Distorted Grid Voltage Conditions Research Articles

Model-Matching Control for Low Switching Frequency Converters Unifying Different Performance Requirements

This paper presents an active damping control for LCL-filtered grid-connected converters to meet strict grid codes and power quality standards. The control strategy allows unifying into a single design procedure different requirements demanded by system operators. A model-matching approach is used to set the reference tracking and disturbance rejection transfer functions separately, which provides an additional degree of freedom to the converter control design. On the other hand, the measurement stage and digital implementation effects are considered in the control design to maximize the control bandwidth. The proposed control strategy uses a reduced number of sensors and is able to compensate unbalanced and distorted grid voltage conditions; it presents a systematic design and is formulated in the discrete-time domain, enabling a direct digital implementation. Performance assessment and comparison of different control strategies are performed using eigenvalue and transient response analyses. The effectiveness and advantages of the proposed strategy are validated through an experimental setup.

Application of Backstepping Control With Nonsingular Terminal Sliding Mode Surface Technique to Improve the Robustness of Stator Power Control of Asynchronous Generator-Based Multi-Rotor Wind Turbine System

The conventional backstepping control (CBC) technique for the asynchronous generator (AG) incorporated in a multi-rotor wind turbine (MRWT) system faces several challenges, including torque and stator power ripples, lack of robustness, and low-quality stator currents. Furthermore, the CBC performs poorly under distorted grid voltage conditions. To address these issues, this study proposes applying backstepping nonsingular terminal sliding mode control (BNTSMC). The controller design involves incorporating a NTSMC into the control law of the CBC technique, where Lyapunov theory was used to analyze the stability of the suggested approach. The synthesis of the suggested controller revolves around two sliding mode surfaces, specifically the stator powers of the AG, which are used to determine the final control laws. The advantages of this technique are excellent robustness against internal and external disturbances based on NTSMC, and the controller exhibits asymptotic stability based on the backstepping technique. Simulation tests in MATLAB software on a 1.5 MW AG-based MRWT system clearly demonstrate the advantages of BNTSMC over CBC.

PV System Linked to an Electrical Network with an Active Power Filter Using DPC Modified Method Under Distorted Grid Voltage Conditions

This article presents an energy system that enhances the quality of electrical energy by injecting photovoltaic (PV) renewable energy into the electrical network in the presence of a polluting load. This system is based on a new control approach known as Zero Direct Power Command (ZDPC). The innovative aspect of the proposed work is the addition of clean energy while simultaneously removing the unsettling harmonics produced by the nonlinear loads provided by distorted voltages. This approach combines a traditional Proportional Integrator (PI) controller for regulating the DC bus voltage with a clever technique (fuzzy logic) for tracking the Maximum Power Point Tracking (MPPT). The current has a harmonic distortion rate of about 1% with unity power factor due to the suppression of undesirable harmonics from the source currents. A PV panel connected in series with a chopper and managed by fuzzy logic via a two-state inverter ensures the injection of PV energy into the electrical network. Software called MATLAB/Simulink is used to model this system. The outcomes demonstrate the reliability and viability of the ZDPC control, which concurrently ensures harmonic current compensation, power factor correction, and the introduction of solar power into the electrical network despite distorted source voltages.

A Dual Fundamental Current Extraction Controller for DSTATCOM in the Distribution System for Power Quality Improvement Under Non-Ideal Source Voltage Condition

Most of the controllers often fail to work properly when encountered with power quality (PQ) issues like unbalanced and distorted grid voltage conditions in three-phase distribution system (TPDS). To address this problem, usually a dynamic static compensator (DSTATCOM) is utilized in TPDS. For enhanced performance, a novel dual fundamental component extraction (DFCE) technique is proposed for generating reference currents (RC) in a TPDS exhausting DSTATCOM. The proposed controller is highly effective for reactive power compensation and source current harmonic suppression under source unbalanced voltages condition with nonlinear load conditions. For computation of RC and phase synchronization, the controller extracts the required fundamental voltage and current at the same time. Consequently, the controller is able to control DSTATCOM operation in TPDS without any phase locked loop (PLL) and low pass filter (LPF). For generation of fundamental components of voltage and current, projected controller utilizes the self-tuning filter (STF). The efficacy of the controller is established in comparison to existing controllers like synchronous reference frame theory, p-q theory and STF-p-q under different grid voltage conditions. To enhance PQ, the suggested controller works efficiently for fundamental current component extraction without using PLL and low/high pass filter in case of grid distorted voltage condition.

Average Model-Based Sliding Mode Control for Quality Improvement of a Grid-Connected PUC Inverter

This paper presents a new Sliding Mode Phase-Shift PWM (SM-PSPWM) based control strategy for a grid-connected Packed U-Cell (PUC) inverter. The proposed control strategy is based on the average model of the PUC inverter while using a multi-carriers phase-shift pulse width modulation (PS-PWM) technique. The use of the PWM strategy, based on the inverter average model, ensures high steady-state performance, low harmonic current, and enhanced thermal properties. In addition, the Sliding Mode Control (SMC) is applied to generate the input signals to achieve a fast dynamical response and high robustness against disturbances. To further improve the robustness under parameter mismatch and distorted grid voltage conditions, an integral term is added to the surface function for better accuracy and stability. To validate the performance of the proposed controller (fast dynamics, global stability, low current THD, ...) shown by the simulation results, a laboratory setup is used to validate the theoretical analysis.

Observer based model predictive current control of distorted grid connected single phase rectifier without AC voltage sensor

In this research, a grid voltage sensorless control scheme under the distorted grid voltage condition for a single-phase rectifier was proposed. For eliminating the grid voltage sensor, a time-varying state observer was designed, which provided direct, accurate, and fast estimation of the grid voltage without subsequent complex calculations, even for the presence of harmonics, a DC component, and frequency fluctuations, and it was implemented in the digital processor through Forward Euler discretization. In addition, the aforementioned observer would be partially reused in another observer to modify the input of the PI controller and then reduce the total harmonic distortion of the input current. To warrant robustness for the real world, the proposed observers were investigated in detail for load resistance variations and grid voltage fluctuations. Several simulations and experimentation tests were conducted to verify the effectiveness of proposed scheme. The obtained results indicated that the proposed observers had fast dynamic responses and low estimation errors. The proposed scheme decreased the THD of the input current.

Multi-resonant Controller Design for a PV-Fed Multifunctional Grid-Connected Inverter in Presence of Unbalanced and Nonlinear Load

The use of Active Power Filters (APFs) in future power grids with high penetration of nonlinear loads is unavoidable. Voltage Source Inverters (VSIs) interfacing Photovoltaic (PV) generator could play the APF role in addition to power supply. In this paper, the control of a PV-fed multifunctional grid-connected three-phase VSI is addressed with nonlinear and unbalanced load. The control objective is threefold. The first one is to deliver the maximum available power from the PV source to the grid satisfying power quality standards. The second one is the Voltage Balancing Control for DC-link capacitors to guarantee correct operation of the VSI. The last one is shunt APF control to compensate for nonlinear and unbalanced load harmonics, reactive power, and unbalanced sequences. A quasi-Proportional-Resonant (PR) controller with harmonic compensators is proposed for the current control loop. The quasi-PR controller parameters are determined through optimization algorithms such as Particle Swarm Optimization (PSO), Genetic Algorithm (GA), and a combination of both PSO and GA. The aim of the objective function is to improve static and dynamic behavior. The different gains at the fundamental resonant frequency and the selected odd harmonics are obtained for the proposed quasi-PR controller. The dynamic characteristics of the optimized quasi-PR controllers show superiority against conventional ones in terms of gain margin, phase margin, overshoot, and robustness. With the proposed control scheme, the harmonics, reactive power, and unbalanced sequences are appropriately compensated. The performance of the PV-fed VSI shunt APF under irradiance change, load change, and distorted grid voltage conditions is validated through numerical simulations performed on hbox {PSIM}^{copyright } software. The results show that the grid currents Total Harmonic Distortion for irradiance change case study are 4.57%, 4.57%, and 3.22% in phase a, b, and c with the proposed control method, while they are 9.25%, 5.65%, and 10.12% with conventional instantaneous p-q theory.

Impact of Inductor Placement on DC Bus Capacitor of Adjustable Speed Drives under Non-Ideal Supply Voltage Conditions

Diode bridge front-end Adjustable Speed Drives (ASDs) generally have an inductor either in the DC side (DC choke) or in the AC side (AC choke). Since both the types of ASDs are commercially widely used, a comparison of their behavior under normal and abnormal grid conditions is of interest. It is identified in this work that there exists a difference in the ripple current through the DC bus capacitor between the two variants. Further, the difference in the ripple current varies depending on the type of the disturbance and its level. As an outcome of the study, it is identified that the stress levels arising in the DC choke variant on the DC bus capacitor are lesser compared to the corresponding AC choke variant under balanced, unbalanced, and distorted grid voltage conditions, for the same value of the equivalent inductance in the AC and DC side. In order to match the stress levels arising in the DC bus capacitor between the ASDs with DC and AC choke, it is found that a 50% higher value of inductance is required in an AC choke variant. The effects are analyzed by simulations and validated using an experimental setup.

Optimum Setting Algorithm Based PI Controller Tuning for SRF-PLL Used Grid Synchronization System

Synchronous reference frame-phase lock loop (SRF-PLL) is most widely used for grid synchronization applications. PLL structure typically consists of a phase detector (PD), loop filter (LF) and voltage control oscillator (VCO). Design and tuning of proportional-integral (PI) controller based LF is a changeling task under distorted grid voltage conditions. Conventional tuning methods such as symmetrical optimum (SO) and Ziegler-Nichols techniques are used to tune the PI controller, resulting in increased phase margin (PM) of the system. However, high bandwidth develops harmonics in the extracted signals under non-ideal grid conditions and affects the stability. In this research article, a new optimum setting algorithm is proposed to tune the PI controller and effective synchronization is achieved. The plant is reduced as a first-order plus dead time delay (FOPDT) model and critical gain; critical frequency of the open loop system transfer function is used to model the LF of the SRF PLL system. The controller parameters are derived such that minimization of time-weighted integral performance criteria is achieved and phase angle, frequency of the grid voltage is accurately extracted. The performance of the proposed optimum setting algorithm for the SRF-PLL grid synchronization technique is discussed in detail and simulation results are presented.

Design of a Repetitive Control for a Three-Phase Grid-Tied Converter under Distorted Grid Voltage Conditions

The paper presents a design of repetitive control (RC) in the current control system of a three-phase grid-tied converter. The goal of the control system is to provide sinusoidal input filter currents under the conditions of distorted and asymmetrical grid voltage. A novel design of the RC is presented, in which the repetitive part is not excited by sharp and non-periodic changes of the reference signal, but it enables high-quality performance under periodic disturbance conditions. In the proposed system. RC cooperates with a discrete state feedback controller. An innovative approach to tuning is proposed in which parameters of the repetitive, as well as the state feedback controller, are selected as a result of the optimization process with the use of a particle swarm algorithm. The proposed control system is verified experimentally on a laboratory test bench. The achieved results confirm the high-quality system performance.

Leveraging half-cycle orthogonal signal generation approach for PLL-less tracking of single-phase grid parameters

This article presents a fast and accurate single-phase grid voltage parameter tracking scheme under distorted grid voltage conditions. The input stage of the proposed scheme consists of a half-cycle moving-window discrete Fourier transform (HC-MWDFT) based pre-filter, whose design guidelines are discussed in the viewpoint of its stability and orthogonal signal generation. Combining a positive feed-forward comb filter and a fractional complex resonator helps to attenuate odd-order harmonics and allows fast extraction of fundamental components. Unlike the phase-locked loop (PLL) techniques, the grid frequency is indirectly estimated using a simple open-loop frequency detector. The proposed method is free from tedious controller tuning aspects and can avoid external phase/frequency feedback loops. The amplitude and phase information are adaptive to the grid frequency by employing a simple linear regression-based error correction technique. The overall efficacy of the proposed algorithm is assessed using numerical and experimental methods.

An Enhanced Control Strategy to Mitigate Grid Current Harmonics and Power Ripples of Grid-Tied PV System Without PLL Under Distorted Grid Voltages

This article proposes an enhanced integrated control strategy to mitigate grid current harmonics and power ripples for a three-phase grid-tied photovoltaic (GTPV) system without a phase-locked loop (PLL) unit. The proposed control strategy consists of an enhanced proportional resonant (EPR) current controller, which suppresses the harmonics of grid injected current under distorted grid voltage conditions. A phase compensated reference current generator (PCRCG) is integrated with the proposed EPR controller and used to eliminate active and reactive power oscillations under grid voltage sags and swells. The proposed integrated control strategy is implemented in the <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> stationary reference frame. The synchronization task is included in the PCRCG and making the system PLL-less. Thus, the associated computational burden and implementation complexity are reduced. Furthermore, the frequency adaptability of the GTPV system is enhanced as compared with the PLL-based control strategy in the presence of grid frequency variations using the proposed control strategy. To ensure the fast maximum power point (MPP) tracking, a modified incremental conductance MPP tracking algorithm is implemented in integration with the proposed control strategy. The effectiveness of the proposed control strategy is validated using OPAL-RT real-time digital simulator and compared with the conventional PLL-based control strategy.

An enhanced finite control set model predictive control method with self‐balancing capacitor voltages for three‐level T‐type rectifiers

An effective finite control set model predictive control (FCS-MPC) is introduced for single-phase three-level T-type rectifiers supplying resistive as well as constant power loads (CPL). The main problem of CPL is the negative resistance phenomenon that endangers the rectifier's stability. Hence, the proposed FCS-MPC method is based on Lyapunov's stability theory such that the stability of the rectifier is guaranteed under all operating points. Unlike the existing FCS-MPC methods, the cost function design in the proposed control method is formulated on the rectifier's stability. According to Lyapunov's stability theory, the rectifier stays stable provided that the rate of change of Lyapunov function is negative. In this case, the derivative of the Lyapunov function can be used as the cost function without utilizing any weighting factor. Therefore, contrary to the existing FCS-MPC methods, the weighting factor requirement is eliminated which leads to easiness in the design and implementation of the controller. Experimental results reveal that the proposed control approach exhibits very good performance with undistorted and distorted grid voltage conditions when the rectifier feeds resistive and CPL loads.

Accuracy assessment of frequency-domain models for harmonic analysis of residential type photovoltaic-distributed generation units

Since Photovoltaic-Based Distributed Generation Units (PV-DGUs) have power electronic interfaces for their grid integration, they can cause harmonic pollution in the system. Thus, the harmonic analysis is required to determine the system’s harmonic constrained PV-DGU hosting capacity and proper planning of the harmonic mitigation devices and PV-DGUs. The accuracy of the harmonic analysis highly depends on the validity of the system equipment models. Accessible literature shows that there is not a report on accuracy assessment of the PV-DGU frequency-domain harmonic models. In this paper, firstly, for numerous distorted grid voltage conditions, the current harmonic distortion nature of the single-phase transformerless two-level H-bridge residential type on-grid PV inverter is examined in an experimental test system. Secondly, for current harmonic analysis of the examined PV inverter, the accuracy of the widely known frequency-domain models is statistically analyzed under numerous distorted grid voltages. For the accuracy assessment of the models, the measurements obtained from an experimental test system are considered as reference values. Finally, to investigate the effect of the models on the voltage harmonic distortion results of the harmonic analysis, the harmonic penetration analysis algorithm is integrated with the models, and it is simulated for IEEE 33 bus test system.

Repetitive Predictive Control for Current Control of Grid-Connected Inverter Under Distorted Voltage Conditions

A repetitive predictive control for grid-connected inverter current control scheme is presented in this paper under voltage harmonic distortion in the stationary reference frame. Predictive control is an approach that uses a receding horizon to achieve the optimal track for the reference. In this approach, the repetitive controller behavior is added to the predictive control to increase its performance under distorted voltage conditions. In addition, to apply the controller in the generation system, controller is designed from the perspectives of the power system. The controller is designed considering the state-space model in the stationary reference frame. The controller performance was verified using a laboratory experimental setup under distorted grid voltage condition. Experimental results confirm that the proposed controller can effectively suppress harmonics under both normal operation and distorted grid voltage conditions, and also satisfy the requirements stipulated in IEEE Std. 1547.2-2008.

An Advanced Proportional Multiresonant Controller for Enhanced Harmonic Compensation With Power Ripple Mitigation of Grid-Integrated PV Systems Under Distorted Grid Voltage Conditions

This article proposes an advanced integrated control strategy for enhanced grid current harmonic compensation with power ripple mitigation in a grid-integrated photovoltaic systems (GIPVS) under distorted grid voltage conditions. The proposed control strategy consists of an advanced proportional multiresonant (APMR) current controller and a compensator for grid voltage sag (CGVS), which is implemented with a phase-locked loop block. The APMR is designed in the α-β reference frame and minimizes the harmonics of the grid injected current. The CGVS, along with the APMR, eliminates the active and reactive power ripples under unbalanced voltage sags conditions. Further, an adaptive step size incremental conductance (ASINC) maximum power point tracking controller in integration with the proposed control strategy is implemented. Along with current harmonic reduction by APMR, the ASINC method ensures effective and faster tracking of maximum power point and negligible oscillations in the steady-state under different environmental conditions. By utilizing the proposed APMR control strategy, the frequency adaptability of the GIPVS is improved compared to the conventional proportional multiresonant (PMR) control strategy under grid frequency deviations. It successfully keeps the grid current total harmonics distortion below the acceptable limits for the grid voltage polluted by several odd harmonics, up to the nineteenth harmonics. The proposed control strategy of APMR with CGVS almost completely eliminates the power ripples in even severe unbalanced voltage sag conditions. Experimental studies are carried out using a real-time digital simulator to verify the effectiveness of the proposed control strategy in comparison with the conventional PMR controller.

Direct active damping control for grid‐connected AC/DC converter with LCL filter using augmented look‐up table

This paper presents a novel approach to adapt the conventional direct power control (DPC) methods that are applied to ac/dc converters with an inductive capacitive inductive (LCL) filter. These new solutions improved the conventional DPC with an active damping algorithm via multi-variable constraint adjustment, which is based on an augmented look-up table and multi-variable comparators. The novel look-up table proposed for the active damping algorithm of the LCL filter allows an optimal voltage vector assignment to be chosen using differences in converter-side current and capacitor voltage. This method is also compared with the DPC method with active damping via the addition of a virtual damping resistor and a harmonic rejection loop. The steady-state (total harmonic distortion of the grid current) and dynamic performances (response time on step changes of the set current), as well as the immunity under distorted grid voltage conditions of the proposed system, are verified through simulation results and experimental measurements.

High quality repetitive control system for a grid-tied converter under distorted grid voltage conditions – design and implementation

High quality repetitive control system for a grid-tied converter under distorted grid voltage conditions – design and implementation

Super Twisting Sliding-Mode Control of DVR With Frequency-Adaptive Brockett Oscillator

This article presents a super twisting sliding-mode control (ST-SMC) for single-phase dynamic voltage restorers (DVRs). Unlike the conventional first-order sliding-mode controller, the proposed ST-SMC technique eliminates the need for differentiating the compensation voltage in the sliding surface function while keeping the merits of first-order SMC. As a consequence of employing ST-SMC, a continuous control signal is achieved from which the pulsewidth modulation (PWM) signals can be generated. In this case, the inverter operates at a constant switching frequency. The stability analysis of ST-SMC is also presented. The reference compensation voltage needed in ST-SMC is estimated by using Brockett oscillator-based frequency-locked loop. Theoretical considerations are verified through experimental results under ideal and distorted grid voltage conditions. The obtained results show that the ST-SMC has good dynamic performance and can maintain the load voltage at a desired level under voltage sag, swell, and harmonically distorted grid voltages.

Backstepping Controller With Dual Self-Tuning Filter for Single-Phase Shunt Active Power Filters Under Distorted Grid Voltage Condition

This article presents the design and hardware implementation of an adaptive nonlinear controller for fast, robust, and stable control of single-phase shunt active power filter. The proposed control system consists of two control loops: an inner harmonic current compensation loop and an outer dc-voltage control loop. The inner loop is realized using self tuning filter based instantaneous power theory (pq theory). The limitations of conventional low-pass filter based fundamental component extraction methods are overcome using self-tuning filter. The outer loop is realized backstepping controller (BSC). The limitations observed in existing dc-link voltage controllers like poor stability margin, steady state error, chattering problem, etc., are overcome by the proposed BSC. The switching loss estimation is introduced in BSC using design estimation rules to enhance the dc-link loss compensation capability. The stability of the system with the proposed controller is studied using Barbalat lemma. A laboratory prototype of BSC based shunt active power filter is implemented. The control algorithm is implemented in a single all on chip field programmable gate array (FPGA). To ensure the effectiveness of the controller in mitigating the harmonic currents and controlling dc-link voltage, the control algorithm is tested under steady state and dynamic conditions.