Methodology for grid voltage unbalance compensation appling a two-converter series DFIG topology

Abstract

This paper presents a new control strategy for the DFIG replacing the conventional Grid Side Converter for a series one, namely series grid side converter (SGSC). The control strategy for operation under grid voltage unbalance intends not only the ideal machine operation, but also to attenuate the grid voltage unbalance. This compensation is performed injecting unbalanced currents in the point of common coupling (PCC). The SGSC promotes a higher operational capacity during grid disturbances as voltage unbalances. The proposed methodology uses the machine electromagnetic power equation decomposed by the symmetrical components to obtain a relationship between the rotor negative sequence current and the stator negative sequence voltage, where the oscillating components of the electromagnetic power are zero. Appling this relationship is possible to reduce the electromagnetic torque oscillations when injecting negative sequence current by the machine stator.

Analytical assessment of subsynchronous resonance ( SSR ) impact on doubly fed induction generator wind turbine behavior and efficient suppression of SSR oscillations

“…However, the potential risk of subsynchronous resonance (SSR) is an important issue raised in the use of series compensation 13‐17 . DFIG‐based wind‐farms, due to the direct connection of the stator winding to the network, are comparatively more sensitive to the power network disturbances such as voltage unbalance, 18 low voltage fault, 19 distortion, 20 and resonance. The SSR in a grid‐connected wind power system is a condition where the wind‐farm exchanges energy with the network at one or more natural frequencies related to the electrical and drive train parts of the combined system, where the frequency of the exchanged energy is below the grid fundamental frequency.…”

Wind Energy Conversion System Based on DFIG with Three-Phase Series Grid Side Converter and Single DC-Link

“…Despite the grid voltage imbalances compensation, the control strategy presented in [40] does not limit the fault currents and does not completely eliminate the oscillations in the active and reactive stator powers and in the electromagnetic torque. In [41] is proposed a control method that reduces the grid voltage unbalance and the oscillations of the electromagnetic torque by injecting negative sequence components of the stator voltage and rotor current, respectively. Although it achieves the proposed objectives, the method presented in this paper may not protect the electrical components of the DFIG by injecting unbalanced currents into the stator and rotor windings.…”

“…The control strategies presented in [22], [39]- [41] do not act on voltage sag compensation, not using the total capacity of the series converter. Differently, the control strategy employed in [42] acts on the grid voltage sag compensation during grid faults.…”