Discernment of transformer oil stray gassing anomalies using machine learning classification techniques

DATA AVAILABILITY : The data that support the findings of this study are available from the corresponding author upon reasonable request.This work examines the application of machine learning (ML) algorithms to evaluate dissolved gas analysis (DGA) data to quickly identify incipient faults in oil-immersed transformers (OITs). Transformers are pivotal equipment in the transmission and distribution of electrical power. The failure of a particular unit during service may interrupt a massive number of consumers and disrupt commercial activities in that area. Therefore, several monitoring techniques are proposed to ensure that the unit maintains an adequate level of functionality in addition to an extended useful lifespan. DGA is a technique commonly employed for monitoring the state of OITs. The understanding of DGA samples is conversely unsatisfactory from the perspective of evaluating incipient faults and relies mainly on the proficiency of test engineers. In the current work, a multi-classification model that is centered on ML algorithms is demonstrated to have a logical, precise, and perfect understanding of DGA. The proposed model is used to analyze 138 transformer oil (TO) samples that exhibited different stray gassing characteristics in various South African substations. The proposed model combines the design of four ML classifiers and enhances diagnosis accuracy and trust between the transformer manufacturer and power utility. Furthermore, case reports on transformer failure analysis using the proposed model, IEC 60599:2022, and Eskom (Specification—Ref: 240-75661431) standards are presented. In addition, a comparison analysis is conducted in this work against the conventional DGA approaches to validate the proposed model. The proposed model demonstrates the highest degree of accuracy of 87.7%, which was produced by Bagged Trees, followed by Fine KNN with 86.2%, and the third in rank is Quadratic SVM with 84.1%.https://www.nature.com/srephj2024Electrical, Electronic and Computer EngineeringSDG-09: Industry, innovation and infrastructur

A modular circuit synthesis oriented modelling approach for non-isolated DC-DC converters in CCM

DATA AVAILABILITY STATEMENT : All generated data is contained in the manuscript.The continued commissioning of DC microgrids in an effort to achieve net-zero carbon levels in the atmosphere demands the large-scale deployment of converters to make the power from renewable energy sources, such as solar PV, usable. To control these inherently non-linear converters using classical linear control methods, averaged modelling techniques are employed. These methods are laborious and easily become intractable when applied to converters with increased energy storage elements. A modular modelling approach is proposed. This approach is based on the synthesis of converters using refined basic building blocks. The refined basic building blocks are independently modelled as two-port networks and used in a circuit synthesis-oriented manner to derive power stage models of commonly used DC-DC converters. It is found that most of the converters considered in the study can be described as a cascade combination of these basic building blocks. As such, transmission parameters are mainly used to model the two-port networks. Moreover, it is also found that using this modelling technique enables the computation of generalized expressions for all power stage models of interest. The use of two-port networks curtails the size of the matrices describing the basic building blocks to 2 2, and thus simplifies the entire modelling procedure. Additionally, two-port network analysis makes this modelling technique modular, thus making it more suited to be employed in DC microgrids. The independence of the two-port models on the circuit topology and functionality makes it possible to even model new converters containing the described basic building blocks solely based on circuit connection.The National Research Foundation (NRF).https://www.mdpi.com/journal/energiesam2024Electrical, Electronic and Computer EngineeringSDG-09: Industry, innovation and infrastructur

Asymmetric neutral point diode clamped topology with reduced component count for switched reluctance machine drive

DATA AVAILABILITY STATEMENT : Simulation files and results data are available on request.The Reduced Asymmetric Neutral Point Clamped converter topology for unipolar driven, multiphase switched reluctance machines is proposed in this paper. This topology shares similarities with the conventional NPC and Asymmetric-NPC topologies, however it is unique in that the components for the capacitor string and outer semiconductor switches are shared among all the phases for a reduced component count. Some switching state combinations are not possible during commutation overlap between motor phases, resulting in minor torque transients during regenerative braking. A custom modulation scheme is implemented with fixed frequency, phase-shifted carrier waveforms that allow for automatic balancing of the neutral point voltage and interleaved switching of the semiconductor switches. A simple torque observer control architecture is used with minor adjustments for arbitrating torque contribution priorities between phases during handover.https://www.mdpi.com/journal/energiesam2023Electrical, Electronic and Computer Engineerin

A review on DC collection grids for offshore wind farms with HVDC transmission system

Abstract: Traditionally, the internal network composition of offshore wind farms consists of alternating current (AC) collection grid; all outputs of wind energy conversion units (WECUs) on a wind farm are aggregated to an AC bus. Each WECU includes: a wind-turbine plus mechanical parts, a generator including electronic controller, and a huge 50-or 60-Hz power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus; consequently, the transformer in each WECU is replaced by a power converter or rectifier. The converter is more compact and smaller in size compared to the transformer. Thus reducing the size and weight of the WECUs, and also simplifying the wind farm structure. Actually, the use of offshore AC collection grids instead of offshore DC collection grids is mainly motivated by the availability of control and protection devices. However, efficient solutions to control and protect DC grids including HVDC transmission systems have already been addressed. Presently, there are no operational wind farms with DC collection grids, only theoretical and small-scale prototypes are being investigated worldwide. Therefore, a suitable configuration of the DC collection grid, which has been practically verified, is not available yet. This paper discussed some of the main components required for a DC collection grid including: the wind-turbine-generator models, the control and protection methods, the offshore platform structure, and the DC-grid feeder configurations. The key component of a DC collection grid is the power converter; therefore, the paper also reviews some topologies of power converter suitable for DC grid applications

Analysis of a DC collector-based power converter topology for an offshore wind farm

The conventional collection grids for offshore wind farm consist of internal AC link comprising of series-parallel connections of identical wind energy conversion units (WECUs). Each WECU includes a 60- or 50-Hz power transformer. The power transformers are replaced by the power converters in this article. The weight of a power converter is much lower compared to that of a power transformer of similar characteristics, thus reducing the weight of the offshore wind farm. Most of the studies being done to design DC collection grids use the conventional full-bridge voltage source converter as the topology of power converter. The efficiency of the wind farm is low due to the switching losses. In this article a cascade connection of a diode rectifier and an interleaved DC-DC boost converter is used for the converter topology. The switches duty cycle is reduced, thus improving the efficiency of the wind farm. The dynamic model of the proposed power converter is derived, including its control schemes. Power Simulator (The Powersim, Rockville, USA) and MATLAB Software (The MathWorks, Natick, Massachusetts, USA) are used to test the performances of the proposed converter model.http://www.tandfonline.com/loi/uemp202016-06-15hb201

A unified rule-based small-signal modelling technique for two-switch, non-isolated DC-DC converters in CCM

DATA AVAILABILITY STATEMENT : All generated data is contained in the manuscript.The inherent non-linear behavior of switch-mode power supplies complicates the task of computing their linear models, which are essential for a model-oriented control design of DC–DC converters. In a model-oriented control design approach, the accuracy of the plant model directly influences the performance of the control system as the plant parameters tend to be linked to the controllers’ gains. Moreover, the extractions of linear dynamic models of high-order non-linear plants such as DC–DC converters are laborious and mathematically intractable. Therefore, in this paper, a generalized expression that represents either the audio-susceptibility or the control-to-output voltage transfer function for voltage-mode control is proposed. The proposed generalization reduces the task of computing the small-signal model of a given converter to simple calculations of coefficients of generalized transfer function/expression. It is shown that the coefficients of the generalized model can be deduced by inspection, directly from the circuit diagram, allowing the whole model to be computed by inspection. Additionally, the proposed modelling technique will be shown to have secondary use of verifying accuracy even when conventional modelling techniques such as state-space averaging or circuit averaging are used.The National Research Foundation (NRF).https://www.mdpi.com/journal/energiesam2023Electrical, Electronic and Computer Engineerin

Dynamic analysis of DC-DC converter internal to an offshore wind farm

This paper investigates the dynamic performance of an active rectifier integrated into a wind park. The small changes in the DC current and the DC voltage are examined. The small variations are caused by the miniature power flow unbalance between the offshore wind park and the grid land. Internal DC collector is considered into the wind park, which provides an internal DC medium voltage bus. The AC output signal from the wind generator to the internal DC collector or DC bus is regulated through active rectifier. An active rectifier is a cascade connection of an uncontrolled full bridge diode rectifier and a controlled DC-DC boost converter. The small changes in the DC current and DC voltage due to power flow unbalance are analysed across the boost converter. The way in which these small variations affect the internal medium DC voltage is determined. The results are presented in the form of small signal transfer functions and are evaluated with MATLAB software.http://ieeexplore.ieee.orgxpl/RecentIssue.jsp?punumber=4159946hb201

A unified analysis of DC–DC converters’ current stress

DATA AVAILABILITY STATEMENT : All generated data are contained in the manuscript.There is always a need to analyze current signals generated by various DC–DC converters. For example, to determine the current stress experienced by semiconductor devices and to evaluate active and reactive power consumption in converters. The study demonstrates that the shape of a current signal dictates the analytical expressions required to determine the average and RMS values of a signal as well as the RMS value of the ripple of that signal. The study also shows that current signals can be treated as composite waveforms comprising various combinations of trapezoidal, rectangular, and triangular pulses. The current literature lacks a unified approach to analyze current stresses in DC–DC converters. This study will propose a unified and generalized analytical technique that is applicable to any type of DC waveform that can be treated as a composite waveform made up of a combination of triangular, rectangular, or trapezoidal sections or sub-intervals. Furthermore, the rectangular and triangular pulses are shown to be a special kind of trapezoidal pulse. This provides the basis for a very broad generalization of current signals’ analysis based on the analysis of a trapezoidal pulse. Additionally, a method for the direct evaluation of signals’ ripple RMS content is developed. This is unlike in the current literature where it is necessary to evaluate the signal’s average and RMS values before ripple content can be evaluated. The technique developed is applicable to continuous and discontinuous conduction modes of operation.https://www.mdpi.com/journal/energiesam2024Electrical, Electronic and Computer EngineeringSDG-07:Affordable and clean energ

High boost-ratio bidirectional converter for interfacing low-voltage battery energy storage system to a DC bus

Supply and demand mismatches in renewable energy systems are addressed by integrating battery banks. Selecting battery bank terminal voltage to match DC-bus voltage (350–450 V for single-phase AC loads), necessitates employing battery banks with long-string connections along with their attendant shortcomings. To employ short-string battery banks, high-boost-ratio bidirectional interfaces are required between the DC-bus and battery bank. Current literature lacks a single source where high-boost-ratio converters’ are categorised and their strengths and weaknesses identified. Comprehensive literature review is hence carried out to determine attributes of various high-boost-ratio DC–DC converters and also categorise them. The key attributes of a topology to interface battery storage to a DC-bus are determined. Based on these a bidirectional tapped-inductor boost converter emerges as the best candidate. Moreover, in order to regulate output voltage, voltage-gain versus duty-ratio characteristics should not be very steep. Since battery terminal voltage varies with state-of-charge, closed-loop control is necessary. Converter's small-signal transfer-functions are derived and a two-loop controller to regulate output voltage and inductor current while allowing bidirectional power flow designed. A novel bidirectional passive lossless snubber circuit is employed to clamp the voltage spikes across the active switches, without altering the normal operation of the converter.https://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=4475725hj2019Electrical, Electronic and Computer Engineerin

An SMC-MRAS speed estimator for sensor-less control of DFIG systems in wind turbine applications

A sliding mode control-based model reference adaptive system (SMC-MRAS) estimator for sensor-less control of doubly fed induction generator (DFIG) systems in wind turbine applications is proposed in this paper. The proposed SMC-MRAS estimator uses the rotor current as a variable of interest. The proposed SMC-MRAS estimator has the advantage of being immune tomachine parameter variations. The SMC parameters are designed using the Lyapunov stability criteria. The performance of the proposed SMC-MRAS estimator is validated using simulations in MATLAB/SIMULINK. A comparative study between the proposed SMC-MRAS estimator and the PI-MRAS estimator is also conducted to demonstrate the superiority of the proposed SMC-MRAS estimator.https://www.mdpi.com/journal/energiesam2024Electrical, Electronic and Computer EngineeringSDG-07:Affordable and clean energ