Voltage Control Loop Research Articles

Application of Fractional Order PI/PID Voltage Controllers for Three-Phase Voltage Source Inverter with Dynamic Load

Purpose: With the depletion of fossil fuels, the landscape of electrical energy generation has witnessed a transformative shift towards alternative sources such as fuel cells and photovoltaic (PV) systems that produce direct current (DC) electricity. In the realm of distributed power generation, three-phase voltage source inverters (VSIs) are extensively utilized for converting energy from DC sources to alternating current (AC) for the grid or loads. The pivotal objective of this study is to investigate and compare the performance of fractional-order Proportional-Integral (PI) and Proportional-Integral-Derivative (PID) control structures against their integer-order counterparts in the voltage control loop of a VSI connected to a dynamic load system.
 Materials and Methods: The research employs frequency response analysis to design and implement both fractional-order PI/PID and integer-order PI/PID control structures for the voltage control loop. The simulation of the controlled system is conducted using MATLAB/Simulink, considering two distinct test scenarios – unstable and stable dynamic load conditions. The primary focus is on analyzing the inverter output voltage in the d-q axis under these scenarios.
 Findings: The analysis of the simulation results reveals noteworthy distinctions between the fractional-order and integer-order PI/PID controllers in the context of controlling the inverter system with dynamic loads. These findings shed light on the advantages of employing fractional order controllers, particularly in dynamic load scenarios, showcasing superior performance in comparison to their integer-order counterparts.
 Implications to Theory, Practice and Policy: The study's outcomes hold significant implications for the theory and practice of voltage control in distributed power generation systems. The superiority of fractional-order PI controllers underscores their potential for enhancing power quality, especially in systems with unstable and time-varying dynamic loads. These insights can inform the development of more effective control strategies for voltage source inverters, influencing both theoretical frameworks and practical applications. Policymakers may consider these findings when formulating regulations and incentives to promote the adoption of advanced control strategies in the evolving landscape of electrical energy generation.

An Optimal Switching Sequence Model Predictive Control Scheme for the 3L-NPC Converter with Output LC Filter

In some applications of microgrids and distributed generation, it is necessary to feed islanded or stand-alone loads with high-quality voltage to provide low total harmonic distortion (THD). To fulfil these demands, an LC filter is usuallyconnected to the output terminals of power electronics converters. A cascaded voltage and current control loop with pulse-width modulation schemes are used to regulate the voltages and currents in these systems. However, these strategies have some drawbacks, particularly when multiple-input–multiple-output plants (MIMO) are controlled using single-input–single-output (SISO) design methods. This methodology usually produces a sluggish transient response and cross–coupling between different control loops and state variables. In this paper, a model predictive control (MPC) strategy based on the concept of optimal switching sequences (OSS) is designed to control voltage and current in an LC filter connected to a three-level neutral-point clamped converter. The strategy solves an optimisation problem to achieve control of the LC filter variables, i.e., currents and output voltages. Hardware-in-the-loop (HIL) results are obtained to validate the feasibility of the proposed strategy, using a PLECS–RT HIL platform and a dSPACE Microlab Box controller. In addition to the good dynamic performance of the proposed OSS–MPC, it is demonstrated using HIL results that the control algorithm is capable of obtaining low total harmonic distortion (THD) in the output voltage for different operating conditions.

Operation of DR–HVdc-Connected Grid-Forming Wind Turbine Converters Using Robust Loop-Shaping Controllers

Off-shore wind power plants can be connected to the on-shore grid using diode rectifier HVdc links. As diode rectifiers are passive converters, off-shore WPPs require grid-forming capability. This paper shows how to improve the WTG dynamic response and the voltage and current harmonic rejection by using H∞-based controllers. The paper explains how to synthesise three different H∞ voltage controllers: the first is a single-loop H∞ controller, the second is a cascaded H∞ controller and the third is a proportional–resonant controller that is optimised using H∞ synthesis. The three H∞-based controllers improve the performance and the robustness obtained with a benchmark case PR controller tuned using the root locus technique. All the controllers are designed in continuous time and implemented in discrete time, applying bilinear discretisation with a sampling rate of 0.25 ms. Detailed PSCAD simulations validate the improvement of the performance and robustness, as well as an improvement in the harmonic rejection. The single H∞ controller shows the best combined characteristics of all tried controllers, at the expense of losing the separation between voltage and current control loops.

Seamless disconnection and reconnection transients for Micro-Grids

The increasing penetration of generation power plants based on renewable energies in the electrical networks has boosted the number of systems connected to the grid. In this scenario the micro-grid, providing advanced functionalities to improve the stability and operation of the network, have become very popular. In this paper a control strategy for the micro-grid management is presented. The proposed system improves the performance of micro-grids and its interaction with the main network, or also with other micro-grids, under grid voltage transients. This technique gives rise to a simple and robust control scheme to ensure the appropriate micro-grid disconnection from the main grid. In the following, the control algorithm for the intelligent connection agent will be shown, and the grid synchronization system, as well as the voltage and current control loops will be detailed. Finally simulation results obtained using the MATLAB/Simulink & Psim platform will be presented and discussed

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.

Modelling, control design, and analysis of the inner control's loops intended for single‐phase voltage‐controlled inverter‐based microgrid

AbstractIn voltage‐controlled voltage source inverters (VSIs)‐based microgrids (MGs), the inner control is of prime interest task for guaranteeing safe and stable operation. In this paper, an in‐depth investigation of the modelling, control design, and analysis of the voltage and current inner control loops intended for single‐phase voltage‐controlled VSIs is established. The main objective of this work is to provide a comprehensive study of the mathematical modelling, control design, and performance evaluation of the inner control's loops considering different proportional‐integral (PI) controller types with and without compensation, and to determine the optimal scheme that can offer better performance in terms of implementation simplicity, robustness, and transient and steady‐state responses. Thus, the mathematical closed‐loop models of designed outer voltage and inner current control schemes based on PI, P, and feedforward controllers with and without compensation are, first, derived. Following this, a systematic and effective control design for tuning the different PI controllers’ parameters is proposed. Furthermore, an analysis revealing the performance of the designed voltage and current control schemes is provided. This analysis enables us to choose a P controller and PI feedforward controller for the current control loop and the voltage control loop, respectively. The chosen P and PI controllers should be simple; meanwhile, they should offer a wide bandwidth. A simulation study is carried out in MATLAB/Simulink software to assess the performance of the adopted inner control scheme for both linear and non‐linear loads. In addition, an experimental setup, based on a TMS320F2837xD μC, of a single‐phase VSI supplying linear and non‐linear loads is built to verify the effectiveness and the robustness of the adopted inner controller. The results demonstrated: (1) the necessity of introducing the compensation term, which is responsible for offering control improvement against voltage perturbation, (2) the high tracking performance of the chosen controller in terms of dynamic and steady‐state responses as well as its simplicity of implementation.

Enhancing Wind Power Plant Integration Using a T-NPC Converter with DR-HVDC Transmission Systems

This paper introduces an innovative control technique tailored for the seamless integration of wind power plants (WPP) into onshore grids. Our approach utilizes Diode-Rectifier-based High Voltage Direct Current (HVDC) system and incorporates a T-type Neutral Point Clamped (T-NPC) converter on the HVDC back, providing solutions for current challenges in wind power integration. While the use of Diode Rectifier (DR) as the Offshore converter for HVDC transmission has proven advantageous in terms of cost and loss reduction, it also introduces new challenges, such as maintaining AC grid voltage and frequency on the Offshore side and achieving the synchronization of the wind energy conversion system (WECS). This paper proposes a comprehensive control strategy to adresse these challenges by developing frequency and AC voltage control loops, ensuring the synchronization of WECS with the DR AC side. A notable distinctive feature of our proposed technique is to ensure the regulation of the DC voltage of the HVDC cable, accomplished through the onshore inverter. This crucial aspect ensures stable and efficient power transmission from wind power plants to the onshore grid.

Control of Distributed Converter-Based Resources in a Zero-Inertia Microgrid Using Robust Deep Learning Neural Network

Considering the evolution of future microgrids (MGs) towards zero-inertia level due to the penetrations of distributed converter-based resources (DCRs), a large number of data produced by these generations will lead the control decisions to be more complicated than conventional power systems. This paper presents a control strategy for a zero-inertia MG with DCRs using a robust deep learning neural network (RDeNN). In a training phase, a sub-space state-based identification method is employed to monitor and analyze the data regarding stability indices, i.e., damping and frequency of dominant modes, and robustness against uncertainties. In addition, a mixed H2/H∞ control strategy is applied to enhance the training efficacy in the frequency and voltage control loops of DCRs. The trained RDeNN is activated to make quick and effective control decisions by using only measured signals from the MG. Simulation results are verified in the zero-inertia MG (or the grid with 100% DCRs) and compared with several existing control techniques. The study results demonstrate the advantages of the proposed RDeNN in many aspects such as low computational time, require-less physical controller models, fast and flexible stabilizing responses, and high robustness against various time delays, data quality issues, and MG uncertainties.

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.

A Second-Order Sliding Mode Voltage Controller with Fast Convergence for a Permanent Magnet Synchronous Generator System

This paper studies an improved super-twisting sliding mode controller (IST-SMC) for the permanent magnet synchronous generator (PMSG) voltage loop to improve the anti-disturbance capability of the system. Compared to conventional voltage controllers, the control algorithm provides advantages in terms of system resistance to load disturbances. Conventional voltage controllers have significant voltage fluctuations and long recovery times during sudden load changes. To solve this problem, a voltage loop controller based on a super-twisting sliding mode (ST-SMC) is designed to enhance the immunity of the system. Also, the ST-SMC was improved to further increase the convergence rate of the system and enhance the dynamic performance. The convergence of the system away from the balance point is accelerated by introducing an exponential term, which in turn provides an improvement in the dynamic performance of the system. The effectiveness of the proposed control scheme was verified on a PMSG.

Improvement of grid injected currents in single-phase inverters rejecting DC link voltage oscillations based on multi-resonant filtering

The operation of grid-tied single-phase inverters generates oscillations in its DC link voltage. If only active/reactive power is controlled by the inverter, this oscillation is at twice the grid frequency. In situations where the inverter output current and/or the grid voltage also has harmonic components, the DC link voltage becomes also polluted by oscillating components at even multiple frequencies of the grid voltage. Such oscillations are propagated through the DC link voltage control loop, increasing the THD index of grid current reference signal. To solve this drawback, this article proposes the insertion of a multi-resonant filter into the DC link voltage control loop, ensuring a harmonic free grid current reference signal. With the proposed filter and without any increase in the power circuit, the use of smaller capacitances in the DC link is allowed, since the DC link voltage oscillation does not interfere in the power quality indexes of the controlled current. A filter tuning methodology is presented to ensure that the design of each filter and of the DC link voltage controller is independent, relating the filter settling time and the system stability. At the same time, the use of the proposed filter allows the voltage controller to be tuned at higher frequencies, improving the DC link voltage dynamic response. The proposal is experimentally validated through a single-phase grid-tied double-stage photovoltaic (PV) system, that can also operate only as a parallel active power filter (PAPF) or, simultaneously as a multifunctional PAPF + PV system.

Efficiency optimization for phase‐shifted full‐bridge converter with variable frequency control

AbstractIn this paper, a control method combined with variable frequency and phase shifting is proposed, the primary purpose of which is to optimize the operating efficiency of the phase‐shift full‐bridge (PSFB) DC‐DC converter. By modeling the losses of the PSFB converter, the optimal switching frequency related to the load conditions is found. A neural network aided variable frequency controller that can work linearly is built to achieve the proposed method. At the same time, the voltage control loop is retained to ensure the stability of the output voltage and the necessary dynamic response of the converter. This paper builds an experimental platform for a 2 kW PSFB DC‐DC converter to verify the effectiveness of the proposed control method. With the control method proposed in this paper, the PSFB converter can always work at the optimum efficiency point under different load conditions. In addition, this paper also analyzes the influence of variable frequency control on PSFB operating states, such as zero‐voltage‐switching (ZVS) constraints, continuous conduction mode (CCM) operating states, and power density, which have an impact on its operating conditions.

Robust and efficient control strategy for single-phase two-stage PV grid-tied system with improved power quality

In the last couple of decades, the integration of photovoltaic (PV) systems with the utility grid has gained significant attention due to its potential to harness renewable energy and contribute to sustainable power generation. However, the integration of the PV system into the grid leads to issues related to power quality and system stability under varying solar irradiation, dc offset, and distorted grid conditions. Therefore, this paper focus on the robust and efficient control of a single-phase, two-stage PV grid-tied system to improve power quality and stable system operation under abnormal grid conditions. These objectives can be achieved using the synchronization method, dc link voltage control, and grid current control. The hybrid PLL is proposed in this paper for synchronization and to mitigate the effect of dc offset and lower-order harmonics in the estimated phase and frequency. Further, the sliding mode voltage controller and integral sliding mode current controller are proposed for dc link voltage regulation and grid current tracking. The stability of voltage and current control loops is analyzed using the Lyapunov stability theory. Furthermore, the proposed schemes are validated using MATLAB simulation and experimental analysis. The proposed approach outperforms conventional methods and exhibits better disturbance rejection capability under both dynamic and steady-state conditions, making it a robust and efficient control strategy for grid-tied systems.

A Decoupling Control Method for Converting DIDO Sequence Admittance Model of Grid-Tied Converter System Into SISO

This paper proposes a decoupling control method to eliminate frequency coupling in three-phase grid-tied system. Firstly, the admittance model of three-phase <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -type three-phase grid-tied converter (GTC) considering DC-bus voltage control (DVC) loop and phase-locked loop (PLL) is established in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -domain frame, and the influence of asymmetrical DVC and PLL control structure on frequency coupling oscillation is analyzed. Then decoupling factors are added to the DVC loop and current control (CC) loop to achieve that the off-diagonal elements of the sequence admittance matrix of the GTC are 0, which means the frequency coupling in the GTC is eliminated. Therefore, the GTC system can be regarded as a single input single output (SISO) system, and the stability can be analyzed by the classic Nyquist criterion in the sequence-domain frame. Compared with the double input double output (DIDO) system, the resulted SISO system convenient for calculating the stability margin of the system and guiding parameter design. In addition, the SISO system is easier to be analyzed on the oscillation and resonance mechanism, and the analysis results can be used to optimize the design of the converter controller, grid planning and operation. Finally, simulation and experiment verify the feasibility and effectiveness of the proposed decoupling control method.

Dynamic Analysis, Stability and Design of Grid Forming Converters With PI-Based Voltage Control in DC and 3-Phase AC Microgrids

This paper analyzes the dynamic behavior of the voltage control loop based on proportional-integral regulators, commonly used for grid-forming converters in 3-phase AC and DC Microgrids and applications that involve a DC-link voltage control. The paper proposes a simple and accurate generalized analysis useful both for the system characterization and design. Two different control schemes, based on linear (Direct Voltage Control, DVC) and quadratic voltage feedback (Quadratic Voltage Control, QVC), are analytically studied, simulated and experimentally tested, demonstrating a superior performance of the QVC under the presence of constant power loads. The operation limits, the system stability and the disturbance rejection capability are analyzed considering the effect of control and plant parameters and the effect of the different types of disturbances and the operating point, taking into account the non-linearities of the system. The analysis is mainly focused on the effect of constant power loads given their negative impact on the system performance. The study provides a generic procedure for the analysis and design of proportional-integral voltage controllers, including the selection of the system capacitance for meeting specific dynamic specifications while considering system characteristics as the load level, the stability margins and the maximum voltage deviation under disturbances.

Capacitor Voltage Feedforward-Based Voltage Loop Control for Grid-Forming Modular Multilevel Converters Under Wide Range of Grid Strength

Capacitor Voltage Feedforward-Based Voltage Loop Control for Grid-Forming Modular Multilevel Converters Under Wide Range of Grid Strength

Analysis of Subsynchronous Oscillation Caused by Multiple VSCs With Different Dynamics Under Strong Grid Connections

It is known that multiple voltage source converters (VSCs) with similar dynamics may induce subsynchronous oscillations (SSOs), particularly under weak grid connections. This study investigated the small-signal stability of multiple VSCs with different dynamics, utilizing both the model with default parameters and the model that accounts for deviations in VSC dynamics. The results indicates that the DC voltage control loops of the VSCs may cause growing SSOs even under strong grid connections. The direction and amplitude of the deviations are affected by the differences between the actual and default parameters. The impact of deviations is the largest when all the parameters are larger or smaller than the default parameters. However, the impact is reduced when some of the parameters are larger and others are smaller than the default parameters. On this basis, two reduced-order stability analysis methods were proposed to quickly and accurately assess the SSO risk in the power system. A sample power system with multiple VSCs is used to demonstrate and evaluate the conclusions and proposed methods.

A Nonlinear Load Current Feedforward Strategy for the Charge-Controlled LLC Converter and Its Digital Implementation to Improve the Dynamic Response

Compared with the traditional single voltage loop control strategy in LLC converters, the charge-controlled LLC converter can significantly improve its dynamic response due to its model having the first-order feature. Generally, a load current feedforward method could improve the converter's dynamic response. However, for the LLC converter, it is almost impractical due to the unclear relationship between the load current and the control variable. To build this relationship, a mathematical expression between the load current change and the control variable is derived that can precisely offset the load disturbance and achieve faster dynamic response. The derived relationship is nonlinear, related to both input and output voltages and switching frequency, and is suitable for digital implementation. To further analyze the dynamic response, the output impedances with and without the load current feedforward control are established and analyzed, which visually demonstrates the dynamic improvement in the time domain through the calculation of the output impedance and load change spectrum in the frequency domain. Finally, a 120W LLC prototype with nominal 48V input and 12V output is built to verify the nonlinear load current feedforward control and its analysis.

Voltage unbalance and inertia based frequency control framework considering intelligent load management for stand-alone operation of microgrid

The less inertial microgrid with high penetration of Renewable Energy Sources (RES) faces several issues including voltage and frequency unbalance. The intermittent nature of RES-based Distributed Generations (DGs) makes the system unstable in the absence of a control mechanism in a stand-alone microgrid. The integration of single-phase loads and sudden load switching on the distribution side make the system prone to several malicious events. The unbalance in voltage among the phases and the high Rate of Change of Frequency (RoCoF) are the major concern for degrading the system’s performance during peak and off-peak hours. The paper addresses a hierarchical dual loop voltage and frequency control mechanism where the primary control is equipped with droop strategy and Positive-Negative Sequence Control (PNSC) to maintain Voltage Unbalance Factor (VUF) and adhere to IEC-61000-3-13. Further, the inertial loop is also implemented to mitigate frequency deviation and reduce transient overshoot. The secondary control mechanism enables intelligent load management by scheduling Thermostatic Control Loads (TCLs) during peak hours. The utmost care is taken for the Customer’s Thermal Comfort (CTC) while scheduling the loads. The rigorous simulation is done in MATLAB Simulink and Digsilent Power Factory with a modified IEEE 13 bus system considering system dynamics. Further, the real-time simulation is also performed with OPAL-RT to validate the efficacy of the above methodology.

Virtual inertia analysis of photovoltaic energy storage systems based on reduced-order model

The problem of non-ideal inertia of the photovoltaic energy storage system (PVESS) may occur due to unreasonable voltage control parameters. In response to this issue, this paper establishes an equivalent reduced-order model (EROM) for PVESS. This EROM considers the current control loop, voltage control loop and the virtual inertia control loop based on low-pass filter. This low-pass filter can effectively enhance the system’s virtual inertia. Since the output impedance of this EROM can visually reflect the external characteristics of the virtual inertia control loop, it is suitable for inertia analysis of PVESS. Furthermore, the impact of voltage control parameters and low-pass filter bandwidth on the system’s inertia is discussed from the perspective of the frequency response of the output impedance. Finally, the switch model of the PVESS is built on the RT-BOX hardware-in-the-loop experimental platform. The validity of the EROM and theoretical analysis is verified by several sets of experimental results.