In grid-connected applications, the synchronous reference frame phase-locked loop (SRF-PLL) is a commonly used synchronization technique due to the advantages it offers such as ease of implementation and robust performance Under ideal grid conditions, the SRF-PLL enables a fast and accurate phase/frequency detection; however, unbalanced and distorted grid conditions highly degrade its performance To overcome this drawback, several advanced PLLs have been proposed, such as the multiple reference frame-based PLL, the dual second-order generalized integrator-based PLL, and the multiple complex coefficient filter-based PLL In this paper, a comprehensive design-oriented study of these advanced PLLs is presented The starting point of this study is to derive the small-signal model of the aforementioned PLLs, which simplifies the parameter design and the stability analysis Then, a systematic design procedure to fine tune the PLLs parameters is presented The stability margin, the transient response, and the disturbance rejection capability are the key factors that are considered in the design procedure Finally, the experimental results are presented to support the theoretical analysis

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Design-Oriented Study of Advanced Synchronous Reference Frame Phase-Locked Loops

The utilization of power electronic inverters in power grids has increased tremendously, along with advancements in renewable energy sources. The usage of power electronic inverters results in the decoupling of sources from loads, leading to a decrease in the inertia of power systems. This decrease results in a high rate of change of frequency and frequency deviations under power imbalance that substantially affect the frequency stability of the system. This study focuses on the requirements of inertia and the corresponding issues that challenge the various country grid operators during the large-scale integration of renewable energy sources. This study reviews the various control techniques and technologies that offset a decrease in inertia and discusses the inertia emulation control techniques available for inverters, wind turbines, photovoltaic systems, and microgrid. This study attempts to explore future research directions and may assist researchers in choosing an appropriate topology, depending on requirements.

Future low-inertia power systems: Requirements, issues, and solutions - A review

Field programmable gate arrays (FPGAs) have established themselves as one of the preferred digital implementation platforms in a plethora of current industrial applications, and extensions and improvements are still continuously being included in the devices. This paper reviews recent advancements in FPGA technology, emphasizing the novel features that may significantly contribute to the development of more efficient digital systems for industrial applications. Special attention is paid to the design paradigm shift caused by the availability of increasingly powerful embedded (and soft) processors, which transformed FPGAs from hardware accelerators to very powerful system-on-chip (SoC) platforms. New analog resources, floating-point operators, and hard memory controllers are also described, because of the great advantages they provide to designers. Software tools are being strongly influenced by the design paradigm shift, which requires from them a much better support for software developers. Focusing mainly on this issue, recent advancements in software resources [intellectual property (IP) cores and design tools] are also reviewed. The impact of new FPGA features in industrial applications is analyzed in detail in three main areas, namely digital real-time simulation, advanced control techniques, and electronic instrumentation, with focus on mechatronics, robotics, and power systems design. The way digital systems are being currently designed in these areas is comprehensively reviewed, and a critical analysis of how they could significantly benefit from new FPGA features is presented.

Advanced Features and Industrial Applications of FPGAs—A Review

Frequency is one of the most important factors for power system harmonic and interharmonic estimation. Accurate spectral analysis relies much on the correct identification of frequencies of the measured signals. In this paper, several commonly used methods for power system stationary and time-varying harmonic and interharmonic estimation are reviewed and compared according to the viewpoint of the frequency identification. It is expected to provide suitable guidelines for the future studies.

Comparative Study of Harmonic and Interharmonic Estimation Methods for Stationary and Time-Varying Signals

Synchrophasor estimation accuracy is a well-known critical issue in systems for smart grid monitoring and control. This paper deals with an in-depth analysis of the effect of both steady-state and dynamic disturbances on single-cycle and multicycle windowed discrete Fourier transform (DFT)-based synchrophasor estimators. Unlike other qualitative or simulation-based results found in the literature, this work provides two accurate and easy-to-use analytical expressions that can be used to determine the worst case range of variation of the total vector error (TVE) due to off-nominal frequency deviations. In such conditions, estimation accuracy is limited by two factors, i.e., the infiltration caused by the input signal image frequency and the scalloping loss associated with the spectrum main lobe of the chosen window. Starting from the aforementioned general analysis, a new two-term window minimizing the detrimental effects of image frequency tone is proposed. The accuracy of the related DFT-based synchrophasor estimator is evaluated under both static and dynamic conditions, which is the most interesting scenario for future smart grids. Moreover, the effect of waveform frequency measurement uncertainty on scalloping loss compensation is quantified. Several simulation results (including the effects of noise, harmonic distortion, and amplitude and phase modulation) confirm that the proposed window can significantly improve the accuracy achievable with a simple single-cycle DFT estimator. Indeed, TVE values much smaller than 1% can be achieved even in the worst case conditions reported in the standard IEEE C37.118.1-2011, when the frequency waveform deviations are within ±4% of the nominal value. In addition, the proposed solution could be useful to improve the performance of more complex dynamic phasor estimators, e.g., those in which the first- and second-order terms of the phasor Taylor series expansion result from the differences of consecutive DFT-based phasor estimates.

Accuracy Analysis and Enhancement of DFT-Based Synchrophasor Estimators in Off-Nominal Conditions

The accurate detection and classification of power quality (PQ) disturbances in power systems is a key step to determine the causes of these events before any proper countermeasure could be taken. This paper presents a new algorithm for detection and classification of PQ disturbances based on the combination of double-resolution S-transform (DRST) and directed acyclic graph support vector machines (DAG-SVMs). The proposed method first employs DRST for an effective feature extraction from power signals. Then, the DAG-SVMs are used to predict the classes of PQ disturbances. The DRST not only has better time–frequency localization and stronger robustness but also reduces the computational complexity without losing the useful information of the original signal in comparison with the traditional S-transform. Through the combined use of DRST and DAG-SVMs, the algorithm can be easily implemented in embedded real-time applications. Finally, the implementation of the proposed algorithm in a digital signal processor + advanced reduced instruction set computing machine-based hardware test platform is introduced. The effectiveness of the proposed method is demonstrated by means of computer simulations and practical experiments with single and combined PQ disturbances.

Detection and Classification of Power Quality Disturbances Using Double Resolution S-Transform and DAG-SVMs

An approach for power system harmonic phasor analysis under asynchronous sampling is proposed in this paper. It is based on smoothing sampled data by windowing the signal with the four-term fifth derivative Nuttall (FFDN) window, and then calculating harmonic phasors in the frequency domain with an improved fast Fourier transform (IFFT) algorithm. The applicable rectification formulas of the IFFT are obtained by using the polynomial curve fitting, dramatically reducing the computation load. The FFDN window can effectively inhibit the spectral leakage and the picket fence effect can be modified by the IFFT algorithm under asynchronous sampling, and the overall algorithm can easily be implemented in embedded systems. The effectiveness of the proposed method was analyzed by means of simulations and practical experiments for multifrequency signals with the fluctuation of the fundamental frequency and with the presence of white noise and interharmonics.

Harmonic Phasor Analysis Based on Improved FFT Algorithm

This paper focuses on the accurate frequency estimation of power signals corrupted by a stationary white noise. The noneven item interpolation FFT based on the triangular self-convolution window is described. A simple analytical expression for the variance of noise contribution on the frequency estimation is derived, which shows the variances of frequency estimation are proportional to the energy of the adopted window. Based on the proposed method, the noise level of the measurement channel can be estimated, and optimal parameters (e.g., sampling frequency and window length) of the interpolation FFT algorithm that minimize the variances of frequency estimation can thus be determined. The application in a power quality analyzer verified the usefulness of the proposed method.

Frequency Estimation of Distorted and Noisy Signals in Power Systems by FFT-Based Approach

This paper focuses on the low-computation harmonic-analysis procedure with sufficient suppression of spectral leakage and picket-fence effect. The interpolated FFT algorithm based on the minimized sidelobe window is considered, and its calculate procedure and formulas are given, which is free of solving high-order equations. The implementation of the proposed algorithm in the digital-signal-processor (DSP) based three-phase harmonic ammeter is also introduced. The proposed algorithm has the major advantages that the calculate formulas for harmonic parameters can be easily implemented by hardware multipliers, making the method a good choice for real-time applications. The simulation and application results validate the accuracy and efficiency of the proposed algorithm.

Simple Interpolated FFT Algorithm Based on Minimize Sidelobe Windows for Power-Harmonic Analysis

A novel approach to harmonic analysis for industrial power systems based on windowed fast Fourier transform is presented. First, the desirable sidelobe window (DSW) is constructed through self-convolution of the maximum decay sidelobe window (MDW) in the time domain. Then, the power system signal parameters, i.e., frequency, phase, and amplitude, are calculated by the DSW-based discrete phase difference correction algorithm. It has been shown that the spectral leakage and harmonic interferences can be reduced considerably by weighting samples with the DSW because of its good sidelobe behaviors, i.e., the peak sidelobe level and the sidelobe rolloff rate are proportional to the order of the window (or the times of self-convolution). The DSW-based discrete phase difference correction algorithm can be easily implemented in embedded systems due to the low computation burden. Both simulative and experiment data tests have been performed to verify the effectiveness and practicability of the proposed method.

Zhaosheng Teng

Papers

Detection and Classification of Power Quality Disturbances Using Double Resolution S-Transform and DAG-SVMs

Harmonic Phasor Analysis Based on Improved FFT Algorithm

Frequency Estimation of Distorted and Noisy Signals in Power Systems by FFT-Based Approach

Simple Interpolated FFT Algorithm Based on Minimize Sidelobe Windows for Power-Harmonic Analysis

Spectral Correction Approach Based on Desirable Sidelobe Window for Harmonic Analysis of Industrial Power System