Behrooz Bahrani

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Vector Control of Single-Phase Voltage-Source Converters Based on Fictive-Axis Emulation

This paper presents an alternative way for the current regulation of single-phase voltage-source dc-ac converters in direct-quadrature (dq) synchronous reference frames. In a dq reference frame, ac (time varying) quantities appear as dc (time invariant) ones, allowing the controller to be designed the same as dc-dc converters, presenting infinite control gain at the steady-state operating point to achieve zero steady-state error. The common approach is to create a set of imaginary quantities orthogonal to those of the real single-phase system so as to obtain dc quantities by means of a stationary-frame to rotating-frame transformation. The orthogonal imaginary quantities in common approaches are obtained by phase shifting the real components by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. In the proposed approach of this paper, the orthogonal quantities are generated by an imaginary system called fictive axis, which runs concurrently with the real one. The proposed approach, which is referred to as fictive-axis emulation, effectively improves the poor dynamics of the conventional approaches while not adding excessive complexity to the controller structure.

Multivariable-PI-Based

This paper presents a linear direct-quadrature current control strategy for voltage source converters (VSCs) in a rotating reference frame (RRF). The described method is based on multivariable-proportional-integral (PI) regulators and provides fast dynamics and a zero steady-state error. Contrary to the well-known conventional PI-based control strategies in RRFs, the presented method provides practically decoupled axes with a superior disturbance rejection capability. Moreover, its implementation is relatively simple and does not impose excessive structural complexity compared to its conventional PI-based competitors. The method is applicable to both single- and three-phase systems and also to anisotropic three-phase systems, e.g., synchronous motors with different direct and quadrature impedances driven by VSCs. Implementing a three-phase test system, the performance of the presented method is experimentally evaluated.

dq

This paper presents a vector control strategy for regulating the current of grid-tied voltage source converters (VSCs) in a rotating reference frame. The proposed approach is based on shaping the open-loop and closed-loop transfer matrices of the system. Solving a constrained convex optimization problem, the shaping is achieved, which guarantees the stability of the closed-loop system. The designed controller results in the desired dynamic performance and decouples the direct and quadrature (dq) current axes. The structure of the proposed controller is similar to that of its predecessors and consists of four proportional-integral controllers. The performance of the method is evaluated based on simulation and experimental results. It is confirmed that its dynamic performance is better than that of the previously proposed approaches, and it results in the decoupled current axes.

Current Control of Voltage Source Converters With Superior Axis Decoupling Capability

The modular multilevel converter (MMC) is a potential candidate for medium/high-power applications, specifically for high-voltage direct current transmission systems. One of the main challenges in the control of an MMC is to eliminate/minimize the circulating currents while the capacitor voltages are maintained balanced. This paper proposes a control strategy for the MMC using finite control set model predictive control (FCS-MPC). A bilinear mathematical model of the MMC is derived and discretized to predict the states of the MMC one step ahead. Within each switching cycle, the best switching state of the MMC is selected based on evaluation and minimization of a defined cost function. The defined cost function is aimed at the elimination of the MMC circulating currents, regulating the arm voltages, and controlling the ac-side currents. To reduce the calculation burden of the MPC, the submodule (SM) capacitor voltage balancing controller based on the conventional sorting method is combined with the proposed FCS-MPC strategy. The proposed FCS-MPC strategy determines the number of inserted/bypassed SMs within each arm of the MMC while the sorting algorithm is used to keep the SM capacitor voltages balanced. Using this strategy, only the summation of SM capacitor voltages of each arm is required for control purposes, which simplifies the communication among the SMs and the central controller. This paper also introduces a modified switching strategy, which not only reduces the calculation burden of the FCS-MPC strategy even more, but also simplifies the SM capacitor voltage balancing algorithm. In addition, this strategy reduces the SM switching frequency and power losses by avoiding the unnecessary switching transitions. The performance of the proposed strategies for a 20-level MMC is evaluated based on the time-domain simulation studies.

Decoupled dq-Current Control of Grid-Tied Voltage Source Converters Using Nonparametric Models

This paper analytically determines the nondetection zone (NDZ) of an active islanding detection method, and proposes a solution to obviate the NDZ. The method actively injects a negative-sequence current through the interface voltage-sourced converter (VSC) of a distributed generation (DG) unit, as a disturbance signal for islanding detection. The estimated magnitude of the corresponding negative-sequence voltage at the PCC is used as the islanding detection signal. In this paper, based on a laboratory test system, the performance of the islanding detection method under UL1741 anti-islanding test conditions is evaluated. Then, determining the NDZ of the method and proposing the countermeasure, the existence of the NDZ and the performance of the modified method to eliminate the NDZ is verified based on simulation results in PSCAD/EMTDC software environment and experimental tests.

Indirect Finite Control Set Model Predictive Control of Modular Multilevel Converters

This paper proposes a multivariable digital control design methodology for the voltage regulation of an islanded single distributed generation (DG) unit microgrid and its dedicated load. The controller design methodology is based on a family of spectral Multi-Input Multi-Output (MIMO) models of the microgrid system and performs open-loop shaping and system decoupling simultaneously by a convex optimization approach. The control design procedure includes: (i) the determination of a family of nonparametric models of the system at various operating points, (ii) the determination of the class of the controller, and (iii) system open-loop shaping by convex minimization of the summation of the square second norm of the errors between the system open-loop transfer functions and a desired open-loop transfer function. Based on the proposed design methodology, two dq -augmented voltage controllers are proposed to regulate the load voltages of a single-DG-unit microgrid. The proposed controllers guarantee the robust stability and satisfactory dynamic response of the system in spite of load parametric uncertainties and also the presence of nonlinear load. This paper describes the theoretical aspects involved in the design procedure of the controllers and evaluates the performance of the controllers based on simulation studies and experiments.

Nondetection Zone Assessment of an Active Islanding Detection Method and its Experimental Evaluation

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: 1) determination of a family of nonparametric models of the SIDO converter at operating points of interest, 2) determination of the class of the controller, and 3) 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.

A Multivariable Design Methodology for Voltage Control of a Single-DG-Unit Microgrid

This paper proposes a vector control strategy for LCL-filter-based grid-connected voltage-source converters (VSCs). The proposed control strategy is inherently capable of attenuating the resonance phenomenon of such systems. This is an advantage over the existing methods, which require additional damping techniques. Moreover, the proposed vector control strategy is able to fully decouple the direct (d) and quadrature (q) components of the current in a rotating reference frame. The design procedure comprises a constrained optimization-based loop shaping. It utilizes the multi-input multi-output (MIMO) nonparametric model of the system along with a high-order linearly parameterized MIMO controller to form an open-loop transfer function matrix. Minimizing the second norm of the error between the open-loop transfer function matrix and a desired one, the coefficients of the controller are optimally determined. Conducting several reference tracking scenarios, the performance of the proposed vector controller is evaluated both by means of time-domain simulation studies in MATLAB/Simulink and experimental results.

Multivariable Control of Single-Inductor Dual-Output Buck Converters

Low system voltage in traction networks, mainly caused by active power absorption of locomotives, adversely affects the performance of the locomotives and also the power transmission capability of the catenary line. This paper introduces a voltage support scheme to compensate for the adverse effects of low system voltage. The proposed method is based on the injection of reactive power through the current-controlled line-side converter of locomotives. Comparing the catenary voltage with its reference value, the error is fed to a high-order controller. The controller generates the quadrature (q)-axis reference value of a current control strategy, which is responsible for the reactive power injection. To design the high-order controller, adopting the nonparametric models of the system at various locations, an optimization-based loop-shaping approach is used. The loop shaping guarantees the stability and the acceptable performance of the closed-loop system for various locomotive positions in the network. The performance of the proposed control strategy is evaluated based on simulation results in MATLAB/PLECS environment. Moreover, implementing a scaled-down laboratory setup, the performance of the proposed scheme is experimentally evaluated.