Allal M. Bouzid

H∞ structured design of a cascaded voltage/current controller for electronically interfaced distributed energy resources

In this paper a new controller design method for inverter-based distributed energy resources in standalone microgrids is proposed. The objective of the design is to exploit the flexibility and fast dynamics of the voltage inverter. The proposed control method is composed of droop, voltage and current controllers. The droop controller is based on V-I droop characteristics, and exhibits significantly faster dynamics compared to the conventional droop method. The voltage and current controllers utilize the structured H∞ PI regulation strategy to track the reference voltage and current. The PI controller parameters are tuned using Hinfstruct optimization function from the MATLAB® Robust Control Toolbox. The performance of the closed-loop system is analyzed to illustrate the capabilities of the new technique. Simulation results are presented to prove validity and verify the efficiency of the proposed control.

Structured H ∞ design method of PI controller for grid feeding connected voltage source inverter

This paper presents a new Structured H∞ Design Method of proportional-integral (PI) controllers for Grid Feeding Connected Voltage Source Converters, based on MATLAB tools for tuning linear control systems in the frequency domain. The study consists in elaborating optimization of an H∞ robust control strategy for a Voltage Source Inverter (VSI) connected to the grid through a LCL filter. The PI controller parameters are tuned using hinfstruct optimization function from the MATLAB® Robust Control Toolbox. A synchronous reference frame PI regulation strategy is used for the grid current control loop. The design of PI is validated by simulations in MATLAB®/Simulink™ and the Simulation results prove the correctness and validity of the proposed control. The robust stability and performance of the closed-loop system are analyzed to illustrate the capabilities of the new technique.

Voltage and frequency control of wind-powered islanded microgrids based on induction generator and STATCOM

This paper presents a comprehensive modeling of a three-phase cage induction machine used as a self-excited squirrel-cage induction generator (SEIG), and discusses the regulation of the voltage and frequency of a self-excited SEIG based on the action of the static synchronous Compensator (STATCOM). The STATCOM with the proposed controller consists of a three-phase voltage-sourced inverter and a DC voltage. The compensator can provide the active and reactive powers and regulate AC system bus voltage and the frequency, but also may enhance the load stability. Moreover, a feed forward control method for the STATCOM is introduced and applied for controlling the SEIGs terminal voltage using a two-degree of freedom RST controller. Simulation results for the steady-state operating condition and transient operating conditions for the system subjected to a wind reference step change, and a step load change are presented to demonstrate the effectiveness of the proposed controller.

A robust control strategy for parallel-connected distributed generation using real-time simulation

In the context of renewable energy sources integration, the use of microgrids (MGs) with multiple distributed generation (DG) based on voltage source inverters is the solution to significantly reduce the influence of the unpredictable nature of the renewable energy sources on the grid. This paper focuses on the problem of uncertainties and disturbances found in such applications with various LCL filter parameters, load conditions and network conditions, and the problem of voltage control of inverter-interfaced microgrids with radial structure. The main objectives of this paper are to (i) design a robust low-order voltage controller which ensures robustness as well as desired performance of the microgrid system in spite of uncertain parameters; (ii) validate the complete model of MG under different operating conditions using a digital real-time simulator. This simulator is a beneficial tool that allows the verification of several scenarios with different operating conditions. It accelerates the testing process as compared to offline simulations, where computation time is much larger.

Simulation of droop control strategy for parallel inverters in autonomous AC microgrids

In this paper, a droop control strategy is presented for accurate power sharing between parallel connected inverters in an AC microgrid in autonomous mode. The proposed strategy is based on the droop control techniques P-f/Q-V and P-V/Q-f, with a virtual impedance and using monitored quantities directly at the inverter. This droop control approach can be used in microgrids where communications are not reliable or not available. It allows for maintaining voltage and frequency stability and for regulating the power flows between the primary energy sources and the common AC bus. The design methodology of the control scheme under study is based on grid-supporting-grid-forming controls using multi-loop dq control. The time domain simulation is conducted, in the MATLAB®/SimPowerSystems environment and the RT-EVENTS toolbox from OPAL-RT is also used to model the inverters. Resulting waveforms show that the proposed method can allocate both real and reactive powers effectively and can enhance the stability of a micro-grid in the autonomous operation mode.

State space modeling and performance analysis of self-excited induction generators for wind simulation

In recent years, it has been proven that one of the main issues in isolated applications is the importance of the dynamic characteristics assessment of three phase self-excited induction generator (SEIG). This paper presents a generalized state-space dynamic model using D-Q stationary reference frame of such a three phase SEIG. The values of self-excitation capacitance and the minimum and maximum speeds necessary to initialize the self-excitation are determined. The effect of variation of excitation capacitance and variation of SEIG loads are determined by simulation. The analysis of the results could help to determine the efficiency solutions for the system to supply isolated areas even when the loads are unbalanced.

H-infinity loopshaping controller design of micro-source inverters to improve the power quality

The introduction of micro grids in distribution networks based on power electronics facilitates the use of renewable energy resources, distributed generation (DG) and storage systems while improving the quality of electric power and reducing losses thus increasing the performance and reliability of the electrical system. The main objective of this paper, is designing H∞ controllers for a control scheme based on decoupled models, which includes an inner current loop and an outer voltage loop for micro-source grid-connected inverter. The robust controller is designed for the grid-forming using structured uncertainties to improve the performance of the system facing parametric uncertainties. The controller is evaluated by simulation to demonstrate the performance of the proposed strategy for the nominal case and perturbed parameters cases.

Comparative study between decoupled control with sliding mode & feedback linearization control applied to STATCOM

This paper presents the study of a static var compensator (STATCOM) connected to the electric power grid. The aim of this work is to compare the performances by the use of two types of controllers: decoupled control using Sliding Mode Controller (SMC) and the input/ output linearization control strategy both applied to reactive current and capacitor voltage regulation in terms of reference tracking and robustness. The STATCOM model and the three controllers are developed using MATLAB/Simulink. The performance analysis showed that the decoupled control using SMC are more robust even under severe variation of some key parameters of the STATCOM. The input/output linearization control presents ripples at current and continuous voltage.

H-∞ loopshaping controller design of micro-source inverters

The range of electric parameters of Distributed Power Generation Systems (DPGSs) is wide and uncertain. The main objective of this paper is to propose H∞ controller design for the grid-forming problem using structured uncertainties to improve the performance of the system facing parametric uncertainties, and to obtain fast transient response with minimum overshoot, precise current control with zero steady-state error, and low distortion and current ripple. The procedure is verified by simulation to demonstrate the performance of the proposed strategy for the nominal case and perturbed parameters cases.