The continued and envisioned large-scale integration of renewable energy sources as a reaction to rising global temperatures and climate change will need a readily available DC grid to increase commissioning and operating efficiency. The effective operation of these grids is predicated on the correct control of its main control points. A plethora of DC-DC converters that find use in DC microgrids act as the main control points. DC-DC converters are non-linear and can operate in different modes with completely unique characteristics. To utilise classical control techniques, laborious equivalent linear models are derived for DC-DC converters using averaging modelling schemes. The application and limitations of these modelling techniques are well captured in the available literature. The most common limitation of the available modelling schemes is that more focus is dedicated to converter attributes like order, functionality and operating mode, even when optimal power flow and voltage regulation within the DC network are of more interest. Structure-based modelling techniques like the use of basic building blocks nullify converter attributes in the modelling process which translates to modelling efficiency. In light of the merits seen with the use of basic building blocks when modelling converters in CCM, the current study extends these merits to converters operation in DCM. Similar to modelling converters in CCM, modelling techniques that are available in the literature continue to consider converter attributes in the modelling process for DCM operation. Moreover, the two modes of operation are treated as unique entities and often modelled in a non-unified manner, which compromise modelling efficiency since the same converter can operate in a different state solely based on loading. The aim is to increase modelling efficiency but also nullify operating mode in the modelling process. The same basic building blocks are now modelled as two-port networks for DCM operation and adopted based on the exact configuration of a specified converter to compute its steady-state and dynamic models. All the advantages seen when modelling converters in CCM using basic building blocks are retained and augmented when considering DCM operation. Thus, any converter with well-defined basic building blocks can be easily modelled solely based on the connection of constituent basic building blocks.

Oil-immersed transformers are expensive equipment in the electrical system, and their failure would lead to widespread blackouts and catastrophic economic losses. In this work, an elaborate diagnostic approach is proposed to evaluate twenty-six different transformers in-service to determine their operative status as per the IEC 60599:2022 standard and CIGRE brochure. The approach integrates dissolved gas analysis (DGA), transformer oil integrity analysis, visual inspections, and two Back Propagation Neural Network (BPNN) algorithms to predict the loss of life (LOL) of the transformers through condition monitoring of the cellulose paper. The first BPNN algorithm proposed is based on forecasting the degree of polymerization (DP) using 2-Furaldehyde (2FAL) concentration measured from oil samples using DGA, and the second BPNN algorithm proposed is based on forecasting transformer LOL using the 2FAL and DP data obtained from the first BPNN algorithm. The first algorithm produced a correlation coefficient of 0.970 when the DP was predicted using the 2FAL measured in oil and the second algorithm produced a correlation coefficient of 0.999 when the LOL was predicted using the 2FAL and DP output data obtained from the first algorithm. The results show that the BPNN can be utilized to forecast the DP and LOL of transformers in-service. Lastly, the results are used for hazard analysis and lifespan prediction based on the health index (HI) for each transformer to predict the expected years of service.

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%.

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 to machine 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.

With affordable and clean energy being one of the Sustainable Development Goals (SDG7), most developing economies are still caught up with the dilemma of inadequate power supply and heavy dependence on fossil fuel. This social menace is premised on rapid population growth, industrialization, modernization, etc. Even though these sources of power appear to be far-fetched from being sufficient, they are noted for creating a significant level of environmental pollution, global warming, and health-related risks. The Conference of Parties 26 (COP26) assembly held in Glasgow, United Kingdom, stressed the need to bring down the rising annual global temperatures to 1.5°, with developing economies having a significant role in achieving this target. This article has presented a review with insight into certain power generation metrics within the context of (SDG7). This span across the investigation of different energy modelling tools, their depth of effectiveness, the general pros and cons of energy policies premised on these tools, and progress made so far towards the development of an affordable and clean power sources mix in developing economies. A deduction was reached that there is an immense potential for power generation from affordable and clean energy sources as this bridges the enormous gap between power demand and supply as well as mitigates greenhouse gases (GHGs) effects.

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.

In this article, a fault ride-through controller is proposed with the use of proportional-integral compensator and electronic sensors for riding-through alternative current faults in a wind power plant collection grid connected with a permanent magnet synchronous generator. Three parallel-connected wind energy conversion units supplying a direct current (DC) load are simulated and discussed. It is found that the sub-module will directly adjust the transmission of power and protect the generator from overheating, which can occur due to voltage fluctuation. Furthermore, a DC voltage controller is designed to ensure a stable closed-loop system, and the system stability is verified using bode plots. The dynamic response of the system is demonstrated with the use of power simulator (PowerSim, Rockville, USA) software to verify the reliability of the proposed rectifier topology.

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.

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.

In this paper, an electromagnetic torque-based model reference adaptive system (MRAS) is proposed for sensor-less control of doubly-fed induction generator (DFIG) systems. The proposed electromagnetic torque based MRAS estimator aims at extracting the rotor speed in the synchronous reference frame. The advantage of the proposed MRAS estimator is its simplicity due to the fact that it requires only the three-phase rotor current and the reference three-phase rotor voltage as inputs. The small signal analysis is used to conduct stability study of the proposed rotor speed estimator. The PI gains of the MRAS estimator are determined using pole placement. The performance of the proposed rotor speed estimator is validated under various operating conditions of the DFIG using MATLAB/SIMULINK.

This paper presents a novel fault ride-through (FRT) capability with the use of active converters (Vienna rectifier-I and five-level three-phase modular multilevel converter (MMC)) integrated with permanent magnet synchronous generator (PMSG) supplying AC load. The DC-link fault is controlled with the use of fault blocking diode and MMC sub-module (SM) based wind power plant (WPP). A 16 kW, 60 Hz, PMSG and wind turbine (WT) integrated with the machine-side converter (MSC), and grid-side converter (GSC) are utilized as wind farm components, along with direct current (DC) transmission grid. This topology contributes to the stability and reliability of the system by ensuring continuous output power under transient fault conditions. The functional response of the system is presented using power simulator (The PowerSim, Rockville, USA) software to demonstrate the performance of the proposed controller.

Transformational techniques unifying synthesis of two-state DC-DC converters and analytical synthesis techniques allowing generation of all possible converters meeting a certain criteria already exist. The analysis of a family of converters derived from a single converter cell has also been unified. Current waveforms generated by the family of converters were shown to be related. However, a concept or basic building blocks that facilitate unified synthesis, analysis, prediction of current waveforms and assignment of switch states over a very wide range of DC-DC converters is still lacking. This study will propose three 3-terminal basic building blocks and one 3-terminal filter block. It will be shown that between them, they are sufficient for realizing all non-isolated DC-DC converters excluding those with coupled inductors. The various DC-DC converters fall into those realized through cascade, stacked, stacked plus cascade, interleaved/paralleled or differential connection of the basic building blocks. A systematic approach for evaluating input-output current gains will be presented. Moreover, a basic building block will be shown to have fixed switching states for proper operation. This gives rise to the generation of a unique set of current waveforms at the three terminals irrespective of where a basic building block is embedded. It has been shown that the effort and time needed to design DC-DC converters can be reduced as switching device stresses can be estimated without the need for tedious first principle derivations.

Multilevel converter topologies are an ongoing research topic only recently extended to the asymmetric, switched reluctance machine drives. These topologies show potential in extending the operable speed range, reducing torque ripple and improving generation efficiency. However, due to the asymmetric, nonconventional nature of these topologies, traditional carrier modulation schemes have yet to be explored. This paper addresses a lack of available literature on the subject by proposing suitable carrier modulation strategies for the asymmetric flying capacitor, asymmetric neutral point clamped and asymmetric cascaded cell half bridge topologies. These strategies make use of interleaved carrier waveforms and can also be implemented with suitable capacitor balancing control strategies. The harmonic distortion of the input current waveform due to the modulation is also considered, providing a suitable criteria for comparing recently proposed topologies with their new modulation schemes.

Supply and demand mismatches in renewable energy systems are addressed by integrating battery banks. Selecting battery bank terminal voltage to match DC-bus voltage (350-450V 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.

An improved topology with a fault ride through (FRT) capability when subjected to a DC-link fault-based wind power plant (WPP) employing a Vienna active rectifier-I is proposed in this paper. The proposed system is capable of mitigating fault occurring on the DC-link side using the PWM-controller technique implemented on the Vienna active rectifier. FRT capability analysis is conducted in this paper, simulation results demonstrate the suitability of the control strategy. Actually, use of proposed wind energy conversion unit (WECU) topology has led to the improvement of system stability by maintaining constant output voltage. Furthermore, the WECU integrating Vienna active rectifier-I is also proven as a feasible technology that can be employed in a large-scale WPP or renewable power generations to realize technical and economical efficient grids integration with high voltage direct current (HVDC) transmission systems.

Practical DC-busses for supplying single-phase inverter loads are rated 350-450V. For generators with terminal voltages below 100V to interface with such DC-busses, boost-ratios in excess of 3 are required. This paper presents the principle of operation and performance analysis for a low-voltage wind energy conversion system (LVWEC) for interfacing a low-voltage wind-generator to a DC distribution system. The proposed LVWEC consists of a three-phase diode rectifier cascaded with an interleaved tapped-inductor boost DC-DC converter. It provides high voltage boost-ratio, low input- and output-side voltage and current ripple. A two-loop controller ensures wide input voltage operation, excellent line and load disturbances rejection and output DC voltage regulation. A good combination of the coupled-inductor turns-ratio and converter duty-cycle is selected to provide the necessary boost-ratio while keeping the device blocking voltages within acceptable limits. The proposed LVWEC has a simple PWM control circuitry and high efficiency. System performance is validated through simulations and experimentally.

High voltage direct current (HVDC) schemes require significant investments. There could be many possible receiving ends (i.e., substations) for an envisaged scheme, with different capabilities to accommodate power without loadflow constraints. Detailed analysis of all the possibilities can be laborious and time-consuming. However, not assessing some of the ends could pose risks to the investment: (i) some excellent ends can be missed, and (ii) decision makers may doubt that the best option has really been identified. In this paper, the authors evaluate the possibility of using negative load representation of power at the receiving end of an HVDC scheme to assess the loadlfow capability of a receiving end (i.e., approach 1). Another set of loadflows is done with HVDC scheme fully modelled (l.e., approach 2). The results from the two approaches are compared and statistical analysis is performed to evaluate the strength of their relationship. The results (i.e., voltages and loading of equipment) from the two approaches showed good agreement. The proposed representation can, therefore, be used to speedily assess capabilities of possible receiving ends, in a transparent manner, to derive a smaller subset that can be studied in detailed.

Conducted EMC noise qualification tests are normally carried out after a prototype has been designed, built and tested and the process is repeated in the event of non-compliance. The ability to determine compliance with EMI standards at the design stage is therefore desirable. This paper will present conducted EMC noise modelling and measurement techniques, yielding simulated and measurement noise results accurate enough to serve as a prequalification test at the design and prototyping stages respectively. Accurate models of the power feed-line connecting the LISN to the DUT (converter), including the load are developed. Predictive conducted EMC modelling is accomplished using detailed active level 3 SPICE-based models, creating a real-time circuit model consisting of a complete converter in its operational state, without the need to separate into equivalent models. Effects of the power feed-line length on EMI noise measurements are investigated as currently available literature has not dealt adequately with this issue. To detect possible radiating frequencies that might emerge with the conducted noise and are detectable in the conducted emission test, results have to be accurate not just in the prescribed frequency band, but all the way up to 100 MHz. The modelling and measurements are performed using software and instruments available in a development laboratory. Keywords— conducted EMC, common-mode, differential-mode, SPICE models, lumped-element, wideband digital measurement, digital noise separation, SiC MOSFET

Effects of polymer heatsink materials on electromagnetic interference (EMI) noise in converters have not been adequately investigated. Heatsinks provide a path to ground for the common-mode (CM) noise. Selecting an appropriate heatsink material can therefore help reduce CM noise by increasing noise path impedance. The critical parameter is the heatsink-to-device capacitance and accurate models of the heatsink impedance need to be developed. This is necessary for results that are accurate enough to allow prequalification of a converter as would happen in an accredited environment. However, simplified CM and differential-mode models of the step-down DC-DC converter are adequate to predict the effects of the heatsink on the conducted noise. This study will demonstrate that compared to a solid aluminium material, using polymer material incorporating conductive fillers can greatly lower the device-to-heatsink capacitance, and still be adequate for heat dissipation in low power converters. This in turn reduces CM noise in the frequency band below 30MHz. The measurement test setup is configured according to MIL-STD-461F standard. It consists of a wideband two-port line impedance stabilising network (LISN), a properly grounded copper sheet, a suspended feed-line from the LISN to the device under test and an oscilloscope recording the data.

Conducted electromagnetic compatibility (EMC) noise qualification tests are normally carried out after a prototype has been designed, built and tested and the process is repeated in the event of non-compliance. The ability to determine compliance with electromagnetic interference (EMI) standards at the design stage is therefore desirable. This study will present conducted EMC noise modelling and measurement techniques, yielding simulated and measurement noise results accurate enough to serve as a prequalification test at the design and prototyping stages, respectively. Accurate models of the power feed line connecting the line impedance stabilisation network to the device under test (converter), including the load are developed. Predictive conducted EMC modelling is accomplished using detailed active level 3 SPICE-based models, creating a real-time circuit model consisting of a complete converter in its operational state, without the need to separate into equivalent models. Effects of the power feed-line length on EMI noise measurements are investigated as currently available literature has not dealt adequately with this issue. The modelling and measurements are performed using software and instruments available in a development laboratory..