Fadia Sebaaly

Design and Implementation of Space Vector Modulation-Based Sliding Mode Control for Grid-Connected 3L-NPC Inverter

This paper presents a closed-loop space vector modulation (SVM)-based sliding mode controller (SMC) for a three-level grid-connected neutral point clamped (3L-NPC) inverter. The nonlinear SMC based on Gaos reaching law has been designed to control the grid current and inject desired amount of active and reactive power into the network. Due to using single dc source at the NPC inverter dc bus, neutral point voltage is controlled through redundant switching states and instantaneous dc voltage feedback integrated into SVM technique. Meanwhile, there is no external voltage controller involved, thus no associated fine tuning issues are existed. The performance of the proposed hybrid controller to inject a desired active/reactive power to the grid is investigated through external perturbations such as change in the line current amplitude/phase shift, ac voltage fluctuation, as well as dc voltage variation. Full converter state-space model was developed and simulated. Experimental results are provided to verify the fast dynamic performance, low content of line current THD%, and good voltage balancing of dc bus capacitors of the NPC inverter.

Three-level neutral-point-clamped inverters in transformerless PV systems — State of the art

Nowadays, the three-level neutral point clamped (3L-NPC) inverter has become more attractive among multilevel inverters topologies, especially in transformerless grid connected photovoltaic systems. Due to the unbalance between the two DC load-side capacitor voltages, the development and improvement of such topology were important topics in recent studies. New structures were derived focusing on better distribution of losses, increasing efficiency, decreasing the switching devices losses and the conduction losses. For this reason, seven derived topologies are discussed in this paper and compared among the conventional inverter. In addition, a part of this study focuses on modifying modulation techniques in order to comply with the neutral point fluctuation problem. Several carrier-based PWM and space vector modulation techniques are considered, discussed and compared.

Sliding Mode Fixed Frequency Current Controller Design for Grid-Connected NPC Inverter

In this paper, a fixed-frequency pulsewidth modulation (PWM) based on sliding-mode current controller is designed and applied to a utility interface three-phase/wire/level neutral-point-clamped inverter. The proposed design methodology of the sliding-mode control is based on a constant switching frequency operation and on Gaos reaching law that allows chattering compensation. The aim of the controller is to inject a controlled active power from renewable energy sources into the grid while controlling the power factor and minimizing supply current harmonics. Moreover, the dc-link voltages across the split capacitors are controlled with a simple proportional-integral (PI) regulator. Experimental results show the advantages of the proposed control algorithm in terms of fast dynamic response, low voltage ripple on the dc bus, low current Total Harmonic Distortion, and robustness toward external perturbations from the dc and ac sides; moreover, a comparison with a PWM-PI current controller is presented.

Sliding-mode current control design for a grid-connected three-level NPC inverter

Three-level neutral point clamped (NPC) inverter topologies are becoming more and more the interest of studies, especially in grid connected systems, due to their advantages compared to other multilevel inverters. Synchronization with an AC source remains a challenge ring current injection to the grid. In order to enhance the performance and the immunity of such grid connected inverters, a sliding mode current controller based on Gaos reaching law has been designed in this paper, and then applied to a three-wire three-level NPC inverter to have a unity power factor system. A PI regulator has been also employed to deal with the split DC-capacitors voltage unbalance problem. Robustness towards external disturbances was verified through simulations using MATLAB.

Finite control set model predictive controller for grid connected inverter design

The grid connectivity requirements imposed today, ask for an accurate controller design able to deal with system constraints and nonlinearities. In order to fulfill grid international standards, this paper details a Finite control set model predictive controller called FCS-MPC as part of controlling an energy conversion system. It uses a discrete time 3L-NPC inverter model (in the α-β frame) to predict the future values of the injected current and DC capacitors voltages. Switching pulses are chosen to ensure the optimum errors between the controlled variables and its desired values. Simulations results for both one-step and two-step horizon have been introduced to verify system robustness and dynamic response against variations of the DC link voltage, AC side current/voltage and reactive power. Low current-distortion levels with an arbitrary power factor prove the system performance when a two-step horizon control algorithm is adopted.

Model predictive controller with fixed switching frequency for a 3L-NPC inverter

This paper introduces a new control scheme based on a Finite Set-Model Predictive Control (FS-MPC) and aims of a fixed switching frequency operation. The proposed scheme allows of obtaining a voltage vector that is responsible of nullifying the cost functions derivative for its optimization. Moreover, a closed-loop space vector modulation (SVM) based on the proposed predictive control scheme is introduced for a three-level grid connected inverter. The proposed algorithm that injects a desired current to the grid is investigated through external perturbations such as change in the line current amplitude as well as DC voltage variation. Discrete-time system model was developed and simulated. Simulation results on MATLAB/Simulink are provided to verify the fast dynamic performance, and good voltage balancing of DC bus capacitors of the NPC inverter.

FCS-MPC with PNSC reference generation method for a 3L-NPC inverter under grid faults

Nowadays, grid faults consist anxious problem for grid inverters controller design. Conventional controllers are based on linear regulators such as PI and PR. This paper proposes a model predictive algorithm based on PNSC reference generation method to control the 3L-NPC inverter under grid voltage sags. The novel algorithm allows the easy inclusion of system nonlinearities, thus no additional fine tuning parameters are required. Moreover, no additional phase-locked loop synchronization technique is needed, reducing system complexity. A SOGI filter is added for voltage sequences extraction. The overall discrete model of the system is developed. Two-step horizon MPC algorithm is introduced for time delay compensation. Simulation results show the advantages of the proposed technique in terms of low harmonic line current content, low switching frequency operation, DC voltages balancing, good dynamic response and robustness to grid sags.

Novel Current Controller Based on MPC With Fixed Switching Frequency Operation for a Grid-Tied Inverter

In this paper, a novel controller based on the predictive technique has been introduced for a neutral-point-clamped grid-tied inverter. The aim of the developed method is to achieve a constant system switching frequency operation. The proposed method generates the voltage vector by nullifying the derivative of the cost function of a finite-set model predictive controller. The developed controller takes in action of injecting the desired grid current while a closed-loop space vector modulator regulates the capacitors voltages. A comparative study with a similar model predictive algorithm with constant switching frequency (MPC-CSF) is introduced and discussed in terms of computational complexity. Experimental results are provided to verify the performance and robustness of the new method over MPC-CSF with its ability of injecting a very low total harmonic distortion to the grid with almost unity power factor during normal and transient operations.

New voltage vector generation method for a MPC algorithm with constant switching frequency operation

This paper Introduces a novel voltage vector generation method for fixed switching frequency scheme of a finite-control set model predictive control (FS-MPC). The main feature of the proposed work is to search the optimum voltage vector responsible of minimizing the cost function. A novel algorithm based on two searching loops is introduced, the first loop optimizes the voltage vector angle while the second one defines its correspondent modulus. The optimal combination is then generated into a FS-MPC algorithm to control a 3L-NPC grid-tied inverter. A closed-loop space vector modulator takes in action of firing the switches and balancing the two DC capacitors of the inverter. The overall discrete model in (dq) frame is developed. The design of the proposed algorithm is then detailed. Simulation results verifies the effectiveness of the proposed method in fulfilling grid requirements during normal operation and against external disturbances.

MPC based on balanced positive-sequence extraction strategy for grid-tied converter control

The massive penetration of renewable energy sources in grid-tied systems has boosted the improvement of conventional control solutions to fulfill the requirements demanded during grid faults. In this paper, balanced positive-sequence reference extraction method (BPSC) is proposed with the predictive technique to control the 3L-NPC inverter under grid sags. Only positive sequence extraction is needed to inject properly the generated reference currents through MPC controller. Moreover, no additional phase-locked loop synchronization technique is required. A Second Order Generalized Integrator filter is added for voltage sequences extraction. The overall discrete system model is developed. Simulation results show the advantages of the proposed algorithm in terms of injecting a set of balanced grid current with low harmonic content during grid sags.