Masoud Karimi-Ghartemani

Addressing DC Component in PLL and Notch Filter Algorithms

This paper presents a method for addressing the dc component in the input signal of the phase-locked loop (PLL) and notch filter algorithms applied to filtering and synchronization applications. The dc component may be intrinsically present in the input signal or may be generated due to temporary system faults or due to the structure and limitations of the measurement/conversion processes. Such a component creates low-frequency oscillations in the loop that cannot be removed using filters because such filters will significantly degrade the dynamic response of the system. The proposed method is based on adding a new loop inside the PLL structure. It is structurally simple and, unlike an existing method discussed in this paper, does not compromise the high-frequency filtering level of the concerned algorithm. The method is formulated for three-phase and single-phase systems, its design aspects are discussed, and simulations/experimental results are presented.

Estimation of Power System Frequency Using an Adaptive Notch Filter

An algorithm based on the concept of adaptive notch filter (ANF) is proposed for estimation of power system frequency. The ANF is a second-order notch filter that is further furnished with a nonlinear differential equation to update the frequency. Performance of the algorithm is compared with that of a newly introduced algorithm which is based on using an enhanced phase-locked loop (PLL) system. Unlike the PLL-based frequency estimator, the proposed algorithm does not employ voltage-controlled oscillator (VCO). This makes its structure much simpler for implementations. Transient response of the proposed estimator is faster than the PLL-based estimator. Computer simulations are presented to highlight the usefulness of this approach in estimating near nominal and off nominal power system frequency

Time-Domain Signal Analysis Using Adaptive Notch Filter

Noise reduction and signal decomposition are among important and practical issues in time-domain signal analysis. This paper presents an adaptive notch filter (ANF) to achieve both these objectives. For noise reduction purpose, the proposed adaptive filter successfully extracts a single sinusoid of a possibly time-varying nature from a noise-corrupted signal. The paper proceeds with introducing a chain of filters which is capable of estimating the fundamental frequency of a signal composed of harmonically related sinusoids, and of decomposing it into its constituent components. The order of differential equations governing this algorithm is 2n+1, where n is the number of constituent sinusoids that should be extracted. Stability analysis of the proposed algorithm is carried out based on the application of the local averaging theory under the assumption of slow adaptation. When compared with the conventional Fourier analysis, the proposed method provides instantaneous values of the constituting components. Moreover, it is adaptive with respect to the fundamental frequency of the signal. Simulation results verify the validity of the presented algorithm and confirm its desirable transient and steady-state performances as well as its desirable noise characteristics

Application of Enhanced Phase-Locked Loop System to the Computation of Synchrophasors

This paper introduces the application of an enhanced phase-locked loop (EPLL) system to the estimation of synchrophasors in a phasor measurement unit (PMU). The major concern is accurate estimation within off-nominal frequency operation of the system. The well-known technique based on discrete Fourier transform (DFT) can provide accurate estimation of the phasors in a three-phase balanced system. However, the negative-sequence component causes errors to the DFT estimates. The DFT cannot accomplish this task in a single-phase system. The EPLL is shown to be a solution for those shortcomings of the DFT technique, both in single-phase and in unbalanced three-phase systems, at the expense of some more complicated structure. Extensive steady-state and dynamic tests are performed and the results are presented and discussed.

Linear and Pseudolinear Enhanced Phased-Locked Loop (EPLL) Structures

In this paper, the enhanced phase-locked loop (EPLL) is modified to achieve a linear time invariant (LTI) and a pseudolinear (PL) EPLL called the LTI-EPLL and the PL-EPLL, respectively. The modification is based on using the estimated amplitude to make the EPLL operation independent from the input signal magnitude. The LTI-EPLL is input-output LTI, and its input-output relationship is represented by a transfer function, a representation that has not been possible for any other type of existing PLL structures. Having a transfer function representation is very useful for design and analysis purposes. The PL-EPLL introduces frequency adaptivity into the LTI-EPLL and is no longer LTI, but its performance is still independent from the input signal magnitude. The transfer function approach also facilitates the development and design of various EPLL extensions to estimate and reject the dc component and the harmonics as well as the extension with controlled attenuation of wideband noises. These topics are discussed in this paper. The presentation is focused on single-phase EPLL, but extension to three-phase PLLs is also briefly presented.

A Unifying Approach to Single-Phase Synchronous Reference Frame PLLs

The three-phase synchronous reference frame phase-locked loop (3-SRF-PLL) is widely used in three-phase power electronics and power system applications thanks to its desirable performance and its simple yet robust structure. Inspired from the 3-SRF-PLL and due to the increasing interest in single-phase applications, multiple single-phase SRF-PLL (1-SRF-PLL) versions have also been proposed in the recent years. This paper presents a unifying approach to the understanding and analysis of the 1 -SRF-PLLs. This paper integrates several 1 -SRF-PLLs having apparently different structures into a single structure. The approach is much useful in understanding various PLLs and also in facilitating further developments in this field.

Derivation and Design of In-Loop Filters in Phase-Locked Loop Systems

This paper addresses the concept of in-loop filters in phase-locked loop (PLL) systems. The in-loop filters are derived from an optimization perspective, and an analytical method to design the controlling parameters of a PLL with in-loop filters is also presented. Such filters can also be selected as conventional window functions in which case they can be tuned to reject certain frequency components similar to the discrete Fourier transform. In this paper, a rigorous method to introduce the concept of in-loop filters and window functions into PLL systems is presented. This method enables smoother estimation of the signal parameters such as phase angle, frequency, and amplitude in the presence of noise and harmonics. The in-loop filters can be adjusted to completely remove specific harmonics. The method is first developed for a single-phase enhanced PLL system and is then extended to three-phase PLLs including the well-known synchronous-reference-frame PLL. Simulation and experimental results are also included.

A Power Control Method With Simple Structure and Fast Dynamic Response for Single-Phase Grid-Connected DG Systems

This paper presents a new method for controlling the exchange of power between a single-phase distributed generation system and the grid. Rather than controlling the active and reactive powers separately and through the media of current signal as is done by the conventional techniques, the proposed controller acts directly on the instantaneous power. This eliminates the conventional need for calculating the active and reactive powers; a calculation that involves filtering/phase-shifting and slows down the system responses and adds to computational complexity. This paper first formulates a nonlinear structure from a purely mathematical approach based on minimizing a cost function. The minimization procedure generates a reference for the current signal which is subsequently used in the current control loop. This paper then derives an equivalent linear counterpart for the nonlinear structure. Moreover, it is also shown that the idea of controlling the instantaneous power does not require a separate loop for the current. Having replaced the nonlinear part with its linear equivalent, a control loop that comprises linear time-varying elements is obtained. This paper further develops a linear time-invariant model of the loop for stability and design purposes. The proposed control system is successfully applied to a photovoltaic system and performance evaluation results (using computer simulations and a laboratory experimental setup) are presented. Desired performance and robustness of the proposed method is verified by testing it within different operating conditions.

Processing of Harmonics and Interharmonics Using an Adaptive Notch Filter

A method for real-time detection and extraction of individual harmonic and interharmonic components in a power signal with potentially time-varying characteristics is presented. The proposed method, which is based on the concept of adaptive notch filter (ANF), adaptively decomposes the measured power signal into its constituting components independent of where their frequencies are located. The algorithm provides instantaneous values of the various estimated frequency components in addition to the values of their frequencies, amplitudes, and phase angles. The structure and mathematical formulation of the proposed technique, including guidelines for its parameter tuning, are presented and its performance is studied in a variety of scenarios where the power signal attributes, such as fundamental frequency and amplitude, undergo variations over time. This study confirms the desirable transient and steady-state performances of the proposed method. Compared with its recently proposed counterpart, the proposed method of this paper obviates the need for using a phase-locked loop (PLL), and hence, offers a more simplified structure which makes it more attractive from an implementation point of view.

A Control Design Approach for Three-Phase Grid-Connected Renewable Energy Resources

This paper presents a method to design a control system for a three-phase voltage source converter (VSC) that connects a renewable energy source to the utility grid through an output L-type or LCL-type filter. The well-known abc/dq transformation method creates coupling terms that are visible and can readily be canceled in the L-type filter. Such terms, however, are very complicated when an LCL filter is used. This paper, first revisits the derivation of the decoupling control method for an L-ype output filter and then, for the first time, derives the decoupling terms for an LCL-type filter. Having successfully decoupled the real and reactive power loops, feedback controllers are presented and designed to achieve desirable performance. The proposed controller provides active damping of the LCL resonance mode, robustness with respect to grid frequency, and impedance uncertainty. Moreover, a new controller is designed to improve the startup transient of the system. The methodology used in this paper is inspired from the feedback linearization theory and it provides a clear design method for the nonlinear systems. Simulation results are presented to confirm the analytical results.