Abstract : This paper briefly describes the direct-drive wind power generation grid-connected inverter from the aspects of control principle, circuit topology, technical features, and experimental analysis. This inverter has a high grid-side power factor, low total harmonic distortion (THD), fast dynamic response, and exhibits current-source characteristics, making it easy to connect multiple units in parallel. Keywords : Wind power generation, direct-drive inverter, total harmonic distortion 0 Introduction Wind power generation is currently the most scalable and commercially viable renewable energy technology. In fact , wind power generation largely depends on the development of variable-speed constant-frequency power generation systems, which have become the mainstream technology for megawatt-class and above wind turbine generators. Variable speed constant frequency (VSC) wind turbines use speed control to ensure the rotor speed follows wind speed changes, maximizing wind energy utilization efficiency and effectively reducing load. Simultaneously, the output frequency of electrical energy remains consistent with the grid frequency as the rotor and its driven motor speeds change. VSC wind power systems are mainly divided into two types: doubly-fed and direct-drive. In doubly-fed systems, the converter is connected in series in the rotor windings of the doubly-fed generator, and its capacity is only 1/3 to 1/4 of the total system power, effectively reducing system costs. Compared to doubly-fed systems, direct-drive systems use a low-speed permanent magnet synchronous generator structure, eliminating the need for a gearbox (or a semi-direct-drive system using a single-stage gearbox) and pulleys. This results in fewer mechanical failures, lower losses, higher operating efficiency, and lower maintenance costs. However, because direct-drive systems transmit the full power of the system, the initial cost is relatively high. Currently, many universities, research institutes, and enterprises in China mainly research, track, and assimilate doubly-fed grid-connected converters. Our company, leveraging its years of experience in researching and developing high-power inverter main circuit topologies and feedback grid-connected control technologies, has focused on developing direct-drive wind power grid-connected control technology. We have successfully developed a product, which has been installed and commissioned in a project in Baotou City, Inner Mongolia Autonomous Region. It has been running successfully for several months without any reported faults. 1. Control Principle The megawatt-level high-power direct-drive grid-connected converter adopts a multi-unit parallel structure. 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 uses a diode uncontrolled rectifier and a Boost voltage regulator circuit, while Figure 2 uses a PWM fully controlled rectifier circuit. [align=center]Figure 1 Topology with Boost Voltage Regulator Circuit[/align] Using the main circuit topology in Figure 1, the Boost voltage regulator circuit effectively controls the input DC voltage of the downstream inverter. Regardless of the magnitude of the output DC voltage variation in the uncontrolled diode rectifier, the DC voltage remains relatively stable after passing through the Boost voltage regulator circuit. This results in a better modulation range for the downstream inverter, improved operating efficiency, and reduced losses. Simultaneously, the Boost circuit can also perform power factor correction on the output side of the permanent magnet synchronous generator. [align=center]Figure 2 Topology with PWM Rectifier Circuit[/align] Using the main circuit topology in Figure 2, the PWM controllable rectification technology effectively addresses the problems of unstable AC voltage, high harmonics, and large DC voltage variations at the generator end. This is the most promising main circuit structure. Both main circuits have their own advantages and disadvantages. The control employs a dual closed-loop vector control technology with an inner current loop and an outer voltage loop. Each unit uses carrier phase-shift multiplexing technology, eliminating the need for additional filters and ensuring that the total harmonic distortion (THD) of the grid-side current is less than the national standard requirement of 5%. 2. Technical Features Utilizing years of experience in developing low-voltage, high-power frequency converters and energy feedback grid-connected technology, a direct-drive wind power grid-connected converter has been successfully developed and applied in wind power projects. This product has the following technical features: (I) The control system employs dual closed-loop vector control of voltage and current, exhibiting current source characteristics. The current loop is the core of the direct-drive wind power grid-connected converter control. (II) The converter exhibits current source characteristics to the grid, facilitating multi-unit parallel connection and high-power assembly. Multiple carrier phase shifting is used between units, significantly reducing the total harmonic distortion (THD) of the grid-side current. (III) The grid-side inverter adopts a three-level circuit topology, adapting to a wide range of grid-side voltages and also contributing to reducing the THD of the grid-side current. (iv) Megawatt-level converters require multiple units connected in parallel. The system control will automatically group the units, making it easy to linearize the grid-connected feedback power and facilitate the overall control of the wind power project system. It also helps reduce the total harmonic distortion of the current. (v) 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 maximize the automatic grid-connected power. (vi) It has fast dynamic response and can instantly meet the requirements of large-scale power changes according to the overall wind power control, with strong adaptability. (vii) It has various protection functions such as temperature, overcurrent, short circuit, bypass, and grid-side voltage abnormality. It has multiple analog and digital interfaces, and interfaces such as CAN bus or RS485 serial bus, which are convenient to connect with other parts of the wind power project and allow for flexible control. 3. Experimental Waveform Analysis Figures 3 and 4 show the grid-side voltage and current waveforms, respectively. Figure 3 shows the grid-side voltage and current waveforms when the grid-connected current is 60A; Figure 4 shows the grid-side voltage and current waveforms when the grid-connected current is 100A. From the two figures, it can be seen that the grid-side current is sinusoidal and out of phase with the grid voltage, exhibiting a negative unity power factor. Simultaneously, it can be observed that as the current increases, the total harmonic distortion (THD) of the grid-side current decreases, meaning the overall efficiency also increases. [align=center] Figure 3 Waveform at 60A[/align] [align=center] Figure 4 Waveform at 100A[/align] 4. Summary The direct-drive wind power grid-connected converter adopts an AC-DC-AC three-level voltage-type main circuit topology, exhibiting control current source characteristics. It is easy to connect in parallel and easily assembled for high power. The sinusoidal grid-side current allows for soft grid connection, has no impact on the grid, and causes no pollution. It can be widely used in renewable energy projects such as wind power generation. Author Introduction : Hu Shunquan graduated from the School of Control Science and Engineering, Shandong University, majoring in Power Electronics and Electric Drive. He is currently working in the R&D department of Shandong Xinfengguang Electronic Technology Development Co., Ltd., engaged in the research and development of power electronic conversion technologies such as frequency converters and renewable energy feedback devices. 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