Investigation On The Modified Control Strategy Of Doubly Fed Induction Generation System Under Universal Harmonic Grid Voltage

With the power capacity provided by wind turbine driven generator in grid becoming more and more as well as the grid voltage tending to more distorted due to large use of power electronics, high-performance wind energy system has received much attention. With the rules for power quality getting more strictly, this also must be an essential demand. Due to its less research under universal grid voltage circumstances has reported unlike under voltage dip or swell. This paper attempts to impove existed strategy or introduce new strategy to improve operating performance of doubly-fed induction generator (DFIG) system under this situation.Aiming at the operation of grid-connected voltage-sourced converters (VSC) under the universal and dynamically varied harmonic voltage conditions existing in reality power grid, this paper proposes a sort of improved sliding-mode-based direct power control strategy under universal harmonic grid voltage. Due to tasks to all harmonics, its implementation has no need of the accurate detection of the concrete order and phase position of harmonics, thus with huge engineering application potentials. Based on the past researches, the integral mathematic model of VSC under universal harmonic circumstances is founded, three different control objectives are devised and the final improved sliding-mode-based direct power control (SMC-DPC) strategy under universal harmonic grid voltage are performed. At last, simulation results of the proposed strategy have verified its effectiveness.To improve the performance of DFIG operating under real power grid where there always exists universal harmonic voltage varying dynamically and instantaneously, this paper proposes a sort of improved sliding-mode-based direct power control (SMC-DPC) strategy to DFIG under universal harmonic grid voltage. As a strategy aiming at all sorts of harmonics, its implementation requires no accurate detection of the concrete order, number and phase position of harmonic voltage in grid. Meanwhile it inherits the edge of traditional SMC-DPC such as strong robustness against systematic parameter uncertainties as well. All these lead to its engineering application potentials. Firstly, the integral mathematic model of DFIG under universal harmonic grid voltage is founded. Then after analyzing the power relationship of DFIG under universal harmonic in detail, the algorithm of compensation power control objectives for three different tasks has be devised. Thus the final improved SMC-DPC strategy under universal harmonic grid voltage could be achieved based on the above analysis. Finally, simulation results have verified its effectiveness of the proposed strategy.This paper proposes an improved coordinated SMC-DPC to wind turbine driven DFIG system for the suppression of outputting harmonic current of its stator and voltage pulsation of its dc-link capacitor under universal harmonic grid voltage circumstance. The improved SMC-DPC strategy is designed based on the detailed analysis of the power relationship of system and corresponding compensation power algorithm under universal circumstance. Finally, comparative simulation results between the traditional and improved SMC-DPC under arbitrary individual order and diverse orders harmonic verify the effectiveness of proposed strategy.This paper presents a sort of nonlinear backstepping-based algorithm combining with direct power control (DPC) to wind turbine driven doubly fed induction generator (DFIG) under normal and especially harmonic grid voltage. Firstly, the power control objectives are analyzed and designed under normal and harmonic grid voltage for the purpose of harmonic stator current suppression; secondly, a unified mathematic model of DFIG under normal and harmonic grid voltage is founded, exploring its power model in detail; finally, the backstepping algorithm is introduced briefly and the backstepping-based direct power control (BS-DPC) algorithm of DFIG is developed under normal and harmonic grid voltage. The comparative simulation results between vector control (VC) with resonant controller, look-up table DPC (LUT-DPC) and BS-DPC under normal grid voltage verify that the proposed BS-DPC realizes the decoupling control of active and reactive power of DFIG, with better dynamic performance than VC, as well as with better steady performance than LUT-DPC. Under harmonic grid voltage, further simulation results verify the effectiveness of BS-DPC for suppressing the harmonic stator current of DFIG in contrast to VC with resonant controller and LUT-DPC with improved control objectives proposed by this paper; meanwhile the adaptability of BS-DPC to minute frequency derivation, harmonic component order and distorted degree of grid voltage under harmonic conditions are verified as well.