New Fengguang direct-drive wind power grid-connected converter device

　　Wind power generation is currently the most scalable and commercially viable renewable energy technology. In fact, wind power generation depends to a large extent on the development of variable speed constant frequency power generation systems, which have become the mainstream technology for wind turbines above MW level. The so-called variable speed constant frequency means that the speed of the wind turbine rotor can follow the change of wind speed through speed control, so as to maximize the utilization efficiency of wind energy and effectively reduce the load. At the same time, when the speed of the wind rotor and the motor it drives changes, the output power frequency is always consistent with the grid frequency.

　　Variable speed constant frequency wind power generation system is mainly divided into two types: doubly fed and direct drive. Since the converter of doubly fed is connected in series in the rotor winding of the doubly fed generator, its capacity is only 1/4~1/3 of the total system power, which effectively reduces the system cost; compared with the doubly fed type, the direct drive type adopts a low-speed permanent magnet synchronous generator structure, without a gearbox (or semi-direct drive, using a primary gearbox), and no pulley, with fewer mechanical failures, less loss, high operating efficiency, and low maintenance cost. However, since the direct drive type uses the full power transmission of the system, the initial cost is relatively high.

　　At present, many domestic universities, research institutes and enterprises are mainly researching, tracking, digesting and absorbing doubly-fed grid-connected converters. In recent years, our company has utilized the advantages of many years of research and development of main circuit topology and feedback grid-connected control technology of high-power inverters to focus on the development of direct-drive wind power grid-connected control technology. We have successfully developed this product, which has been installed and debugged in Baotou City, Inner Mongolia. It has been successfully running for several months with no fault records.

　　1 Control Principle

　　The MW-class high-power direct-drive grid-connected converter adopts a multi-unit parallel structure, and the main circuit topology of a single unit adopts an AC-DC-AC voltage type structure, as shown in Figure 1 or Figure 2. Figure 1 adopts a diode uncontrolled rectification and Boost voltage stabilization circuit, and Figure 2 adopts a PWM fully controlled rectification circuit.

　　Using the main circuit topology of Figure 1, the input DC voltage of the back-end inverter can be well controlled through the Boost voltage regulator link. That is, no matter how much the output DC voltage of the diode uncontrolled rectifier changes, after passing through the Boost voltage regulator circuit, its DC voltage is basically stable, so that the modulation range of the back-end inverter is good, the operating efficiency is improved, and the loss is reduced. At the same time, the Boost circuit can also perform power factor correction on the output side of the permanent magnet synchronous generator.

　　The main circuit topology shown in Figure 2 can be used to deal with the problems of unstable AC voltage, large harmonics and large voltage changes on the DC side at the generator end through PWM controlled rectification technology. It is the most promising main circuit structure. The two main circuits have their own advantages and disadvantages. The control adopts the current inner loop and voltage outer loop double closed loop vector control technology.

　　Each unit adopts carrier phase shift multiplexing technology, which can make the grid-side current harmonic distortion coefficient THD < the national standard requirement of 5% without adding additional filters.

　　2 Technical features

　　Shandong New Wind Photovoltaic Electronic Technology Development Co., Ltd. has successfully developed and used the direct-drive wind power grid-connected converter in wind power generation projects by using the main circuit topology and energy feedback grid-connected technology of low-voltage high-power inverters developed for many years. Its technical features are:

　　1) The control adopts voltage and current dual closed-loop vector control, showing current source characteristics. The current loop is the core of the direct-drive wind power grid-connected converter control;

　　2) The converter presents current source characteristics to the grid, making it easy to connect multiple units in parallel and assemble them in high power. Multiple carrier phase shifting is used between the units, which greatly reduces the total harmonics of the grid-side current.

　　3) The grid-side inverter adopts a three-level circuit topology, which can adapt to a wide range of grid-side voltages and is also beneficial in reducing grid-side harmonic currents;

　　4) MW-level converters require multiple units to be connected in parallel. The system control will automatically work in groups, which makes it easy to linearize the grid-connected feedback power, facilitate the system control of the entire wind power project, and help reduce the total harmonics of the current;

　　5) The grid-connected converter adopts advanced PWM control technology, which can flexibly adjust the active and reactive power of the system, reduce switching losses, improve efficiency, and automatically maximize the grid-connected power;

　　6) Fast dynamic response. According to the overall control of wind power, it can instantly meet the requirements of a wide range of power changes and has strong adaptability;

　　7) It has various protection functions such as overheating, overcurrent, short circuit, bypass, abnormal grid-side voltage, etc. It has multiple analog and digital interfaces, and has interfaces such as CAN bus or RS485 serial bus. It is easy to connect with other parts of the wind power project and has flexible control.

　　3 Experimental waveform analysis

　　Figure 3 is the grid-side voltage and current waveform when the grid-connected current is 60 A, and Figure 4 is the grid-side voltage and current waveform when the grid-connected current is 100 A. It can be seen from the two figures that the grid-side current is sinusoidal and in antiphase with the grid voltage, showing a negative unity power factor. At the same time, it can be observed that as the current increases, the harmonic distortion coefficient (THD) of the grid-side current becomes smaller and smaller, that is, the overall efficiency becomes higher and higher.

　　4 Conclusion

　　The direct-drive wind power grid-connected converter adopts an AC-DC-AC three-level voltage-type main circuit topology, presents a controlled current source characteristic, is easy to connect in parallel, easy to assemble in high power, has a sinusoidal grid-side current, can be softly connected to the grid, has no impact on the grid, and is pollution-free. It can be widely used in renewable energy projects such as wind power generation.