Detailed explanation of photovoltaic system optimization based on a new inverter

With the growing demand for green energy, the rapid development of solar power generation in recent years has attracted widespread attention from all parties. Such a high growth rate forecast is based on the following factors: the current excess production capacity has reduced the average manufacturing cost of photovoltaic systems by 25%; the installation price of photovoltaic systems is continuing to decline; and government subsidies in countries and regions around the world. China has abundant solar energy resources, and the country has recently introduced subsidy support policies. For example, the most influential Golden Sun Project proposes to provide 50% and 70% financial subsidies to photovoltaic grid-connected projects and off-grid photovoltaic power generation projects in areas without electricity, respectively.

Circuit topology

To convert the "rough electricity" (fluctuating DC voltage) output by solar panels into a constant and reliable sinusoidal AC mains electricity, the implementation methods are usually divided into two architectures: single-stage conversion and two-stage conversion, also known as DC-free and DC-with chopping. Sometimes it is also beneficial to the selection of power semiconductor devices and system cost optimization. Therefore, more and more manufacturers are developing or evaluating single-stage conversion architectures, even though this will face more complex inverter control and potentially higher device tolerance requirements. Among the new topologies, HERIC and multi-level structures have attracted more attention in the industry and are expected to become the mainstream topology, especially when connected to the power grid.

Figure 2: Three-level clamped diode topology. As shown in Figure 1, the structure of the HERIC inverter is based on the traditional single-phase inverter full bridge, with a pair of diode series switches connected in anti-parallel as the output. The switching devices in the newly added circuit switch at the power frequency cycle speed, and there is no special requirement for the device speed. After applying appropriate phase control, this circuit can handle reactive power more effectively, thereby improving the efficiency of the entire system.

The three-level diode clamped inverter is a new type of solar inverter circuit topology that has received special attention recently. It has been successfully used in high-voltage centralized solar power generation applications. Each bridge arm of the three-phase three-level circuit shown in Figure 2 is composed of four switches with anti-parallel diodes in series, and each phase has a diode phase arm connected across the main switches, and its midpoint is directly connected to the neutral point of the DC bus. This diode phase arm acts as a voltage clamp to ensure that when the circuit is working, the maximum voltage stress on each main switch device is half of the bus voltage. Due to this special topological structure, the three-level output has the advantages of low harmonics (AC output is closer to sine), low electromagnetic noise level, low voltage tolerance of the required switch devices, and expandable levels. When solar grid-connected power generation, it is particularly suitable for three-phase high-power and high-voltage occasions. It is reflected in three aspects: First, practice has proved that the price of high-voltage semiconductor switching devices is twice that of low-voltage devices with the same current tolerance and half the voltage tolerance, so the device cost of the three-level circuit is lower; second, the harmonics of the output voltage are small, and the size of the required filter magnetic components is greatly reduced, thereby reducing the cost of the filtering equipment; finally, due to the increase in the number of open tubes, even in the pulse width modulation mode, some of the main switches of the three-level can be switched at a low frequency, so relatively economical switching devices can be used.

Common types and selection principles of power electronic devices

The power semiconductor devices used in the broad sense of solar inverters mainly include MOSFET, which is a metal-oxide-semiconductor field-effect transistor, referred to as metal-oxide-semiconductor field-effect transistor (MOSFET), which is a field-effect transistor that can be widely used in analog circuits and digital circuits. According to the polarity of its "channel", MOSFET can be divided into n-type and p-type MOSFET, which are usually called NMOSFET and PMOSFET. Other abbreviations include NMOSFET, PMOSFET, nMOSFET, pMOSFET, etc. IGBT, Super Junction MOSFET. Among them, MOSFET has the fastest speed, but the highest cost. In contrast, IGBT has a slower switching speed, but has a higher current density, so it is cheap and suitable for high current applications. Super junction MOSFET is between the two. It is a product with a compromise between performance and price and is widely used in practical designs. To help designers quantify the relationship between efficiency and device cost, Table 1 lists the power loss and price factors of three semiconductor switch devices. For ease of comparison, all parameters are normalized to the MOSFET case.

Table 1 Performance and price comparison of commonly used switching devices (all figures are normalized to MOSFET) Single-phase full-bridge hybrid device module and three-level hybrid device module

The hybrid single-phase full-bridge power module shown in Figure 3 is a product dedicated to solar single-phase inverter . With the unipolar modulation method, the two switch tubes of each bridge arm work in completely different switching frequency ranges. Taking the figure as an example, the upper tube always switches on and off at the power frequency, while the lower tube always operates at the pulse width modulation frequency. According to this working characteristic, the upper tube always uses a relatively cheap gate trench IGBT to optimize the on-state loss. IGBT (Insulated Gate Bipolar Transistor), an insulated gate bipolar transistor, is a composite fully controlled voltage-driven power semiconductor device composed of BJT (bipolar transistor) and MOS (insulated gate field effect transistor). It has the advantages of both MOSFET's high input impedance and GTR's low conduction voltage drop. GTR has a low saturation voltage drop and a large current density, but a large drive current; MOSFET has a very small drive power and a fast switching speed, but a large conduction voltage drop and a small current density. IGBT combines the advantages of the above two devices, with low drive power and low saturation voltage drop. It is very suitable for use in converter systems with a DC voltage of 600V or above, such as AC motors, inverters, switching power supplies, lighting circuits, traction drives, and other fields. The lower tube can select a non-punch-through (NPT) IGBT to reduce switching losses. This topology not only ensures the highest system conversion efficiency but also reduces the cost of the entire inverter equipment. Figure 4 shows the conversion efficiency curves of different device combinations to verify the superiority of this solar power module. It can be found that this hybrid device configuration can achieve a conversion efficiency of more than 98% under different loads.

Figure 3: Hybrid device solar inverter module.

Figure 4: Comparison of inverter efficiency with different device combinations.

In Microsemi's three-level inverter module, a hybrid device mechanism is also introduced. Its main purpose is to make full use of the fact that the switching frequency of the devices at the two ends is much higher than that of the two adjacent devices in the middle. Therefore, the APTCV60 series three-level module uses a structure of super junction MOSFETs at both ends and Trench IGBT in the middle to further improve efficiency.

in conclusion

Improving conversion efficiency and reducing costs are long-term issues in solar inverter design and the biggest challenges facing engineering designers. This article discusses the design principles, typical topologies and selection methods of switching devices for centralized solar inverters based on how to design an optimized new generation of solar power conversion systems. It also explains how design engineers can use new technologies at all levels of devices, circuits and systems to optimize inverter system design. Practice has proved that many of Microsemi's related new products can optimize system performance from multiple aspects, providing efficient, reliable and economical system solutions for the solar inverter market.