Regardless of the technology used, the basic design of the inverter is clear and very similar. The core is the process of converting DC voltage (solar cell modules) into AC voltage (grid connected). In the process of transformation, the positive and negative poles of DC electricity are continuously converted to form alternating current with directional changes. So, the key component of the inverter is the bridge switch (power device), as shown in Figure 1 (a). One side of this switch bridge is connected to the input DC power supply, and the other side is connected to the AC power grid. During the work process, only two opposing switches can be turned off simultaneously.
If the switching speed of this bridge is set to be the same as the grid frequency, theoretically the output side of the bridge can be connected to the grid. However, since the output current is a square wave with no change in intensity, it is necessary to install an inductor with an iron core at the output end to control the output current into a sine wave shape. The disconnection of the bridge is carried out using a pulse process, resulting in a smaller current component related to the pulse. This current component can control the current of the inductor. The frequency of the pulse is generally 20kHz, which can completely form a 50Hz current, as shown in Figure 1 (b).
For photovoltaic inverters, there is another very important device that cannot be overlooked: the capacitor at the input end, as shown in Figure 1 (c). The function of capacitors is to store electrical energy, ensure that the current from the power generation side is continuously and consistently supplied to the bridge switch, and enter the grid through a bridge that changes synchronously with the grid frequency. Only when the capacity of the input capacitor is large enough can the continuous and normal operation of the photovoltaic power generation system be guaranteed.
In practical applications, the range of input voltage has certain limitations. For grid connected power generation applications, the input voltage must always be higher than the peak voltage of the grid. When the effective value of the grid voltage is 250V, in order to achieve normal grid connection, the minimum voltage on the power generation side is required to be 354V.
Unlike the basic design of standard inverters, there are many ways to adjust or increase the input voltage range for direct grid connected inverters. The commonly used inverter technology solutions and structures are all different. The topology structure of the inverter mentioned above not only differs in electrical isolation, but also in achievable efficiency, voltage dependence, and other aspects. Therefore, there is no unified formula to define which inverter design is the best, and the specific characteristics of the inverter used must be considered in the design.
Another trend in the design of photovoltaic inverters is to expand the input voltage range, which can lead to a decrease in input current at the same power level or an increase in power level at the same input current. When the input voltage is relatively high, IGBT with a higher rated voltage (within the range of 1200V) needs to be used, resulting in greater losses. One way to solve this problem is to use a three-level inverter
By using two series connected electrolytic capacitors, the high input voltage can be split in half, and the midpoint can be connected to the neutral line. In this case, a 600V switch can be used. A three-level inverter can convert between three levels:+ Vbus, 0V, and - Vbus. In addition to being more effective than the 1200V switch structure solution, the three-level inverter also has an advantage of significantly reducing the output inductance. The three-level inverter has two significant characteristics:
① The output voltage sine waveform synthesized by multiple level steps significantly reduces harmonic content and improves the output voltage waveform compared to traditional two-level inverters under the same switching frequency conditions;
② The voltage rating of the switching tube is only half of the voltage on the DC bus, allowing low-voltage switching devices to be applied in high-voltage converters.
However, the disadvantages of three-level inverters are complex control strategies and the problem of unbalanced midpoint voltage, which is a fatal weakness of three-level inverters. Obviously, if the midpoint voltage of two capacitors connected in parallel on the DC bus of the inverter is unstable during operation, it will cause changes in the output three-level voltage, which not only distorts the output voltage waveform and increases harmonics, but also makes the three-phase output current asymmetric, losing the advantage of the three-level inverter. However, there is currently no fundamental solution to the problem of unbalanced midpoint voltage. One representative method is to use improved hardware circuits to achieve midpoint voltage balance; The second is to achieve voltage balance by changing the timing of switches or controlling the duration of vector voltage. But there are problems with complex circuits and unsatisfactory control effects.
At present, as long as the photovoltaic power generation system is designed reasonably, it can operate economically. Transformer free inverters that are directly integrated into the power grid are increasingly being valued for their low cost and high efficiency. Transformers convert electrical energy into magnetic energy, and then convert magnetic energy into electrical energy. The energy loss caused by the electrical isolation device installed between the input and output terminals can reach 1%, or even up to 2%. Therefore, the operating efficiency of transformerless inverters is higher than that of transformer inverters, and this technology has many other advantages, such as low material consumption and light weight.