With the rapid development of offshore wind power, the doubly fed induction generator and permanent magnet synchronous generator cannot meet the increasing request of power capacity. Therefore, superconducting generator should be used instead of the traditional motor, which can improve generator efficiency, reduce the weight of wind turbines, and increase system reliability. This paper mainly focuses on nonlinear control in the offshore wind power system which is consisted of a wind turbine and a high temperature superconductor generator. The proposed control approach is based on the adaptive backstepping method. Its main purpose is to regulate the rotor speed and generator voltage, therefore, achieving the maximum power point tracking (MPPT), improving the efficiency of a wind turbine, and then enhancing the system’s stability and robustness under large disturbances. The control approach can ensure high precision of generator speed tracking, which is confirmed in both the theoretical analysis and numerical simulation.
Because of the reduction of fossil fuels and increasingly serious environmental pollution, wind energy is world-widely recognized as a cost-effective, environmental-friendly solution to the energy shortage [
Wind power total capacity, 1997–2013.
Total Installed Capacity 2013 [MW].
At present, most wind turbines are operated onshore, making huge noise, which results in a negative impact to people’s life. The wind farms occupy a large area of land, so the onshore wind power technology is believed to be reliable. Due to transportation restrictions, the size of the onshore turbine is expected to be lower than the current size of 3 to 5 MW [
Doubly fed induction generators (DFIGs) are mainly applied to the offshore wind turbine system nowadays [
Solutions to the problem of wind energy conversion system (WECS) development experience and grid connected wind turbines that require different control strategies. Therefore, a global optimization control method is necessary, in order to ensure energy conversion functionality and effectiveness of the system as a whole and to provide the quality of energy. Considering the poor atmospheric conditions and random nature of the primary energy, wind, these are not trivial tasks [
In this paper, a high temperature superconductor generator (HTSG) unit is embedded into the system to enhance the generated power quality. This is achieved by properly regulating the HTSG speed to maintain constant speed of the generator over a wide range of wind speed. It should be noted that DFIG has been widely used in high performance servo system where a rapid and accurate torque response is required because of their inherent advantages such as low inertia, high efficiency, and high power density compared to other turbines with the same capacity. The main task of the control system is to adjust the HTSG speed so as to improve power quality.
Reference [
The rest of the paper is divided into the following several sections. Section
A wind turbine extracts kinetic energy from the swept area of the blades. The power in the airflow is given by [
One has
From (
Direct drive HTSG wind tusrbine used the single-mass model representation, as shown in Figure
Wind turbine drive train.
A single mass for the wind turbine can be defined as
Since the early 1960, high temperature superconducting winding has hopes to become the next generation of generator stator and rotor winding [
The characteristics of superconductor mainly have the following several aspects. Firstly, zero resistance phenomenon. Superconductors under the low temperature environment, resistance suddenly disappeared. Secondly, Meissner effect, when the high temperature superconductivity at low temperatures, the resistance is zero, then the applied magnetic field so that the appropriate zero resistance disappears. Thirdly, critical magnetic field. Superconducting zero resistance phenomenon disappear with magnetic field intensity (see (
There are three main types of wind turbine generator; the first stage is DFIG. The system has two advantages: the generator slip ring is unnecessary that required regular maintenance and suitable converter capacity which can provide better low voltage through ability. DFIG-based wind turbines have some advantages such as the variable speed operation, active power, and reactive power-independent control [
The second stage is PMSG. Compared with gear growth mode, the direct-drive wind turbine system can simplify the drive train to increase energy output and reliability. PMSG system uses permanent magnet excitation and no additional power supply will need to provide excitation, which has high efficiency. However, due to the low-speed operation, direct-drive PMSG may have deficiencies such as the large size, heavy weight, and high cost of generators [
The third stage is the HTSG. The increase of the volume and weight has inversely proportional relationship with the motor’s rated speed decreases. The system as a whole benefits from the advantages of lower quality and size, and no gearbox. HTSG is able to solve the problem of DFIG and PMSG capacity (saturated magnetic field) and a superconducting generator with higher efficiency, lighter mass, and smaller volume. Through the use of high current density of a series of superconductors, produced by the high air gap magnetic flux density, will allow generator more compact [
The basic structure of HTSG.
The electrical model of HTSG in the stator reference
One has
The electromagnetic torque is obtained as
From the electromagnetic torque equation, it can be seen that the torque control can be obtained by the regulation of
Generator torque is calculated by means of look-up table; the generator speed control area after filter is divided into four parts: 1, 2, 2.5, and 3 [
Wind turbine control region.
In regions 2-3, the demand torque is given by
A HTSG-based wind power generation system schematic diagram is shown in Figure
Configuration of a HTSG wind turbine.
Superconducting windings have a constant magnetic field with external conditions (temperature, currents), from (
In order to facilitate the calculation, then from (
Equation (
The error variable is defined as
To make the subsystems is stable, select the Lyapunov function as
From (
From (
Select the stable function as
Then
Because (
Estimation error is defined as
Equation (
From (
Step
Then
From (
Therefore,
Equation (
From (
Select the Lyapunov function as
Therefore,
The adaptive law is designed as
Therefore, we can obtain
Therefore,
The HTSG wind turbine parameter used in the simulation is shown in Table
NREL 5 MW baseline wind turbine model properties.
Properties | Value |
---|---|
Rating power | 5 MW |
Rotor orientation, configuration | Upwind, 3 blades |
Control | Variable speed, variable pitch |
Hub height | 90 m |
Rated rotor | 12 rpm |
Rated generator efficiency | 98% |
Tip speed ratio | 6.5 |
Power coefficient | 0.46 |
Stator resistance | 150 |
Using the turbine model data from [
With the rapid development of offshore wind power industry, machine capacity of wind turbines is gradually increased. Due to the shallow sea area construction of wind field affect human life. The siting of wind field is established in the deep water area is a better choice. Therefore, floating offshore wind turbines generator using HTSG can reduce offshore wind farm construction and maintenance costs [
NREL 5 MW baseline wind turbine model properties.
Properties | Value |
---|---|
Rating power | 5 MW |
Rotor orientation, configuration | Upwind, 3 blades |
Control | Variable, collective pitch |
Drivetrain | Low speed, direct drive |
Rotor, hub diameter | 126 m, 4 m |
Hub height | 90 m |
Cut-in, rated, cut-out wind speed | 3 m/s, 11.4 m/s, 25 m/s |
Cut-in, rated rotor speed | 6.9 rpm, 12.1 rpm |
Rotor mass | 110000 kg |
Nacelle mass | 24000 kg |
Tower mass | 37460 kg |
Coordinate Location of overall CM | (−0.2 m, 0.0 m, 64.0) |
Using the turbine model data from [
Physical properties of the floating platform.
Properties | Value |
---|---|
Size ( |
40 m × 40 m × 10 m |
Moon pool ( |
10 m × 10 m × 10 m |
Draft, freeboard | 4 m, 6 m |
Water displacement | 6,000 m3 |
Mass, including ballast | 5452000 kg |
CM location below SWL | 0.281768 m |
Roll inertia about CM | 726900000 kg·m2 |
Pitch inertia about CM | 726900000 kg·m2 |
Yaw inertia about CM | 1453900000 kg·m2 |
Anchor (water) depth | 150 m |
Separation between opposing anchors | 773.8 m |
Unstretched line length | 473.3 m |
Neutral line length resting on seabed | 250 m |
Line diameter | 0.0809 m |
Line mass density | 130.4 kg/m |
Line extensional stiffness | 589000000 N |
Using the turbine model data from [
Floating wind turbine.
Floating offshore wind energy conversion system including wind turbines, driving chain, electronic power converter, nacelle, tower, floating platform. The external environment has the wind and waves. The generality of each module also ensures that the overall simulation tool is universal enough to analyze a variety of wind turbine, support platform, and mooring system configurations. Figure
Floating offshore wind power system.
In order to evaluate the performance of the adaptive backstepping control algorithm which is proposed, the effectiveness of the proposed torque control strategy is verified through the Matlab/Simulink code (Figure
Block diagram of WECS in Matlab.
Verify that the paper design control algorithms, the wind and sea conditions offshore wind turbine is shown in Figures
Wind speed.
Wave height.
Comparison of tip speed ratio tracking.
Power coefficient.
The simulation results of the advanced MPPT controller with adaptive backstepping control method are demonstrated in Figures
The author presented two kinds of generators that can be used to convert wind energy into electric energy. Power capacity and weight are limited in traditional technology which is used in wind applications. With the increasing of the wind power generation unit capacity, superconducting technology is a good choice for wind turbines. Superconducting technology is relatively mature to develop the wind turbines of high efficiency and power density compare with DFIG or PMSG.
A model-independent control scheme based on adaptive backstepping control method has been developed for torque tracking of HTSG system. We establish torque and flux tracking of proposed adaptive backstepping control scheme according to Lyapunov stability theory. Adaptive backstepping control algorithm is put forward for its prominent features such as the robustness of parameter disturbance in the turbines and the electric grid disturbance. Numerical analysis and simulation results clearly show that the adaptive backstepping control method is more effective than the traditional methods in terms of MPPT.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported in part by the National High Technology Research and Development Program of China (SS2012AA052302), Fundamental Research Funds for the Central Universities (ZYGX2012J093), and National Natural Science Foundation of China (51205046).