Key Technology Of Doubly-fed Induction Generation System For Power Quality Compensation

With the rapidly increasing penetration of wind power generation,the traditional wind power operation mode that simply hehaves as a power source gradually becomes difficult to meet the demand of the power grid.In order to build a more friendly wind power generation system for the grid and promote the wind power consumption,it is necessary to explore the potential of the wind power generation system and realize the power quality support to the power grid.On the one hand,because wind farm interconnection points are often at the end of the power grid,which usually presents as weak power grids,and the grid voltage is extremely vulnerable to power imbalances and harmonics due to non-ideal local loads,there is a current demand for the power quality improvement;On the other hand,due to the inherent uncertainty of wind energy and the serious wind power abandon problems,wind power generators are usually not full-loaded,so the installed capacity tends to be more redundant and the grid power quality control is achievable..Based on the above two reasons,the use of redundant capacity of wind power generation system to participate in grid power quality control has prominent practical significance and considerable research prospects,and is receiving more and more attention from researchers at home and abroad.The doubly fed induction generator(DFIG)based wind power generation system is the mainstream wind turbine because of its advantages of small converter capacity and low cost.This paper is focused on the theme of wind power generation system participating in grid power quality improvement.Through the theoretical analysis,control scheme design and experimental research,the following innovations have been obtained:1.Proposed a DFIG control strategy to eliminate voltage unbalance of the grid.The traditional DFIG control strategies for voltage unbalance compensation are all designed according to the condition of purely inductive power grid.The compensation loop is based on the double-layer PI structure of voltage and current in both positive and negative sequence reference frames,which makes the traditional control strategy lack adaptability to the grid impedance,and leads to poor compensation performance and even instability in the resistive power grid.Meanwhile,the control structure is complex and depends on multiple frame transformations and extraction of positive and negative sequence components,thereby increasing the computational burden and introducing phase deviation.In this paper,we first consider the influence of grid impedance and propose a decoupled voltage unbalance compensation strategy that adapts to the actual grid impedance characteristics.This strategy can effectively compensate the grid voltage unbalance under various grid impedances.Then,aiming at the problem of complex control structure,a voltage unbalance compensation strategy based on direct resonant regulator is proposed.The resonant controller can be used to form a single-layer compensation loop in the positive synchronous rotating reference frame to compensate unbalanced grid voltage and avoid multiple reference transformation and positive and negative sequence component extraction,as well as reducing the difficulty of the parameter tuning.2.Proposed a flexible grid voltage compensation strategy based on a novel hybrid virtual impedance method.Under unbalanced or harmonic grid conditions,the traditional DFIG control strategies can be divided into two categories.The first category focuses on improving the operation performance of the unit itself,such as eliminating the unbalance and harmonics of the output current,but it can not provide any power quality improvement for the power grid.The second kind pays attention to the power quality improvement of the interconnection point,but it only compensates the power quality in a non-adjustable manner,which may risk the converters,drive trains and DC bus capacitors.In order to balance the operation performance of the generator and the grid power quality,a flexible compensation strategy is proposed in this paper to achieve a continuous and flexible trade-off between the voltage quality at the point of common coupling and the output current quality of the DFIG.This strategy essentially requires flexible reshaping of the negative sequence and harmonic output impedances of a DFIG in a large range,which is unattainable with conventional virtual impedance methods because of the inevitable amplification of feedback and feedforward gains during large-range impedance reshaping,and then introduce instability and over-modulation problems.In this paper,a hybrid virtual impedance method is proposed to avoid the instability and over-modulation problems while adjusting the output impedance from zero to infinity.3.Proposed an autonomously coordinative strategy for multi-DFIG units to participate in the grid voltage unbalance compensation.Because of the decoupling of the DC bus capacitance,the DFIG generator and the grid-side converter can be considered as two separate sources for negative sequence components.In this paper,multiple DFIG generators and their grid-side converters are controlled to coordinate and autonomously participate in grid voltage unbalance compensation.The following two objectives can be achieved at the same time:firstly,the voltage unbalance degree at the interconnecting point is flexibly controlled with a closed loop,which can adjust the overall compensation degree of the whole wind farm;secondly,the negative sequence compensation current is properly distributed among all the DFIG generators and grid-side converters in accordance with their remaining capacity,which can best protect all the generators.The control strategy is completely implemented in the local controllers of each unit without the need of real-time communication,avoiding the cost problem of high-speed communication system construction and the stability problem caused by communication delay.