Outer Voltage Control Loop Research Articles

Active Disturbance Rejection Control Combined with Improved Model Predictive Control for Large-Capacity Hybrid Energy Storage Systems in DC Microgrids

In DC microgrids, a large-capacity hybrid energy storage system (HESS) is introduced to eliminate variable fluctuations of distributed source powers and load powers. Aiming at improving disturbance immunity and decreasing adjustment time, this paper proposes active disturbance rejection control (ADRC) combined with improved MPC for n + 1 parallel converters of large-capacity hybrid energy storage systems. ADRC is utilized in outer voltage control loops, and improved MPC is employed in inner current control loops of n battery converters. Droop control is adopted to obtain power distribution between n battery converters, and a DC bus voltage compensator is used to compensate voltage deviations and maintain constant DC bus voltage. The low-pass filter (LPF) is adopted to obtain high-frequency power as the reference for the supercapacitor converter, ADRC is also utilized in the outer power control loop, and MPC is employed in the inner current control loop. Compared with traditional observers, the voltage expansion state observer of the proposed ADRC control is independent of the system model and parameters and consequently has strong disturbance immunity, and significantly reduces voltage overshoots during power fluctuations. The MPC-based inner current control loops of n + 1 converters accelerate current response speed and significantly decrease switching losses. Simulation and experimental results indicate that utilizing the proposed control strategies, large-capacity HESS has stronger anti-interference ability, shorter regulation time, smaller switching loss, and simultaneously maintains the stability of the DC bus voltage.

Decoupled average model-based sliding mode current control of LC-filtered inverters in rotating frame

Recently, LC-filtered inverter has been a trendy research object towards the efficient exploitation and use of sustainable energy sources. The nested control scheme of the system includes a voltage control loop and a current one, which can be developed on the rotating frame. The conventional PI controller is employed for the outer loop voltage controller and it performs effectively. However, the PI current controller introduces delay to the system and overshoot occurs frequently due to the influence of nonlinear load. This paper proposes a novel method of decoupled average model-based sliding mode current control performing faster response and better tracking performance of the desired current. Besides, the comprehensive model of the nonlinear load is applied to examine the robustness of the proposed control scheme. This method is developed on the average model, fixed-frequency PWM can therefore be used to modulate the inverter. The novel method is compared with a conventional PI current controller. Offline simulation on MATLAB/Simulink and real-time simulation using Opal-RT experimental platform prove the advantages of the proposed current controller considering critical scenarios with the nonlinear load.

Sensorless integral backstepping control of a single-stage photovoltaic system connected to the single-phase grid using a multi-level NPC inverter with an LCL filter

In this paper, the control problem of a single-phase, grid-tied photovoltaic (PV) system consisting of a three-level neutral point clamped (3L-NPC) inverter and an LCL filter is considered. The proposed controller aims at achieving threefold control objectives: (i) guaranteeing the PF correction (PFC) objective by enforcing the current of the grid to track a sinusoidal reference signal in phase with the grid voltage; (ii) controlling the DC bus capacitor voltages to follow a reference value provided by the maximum power point tracking (MPPT) algorithm to achieve the maximum available PV power; (iii) ensuring a balanced power exchange through the regulation of the two input capacitor voltages to the same values. To achieve these aims, a multi-loop nonlinear controller is synthesized, including three control loops, specifically the inner PFC requirement loop designed through the integral backstepping approach, the outer voltage control loop formulated using a filtered proportional integral (PI) regulator, and the power balance loop built up using a PI regulator. The control problem under consideration entails several difficulties as the existence of numerous state variables that are inaccessible to measurements. This challenge is addressed by adding a high-gain observer to estimate the current of the solar panels. The performance of the proposed controller against climate changes is demonstrated via numerical simulations using the MATLAB/Simulink platform.

Improving the Synchronous Stability Control Strategy of Virtual Synchronous Generators

The microgrid dominated by new energy has small inertia and weak damping, while a virtual synchronous generator (VSG) is an effective means to enhance inertia and damping to improve the stability of the microgrid. However, it also introduces power angle oscillation problems similar to that of synchronous machines. This paper is based on the current inner loop and voltage outer loop control of the three-phase LCL inverter. It introduces the virtual inertia and damping coefficient of the virtual synchronous generator, achieving a good grid connection effect. While a significant change in active power occurs, the system can lose synchronization and stability. Therefore, the paper proposes an improved synchronization stability control strategy for virtual synchronous generators by introducing a frequency-proportional integral feed-forward link in the reactive power loop to suppress power angle oscillation and improve system synchronization stability. Theoretical analysis was conducted on the synchronous stability of the improved control strategy in case of sudden changes in active power load. The effectiveness of the strategy was verified through simulation.

Design of PFC converter with stand-alone inverter for microgrid applications

The proposed topology is used to connect a single-phase and a three-phase renewable energy resources to the grid. The single-phase source is coupled to a single-phase PFC boost converter, which enhances the input PF utilizing two feedback loops: outer voltage loop control and inner current loop control. The basic hightlight is to study the PFC converter in microgrid application where renewables are integrated with the systems. The basic aim is to observe the overall performance of the converters with various disturbances such as load variations, etc. Here, the single-phase and three-phase stand-alone inverter is used to get the the output of the PFC boost converters. A symmetrical sinusoidal output voltage waveform should be produced and maintained by the inverter. The three-phase source is also coupled to a PFC buck converter, which enhances the input PF utilizing two feedback loops: outer voltage loop control and inner current loop control. The single-phase stand-alone inverter receives the output of the PFC buck converter. The transformer receives the outputs of both the inverters as it is a multi-winding high-frequency transformer and offers isolation between the grid and the source. The pulses for the switches in the single-phase inverter coupled stand-alone system were generated using a sinusoidal pulse width modulation approach. Both the PI controllers are implemented to maintain the regulations. The simulation results are achieved by varying the load, maintaining a constant voltage, and observing whether the current varies as the load changes. It also provides the efficacy of the study.

Small Signal Stability of Phase Locked Loop Based Current-Controlled Inverter in 100% Inverter-Based System

Present-day inverter control mainly employs phase locked loop (PLL) based current-controlled strategies. With growing penetration of inverter-based resources in power systems, stability issues associated with inverter control have been reported in weak grid scenarios. A natural and fundamental question to answer is whether this instability is inherent to PLL based current-controlled strategies or it can be resolved with proper frequency and voltage control loop design. This paper addresses this question by analyzing the small signal stability of the current-controlled inverter in island and weak grid operation scenarios. Torque analysis, which is well known for synchronous machine stability analysis, is adapted to uncover the stability enhancement effect of properly designed and parameterized outer-loop controls. Different types of loads are considered and investigated in the study. Electromagnetic transient (EMT) simulations are conducted on a test network to validate the analytical results. The results from this study indicate that with appropriate outer-loop voltage and frequency control design, the PLL based current-controlled inverter can be small signal stable with high inverter penetration, even in a 100% inverter-based system.

Super Twisting Control Design for HSPMSG Voltage Stabilization Based on Disturbance Observation Compensation

For the purpose to solve the problem of poor anti-disturbance performance and large voltage fluctuations of the high-speed permanent magnet synchronous generator (HSPMSG), this paper proposes a compound control method. The method combines the super-twisting (STW) voltage outer loop controller with the super-twisting disturbance observer (STWO). First, in order to overcome the problem of poor anti-disturbance performance under the conventional PI control method, a voltage outer loop controller based on the STW is designed. Second, a load disturbance observer based on the STW is developed for further improving the anti-load disturbance capability of HSPMSG. The estimated value of the disturbance observer is feedforward into the STW controller as a compensation term. Finally, both simulations and experiments are performed on a HSPMSG to verify the effectiveness of the proposed control strategy.

Research on the Control Method of Micro-gas Turbine Power Supply Modulation System for Primary Energy in Electromagnetic Launch Technology

Combining the advantages of micro-gas turbine generation system (MTGS) and power converters, moreover, comparing the various design ideas for the primary energy of electromagnetic launch technology, we design a primary energy to meet the reliability, stability, safety and portable of electromagnetic launch technology. We discuss the control ideas and methods to be adopted in this system, the first step is to output a stable DC bus voltage through rectification, and then to step up the voltage through a series resonant converter to achieve the desired output voltage. Through theoretical analysis and simulation, it reveals the effect of this load on the performance of MTGS, and the parameter optimization design is combined with the voltage outer loop control strategy to achieve the purpose of outputting stable voltage. We used a ARM controller to implement the control of this system to measure the response curve of the system under different input commands. According to the theoretical analysis, numerical simulation and experimental results, the control strategy designed in this paper can better meet the performance index requirements of micro-gas turbine engine (MTE) and power converter as well as the control method is simple and easy to implement in engineering. The results of this paper confirm the feasibility of this primary energy application for electromagnetic launch technology.

The Design and Processor-In-The-Loop Implementation of a Super-Twisting Control Algorithm Based on a Luenberger Observer for a Seamless Transition between Grid-Connected and Stand-Alone Modes in Microgrids

The abrupt transfer from grid-connected (GC) to stand-alone (SA) operation modes is one of the major issues that may threaten the stability of a distributed generation (DG) system. Furthermore, if the islanding mode happens, it is vital to take into consideration the load voltages or load current waveforms as soon as feasible. This paper develops an advanced control technique based on a super-twisting sliding mode controller (ST-SMC) for a three-phase inverter operating in both the GC and SA modes. This control scheme is proposed to ensure a smooth transition from the GC to SA mode and enhance the load voltage waveforms under the islanding mode. In addition, to minimize the operational costs of the system and the complexity of the studied model, a digital Luenberger observer (DLO) with a proper design is adopted for estimating the inverter-side current. The control scheme of the whole system switches between a current control mode during the GC mode and a voltage control mode during the SA mode. The super-twisting control algorithm is applied to the outer voltage control loop involved in the cascaded voltage/current control scheme in the SA mode. Simulation tests of a three-phase inverter are performed for the purpose of assessing the suggested control performance by using the PowerSim (PSIM) software and comparing it with a classical PI controller. Furthermore, a processor-in-the-loop (PIL) implementation in a DSP board TMS32F28335 while debugging is conducted using code composer studio 6.2.0. The obtained results show efficient control properties, such as a smooth transition among the microgrid (MG) operating modes, as well as effectiveness and robustness during both the GC and SA operation modes.

Enhancement of Voltage Regulation and Transient Stability of Six Phase STATCOM using Decoupled Current Control Strategy

STATCOM is used in a six-phase transmission system to improve voltage regulation and transient stability. Six-phase pulse width modulated voltage source converters are used in the design of STATCOM, and a decoupled current control approach is used to operate it. The voltage references used by the voltage source converters' pulse generator are produced using a decoupled control method. This control technique allows for the decoupling of the coupling effect or the dependence of the d and q currents on one another, which improves system performance under unusual and abnormal conditions. By using the pulse generator of voltage source converters, the inner current control loop and the outer voltage control loop are intended to produce the necessary reference voltages. Simulation results are presented using MATLAB/SIMULINK to check efficacy of proposed six phase STATCOM.

Design and Implementation of Hybrid Controller for Dynamic Power Management in a DC Microgrid

Nowadays more and more devices and appliances are operated with electricity, thus the electrical crisis is increasing exponentially day by day. In order to avoid the occurrence of electricity crisis, various power generation resources are used at the utility side to enhance the power generation to meet the consumers’ demand for electricity. Hence, a suitable control scheme has to be implemented at the microgrid to reduce the electrical fluctuation, power loss and manage the power quality. The Adaptive Proportional Integral Voltage Controller (APIVC) and hysteresis current controller (HCC) are integrated to enhance the quality of power generated. The electrical fluctuation is reduced by the proposed efficient hybrid parallel source controller model for DC Microgrid. The proposed model exerts decentralized control, which is an advanced droop control where communication is not required. The outer voltage control loop and inner current control loop provide faster control to maintain the grid voltage constant. The grid voltage is set as the reference value and the actual value is sensed to generate error value, which sets the reference value of current. The error signal is processed to provide switching signals for the converters. The performance analysis and simulation results show that the proposed mechanism performed better than the conventional methods such as Hysteresis Band Current Controller (HBCC) with Pulse Width Modulation (PWM) and Proportional Integral Voltage Controller (PIVC) with Hysteresis Current Controller (HCC), in terms of the electrical fluctuation, power loss and manage the power quality in the microgrid.

AC-DC Fuzzy Linear Active Disturbance Rejection Control Strategy of Front Stage of Bidirectional Converter Based on V2G

Aiming at the problems of output voltage fluctuation and current total harmonic distortion (THD) in the front stage totem-pole bridgeless PFC of two-stage V2G (Vehicle to Grid) vehicle-mounted bi-directional converter, a fuzzy linear active disturbance rejection control strategy for V2G front-stage AC-DC power conversion system is proposed. Firstly, the topological working mode of the totem-pole bridgeless PFC is analyzed, and the mathematical model is established. Combined with the system model and the linear active disturbance rejection theory, a double closed-loop controller is designed with the second-order linear active disturbance rejection control as the voltage outer loop and PI control as the current inner loop. The controller can realize self-adaptive tuning of the proportional gain coefficient of the active disturbance rejection controller through fuzzy reasoning and realize self-adaptive control. Simulation and experimental results show that this method can better solve the problems of slow system response and high total harmonic distortion rate of input current and effectively improve the system’s robustness.

Improved Performance in the Control of DC-DC Three-Phase Power Electronic Converter Using Fractional-Order SMC and Synergetic Controllers and RL-TD3 Agent

In this article, starting from a benchmark represented by a Direct Current-to-Direct Current (DC-DC) three-phase power electronic converter used as an interface and interconnection between the grid and a DC microgrid, we compare the performances of a series of control structures—starting with the classical proportional integrator (PI) type and continuing with more advanced ones, such as sliding mode control (SMC), integer-order synergetic, and fractional-order (FO) controllers—in terms of maintaining the constant DC voltage of the DC microgrid. We present the topology and the mathematical modeling using differential equations and transfer functions of the DC-DC three-phase power electronic converter that provides the interface between the grid and a DC microgrid. The main task of the presented control systems is to maintain the DC voltage supplied to the microgrid at an imposed constant value, regardless of the total value of the current absorbed by the consumers connected to the DC microgrid. We present the elements of fractional calculus that were used to synthesize a first set of FO PI, FO tilt-integral-derivative (TID), and FO lead-lag controllers with Matlab R2021b and the Fractional-order Modeling and Control (FOMCON) toolbox, and these controllers significantly improved the control system performance of the DC-DC three-phase power electronic converter compared to classical PI controllers. The next set of proposed and synthesized controllers were based on SMC, together with its more general and flexible synergetic control variant, and both integer-order and FO controllers were developed. The proposed control structures are cascade control structures combining the SMC properties of robustness and control over nonlinear systems for the outer voltage control loop with the use of properly tuned synergetic controllers to obtain faster response time for the inner current control loop. To achieve superior performance, this type of cascade control also used a properly trained reinforcement learning-twin delayed deep deterministic policy gradient (RL-TD3) agent, which provides correction signals overlapping with the command signals of the current and voltage controllers. We present the Matlab/Simulink R2021b implementations of the synthesized controllers and the RL-TD3 agent, along with the results of numerical simulations performed for the comparison of the performance of the control structures.

Linear Active Disturbance Rejection Control of New Double Full-Bridge ZVZCS Converter for Beam Supply

In view of the unstable output voltage of the new double full bridge ZVZCS converter of beam power supply when it is disturbed by external disturbance or internal parameter change in practical application, combined with the characteristics of the beam power supply system model, this paper proposes a beam power supply control strategy based on linear active disturbance rejection control (LADRC). Firstly, the working principle of the new double full bridge ZVZCS converter of beam power supply is analyzed, and considering the parasitic parameters and effective duty cycle of circuit components, the small signal equivalent model under two working modes is established. Secondly, combined with the system model and linear active disturbance rejection control theory, a double closed-loop controller is designed, which takes the second-order LADRC as the outer voltage loop and PI control as the inner current loop. Finally, the simulation and experimental results show that the designed controller has stronger robustness, anti-interference and better dynamic response speed than the traditional double closed-loop PI controller in load switching and dual-mode switching, and has a good development prospect.

Energy management and nonlinear control strategy of hybrid energy storage system for electric vehicle

The energy management of hybrid energy storage system (HESS) and the nonlinear control strategy of the interface circuit are studied in this paper. In order to realize the reasonable distribution of load power between the battery and the supercapacitor, the energy management strategies (EMS) are used to decompose the load power required for electric vehicles under different driving cycle conditions, aiming at choosing an optimal power allocation strategy. Based on the local modeling, the global mathematical model of two bidirectional DC/DC converters under the HESS with active control topology is established, and the nonlinear control strategies are studied for the interface circuit and then the target controller is designed to realize the EMS. The current inner loop controller based on synergetic control theory is to track the battery current and supercapacitor current. The voltage outer loop controller takes power conservation as the criterion and adds the DC bus voltage PI controller to construct the Lyapunov function, where the connection between the DC bus voltage stability and the supercapacitor reference current is established. The supercapacitor replaces the battery to overcome the fluctuation of the DC bus voltage caused by the sudden change of the load power, maintaining the stability of the entire control system, and at the same time the case that the power allocated by the supercapacitor is consistent with the EMS is ensured. Two typical vehicle driving cycle conditions, IM240 and ECE-15, are selected to test the effectiveness of the current controller and the voltage controller. The experimental platform of a small-capacity HESS is built to conduct experimental verification. The simulation and experimental results show that the nonlinear controllers are feasible and have strong robustness and good adaptability under different driving conditions.

A Probabilistic Framework for the Robust Stability and Performance Analysis of Grid-Tied Voltage Source Converters

This paper proposed a probabilistic framework that could be used for the sensitivity assessment of grid-connected voltage source converters (VSCs), where uncertainties in the grid short circuit ratio (SCR) and operating point conditions, as well as control-loop interactions, were considered. The proposed method tried to broaden the available knowledge on the small-signal stability analysis of VSCs and provide a probabilistic point of view of this subject. It considered the probability of different operational conditions in order to obtain less conservatism and more accurate results. Based on uncertain inputs and the employed stability model, the proposed model produced statistical distributions of the critical mode and its damping factor and ratio, which were not accessible by existing deterministic methods. Crucial statistical information measures how much system stability and performance are maintained or changed over the system uncertainties and disturbances, as well as provides a clear insight into the system stability problem. For instance, as concluded in this paper, for the conventional control system design, fast dynamic parts of a VSC, such as the current controller and control delay, significantly impact the minimum damping ratio. Furthermore, slow dynamic parts, such as outer voltage control loops and the synchronization block, influence the maximum damping factor. For strong grids, the AC voltage magnitude controller (AVC) significantly impacts the maximum damping factor due to its lower bandwidth among all control loops. For weak grids, the damping factor of the critical mode is highly affected by interactions between the VSC, the power grid, and different control loops due to the synchronization mechanism. The other contributions of this paper were the introduction of robust stability and performance definitions and indices; explanations of the pros and cons of probabilistic assessment methods and their applicability; interpretation of the obtained results; and, finally, a link was provided between system stability and reliability, which will be crucial for future power system design.

Atom Search Optimized FOPI Controller of the DC–DC SEPIC Model with Matignon’s Theorem Stability Analysis

In this work, the design and implementation of an optimized fractional-order proportional-integral (FOPI) controller for a DC–DC single-ended primary inductance converter (SEPIC) is considered to fulfill the application of power factor correction and voltage regulation along with improved robustness, efficiency, and performance characteristics of the converter. An Atom Search Optimization technique is proposed to optimize the parameters of the controller in the outer voltage and inner current control loop of the DC–DC SEPIC model. The model is developed using MATLAB –Simulink, and the simulation analysis is carried out at rated load, setpoint tracking, source voltage, and load variation. The effectiveness of the proposed atom search optimized FOPI controller is verified by comparing its performance with the particle swarm optimized FOPI controller. Furthermore, Matignon’s theorem-based closed-loop stability analysis is carried out for the tuned values of the FOPI controller with the SEPIC model. According to the simulation results, the proposed technique provides a more dominant dynamic response, an improved efficiency, and a power factor than the particle swarm-tuned controller of the SEPIC model. In addition, better voltage regulation is achieved in the case of source voltage and load variation. Furthermore, the closed-loop stability analysis reveals that the proposed Atom Search Optimization-tuned FOPI controller performance is more stable than the particle swarm optimization method. The hardware-in-the-loop implementation is carried out to validate the proposed controller in real-time.

Design of super-twisting algorithm control and observer for three-phase inverter in standalone operation

This paper develops a new control algorithm of a distributed generation system in the standalone operation. Behaviour of three-phase voltage source inverter is investigated and the guidelines for tuning the control parameters are presented. Based on Super-Twisting algorithm, the proposed controller guarantees the load voltage performance under different types of loads. The proposed controller is established for an inner-loop current controller and an outer-loop voltage controller in a dual control scheme. The proposed scheme is very simple, thus tuning control parameters is easy and the computational burden of the controllers is low. In order to validate the load current of the proposed system feasibility, a reduced-order observer is adopted. The simulation results indicate a more reliable and efficient performance compared to the standard sliding control and the adaptive control.

Comprehensive Modeling, Analysis, and Comparison of State-Space and Admittance Models of PLL-Based Grid-Following Inverters Considering Different Outer Control Modes

The phase-locked loop (PLL)-based grid-following inverters have been widely used as the interface between renewable energy resources (e.g., wind and solar power) and the utility grid. In addition to the basic PLL and inner current control loop that are responsible for tracking the phase angle of the point of common coupling (PCC) voltage and regulating the current injection according to the estimated phase angle, respectively, these grid-following inverters are commonly also equipped with outer power and voltage control loops. Some examples are outer active power/dc-link voltage control loops and outer reactive power/PCC voltage control loops, which provide the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -axis current references for the inner current control loop. However, the existing literature generally focuses on specific outer control modes within a specific small-signal modeling framework, e.g., state-space or admittance modeling framework. In addition, different research groups usually repeatedly derive these small-signal models without a systematic modeling and analysis procedure. Furthermore, the systematic and comprehensive comparative modal and admittance analysis of these outer control modes is still missing in the literature. To avoid reinventing the wheel, provide the small-signal stability analysis community with handy both state-space and admittance models, and investigate the effects of various outer control loops on the small-signal models, this article presents a comprehensive and systematic small-signal modeling, analysis, and comparison framework of the PLL-based grid-following inverters, where the state-space and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -domain admittance models of eleven control modes are derived step by step in a completely followable style. All of these derived small-signal models are verified by the time-domain step responses and frequency-domain <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -domain admittance measurement. The insights into the effects of these outer control modes on the poles and residues of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -domain admittance model are also explored, which help to understand how these outer control modes contribute to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -domain admittance shaping.

Virtual Conductance Based Cascade Voltage Controller for VSCs in Islanded Operation Mode

Voltage source converters have become the main enabler for the integration of distributed energy resources in microgrids. In the case of islanded operation, these devices normally set the amplitude and frequency of the network voltage by means of a cascade controller composed of an outer voltage control loop and an inner current control loop. Several strategies to compute the gains of both control loops have been proposed in the literature in order to obtain a fast and decoupled response of the voltages at the point of common coupling. This paper proposes an alternative and simple methodology based on the introduction of a virtual conductance in the classic cascade control. This strategy allows to design each control loop independently, obtaining a closed-loop response of a first-order system. In this way, the gains of each control loop are easily derived from the parameters of the LC coupling filter and the desired closed-loop time constants. Furthermore, a state observer is included in the controller to estimate the inductor current of the LC filter in order to reduce the number of required measurements. A laboratory testbed is used to validate and compare the proposed controller. The experimental results demonstrate the effectiveness of the proposal both in steady-state and transient regimes.