Research On The Interactions Between DFIGs Based Wind Farms And Power Grid And Related Optimal Control Strategies

The proportion of installed wind power capacity in power grid continues to increase, which makes the closely coupled interactions between wind generators and power grid becomes more severe, then the operation characteristics of power grid during fault may change. In order to study the operation characteristics and control problem of power grid formed by the interconnection of asynchronous power source based on power electronics equipment, taking doubly fed induction generators(DFIGs) as a research subject. The operation characteristics of DFIGs-integrated system are revealed based on theoretical analysis and time domain simulation, and some wind power optimized control strategies for improving the performance of DFIGs-integrated system are proposed.The paper summarizes the domestic and foreign status on the fault ride through (FRT) control technology of DFIGs, the oscillation characteristics of DFIGs-integrated system in a region, and the stability of DFIGs-integrated system. The detailed DFIGs and overall control system and typical ancillary control system model are given through researching the operation and control principle of DFIGs.The expressions of the terminal voltage of DFIGs when a grid fault occurs are given in this paper by analytical method, on the basis of these, and the detailed research and analysis on terminal voltage phase angle jump characteristics and the sensitivity of influence factors to which are given. The phase angle jump will influence the lock accuracy of PLL, break the decoupling of power control, weaken the control effect of traditional fault ride-through control, reduce the fault ride-through capability of DFIGs. According to the analysis, a novel adaptive FRT control strategy is proposed, the proposed FRT control strategy keeps the phase angle constant to reduce the phase angle jump value during the grid fault, and fast track the terminal voltage phase angle change to increase the control accuracy of DFIGs after clearing the grid fault, then the FRT capability of DFIGs is improved, the transient characteristics of DFIGs are optimized.For the oscillation problems of DFIGs-integrated system in a region, the voltage dynamic characteristic at PCC of DFIGs in case of weak grid connection is studied based on sensitivity analysis method, further the interactions between DFIGs and power grid and its influence factors are revealed. The dynamic response characteristics of DFIGs or SVG to a grid disturbance are derived respectively, and then the interactions between DFIGs and SVG and its influence factors are revealed. The weak grid connection, unreasonable parameter set of DFIGs or SVG are the reason of oscillation phenomenon of DFIGs-integrated system, at last the simulation results of an actual DFIGs-integrated system prove the analysis conclusions.The impacts of DFIGs on inertia characteristic and primary frequency control characteristic of power grid are discussed. On the basis of studying operation and control characteristic of DFIGs, the definition of effective energy storage and a calculation method of equivalent virtual inertia time constants are presented, then a variable parameter virtual inertia control strategy based on the effective energy storage of DFIGs is proposed, which coordinates the frequency deviation and operating state of DFIGs to control wind turbine output power. According to the advantages of speed control and pitch control respectively, a primary frequency controller of DFIGs based on participation factor is proposed, which can provide controllable frequency response characteristics of DFIGs and dynamic active power support for power grid, at the same time the stability of DFIGs is also maintained in the process of frequency regulation. At last, the change of expressions of synchronous generator output power is given when the wind turbine is integrated to the power grid, and the impact of fault behaviors DFIGs on transient stability of power grid is analyzed.