Enhanced Control of DFIG Based Wind Energy Conversion System Under Unbalanced Grid Voltages Using Mixed Generalized Integrator

 Abstract

The operation of a wind energy conversion system based on a doubly fed induction generator (DFIG) is found to be extremely sensitive to unbalanced grid voltages. In this context, this article presents an enhanced control strategy for a DFIG operating under unbalanced grid voltages, using a mixed generalized integrator (MGI). The MGI is a hybrid form of the second and third-order generalized integrators and is employed for positive and negative sequence calculation. The MGI-based enhanced control strategy utilizes the hysteresis band control of rotor currents and grid currents, to generate switching sequences for rotor side converter and grid side converter, respectively. In addition to superior dynamic tracking of reference currents, this scheme eliminates the need for implementation and careful tuning of eight distinct proportional-integral inner current regulators, as is the case with conventional control strategy for the DFIG operating at unbalanced grid voltages. Furthermore, MGI improves the grid synchronization in DFIG system amidst the presence of dc offset and unbalance in the supply voltages. The overall coordinated control strategy is designed to realize seven separate control objectives, and its effectiveness is verified through simulation and experimental results.

Index Terms

Doubly-fed induction generator (DFIG), power quality, unbalanced voltages, wind energy conversion system (WECS).

Block Diagram:

Fig. 1. System configuration of DFIG-based WECS.

Expected Simulation Results:

                Fig. 2. Simulation results for DFIG system with unbalance in one phase of grid voltages.

Fig. 3. Simulation results for the DFIG system with unbalance in two phases of grid voltages.

CONCLUSION

An MGI-based coordinated control strategy for aDFIG-based WECS operating at unbalanced grid voltages has been presented in this article. Initially, the DFIG system has been modeled with unbalanced grid voltages. Then, negative sequence reference values for rotor currents and grid currents, for realizing a set of seven pre-defined control objectives, have been identified. The MGI has been utilized to estimate the reference current values through effective PNSC of voltages and currents. Moreover, enhanced grid synchronization capability, stability analyses and parametric tuning of MGI have also been examined. Hysteresis band control of rotor and grid currents has been utilized in MGI-based coordinated control strategy, to reduce the control complexity by eliminatingmultiple PI controllers and employing fewer rotational transformations. The effectiveness of the presented scheme has been validated through simulation, experimental, and comparative results. Moreover, extensive assessment of the performance improvements in MGI-based scheme at unbalanced grid operation reveals target-II (balanced stator currents) and target-V (balanced grid currents), as optimal targets for RSC control and GSC control, respectively.

REFERENCES

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