Anti-backflow (anti-reverse charge) and bypass diode
In the photovoltaic square array, the diode is a very important element. The commonly used diode is a silicon rectifier diode. When selecting it, pay attention to its specification parameters and leave a margin to prevent breakdown damage. Generally, the reverse peak breakdown voltage and maximum operating current must be more than twice the maximum operating voltage and operating current. Diodes are mainly divided into two categories in solar photovoltaic power generation systems.
(1) Anti-backflow (anti-reverse charge) diode
One of the functions of the anti-backflow diode is to prevent the current of the battery in the off-grid system from being reversed to the module or the square when the battery module or the square array is not generating electricity. This not only consumes energy, but also heats up the module or the square array or even damages it. The second function is to prevent the current flow between the branches of the square array in the photovoltaic array. Because the output voltage of each branch in series cannot be absolutely equal, the voltage of each branch always has a high and low difference, or the output voltage of a branch is reduced due to a fault, shadow mask, etc., and the current of the high-voltage branch will flow to the low-voltage branch, which will reduce the overall output voltage of the square matrix in severe cases. Connecting anti-backflow diodes in series in each branch can avoid this phenomenon.
In the off-grid photovoltaic power generation system, the anti-reverse charge diode is already connected to the circuit of the general photovoltaic controller, that is, when the controller has an anti-reverse charge function, the module output does not need to be connected to the diode. In the same way, in grid-connected photovoltaic power generation systems, anti-backflow diodes are generally connected to the DC combiner box or inverter input circuit, so there is no need to connect the diode to the output of the module.
(2) Bypass diode
When a module or part of a module in the square array string is shaded or fails to stop power generation, a forward bias voltage will be formed at both ends of the bypass diode of the module to turn on the diode. The working current of the module string bypasses the failed module and flows through the diode bypass, which will not affect the power generation of other normal modules. At the same time, it also protects the bypass modules from high forward bias voltage or damage due to heating due to “hot spot effect“.
Bypass diodes are generally installed directly in the module junction box. According to the power of the module and the number of cell strings, install 1-3 diodes, as shown in Figure 1. Among them, Figure 1 (a) uses a bypass diode, when the module is blocked or malfunctions, the module will be all bypassed; Figure 1 (b) and (c) use 2 and 3 diodes to bypass the battery module in sections. When a certain part of the module fails, only half or 1/3 of the module can be bypassed, and the rest can still continue to work.
Bypass diodes are not required in any situation. When the modules are used alone or in parallel, there is no need to connect diodes. For occasions where the number of modules in series is small and the working environment is good, bypass diodes can also be considered.
The circuit of the photovoltaic array
The basic circuit of the photovoltaic array is composed of battery module strings, bypass diodes, anti-reverse charging diodes, and DC combiner boxes with lightning arresters. Common circuit forms include parallel square array circuits, series square array circuits and series and parallel hybrid square array circuits, as shown in Figure 2.
Energy loss of photovoltaic array combination
The photovoltaic array is composed of several battery modules and thousands of solar cells. This combination inevitably has various energy losses, which can be summarized as follows.
(1) Connection loss: The loss caused by the resistance of the connecting cable and the poor connection of the plug.
(2) Discrete loss: It is mainly due to the power loss caused by the different performance and attenuation of battery modules and the inconsistent parameters. The selection of different manufacturers, different factory dates, different specifications, and different brands of cells for the square array combination will cause discrete losses of the photovoltaic square array.
(3) Series voltage drop loss: The internal resistance of the battery slice and the battery module itself cannot be zero, that is, the PN junction constituting the battery slice has a certain internal resistance, which causes the voltage drop loss after the series connection of the modules.
(4) Parallel current loss: The reverse resistance of the battery slice and the battery module itself cannot be infinite, that is, the PN junction constituting the battery slice has a certain reverse leakage current, which causes the leakage current loss after the modules are connected in parallel.