Linear Parameter-Varying Control of A Power-Synchronized Grid-Following Inverter

This paper proposes a Linear Parameter-Varying loop-shaping controller for a power-synchronized grid-following inverter (PSGFLI). This control strategy regulates the inverter output active and reactive power at the terminal instead of the point of connection and does not require a phase-locked loop (PLL) for extracting the voltage phase angle. Hence, the prevalent stability issues exhibited when GFLIs are connected to weak grids are not present, and the proposed PSGFLI control strategy can work under both very weak and strong grid conditions without being prone to instability. In this approach, the controller parameters are functions of the operating point and are changed during the real-time operation such that the closed-loop performance is preserved in all operating points. Furthermore, since the grid impedance is a factor in the design process, a robustness analysis against grid impedance estimation error is conducted, and it is shown that discrepancies in the estimated and real grid impedances are unlikely to make the system unstable. The performance of the proposed control design is validated in MATLAB/PLECS and experiments for both strong and weak grids.

Convertisseur de fréquence indirect à rapport de tension fixe

Aziza Benaboud

Many electric power generators use gas turbines as power sources. Typically they are connected to the turbines through a mechanical gearbox in order to adapt their synchronous speed to the optimal rotation speed of the turbine, which is very often much higher than the synchronous speed. However, due to direct network connection, the generator speed cannot be variable : it is imposed by the network and constant. To overcome this problem, we propose to replace the mechanical gearbox by a flexible electronic solution which offers a high efficiency. Using this approach, the turbine is directly connected to the synchronous generator, which is connected to the grid through an indirect static frequency converter with an intermediary DC circuit. However, this type of converter is not common in this application because of very high switching losses due to the high frequency of the PWM technique normally used for its control. In this dissertation, a new control strategy is proposed for the three level Neutral Point Clamped converter, characterized by its high efficiency due to the use of square-wave operation mode. The main advantage of this mode is the quasi absence of switching losses. In this mode, only the frequency can be varied between the input and the output voltage, but their magnitudes are not freely controllable. A voltage magnitude adaptation can be done by the generator's excitation. The produced active and reactive power can be controlled by the generator excitation as well as both the angle shift between the generator and rectifier voltages and between the inverter and network voltages. The capacitive intermediary circuit brings the advantage of decoupling of harmonics between the generator and the network currents. A control method is also proposed to resolve some problems incurred by using square wave operation mode, in particular to reduce the harmonics distortion of the output inverter voltage and current. As second important contribution, this thesis proposes a new fast-ramping DC-component elimination strategy for AC currents. In comparison to the usually slow transient that characterizes a DC component free current transient, we achieve that much faster transients also without DC-component, simply by choosing a well defined transition period. Simulation and experimental results for different operating points and transitions between them highlight the capabilities of the proposed control strategy. These include the ability to operate with unity power factor and better current quality without continuous component and less harmonics.

EPFL2007

Multivariable Control of Single-Inductor Dual-Output Buck Converters

Behrooz Bahrani, Alireza Karimi, Alfred Rufer

Cross regulation is the main technical challenge of a Single-Inductor Multiple-Output (SIMO) dc-dc converter. This paper proposes a multivariable digital controller to suppress the cross regulation of a Single-Inductor Dual-Output (SIDO) buck converter in Continuous Conduction Mode (CCM) operation. The controller design methodology originates from the open-loop shaping of the Multi-Input Multi-Output (MIMO) systems. The control design procedure includes: (i) determination of a family of nonparametric models of the SIDO converter at operating points of interest, (ii) determination of the class of the controller, and (iii) system open-loop shaping by the convex minimization of the summation of the square second norm of the errors between the system open-loop transfer function matrices and a desired open-loop transfer function matrix. The proposed controller minimizes the coupling between the outputs of the SIDO converter and provides satisfactory dynamic performance in CCM operation. This paper describes the theoretical aspects involved in the design procedure of the controller and evaluates the performance of the controller based on simulation studies and experiments.

Ieee2014

Convertisseur de fréquence indirect à rapport de tension fixe

Aziza Benaboud

EPFL2007