Grid Synchronization Algorithm Research Articles

Enhanced PLL-less grid synchronization algorithm amidst unbalanced and distorted three-phase grid conditions

Fast and precise tracking of grid voltage attributes is essential for the smooth and reliable control and operation of grid-side converters. The synchronous reference-frame phase-locked loop (SRF-PLL) approach is commonly used for this purpose, although it has issues related to the stability limit and sluggish transient response. Consequently, PLL-less grid synchronization approaches are becoming increasingly prevalent. In this context, in the present paper, a discrete Fourier transform-based band pass filter (DFT-BPF) is designed in the prefiltering stage for the purpose of extracting the fundamental frequency positive sequence component (FFPS) from unbalanced and distorted grid conditions. Despite being widely used, the SRF-PLL is struggling with stability issues and demands tedious tuning effort. For this objective, a simple frequency estimator has been developed, which is capable of accurately estimating the fundamental frequency. Additionally, a curve fitting based error correction technique is employed to adaptively estimate the amplitude and phase-angle of the fundamental component without using frequency feedback loop. The experimental results validate the efficacy of the proposed scheme.

Miniaturized Distributed Generation for a Micro Smart Grid Simulator

In this paper, a miniaturization method is proposed for developing micro distributed generation for a micro smart grid simulator. The micro smart grid simulator is a fault simulator that was built to test and verify the new operation control algorithms for smart grids in the laboratory and has a size downscaled to one-thousandth of that of an actual smart grid. The micro distributed generation was designed in a multi-layered structure (dimension: 13 × 20 cm2), in which each function is implemented in several layers, to satisfy the size requirements. Next, the grid synchronization and PQ control algorithms required for the distributed generation were developed. A three-phase 19 V power system was built, and a 19 V–7.5 W three-phase micro distributed generation was realized through experimental verification. In addition, by verifying the effectiveness through grid synchronization and 7.5 W PQ control experiments, it was confirmed that the micro distributed generation based on the proposed miniaturization method can be implemented in a micro smart grid simulator.

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.

Hierarchical Energy Management in Islanded Networked Microgrids

Networked microgrids have many advantages for consumers and small energy producers, including higher reliability than non-networked microgrids. However, energy transaction, network interconnection, the intermittent nature of renewable sources, and other problems lead to challenges in the practical implementation of networked microgrids. Despite its favorable use of space, a floating PV system presents challenges that differ from those associated with its land-based counterparts because it is prone to the motion of the surface of the water, resulting in an unpredictable power output. This work presents a hierarchical energy management system (EMS) to address these issues. In level 1 of the EMS, which is for the overall management thereof, a blockchain model is used to manage transactions among microgrids. A grid synchronization algorithm is implemented in level 2 of the EMS, which manages the interconnection of microgrids. Level 2 is activated when an energy transaction between microgrids is needed. An on-line recurrent neural network (RNN)-based controller for an energy storage system (ESS), which is designed specifically to mitigate the problem caused by a floating PV platform, is deployed in level 3 as a local controller. The results of a hardware-in-the-loop (HIL) simulation demonstrate that the EMS can properly coordinate the levels in the hierarchical scheme to interconnect and provide power support between the microgrids. Real-time simulation results show that the ESS controller responds well, proving the viability of the hardware controller. According to these findings, the hierarchical EMS that is proposed in this work can solve the considered problems.

Sensorless Voltage Control Schemes for Brushless Doubly-Fed Induction Generators in Stand-Alone and Grid-Connected Applications

The aim of this paper is to investigate the sensorless voltage control schemes for the promising wound-rotor brushless doubly-fed induction generator (BDFIG) in both the stand-alone and grid-connected systems. For sensorless target, a new rotor position observer using the space vector flux relations of BDFIG is suggested. The proposed sensorless algorithm does not require any additional observers for obtaining the generator speed by directly detecting the rotor position, which may reduce the cost of the overall control system. For the voltage control purpose, a direct voltage control (DVC) scheme is applied for the stand-alone systems. However, this control strategy did not handle the control of the generated-voltage phase angle and also did not consider the phase sequence issue which are very essential provisions for the smooth connection target in grid-connected applications. Hence, DVC strategy cannot guarantee all the intended provisions for a soft and smooth connection process with the grid side and therefore, an effective grid-synchronization algorithm is proposed. A comprehensive analysis to confirm the functionality of the proposed control strategies is discussed through both simulation and experimental results. The presented study in this paper verifies the efficacy of the proposed schemes for sensorless voltage control target of BDFIG in both the stand-alone and grid-connected applications with a good transient behavior.

Benchmarking of phase lock loop based synchronization algorithms for grid-tied inverter

In renewable energy-based generation sources a phase locked loop is one of the most popular synchronization techniques. A rapid and precise grid voltage phase and frequency estimation under a wide spectrum of possible grid disturbances is its main objective. This paper compares popular grid synchronization algorithms on grid voltage anomalies. The compared algorithms are divided in three groups: without filtering, with filtering in synchronous reference frame and with filtering in stationary reference frame. The behaviour of the algorithms is tested in a laboratory, using dSpace 1103 as a platform on which the algorithms are compiled and OMICRON CMC 356 as a programmable grid voltage generator. The benchmarks conducted in this paper include voltage sags, grid voltages harmonics, DC offset and frequency step change. The obtained results show that there are significant differences in tested PLL responses for some networks disturbances.

Effects of post-filtering in grid-synchronization algorithms under grid faults

The spread of distributed generation systems has reinforced concerns and requirements on grid-tied power converters. The synchronization with the utility voltage vector is a major concern. In the literature, algorithms based on Phase Locked Loops are extensively presented. In spite of adequate performances under ideal and balanced grid conditions, under grid faults great inaccuracies arise. Shortcomings are overcome by advanced algorithms at the expenses of the complexity and computational cost. In this paper, grid synchronization algorithms are addressed. A solution is proposed by introducing a new post-filter stage in a Decoupled Double Synchronous Reference Frame not affecting, at the same time, the complexity of implementation, the detection time and damping of the conventional DDSRF solution. The post-filtering stage design criteria are described based on a mathematical derivation of the phase error in a conventional DDSRF algorithm under distorted grid utility. A comparison of system performances with the conventional DDSRF algorithm is carried out under distorted, balanced and unbalanced utility conditions, validating the benefits brought by the proposed solution.

A Novel Open-Loop Frequency Estimation Method for Single-Phase Grid Synchronization Under Distorted Conditions

In this paper, a new open-loop architecture with good dynamic performance and strong harmonic rejection capability is proposed for single-phase grid synchronization under distorted conditions. Different from previous single-phase grid synchronization algorithms based on the phase-locked loop technique, the proposed method is to estimate the frequency and phase angle of the grid voltage in an open-loop manner so that fast dynamic response and enhanced system stability can be achieved. First, an open-loop frequency estimation algorithm is introduced under ideal grid condition. Then, it is extended to distorted grid voltages through the combination of the developed frequency estimation unit and a prefiltering stage consisting of a second-order low-pass filter and a cascaded delayed signal cancellation (DSC) module. In addition, a transient process smoothing unit is designed to achieve smooth frequency transients in cases where the grid voltage experiences fast and large changes. The working principle of the new frequency estimation algorithm and the developed single-phase grid synchronization approach is given in detail, together with some simulation and experiment results for verifying their performance.

A Fast and Robust Grid Synchronization Algorithm of a Three-phase Converters under Unbalanced and Distorted Utility Voltages

In this paper, a robust and fast grid synchronization method of a three-phase power converter is proposed. The amplitude and phase information of grid voltages are essential for power converters to be properly connected into the utility. The phase-lock-loop in synchronous reference frame has been widely adopted for the three-phase converter system since it shows a satisfactory performance under balanced grid voltages. However, power converters often operate under abnormal grid conditions, i.e. unbalanced by grid faults and frequency variations, and thus a proper active and reactive power control cannot be guaranteed. The proposed method adopts a second order generalized integrator in synchronous reference frame to detect positive sequence components under unbalanced grid voltages. The proposed method has a fast and robust performance due to its higher gain and frequency adaptive capability. Simulation and experimental results show the verification of the proposed synchronization algorithm and the effectiveness to detect positive sequence voltage.

Controller Area Network Assisted Grid Synchronization of a Microgrid With Renewable Energy Sources and Storage

Microgrids are emerging as an important constituent of large-scale smart grids. They are equipped with the controls necessary for managing the operation in islanded or grid connected mode and serve the loads with clean, reliable, and uninterruptible power. Microgrid must be equipped with a robust grid synchronization (GS) algorithm so as to allow a smooth transfer from islanded mode to grid tied mode. This gets even more critical when the availability of the power grid is irregular or the power grid is weak. This paper proposes a simple yet highly reliable GS technique based on controller area network (CAN) communication. The grid synchronizer block senses the three phase grid voltages, derives the phase angle ( $\boldsymbol {\theta }$ ) using synchronously rotating reference frame-based phase lock loop, and transmits this information on the CAN network to all micro-sources (MS). The local controller associated with each MS receives the data with small but known and definite time latencies. This allows each MS to energize simultaneously in synchronization with the grid, facilitating easy GS of all the MS, minimizing the time required for GS of the complete microgrid. All the analytical, simulation, and hardware results related to this paper are presented.

Grid Synchronization Algorithm For DG Systems Using DSRF PLL

Distributed Generation (DG) System is a small scale electric power generation at or near the user’s facility as opposed to the normal mode of centralized power generation. In order to ensure safe and reliable operation of power system based on DS, grid synchronization algorithm plays a very important role. This paper presents a novel Double Synchronous Reference Frame (DSRF) PLL (based on synthesis circuit) for grid connected inverter system under grid disturbances. Due to flexible in characteristics, DSRF PLL can accurately detect the phase irrespective of the grid conditions. Further, it demonstrates how the PLLs track the phase angle during some of the major abnormal grid conditions like voltage unbalance, line to ground fault and voltage sag etc.

A Stationary Reference Frame Grid Synchronization System for Three-Phase Grid-Connected Power Converters Under Adverse Grid Conditions

Grid synchronization algorithms are of great importance in the control of grid-connected power converters, as fast and accurate detection of the grid voltage parameters is crucial in order to implement stable control strategies under generic grid conditions. This paper presents a new grid synchronization method for three-phase three-wire networks, namely dual second-order generalized integrator (SOGI) frequency-locked loop. The method is based on two adaptive filters, implemented by using a SOGI on the stationary αβ reference frame, and it is able to perform an excellent estimation of the instantaneous symmetrical components of the grid voltage under unbalanced and distorted grid conditions. This paper analyzes the performance of the proposed synchronization method including different design issues. Moreover, the behavior of the method for synchronizing with highly unbalanced grid is proven by means of simulation and experimental results, demonstrating its excellent performance.