Different classification methods of photovoltaic grid-connected power generation circuit topology can be obtained from different angles. In practical applications, it is necessary to select an appropriate circuit topology according to different usage occasions and grid-connected requirements. The key index factors of the topology structure of power electronic conversion circuit are:
◆High efficiency (power loss, number of conversion stages, soft switching);
◆Low cost;
◆Reliable, long life (affected by electrolytic capacitors), easy to maintain (modular);
◆Power quality (control scheme);
◆Low noise (high frequency);
◆Small size (wireless transformer, related filters);
◆ Power isolation (transformer) requirements;
◆Network monitoring capability.
Based on the above index factors, the general design principles are summarized as follows:
(1) Efficiency
For photovoltaic inverters, the conversion efficiency of the entire device is an important indicator to measure the performance of photovoltaic inverters. European efficiency is the most representative conversion efficiency evaluation method, which is mainly proposed for photovoltaic inverters, and its definition is different from what is usually called average efficiency or maximum efficiency. It fully takes into account the change of solar radiation intensity, the photovoltaic inverter will not always work under the rated power, but more in the light load situation. The European efficiency is obtained by accumulating the efficiencies under different load conditions according to different weighting factors, as shown in Table 1. It can be seen that the half-load efficiency accounts for a large proportion, and the rated load only accounts for 20%, which fully considers the situation of the photovoltaic inverter running at light load.
Since efficiency has always been a key indicator of photovoltaic grid-connected power generation systems, circuit efficiency should first be taken as a basic consideration in selecting circuit topology. The lower the power loss of the circuit, the better, and the fewer transformation stages the better. At present, insulated gate bipolar transistors (IGBTs) and power MOSFETs are usually used as the main power devices. The IGBT turn-on voltage drop is usually around a constant value, and it does not increase significantly with the increase of current. The on-resistance of the power MOSFET is basically unchanged, and its on-voltage drop is proportional to the current.
Therefore, using IGBTs in high power situations has lower conduction losses and higher inverter efficiency. Conversely, the power MOSFET has lower conduction losses and the inverter efficiency is higher in the case of low power. Most photovoltaic inverters work under light load conditions. Therefore, power MOSFETs are the preferred power devices for low-power photovoltaic inverters, and IGBTs are usually used as the preferred power devices for medium and high-power photovoltaic inverters.
Using a three-level inverter circuit instead of a two-level inverter circuit, or using a unipolar PWM modulation technique instead of a bipolar PWM modulation technique, is conducive to reducing filter losses under the same switching frequency. Appropriate use of soft switching technology to reduce the switching loss of the circuit is also a way to improve the efficiency of photovoltaic inverters.
(2) Cost control
Controlling costs is the guarantee of maximizing profits, so the simpler the circuit structure, the more advantageous, and the fewer circuit components the better. The main power device should be a power device that is easy to buy and has strong versatility. The lower the withstand voltage level and overcurrent level of the power device, the better. The control CPU should choose the mainstream chips in the market. Other products such as relays, contactors, terminals, EMI filters, lightning arresters, fuses, etc. are best to choose products with various certifications, so as to successfully pass the certifications of German TUV, EU CE, Italian DK5940, and China Golden Sun.
(3) Circuit topology
If isolation is not required, the non-isolated circuit topology is preferred due to its high efficiency. If there are isolation requirements without strict requirements on volume and weight, the power frequency isolation circuit topology can be used because of its higher efficiency. Otherwise, a high-frequency isolated circuit topology can be used, because its efficiency is not easy to improve, but it should be noted that it is only suitable for small and medium power ranges.
(4) Power quality
Various grid-connected standards stipulate that photovoltaic inverters should have good power quality. Usually, the filter also has a great impact on the volume and weight of the device and the quality of grid-connected current. Increasing the switching frequency or using frequency multiplication control technology is beneficial to reduce noise and also benefit filter design. At the same time, it is a good choice to use multi-level technology instead of two-level technology [107-111], or LCL filter to replace LC filter. On the basis of this hardware circuit, using advanced phase-lock control, current control and other technologies, the system can obtain good dynamic and static performance.
(5) Safe and reliable, long life and easy maintenance
Safety and reliability, long life and easy maintenance are also considerations in choosing a circuit. Replacing ordinary aluminum electrolytic capacitors with organic film capacitors or long-life electrolytic capacitors can greatly extend the life of photovoltaic grid-connected power generation systems. The use of power modules to replace discrete components and intelligent drives to replace conventional drives can improve the reliability of the system. Modular circuit design is not only conducive to installation and maintenance, but also to improve the redundancy and reliability of the system.
(6) Network monitoring capability
Generally, photovoltaic grid-connected power generation systems have network communication capabilities, and the host computer performs online control and monitoring of photovoltaic grid-connected power generation systems through various communication methods. Communication methods include wireless communication, power carrier communication, RS485, RS232, Ethernet, etc. Wireless communication can use ZIGBEE, WiFi and other technologies, data transmission is convenient, no data cable is required. For ZIGBEE, the communication distance generally does not exceed 30m without an amplifier; for WiFi, the maximum transmission range is 300m outdoors and 100m indoors with obstacles. Wireless communication can also rely on the relay transmission technology of microwave stations and data communication services based on GSM/GPRS.
Power carrier communication does not require additional electricity, and the communication can be realized by using the existing power grid and the corresponding modem, and the communication distance is generally not more than 1000m. RS485 communication distance does not exceed 1200m, RS232 communication distance does not exceed 12m. Ethernet is a standard communication method for computers. Through the Ethernet communication module, it can be extended infinitely according to needs, and the connection with the network is simple.
In short, in a photovoltaic grid-connected power generation system, it is impossible to meet the above general design principles at the same time, and it is necessary to consider a compromise to design a more satisfactory circuit topology.
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