Hot spot effect and series and parallel combination of battery modules

The photovoltaic array is a photovoltaic array structure formed by several battery modules connected in series and parallel to meet the requirements of high-voltage and high-power power generation, and fixedly combined by a certain mechanical method. In addition to the series-parallel combination of battery modules,

The photovoltaic array also needs anti-reverse (anti-backflow) diodes, bypass diodes, cables, etc. to electrically connect the battery modules. It also needs to be equipped with a dedicated DC combiner box with a lightning arrester and a DC lightning protection distribution box. Sometimes in order to prevent bird yellow from contaminating the surface of the photovoltaic array and causing the “hot spot effect”, a bird repellent is installed on the top of the array. In addition, the entire photovoltaic array must be fixed on the photovoltaic bracket, so the bracket must have sufficient strength and rigidity, and the entire bracket must be firmly installed on the base of the bracket.

1. The hot spot effect of battery modules

When the battery module or a part of the surface is dirty, scratched, or covered or obscured by bird droppings, leaves, building shadows, cloud shadows,

The solar radiation obtained by the covered or shaded part will be reduced, and the output power (power generation) of the corresponding cell will naturally be reduced, and the output power of the corresponding module will also be reduced accordingly. Since the output power of the entire module is not linearly related to the shaded area, even if only one cell in a module is covered, the output power of the entire module will be greatly reduced. If the shielded part is only the parallel part of the square array module string, then the problem is relatively simple, but the output current of this part will be reduced. If the part that is blocked is the series part of the square array module string, the problem is more serious. On the one hand, the output current of the entire module string will be reduced to the current of the blocked part. On the other hand, the shaded cell not only cannot generate electricity, but also is used as an energy-consuming device to consume the energy of other illuminated cell modules by heating. Long-term blocking will cause repeated local overheating of the cell module, resulting in a hot spot effect. This effect can severely damage the cells and modules, may cause the solder joints of the modules to melt, damage the packaging materials, and even cause the entire module to fail. In addition to the above, the cause of the hot spot effect is that individual cells of poor quality are mixed into the battery module, the electrode pads are welded, the cell is cracked or damaged, and the performance of the cell is deteriorated.

2. Serial and parallel combination of battery modules

There are several ways to connect photovoltaic arrays in series, parallel, and mixed series and parallel. When the performance of each single battery module is consistent, the series connection of multiple battery assemblies can increase the output voltage of the entire square array proportionally without changing the output current; When the modules are connected in parallel, the output current of the entire square matrix can be increased proportionally without changing the output voltage; When the series and parallel connections are mixed, the output voltage of the square matrix can be increased, and the output current of the square matrix can be increased. However, the performance parameters of all battery modules forming the square matrix cannot be completely consistent, and the contact resistance of all connecting cables, plugs, and sockets cannot be the same. So the operating current of each series battery module is limited to the module with the smallest current, and the output voltage of each parallel battery module is also clamped by the battery module with the lowest voltage. Therefore, the square matrix combination will produce a combined connection loss, so that the total efficiency of the square matrix is always lower than the sum of the efficiencies of all individual modules. The size of the combined connection loss depends on the discreteness of the performance parameters of the battery module. Therefore, in addition to improving the consistency of the performance parameters of the battery modules as much as possible during the production process of the battery modules, the battery modules can also be tested, screened, and combined, and battery modules with similar characteristics can be combined. For example, the operating currents of the modules of the series combination should be as close as possible, and the total operating voltage of each string and each string should also be considered as similar as possible to minimize the combined connection loss. Therefore, the square matrix combination connection should follow the following principles.

(1) When connecting in series, modules with the same working current are required, and several bypass diodes are connected in parallel in the junction box of each module.

(2) modules with the same working voltage are required for parallel connection, and anti-backflow diodes are connected in series in each parallel circuit.

(3) Try to make the module connection line as short as possible, and use a wire that meets the current-carrying capacity.

(4) Strictly prevent individual battery modules with deteriorated performance from being mixed into the photovoltaic array.