A Control Parameters Design Method With Multi-Constrains for an LCL-Filtered Grid-Connected Inverter in a Weak Grid

Under weak grid conditions, the variation of the grid impedance will affect the steady-state and dynamic performance of the LCL-filtered grid-connected inverter and even make the inverter unstable. To ensure the system stability and further improve the dynamic performance in a weak grid, a control parameter design method with multi-constrains considering the system bandwidth for the current controller and active damping is proposed in this paper. First, based on the current controller and active damping with only grid current feedback, the effects of control parameters and grid impedance on the LCL resonant suppression and the performance of the inverter are analyzed. Moreover, the parameter constraints of the controllers are derived considering the grid impedance, including stability, resonance suppression, and margin constraints. Furthermore, as the system bandwidth affects the dynamic performance of the inverter, combined with the obtained multi-constraints, the optimal control parameters are determined by achieving the maximum bandwidth of the system against the impedance variation. Compared with other two methods, when the proposed method is applied, the system can operate with a better dynamic and steady-state performance. Finally, experiments are performed on a 2 kW three-phase grid-connected inverter in the weak grid, which verify the effectiveness of the parameter design method proposed in this paper.

Modeling and Analysis of STATCOM for Renewable Energy Farm to Improve Power Quality and Reactive Power Compensation

Power Quality (PQ) improvement in grid-integrated photovoltaic (PV) and wind energy hybrid systems for effective power transfer is presented in this paper. Due to interlinked hybrid renewable energy resources and nonlinear loads, various issues arise which affect the power quality, i.e., voltage sag, harmonic distortion increases, and also reactive power demand. In order to mitigate these issues, flexible alternating current transmission system (FACTS) devices are utilized. In this paper, hysteresis band current controller (HBCC)-based static synchronous compensator (STATCOM) is modeled to reduce PQ problems. HBCC is a robust and simple technique to improve voltage profile, reduce total harmonic distortion (THD) and fulfill the reactive power demand. Two case scenarios of the hybrid system, i.e., (I) grid integrated hybrid system without HBCC (II) grid integrated hybrid system with HBCC, are tested. Results demonstrate that under scenario II, load bus voltage is regulated at 1.0 p.u., THD of system voltage and current is reduced 0.25% and 0.35%, respectively, and reactive power demand of 30 kVAR is fulfilled. The HBCC was designed for reducing THD of the system with the limits specified by standards IEEE 519-1992 STATCOM using hysteresis band current controller to improve power quality in the distribution system which is simulated using MATLAB/SIMULINK. After that, the performance of the system is better in terms of power quality.

Dual Sequence Controller with Delayed Signal Cancellation in the Rotating Reference Frame

Manuscript submitted to the Twenty-second IEEE Workshop on Control and Modeling for Power Electronics (COMPEL 2021).<div>Abstract: Dual-sequence current controllers of voltage source converters (VSCs) feature two separate rotating reference frames (RRFs), commonly named dq frames, and rely on techniques that isolate the positive and negative sequences of three-phase measurements. One of these techniques is the delayed signal cancellation (DSC). It is performed in the stationary reference frame (SRF), also known as αβ frame. The DSC combines old values of one axis with new values of the other axis of the SRF. The results are, then, transformed into the RRFs for use in the current controller. This filtering process introduces an extra layer of complexity for dual-sequence current controllers, which could otherwise operate solely in the RRFs. This paper introduces a frequency adaptive DSC method that operates directly in the RRF. Moreover, an averaging of two of the proposed DSC filters with contiguous integer delays is employed for reducing discretization errors caused by grid frequency excursions. A formal proof of the equivalence between the αβ and dq DSC methods is presented. Furthermore, computer simulations of a case study support the interpretation of the results.</div>

Dual Sequence Controller with Delayed Signal Cancellation in the Rotating Reference Frame

Manuscript submitted to the Twenty-second IEEE Workshop on Control and Modeling for Power Electronics (COMPEL 2021).<div>Abstract: Dual-sequence current controllers of voltage source converters (VSCs) feature two separate rotating reference frames (RRFs), commonly named dq frames, and rely on techniques that isolate the positive and negative sequences of three-phase measurements. One of these techniques is the delayed signal cancellation (DSC). It is performed in the stationary reference frame (SRF), also known as αβ frame. The DSC combines old values of one axis with new values of the other axis of the SRF. The results are, then, transformed into the RRFs for use in the current controller. This filtering process introduces an extra layer of complexity for dual-sequence current controllers, which could otherwise operate solely in the RRFs. This paper introduces a frequency adaptive DSC method that operates directly in the RRF. Moreover, an averaging of two of the proposed DSC filters with contiguous integer delays is employed for reducing discretization errors caused by grid frequency excursions. A formal proof of the equivalence between the αβ and dq DSC methods is presented. Furthermore, computer simulations of a case study support the interpretation of the results.</div>

Linear Quadratic Regulator and Fuzzy Control for Grid-Connected Photovoltaic Systems

This work presents a control scheme to control the grid-connected single-phase photovoltaic (PV) system. The considered system has four 250W solar panels, a non-inverting buck-boost DC-DC converter, and DC-AC inverter with LCL filter. The control system aims to track and operate at the maximum power point (MPP) of PV panels, regulate the voltage of DC link, and supply the grid with a unity power factor. To well achieve these goals, the proposed control system consists of three parts, that are MPP tracking controller module with a fuzzy-based modified incremental conductance (INC) algorithm, a DC-link voltage regulator with a hybrid fuzzy proportional-integral (PI) controller, and a Current Controller module using the linear quadratic regulator (LQR) for grid-connected power. Based on fuzzy control and LQR, this work introduces a full control solution for grid-connected single-phase PV systems. The key novelty of this research is to analyze and prove that the newly proposed method is more successful in numerous aspects by comparing and evaluating the previous and present control methods. The designed control system settles quickly, which is critical for output stability. In addition, as compared to backstepping approach used in our past study, the LQR technique is more resistant to sudden changes and disturbances. Furthermore, backstepping method produces the larger overshoot, which has a detrimental impact on efficiency. Simulation findings under various weather conditions were compared to theoretical ones to indicate that the system can deal with variations in weather parameters.

Harmonic Suppression Strategy of Photovoltaic Grid Connected Inverter Based on Repetitive and PI Control

To address the serious harmonic problem of grid connected current in photovoltaic grid-connected inverter, a harmonic suppression strategy based on Repetitive and PI control is proposed in this thesis. According to this strategy, the mathematical model of LCL photovoltaic grid-connected inverter is established with the harmonic mechanism analyzed, the repetitive and PI control is added into the current controller, and the capacitive-current feedback is added to enhance the system damping. Furthermore, it puts PI control on the inner loop and the repetitive control on the outer loop to improve the dynamic performance and achieve the harmonic suppression of the system. Moreover, the simulation results show that this method improves the dynamic response ability of the system, and effectively suppress the grid-connected current harmonics as THD of grid-connected current is 19.65% lower than that of PI controller.