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Abstract
Open-loop modal coupling is the closeness of an open-loop oscillation mode of a grid-connected DFIG to an open-loop electromechanical oscillation mode of a power system on the complex plane. When the open-loop modal coupling occurs, the dynamic interactions between the DFIG and the power system become strong and may degrade the damping of the low-frequency electromechanical oscillations (LEPOs) of the power system. In this article, analysis is carried out to explain why the open-loop modal coupling may cause the damping degradation of the LEPOs. By deriving the simplified open-loop transfer function matrices of the DFIG, existence of open-loop DFIG oscillation modes to cause the open-loop modal coupling is analytically examined. Conclusions of analytical examination are: (1) If the rotor speed is used as the input signal to the active power control outer loop of the rotor side converter (RSC) or the generic type 3 model is adopted by the reactive power control outer loop of the RSC, the DFIG may have an open-loop oscillation mode to cause the open-loop modal coupling; (2) The PLL may also contribute to an open-loop oscillation mode of the DFIG. Study cases are presented to demonstrate and evaluate the analysis and conclusions.
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Wenjuan Du
Wenjuan Du (M’14) received the Ph.D. degree from the University of Bath, U.K., in 2009. She is now a full Professor with the School of Electrical and Electronic Engineering, North China Electric Power University, Beijing, China. Her main research interests are power system stability analysis and control, including energy storage systems, FACTS, EV, and renewable power generations.
Xiao Chen
Xiao Chen is a Ph.D. student at the School of Electrical and Electronic Engineering, North China Electric Power University, Beijing, China. His main research interests are power system stability analysis and control considering wind power integration.
Haifeng Wang
Haifeng Wang (M'96–SM'02) is a full Professor with the School of Electrical and Electronic Engineering, North China Electric Power University, Beijing, China. He had worked in the U.K. for years and was the Chair in Electrical Engineering and full Professor at the Queen's University of Belfast, U.K., before he moved to Beijing. His main research interests are power system stability analysis and control.