Direct-drive wind power grid-connected converter device

Variable speed constant frequency wind power generation systems are mainly divided into two types: doubly 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/ 3-1 /4 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, does not require a gearbox (or semi-direct drive, using a primary gearbox), and has no pulleys, fewer mechanical failures, less loss, high operating efficiency, and low maintenance costs. 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 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 generation grid-connected control technology, successfully developed products, and installed and debugged them in the Baotou City, Inner Mongolia Autonomous Region project. It has been successfully operated for several months with no fault records.

1 Control principle

Megawatt-class high-power direct-drive grid-connected converters adopt 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.

Figure 1: Topology of Boost voltage regulator 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.

Figure 2 PWM rectifier circuit topology

The main circuit topology shown in Figure 2 can be used to deal with the unstable AC voltage at the generator end, large harmonics and large voltage changes on the DC side 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, and the total harmonic THD of the grid-side current can be less than the national standard requirement of 5% without adding additional filters .

2 Technical Features

By utilizing the main circuit topology and energy feedback grid-connected technology of low-voltage high-power inverters developed over many years, we have successfully developed a direct-drive wind power grid-connected converter, which has been successfully used in wind power generation projects. This product has the following technical features:

(I) 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.

(ii) The converter presents current source characteristics to the power 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.

(III) The grid-side inverter adopts a three-level circuit topology, which can adapt to a wide range of grid-side voltage and is also beneficial in reducing the total harmonics of the grid-side current.

(IV) Megawatt-class converters require multiple units to be connected in parallel. The system control will automatically work in groups, making it easy to linearize the grid-connected feedback power, making it easy to control the entire wind power project system, and at the same time helping to reduce the total harmonics 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 automatically maximize the grid-connected power.

(6) It has fast dynamic response and can instantly meet the requirements of a wide range of power changes based on the overall control of wind power, and has strong adaptability.

(VII) It has various protection functions such as temperature, overcurrent, short circuit, bypass, abnormal grid 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 and Figure 4 are grid-side voltage and current waveforms, respectively. Figure 3 is the grid-side voltage and current waveform when the grid-connected current is 60A ; Figure 4 is the grid-side voltage and current waveform when the grid-connected current is 100A . 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 also be observed that as the current increases, the total harmonics ( THD ) of the grid-side current becomes smaller and smaller, that is, the overall efficiency also increases.

Figure 3 Waveform when current is 60A

Figure 4 Waveform when current is 100A

4 Summary

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.