Mohammad Sleiman

A survey on modeling, control, and dc-fault protection of modular multilevel converters for HVDC systems

The modular multilevel converter (MMC) is becoming one of the most promising topology in multilevel converter series especially for high-voltage and high-power applications. This valuable features, nominates MMCs to interface high-voltage and high-power renewable energy resources into modern HVDC electric grids for more penetration of renewable energy. This paper investigates recent modelling, control, and dc-fault protection techniques that have been applied to MMCs.

A new 7L-PUC multi-cells modular multilevel converter for AC-AC and AC-DC applications

In this paper, a new cell based Modular Multilevel Converter (MMC) for AC-AC and AC-DC applications is presented. The new topology makes use of an efficient Packed U-Cells (PUC) structure to form the Multi-Cells Modular Multilevel Converters (M3C). It is a member of MMC root family, with extended operational capability covering therefore AC-AC and AC-DC modes of operation. A dynamic model of the PUC and the single phase M3C will be used along with predictive control method to validate the effectiveness of different operation modes of the converter.

Modeling, control and simulation of DFIG for maximum power point tracking

This paper deals with the modeling, analysis, control and simulation of a doubly-fed induction generator (DFIG) driven by a wind turbine. This grid connected wind energy conversion system (WECS) is composed of DFIG and two back-to-back PWM voltage-source converters in the rotor circuit. A mathematical model of the machine, derived in an appropriate dq reference frame is established. The grid voltage oriented vector control is used for the grid side converter (GSC) in order to maintain a constant DC bus voltage and to compensate for reactive power at the power network. The stator voltage orientated vector control is adopted in the rotor side converter (RSC) control strategy, providing efficient handling of active and reactive power at the stator, as well as a maximum power point tracking (MPPT) method for the DFIG-based wind turbine. The proposed system is simulated for different operating conditions to illustrate the reliability of the control technique. Corresponding system simulation results under nonlinear load variations and wind speed transients are presented to demonstrate the significance of MPPT in WECS, and the effectiveness of adopted control technique.

Model predictive control of a dual output seven-level rectifier

In this paper, a component efficient multilevel rectifier is modeled and controlled using Model Predictive Control (MPC) technique. The recently introduced Capacitor Tied Switches (CTS) converter structure features two distinct dc-load terminals which can operate at different voltage levels while experiencing high voltage levels at the ac-side terminals. The CTS converter structure inherits attractive features from the efficiency and power quality perspectives, with excellent topology performance which was validated through different simulation scenarios.

Energy equalization module for modular multilevel converters in variable speed motor drives

Modular Multilevel Converters (MMC) processing phase quantities at low frequency are vulnerable to high cell voltage ripples. MMC upper and lower arms processes pulsating power at fundamental and second harmonic phase components. As a consequence, arms are expected to buffer the energy difference exchanged between DC-link and each AC-phase. This buffered energy is translated into increased cell voltage ripple when operating at low phase frequency. Several remedies have been presented in literature, which can be classified under software and hardware methods. To address this short-come we introduced a plug-in hardware alternative that permit energy exchange between cells from upper and lower arms thus remarkably reducing capacitor voltage ripples caused by out of phase energy variations. The proposed method can improve MMC performance over the full operation region and is mainly attractive to low frequency operation such as in variable-speed motor drives where higher-than-nominal torques might be required for a wide speed range. Performed simulations over a synchronous machine using maximum torque per current control technique, conforms well with the analytical findings and shows the effectiveness of the proposed method.

A simple control method for modular multilevel converters

In this paper a simple control method involving new insertion index selection scheme for control of Modular Multilevel Converters (MMC) is proposed. In contrast with classical closed-loop methods, where all available arm voltages are measured and used in the generation process of insertion indices, this method uses available average voltage, i.e. the arithmetic mean of available upper and lower arm voltages per leg. Thus, number of measured signals to be fed back are reduced to half. Moreover, development of available arm voltage ripple expressions along with the thorough evaluation of the proposed insertion method impact on arm voltages driving input- and output-currents are presented and discussed. Finally, analytical findings along with simulation results proved the effectiveness of the proposed method.

Numerical stability of multi-rate system using Lyapunov's theorem: Applied to real-time simulation

This paper proposes a method to analyse stability of multi-rate system. Multi-rate systems are used to simulate different part of the circuit with different sampling rate. This allows to reduce computational burden of stiff system; by choosing the most appropriate time-step according to the time constant of the phenomena studied. Traditionally, stability of discrete system is done by studying the eigenvalues of the system which can only uses a single sampling time and therefore cannot be applied to multi-rate systems. Many examples, where simulation is done with multiple rate, can be found in literature with no proof of stability other than simulation results. In this paper, multi-rate system are first represented as non-linear system using a single time-step and their stability is then demonstrated using Lyapunovs theorem. The proposed method is supported by a numerical example.

Insertion Index Generation Method Using Available Leg-Average Voltage to Control Modular Multilevel Converters

This paper presents a new insertion index generation method to control modular multilevel converters (MMC) topology. Unlike classical closed-loop methods, where the sum of all measured capacitor voltages per arm is used in the generation process of insertion indices, the proposed method uses available leg average voltage, i.e., the arithmetic mean of per leg available upper and lower arm voltages. Full mathematical development of available arm voltage ripple expressions is presented. Impact of the insertion method on arm voltages driving input and output currents is thoroughly evaluated and discussed. The proposed method inherently stabilizes arm stored energy; thus, no additional control loops are required to achieve regulation of arms energy difference. Moreover, the number of measured signals to be fed back to a high-level controller is reduced to half. Analytical findings along with experimental results obtained on a 6-kVA, 10-cells-per-arm MMC proved the effectiveness of the proposed method.

A new insertion index selection method to control modular multilevel converters

This paper introduces a new insertion index selection method to control a Back-to-Back Modular Multilevel Converter (MMC) laboratory platform. The test bench is based on a Power-Hardware-In-the-Loop (PHIL) setup constructed from two 6kVA, 3-phase, 10 cells per arm MMC systems connected in Back-to-Back configuration. The proposed insertion index selection method uses the arithmetic mean of available upper and lower arm voltages in a leg to generate insertion indices; unlike classical closed-loop methods which use the available arm voltage of each arm (i.e. sum of measured capacitor voltages in an arm). For this purpose, a detailed mathematical derivation of available arm voltage ripple equations is introduced. Furthermore, impact of the proposed insertion method on inserted arm voltages that drives input and output currents is thoroughly explored. No additional control loops for arm energy difference are required, as the proposed method inherently achieves arm energy stabilization. Nevertheless, the number of measured signals to be fed back to a high-level controller is reduced to half. The PHIL setup is formed of MMC-1 which emulates an AC grid and MMC-2 which is controlled as a grid tied converter. Analytical findings along with experimental results obtained from the Back-to-Back PHIL setup proves the effectiveness of the proposed method.

Efficient MMC Devices with Reduced Radiated and Conducted Interferences for Electric Vehicles Application

This paper presents a new cell module for modular multilevel converter (MMC) used in electric vehicles application. Traditionally, MMC has been used in high-power high-voltage application. The large number of power electronic devices required is rather used for reducing their voltage stress than the harmonic contents. Using the proposed cell topology, higher numbers of levels are achieved while the number of component remains low. This characteristic makes it an ideal configuration for a low switching high-efficiency AC/DC power converter, required for the soon to be common for plug-in vehicles application. Such converter can play two roles, not only can it be used as an active bidirectional charger, but also as active filter increasing network reliability. A predictive control algorithm is also presented in this paper, which will demonstrate the possibility to impose a charging curve on the DC side, even when the converter is used as active filter.