The investigation in the literature found that the injection of appropriate reactive currents through suitable control can contribute to the small-signal stabilization of the system. In, a study was conducted on the low-frequency oscillation phenomenon between the grid-side converter and the transmission line. The injected reactive current contributes to the grid’s stability during grid faults. The converter’s active current output should be reduced if the converter’s limited current-carrying capacity is reached. For example, in, when the voltage was lower than 0.9 p.u., the converter was required to inject 2% of the nominal current as reactive current into the grid for each 1% of voltage reduction. The amount of reactive current injection usually is related to the residual voltage at the point of common coupling (PCC) during the voltage dip. The requirements for reactive current injection by converters during low-voltage-ride-through (LVRT) are defined in various grid connection codes. The converter’s injected reactive current helps to maintain the grid voltage during faults, and enhances the grid’s voltage stability performance. The results and the optimized strategies were validated via controller hardware-in-the-loop tests. Furthermore, an easily implementable strategy for reactive current injection, leading to minimum power oscillations, was presented. On this basis, this study also focused on the reactive current reference’s influence during and after fault clearing.
The investigation considered the influence of the converter’s phase-locked loop (PLL), responsible for phase tracking, as well as that of the DC link on the converter-grid system, which has a major influence on the active power exchange with the grid. Using large-signal modelling, this study investigated the converter’s dynamic processes during and after such fault situations. This leads to oscillations of both the active and reactive power fed into the grid. During severe grid faults, large disturbances cause the converter’s operating point to move away from the stable equilibrium point during normal operation.
The precise control of output power by grid-connected converters relies on the correct identification and tracking of a grid voltage’s phase at the converter terminal.
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