Conference Paper

Tool for detecting waveform distortions in inverter-based microgrids: A validation study

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... Frequency is an elemental descriptor of an oscillation, and one might expect different instability modes to show distinct frequency characteristics, yet the ordinary Fourier spectra may be not enough in this context. To be specific, frequency modulation (FM) effect has been reported in both control-involved waveform distortion [2,3] and harmonic oscillation [4], indicating that the instantaneous frequency (IF) [5,6] of VSC is constantly varying, rather than discrete frequency components, as is commonly recognized. Besides, the site signals can be rather complex that both the amplitude and the frequency are vary-ing with time. ...
... where: ̄= �̄2 +̄2 , 3 = arctan(̄⁄ ) (4) indicates that the PCC voltage consists of three frequency-modulated signals, as is shown in Table I. The center frequencies of these FM signals are , ( + ℎ ), ( − ℎ ), that is one center fundamental frequency (1 + 3 cos( ℎ + 2 )) + ℎ 1 − 2 2̄ ...
Conference Paper
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With the increasing penetration of voltage-source-converter (VSC) -interfaced distributed generations(DGs) in power systems, oscillation issues have been widelyconcerned whereas the root cause and nature of the oscilla-tion is sometimes not clear in real cases. This paper focuseson the instability mode recognition (IMR) based on a com-plete data-driven approach applied to the oscillation waveswhich can be obtained from the on-site recordings. To thisend, we explore the Hilbert-Huang Transform (HHT) fordiagnosing the root cause of instability, using only raw datasuch as the current and voltage waveforms which are acces-sible by operators. Special attention is paid to distinguishbetween the sub-synchronous oscillation (SSO) and the lossof synchronization (LOS) as they are two primary instabil-ity forms of grid-tied VSC which manifest with very similarwaveforms. The pros and cons of the considered signalanalysis tools in SSO and LOS recognition are discussed andnew lines of investigations are proposed. The analysis andresults presented in this paper could shed light on futuredata-driven analysis, e.g., serving as model-free or hybridmodel database for artificial intelligence-based stabilitydiagnosis and recognition. (PDF) Instability Mode Recognition of Grid-Tied Voltage Source Converters with Nonstationary Signal Analysis. Available from: https://www.researchgate.net/publication/360005412_Instability_Mode_Recognition_of_Grid-Tied_Voltage_Source_Converters_with_Nonstationary_Signal_Analysis [accessed Apr 18 2022].
... This thesis is based on and will elaborate a validation study where we tested the hypothesis by investigating the underlying system behind the microgrid [8]. The validation procedure consists of comparing results observed in the field data with an analytical model of the distortion, supported by simulation data from a detailed Matlab model which then are used to discuss the hypothesis. ...
... 8 shows v ac,mg (t)'s Hilbert Spectrum in greater detail. Its frequency has a semi-periodic behavior with repetition every 10 ms. ...
Thesis
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This thesis presents a validation study based on data analysis aimed at identifying the cause of atypical behavior observed in a microgrid’s data. The data of a stand- alone PV microgrid in Bhutan is analyzed. The analysis of the voltage waveform distortions found in the data is utilizing the Hilbert-Huang Transform (HHT) and periodograms. The Hilbert-Huang Transform shows an oscillating frequency with fluctuations of 10 ms, while the fundamental frequency component’s period is 20 ms. As we couldn’t attribute this distortion to a nonlinear load, we first argue that the most probable cause of the distortion is a 100 Hz oscillation commonly known to exist on the dc bus. The Periodogram shows a dominance of odd harmonics, with particular emphasis on 50 and 150 Hz. The analysis reveals that the dominance of the third harmonic can be explained by multiplication in the microgrid’s controller. There are two interpretations for the other odd harmonics. The pulse-width modulation and unideal low pass filtering cause odd harmonics on the ac side of the inverter. The same does a harmonic feedback from the ac bus to the dc via the inverter. The harmonic feedback creates a positive feedback loop through the controller resulting in odd harmonics on the ac side and even harmonics on the dc side. The thesis will also compare the voltage data measured on a physical grid with a mathematical model of the system and a simulation to test the hypothesis of the distortion’s origin. This test goes far in confirming the hypothesis of the source of the distortions.
... The HHT, in fact a NASA designated name, combines EMD and the aforementioned HT, and is well suited for analysis of non-stationary signals. The use of HHT has proven useful for obtaining instantaneous amplitude, frequency and power in power systems and isolated microgrids [1,5,38,39]. As will be outlined in the next subsection, the EMD decomposes signals into monocomponents/IMFs. ...
Thesis
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Several cases with time varying frequencies have been reported in isolated electrical systems such as stand-alone microgrids and marine vessel power systems. This thesis studies the use of several types of Kalman filters (KF), Hilbert-Huang Transform (HHT) and the proposed method of merging empirical mode decomposition (EMD) and KF for the purpose of tracking instantaneous values of voltage- and current waveforms in isolated microgrids with the aforementioned challenges. Both synthetic signals and real measurements from a marine vessel power system were used to validate the methods. The algorithms and methods were implemented in Matlab and Simulink. In varying degrees, the methods did all prove to be viable options for tracking of the fundamental frequency on the marine vessel. The proposed method turned out to be particularly powerful to decompose multicomponent signals consisting of several time-varying monocomponents, and track their instantaneous amplitude and frequency.
... Time varying frequencies have been observed in data collected from stand-alone power systems (e.g. isolated microgrids, marine vessel power systems) and they originate from sources that are fundamentally different from the sources of frequency variations in classical power systems [2], [3]. In stand-alone power systems like microgrids, the generation source is usually interfaced with the load by "static" power electronic equipment (e.g. ...
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The paper addresses the impact that time varying angular frequencies observed in electrical signals can have on the calculation and separation of components from the instantaneous electric power signal. Instantaneous power theories provide various methods for calculating the instantaneous power components in an electrical network. These methods are based on the basic assumption of constant fundamental frequency and harmonics that are multiple of the fundamental frequency. Recent field measurements in isolated electrical systems have however reported the existence of time varying angular frequencies or instantaneous frequencies. This new observation will affect the very foundation of the established methods for instantaneous power calculation and components separation. This paper analyses the separation of instantaneous average and oscillatory components of powers by using linear and non-linear filtering approaches in systems that exhibit time varying angular frequencies. The results of this comparison reveals the limitations of the assumption of fundamental and harmonic frequency when using linear filtering techniques in the presence of time varying angular frequencies. Non-linear filtering may offer a more robust and accurate estimation of the instantaneous values of powers and a power quality assessment that better reflects the actual system conditions.
... In previous works on the same system, it was shown how oscillations on the dc side can explain oscillations on the ac side [2], [3]. ...
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This paper presents an analytical method developed to explain the mechanism of harmonic transfer between the ac and dc sides of a single-phase inverter in a PV microgrid. The model explains how the feed-forward of the current from the ac side of the PV-inverter into the control system, causes even harmonics on the dc voltage. It further shows how the controller's feedback of the dc bus voltage carrying even harmonics results in odd harmonics on the ac side of the PV-inverter. This harmonic propagation model is verified with a simulation of the PV microgrid system. The results of this simulation study provides consistency by verifying that odd harmonics on the ac voltage causes even harmonics on the dc bus, and that even harmonics on the dc bus again causes more odd harmonics on ac voltage.
Research Proposal
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Aim of the project: David against Goliath: Could Small Data from single-channel or low-density EEG compete in the Big Data contest with high density EEG on equal footing or could David and Goliath join forces? Yes, they can and FlexEEG will show how. FlexEEG proposes a new concept of dry single-channel EEG with enriched information extraction that will materialize into a sensor-embedded data-driven approach to the real-time localization of brain activity. Level of impact and excellence: While a laboratory setting and research-grade electroencephalogram (EEG) equipment will ensure a controlled environment and high-quality multiple-channel EEG recording, there are situations and populations for which this is not suitable. FlexEEG aims at validating a new concept of single-channel or low-density EEG system that while being portable and relying on dry-sensor technology, will produce recordings of comparable quality to a research-grade EEG system but will surpass the capabilities and scope of conventional lab-based EEG equipment: In short, a single more intelligent EEG sensor could defeat high-density EEG. Conventional EEG is challenged by high cost, immobility of equipment and the use of inconvenient conductive gels. Ease of use and quality of information extraction are much awaited in a new EEG concept that produces recording of comparable quality to a research-grade system but that puts EEG within the reach of everyone. FlexEEG will bring that. This project will exploit methods of inverse problems, data-driven non-linear and non-stationary signal analysis 1,2,3 combined with dry-sensor technology to develop a pioneering system that will enable a single, properly localized EEG channel, to provide research-grade information comparable to and surpassing the capabilities of high density-channel EEG. Through this, the range of applications of EEG signals will be expanded from clinical diagnosis and research to healthcare, to better understanding of cognitive processes, to learning and education, and to today hidden/unknown properties behind ordinary human activity and ailments (e.g. walking, sleeping, complex cognitive activity, chronic pain, insomnia). This will be made possible by the implementation of adaptive non-linear and non-stationary data analysis tools in combination with inverse modelling to solve the brain-mapping problem.
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This paper describes a platform for obtaining and analyzing real-time measurements in Microgrids. A key building block in this platform is the Empirical Mode Decomposition (EMD) used to analyze the electrical voltage and current waveforms to identify the instantaneous frequency and amplitude of the monocomponents of the original signal. The method was used to analyse the frequency fluctuation and obtain information about the linearity of electrical current and voltage waveforms measured in the field. Comparison between grid-connected and stand-alone microgrid voltage and currents’ monocomponents were conducted. Fluctuations in the grid frequency occurred in both the grid-connected and stand-alone microgrid, but the degree of the observed fluctuations were different, revealing more apparent nonlinear distortions in the latter. The observed instantaneous frequency from the collected data indicates potential nonstationary electrical signals when compared to synthetic data containing periodic signals coming from nonlinear loads. This observation leads us to expect the next generation of real-time measuring devices for the micro power grids to be designed on the principle of instantaneous frequency detection. Further efforts will be directed to a more rigorous characterization of the nonstationary nature of the signals by analyzing more and longer set of data.
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A new method for analysing nonlinear and non-stationary data has been developed. The key part of the method is the `empirical mode decomposition' method with which any complicated data set can be decomposed into a finite and often small number of 'intrinsic mode functions' that admit well-behaved Hilbert transforms. This decomposition method is adaptive, and, therefore, highly efficient. Since the decomposition is based on the local characteristic time scale of the data, it is applicable to nonlinear and non-stationary processes. With the Hilbert transform, the 'instrinic mode functions' yield instantaneous frequencies as functions of time that give sharp identifications of imbedded structures. The final presentation of the results is an energy-frequency-time distribution, designated as the Hilbert spectrum. In this method, the main conceptual innovations are the introduction of `intrinsic mode functions' based on local properties of the signal, which make the instantaneous frequency meaningful; and th
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The concept of instantaneous frequency (IF), its definitions, and the correspondence between the various mathematical models formulated for representation of IF are discussed. The extent to which the IF corresponds to the intuitive expectation of reality is also considered. A historical review of the successive attempts to define the IF is presented. The relationships between the IF and the group-delay, analytic signal, and bandwidth-time (BT) product are explored, as well as the relationship with time-frequency distributions. The notions of monocomponent and multicomponent signals and instantaneous bandwidth are discussed. It is shown that these notions are well described in the context of the theory presented
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With an imperative demand of clean and reliable electricity generation in some countries, the increasing adoption of new photovoltaic (PV) systems pushes the Distribution System Operators (DSOs) to expand the transmission/distributed lines. However, the potential cost brought by such extensions and increased maintenances introduce new obstacles. In view of this concern, the DSOs starts to reduce PV installations in order to avoid an extension of the power infrastructure. Besides, another alternative solution is to limit the maximum feed-in power of the existing PV systems to a certain level. It can contribute to a weakened requirement of grid expansion and at the same time an increased penetration level. Therefore, to meet the need of this emerging ancillary service provided by future PV systems, a Constant Power Generation (CPG) control concept of PV inverters is proposed in this paper. Accordingly, it is worth investigating into two main issues: a) analyzing the reduction of the energy yield due to CPG control to study its feasibility from an economic point of view and b) developing robust CPG control methods, otherwise, it may introduce instabilities. Thereby, the implementation possibilities for PV systems in CPG operation mode are also discussed in this paper. Additionally, the loss of energy is calculated to reveal the viability of the proposed CPG control method. Operation examples of a PV system are presented to show the effectiveness of the CPG control method to unload the distributed grid.
Grid Converters for Photovoltaic and Wind Power Systems
  • Remus Teodorescu
  • Marco Liserre
  • Pedro Rodriguez
Remus Teodorescu, Marco Liserre, and Pedro Rodriguez. Grid Converters for Photovoltaic and Wind Power Systems. Wiley, Sussex, UK, 2011.
Why-it-Matters CleanEnergy 1p.pdf. [Online; accessed 30
United Nations Sustainability Goals. Affordable clean energy: why it matters. http://www.un.org/sustainabledevelopment/wp-content/ uploads/2016/06/Why-it-Matters CleanEnergy 1p.pdf. [Online; accessed 30-June-2016].