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

0 Introduction
Wind power generation is currently the most scalable and commercial 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 changes in 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 systems are mainly divided into two types: double-fed and direct-drive. Since the converter of the doubly-fed type 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 type, using a primary gearbox), and without a pulley, with fewer mechanical failures, small losses, high operating efficiency, and low maintenance costs. However, since the direct-drive type uses full system power transmission, the initial cost is relatively high.
At present, many domestic universities, research institutes and enterprises mainly study, track, digest and absorb doubly-fed grid-connected converters. In recent years, our company has taken advantage of many years of research and development of high-power inverter main circuit topology and feedback grid-connected control technology, focusing on the development of direct-drive wind power grid-connected control technology, and successfully developed this product, which has been installed and debugged in Baotou City, Inner Mongolia. It has been successfully operated 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. Using the main circuit topology of Figure 2, through PWM controlled rectification technology, it can well deal with the problems of unstable AC voltage, large harmonics and large DC side voltage changes at the generator end, which 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 dual closed-loop vector control technology.
Each unit adopts carrier phase shift multiplexing technology, without the need to add additional filters, it can make the grid-side current harmonic distortion coefficient THD < the national standard requirement of 5%.
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 control of direct-drive wind power grid-connected converter;
2) The converter shows current source characteristics to the power grid, which is easy to parallel multiple units and easy to assemble in high power. Multiple carrier phase shifting is adopted between each unit, which greatly reduces the total harmonics of the grid-side current;
3) The grid-side inverter adopts a three-level circuit topology, which is suitable for a wide range of grid-side voltages and is also beneficial to reduce the harmonic current on the grid side;
4) MW-level converters require multiple units to be connected in parallel, and the system control will automatically work in groups. It is easy to linearize the grid-connected feedback power, which is easy for the system control of the entire wind power project and is beneficial to 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 large-scale power changes and has strong adaptability.
7) It has various protection functions such as overheating, overcurrent, short circuit, bypass, abnormal grid voltage, etc. It has a variety of analog and digital interfaces, and interfaces such as CAN bus or RS485 serial bus. It is convenient 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 is in reverse phase 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 the AC-DC-AC three-level voltage type main circuit topology, presents the characteristics of controlled current source, is easy to connect in parallel, easy to assemble in high power, has 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.