Grid Synchronization Research Articles

Seamless Operation of Master–Slave Organized AC Microgrid With Robust Control, Islanding Detection, and Grid Synchronization

In this article, islanding detection, control, grid synchronization, and power share techniques have been considered for the seamless operation of ac microgrid in grid-connected mode, islanded mode, and during the transition between both modes. A master–slave organization coordinates the power distribution among microgrid sources. The control scheme of each source is designed with a nested loop robust linear quadratic regulator and mixed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$H_{2}/H_{\infty }$</tex-math></inline-formula> optimal controller. The islanding detection algorithm is developed with synchro-extraction transform and optimal support vector machine (OSVM). An optimal linear Kalman filter-based synchronous reference frame phase locked loop (OLKF-SRFPLL) method is considered for the reconnecting of the microgrid to the utility grid. For optimal parameter tuning in the OSVM and LKF-SRFPLL methods, a particle swarm optimization technique is used. The seamless operation of the proposed ac microgrid system is verified in MATLAB/SIMULINK, and the practical feasibility is verified with an experimental photo-type setup and dSPACE 1202 control kit. Simulation and experimental results are presented to illustrate the seamless operation of the proposed ac microgrid system.

Interactive Grid Synchronization-Based Virtual Synchronous Generator Control Scheme on Weak Grid Integration

While high volume of power electronic converters with renewable energy are being integrated into the grid, low system strength of grid causes reduction in system inertia and loss of synchronization with power converters. This paper proposes a novel interactive control scheme of voltage source converter (VSC) connected to weak grid. Firstly, a grid synchronization approach is performed using VSC side voltage transformed on rotating reference frame with respect to grid voltage, which is carried out based on lowest short circuit ratio (SCR) as approved by utilities such as Australian Energy Market Operator’s system strength report. The transformed voltage is then fed into the active power and reactive power control through PLL. Secondly, the grid synchronization approach through PLL is made interactive by adding virtual synchronous generator (VSG) control into the outer loop, which generates virtual inertia to mimic the synchronous generators of a power system. The design of the VSG control parameters is realized based on small signal stability analysis. Furthermore, the control method is evaluated as per the National Electricity Rules clauses S5.2.5.1-S5.2.14 under AEMO and National Service Providers of Australian strict Industry guidelines. The time-domain simulation results are conducted on PSCAD platform.

Evaluating Small-Signal Synchronization Stability of Grid-Forming Converter: A Geometrical Approach

Grid synchronization stability is an important research topic for grid-forming converters. To prevent instability and undesired oscillations, the small-signal modeling and stability analysis must be performed, which requires the line impedance information. Nevertheless, the line impedance may be unknown in most cases. As a consequence, the designed control parameters may fail to satisfy the stability requirement. To fill in this research gap, this article presents a geometrical approach to evaluate and improve the small-signal synchronization stability of grid-forming converters. The proposed analytical approach maps the line impedance variation on a two-dimensional plane and identifies the stability boundary in a geometrical way. As a benefit, the controller of grid-forming converters can be explicitly designed to achieve the robust synchronization stability. Finally, the feasibility of the proposed technique is verified by simulation and hardware experimental results.

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.

Open-Loop Synchronization Systems for Grid-Tied Power Converters: Literature Overview, Design Considerations, Advantages, and Disadvantages

Primitive grid synchronization systems of power converters were based on a zero-crossing detection (ZCD) algorithm, which is an open-loop synchronization (OLS) system. ZCD algorithms, however, could cause some stability problems for power converters. Therefore, they were gradually replaced by closed-loop synchronization systems. Recently, in parallel with developing closed-loop synchronization systems, designing efficient OLS systems has received much attention. This article aims to provide an overview of recent developments in designing OLS systems, analyze the systems’ advantages/disadvantages, and formulate their design procedure into some general yet simple steps.

Multiple Delayed Signal Cancellation Filter-Based Enhanced Frequency-Locked Loop Under Adverse Grid Conditions

An accurate and prompt estimation of phase angles, frequency, and magnitudes of the grid voltage is extremely important in paving a good platform for a fast and reliable operation of power grid monitoring, protection, and control. The phase-locked loop (PLL) served this purpose for decades, while the concept of the frequency-locked loop (FLL) has been proposed more recently. Even though the FLL and the PLL share some similarities, some differences still exist. Further research on FLLs can boost their capabilities and open up new windows in designing devices for grid synchronization. Multiple delayed signal cancellation (MDSC) filters are a peculiar system of filters. Their applications to FLLs can enrich FLL’s performance. This article focuses on applying the stationary reference frame MDSC filters at in-loop and prefiltering stages of the FLL and compares its effectiveness with widely used cascaded delayed signal cancellation (CDSC) and complex bandpass filter (CBF) based filtering techniques. The application of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$K$</tex-math></inline-formula> -factor-methodology-based controller design to FLLs has also been explored for dynamic performance enhancements. A new cascaded multiple delayed signal cancellation (CMDSC) filter is proposed and is compared with its CDSC counterpart proposed in the literature to address the effects of dc offsets and imbalances under adverse grid conditions. The effectiveness of the proposed method is verified through OPAL-RT real-time simulations and dSPACE ds1104 and TMS320F28335 microcontroller experimental test bench.

Back‐electromotive‐force observer (BEMF observer) based symmetrical PLL for grid synchronization stability enhancement under weak grid conditions

The synchronous reference frame phase-locked loop (SRF-PLL) is widely utilized for grid synchronization in voltage-source converter (VSC) system. However, it fails to eliminate the effect of high grid impedance leading to instability in weak grids. To address this issue, this paper introduces the back-electromotive-force observer (BEMF observer), commonly being utilized in DC motor, to modify the input of the SRF-PLL. To be specific, the equivalent internal electromotive force calculated by the BEMF observer rather than the PCC voltage is used to capture the phase information. Since the BEMF observer is designed in α β $\alpha \beta$ -frame while the SRF-PLL is in d q $dq$ -frame, the conventional stability analysis method suffers from complicated process. Thus, this paper further reconstructs the SRF-PLL with the BEMF observer to be symmetrical for simplification. Apart from the symmetrical structure, the stability criterion is accordingly modified. As a result, the proposed back electromotive force based symmetrical PLL (BEFS-PLL) offers the following advantages: (i) it effectively enhances the grid synchronization stability and simplifies the stability analysis process simultaneously, (ii) it exhibits a strong robustness even with the calculation error up to 50%, and (iii) it is characteristic of clear physical meanings. Finally, the effectiveness of the BEFS-PLL is verified by simulation.

Enhanced Harmonics Reactive Power Control Strategy Based on Multilevel Inverter Using ML-FFNN for Dynamic Power Load Management in Microgrid.

The shift of the world in the past two decades towards renewable energy (RES), due to the continuously decreasing fossil fuel reserves and their bad impact on the environment, has attracted researchers all around the world to improve the efficiency of RES and eliminate problems that arise at the point of common coupling (PCC). Harmonics and un-balance in 3-phase voltages because of dynamic and nonlinear loads cause a lagging power factor due to inductive load, active power losses, and instability at the point of common coupling. This also happens due to a lack of system inertia in micro-grids. Passive filters are used to eliminate harmonics at both the electrical converter’s input and output sides and improve the system’s power factor. A Synchronous Reference Frame (SRF) control method is used to overcome the problem related to grid synchronization. The sine pulse width modulation (SPWM) technique provides gating signals to the switches of the multilevel inverter. A multi-layer feed forward neural network (ML-FFNN) is employed at the output of a system to minimize mean square error (MSE) by removing the errors between target voltages and reference voltages produced at the output of a trained model. Simulations were performed using MATLAB Simulink to highlight the significance of the proposed research study. The simulation results show that our proposed intelligent control scheme used for the suppression of harmonics compensated for reactive power more effectively than the SRF-based control methods. The simulation-based results confirm that the proposed ML-FFNN-based harmonic and reactive power control technique performs better in terms of MAE, for the case of MSE, and when evaluating based on the RMSE.

A Novel Control Strategy in Grid-Integrated Photovoltaic System for Power Quality Enhancement

The integration of solar photovoltaic (PV) systems and utility grids has gradually gained significant interest in improving the sustainability of clean power supply for society. However, power quality remains a challenge due to partial shading conditions and harmonics. To overcome these drawbacks, a flexible radial movement optimization based on a dynamic safety perimeter maximum power point tracking algorithm is employed to track maximum power out of a PV system and to ensure that the optimum voltage level at the common DC bus is obtained under partial shading conditions using fixed-tilt installation configuration. Furthermore, a novel inverter control loop system with a double second order generalized integrator phase-locked loop (DSOGI-PLL) is also proposed to mitigate harmonics and improve the power quality of the grid interfacing PV system using MATLAB SIMULINK software. The proposed system has several merits, such as better harmonic suppression capability, control adaptivity, rapid tracking speed, low computational burden and phase and grid synchronization.

Grid Synchronization of Equivalent Wind Farm Equipped with DFIG Model for Transient Stability by Using Nonlinear Integral Backstepping Control

Grid Synchronization of Equivalent Wind Farm Equipped with DFIG Model for Transient Stability by Using Nonlinear Integral Backstepping Control

Fast harmonic analysis for PHIL experiments with decentralized real-time controllers

This paper proposes and implements, a harmonic analysis technique used in microgrids for inverter power control when measured voltage and current signals are passed over a communication link with considerable latency. Using frequency-shifting and filtering techniques, the measurement is converted to magnitude and phase information and passed over an asynchronous communication link to another controller, where the original waveform is recovered with delay compensation. The method allows accurate power calculations and grid synchronization over distributed prosumer controllers. The proposed method can work at different execution rates, depending on real time (RT) workload, and is shown to be robust against step changes, harmonics and communication delays. The method is demonstrated with two PHIL experiments at the CoSES, TU Munich lab in grid connected and island mode.

Wind energy conversion system using perturb &amp; observe-based maximum power point approach interfaced with T-type three-level inverter connected to grid

Abstract In this paper, the performance of a permanent magnet synchronous generator (PMSG)-based wind energy conversion system (WECS) supplied to an uncontrolled rectifier-fed boost converter (BC) interfaced with a three-phase T-type three-level inverter (TLI) has been analysed. The proposed WECS involves three converters, namely an uncontrolled rectifier that is used for conversion from AC to DC; a BC supplied by a PMSG-fed rectifier used to enhance the voltage gain; and a grid-connected three-phase T-type TLI is proposed to eliminate power-quality issues with synchronization of grid voltage and current. The main goal of this research is to model and control the grid-connected T-type TLI using a d–q synchronous frame for wind energy for regulating the DC-link voltage and transferring the generated wind power from the BC to the grid. Furthermore, the perturb &amp; observe (P&amp;O)-based maximum power point (MPP) approach is recommended to keep track of the MPP for a BC that is supplied from a PMSG-based WECS under constant and variable wind speeds. The proposed PMSG-based WECS interfaced with grid-connected T-type TLI using d–q control has been computationally modelled, simulated and validated with constant and variable speeds using MATLAB® and Simulink®. It is confirmed that the P&amp;O-based MPP approach ensures maximum power for varying wind speeds, and the total harmonic distortion of the T-type TLI grid current value is 3.18%, which is within IEEE-519 limits. Furthermore, with grid synchronization, the power factor of the T-type TLI is maintained at unity to avoid power-quality issues.

Performance improvement of a three‐phase PLL under distorted grid conditions based on frequency adaptive hybrid pre‐filtering

The phase-locked loop (PLL) is widely used for grid synchronization in converter control. The moving-average filter (MAF)-based PLL has recently attracted considerable attention. However, the response time is drastically increased due to the MAF's large window width, determined by the lowest harmonics. Furthermore, the filtering performance of MAF is affected by frequency fluctuation. This paper presents an improved PLL with a hybrid pre-filtering, frequency-adaptive MAF-based PLL (FAMAF-PLL), ingeniously cascading the MAF and the dual-second-order generalized integrator frequency-locked loop (DSOGI-FLL). The DSOGI-FLL measures the frequency and filters out the lowest harmonics caused by the three-phase imbalance. Thus, the delay time of MAF is reduced by narrowing the window width, and the filtering performance of the MAF is greatly improved through frequency adaptation based on the DSOGI-FLL. The proposed FAMAF-PLL significantly enhances the performance and accelerates the dynamic response compared with MAF-PLL. Experiments were carried out to validate the effectiveness and robustness of the proposed method.

Comparative analysis of dual second-order generalized integrator-phase locked loop based series hybrid filter and static synchronous compensator for load voltage compensation and power quality improvement with grid synchronization in islanded microgrid

PurposeThe purpose of this study is to eliminate voltage harmonics and instantly measure the positive sequence fundamental voltage during unbalanced grid conditions, the dual second-order generalized integrator-phase locked loop used in series hybrid filter structures is often used in grid synchronisation in three-phase networks. The preferred series active hybrid power filter simultaneously compensates for voltage balancing and current harmonics generated by non-linear loads.Design/methodology/approachThis paper examines the use of renewable energy–based microgrid (MG) to support linear and non-linear loads. It is capable of synchronising with both the utility and the diesel generator unit. Power is transferred from the grid throughout a stable grid situation with minimum renewable energy generation and maximum load demand. It synchronises with diesel generator set to supply the load and form an AC MG during outages and minimum renewable power generation. In islanded and grid-connected mode, the voltage and power quality issues of the MG are controlled by static synchronous compensator and series hybrid filter.FindingsBecause of the presence of non-linear loads, reactive loads in the distribution system and the injection of wind power into the grid integrated system result power quality issues like current harmonics, voltage fluctuations, reactive power demand, etc.Originality/valueThe voltage at the load (linear and non-linear) is regulated, and the power factor and total harmonic distortions were improved with the help of the series hybrid filter.

Advanced Grid Synchronization Scheme Based on Dual eSOGI-FLL for Grid-Feeding Converters

In this article, an improved grid synchronization method for three-phase grid-feeding converters using a dual enhanced second-order generalized integrator (eSOGI) frequency-locked loop (FLL) is proposed. The eSOGI, of which gains can be adjusted based on the positive-sequence component of the grid voltage, is designed to estimate the grid voltage accurately. Moreover, to improve the grid frequency estimation under grid voltage sags, frequency rate limits is suggested in the FLL structure. The selection of the eSOGI-FLL parameters is described as well. After the estimated grid voltages are obtained using the dual eSOGI-FLL, they are used to synthesize the references in the dual-current controllers for positive- and negative-sequence components. Through the simulation and experimental results under grid voltage sags and distorted grid conditions, the improved power control performance of the proposed method has been demonstrated in comparison with those of the existing methods.

Self-Tuning PLL: A New, Easy, Fast and Highly Efficient Phase-Locked Loop Algorithm

A phase-locked loop or phase lock loop (PLL) is, essentially, a closed loop control system, where its output signal (regulated) maintains a direct (chosen) relationship to the input signal (unregulated). If the phases of the input and output signals remain the same, their frequencies will also match. In its usual way, a PLL is an electronic circuit with filters and controllers that must be tuned each time, according to the input signal parameters. As a contribution and novelty, this paper introduces a new, easy, fast and highly efficient PLL algorithm, that it does not need to adjust every time the input signal changes. This makes it independent of the input signal it receives and, therefore, and in a certain way, universally applicable. In addition, the proposal is implemented exclusively by software, housed in a microcontroller, which also represents another novelty. As another remarkable feature, the proposed algorithm is very simple, with a very low computational cost, which makes it very stable, practically insensitive to noise, and very fast. These characteristics and the performance shown over a wide range of input signal frequencies, make the proposed algorithm suitable for use in different applications, from the electricity grid synchronization to the demodulation of frequency modulation, DC motor drives, etc. In addition to frequency and phase tracking, the developed algorithm generates the signal corresponding to the fundamental frequency of the input signal. Furthermore, the algorithm behavior is not affected by the input signal distortion and is independent of its initial phase. The performance and excellent behavior of the developed algorithm are evaluated through simulations and experimental tests.

Inertia Emulation and Fast Frequency-Droop Control Strategy of a Point-to-Point VSC-HVdc Transmission System for Asynchronous Grid Interconnection

This article proposes a novel inertia emulation and fast frequency-droop (IEFF) control strategy for a point-to-point voltage source converter-based high voltage direct current (VSC-HVdc) transmission system. It allows asynchronous ac grid synchronization in forms of inertial and fast frequency-droop responses provided to “both” sides of the VSC-HVdc interconnected grids, different from the historically reported schemes only providing these responses to one grid. Under the IEFF scheme, the power-regulating VSC (PR-VSC) station emulates a synchronous generator directly through its active power control, whereas the dc-voltage-regulating VSC (DR-VSC) station employs a dc voltage/frequency droop control to convey its connected grid frequency communication-free to the remote PR-VSC for inertial and fast frequency-droop oriented power variations. To avoid the clash of the PR-VSC IEFF and DR-VSC IEFF control under simultaneous frequency events in the asynchronous grids, a novel frequency support mode selection scheme is designed to effectively prioritize the IEFF support with a higher frequency gradient. The key parameters of the IEFF controller are optimized through small signal stability analysis. The effectiveness of the proposed IEFF scheme is verified through controller hardware-in-the-loop experiments. The results show that the proposed IEFF scheme effectively promotes synchronization and enhances overall frequency stability for the two interconnected asynchronous grids.

A novel approach of synchronization of the sustainable grid with an intelligent local hybrid renewable energy control

A novel approach of synchronization of the sustainable grid with an intelligent local hybrid renewable energy control

Power-Sharing Analysis of Hybrid Microgrid Using Iterative Learning Controller (ILC) considering Source and Load Variation

In this study, a novel iterative learning controller is proposed to maintain power balance in hybrid microgrid considering source and load variation. A hybrid microgrid comprising of solar, battery, utility grid, and AC and DC loads are developed with ac and dc bus in MATLAB Simulink environment. The primary or local level control is implemented for solar and battery for maintaining stable dc bus voltage. The secondary or system level control is implemented by controlling the interlinking converter placed between ac and dc bus to supply stable voltage along with the smooth grid synchronization and ensuring the proper real power sharing between dc/ac buses. The simulation studies reveal that the proposed iterative controller has efficient and effective control over the voltage and power in hybrid microgrid under various modes of operation such as autonomous and grid connected modes.

Fifth-Order Generalized Integrator for a Double-Stage Single-Phase Grid-Interfaced Photovoltaic Supply System

This paper presents a fifth-order generalized integrator (FOGI) as an orthogonal signal generator (OSG) to obtain a grid synchronization signal for a double-stage single-phase grid-interfaced photovoltaic supply system (GIPVSS). The proposed GIPVSS consists of a single-ended primary inductor converter (SEPIC) and a H-bridge voltage source converter (HBVSC) to transfer solar power to the grid system. The SEPIC controls maximum power delivery (MPD) from the Photovoltaic (PV) array and feeds that power to the grid via an HBVSC. In this study, all energy storage elements (inductors and capacitors) of SEPIC are considered with their effective series resistance (ESR). The dynamic modeling of SEPIC is obtained using a small signal analysis. The control of VSC allows harmonics’ mitigation and power factor improvements. A FOGI and a second-order adaptive filtering approach are used in controling VSC. The FOGI is used to obtain a grid synchronization signal from sensed grid voltage under disturbances. The second-order adaptive filtering approach is used to obtain an active load current component. The comparative analysis of FOGI with other reported algorithms is carried out under harmonics in sensed grid voltage. The effectiveness of the proposed system and its control is validated in a laboratory prototype using a Launch Pad TMS320F28379D digital signal processor (DSP).