Transient Characteristics Analysis And Fault Ride-through Schemes For Doubly-fed Induction Generators

Wind power is an important type of renewable energy, because of its prominentrole in reducing of environmental pollution and easing future energy tense, more andmore attention of the world has been attracted. In recent years, with the increasingproportion of wind power installed capacity in China’s power system, wind power hasrandomness characteristics along with some other issues, brought a great impact to thesafe and stable operation of the power system, especially the low voltage ride throughproblems. Therefore, how to improve the low voltage ride-through capability for windturbines to ensure that the turbines will not be off-grid even if the voltage drops,which problem is worthy of further study.The doubly-fed wind turbine was deeply studied in this paper, including theirmathematical models, control schemes and electromagnetic transient analysis methods,etc. The mathematical model is the basis of theoretical analysis, and doubly-fedinduction generator is the core element for the entire wind turbine. Account for theabove, a mathematical model for a doubly-fed induction generator in the synchronousrotating reference frame was built, and an electromagnetic transient simulationprogram for a doubly-fed induction generator was written in MATLAB at the sametime. In addition, to study the dynamic characteristics for an entire doubly-fed windturbine, a grid-connected simulation model was also built in DIgSILENT. Besides, toanalysis the electromagnetic transient characteristics for a doubly-fed inductiongenerator during voltage sags, a theoretical analysis method based on thesuperposition principle was proposed in this paper, which regarded the transientprocess during the fault as a superposition of different states, considering the statorside voltage fault component, the rotor side voltage fault component and the transientshocks caused by crowbar invested, which has a higher accuracy compared totraditional methods. Moreover, to set the crowbar resistance legitimately, therelationship-function between the rotor voltage peak and the crowbar resistance wasderived, so an appropriate crowbar resistance can be determined using the mappingmethod. Finally, in order to enhance the ability of low voltage ride through for adoubly-fed wind turbine, a control strategy was proposed, in which the active and the reactive power reference values will be adjustment adaptively according to the depthof voltage sags. The simulation results showed that both of the new crowbarresistance determined method and the low voltage ride-through scheme are reasonable,for it can help to ride through voltage sags, and generate a certain reactive power,which is helpful for voltage restore.