Abstract
Rapidly increasing demand for renewable energy has created a need for the photovoltaic and wind farms to be placed in various locations that have diverse and possibly time-variant grid conditions. A mismatch between the grid impedance and output admittance of an inverter causes impedance-based stability issues, which appear as power quality problems and poor transient performance. Grid synchronization with phase-locked loop (PLL) introduces a negative-resistance-like behavior to inverter output admittance. High control bandwidth of the PLL makes the system sensitive to impedance-based stability issues when the inverter is connected to a weak grid that has high impedance. However, very conservative tunings lead to overly damped dynamic responses in strong grids, where the control performance and power quality can be improved by applying higher PLL control bandwidths. Continuous evaluation of grid conditions makes it possible to avoid the risk of instability and poor dynamic responses, as the inverter output admittance can be re-shaped online to continuously match the grid conditions. The present work proposes method for adaptive control of the PLL based on the real-time measurements of the grid impedance, applying pseudo-random binary sequence (PRBS) injections. The method limits the PLL bandwidth in weak grids to avoid stability issues and increases the control bandwidth in strong grids to improve voltage-tracking, and thus overall control performance. The method is verified through simulations and experimental laboratory tests in a kW-scale system. The results show that optimizing the PLL bandwidth with respect to the grid conditions is highly beneficial for system performance and stability.
Highlights
The increasing penetration level of the inverter-connected renewable energy has caused stability and power-quality issues in power grids [1,2]
This paper extends the research in [28] by introducing a practical implementation of the adaptive phase-locked loop (PLL) that optimizes the trade-off between the system robustness and PLL control performance based on continuous evaluation of the impedance-based sensitivity function
By adaptively adjusting the PLL bandwidth based on changes in the grid impedance, the sensitivity peak can be kept at a constant value which should be chosen as a good trade-off between the control performance and robustness
Summary
The increasing penetration level of the inverter-connected renewable energy has caused stability and power-quality issues in power grids [1,2]. This paper extends the research in [28] by introducing a practical implementation of the adaptive PLL that optimizes the trade-off between the system robustness and PLL control performance based on continuous evaluation of the impedance-based sensitivity function. The PLL bandwidth is controlled so that the maximum value of the system sensitivity function is kept constant regardless of the grid conditions This is equivalent to constant stability margins with the highest possible PLL bandwidth, satisfying the desired system robustness. More sophisticated and systematic adaptive control of the PLL is provided that considers the optimal control performance in addition to the system stability This results in improved power quality and overall dq-domain control performance under varying grid conditions.
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