Controlled Voltage Source Inverter Research Articles

A control strategy of microgrid voltage source inverter based on self‐learning sliding mode control

AbstractIn island mode, voltage source inverter (VSI) supports the frequency and voltage of microgrid. After the complex load is connected, the VSI control performance is degraded, and the output voltage has deviation, negative sequence, waveform distortion and other problems, which further deteriorate the power quality of the microgrid. Different from the traditional strategy that only focus on a single problem, the strategy proposed in this paper can deal with these three power quality problems simultaneously. In this paper, a self‐learning sliding mode control strategy is proposed. First, a nonlinear smooth function is used to design an expansion observer, which can estimate the expansion state of the internal uncertainties and external disturbances of the control system. Second, the extended observer is combined with the self‐learning synovial control technology to realize the self‐learning synovial disturbance rejection control of VSI control strategy. This strategy can improve the stability of voltage control under various working conditions without precise mathematical model. The simulation results show that, compared with the traditional control strategy, this strategy has good robustness under different working conditions.

Utility Interactive Renewable Microgrid with Backup Power Functionality

This paper deals with three-phase wind-solar-photovoltaic microgrid (WPV-MG), which is capable of performing in grid-tied and island modes of operation under steady state and in DSTATCOM (Distribution Static Compensator) mode, utility-interactive and disconnecting modes under dynamic state. This microgrid (MG) uses a single VSI (Voltage Source Inverter) with reduced power converters and switches. At the DC bus of VSI, a battery bank is used to provide load leveling either at less renewable generation or at light loads. The control of VSI is also able to maintain power balance among various units and provides PQ (Power Quality) improvement in the MG under local nonlinear loads. During no renewable power generation, it works as a DSTATCOM to improve PQ and provides power factor correction and reduces voltage harmonics in PCC (Common Coupling Point) under local linear/nonlinear loads. During abnormal grid conditions, it provides power backup to local consumers under island operation. For this three-phase WPV-MG, a laboratory prototype is developed and its performance under different operations is validated through test results.

Sliding Mode Control for Single-Phase Grid-Connected Voltage Source Inverter with L and LCL Filters

This paper presents an analysis of the sliding mode control (SMC) method applied to a single-phase grid-connected voltage source inverter (VSI) with L and LCL filters. First, simulation results were presented for the L filter, and then, after some adjustments, the same theory was applied to the LCL VSI with active damping. To improve the obtained results for the SMC control, we adopted the hyperbolic tangent function and the explicit establishment of the modulation index m in the mathematical procedure to help reduce the chattering phenomena; later on, the same function was replaced by the usage of a proportional plus resonant controller to continuously improve the control system responses.

Power Decoupling Strategy for Voltage Modulated Direct Power Control of Voltage Source Inverters Connected to Weak Grids

The grid voltage modulated direct power control (GVM-DPC)-based inverter is an attractive solution to regulate the instantaneous real and reactive powers injected into power grids. However, the power coupling in GVM-DPC, i.e., interactions between the real and reactive power loops, may deteriorate its transient performance. In this paper, a power decoupling strategy for GVM-DPC is proposed based on a dynamic feedforward power compensation (DFPC) algorithm. It is proved that the proposed strategy has better decoupling performance than that of the traditional virtual impedance method (VIM). Firstly, power coupling characteristics in GVM-DPC are analyzed, indicating that the ignorance of voltage angle variation at the point of common coupling (PCC) will result in severe power coupling. Then, power coupling magnitudes are derived according to relationships among the real power, reactive power and PCC voltage. Next, the coupling magnitudes are compensated into the power control loop of the GVM-DPC for the better decoupling performance. The transient performance of inverter using DFPC is studied based on the impedance analysis. Also, the relationship between coupling magnitude and grid side impedance is studied, which indicates the limited power decoupling capability of VIM. Finally, the proposed method is validated by the simulations and a hardware-in-loop system.

A Multivector-Based Model Predictive Current Control of PMSM Drive With Enhanced Torque and Flux Response

Model predictive current control (MPCC) is a modern current control technique with the provision to include multivariable objectives and nonlinearities. In this article, a multivector-based MPCC is proposed for the speed control of a two-level voltage source inverter (VSI) fed permanent magnet synchronous motor (PMSM) drive, to improve the steady-state (SS) torque and flux response. The multivector operation is obtained using an extended control set (ECS) that utilizes two adjacent-active voltage-space vectors (VSVs) with an appropriate distribution coefficient. However, the augmentation of the control set (CS) catalyzes an undesirable increase in the computational burden. To address this limitation, a lookup table-based VSV preselection scheme based on stator current error is employed. The VSV grouping is based on the gradient of stator current error and effectively replicates the control scheme with all ECS-VSVs. Subsequently, the amplitude of the selected optimum ECS-VSV is optimized using an average error minimization technique. The optimum duration of the ECS-VSV is determined without employing space vector modulation (SVM). Hence, the complexity of the proposed scheme is minimized, and the SS torque and flux performance are enhanced without compromising the dynamic performance of the drive. The effectiveness of the proposed scheme is verified by conducting comprehensive comparisons with conventional MPCC and recently published literature.

Research and Development of Adjustable Discontinuous Pulse Width Modulation Method for Three-Phase Voltage Source Inverter

Continuous pulse width modulation (CPWM) and discontinuous pulse width modulation (DPWM) strategies are used to control voltage source inverter operation. CPWM strategies allow for the reduction of total harmonic distortion values, while DPWM strategies provide a more effective reduction in inverter switching losses. The paper is devoted to the problem of improving the three-phase voltage source inverter efficiency by a controlled transition from CPWM to DPWM. The article proposes an adjustable discontinuous pulse width modulation (ADPWM) method, which means a transition from space vector PWM (refers to CPWM strategies) to DPWM in all inverter phases when the switches’ temperatures exceed the allowable value in at least one of the inverter phases. The method flowchart is presented and an algorithm for modifying the envelope curve within one sector is given. In order to test the ADPWM method a Simulink-model of a three-phase voltage source inverter and its control system were developed. A study of the proposed ADPWM method’s efficiency in comparison with CPWM, PCDPWM and NCDPWM methods was carried out using a Simulink-model. It was established that the proposed ADPWM method provides dynamic losses reduction in the inverter switches by 2.86 times compared to CPWM and by 1.89 times compared to PCDPWM and NCDPWM methods. Power supply systems for medium and high-power AC motors provide a promising area for application of the proposed ADPWM method.

Double-Vector Model-Free Predictive Control Method for Voltage Source Inverter With Visualization Analysis

Strong parameter dependence and large current ripple are two shortcomings that obstruct the development of model predictive control for voltage source inverters (VSIs). To solve these problems, in this article,a double-vector model-free predictive control (MFPC) method for VSI with visualization analysis is proposed. First, the ultralocal model of the VSI is established and a full-order sliding model observer is designed to estimate the lumped disturbance. MFPC is achieved with enhanced parameter robustness. Then, a double-vector MFPC method is further proposed. By applying two vectors per control period, the current ripple is reduced significantly. A detailed visualization analysis method is proposed, which verifies the effectiveness of the proposed double-vector MFPC method in theory. Besides, the proposed visualization analysis method has the potential to be used to analyze the effectiveness of other types of multivector model predictive control method with different cost functions. Detailed comparative experimental studies verify the effectiveness of the proposed double-vector MFPC method.

A Contemporary Design Process for Single-Phase Voltage Source Inverter Control Systems.

This paper presents an overview of contemporary voltage source inverter control system design. Design begins with the theoretical considerations that lead to the creation of the system’s differential control law. This stage does not include scaling coefficients for the output voltage, output current, and filter inductor current. Following this, the inverter is modelled in MATLAB’s Simulink environment with an appropriate load and control system. If the resultant simulation provides satisfactory results, a hybrid system consisting of MATLAB’s Simulink and dSpace libraries with the MicroLabBox device is used to interface the simulation with an experimental hardware model in real-time. This allows the hardware plant and measuring traces to be validated. ControlDesk is used to scale the relevant coefficients. During the final stage of the design process, a microprocessor is programmed to control the inverter according to the dSpace simulation results. This requires new scaling values. Throughout every stage of the design process, too high a value of the modulation index disables the reduction of output voltage distortions. This paper details the entire design process for both single-input and multi-input control systems, explaining the scaling process and the required software. Such a modern design process ensures the shortest time between conceptualization and the final product.

Adaptive super-twisting sliding mode for DC-AC converters in very weak grids

ABSTRACT This paper presents the design of a direct-type adaptive control structure for current regulation of a single-phase grid-tied Voltage-Source Inverter (VSI) with an LCL filter under very weak grid condition. The initial content of this work was previously published, where the controller was presented and its discrete-time stability proof was provided. Power converters connected to weak grids bring challenges to the control system, since the grid can change abruptly from a strong to a very weak condition , degrading system power quality. This study evaluates the feasibility of a RMRAC-based robust adaptive Super-Twisting Sliding Mode controller for current control of VSI when the grid inductance changes substantially, from 0 mH (strong grid) to 10 mH (very weak grid). To show the benefits of the control strategy and its robustness, hardware-in-the-loop (HIL) simulation results are presented, comparing its performance with two other controllers..

Fixed-time control of voltage dynamics of three-phase voltage source inverters with LC output filter

<p>This paper puts forward a fixed-time cascade voltage control system for threephase voltage source inverters (VSIs) with LC output filter.First, a feedforward decoupling mechanism is used to elminate the dependency between the d-q parts of the control system. Then, proportional-integral (PI) regulators are used for current control in the inner loop. The current reference is provided by a novel VSI control technique developed for fixed-time voltage regulation and reference tracking. The approach suggested in this work tracks and maintains the voltage magnitude at its normalized value in a finite-time and before a maximum settling-time fixed in advance and independent of the system’s initial state. The voltage controller also maintains current stability by providing a smoother and smaller current reference. A comparative study with VSI conventional PI control for linear, nonlinear and unbalanced loads confirms the theoretical results.</p>

Adaptive Virtual Resistance for Postfault Oscillation Damping in Grid-Forming Inverters

Postfault oscillations in active power and voltage responses of grid-connected voltage source inverters (VSIs) have been reported in the literature. They are caused by nonideally tuned controllers and the implemented current limitations used for protecting the VSIs from overcurrents. These oscillations become even more significant when the VSI is connected to a weak grid, deteriorating the recovery process of the VSI. This article presents a method for damping the postfault oscillations by using an adaptive virtual resistor (VR). Even though these oscillations are observed with both grid-following and grid-forming inverters, this article focuses on droop-based grid-forming inverters. The proposed method dynamically integrates a VR into the VSI control and removes it in the normal operation mode of the VSI. This method is implemented in the synchronous reference frame, which is commonly used due to its decoupled active and reactive power control. The amount of virtual resistance used for oscillation damping is adaptive to the recovery rate of the VSI. Hence, the proposed method is robust against changes in grid strength. Finally, the performance of the method is evaluated in PSCAD/EMTDC and also experimentally validated.

Resonant and a new disturbance-observer combined control for off-grid voltage source inverter

<span lang="EN-US">This paper presents a new control strategy based on the combination between the resonant controller and the disturbance-observer (RSCDO) for stand-alone four-leg Voltage Source Inverter (VSI). The aim of the new controller is to regulate the inverter output voltage, reducing as much as possible the voltage Total Harmonic Distortion (THDv) in every load condition. The resonant controller allows to adjust the output voltage fundamental harmonic, whereas the disturbance observer is designed for compensating output voltage harmonics when non-linear loads have to be fed. The proposed control strategy has been tested using a 40 kVA three-phase 4-leg VSI and a dedicated control board, which uses the National Instruments System-on-Module sbRIO-9651. Experimental results demonstrate good behavior of the RSCDO controller to reject the disturbance in the output voltage, as well as a good dynamic response.</span>

Constrained Modulated Model Predictive Control for a Three-Phase Three-Level Voltage Source Inverter

In the last decade, model predictive control (MPC) has been widely studied in power converters, such as voltage source inverters (VSIs). Unfortunately, MPC often presents a high computational burden that limits their applicability, especially when driving multilevel inverters (MLIs) because of their higher number of switching combinations than two-level inverters. As a result, some strategies have been developed to reduce the computational complexity of MPC. One of the most relevant is the use of artificial neural networks (ANNs) to approximate the behavior of an MPC. However, ANNs require to be evaluated at bounded inputs. Otherwise, their response cannot be guaranteed to be a good approximation of the controller they learned from. Furthermore, when driving an LC-filtered VSI, the inductor current can present high peaks due to the cross-coupling effect between the inductor and the capacitor. These current peaks can cause physical damages and loss of performance of an ANN-emulated MPC. This paper presents a new constrained modulated MPC (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> PC), better suited for ANN emulation, to overcome these issues. The proposed strategy evaluates the cost function once per switching region, allowing easy and intuitive constraint inclusion. Additionally, an overmodulation stage is used to handle negative duty cycles and enhance disturbance rejection. Finally, the proposal is validated through simulations, using MATLAB-Simulink, taking into account different load conditions. Simulations show that the constrained M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> PC keeps the inductor current below its desired limit while having a good performance (low harmonic distortions and fast dynamics) even when the inverter operates near its boundaries.

An Improved Dynamic Control and Resonance Damping for Voltage Source Inverters in Microgrids Through a Novel Active Damping Technique

Microgrids have the potential to provide customers with improved power quality and reliability. However, there are technical challenges related to the operation and control of microgrids and the distributed energy source (DERs) that constitute them. Resonance caused by the low-pass filters connected to the voltage source inverters of the DERs is one of those issues in the operation of microgrids. This paper proposes a novel technique of Active Damping Algorithm (ADA) for resonance damping and improving the dynamic stability of a microgrid composed of multiple converter-interfaced DERs. The paper considers two cases; one is where the resistor and the filter capacitor are connected in parallel, and the other is where they are connected in series. The proposed active damping method is based on output current feedback with automatically calculated and adjusted damping coefficients to suppress resonance. The proposed method was verified through simulation and experimental tests carried out using PSCAD/EMTDS and an operational microgrid in Beijing, China. The results from both the experiment and simulation analysis showed that the devised method was able to achieve the intended target; where the filter resonance is effectively surpassed and the voltage and frequency dynamic responses are quite stable and within the acceptable range during large power changes in the microgrid system.

Optimized Fractional Order Based Droop Control with Improving the Flexibility of a Microgrid

This work presents an improved four-levels hierarchical control strategy for flexible microgrid based on three-phase voltage source inverters (VSIs) connected in parallel. In the proposed strategy, zero-level control is required to handle current and voltage control of VSIs. The primary-level that consists of a decentralized controller is based on a modified universal droop control by introducing a fractional-order derivative. It is used to enhance the power sharing quality during islanded mode and the desired power generation in grid-connected mode. In the secondary centralized control, the voltage and frequency deviations caused by the primary-control are restored. Also, this level includes the synchronization control loop that enables a seamless transition between both operation modes. In order to improve the flexibility of the microgrid, a sequential logic approach is proposed in the tertiary level. It is exploited to manage both the restoration and the synchronization loops at each transition, as well as the static switch (SS) which is used to connect/disconnect the microgrid to/from the main grid. The control parameters are optimized by using the self-learning particle swarm optimizer (SLPSO) algorithm. Simulations were performed to highlight the performances of the proposed hierarchical control approach compared with the well-known three-levels control scheme.

Optimized sliding surface predictive control of a voltage source inverter with improved steady-state performance

In this paper, an optimized sliding surface predictive control of a three-phase voltage source inverter is introduced. In power electronics, the model predictive control method (MPC) is broadly used and applied to a wide range of energy conversion systems. However, analyzing the stability of the MPC is not a straightforward task, and Lyapunov-based approaches are used to examine the stability characteristics in most cases. MPC is a nonlinear control technique, and the traditional frequency-domain stability tools cannot be used to examine the closed-loop stability. Therefore, a poor design of the MPC without considering the stability may worsen the system performance. Even the norm choice of the objective function leads to closed-loop instability, for example, ℓ1 norm is not a sufficient choice to guarantee the global asymptotical stability. Even though ℓ1 norm offers a low computational burden during the online optimization process, the system may suffer from closed-loop instability. For all these reasons, a stability-guaranteed objective function design procedure is proposed in this paper. The proposed objective function selection process is based on the sliding-mode control theory. The objective function is reformulated as a sliding surface function, and the switching combination that satisfies the sliding mode control stability criteria is selected as an optimum input. The mathematical concepts are experimentally validated, and the results demonstrate the potency of the proposed strategy.

Advanced single-loop discrete-time control for T-type voltage source inverter with minimum capacitor voltage ripple modulation

This research focuses on improving the performance of the voltage source inverter (VSI), which has been widely used in practical applications such as uninterruptible power supply (UPS), photovoltaic (PV) systems in standalone mode. To reduce the total harmonic distortion (THD) of the output voltage without increasing the switching frequency, the T-type three levels inverter with enhanced modulation strategy, which minimizes the voltage ripple of the two input capacitors, is employed. In addition, a new single-loop fully digital control strategy in which various advanced control techniques such as proportional-integral observer, one-step ahead minimum prediction error, and model-based current command generator is also proposed to fulfill the strict requirements of the output voltage. The proposed control strategy is capable of achieving excellent output voltage regulation and high robustness against the parameters variation with low computational cost. Real-time simulations based on hardware in the loop (HIL) platform are conducted to confirm the validity of the method.

Sensorless predictive control of voltage source inverters for renewable energies integration under unbalanced and distorted grid conditions

Sensorless predictive control of voltage source inverters for renewable energies integration under unbalanced and distorted grid conditions

A Comparison of Instantaneous and Extension p-q Methods for Current Controlled Voltage Source Inverter in Microgrid

ABSTRACT With the advancement in technology and population growth, the motivation of integrating utility grid with the microgrid through power electronic converters increases. This paper presents the control of power electronic converter connected at the grid side. This converter regulates the power flow between renewable energy system (RES), load and grid in addition to the compensation of harmonic and reactive power. The reference currents are generated to decide the power flow from RES to the load and to suppress the harmonic and reactive power in the grid making grid current sinusoidal. In this paper, the reference current generation techniques like instantaneous p-q method and extension p-q method are implemented and compared. Linear Quadratic Regulator (LQR) is used to track the reference current. The control law of LQR is designed in a simple way using simplified equivalent circuit of a three-phase grid integrated renewable energy system. The system is tested experimentally under balanced and unbalanced conditions in two different modes such as insufficient renewable power and excess renewable power. The supremacy of extension p-q method over instantaneous p-q method is proved by getting low total harmonic distortion and better power factor under balanced and unbalanced conditions experimentally.

A Novel H∞ Robust Control Strategy for Voltage Source Inverter in Microgrid

The voltage control performance of the voltage source inverter (VSI) in a microgrid may change under different load conditions. However, in the case of traditional control strategies, the robustness of VSI is insufficient. In response to the above problems, a novel robust control scheme for VSI in the microgrid based on H∞ hybrid sensitivity is proposed in this study. The grid-side interference during the VSI operation is taken as the variable, and the sensitivity function is designed to build a H∞ robust voltage controller for VSI. In addition, an adaptive virtual impedance group is designed to further improve the voltage control robustness under a variety of operation conditions. Finally, comparative simulation experiments are carried out to verify the anti-interference ability of the proposed control strategy under different working conditions.