Analysis On Harmonic Characteristics Of Fault Current Of Doubly-fed Wind Turbine And Its Effect On Relay Protection

Doubly-fed wind turbine, based on the double fed induction generator (DFIG), isthe main equipment for wind power utilization. The capacity of this type of windgenerator accounts for more than50%of capacity of grid connected wind power. Statorwinding of DFIG is directly connected to the grid and rotor winding is connected to gridby the AC-DC-AC excitation converter, which is virtually different to the conventionalsynchronous generator. As the wind energy gradually develops from added energy to themainstream energy, its influence to power system has becomes more and more serious.Especially, during low voltage ride through (LVRT), due to the mutual electromagneticcoupling between DFIG and power grid, the transient characteristics of DFIG becomesmuch complicatedAnalysis on harmonic characteristic is an important part of transient ananlysis ofDFIG. By researching on the constituent and content of harmonic current, it is useful todeepen the understanding of fault transient characteristics and get a full grasp oncharactersitics of short circuit current. The research can offer the theoretical guidance toanalyze the effect on relay protection caused by short circuit current of DFIG.Considering the development and features of wind power, the research oncharacteristics of short circuit current and effect on relay protection by short circuitcurrent of DFIG are comprehensively reviewed in this paper. After collecting andarranging the related articles, the features and problems of existing research is analyzed,which points out that the research on harmonic characteristic of short circuit current hasnot attracted enough attention.Based on the structure characteristics of DFIG, the instantaneous model and vectorspace model is set up in three-phase static coordinate and synchronous rotatingcoordinate respectively by using coordinate conversion, and the theoretical basis of eachmodel is researched; considering the electromagnetic coupling between stator windingsand rotor windings, the equivalent circuit of harmonic current is built up; then, thecontrol model of rotor side converter and grid side converter, on the basis of vectorcontrol, is researched in detail, which explicitly points out the regulating mechanism ofconverter control to the output of DFIG.In order to fully analyze and calculate the harmonic component of rotor faultcurrent of DFIG during the grid fault, this paper considers the excitation control of rotor side converter that may influence on the harmonic characteristics of rotor fault current.By obtaining the expression of rotor command voltage and rotor command current incontrol loop of rotor side converter, the rotor harmonic current is derived, and thegenerating mechanism of some particular rotor harmonic current is analyzed. Thetheoretical analyses are verified by several examples of short-circuit test and simulationresults.To analyze the effect of harmonic component of stator short circuit current ofDFIG on power system relay protection, this paper studies the harmonics characteristicsof stator short circuit current of DFIGduring gird fault. Based on the comprehensivevector model of DFIG, the harmonic current with frequency(1s)fsof stator shortcircuit current is analyzed; Based on the impacts of rotor-side converter on the harmoniccomponents of stator short circuit currents, the generating mechanisms and analyticalexpression of stator second harmonic currentare proposed. It is pointed out that with thevector control of converter, the stator short circuit current of DFIG may cause themaloperation of the second harmonic escapement of transformer protection. Thetheoretical analyses are verified by several examples of short-circuit test and simulationresults.