power stations composed of single or multiple string photovoltaic inverters. It adopts the built-in or external anti-PID repair function module of the inverter, which is powered by the AC side. The positive and negative poles are added with forward bias voltage to repair the PID effect, and three output modes can be provided: automatic mode, night mode and continuous mode. Generally, the default is automatic mode output, and automatic mode output is the highest voltage of the system.

At present, the PID effect has been recognized in the industry as an important factor affecting the power of photovoltaic modules. Especially in the face of complex environments such as high temperature and high humidity, the PID effect of photovoltaic modules will intensify. Therefore, on the solar module side, we can use raw and auxiliary materials with better weather resistance for packaging, increase the insulation resistance of the external circuit and the internal cells, and reduce the leakage current phenomenon; in addition, the frameless module has better performance than the framed module in the experiment. Good anti-PID characteristics, so the border is also an important factor in our study of PID. On the inverter side, a virtual grounding scheme can be used, or a built-in or external anti-PID function module can be selected to solve the PID effect.

Through the above scheme, although the effect of completely avoiding the PID effect cannot be achieved, the losses caused by the PID effect can be minimized.