Introduction to the Functions and Basic Principles of Wind Power Converters

The wind power converter is the excitation device added to the rotor side of the doubly-fed wind generator. Its main function is to control the amplitude, phase, and frequency of the excitation through the converter when the rotor speed n changes, so that the stator side can input constant frequency power to the smart grid. Including power module, control module and grid-connected module.

1. Development of wind power converters

According to the "China Wind Power Converter Market Research Report", the global wind energy industry created an output value of nearly 40 billion euros (55 billion U.S. dollars) in 2010, compared with 50 billion euros (70 billion U.S. dollars) in the same period in 2009. The decline in the output value of the global wind energy industry in 2010 is related to the decrease in the cost of wind power generation and the transfer of the wind energy industry to China. In 2010, China became the country with the largest installed capacity of wind energy and became the center of the international wind energy industry. China's share in the global wind energy market exceeded 50%.

In terms of offshore wind power generation, China established its second major offshore wind farm off the coast of Shanghai in 2010, with an installed capacity of 100MW. Compared with China's total installed wind energy capacity, offshore wind energy is still insignificant in China (its new installed capacity only accounts for 0.5% of the total new installed capacity). Under the three scenarios of low growth, medium growth and high growth, China's installed wind power capacity will reach 100 million kW, 150 million kW and 200 million kW in 2020, respectively. At the same time, the proportion of wind power in total energy consumption will reach 1.6%, 2.5% and 3.3% respectively.

The rapid growth of my country's wind power installed capacity has provided a strong impetus for the development of my country's wind power converter industry. In 2009, the market capacity of my country's wind power converters was 6 billion yuan. Among them, the demand for direct drive wind turbines for full-power inverters is 2202MW, and the market capacity is 1.4 billion yuan; the market demand for double-fed converters for the newly added capacity of double-fed installed capacity is 11608MW, and the market capacity is 4.6 billion yuan. In 2010, the demand for full-power converters of direct drive wind turbines was about 3230MW, and the market capacity was about 1.9 billion yuan; the demand for doubly-fed converters of double-fed wind turbines was about 9770MW, and the market capacity was about 3.5 billion yuan . In 2010, the total capacity of my country's wind power converter market was about 5.5 billion yuan.

In 2009, there were about 3,000 sets of megawatt-level wind power converters in my country, and the converters produced by most enterprises were mainly 1.5MW. Enterprises with 1.5MW production capacity mainly include Tsingneng Huafu, Kono Albert, Sungrow, Hopewind Electric, CSR Times, etc. However, in 2010, they still focused on small batch production. From the perspective of planned production capacity, major domestic enterprises will form a total production capacity of more than 4,000 units in 2011. In 2011, the production capacity of Sungrow, Hopewind Electric and CSR Times is expected to reach 1,000 wind power converters.

In 2015, the wind power converter independently developed by the 18th Institute of Aerospace First Academy completed the low-voltage ride-through test. The success of the low-voltage ride-through test indicates that my country has independently mastered the core technology and key algorithms of wind power converters.

2. Functions of wind power converters

The converter excites the rotor of the doubly-fed asynchronous wind turbine, so that the amplitude, frequency and phase of the output voltage on the stator side of the doubly-fed generator are the same as those of the grid, and independent decoupling of active and reactive power can be performed as required control.

The converter controls the doubly-fed asynchronous wind generator to achieve soft grid connection, reducing the adverse impact of the grid connection surge current on the motor and the grid.

The converter provides a variety of communication interfaces, such as Profibus, CANopen, etc. (expandable according to user requirements), through which users can easily realize the integrated control of the converter, the system controller and the remote monitoring system of the wind farm.

The power distribution system of the converter provides protection functions such as lightning strike, over-current, over-voltage, and over-temperature.

The converter provides a real-time monitoring function, and the user can monitor the running status of the wind turbine converter in real time.

The converter can be specially designed according to the altitude, and can be customized to meet the operating requirements of low temperature, high temperature, dustproof, salt spray and so on.

3. Basic principles of wind power converters

The converter adopts three-phase voltage type AC-DC-AC bidirectional converter technology, and the core control adopts a "dual DSP full digital controller" with fast floating-point computing capability; the stator converter is implemented on the rotor side converter of the generator Field-oriented vector control strategy, the grid-side converter realizes the grid voltage-oriented vector control strategy; the system has the functions of adjustable input and output power factors, automatic soft grid connection and maximum power point tracking control. The power module adopts IGBT power devices with high switching frequency to ensure good output waveform. This rectifier and inverter device has the advantages of simple structure and less harmonic content, and can obviously improve the operating state and output power quality of the doubly-fed asynchronous generator. The doubly-fed asynchronous generator excitation control system of this voltage-type AC-DC-AC converter realizes the decoupling control of the active and reactive power of the generator based on the maximum power point tracking of the wind turbine, and is the first choice for doubly-fed asynchronous wind turbines. One represents the direction.

4. System composition of wind power converter

The converter is composed of main circuit system, power distribution system and control system. Including stator grid-connected switch, rectifier module, inverter module, input/output filter, IGBT, PLC, JCE1000-AXS current sensor, fan, active crowbar circuit, controller, monitoring interface and other components.

The main circuit system of the converter includes the following basic units:

Rotor side inverter, DC bus unit, grid side rectifier.

The power distribution system is composed of grid-connected contactors, main circuit breakers, relays, transformers, etc. It is integrated with a grid-connected control system. Users do not need to configure grid-connected cabinets, which improves system integration and saves engine room space. Provide 220V power supply for on-site debugging.

The control system is composed of high-speed digital signal processor (DSP), man-machine interface and programmable logic controller (PLC). The entire control system is equipped with an uninterruptible power supply (UPS), which facilitates the system to have uninterrupted operation capability when the voltage drops.

The above functions are allocated to the control cabinet, power cabinet and grid-connected cabinet:

Control cabinet: The control cabinet mainly analyzes the collected analog and digital signals, issues control commands, and controls the operating status of the converter. The control cabinet is mainly composed of the main control box, PLC, filter, power module and so on.

Power cabinet: mainly responsible for the transfer of rotor slip energy. The power cabinet is mainly composed of power module, IGBT, PLC, JCE1005-FS current sensor, fan, active crowbar, etc.

Grid-connected cabinet: It is mainly used for the connection control of the converter and the generator system and the grid, the collection of some control signals and the configuration of the secondary circuit. The grid-connected cabinet is mainly composed of circuit breakers, contactors, signal acquisition components, UPS, heaters, signal interface parts, etc.

5. Technical characteristics of wind power converters

Wind power converters can optimize the operation of wind power generation systems, realize variable speed and constant frequency power generation within a wide wind speed range, improve fan efficiency and transmission chain working conditions, reduce generator losses, improve operating efficiency, and increase wind energy utilization.

Wind power converters have the following characteristics:

5.1 Excellent control performance

5.2 Complete protection function

5.3 Good grid adaptability

5.4 High reliability, suitable for operation in harsh areas such as high and low temperature, high altitude

5.5 Modular design, combined structure, easy installation and maintenance