In this paper, we discuss the compression of waveforms obtained from measurements of power system quantities and analyze the reasons why its importance is growing with the advent of smart grid systems. While generation and transmission networks already use a considerable number of automation and measurement devices, a large number of smart monitors and meters are to be deployed in the distribution network to allow broad observability and real-time monitoring. This situation creates new requirements concerning the communication interface, computational intelligence and the ability to process data or signals and also to share information. Therefore, a considerable increase in data exchange and in storage is likely to occur. In this context, one must achieve an efficient use of channel communication bandwidth and a reduced need of storage space for power system data. Here, we review the main compression techniques devised for electric signal waveforms providing an overview of the achievements obtained in the past decades. Additionally, we envision some smart grid scenarios emphasizing open research issues regarding compression of electric signal waveforms. We expect that this paper will contribute to motivate joint research efforts between electrical power system and signal processing communities in the area of signal waveform compression.

The Compression of Electric Signal Waveforms for Smart Grids: State of the Art and Future Trends

The discrete Fourier transform (DFT) is still a widely used tool for analyzing and measuring both stationary and transient signals in power system harmonics. However, the misapplications of the DFT can lead to incorrect results caused by some problems such as an aliasing effect, spectral leakage, and picket-fence effect. A strategy of recursive group-harmonic power minimizing algorithm is developed for systemwide harmonic/interharmonic evaluation in power systems. The proposed algorithm can restore the dispersing spectral leakage energy caused by the DFT and regain its harmonic/interharmonic magnitude and respective frequency. Every iteration loop for harmonic/interharmonic evaluation can guarantee to be convergent using the proposed group-harmonic bin power algorithm. Consequently, not only high precision in integer harmonic measurement can be retained but also the interharmonics can be accurately identified, particularly under system frequency drift. The numerical example is presented to verify the proposed algorithm in terms of robust, fast, and precise performance.

Power Harmonics and Interharmonics Measurement Using Recursive Group-Harmonic Power Minimizing Algorithm

This paper presents the results of comprehensive testing and subsequent detailed analysis of the obtained test results, evaluating harmonic and interharmonic performances of photovoltaic inverters (PVInvs) for a range of different operating conditions. The presented results indicate significant power-dependent changes in harmonic and interharmonic emissions of tested PVInvs for different supply voltage conditions (presence of voltage waveform distortions and various source impedance values). To correctly quantify and describe these changes in PVInv performance, this paper discusses and applies measurement procedures and metrics for evaluating harmonic and interharmonic emission recommended in existing standards, as well as some additional metrics and indicators. For some operating conditions, tested PVInvs significantly increase both harmonic and interharmonic emissions, and this paper also discusses the impact of PVInv control (e.g., maximum power point tracking control) as a possible origin of the interharmonic distortion.

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Experimental-Based Evaluation of PV Inverter Harmonic and Interharmonic Distortion Due to Different Operating Conditions

Interharmonics are emerging power quality challenges in grid-connected photovoltaic (PV) systems. Previous studies and field measurements have confirmed the evidence of interharmonic emission from PV inverters, where the maximum power point tracking (MPPT) is one of the main causes for interharmonics. In that regard, the MPPT parameters such as their sampling rate has a strong impact on the interharmonic characteristic of the PV system. In general, there is a trade-off between the interharmonic emission and the MPPT performance when selecting the sampling rate of the MPPT algorithm. More specifically, employing a faster MPPT sampling rate will improve the MPPT efficiency, but it will also increase the interharmonic emission level. To solve this issue, a new mitigating solution for interharmonics in PV systems is proposed in this paper. The proposed method modifies the MPPT algorithm in a way to randomly select the sampling rate between the fast and the slow value. By doing so, the interharmonics in the output current can be effectively reduced due to the distribution of the frequency spectrum. On the other hand, the MPPT performance of the proposed method can be maintained similar to the case when employing a fast MPPT sampling rate. The effectiveness of the proposed interharmonic mitigation has been validated experimentally on a single-phase grid-connected PV system.

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Mitigation of Interharmonics in PV Systems With Maximum Power Point Tracking Modification

The recent trends to design more efficient and versatile maritime (both marine and offshore) vessels have attracted significant attention toward high penetration of power electronics systems in electric ship systems, which trigged a variety of power system architectures in civilian and naval ships. The availability of advanced power electronics converters further supported to improve maneuverability, efficiency, and compactness at reduced greenhouse gas emission in marine vessels. The fast-growing penetration of these power electronics converters adds a number of advantages to the ship power system. However, risk factors associated with the quality and reliability of the whole system should be considered. Power quality issues in marine networks have been reported from recent field accidents, therefore, the marine regulatory bodies need to revise and/or develop new power quality standards to ensure the reliability and scrutinize the safety of the whole ship system and crews. This paper presents 1) a classification of marine vessels and their power system architectures; 2) power electronics converters topologies and their non-linear characteristics; 3) control and protection architecture; 4) energy efficiency indicators; 5) a comprehensive case study to elaborate power quality in the marine system, and; 6) extensive discussion about power quality standards and highlights the urgency to update existing power quality standards.

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A Comprehensive Review of Maritime Microgrids: System Architectures, Energy Efficiency, Power Quality, and Regulations

Some of the most remarkable issues related to interharmonic theory and modeling are presented. Starting from the basic definitions and concepts, attention is first devoted to interharmonic sources. Then, the interharmonic assessment is considered with particular attention to the problem of the frequency resolution and of the computational burden associated with the analysis of periodic steady-state waveforms. Finally, modeling of different kinds of interharmonic sources and the extension of the classical models developed for power system harmonic analysis to include interharmonics are discussed. Numerical results for the issues presented are given with references to case studies constituted by popular schemes of adjustable speed drives.

M.F. Akram

Papers

Interharmonics: Theory and Modeling