Environmental Engineering Reference
In-Depth Information
induction generator (DFIG) was proposed in [
5
]. The tasks of grid synchronization
and power control were undertaken by two different algorithms, designed to
command the rotor-side converter (RSC) at fixed switching frequency. Despite
conventional synchronous generators, permanent magnet synchronous generators,
and doubly- fed induction generators, the switched reluctance generator (SRG) can
also be considered as a wind generator. In [
6
], a novel speed control of SRG by
using adaptive neural network (ANN) controller was presented. The SRG is driven
by variable-speed wind turbine and it is connected to the grid through an asym-
metric half bridge converter, DC-link, and DC-AC inverter system. Among a lot of
control methods, SMC offers some superior properties including fast and finite-
time convergence, and high steady-state precision [
7
] and has been used in WECS.
This chapter describes a control strategy for wind energy integration into power
network. A typical WECS and its mathematical model are analyzed. For a wind
turbine to have the maximum active power extraction from the wind at any given
instant, the electrical load on the generator is regulated using the MPPT method.
For wind energy integration into power network, the structure of dual PWM
inverter is utilized. Both the voltage and currents in the WECS are measured and
control signals for both two PWM inverters are generated based on the control
algorithms. An improved outer-loop control strategy is designed to control the
square of the DC-link voltage.
Compared to other control methods, sliding-mode control (SMC) has many
important features, such as simplicity for implementation, high robustness to
external disturbances and low sensitivity to the system parameter variations [
8
-
11
].
SMC includes conventional linear sliding-mode (LSM) control and nonlinear ter-
minal sliding-mode (TSM) control. The former are asymptotically stable, and the
latter are finite-time stable. Compared to traditional LSM control, TSM control
exhibits various superior properties such as fast and finite-time convergence, and
smaller steady-state tracking errors [
12
,
13
]. In the chapter TSM is used to make the
error of the current and DC-link voltage reach zero in finite time. Meanwhile, high-
order sliding-mode technique is utilized to eliminate the chattering phenomenon
existing in sliding-mode control. The actual control signal is soften to be continuous
and smooth. The TSM control strategy described in the chapter can improve the
performance of the grid-side PWM converter of the wind power system.
3.2 Model of Wind Turbine
A typical WECS is shown in Fig.
3.1
. It consists of a wind turbine, a gearbox, a
generator, a machine-side PWM inverter, an intermediate DC circuit, a grid side
PWM inverter, a transformer, and a control system. The generator may be a syn-
chronous, permanent magnet synchronous, doubly-fed induction, or induction
generator.
