7/23/2019 Control Dynamics of a Doubly Fed Induction Generator
Under Sub and Super-Synchronous Modes of Operation
1/9
1
Abstract-- Depending on wind speed, a doubly fed induction
generator (DFIG) based variable speed wind turbine is capable
of
operating in sub- or super-synchronous mode of operation
using
a back to back PWM converter. A smooth transition between
these two modes of operation is necessary for reliable
operations
of the wind turbine under fluctuating wind. This paper
presents
the analysis and modeling of DFIG based variable speed wind
turbine and investigates the control dynamics under two
modes
of operation. A battery energy storage (BESS) with a
bidirectional dc-dc converter is added for a smooth
transition
between the modes. Mathematical analysis and corresponding
modeling results show that the power flow in the rotor
circuitunder two modes can be controlled by changing current
and
voltage phase sequence through the rotor side converter
(RSC)
and line side converter (LSC). A coordinated control among
RSC, LSC and DC link storage system ensure variable speed
and
maximum power extraction from the fluctuating wind and
reduce
the possibility of instability around synchronous speed.
Extensive
simulations have been conducted to investigate control
dynamics
under the two modes of operation and during transitions.
Index Terms-- DFIG, Sub-and super-synchronous, rotor side
converter, Line side converter.
I. INTRODUCTION
HE contribution of renewable based distributedgeneration has
been increasing dramatically into the
power system for last two decades [1]. A variable speed
generator based wind turbine can extract more power from the
wind than a fixed speed wind turbine [2]. Doubly fed
induction generator (DFIG) is a popular choice for variable
speed wind turbine application, as it is able to generate
power
at constant voltage and frequency while the rotor speed
varies.
A decoupled control of the real and reactive power is
possible
[3, 4]. Moreover, a fraction of total system power needs to
be
controlled, resulting in the reduction of the power losses
and
the cost of the converters, filters and EMI filters [2, 5].
Generally, the stator of the DFIG is connected to the
griddirectly, and the rotor is fed through bi-directional
back-to-
This research has been financially supported by the Australian
Research
Council under ARC Linkage Grant K0014223 Integration of
Distributed and
Renewable Power Generation into Electricity Grid Systems,
collaboration
with Aurora Energy, Tasmania.
Md. Aktarujjaman, Dr. Md Enamul Haque, Dr. Kashem Mohammad
Muttaqi, and Prof. Michael Negnevitsky are with the School of
Engineering,
University of Tasmania, Hobart, Australia; and Prof. Gerard
Ledwich is with the
School of Engineering Systems, Queensland University of
Technology, Brisbane,
Australia, (emails: [email protected]; [email protected];
[email protected]; [email protected]).
back PWM converters [6]. These converters are used for
exchanging the slip power to and from the grid for variable
speed operation [7]. It is possible to control rotor current
injection using fully controlled electronic converters to
ensure
effective operation in both sub- and super-synchronous modes
[8, 9]. In sub-synchronous mode, the RSC works as an
inverter and LSC as a rectifier and controls the power flow
into the rotor. In the case of super-synchronous mode, RSC
acts as a rectifier and LSC as an inverter, the direction of
power flow is out of the rotor. In reference [1-3], maximum
wind power extraction capability by using DFIG has been
demonstrated. Active and reactive power control technique
using vector controls are presented in [4-5]. Authors in [7]
have described the ability of DFIG to compensate unbalance
situation. The effect of commutation angle of the converters
during sub- and super-synchronous operations is reported in
[9-10].
For effective control, it is necessary to understand the
control dynamics of DFIG based variable speed wind turbine
and converters action under sub- and super-synchronous
modes of operation and during the transition of these two
modes. This paper investigates the control dynamics of a
DFIG based variable speed wind turbine during two modes of
operation and subsequent transition period with a
batterystorage.
This paper is organized as follows: In section II, system
configuration is presented. The model of the
turbine-generator
system is given in section III. In section IV, Operation and
control issues in sub- and super-synchronous modes are
discussed. Simulation and discussion are given in section V.
Finally, conclusions are made in section VI.
II. SYSTEM CONFIGURATION
A typical DFIG based wind turbine system shown in Fig. 1
consists of a wound rotor induction machine with slip rings,
and a bidirectional back-to-back PWM converter between therotor
slip-rings and the grid. The stator is directly connected
to the grid. Both the stator and rotor windings are able to
supply power to the grid. The direction of the power flow in
the rotor circuit depends on the variation of the wind
speed.
The bidirectional converter controls both the direction and
magnitude of the power flow of the machine. In sub-
synchronous mode, the converter feeds the rotor windings
from the grid, whereas the rotor supplies power to the grid
in
Control Dynamics of a Doubly Fed Induction Generator
Under Sub- and Super-Synchronous Modes of Operation
M. Aktarujjaman, StudentMember, IEEE, M.E. Haque,Member, IEEE,
K. M. Muttaqi, Senior
Member, IEEE, M. Negnevitsky,Member, IEEEand G. Ledwich,
SeniorMember, IEEE
T
2008 IEEE.
