This paper analyzes the dynamic response of wind power generators in case of voltage dips. When a fault occurs in the network, a voltage dip is applied to the wind power generator common coupling point. Wind power generators are equipped with minimum voltage protections that can trip the unit in case of a long voltage dip to protect both the machine and the power system. If wind power generators are connected to the transmission network, a fault in it may result in voltage dip applied to many wind power generators. A large amount of wind power generation can be tripped if the dip is long enough, leading to system collapse. The capability of wind power generators to withstand voltage dips is a key feature of them since it is currently determining the maximum admissible wind power generation in systems with high wind penetration. The response of wind power generators to voltage dips depends on the technology of the electromechanical energy conversion device. Three technologies are mainly used and will be analyzed and compared in this paper: squirrel cage induction generators, doubly fed induction generators and multi-pole synchronous generators. The fundamental response of the generators will be obtained as the basis for the design of protection schemes. Despite their limitations, wind power generators based on squirrel cage induction machines are widely used. A voltage dip results in a reduction of the electromagnetic torque and in a subsequent rotor acceleration. Depending on the voltage dip duration the machine may either come back to the original operating point or lose stability. During the fault and the deceleration process the machines consumes reactive current that it is limited by the transient reactance and the equivalent external impedance. Doubly fed induction generators are the most widespread electromechanical energy conversion devices in wind power generation due their speed regulating capability that allows them to operate in the optimal operating point of the wind turbine. In addition, the network side power electronic converter provides them continuous reactive power compensation capability. The response of doubly fed induction generators in case of voltage dips is limited by the overvoltages in the dc link and the overcurrents in the rotor converter. In addition, the doubly fed induction machine only consumes reactive power during the voltage dip. Dc link overvoltages and rotor converter currents can be reduced with a crowbar system connected to the machine rotor. Wind power generators based on multi-pole synchronous generators have the advantage that do not require a gear box. Multi-pole synchronous generators equipped with voltage source power electronic converters have speed regulating capability to be able to operate in the optimal operating point of the wind turbine. As in doubly fed induction generator, the network side power electronic converter provides them continuous reactive power compensation capability. The response of multipolesynchronous generators in case of voltage dips is limited by the overvoltages in the dc link. In addition, multipole synchronous generators do not draw reactive power during the voltage dip. Dc link overvoltages can be reduced connecting a crowbar system in the dc link.
Keywords: Wind power generators, dynamic response, voltage dip, squirrel cage induction generators, doubly fed induction generators, synchronous generators.
IEEE Power Engineering Society General Meeting. Montreal, Canada, Paper 06GM1126. 18-22 June 2006
Published: June 2006.