Enhanced Direct Power Control of DFIGs driven by Wind Turbine under
Unbalanced Grid Voltage
Abstract
Undesired oscillation components appear in active and reactive powers,
electromagnetic torque and DC-link voltage of doubly fed induction
generators (DFIGs) connected to unbalanced grid voltage. These
components oscillate at double source frequency as a result of negative
sequence components in voltage and current. Different direct power
control (DPC) techniques were studied in literatures to damp these
oscillations. However, these techniques require sequence decomposition
process, axes transformation of stator voltage/current and estimation of
different power components which complicate the overall control system.
This paper presents a simplified DPC of DFIGs in stationary reference
frame under normal and unbalanced grid voltage. Decomposition process,
axes transformation and compensation power terms are totally eliminated.
Vector proportional- integral (VPI) controllers are designed to regulate
stator active and reactive powers. The performance of the proposed DPC
scheme using VPI and proportional-integral-resonant (PIR) controllers is
analyzed and compared under different operating conditions. Bode diagram
of open loop and closed loop control using VPI and PIR are studied to
illustrate stability, steady state and transient response of the two
controllers. Also, the performance of proposed technique and previous
DPCs designed in synchronous reference frame is compared to prove the
validity of proposed one. The results show that proposed DPC using VPI
has superior performance in steady state and transient conditions with
simple implementation.