Improved Direct Power Control of a Wind Turbine Driven Doubly Fed Induction Generator During Transient Grid Voltage Unbalance

Abstract

This paper proposes an improved direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind power generation system under unbalanced grid voltage dips. The fundamental and double grid frequency power pulsations, which are produced by the transient unbalanced grid faults, are mathematically analyzed and accurately regulated. Five selectable control targets, with proper power references given, are designed for different applications during network unbalance. In order to provide enhanced control performance, two resonant controllers, which are tuned to have large gain at the power pulsation frequencies, are applied together with the proportional-integral controller to achieve full control of the DFIG output power. The effectiveness of the proposed DPC strategy is verified by the experimental results of a 5-kW DFIG system under different unbalanced voltage dips, which are generated by a specially designed voltage dip generator.Index Terms-Direct power control (DPC), doubly fed induction generator (DFIG), grid voltage unbalance, resonant controller, wind turbine.

Control& Implementation of Fuzzy Based DFIG Wind Power Generators in Unbalanced Microgrids

“…However, in these methods, the calculation of current references based on the power pulsations also requires the positive and negative sequence components of the machine stator voltage, current, and flux. Direct power control (DPC) methods have been also suggested for unbalanced voltage condition which relatively reduce the complexity of the control method compared to the vector control scheme [11], [18]- [20]. However, the DPC methods similar to the unbalanced vector control methods still need decomposition of positive/negative sequences and compensation for the filters delays.…”

“…Practically, this decomposition can be realized by transferring the current to the synchronous reference frame and using digital filters, or signal delay cancelation technique. These methods introduce time delays and obvious errors in amplitude and phase which adversely affect on the dynamic performance of the control system [20].…”

“…In these methods, the current references are calculated according to the power pulsations in a feed-forward manner so the stator voltage, current, and flux have to be decomposed into the positive and negative sequences for calculating the rotor current references [20].…”

Fault Current Characteristics of the DFIG under Asymmetrical Fault Conditions

“…Among them, the simulation model of the DFIG's control system is derived from reference [17] and the control target for balanced stator current of the DFIG is derived from reference [26].…”

“…Under unbalanced fault conditions, the conventional phase locked loop (PLL) techniques cannot present accurate phase angle estimation because the second harmonics produced by the negative-sequence component of the grid voltage will propagate through the PLL system and will be reflected in the extracted phase angle [16]. In the engineering practice, notch filters are usually used to extract the positive-and negative-sequence components [17,18]. This means that the transient behaviors of the DFIGs are changed by the notch filters and the research results presented in [14,15], do not agree with reality very well.…”