Grid Voltage Swell Research Articles

A Direct AC–AC Switched-Capacitor Converter With Input-Series Output-Parallel and In-Phase/Out-of-Phase Capabilities

In this paper, a bidirectional transformerless multi-level direct ac-ac switched-capacitor converter (SCC) is proposed with both in-phase and out-of-phase output voltage capabilities. This converter can be used for series compensator applications, such as dynamic voltage restorers (DVRs), to compensate grid voltage sag and swell phenomena by incorporating its input-output common ground. Its input-series output-parallel capabilities reduce voltage and current stresses of the converter components in multi-level configurations, which lead to higher efficiency and lower manufacturing costs for high-power applications. Furthermore, proper utilization of an interleaving control scheme can provide lower input and output currents ripples values and smaller filter sizes. The proposed SCC operation principle and its mathematical analyses are given, in detail. Additionally, its switching patterns have been developed with a safe-commutation strategy to obtain various output voltages. Subsequently, both single- and two-level SCC prototypes are implemented as series compensators for DVR applications. Moreover, experimental results are given to generate in-phase and out-of-phase regulated output voltages. The experimental results at 2 kW show that the output voltage changes are less than 3% under the steady-state conditions, despite the large input voltage changes from 140 to 300V. The converter maximum efficiency value is equal to 95.4% at maximum output power value.

A Bidirectional Onboard Charger With Multistep Constant Current Charging Capability

This article realizes a bidirectional onboard charger (OBC) for light electric vehicles (LEVs), with multistep-constant-current (MSCC) charging capability. In this MSCC charging operation, the battery current is reduced during the peak-load hours and resumes the rated charging operation amid the off-peak load hours. Since this MSCC operation is a wide load range operation, the controllers of the front-end converter (FEC) and back-end converter are modified. In this context, a Hermite polynomial-derived functional link neural network (H-FLNN)-based deadbeat predictive current controller (DPCC) is introduced to improve the performance of the FEC of the OBC. Moreover, the HFLNN-based DPCC for the FEC regulates the grid current quality during MSCC and distinct operating conditions such as grid voltage sag and swell, distorted grid voltage, and the presence of dc offset error in the measured grid voltage. A laboratory prototype of a 1.1-kW rating is developed to verify the efficacy of the H-FLNN DPCC to keep the grid current power quality (PQ) within the International Electrotechnical Commission (IEC)-61000-3-2 standard throughout the operation. Moreover, the effectiveness of the design is verified amid the grid voltage sag and swell and distinct operating conditions. The experimental analysis depicts that the rated efficiency of the developed laboratory prototype is 90.18% and the analytical power density is 64.16 W/in3. Furthermore, the vehicle to grid (V2G) operation validates the satisfactory design of OBC for bidirectional operation. At last, the comparison of the presented system with existing systems is given to summarize the advantages of the presented system over the existing systems.

Coordinated Control of Grid-Connected PMSG Based Wind Energy System With STATCOM and Supercapacitor Energy Storage

In this paper, two coordinated control schemes have been designed for a grid-connected permanent magnet synchronous generator-based wind energy conversion system with static synchronous compensator (STATCOM) and supercapacitor energy storage (SCES) under various operating conditions. The coordinated control scheme-I (CCS-I) integrated a SCES into the DC-link through a bidirectional DC/DC converter to stabilize the DC-link voltage under varying wind speed and grid disturbances. The coordinated control scheme-II (CCS-II) implemented a STATCOM at the point of common coupling (PCC) to enhance the grid voltage stability during the grid voltage sag or swell. In CCS-II the SCES balances the active power while reactive power support is extracted from the grid side converter (GSC). The STATCOM is only activated when the reactive power requirement by the PCC exceeds the support capacity of the GSC. The proposed coordinated control schemes have been experimentally implemented with a dSpace MicroLabBox. Results show that CCS-I and II ensured the uninterrupted operation of the wind energy system and satisfied grid code requirements under various operating conditions and grid disturbances.

Efficient HVRT Scheme of DFIG Using a SSFCL-Chopper Considering Grid Voltage Recovery

To enhance the dynamic high-voltage ride-through (HVRT) performance of doubly fed induction generator (DFIG) considering both the grid voltage swell and recovery influences, an efficient HVRT scheme is proposed by integrating a stator-side superconducting fault current limiter (SSFCL) with a dc-link chopper. The basic principle, circuit modeling, resistance setting, current-limiting characteristics design and coordinated control strategy of the SSFCL and chopper are presented to form an integrated protection scheme. As compared to a SSFCL or chopper-based HVRT scheme, the results obtained in the proposed SSFCL-chopper-based scheme show that the amplitudes of external and internal electromagnetic transient oscillations of the DFIG are largely reduced with shorter recovery time. This enhances significantly the HVRT behaviors in the DFIG considering grid voltage recovery.

Multi-Mode Voltage Sag/Swell Generator Based on Three-Phase Inverter Circuit

The voltage ride through capability of the major auxiliary variable-frequency drive (VFD) in large thermal power plants is the key technical issue of power grid and source coordination. In order to test the high voltage ride through (HVRT) and low voltage ride through (LVRT) capability of the auxiliary VFD, it is necessary to develop a power supply to simulate different grid voltage sag and swell accurately. In this paper, a generator (VSSG) based on the common three-phase inverter circuit that can simulate multi-mode voltage sag/swell is proposed. The designed main circuit consisting of transformer, rectifier, DC split capacitors, three-phase inverter, and LC-filter can generate single-phase and three-phase voltage sag, swell, and phase angle jumping flexibly. The developed control strategies composed of the double closed-loop control and the neutral voltage balance control achieve accurate output, fast dynamic response, and step-less adjustment. Simulation and experiment results verify the multi-mode voltage simulation performances of the proposed VSSG, which can be effectively used to emulate power grid voltage sag and swell phenomena under the IEEE 1159 and IEC standards.

Proportional-Resonant and Unipolar Switching Control of Single-Stage Solar Photovoltaic Grid Interfaced System

ABSTRACT In the proposed work, a new control is used with inner loop as a current control and the outer loop as a voltage control of a solar photovoltaic (PV) array-fed single-phase grid interfaced system, using a voltage-source converter (VSC). It deals with the power quality mitigation, which includes power factor correction, harmonic mitigation in abnormal conditions such as grid voltage sag and swell and irradiation changes. The PI (Proportional–Integral) controller is used as an inner current loop control for reference and sensed DC link voltage balancing and to generate the command for the harmonics accurately, which generates the reference current. The PI controller is used for generating reference current and PR (Proportional-Resonant) is used for generating voltage reference, which, in turn, produces modulating signal within the limit of carrier wave. A comparison between reference voltage and carrier gives unipolar switching, attaining three levels for the VSC. Here, the maximum power point tracking (MPPT) is achieved by an incremental conductance (INC) method. Experimental performance is studied at dynamic abnormal conditions such as voltage sag and voltage swell in order to investigate the total harmonic distortion (THD) of the grid current and point of common coupling (PCC) voltage, which are observed within the IEEE-519 standard.

Coordinated Droop Control and Adaptive Model Predictive Control for Enhancing HVRT and Post-Event Recovery of Large-Scale Wind Farm

The wind turbine (WT) terminal overvoltage during grid voltage swell events may result in tripping the WT and consequently threaten the secure operation of large-scale wind farms (WFs). In this paper, an optimal coordination of droop control and adaptive model predictive control (MPC) scheme is proposed to enhance the high-voltage ride-through (HVRT) and post-event recovery of large-scale WFs. During the HVRT, the reactive power reference is generated in each WT controller by following an optimal droop coefficient to realize a fast voltage reduction at the WT terminal. The droop coefficients are pre-calculated by taking the WF collection system topology and voltage swell magnitude into consideration. At the post-event recovery stage, an adaptive MPC-based voltage recovery control scheme is proposed to improve post-event voltage dynamic restoration performance. The droop coefficients of the WT controllers are optimized based on the voltage sensitivity coefficients and voltage swell magnitude. With the proposed control scheme, all the WT terminal voltage can be maintained within their feasible range and the response time of post-event voltage recovery is significantly shortened. The proposed control scheme is validated and tested under various operating scenarios.

MPC-based DC-link voltage control for enhanced high-voltage ride-through of offshore DFIG wind turbine

To deal with the DC-link overvoltage during high-voltage ride-through (HVRT) of the wind turbine, a coordinated DC-link voltage control (CDVC) scheme is proposed for enhancing the HVRT performance of offshore doubly-fed induction generator (DFIG) wind turbines equipped with the supercapacitor energy storage system (SESS). This scheme is formulated as a two-stage control problem depending on the depth of grid voltage swell, which can prevent damaging the converter resulting from the insufficient reactive support in the first stage, and achieve efficient coordinated control under various magnitude and timescale of grid voltage swell. In the first stage, considering the dynamic behavior of the DFIG during HVRT, a voltage-dependent reactive current control (VRCC) scheme is designed to regulate the current reference of the wind turbine. The first stage aims to supply fast reactive current support without the DC chopper. In the second stage, once the DC-link voltage exceeds the predefined threshold, a coordinated control scheme considering the SESS and the wind turbine operation is implemented using model predictive control (MPC), aiming to regulate the excessive active power by tracking the current reference from the first stage. The proposed control scheme is validated in PSCAD/EMTDC under various operational scenarios. Case study results show that the proposed scheme can effectively mitigate the DC-link overvoltage and enhance HVRT capability under asymmetrical and symmetrical voltage swells.

Three-Phase Four-Wire PV System for Grid Interconnection at Weak Grid Conditions

A photovoltaic (PV) system for reliable interconnection of a PV array to the grid is presented in this article. A boost converter ensures operation of the PV array at its maximum power point, and it also enables the adaptive adjustment of the dc link voltage of the interfacing voltage source converter according to the variations in the grid voltages. This prevents system tripping during the voltage sag, and helps to maintain the grid current power quality at voltage swell conditions. Even in the weak grid scenarios of unbalanced or distorted voltages, the grid currents are maintained distortion free and balanced. Furthermore, the system eliminates the dc offset in the sensed grid voltages, and ensures absence of dc offset in the grid currents. The fast dynamic response of the system to sudden load variations is obtained by employing a total least squares-based control technique, which swiftly extracts the fundamental active weights from the distorted load currents. Moreover, the PV system performs grid power quality conditioning, such as neutral current mitigation, harmonics reduction, power factor correction, and balancing of grid currents, even under the absence of PV power, such as during nights. The system performance is validated by test results at unbalanced load currents, unbalanced grid voltages, distorted grid voltages, grid voltage sag and swell, PV power variations, and dc offset in the sensed grid voltage.

Power Quality Improvement Using Three-Phase Solar PV Integrated UPQC

This paper proposes the application of a Dynamic Voltage Restorer (DVR) to enhance the power quality and analyzed the solar PV integrated unified power quality conditioner (PV-UPQC).The PV-UPQC is employed to maintain power quality under various current and voltage distortion. The PV-UPQC consists of a shunt and series compensators connected back to back with common DC-link. The shunt compensator performs the dual function of extracting power from PV array apart from compensating for load current harmonics. The series compensator compensates for the grid side power quality problems such as grid voltage sag and swell. In proposed system Dynamic Voltage Restorer (DVR) is used as a series compensator and also it enhance the power quality. The UPQC is eliminate power quality issues like unbalanced grid voltage with harmonics and load harmonics. The performance of the system are evaluated by simulating in MATLAB/Simulink environment under a nonlinear load.

Frequency Adaptive Multistage Harmonic Oscillator for Renewable-Based Microgrid Under Nonideal Grid Conditions

Diurnal and seasonal variability associated with the renewable power generation is disinterested by grid integration. However, for the cases of developing and underdeveloped countries, the utility grids are relatively weak and exacerbate the issues related to power quality, frequency jumps, grid voltage distortion, sag, and swell. To deal with these issues of weak grid-integrated hybrid-renewable-power-generation-based microgrid and to ensure reliable and effective operation, the frequency adaptive multistage harmonic oscillator (MSHO) control is used in this article. This control comprises of multistage filter with dc bias rejection loop and frequency-locked loop. The multistage filter is tuned to dominant lower order harmonic and allows the selective elimination of the harmonics. An MSHO is used for assessing the fundamental constituent from harmonics and dc-bias-infected grid voltage. It also extracts the positive-sequence constituent from distorted/unbalanced grid voltages while maintaining the balanced grid currents. It effectively overcomes the frequency perturbations and nonlinear load compensation and significantly increases the overall reliability and quality of power. A set of back-to-back associated power converters provide a flexible speed operation of the wind power generation system. Different functionality modes are incorporated in order to address round the clock operation of the microgrid, enabling the reactive power compensation capability and mitigation of the harmonics demanded by the nonlinear load connected at the distribution side. The maximum power operating point for both wind and solar photovoltaic arrays is acquired by the individual implementation of the perturb and observe technique. The performance investigation of the system is carried out on a developed laboratory prototype. The aptness and the functionality of the control algorithms are noticeable from the experimental results. The issues of loss of supply, estimation of grid voltage signal, effects of weak grid, variability of renewables with curtailable load are addressed, and the power quality standard IEEE-519-2014 is satisfied.

An Adaptive Differentiation Frequency Based Advanced Reference Current Generator in Grid-Tied PV Applications

In this article, an advanced reference current generator employing a combination of novel dual multi-layer fifth-order generalized integrator and adaptive differentiation frequency locked loop scheme (DMFOGI-dFLL) is proposed for mitigation of harmonics present in a grid-tied photovoltaic (PV) system. The proposed DMFOGI reference current generator is utilized to separate the fundamental constituents from grid voltage distortions, sag, swell, unbalance, frequency variation, and dc offset conditions. In addition, a dFLL is implemented to estimate the system frequency adaptively at grid transient conditions, which is fed as feedback to the pre-filtering unit. Similarly, a fuzzy tuned proportional integral derivative (FPID) voltage controller is incorporated in the proposed controller to maintain the power balance between dc and ac sides by controlling the dc-link voltage. An incremental conductance (IC) control technique is additionally utilized to generate maximum power from the PV source through a boost converter. The proposed system is simulated on MATLAB/SIMULINK under various transient conditions, such as steady state, voltage sag, swell, distorted, unbalance, dynamic load, and load shedding conditions. Moreover, the overall system is developed on a laboratory prototype experimental setup. The simulation outcomes are validated through an experimental platform under above said conditions in respect of total harmonic distortion (THD) of grid current and voltages. Results of the system are found well and also maintained within the standard limits of IEEE-519.

Grid Synchronization of a PV System With Power Quality Disturbances Using Unscented Kalman Filtering

This paper proposes a new grid integration control scheme forasingle-stage PV system connected to a three phase grid, that implements an Unscented Kalman Filter algorithm (UKF) to abstract the fundamental components of the PCC voltage and load current. The dc link capacitor voltage and the PCC voltage are regulated using a Proportional Integral (PI) controller. UKF effectively extracts the fundamental components of the PCC voltage and the load current. The simulation and real-time studies show that the proposed UKF-PI control scheme effectively extracts the maximum PV power and mitigates the load current harmonics by decreasing the Total Harmonic Distortion (THD) of the utility grid current. The THD is within the limits prescribed by the IEEE 519. The performance of the proposed scheme is verified under grid voltage sag and swell conditions. The results obtained envisage that effective grid synchronization is achieved with Power Quality Disturbances, i.e., the grid current is sinusoidal while the PCC voltage is regulated by providing appropriate reactive power.

Weak Grid Intertie WEGS With Hybrid Generalized Integrator for Power Quality Improvement

The wind energy generating system (WEGS) in this paper, aims to provide the required active peak power without high frequency fluctuations even under variable wind speed conditions. The generator speed is adjusted according to the intermittent wind speeds through two voltage source converters (VSCs) connected back to back, namely, machine-side VSC (MSVSC) and utility grid side VSC (UGVSC) across the dc-link capacitor. In this paper, the hybrid generalized integrator control is proposed for switching UGVSC and providing dc offset rejection and immunity against oscillatory errors due to subharmonics, thereby improving the power quality (PQ). Fuzzy logic controller (FLC) is implemented for the speed control of the salient pole synchronous generator (SG) driven by the wind turbine. The FLC provides the tracking of the reference speed under high overshoot transient conditions and narrow bandwidth. The switching of MSVSC is obtained by field oriented control. The dynamic performance is improved by the wind feed-forward term, which reduces the oscillation, ensuring balanced and sinusoidal grid currents. The generated power from the WEGS is fed to the grid. The weak grid conditions, namely, grid voltage unbalance, voltage sag, voltage swell, and grid voltage distortion, are considered. The performance of the system is tested on a laboratory prototype. Test results provide the effectiveness of the system with increased wind penetration and performance under weak grid conditions. Moreover, improving the PQ, the grid current total harmonic distortion meets the requirement of IEEE-519 standard, and is found to be less than 5%.

A Single-Phase Buck-Boost Matrix Converter with Low Switching Stresses

The suggested single-phase ac-to-ac matrix converter operated with inverting and noninverting characteristics may solve the grid voltage swell and sag problem in power distribution system, respectively. It is also employed as a direct frequency changer for domestic induction heating. The output voltage is regulated through duty cycle control of high frequency direct PWM (DPWM) and indirect PWM (IDPWM) switching devices. The DPWM control switches control the switching states of IDPWM switching devices. The inverting and noninverting characteristics are achieved with low voltage stresses and hence low dv/dt across the high and low frequency-controlled switches. This reduces their voltage rating and losses. The high voltage overshoot problem in frequency step-up operation is also analyzed. The sliding mode (SM) controller is employed to solve this problem. Pulse selective approach determines the power quality of load voltage. The validity of the mathematically computed values is carried out by modelling the proposed topology in MATLAB/Simulink environment and through hardware results.

Control strategy of doubly‐fed induction generator during the grid voltage swell

Control strategy of doubly‐fed induction generator during the grid voltage swell

A Simple Operation Approach for Modular Multilevel Converter Under Grid Voltage Swell in Medium-Voltage Microgrids

Modular multilevel converter (MMC) is drawing more and more attention in medium-voltage microgrid applications. Large-scale switching-off loads and grid faults may lead to the voltage swell in microgrid, where MMC should have the ride-through ability. In this paper, a simple operation approach with the fundamental and irregular zero-sequence voltage (ZSV) injection is proposed for MMC under grid voltage swell condition. First, the fundamental ZSV injection is applied to adjust the modulation margin of three phases where the amplitude values of modulation waves in each phase can be the same. Then, if the amplitudes with fundamental ZSV injection are still in over-modulation status, the new irregular ZSV injection is added to further enlarge the operation region. In addition, the limitation of the irregular ZSV injection is analyzed. Simulation and experimental results of MMC under different grid voltage swell conditions verify the effectiveness of the proposed operation approach.

Analysis of Electromagnetic Transient Characteristics of Doubly-Fed Induction Generator Under Grid Voltage Swell

The electromagnetic transient characteristics (ETC) of doubly-fed induction generator (DFIG) under grid voltage dip have been extensively studied theoretically, and various control strategies have been proposed. However, the ETC of DFIG under grid voltage swell are less studied. In order to study the influence on the ETC of DFIG under grid voltage swell and the corresponding control strategy, this paper compares and analyzes the steadystate and transient components of the stator flux linkage, rotor induced voltage and rotor current of DFIG under grid voltage swell and dip. It is deduced that DFIG is less prone to rotor overcurrent under grid voltage swell so the crowbar protection circuit is not needed, and the grid voltage amplitude changes little, the rotor side inverter is less prone to overmodulation. An experimental setup and simulation model is implemented with high-voltage ride-through of 1.5 MW doubly-fed wind Turbine. Simulation in addition to experimental results verifies the correctness and effectiveness of the theoretical analysis.

Unified Non-Inverting and Inverting PWM AC–AC Converter With Versatile Modes of Operation

This paper proposes unified non-inverting and inverting ac–ac converter (UNI-AC) using pulse width modulation (PWM) for the utility voltage compensation. It offers four effective switching states to regulate the output voltage in bipolar manner, facilitating versatile modes of operation with different number of switching states being modulated. Each mode of the proposed UNI-AC is able to compensate both the grid voltage sag and swell problems due to its bipolar voltage gain. The operational principle and comparison for all these modes are investigated in details. Also, the UNI-AC is reversible and compatible with full range of power factor. Other technical merits offered by the proposed approach include the compact hardware installation, reduced switch voltage stress (also low dv/dt) and decreased control complexity. Detailed analysis and experimental verification are presented in this paper.

Reactive Current Assignment and Control for DFIG Based Wind Turbines during Grid Voltage Sag and Swell Conditions

This paper proposes a reactive current assignment and control strategy for a doubly-fed induction generator (DFIG) based wind-turbine generation system under generic grid voltage sag or swell conditions. The system’s active and reactive power constrains during grid faults are investigated with both the grid- and rotor-side convertors (GSC and RSC) maximum ampere limits considered. To meet the latest grid codes, especially the low- and high-voltage ride-through (LVRT and HVRT) requirements, an adaptive reactive current control scheme is investigated. In addition, a torque-oscillation suppression technique is designed to reduce the mechanism stress on turbine systems caused by intensive voltage variations. Simulation and experiment studies demonstrate the feasibility and effectiveness of the proposed control scheme to enhance the fault ride-through (FRT) capability of DFIG-based wind turbines during violent changes in grid voltage.