Fault Analysis and Protection of Doubly Fed Induction Generator-Based Wind Farms

Abstract

The purpose of this thesis is to study the fault current behavior of a wind farm with Doubly Fed Induction Generators (DFIG), and identify the electrical protection requirements for a large wind farm. Brief description of wind energy conversion systems including types of wind turbine generators are given in the beginning of the thesis followed by a review of induction machine modeling and the special modeling considerations for a DFIG during a short circuit condition. Fault calculations using symmetrical components and factors which determine the fault current contribution are also discussed. A nine MW wind farm with six units of 1.5 MW DFIGs are modeled using Matlab/Simulink/SimPowerSystems and simulated for symmetrical and asymmetrical faults within the wind farm and in the power system close to the wind farm. Voltage and current profiles are observed for pre-fault, fault and post- fault periods and the results are presented and discussed. The simulated voltage and current waveforms during faults nearly conform with the ideal conditions expected. This gives a clear view of each phase current and phase voltage behavior during different fault conditions. Knowledge of fault levels in a wind farm is important in determining the ratings of current transformers, voltage transformers and circuit breakers. Further, protection functions are graded and settings are selected based on the fault current contribution from the wind turbine generators and the power system.