Control Strategy Of Converter Research Articles

Improving Dynamic Performance of Low-Inertia Systems Through Eigensensitivity Optimization

An increasing penetration of renewable generation has led to reduced levels of rotational inertia and damping in the power network. The consequences are higher vulnerability to disturbances and deterioration of the dynamic response of the system. To overcome these challenges, novel converter control schemes that provide virtual inertia and damping have been introduced, which raises the question of optimal distribution of such devices throughout the network. This paper presents a comprehensive framework for performance-based allocation of virtual inertia and damping to the converter-interfaced generators in a detailed low-inertia system. This is achieved through an iterative, eigensensitivity-based optimization algorithm that determines the optimal controller gains while simultaneously preserving small-signal stability and ensuring that the damping ratio and frequency response after disturbances are kept within acceptable limits. Two conceptually different problem formulations are presented and validated on a modified version of the well known Kundur's two-area system as well as a larger 59-bus South-East Australian network.

Research on Control Strategy of Grid-side Converter of Permanent Magnet Direct-drive Wind Turbine

In order to make the permanent magnet direct-drive wind turbines stable and grid-connected operation, a double closed-loop control strategy based on grid voltage-oriented vector and current decoupling is proposed. First, the topology of the grid-side converter of the wind turbine is determined, then the mathematical model of the control strategy is derived, and the parameters of the PI regulator are calculated. Finally, the control strategy is verified by the simulation results. Through the analysis of the experimental results, it is found that the control strategy can achieve the functions of stabilizing the DC bus voltage of the fan and controlling the input power factor, ensuring the stable operation of the fan.

Artificial Intelligence-Aided Minimum Reactive Power Control for the DAB Converter Based on Harmonic Analysis Method

With the aim of reducing the reactive power for the dual-active-bridge (DAB) converter, this letter proposes an artificial intelligence (AI) aided minimum reactive power control scheme based on the harmonic analysis method. Specifically, as an advanced algorithm of the deep reinforcement learning (DRL), the deep deterministic policy gradient (DDPG) is used to train an agent off-line. During the training of DDPG algorithm, the three-phase-shift (TPS) modulation is adopted and the zero-voltage-switching (ZVS) constraints are considered. Thus, the trained agent of the DDPG which likes an implicit function, can provide optimal control strategies for the DAB converter in real-time with the minimum reactive power and soft switching performance in the continuous operation range. Finally, experimental results validate the feasibility and correctness of the proposed AI based optimized method.

Review of Electric Vehicle Converter Configurations, Control Schemes and Optimizations: Challenges and Suggestions

Electric vehicles are receiving widespread attention around the world due to their improved performance and zero carbon emissions. The effectiveness of electric vehicles depends on proper interfacing between energy storage systems and power electronics converters. However, the power delivered by energy storage systems illustrates unstable, unregulated and substantial voltage drops. To overcome these limitations, electric vehicle converters, controllers and modulation schemes are necessary to achieve a secured and reliable power transfer from energy storage systems to the electric motor. Nonetheless, electric vehicle converters and controllers have shortcomings including a large number of components, high current stress, high switching loss, slow dynamic response and computational complexity. Therefore, this review presents a detailed investigation of different electric vehicle converters highlighting topology, features, components, operation, strengths and weaknesses. Moreover, this review explores the various types of electric vehicle converter controllers and modulation techniques concerning functional capabilities, operation, benefits and drawbacks. Besides, the significance of optimization algorithms in electric vehicle converters is illustrated along with their objective functions, executions and various factors. Furthermore, this review explores the key issues and challenges of electric vehicle converters, controllers and optimizations to identify future research gaps. Finally, important and specific suggestions are delivered toward the development of an efficient converter for future sustainable electric vehicle applications.

Design and Implementation of a PV Generated Feed Forward Control Strategy of a Sepic Converter

Design and Implementation of a PV Generated Feed Forward Control Strategy of a Sepic Converter

Experimental Implementation and Validation of a Broadband LTI Energy-Maximizing Control Strategy for the Wavestar Device

This study addresses the experimental validation of a linear time-invariant (LTI) energy-maximizing control strategy for wave energy converters (WECs), applied to a 1/20 scale Wavestar WEC. To fulfill this objective, system identification routines are utilized to compute a mathematical (parametric) model of the input–output dynamics of the device, suitable for control design and implementation. With this parametric model, the so-called LiTe-Con energy-maximizing strategy, recently published in the literature, is designed, synthesized, and tested under irregular wave excitation in the wave basin at Aalborg University. Given that the LiTe-Con requires instantaneous knowledge of the wave excitation effects, estimates are provided by means of an unknown-input Kalman filter, designed in close synergy with the so-called internal model principle. For the experimental assessment, both controller and estimator are directly implemented in a real-time architecture. The performance of the LiTe-Con is evaluated in terms of energy-absorption, showing consistent results with respect to those obtained in numerical simulation, hence validating the LiTe-Con controller in a realistic real-time scenario.

Observer-Based Sliding-Mode Control for Grid-Connected Power Converters Under Unbalanced Grid Conditions

This article proposes a novel robust control strategy for three-phase power converters operated under unbalanced grid conditions. A consolidated control objective is obtained in the stationary <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha \beta$</tex-math></inline-formula> frame, which can be flexibly adjusted according to the degree of oscillation in the active and reactive powers and the balance of the three-phase current. Based on the dynamics of the converter and control objective, a control scheme in a cascaded framework is presented, in which an adaptive observer is applied to estimate the positive and negative sequences of the grid voltage. In the current tracking loop, a super-twisting algorithm current controller coupled with a super-twisting differentiator is implemented to track the current references, featuring rapid dynamics, and improved robustness. Additionally, in the voltage regulation loop, an effective composite controller is developed to regulate the dc-link voltage, where a super-twisting observer is used to estimate the load disturbance, thereby improving the performance of the converter. The experimental results are provided to confirm the effectiveness and superiority of the proposed control strategy.

Optimized Control for Modified Push-Pull Dual Active Bridge Converter to Achieve Wide ZVS Range and Low Current Stress

Compared with the traditional dual active bridge (DAB) converter, the push-pull DAB converter can reduce the number of switching devices and is suited for low voltage applications. In order to extend the zero voltage switching (ZVS) range and reduce the current stress within wide voltage range, this paper modifies output rectifier circuit, and proposes an optimized control strategies for push-pull DAB converters. Unlike conventional modulation that only utilizing the primary duty cycle and phase shift angle to adjust the power, the duty cycle for the secondary side is also utilized for the PWM modulation. With the modified PWM modulation, a control law for achieving minimum root mean square (RMS) current within the precondition of achieving wide range ZVS is proposed. The working modes under the modified PWM modulation, current and power analysis and optimized control law are analyzed in depth. Finally, the experimental results using a 1 kW prototype verify the effectiveness of the proposed converter.

Decoupling Control Optimization Based on Analysis and Compensation of Back EMF Disturbance in High Precision Current Converter

In high performance applications, LC filter usually is added to power converter for optimizing the output current ripple characteristics of the system. However, the LCL link composed of LC filter and resistor-inductance load causes the resonance, which leads to the traditional closed-loop not appropriate to be applied directly. Consequently, the active damping compensation method based on harmonic current is considered to suppress the resonance, and the suppression effects of different damping parameters on the resonant peak are compared and analyzed. Moreover, back EMF treated as a nonlinear disturbance affects the accuracy of current. Therefore, the current harmonic suppression method based on feedforward control for resonant pole-based dual buck soft-switching converter (RPDBSC) is presented, and the current suppression method of feedforward control is analyzed by the decoupling control model of bias current. This paper focuses on the improvement of the closed-loop control strategy for high precision current converters, and the effectiveness of the proposed method is verified by simulation and experiment.

Comparison of Grounding Modes of MMC-Based Flexible DC Distribution System

DC distribution system has a high power quality and power reliability. It is conducive to the access of renewable energy and battery-based storage system. There are many fault types in DC distribution system. The fault characteristics are complex and closely related to the grounding mode. The investigation on the grounding mode is of great significance. Firstly, the basic structure, the operating principle and the control strategy of Modular Multilevel Converter (MMC) are briefly introduced. Secondly, the topology of DC distribution network is presented. The fault characteristics of single-phase-to-ground fault both in AC side and DC side are analyzed. The advantages and disadvantages of these grounding modes are discussed. The results provide references for flexible DC distribution system (FDDS) design.

Statistical Tools to Evaluate the Performance of Current Control Strategies of Power Converters and Drives

The proposal of new current control techniques for power converters and electric motor drives has been one of the main research topics in the fields of power converters and drives during the past years. Usually, when evaluating and comparing the performance of current controllers, various figures of merit (FMs) are used, e.g., the mean squared error or the absolute error between the reference and the measurement. Here, it is shown that such FMs have a random nature. Nevertheless, only one result is reported in many published articles, for each FMs. Also, it is not indicated whether or not more than one trial has been performed to obtain the FM. In that case, opposite conclusions can be reached when two current controllers are compared, depending on the chosen results. In this sense, the number, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> , of experimental runs required to accurately compare any FM, is proposed in order to address this problem. Likewise, a statistical comparison procedure is introduced to evaluate the relative performance of two controllers using any FM. Also, based on the proposed statistical comparison methodology compared to other criteria, an exhaustive simulation analysis is presented comparing the accuracy of decision-making. Finally, a real data set application based on experimental results is used to illustrate the proposed procedure.

Research on Coordinated Control Strategy of AC-DC Hybrid Microgrid Bidirectional Converter

Based on the improved droop control of the bidirectional converter controlled by the DC (direct current) side power quantity, a coordinated control scheme with energy storage is proposed. Not only can the two-way converter automatically and smoothly switch in the rectification, inverter, and idle modes, but also the energy storage device can perform coordinated control in the DC side idle mode to maintain the DC bus voltage stability. Using DC side power control, there is no need to switch control strategies to meet islanding and grid-connected modes. The simulation results show that improved droop control can reduce frequent actions of power electronic devices caused by small-range fluctuations in DC bus voltage and AC (alternating current) frequency. Coordinated control can make the DC bus voltage more stable, ensure stable operation of the AC and DC hybrid microgrid, and improve system reliability.

Dynamic Stability Improvement of Power System by Means of STATCOM With Virtual Inertia

The paper investigates the application of Static Synchronous Compensator (STATCOM) with synchronverter control to enhance dynamic stability. Synchronverter is a control strategy for voltage source converters that emulates a synchronous generator, therefore providing virtual inertia. A thorough analysis of system stability with STATCOM controlled using synchronverter is presented. Furthermore, a comparison to vector control is provided. The analysis was conducted for a commonly known Single Machine Infinite Bus (SMIB) test case. The authors also compare the synchronverter and vector control performance using different mathematical tools such as eigenvalue analysis, numerical simulation, and lyapunov theory. Synchronverter algorithm improves the damping of the system, as small-signal analysis shows. The results of numerical simulations demonstrate the improvement of dynamic stability. Besides, the stability region also improves in the case of synchronverter. Finally, the paper demonstrates on the IEEE 39 bus system that the operation of STATCOM with a synchronverter control strategy is feasible and improves dynamic stability. Synchronverter brings the advantages of artificially adding inertia to the system, an essential issue in modern power systems.

Grid-Forming Converters: Control Approaches, Grid-Synchronization, and Future Trends—A Review

In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for use in wider interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the expected behaviour of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e. angle stability, fault ride-through (FRT) capabilities, and transition from islanded to grid connected mode are discussed. Perspectives on challenges and future trends are lastly shared.

Research on Operating Characteristics of Two-Terminal Flexible Direct Transmission with Improved Deadbeat Control

Traditional DC transmission technology has obvious defects in power adjustment speed, loss reduction, protection and control, etc., and it has been unable to meet the diverse needs of electricity consumption. The flexible DC transmission system not only solves the above shortcomings, but also has the advantages of independently controllable power and power supply to passive network. It is widely applied in distributed power generation by new energy, micro grid and other fields. This paper takes the two-terminal flexible DC transmission system as the research object, establishes the mathematical mode of two-terminal flexible DC transmission system, and studies the basic operation mode of the system and the voltage source converter (VSC) control strategy on this basis. Considering the problems of control delay and poor responsiveness of current closed-loop control when the traditional deadbeat control is applied to VSC converters, an improved deadbeat control strategy is proposed to solve the above-mentioned defects by predicting the current sampling at k+2 moment. Finally, the co-simulation is used to verify the correctness and reliability of the above-mentioned method.

A Time-Domain Comparator Based Skipping-Window SAR ADC

This paper presents an energy efficient successive-approximation register (SAR) analog-to-digital converter (ADC) for low-power applications. To improve the overall energy-efficiency, a skipping-window technique is used to bypass corresponding conversion steps when the input falls in a window indicated by a time-domain comparator, which can provide not only the polarity of the input, but also the amount information of the input. The time-domain comparator, which is based on the edge pursing principle, consists of delay cells, two NAND gates, two D-flip-flop register-based phase detectors and a counter. The digital characteristic of the comparator makes the design more flexible, and the comparator can achieve noise and power optimization automatically by simply adjusting the delay cell number. An energy efficient digital-to-analog converter (DAC) control scheme suitable for the skipping window technique is also developed to reduce the switching energy during SAR conversion. Together with the skipping-window technique, the linearity and the power consumption of the SAR ADC are improved. The impact of different window sizes on comparison cycles, DAC switching energy and the overall energy efficiency is analyzed. Simulation results show that the proposed skipping-window technique can improve the overall energy-efficiency of the SAR ADC, as well as the linearity, and the optimized window size for the overall energy efficiency will vary with the DAC switching energy.

Internal Model Power Synchronization Control of a PV-Based Voltage-Source Converter in Weak-Grid and Islanded Conditions

The power synchronization control strategy for grid-connected voltage-source converters (VSCs) provides an operation similar to synchronous machines. It is able to avoid the instability caused by a standard phase-locked loop in integration into weak grids. However, the non-minimum phase phenomenon in the developed dynamics places a fundamental limitation on the ac system's stability. This paper proposes a one-degree-of-freedom internal-model-based control methodology. It introduces a control approach to incorporate the dynamics of the system's nominal model in the control structure. It also rectifies the unwanted effects of the right-half plane zeros. The explicit incorporation of the model enhances the tracking capabilities of the controller in a PV-based VSC. Besides, this article shows that a single-loop of control will suffice to regulate active and reactive power. Validating results are generated via a hardware-in-the-loop system based on a Xilinx Zynq-7000 SoC field-programmable gate array (FPGA). Furthermore, experimental results are conducted for low-power prototyping to examine the satisfactory performance of the proposed control architecture.

Neural network based MPPT control with reconfigured quadratic boost converter for fuel cell application

An artificial neural network (ANN) based maximum power point tracking (MPPT) technique for proton exchange membrane fuel cell (PEMFC) is analysed and proposed in this paper. The proposed ANN technique employs Radial basis function network (RBFN) based MPPT strategy to extract the maximum available power from fuel cell in different operating condition. In order to achieve high voltage rating, a novel high step up DC/DC converter is incorporated in the proposed configuration. To validate the performance of the proposed configuration, the result is compared with different DC/DC converter and MPPT control strategy. The proposed system is simulated in MATLAB/Simulink platform to analyse the performance of the system.

Modeling and Shaping of the DC-Side Admittance of a Modular Multilevel Converter Under Closed-Loop Voltage Control

The dc-side admittance of a modular multilevel converter can be used in assessing the stability of the dc system by means of impedance-based stability criteria. An accurate mathematical representation of the small-signal admittance can be given using harmonic linearization. To this end, the effect of the internal dynamics of the converter, e.g., the circulating current, the converter control scheme, and the controller parameters on the admittance of the converter should be analyzed. In this article, a linear analytical model for the dc-side admittance of the converter is derived based on a combination of harmonic linearization and frequency-domain representation, which incorporates different control schemes. Moreover, an admittance model is given for the closed-loop voltage control mode of the converter, where the ideal insertion indices are applied. To this end, the impact of an arm-balancing controller and its parameters on the dc-side admittance of the converter is investigated. Finally, experiments are carried out on a down-scaled prototype to validate the accuracy of the analytical model.

Connection and Control Strategy of PV Converter Integrated into Railway Traction Power Supply System

In order to supply the single-phase locomotive load and mitigate the negative sequence current, this paper develops a V/V transformer-based connection and control strategy of three-phase photovoltaic (PV) converters integrated into railway traction power supply systems. In this V/V transformer-based connection, the two-phase traction voltage is converted into the three-phase voltage. This approach can offer a common low voltage AC bus, which is more convenient for more access to three-phase PV converters. Based on this V/V transformer-based connection, an individual phase current control strategy with the hybrid current reference is fully designed. In this control strategy, the current reference, containing two parts, is generated. One is the asymmetrical part for powering the single-phase locomotive load and mitigating the negative sequence current. The other is the symmetrical part for feeding the surplus power back to the utility grid. Then, each phase current replaces the dual-sequence current to be controlled to track the corresponding phase current reference. Consequently, PV converters can flexibly inject the symmetrical and asymmetrical currents without the dual-sequence extraction for a simpler implementation. Finally, the effectiveness of the developed connection and control strategy is validated by the simulation studies.