â‘ Photovoltaic module and array model
Solar cell unit is the smallest unit of photovoltaic cell, its working voltage is generally only 0.5~1.0V, and the output power is small. In order to meet the power requirements of practical applications, solar panel manufacturers generally combine and package solar cells in series and parallel to form photovoltaic modules with power ranging from tens of watts to two or three hundred watts, which can be used alone.
When building a photovoltaic module model, it is generally assumed that the characteristics of all battery cells in the module are the same. As shown in Figure 1, the current of the module is the sum of the currents of the single cells in parallel, and the voltage is the sum of the voltages of the single cells in series. The characteristics of IU are the same as that of a single battery.
Photovoltaic branch refers to a series of modules, also called string. In order to prevent the current from flowing backward when the branch circuit voltage is too low, causing damage to the battery, the branch circuit is connected in series with blocking diodes. Figure 2 is the structure diagram of a single photovoltaic branch, which uses 4 photovoltaic modules M1, M2, M3, and M4 in series; VDb is a blocking diode, and VD1~VD4 is a bypass diode
The photovoltaic array is formed by a series of modules in series and parallel according to the actual load capacity requirements. It has a large output power and is often used in ground photovoltaic power stations or rooftop photovoltaic systems. A common series-parallel photovoltaic array is shown in Figure 3.
â‘¡Maximum output power of solar photovoltaic array
Photovoltaic cells are extremely unstable. The output characteristics of photovoltaic cells are greatly affected by light intensity and ambient temperature, and have obvious non-linear characteristics. Therefore, the maximum power can be output only at a certain voltage. In order to make full use of solar energy, increase the power of photovoltaic cells. For the output power, the corresponding control model and strategy method should be added to the photovoltaic cell circuit, so that the photovoltaic array can still obtain the maximum power output when the irradiance and temperature change. According to past experience, the MPPT (Maximum Power Point Tracking strategy) It can increase power generation by 5% to 20%.
Figure 4 is a schematic diagram of the operating point of the photovoltaic cell under working conditions. The curve represents the output I/U characteristic, the straight line represents the I/U characteristic of the load resistance, and the intersection of the two lines is the photovoltaic cell operating point.
It can be seen from Figure 4 that when working at the maximum power point, the impedance of the photovoltaic cell matches the impedance of the applied load, and the output power of the photovoltaic cell reaches the maximum at this time. When the sunlight intensity and the ambient temperature change, the output voltage and current of the photovoltaic cell change in a non-linear relationship, and the output power also changes accordingly, and when the photovoltaic cell is applied to different loads,
Because the output impedance of the photovoltaic cell does not match the load impedance, the output power of the photovoltaic system cannot reach the maximum value. An effective way to solve this problem is to add a switching conversion circuit (DC/DC) between the output of the photovoltaic cell and the load. Using the switching principle of the impedance conversion of the switching conversion circuit, the equivalent impedance of the load follows the output impedance of the photovoltaic cell, so that the output power of the photovoltaic cell is maximized. Figure 5 is a schematic diagram of the circuit.
There are many MPPT algorithms at present, and the commonly used MPPT methods are the disturbance observation method and the incremental conductance method. The perturbation method is mainly to continuously apply small perturbations to the output voltage of the photovoltaic cell, measure the output power, compare the changes of the output power before and after the perturbation, determine the direction, the output power is larger than before, continue to add the positive perturbation, the output power becomes smaller, and the negative perturbation , The disturbance observation method is used here, which has a small amount of calculation and is easy to implement.
The disturbance observation method is divided into inverter input parameters and inverter output parameters. The input parameters refer to the output voltage and current of the photovoltaic cell. After collecting the voltage and current, the output power is calculated, and small disturbances are added to determine the output power conversion; the output parameters refer to the inverter output power. The selection in this book is the inverter input parameter Disturbance observation method, Figure 6 is the flow chart of the disturbance observation method.
