| Utility codes for grid-connected Wind Energy Conversion Systems (WECS) have become a standard feature since they are becoming a significant portion of the power grid. However, Doubly-Fed Induction Generators (DFIGs) which make up the bulk of all wind power installations are known to be susceptible to grid disturbances such as voltage sags. In the literature, one of the most robust low voltage ride-through (LVRT) solution for both balanced and unbalanced sags is based on the principle of series voltage compensation used extensively in dynamic voltage restorers (DVRs) and unified power quality conditioner (UPQC) systems. However, the voltage compensation LVRT is expensive as an additional converter is required. This converter however is underutilized idling during normal grid conditions but increases the complexity, cost, size and losses of the entire system. The component saving topological feature of the Nine-switch converter fits well into this type of application as it reduces cost, size and losses compared to using two classical grid side converters. A detailed mathematical model of the Nine-switch converter is needed to fully utilize the performance advantages of this converter which is virtually nonexistent. The mathematical model of this converter is derived and its space vector and carrier-based pulse-width modulation methodologies are developed. In addition, as an extension of this converter topology, the model of the generalized 3n+3-switch converter is also presented. It is shown that injecting the neutral voltages of the converter into the modulation signals has a dual role of shaping the modulation scheme as well as ensuring that the switching function constraints of the converter are not violated. The Nine-switch converter so developed is used for the proposed LVRT for a grid-connected DFIG wind turbine to serve the dual role of parallel grid side and series grid side converters. The principle of ride-through, sag detection, and controllers are described. The controllers are designed based on the stationary reference frame using hybrid proportional and resonance controllers. Operation is demonstrated through simulation and verified by experimentation using a laboratory scale 5-hp wound rotor induction machine and a 5-hp DC machine as the wind turbine emulator. |
