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The growth and integration of renewable energy and control systems in the utility grid represents a challenge for the electric power industry. Power quality monitoring plays a critical role in modern electrical systems for both standards compliance and system security reasons. This paper presents a novel technique for estimating subharmonics, harmonics, interharmonics, supraharmonics, and DC offset in electrical systems. This extended spectral fitting (ESF) approach is a combination of the numerical Laplace transform (NLT) and a modified vector fitting (VF) which we have denominated extended vector fitting (EVF). In this novel approach, it is assumed that frequencies of the harmonics and supraharmonics are known poles and that frequencies of the subharmonics and interharmonics are unknown poles in the frequency domain (FD), resulting in a rational approximation that considers known poles and unknown poles. The advantages of the proposed methodology are demonstrated (1) for synthetic test signals and (2) in an AC-DC-AC converter simulation. Results show that the ESF approach can fully decompose a signal into the aforementioned components with a high degree of accuracy. [ABSTRACT FROM AUTHOR]