summary:
As one of the most commercially viable renewable energy technologies, wind power is receiving increasing attention from countries around the world. This paper takes a 2.0MW doubly-fed wind turbine converter as an example, introduces the commonly used topologies of doubly-fed wind turbine converters, calculates and selects the parameters of the main components, and then explains the system control strategy.
Abstract:
Wind energy as one of the renewable power technology which was the most close to commercialization has been more and more attention by the countries all over the world. In the case of 2.0MW Doubly-fed Wind Power Converters, this discussion introduces the topologies that doubly-fed wind power converters most commonly used, and the calculation and choice of the main device parameter, then demonstrates system control strategy.
Keywords: Wind power generation; Doubly fed induction generator (DFIG); Grid-side filter; Control strategy;
Key words: wind power; double-fed converter; grid side filter; control stategy
1 Introduction
With the increasing depletion of conventional energy sources on Earth, the development of renewable energy has become an inevitable trend. Major developed and developing countries have all adopted the development of renewable energy sources such as wind and solar power as important means to address the dual challenges of energy and climate change in the new century. However, among all renewable energy sources besides hydropower, wind power is undoubtedly recognized worldwide as one of the most commercially viable renewable energy technologies. With the continuous development of wind power technology, doubly-fed induction generators (DFIGs) have been adopted by most wind farms due to their advantages such as small size and low cost, and DFIGs have gradually become a research hotspot in wind power systems.
2. Basic Working Principle of Doubly Fed Wind Power Converter System
2.1 Introduction to Doubly Fed Wind Power Converter System
The schematic diagram of the doubly-fed wind power converter system is shown in Figure 2-1. The generator used is a doubly-fed generator with rotor AC excitation, and its structure is similar to that of a wound-rotor induction motor. The stator winding is directly connected to the power grid, and the rotor winding is supplied with three-phase low-frequency excitation current by a power source with adjustable frequency, amplitude, and phase. This forms a low-speed rotating magnetic field in the rotor, and the rotational speed of this magnetic field, when added to the mechanical speed of the rotor, equals the synchronous speed of the stator magnetic field. This induces a power frequency voltage in the generator stator winding.
Since this variable speed constant frequency control scheme is implemented in the rotor circuit, the power flowing through the rotor circuit is the slip power determined by the operating speed range of the AC excitation generator. This slip power is only a small part of the stator rated power. Therefore, the capacity of the bidirectional frequency converter is only a small part of the generator capacity, which greatly reduces the cost and size of the frequency converter.
This control scheme can also change the generator's power angle by controlling the rotor current amplitude and phase. Therefore, by adjusting the excitation, independent control of active and reactive power can be achieved, enabling flexible adjustment of the power factor. For the power grid, outputting reactive power can serve as reactive power compensation; absorbing reactive power can suppress overvoltage, thus contributing to the stability of the power grid.
The disadvantage of this approach is that the AC excitation generator still has slip rings and brushes; the mathematical model and energy relationships of the AC excitation doubly-fed generator are complex, and the control strategy is also relatively complex.
2.22.0MW Doubly Fed Converter System Main Circuit Description
As shown in Figure 2-1, this system is designed with a pre-charging branch controlled by KM2. When KM2 is closed, the capacitor on the DC bus is slowly charged through the current-limiting resistor. When the bus voltage reaches a certain value, KM1 is closed and the pre-charging branch is disconnected. This prevents the impact on the converter bus capacitor when the grid-side KM1 is directly closed.
The grid-side LC filter unit (U5) can filter the switching harmonics in the converter output voltage.
When a momentary power grid failure occurs, the braking unit (U3) and the Crowbar unit (U6) work together to keep the converter connected to the grid and prevent it from disconnecting.
The rotor-side LC filter unit (U4) suppresses voltage switching harmonics in the machine-side output and controls dv/dt within a certain range.
The grid-side converter unit (U2) is a three-phase PWM rectifier that provides a stable DC voltage to the rotor-side converter unit (U1); while the rotor-side converter is actually an inverter that provides AC excitation to the doubly-fed motor rotor. Both the grid-side converter unit and the machine-side converter have their own DSPs responsible for their real-time control.
When the converter control system detects that the generator set meets the grid connection conditions, it controls the grid connection switch (QF2) to close, thereby enabling the generator set to generate electricity by grid connection.
3. Calculation of main component parameters of 2.0MW doubly-fed wind power converter
3.1 Grid-side filter design
3.1.1 Design of Inductor for Grid-Side Filter
The design of the AC input inductor of a grid-side PWM rectifier should first meet the requirements of the rectifier's output active (reactive) power, while also meeting the requirements of suppressing current harmonics and achieving fast current tracking. The constraints on the inductor when the PWM rectifier operates at unity power factor are given below.
In this 2.0MW doubly-fed converter system, the total power of the three-phase PWM rectifier is 720kW, the AC input voltage is 690V, the DC bus voltage is 1100V, the switching frequency is 2.5kHz, and the grid-side rated current is 600A. When selecting the total inductor, the maximum allowable value of harmonic current ripple can be amplified to 10% to 20% of the rated current. In this system, the maximum allowable value of current harmonic ripple is adopted.
3.1.2 Grid-side filter capacitor design
Let the total power of the three-phase PWM rectifier be , then the reactive power consumed by the capacitor is:
The minimum DC bus voltage is 1073V. Therefore, a DC bus voltage of 1100V is selected.
Considering the above factors and the supply situation of IGBTs on the market, with a certain margin, we chose 1700V, 1000A IGBTs connected in parallel, with two IGBTs connected in parallel on each phase unit on the grid side and three IGBTs connected in parallel on each phase unit on the generator side, to meet the requirements of this system.
3.3 Calculation of DC bus filter capacitor
The selection of the DC bus filter capacitor C mainly considers the following three factors:
(1) The capacitance value can meet the expected ripple voltage.
(2) Rated voltage of the capacitor
(3) Rated ripple current of the capacitor
Based on the analysis of the relationship between ripple current and the heat generation and lifespan of electrolytic capacitors, ripple current plays a crucial role in constraining the selection of filter capacitor capacity in power electronic devices. Therefore, the filter capacitor capacity can be calculated based on the maximum allowable value of ripple current. The calculation of the filter capacitor is based on maintaining voltage ripple within the allowable range under the condition of most severe DC current ripple.
Assuming the load current is sinusoidal, the pulsating DC current in the DC circuit also follows a pattern.
4. Overall Software Framework
Since the main topology of the 2.0MW wind power converter adopts an AC-DC-AC structure and the hardware adopts independent control between the generator side and the grid side, its software is mainly divided into two parts: grid-side control software and generator-side control software.
The software design adopts a modular approach, dividing the program into functional modules based on its purpose. The main components of the generator-side/grid-side program include: a system initialization module, an AD sampling and filtering conversion module, a CAN communication module, a waveform display module, a soft-start module, a voltage fault acquisition module, a low-voltage ride-through processing module, a generator-side/grid-side control algorithm module, and an SVPWM generation module. The program uses a timer interrupt method for AD acquisition and corresponding closed-loop control, executing in a polling manner during AD interrupts and main timer interrupts. The algorithms in the program mainly include the grid-side voltage outer loop and dual-current closed-loop control, and the generator-side grid voltage-oriented vector control.
4.1 Network-side control strategy
The grid voltage-oriented vector control employs a dual-closed-loop cascaded control structure: an outer voltage loop and an inner current loop. The main function of the voltage loop is to control the DC bus voltage; the current loop controls the AC input current based on the current command given by the voltage loop, achieving unity power factor operation. See the diagram below.
To detect asymmetrical voltage drops in the power grid, the system acquires negative sequence voltage and current, and performs independent control through a dual-current closed-loop system. Voltage dips are addressed by acquiring the dq components in synchronous coordinates under coordinate transformation, filtering them, and then sending them to the current calculation stage to provide reactive power support to the power grid.
4.2 Machine-side control strategy
The AC-excited doubly-fed induction generator (DFIG) is flexibly connected to the power grid. By controlling the generator rotor current, grid connection conditions can be met at any speed during variable-speed operation, achieving successful grid connection. The grid connection condition for the DFIG is that the stator voltage and the grid voltage are identical in amplitude, frequency, and phase; therefore, the stator voltage must be adjusted before grid connection. After successful grid connection, the wind turbine adjusts its active and reactive power in real time according to actual wind speed, wind direction, and grid dispatch requirements. Therefore, the power of the DFIG must be adjusted after grid connection.
The machine-side segmented grid-connected control strategy consists of three phases: the rotor initial position error compensation phase, the stator voltage establishment phase, and the doubly-fed induction generator (DFIG) grid-connection phase. During grid connection, the dq components of the grid voltage and rotor voltage and current are extracted and filtered in real time. When the bus voltage is over-voltage, or the rotor experiences overcurrent or overvoltage, the machine-side PWM signal is blocked, and the Crowbar circuit is activated to protect the motor rotor. After the current stabilizes, the machine-side converter is restored, and a reactive power support algorithm is adopted. When the voltage recovers, the machine-side converter continues to perform power control.
5. Conclusion
This paper takes a 2.0MW doubly fed wind power converter as an example to introduce the calculation and selection of parameters of devices such as grid-side filters, bus capacitors, and IGBTs in the main circuit of the system. Then, it explains the overall software framework of the system and the grid-side and machine-side control strategies.
About the author:
Zhang Zhenan is a research and development staff member at the New Energy Business Unit of Harbin Jiuzhou Electric, mainly engaged in the research of high-power wind power converters.
