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This article presents a new current controller in the synchronous reference frame and its associated design for enhancing the performance of three-phase grid-connected inverters, especially against weak-grid conditions. The existing controllers do not perform strongly during high-impedance grid conditions and lead to oscillations and instability issues due to the interactions between the synchronization and control units. The proposed controller addresses this issue by 1) deriving a linear model of the three-phase phase-locked loop (PLL), 2) integrating the PLL model into the current controller design, 3) using a multivariable control design for multi-input multi-output systems, and 4) designing the controller gains using optimal linear quadratic theory. The proposed controller has superior performance over a substantially wider range of weak-grid conditions compared to conventional controllers. Extensive simulation and experimental results are presented in order to validate and reveal the desirable performance of the proposed controller. [ABSTRACT FROM AUTHOR]

A novel system for decomposing a set of three-phase signals into its constituting symmetrical components is proposed. The system also estimates the frequency, the magnitudes and phase-angles of the sequence components. The system is useful for online estimation of instantaneous symmetrical components and their attributes in the presence of frequency variations. The proposed system can be considered as a fundamentally improved version of the conventional three-phase Phase-Locked Loop (PLL) system which is widely used in power system applications. From this viewpoint, the proposed system obviates the inherent shortcoming of the conventional PLL system which is its erroneous response in the presence of negative-sequence component. Mathematical derivations and computer simulations are presented to describe the principles of operation and the performance of the proposed system. Examples of applications of the proposed system are in Flexible ac Transmission Systems (FACTS), HVDC systems, and Custom Power Controllers. Index Terms--Dynamic phasors, Flexible ac Transmission Systems (FACTS), frequency estimation, phase-angle detection, Phase-Locked Loop (PLL), synchronization, time-domain symmetrical components.

An adaptive notch filter (ANF) is presented to achieve noise reduction and signal decomposition. It verifies the validity of the presented algorithm and confirms its desirable transient and steady-state performances as well as its desirable noise characteristics.

Three independent research groups have proposed three apparently different architectures for an improved phase-locked loop (PLL) structure through three entirely different approaches. It is shown that all three PLLs are structuraly and mathematically the same. A linear analysis is performed and concludes that the available theory for the design of conventional PLLS can be leverages into the design of the new class of PLLS. Index Terms--Costas loop, frequency estimation, phase detection, phase-locked loop (PLL), signal decomposition.

A novel method of extraction and measurement of individual harmonics of a signal with time-varying frequency is presented. The proposed method is based on a nonlinear, adaptive mechanism. Compared with the well-established techniques such as DFT, the proposed method offers (i) higher degree of accuracy, (ii) structural/performance robustness, and (iii) frequency-adaptivity. The structural simplicity of the algorithm renders it suitable for both software and hardware implementations. The limitation of the proposed method as compared with DFT-based methods is its slower transient response. Based on simulation studies, performance of the method is presented and its accuracy and response time are compared with a DFT-based method. Index Terms--Harmonics detection and extraction, harmonics measurement, inter-harmonics, power quality, time-varying frequency.

This paper presents a new synchronization method which employs an enhanced phase-locked loop (EPLL) system. The operational concept of the EPLL is novel and based on a nonlinear dynamical system. As compared with the existing synchronization methods, the introduced EPLL-based synchronization method provides higher degree of immunity and insensitivity to noise, harmonics and other types of pollutions that exist in the signal used as the basis of synchronization. The salient feature of the EPLL-based synchronization method over conventional synchronization methods is its frequency adaptivity which permits satisfactory operation when the centre frequency of the base signal varies. The proposed EPLL-based method of synchronization is also capable of coping with the unbalanced system scenarios. Structural simplicity of the EPLL-based method greatly simplifies its implementation in digital software and/or hardware environments as an integral part of a digital control platform for power electronic converters. The primary application of the proposed synchronization method is for the distributed generation units, e.g., wind generation systems, which utilize power electronic converters as an integral part of their systems. Index Terms--Distributed generators, phase angle estimation, synchronization, PLL, power systems.

A signal processing system for extraction of harmonic and reactive current components is introduced and its performance is evaluated. The extraction system is adopted as part of the control system of a single-phase active power filter (APF) to provide the required signals for harmonic filtering and reactive power compensation. Performance of the overall system is evaluated based on digital time-domain simulation studies. The APF control system including the signal processing algorithms are implemented in Matlab/Simulink Fixed-Point Blockset to accommodate bit-length limitation which is a crucial factor in digital implementation. The power system including the APF, load and the supply system are simulated with the PSCAD/EMTDC software to which the Matlab-based control model is interfaced. The simulation results indicate that the signal processing unit can provide the required signals for APF to perform filtering/compensation within the transient period of 2 to 3 cycles. Index Terms--APF, FACTS, harmonics, power factor adjustment, reactive current, STATCOM.

An alternative method of time-frequency signal analysis is introduced and is compared with the conventional discrete Fourier transfer (DFT)-based methods. The structure of the proposed algorithm and its mathematical properties are presented. The proposed algorithm has a nonlinear structure that provides a frequency-adaptivity features.

This paper introduces a new phase-locked loop (PLL) system. The proposed system provides the dominant frequency component of the input signal and estimates its frequency. The mechanism of the proposed PLL is based on estimating in-phase and quadrature-phase amplitudes of the desired signal and, hence, has application advantages for communication systems which employ quadrature modulation techniques. The studies demonstrate that the proposed PLL also provides a superior performance for power system applications. Derivation of the mathematical model and theoretical stability analysis of the proposed PLL are carried out using dynamical systems theory. Advantages of the proposed PLL over the conventional PLLs are its capability of providing the fundamental component of the input signal which is not only locked in phase but also in amplitude to the actual signal while providing an estimate of its frequency. Computer simulation is used to evaluate its performance. Evaluations confirm structural robustness of the proposed PLL with respect to noise and distortions. Index Terms--Frequency estimation, peak detectors, phase detection, phase-locked loops (PLLs), power system state estimation, quadrature amplitude modulation, synchronization.

A new approach for measuring the peak value of the fundamental component of a distorted sinusoidal signal for power system applications is presented. The method is applicable to single-phase as well as three-phase systems. While maintaining structural simplicity, the proposed approach is highly robust with respect to noise and distortion due to disturbances and unbalanced conditions of the system. The method is also highly tolerant of uncertainties in the setting of its internal parameters. The salient feature of the proposed approach is its capability of adapting to the variations in the center frequency of the input signal. The method is suitable for environments that frequency excursions are experienced and conventional discrete Fourier transform (DFT)-based methods do not provide satisfactory results. Speed and accuracy of the response can also be controlled. Structural simplicity and robustness of the proposed scheme make it well suited for digital implementation on software and hardware platforms. Performance of the proposed method is presented based on simulation studies in the MATLAB environment and an experimental setup. Index Terms--Amplitude measurement, frequency adaptive, peak measurement.

A new method for estimation of power frequency and its rate of change is presented. Unlike conventional methods which are based on the concept of linearization, the proposed scheme accommodates the inherent nonlinearity of the frequency estimation problem. This makes the method capable of providing a fast and accurate estimate of the frequency when its deviation from the nominal value is incremental or large. The estimator is based on a newly developed quadrature phase-locked loop concept. The method is highly immune to noise and distortions. The estimator performance is robust with respect to the parameters of its structure. Structural simplicity and performance robustness are other salient features of the method. Index Terms--Frequency estimation, nonlinear systems, notch filters, phase-locked loops.

This paper provides an adaptive filter for synchronous detection and extraction of harmonics. The filter can he used as integral part of the control system of a power electronic apparatus (e.g., STATCOM, APF, and UPFC) to generate the desired control signals. Stability and convergence analyses of the adaptive filter are presented based on dynamical systems theory. Performance of the filter is verified as a means for reference signal generation in a shunt active power filter. Index Terms--Active power filter, adaptive filter, averaging theorem, synchronous harmonic detection.

This paper presents a unified power signal processor (PSP) for use in various applications in power systems. The introduced PSP is capable of providing a large number of signals and pieces of information which are frequently required for control, protection, status evaluation, and power quality monitoring of power systems. The PSP receives a set of locally measured three-phase voltage and current signals and provides their fundamental components, amplitudes, phase angles, frequency, harmonics, instantaneous and stationary symmetrical components, active and reactive currents and powers, power factor, and the total harmonic distortion. Simplicity and integrity of its structure as well as its robustness with respect to internal parameters and external disturbances and noise render the proposed scheme very attractive for practical implementations. Index Terms--Amplitude detection, frequency estimation, harmonic analysis, phase detection, PLL, power signal processing, reactive current extraction, symmetrical components, synchronization, THD.

This article presents the complete design of a local controller for a grid-supportive battery energy storage (BES) system. The controller's objectives are: 1) to execute commands issued from the secondary controller; 2) to provide grid support; 3) to prevent converter overcurrent during transient low-voltage incidents; and 4) to maintain the state of charge of the battery. The grid support is divided into static, i.e., the droop-based, and dynamic, i.e., the inertia-based, voltage supports. The proposed controller uses an optimally designed full-state feedback approach, which merges the voltage and current controllers. This makes the design more systematic, flexible, and with better tradeoff in speed and damping. The proposed BES is autonomous, meaning that it does not have to be colocated with a renewable resource. This autonomous feature allows the BES to connect to any critical node in a system, detect disturbances through local measurements, and provide support regardless of the origin of the disturbance. Mathematical derivations, computer simulations, and laboratory experimental results of the proposed controller are presented in this article. [ABSTRACT FROM AUTHOR]

This paper presents various applications of a nonlinear adaptive notch filter which operates based on the concept of an enhanced phase-locked loop (PLL). Applications of the filter for online signal analysis for power systems protection, control and power quality enhancement are presented. The proposed scheme can be applied for signal analysis both under stationary and non-stationary conditions. Based on digital time-domain simulations, applications of the filter for a) sinusoidal waveform peak detection, b) harmonic identification/detection, c) detection/extraction of individual components of a signal, d) instantaneous reactive current extraction, e) disturbance detection, f) noise reduction in zero-crossings detection, and g) amplitude (phase) demodulation for flicker estimation, are presented. Index Terms--Amplitude/phase demodulation, flicker estimation, harmonic extraction, notch filter, phase-locked loop (PLL), reactive current extraction, sinusoidal peak detector, spectrum analysis, zero-crossings detection.