Mohamed Moez Belhaouane

On the Backstepping Approach for VSC-HVDC and VSC-MTDC Transmission Systems

Abstract This article presents a backstepping control design strategy for the voltage source converter (VSC)-based high-voltage direct current (HVDC). First, a dynamic model is derived based on the state space description. Subject to the backstepping control design procedure strategy, a non-linear control scheme is developed in the sense of Lyapunov stability theory in order to satisfy various objectives of a stable HVDC system and guarantee a grid connection with a unity power factor. Then, the proposed control method is extended for multi-terminal (MT) HVDC transmission systems based on VSCs. In order to improve the dynamic behavior of the controlled DC bus voltage and the stability of MTDC systems, a backstepping control strategy accorded to each VSC is proposed and integrated into the voltage droop control strategy. The designed advanced controller allows to improve the overall DC grid stability and to reach the droop values, designed on static considerations, with satisfying dynamic behavior. Compared to the conventional control, the use of a backstepping control allows to exhibit excellent transient response over a wide range of operating conditions.

MMC Stored Energy Participation to the DC Bus Voltage Control in an HVDC Link

The modular multilevel converter (MMC) is becoming a promising converter technology for HVDC transmission systems. Contrary to the conventional two- or three-level VSC-HVDC links, no capacitors are connected directly on the dc bus in an MMC-HVDC link. Therefore, in such an HVDC link, the dc bus voltage may be much more volatile than in a conventional VSC-HVDC link. In this paper, a connection between the dc bus voltage level and the stored energy inside the MMC is proposed in order to greatly improve the dynamic behavior in case of transients. EMT simulation results illustrate this interesting property on an HVDC link study case.

Optimal control design for Modular Multilevel Converters operating on multi-terminal DC Grid

This paper proposes an advanced control strategy for Modular Multilevel Converters (MMC) integrated in Multiterminal DC grid. In this present work, a three terminal MMC-MTDC system connecting onshore AC systems with an offshore wind farm is setup. Firstly, the voltage droop control associated to the conventional cascaded controllers for MMC stations is studied, the dynamic behavior of the DC voltage is analyzed and some drawbacks are outlined. In order to improve the dynamic behavior of the controlled DC bus voltage and the stability of MTDC system, an optimal multivariable control strategy of each MMC converter is proposed and integrated in a voltage droop controller strategy. The designed advanced controller allows to improve the overall DC grid stability and to reach the droop values designed on static considerations with acceptable dynamic behavior. By means of numerical simulations in EMTP-RV software, it is shown that the proposed control strategy performs well the stability of MTDC grid with 400-level model for MMC compared with the classic existing control methods.

Impact of control algorithm solutions on Modular Multilevel Converters electrical waveforms and losses

Modular Multilevel Converters (MMC) are becoming increasingly popular with the development of HVDC connection and, in the future, Multi Terminal DC grid. A lot of publications have been published about this topology these last years since it was first proposed. Many of them deal with converter control methods, other address the method of estimating losses. Usually, the proposed losses estimation techniques are associated to simple control methods For VSC (Voltage Sources Converters) topology, the losses minimization is based on the limitation of the RMS currents values. This hypothesis is usually extended to the control of MMC, by limiting the differential currents to their DC component, without really being checked. This paper investigates the impact of two control algorithms variants on electrical quantities (currents, capacitor voltages ripple, losses). From the published results, it is shown that in some cases the usual choice is not the best one.

Finite-time stabilisation of some power transmission systems

This paper presents the finite time stabilisation strategy of two problems: the first one is the control of the high voltage direct current based on voltage source converter, while the second is the control of the multi-terminal direct current transmission systems. Subject to finite-time control design strategy, a linear and nonlinear dynamic model are derived based on the state-space description. Furthermore, continuous or discontinuous finite-time feedbacks are proposed to ensure the tracking of the output variables and to enhance the stability of the studied high voltage direct current system. In addition, the proposed control strategy is extended for the multi-terminal direct current system. A comparative study between various approaches (Proportional-Integral control, continuous or discontinuous stabilising finite-time controllers and control by backstepping) is presented and shows that the finite-time continuous feedback gives an excellent transient response.

Multivariable optimal robust control strategy for MMC converter

The Modular Multilevel Converter (MMC) is a newly introduced switch-mode converter topology with the potential for High-Voltage Direct Current (HVDC) transmission application. To provide a good dynamic performance and improve the anti-disturbances ability of MMC, a robust control approach is designed to reduce the effects of the unmodeled disturbances in the AC side of the converter. Therefore, in this paper, a multivariable optimal guaranteed cost control with input constraints is proposed in order to control the MMC with norm bounded time-varying parameter uncertainties and constraints. For this aim, a small-signal state-space model is developed. Furthermore, a new optimal guaranteed cost controller is designed based on a convex optimization problem. Finally, the effectiveness of the strategy is conformed using Matlab/Simulink platform.

Control of MMC converter integrated in HVDC link based on quadratic optimization approach

This paper proposes an energy and dc-bus voltage control strategy for the MMC converter, connected to HVDC link, using online quadratic optimization. The proposed approach leads to regulate simultaneously the energy sum and difference in “abc” frame and to maintain the DC voltage level around the operating point. The aim of this paper concerns the development of a reliable strategy based on optimization approach that is able to control the average energy distribution inside de MMC and further regulate the dc-bus voltage in dc-side. A combined controller between classic and advanced control methods is given to take advantage of both methods. Performance of the proposed optimal control strategy is evaluated based on time-domain simulation study. Using quadratic optimization approach, the obtained results prove a satisfactory response of MMC converter operating in voltage control mode under various operating conditions.

Nonlinear modeling and control of a VSC-HVDC transmission systems

This paper presents a new modeling and control strategy of VSC-HVDC transmission system in order to improve system damping oscillations, and enhancing transient and voltage stability. This control strategy is based on a linear and bilinear state space deviation models. The structure of the control method is designed in order to minimize the number of the control loops (inner and outer control loops) in the classical strategy to one control loop for each output. These control loops are active power or DC voltage for the first output, and the reactive power for the other output. Simulations for VSC-HVDC transmission system model show the validity of the derived models and the effectiveness of the control strategies.

Nonlinear Control Design of VSC-MTDC Systems based on Backstepping Approach

This paper deals with the nonlinear control approach of Voltage Source Converter (VSC) based on MTDC (multi-terminal direct current) transmission systems. A nonlinear control approach based on Backstepping method is proposed for two different control methods: active power and DC voltage. The proposed control approach, based on Lyapunov theory, is capable of analytically obtaining a control laws in order to regulate the active power and dc bus voltage in an MTDC system. Furthermore, the dynamic interactions between the active power nonlinear control design and the DC voltage droop control are examined. Finally, the validity of the proposed control design approach is verified by time-domain simulations under the Matlab/ Simulink environment.