During the past two decades, the Doubly Fed Induction Generator (DFIG) based wind farm has been under rapid growth, and the increasing wind power penetration has been seen. Practically, these wind farms are connected to the three-phase AC grid through long transmission cable which can be modelled as several Π units. The impedance of this cable cannot be neglected and requires careful investigation due to its long distance. As a result, the impedance interaction between the DFIG based wind farm and the long cable is inevitable, and may produce High Frequency Resonance (HFR) in the wind farm. This paper discusses the HFR of the large scale DFIG based wind farm connected to the long cable. Several influencing factors, including 1) the length of the cable, 2) the output active power and 3) the rotor speed, are investigated. The transformer leakage inductances in the transmission system are taken into consideration when investigating the HFR. Simulation validations using MATLAB / Simulink have been conducted to verify the theoretical analysis.Index Terms -DFIG system; wind farm; Π unit model based long transmission cable; high frequency resonance; transformer leakage inductance.
Compared with the conventional power systems, large-scale power electronics based power systems present a more complex situation, where harmonic instability may be induced by the mutual interactions between the inner control loops of the converters. This paper presents an approach to locate which power converters and buses are more sensitive and have a significant contribution to the harmonic instability. In the approach, a power electronics based system is introduced as a multi-input multi-output (MIMO) dynamic system by means of a dynamic admittance matrix. Bus participation factors (PFs) are calculated by the oscillatory mode sensitivity analysis versus the elements of the MIMO transfer function matrix. The PF analysis detects which power electronic converters or buses have a higher participation in harmonic instability excitation than others or at which buses such instability problems have a higher impact. In order to confirm the effectiveness of the presented approach, time-domain simulation results are provided for a 400-MW wind farm in PSCAD software environment.
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