Grid Synchronization Technique Research Articles

Simple solution of DC-offset rejection based phase-locked loop for single-phase grid-connected converters

Distributed Generators (DG) systems based on Renewable Energy Sources (RES) such as hydro, wind, and solar power plants have been spread widely due to their lower cost and the advanced capability of connecting them with the grid. The power generated from the DG must be shaped to be interfaced with the grid employing power electronics converters. The grid-connected power electronics converters must be synchronized with the grid (i.e., the same fundamental component of the grid frequency, phase, amplitude, and sequence). Synchronization techniques are employed to achieve accurate and fast grid synchronization between the converter and the grid. The existence of (DC-offset) in the input of Phase Locked Loop (PLL) caused synchronization problems as it causes oscillations in the estimated fundamental grid phase, frequency, and amplitude. In addition, the closed-loop system stability can be affected. This work proposes a simple technique for grid synchronization based on PLL with a phase angle correction. The proposed method was developed using Transfer Delay (TD) and Delay Signal Cancelation (DSC) operators; then, the small single model and stability analysis was employed. Several scenarios were developed to compare the proposed method with previous methods using MATLAB/Simulink tool. The scenarios involve introducing phase jumps, DC offsets, and amplitude changes to the grid voltage. Additionally, the grid frequency was also changed. The results show that the proposed PLL can solved the DC-offset problem using any delay time and fully synchronized with the grid. Moreover, the proposed PLL has the fastest dynamic response and shortest synchronization time over the other methods from literature.

A Unified Control of Grid-Interactive Off-Board EV Battery Charger With Improved Power Quality

This article presents a self-tuning filter (STF) and sliding mode control (SMC)-based control algorithm for a grid-interactive off-board electric vehicle (EV) battery charger to power the EV batteries and simultaneously improve the grid power quality. In grid-connected operation (GCO), the charger uses the STF-based control strategy to estimate the fundamental load current and synchronize voltage templates to generate a pure sinusoidal reference current. Compared to the phase-locked loop (PLL) and the second-order generalized integrator (SOGI), the STF-based technique precisely estimates the synchronizing voltage templates during nonideal grid voltage conditions. Therefore, in GCO, the charger reference current ensures the unit power factor in the grid-to-vehicle/vehicle-to-grid (G2V/V2G) operation mode and the harmonic free grid current in the charger-for-grid (C4G) operation mode. Furthermore, the charger operates in the vehicle-to-load (V2L) operation mode during grid outages to maintain an uninterrupted supply to the residential load. The control algorithm also includes a grid synchronization technique to achieve a smooth transition between GCO and V2L. Furthermore, an SMC-based dc-link voltage controller (SMC-DLVC) is proposed to reduce dc-link voltage overshoot with finite-time convergence during external disturbances in all operation modes. The performance of the proposed control algorithm is validated in a 12.6-kVA off-board charger laboratory prototype under ideal/nonideal grid voltage conditions.

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.

Modified LMS synchronization technique for distributed energy resources with DC-offset and harmonic elimination capabilities

Grid synchronization techniques are needed to improve the distribution system’s power quality with the Shunt Active Power Filter (SAPF). The traditional synchronization technique or the Phase Locked Loop (PLL) works well under ideal grid conditions; however, its performance deteriorates under non-ideal grid conditions. In this paper, a new scheme has been proposed to function as PLL. The Modified Least Mean Square (MLMS)-PLL has been proposed for tracking the phase angle under non-ideal grid conditions such as phase shift, frequency deviation, harmonics in the signal, DC offset and combinations of these grid voltage issues. The proposed method has added the advantage of DC-offset estimation and perfect synchronization template generation over conventional PLL. The MLMS-PLL algorithm precisely estimates phase, amplitude and frequency information and generates a synchronizing signal under abnormal grid conditions. The designed algorithm is extensively tested and shows advantages such as least steady-state error, faster dynamics and DC-offset rejection capability. Experimental findings of the proposed synchronizing technique show the effectiveness of the proposed technique and experimental results are also compared with other modern synchronization techniques. The application of MLMS-PLL for synchronization, reactive power compensation and harmonic elimination in a grid-tied PV system has been validated in the experimental prototype.

PV Integrated Multifunctional Off-Board EV Charger With Improved Grid Power Quality

This article presents an adaptive notch filter (ANF) based efficient control algorithm for multifunctional grid connected solar photovoltaic (PV) powered electric vehicle (EV) charger to power the EV batteries and simultaneously improve the grid power quality. Furthermore, a multilevel topology is adopted for the grid-facing converter to improve output voltage quality. The ANF accurately estimates the fundamental EV current and grid voltage for generating pure sinusoidal reference current and synchronizing voltage template, respectively. The ANF-based voltage template estimator precisely estimates in-phase and in-quadrature synchronizing voltage templates compared to a phase-locked loop and second-order generalized integrator during nonideal grid voltage conditions. The charger is designed to operate in grid connected operation (GCO) and standalone operation (SO) to optimize the PV generation. In GCO, the charger provides grid current harmonic compensation and reactive power support to the grid. Furthermore, it offers backup power to the residential load during an emergency. The charger control algorithm also includes a grid synchronization technique based on phase error minimization to achieve a smooth and seamless transition from SO to GCO and vice versa. The effectiveness of the proposed control algorithm is validated in a 12.6-kVA <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> -board EV charger laboratory prototype. The obtained results verify the charger performance complies with IEEE 1547 standard.

A high-speed frequency adaptive grid synchronization technique for single-phase inverter under distorted grid conditions

A high-speed frequency adaptive grid synchronization technique for single-phase inverter under distorted grid conditions

Enhanced Grid Synchronization Technique Based on Frequency Detector for Three-Phase Systems

An adaptive cascaded delayed signal cancellation (CDSC) method relying on a simple frequency detector is combined with a third-order function for three-phase grid synchronization application. In this article, the proposed synchronization technique shows a high degree of insensitivity to harmonics at both nominal and off-nominal frequency environments. When compared to a similar method relying on adaptive CDSC and phase-locked loop (CDSC-PLL), the proposed technique generates alike results but demands less computational effort due to avoiding real-time evaluation of computationally demanding trigonometric functions and square root operation. Moreover, unlike the CDSC-PLL method, the proposed one eliminates the interdependent loops exist between the phase and frequency estimation, thus a simpler tuning process is achieved. In addition, the proposed method stays away from the tuning of the loop filter in the PLL by removing the phase loop. The usefulness of the proposed method is established by both simulation and experimental results.

Benefits of Fuzzy Logic on MPPT and PI Controllers in the Chain of Photovoltaic Control Systems

This paper presents a comparative study between two maximum power point tracking (MPPT) algorithms, the incremental conductance algorithm (InC) and the fuzzy logic controller (FLC). The two algorithms were applied to a low photovoltaic power conversion system, and they both use different PI controllers and grid synchronization techniques. Moreover, both InC and FLC methods have Clarke and Park Transformation. To some extent, the incremental conductance and fuzzy logic controller approaches are similar, but their control loops are different. Therefore, the InC has classic Proportional Integrative (PI) controllers with simple phase-locked loops (PLL). At the same time, the FLC works with fuzzy logic PI controllers linked with the Second Order Generalized Integrator (SOGI). The proposed techniques examine the solar energy conversion performance of the photovoltaic (PV) system under possible irradiance changes and constant temperature conditions. Finally, a performance comparison has been made between InC and FLC, which demonstrates the effectiveness of the fuzzy controller over the incremental conductance algorithm. FLC turns to convert photovoltaic power easily, decreasing fluctuations, and it offers a quick response to the variation of solar irradiance (shading effect). The simulation results show a superior performance of the controller with fuzzy logic, which helps the inverter convert over 99% of the power generated by the photovoltaic panels. In comparison, the incremental conductance algorithm converts around 80%.

Accurate Phase Detection System Using Modified SGDFT-Based PLL for Three-Phase Grid-Interactive Power Converter During Interharmonic Conditions

Grid synchronization algorithm is used to track the fundamental phase angle and frequency of the grid voltages/ currents accurately, to generate the reference currents for control of grid-interactive power converters. Phase-locked loop (PLL) is vital in the grid synchronization technique. It is mandatory to track the phase angle and filter out the harmonics and interharmonics during distorted grid conditions. In this article, a modified sliding Goertzel discrete Fourier transform (SGDFT)-based PLL is proposed to extract the fundamental component during interharmonics conditions. The proposed PLL grants three degrees of freedom (DOFs), i.e., vigorous against several distorted grid conditions, namely, voltage sag, high-frequency harmonics, interharmonics, and dc offset. Furthermore, the proposed PLL precisely extracts the fundamental frequency and phase angle with the faster transient response and attains superior system stability. The detailed mathematical modeling of recommended PLL is analyzed with existing PLLs, such as harmonic, interharmonic, and dc offset PLL (HIHDO PLL), hybrid prefiltering stage PLL (HPFS PLL), and SGDFT-based PLL. The numerical simulations for the proposed PLL are carried out using MATLAB <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> /Simulink environment and the performance of the recommended PLL is tested experimentally through the dSPACE 1104 DSP platform.

Enhanced Frequency Adaptive Demodulation Technique for Grid-Connected Converters

This article presents an enhanced frequency adaptive demodulation technique for grid-synchronization of grid-connected converters in variable frequency condition. Demodulation works by generating demodulated voltages which contain undesired double frequency components. As a result, high-order low-pass filters with high cutoff frequency are required to eliminate the undesired components. This reduces the dynamic performance. Frequency adaptive demodulation technique enhances the dynamic performance by rejecting the double frequency components as opposed to filtering, however, at the cost of additional computational complexity. This article overcomes this problem by using double demodulation without recreating the double frequency component for rejection purposes. This reduces the computational complexity significantly. Suitability of the proposed method is verified through numerical simulation and experimental study. Comparative study with existing frequency adaptive demodulation and second-order generalized integrator phase-locked loop techniques demonstrate the validity and performance improvement by the proposed technique.

PV integrated grid synchronization technique using modified SOGI-FLL and zero-crossing detector

PV integrated grid synchronization technique using modified SOGI-FLL and zero-crossing detector

DC offset elimination method of Phase-locked loop based on novel generalized integrator

When the renewable energy power is connected to the grid, distortion voltage interference and grid voltage frequency fluctuation will occur, which will cause the inverter output current to be abnormal. So the grid-connected environment of weak grid puts forward higher requirements on the performance of phase-locked loop (PLL). The synchronous reference frame PLL(SRF-PLL) has become a usually adopted method in grid synchronization technique for its simple structure and strong stability. However, the conventional SRF-PLL has insufficient capability to suppress DC offset, which easily causes fundamental frequency and phase oscillations. Therefore, this paper presents a novel generalized integrator(NGI), and according to the presented NGI structure, a modified novel generalized integrator (MNGI) that can eliminate DC offset voltage is proposed. Then a novel PLL based on dual MNGI(DMNGI) is proposed. This method improves the capability of the phase-locked loop to suppress DC offset. At the same time, this paper designs the parameters of the PLL, and the rationality of the novel PLL is tested effectively through the experimental comparison with the other two methods.

Enhanced Third-Order Generalized Integrator-Based Grid Synchronization Technique for DC-Offset Rejection and Precise Frequency Estimation

An enhanced third-order generalized integrator (TOGI) method has been proposed for the estimation of grid frequency and the synchronizing signals. The proposed method precisely estimates grid frequency and synchronizing signals in the presence of high harmonic content, phase change, frequency change, noise, and DC-offset in the grid voltage. A robust frequency estimator based on three consecutive sampling method has been integrated with TOGI algorithm for accurate frequency estimation. The proposed technique gives zero steady-state error in case of frequency estimation under different grid voltage conditions. This paper demonstrates the developed enhanced TOGI technique for synchronization in a single-phase system. The efficacy of the proposed algorithm has been tested and analyzed in MATLAB/Simulink and verified with experimental results.

An intelligent grid synchronization technique for microgrid system with smooth power quality using chaotic grey wolf optimization‐random forest algorithm scheme

This manuscript proposes an intelligent grid synchronization technique for microgrid system with smooth power quality using proposed approach. The proposed approach is the consolidation of chaotic grey wolf optimization (CGWO) and random forest algorithm (RFA), hence it is called CGWO-RFA approach. The major purpose of this work is to efficiently differentiate the phase angle utility signal, monitor the phase with smooth frequency varieties, take out disruptions with harmonics, and consider the power to load uninterruptedly. In the proposed approach, the CGWO method is utilized to set the optimum control signals of the converter in offline mode depending on the power variation between the source and load side. Similarly, the limits of the proposed technique are voltage variation, frequency, and phase angle between the associated phases of the generator output and grid supply. The RFA predicts the optimal control signals depending on the accomplished dataset in the online way. The proposed CGWO-RFA method is executed in MATLAB/Simulink work site and the performance is analyzed with different existing methods, like CGWOANN, DSOGIPLL, DDSRFPLL, and DSOGIFLL. Moreover, the statistical analysis, GMPPT searching duration, and the brief comparison of synchronization scheme for explicit applications are likewise dissected and analyzed. Here, the GMPPT searching duration of the proposed technique is 0.413 seconds. The experimental outcomes demonstrate that the CGWO-RFA technique has high accuracy, high response, and robust to the faulty conditions.

A Third-Order MAF Based QT1-PLL That is Robust Against Harmonically Distorted Grid Voltage With Frequency Deviation

The quasi-type-l phase-locked loop (QT1-PLL) is a grid synchronization technique that has become very popular in recent years thanks to its attractive performance such as easy implementation, fast dynamic response, and good accuracy in steady-state operation. However, it is still vulnerable to operation under harmonically distorted grid voltages with frequency drift. This paper proposes a novel QT1-PLL based synchronization algorithm that makes an appropriate combination of two filters’ types: an in-loop third-order moving average filter (MAF) with a reduced window width, and a simplified second-order fast delayed signal cancellation (FDSC) based prefiltering stage. The proposed PLL is named third-order MAF based QT1-PLL (TQT1-PLL). Though both TQT1-PLL's filters do not need any adaptive algorithm, it is able to reject non-triplen odd-harmonics and the fundamental frequency negative sequence (FFNS) even under grid frequency drift. Its correct operation is confirmed through numerical simulations and real-time implementation on a digital signal processor (DSP). Moreover, the obtained results confirm its ability to reduce the ripple in the estimated frequency and phase under distorted grid voltages and off-nominal frequency operation. Authors show also through an analytical development that the topology of the proposed TQT1-PLL can be extended to enable the rejection of the DC-offset.

Enhanced Fault Ride-Through of Power Converters Using Hybrid Grid Synchronization

Inaccurate phase-angle jump (PAJ) estimation of the grid voltage during faults is one of the major causes for poor fault ride-through (FRT) performance of converters. To overcome this issue and make the converter’s current controller robust, this article proposes a hybrid grid synchronization transition technique. In this concept, a synchronous reference frame-based phase-locked loop (SRFPLL) grid synchronization method is used during normal grid operation and switched to an arctangent-based phase-angle estimation during grid faults. Simultaneously frequency estimation is switched to the arctangent derived frequency. A common transition method, which depends on the phase-angle error between the two-phase estimation techniques, is proposed to ensure a smooth transition between the hybrid phase-angle and frequencies. The transition technique is implemented using the current control of a three-phase voltage source converter in the SRF. The performance of the converter during both symmetrical and asymmetrical grid faults along with the FRT strategies is tested using real-time experiments. It is observed that the transition-based hybrid grid synchronization technique reduces the loss of synchronism duration. Additionally, it offers a more robust converter current control performance compared to the SRFPLL technique over a wide range of voltage sags and PAJs.

Adaptive Observer-Based Grid-Synchronization and Sequence Extraction Techniques for Renewable Energy Systems: A Comparative Analysis

Various faults and disturbances often make the grid unbalanced and distorted. To make the grid-connected converters resilient to unbalanced and distorted grid, real-time estimation of grid voltage sequences are essential. Adaptive-observer based grid-synchronization techniques are very suitable for this purpose. In this paper, a comparative analysis of three recently proposed adaptive observer-based grid-synchronization technique are presented for frequency estimation and sequence extraction. The considered techniques are: global adaptive observer, gain normalized adaptive observer, and second-order generalized integrator type adaptive observer. Following a fair and systematic gain tuning process, the performances are verified through extensive numerical simulation and experiments. Experimental results demonstrate the merits and demerits of the individual techniques. Academic researchers and industrial practitioners will find the results useful for the selection of adaptive observer-based grid-synchronization and sequence extraction techniques.

An Enhanced Frequency-Adaptive Single-Phase Grid Synchronization Technique

Second-order generalized integrator phase-locked loop (SOGIPLL) relies on the frequency feedback from the PLL estimation. The frequency is used to generate the equivalent in-phase and in-quadrature signals of the single-phase grid voltage. The phase-angle-coupled frequency feedback makes the SOGIPLL structure vulnerable to various grid voltage transients such as voltage sag, phase-angle jumps (PAJs), and frequency variations. This article proposes a PLL independent frequency estimation technique. It estimates the frequency using the simplified teager energy operation on the normalized in-phase voltage component output of the SOGI. The phase-angle is estimated using the third-order polynomial approximated arctangent function on the in-phase and in-quadrature outputs of the proposed frequency-adaptive SOGI. The grid synchronization technique presented in this work avoids the use of any trigonometric operations and PLL gain tuning issues. Additionally, the impact of steady-state grid disturbances, namely, harmonics and dc offset on the proposed synchronization technique is investigated. Finally, its performance robustness is compared with other techniques during both grid transients and steady-state disturbances using both simulation analysis and experimental validation.

Linear Kalman Filter-Based Grid Synchronization Technique: An Alternative Implementation

Grid synchronization techniques play a significant role in integrating renewable energy sources to the electric power grid. In this context, estimating the phase and frequency of the grid voltage signal is an interesting problem. Out of various techniques available in the literature, linear Kalman filter (LKF) is one of the most popular one. In this article, we propose an alternative implementation of the LKF for grid synchronization application. The proposed implementation uses a linear parametric model of the grid voltage signal including dc offset. It does not involve any quadrature signal generation, rather it works by estimating the phase angle. This helps to estimate the unknown grid frequency directly from the phase angle. This clearly differentiates the proposed alternative implementation with respect to the existing implementations. Performance improvement by the proposed technique is verified extensively through comparative numerical simulation and experimental studies. Comparative results demonstrate the suitability of the proposed technique with respect to other state-of-the-art techniques namely second-order generalized integrator phase-locked loop and enhanced phase-locked loop.

Digital implementation methods for grid synchronization using an integrated filter

The steady rise in the deployment of grid-connected inverters is driving a need for more efficient and accurate grid synchronization techniques. In this paper, digital implementation methods are proposed to increase grid synchronization accuracy when an integrated filter is used. First, a method to compensate for the computational delays in the implementation of integrated filters with numerical integrations is presented. Second, a simple method to estimate the grid frequency is introduced for updating the center frequency of integrated filters. The proposed methods have been implemented in a digital signal processor and tested with a real-time simulator. They achieve ripple errors of 0.14 mHz and 0.008° in the frequency and phase angle estimations, respectively.