# A Review of Harmonic Detection, Suppression, Aggregation, and Estimation Techniques
## 5 Harmonic Estimation Models
A key advantage of this method is that the parameters can be estimated from measurements taken under different voltage conditions. A mathematical signal model is developed in [68] with a factor increase in all harmonics except the fundamental frequency component. A new Taylor-Kalman-Fourier (TKF) filter is proposed to estimate dynamic phasor and time-varying harmonic components while reducing total harmonic distortion (THD) in power systems. In [69], a  
Figure 12: Flow chart of adaptive Kalman filter. Figure 13: Equivalent charging circuit. state-estimation-based model is presented that determines the harmonic contribution at the point of common coupling (PCC) from iron and steel plants, where non-linear characteristics are modeled as current sources and the harmonic fingerprint of the loads are separated from the upstream. The flowchart of this state-based algorithm is presented in Figure 14. The main benefit of this model is that it removes the need for expensive and cumbersome resistive-capacitive voltage transformers (RCVTs) for continuous measurements to accurately measure harmonic voltages. State-estimation-based corrections are applied to harmonic voltage measurements obtained from conventional voltage transformers (CVTs). In [70], an adaptive observer-based, closed-loop feedback approach is introduced for estimating single-phase grid voltage parameters that include harmonics. This proposed estimation method is robust against harmonic voltage disturbances due to its closed-loop feedback design. It ensures zero steady errors even when the grid's fundamental frequency changes significantly from its nominal value. The adaptive observer method is further extended in [71] to provide an asymptotic estimate of three-phase grid voltage parameters, including amplitude, frequencies, and phase angles under unbalanced grid faults and harmonic voltage disturbances. Nodes and devices in electric distribution networks are often nonlinear and unbalanced, making precise measurement devices and methods necessary to monitor harmonic propagation and power quality (PQ) phenomena. A complete or redundant monitoring system may not be feasible for estimating the harmonic status of the entire grid due to technical and economic factors. In [72], the authors present an efficient compressive sensing harmonic detector (CSHD) for identifying and estimating the main sources of pollution. In [73], a noninvasive and multiparametric estimation algorithm is proposed for grid-connected voltage-source converter systems, based on time-domain models of voltage-source converters. This proposed method performs harmonic estimation using the steady-state extended harmonic domain (EHD) model in combination with the nonlinear least squares (NLSQ) fitting algorithm; the flowchart is presented in Figure 15. The EHD model reduces the necessity of synchronized measurements, which eventually enhances the performance and robustness of the NLSQ model. This proposed system is a flexible, less complex, and reliable algorithm under measurement uncertainties that can identify grid equivalent, ac filter, and switching and conduction parameters from a reduced number of non-synchronized measurements. An estimation of the harmonic synchro phasor is proposed in [72], utilizing the flat-top (FT) finite-impulse-response (FIR) filter. Analysis under both dynamic and steady-state conditions was conducted to determine the properties of the proposed algorithm, which is based on convolution and can be implemented in common embedded systems. Harmonic distortion, intermodulation, and noise affect the overall linearity of electronic circuits [74]. Consequently, the authors in [75] present a time-domain harmonic distortion estimation method that offers a compact and systematic approach to modeling complementary metal oxide semiconductor (CMOS) transistor stages, estimating the harmonic contribution at each node in CMOS circuits with any number of stages. Figure 14: Flow diagram of state-estimation-based harmonic current contribution determination method.