The modular multilevel converter (MMC) is attractive for medium- and high-power applications because of its high modularity, availability and power quality. In this study, a quadruple sampling phase-shifted-carrier pulse-width modulation (PSC-PWM) strategy based on hierarchical distributed control architecture for MMC in medium-voltage power distribution static synchronous compensator (DSTATCOM) application is presented. Compared to centralised control architecture, the multi-digital signal processors master–slave hierarchical control architecture can enhance the scalability of MMC-DSTATCOM and the sub-module (SM) number can expand without limitation. To take advantage of the proposed hierarchical architecture, the multiple sampling PSC-PWM technique is employed to improve the converter performance. The spectrum of the electromotive force of MMC with quadruple sampling is almost the same as that of natural sampling, which leads to fewer harmonics in reactive and harmonic current compensating. Besides, it also raises the sampling frequency so that the MMC-DSTATCOM can compensate harmonic at the low SM switching frequency. The effectivenesses of the proposed architecture and modulation method are verified by experimental results obtained from a 60 V/2 kVA downscaled prototype.

To obtain a resilient and flexible DC grid, reliable means for interfacing its parts operating under different voltage levels must be ensured. This paper proposes bidirectional, isolated, DC–DC converter designed to connect either high/medium voltage bipolar DC grid or two separate high/medium voltage DC grids with another DC grid of similar or different voltage level. In order to achieve galvanic separation among converter stages, while providing redundancy in case of the converter parts malfunction or a DC feeder loss, Scott transformer connection is employed. So far, Scott transformer connection has mostly been used in the railway applications, implying low-frequency operation, in order to obtain two single-phase voltages from a symmetrical three-phase grid. However, its use in the field of the DC–DC conversion has never been analysed in the literature. Further, this paper emphasises operating frequency shift towards the medium frequency range, following the trend of the solid-state transformers which have drawn the attention of both academia and industry worldwide. To control the converter power flow, phase shift control principles are adopted. Consequently, control simplicity is retained, while achieving system complexity reduction. At last, in order to evaluate proposed converter performance, simulation results of a 40 kV/1.5 kV, 10 MW converter are presented, illustrating excellent operating performances.

Reliability is an important issue related to the modular multilevel converter (MMC)-based medium voltage (MV) drives. The design for reliability (DFR) approach has been discussed in many power electronic systems in recent years. MMC-based MV drives power losses and thermal cycling are strongly affected by start-up and low-speed operation, since zero-sequence current injection must be applied. In order to implement fast thermal simulations, the traditional DFR approach does not consider the heat sink thermal capacitance, which can significantly affect the MMC lifetime prediction. This article discusses how the heat sink realisations (material, thickness) affect the MMC-predicted lifetime and evaluates how the drive start-up time affects the damage in the power converter. The case study is based on a 1.4 MW slurry pump system driven by a three-phase induction motor. The lifetime evaluation of the MMC is realised through the Monte–Carlo simulation. The results show that the traditional approach results in an underestimation of up to 42.5% of the converter lifetime. In addition, long start-up times lead to a longer lifetime when compared to the shorter start-up times, due to the reduced thermal stresses in the semiconductors.

Metaheuristic optimisation techniques such as the Grey Wolf optimiser (GWO) and artificial bee colony (ABC) algorithms have been developed for enhancing the dynamic behaviour of wind energy conversion system. The stand-alone doubly fed induction generator (DFIG) control system based on the direct voltage control is experimentally validated for the robust independent control of the stator voltage amplitude and the consequent rotor current regulation. The GWO and ABC are used for selecting the optimal gains of the proportional–integral (PI) regulator to improve the dynamic performance and the robust stability of the DFIG system in the presence of step voltage variation and sudden load operation. Through, MATLAB™/Simulink numerical simulations, the dynamic performances of the GWO-PI and ABC-PI applied to stand-alone DFIG systems are compared with the conventional PI controller under different disturbances. Using a 3 kW DFIG test bench DSPACE DS1104 card prototype, the experimental validation justifies the superiority of proposed novel GWO-PI and ABC-PI controllers over the conventional PI control in terms of steady-state errors, maximum overshoots, settling time and rise time.

In this study, a new multi-point clamped (MPC), three-phase two-leg, five-level unidirectional front end high-power-factor converter is proposed for directly connected wind energy conversion system with a permanent magnet synchronous generator. The proposed topology can be modelled and modulated similarly to other existing MPC structured converters with the omission of clamping diodes and bidirectional switches. The proposed converter has an advantage of reduced power semiconductor device count and maximum device stress. A factor (normalised index) is introduced and comprehensive comparative analysis with N level existing topologies is carried out. The minimum value of the normalised index makes the proposed topology as a suitable option for medium-voltage and high-power applications. Also, the fault ride through capability is one of the key performance indicators is also investigated on proposed topology under voltage sag condition. Further, steady state and transient performance of topology during load as well as dc-link voltage change are presented. Minimum distorted and balanced line currents with unity power factor are drawn from supply by implementing negative sequence elimination control algorithm. The validation of proposed topology is verified with simulation and a down-scaled experimental setup.

The authors intent using a boost multi-level converter for the doubly fed induction machine (DFIM) used in variable-speed pumped storage plants (VS-PSP). Voltage-source converters connected on the rotor side of the machine control the active and reactive powers of the unit. The proposed boost neutral point clamped (NPC) converter topology provides a voltage output two times larger than a conventional three-level NPC (3L-NPC) with similar DC-link voltage and equal number of switches. Hence, it increases the speed variation of the unit, which improves the efficiency during generation and pumping modes. Moreover, it reduces the starting period of the unit at the pumping mode, which is significant during mode changeover time. Furthermore, it reduces switching and conduction losses in the converter. It also reduces the total harmonic distortion in the output current, as it provides five output voltage levels. These improvements show that the boost NPC converter topology is better among VS-PSP project authorities. In addition, the reliability of the proposed topology is investigated, where converter redundancy is a challenging issue in asynchronous VS-PSP units. The proposed boost NPC was compared with the conventional 3L-NPC system by examining a 250 MW DFIM hydro-generating unit.

This work develops a combined control for a stand-alone permanent magnet synchronous generator (PMSG) working with variable source and output demand. The power circuit consists of one PMSG, driven by uncontrolled prime mover supplying load through two voltage source converter (VSC) connected through a dc link capacitor. The main objective of the control is to supply load at constant voltage and frequency, even with variation in source speed and power, and also to accommodate the change in load transients. The generator side converter control is developed in rotor flux oriented synchronously rotating reference frame, to keep the dc link voltage, of the back to back VSC, constant. The load side converter control is also developed in the synchronously rotating reference frame to supply load at constant voltage and frequency. The developed combined control strategy is implemented in MATLAB/SIMULINK environment and found to be successful under all considered operating conditions. The theoretical findings are experimentally verified with a real-time prototype.

Grid-connected medium-voltage converters are typically operated at switching frequencies of several hundred hertz per switch position, requiring bulky and expensive LCL filters in order to meet the harmonic limits given by the grid code. Commonly, semiconductor current derating and increased switching frequencies are used to reduce the LCL-filter costs, leading to a reduced utilisation and efficiency of the converter system. To overcome these disadvantages of conventional converter systems, the presented hybrid converter uses a parallel voltage-source active output filter and thus allows a significant reduction of the passive component demand. The harmonic performance is improved for the operation with small passive filter components, revealing the potential for increasing the utilisation and efficiency of high power medium-voltage converters. As a result, significant reductions of the filter losses and passive components as well as an increased output power are achieved compared to a reference LCL-filter-based converter system.

This study presents the structure and the space vector pulse-width modulation (SVPWM) for power electronic transformer (PET) based on two seven-level cascade H-bridge (CHB) inverters. The DC links of CHB inverters are coupled with nine dual-active bridge (DAB) converters with medium-frequency transformers. The DC-link voltages are equalised with two methods – through the control of DAB voltages and through the modulation strategy applied to both CHB inverters. In the proposed SVPWM, the influence of vector sequences on predicted DC-link voltages is analysed, and the optimum vector sequence is selected to equalise them. Regardless of this, the proposed SVPWM strategy enables the proper generation of output voltage vector also in the case of DC-link voltage imbalance – the calculation of the space-vector area takes into consideration the inequality of the DC-link voltages and its influence on the lengths and positions of active vectors. To simplify the modulation algorithm, the multilevel CHB inverter is considered as a set of three-level inverters connected in series. Each of them is controlled using the same SVPWM algorithm. The proposed modulation method reuses the H-bridges with zero duty cycles determined in the initial stages of the output voltage generation process. This enables the optimal management of the DC-link voltage distribution. The experimental research was carried out on a 600 kW/3.3 kV PET. The results are presented in this study.

Medium-voltage-direct-current collection and distribution grids are being considered for emerging applications, and require enabling power electronic technologies. Medium-voltage high power DC–DC converters are one of the solutions that will play an important role in future energy systems. This study presents a novel bidirectional multiport power electronic converter, interfacing medium-voltage and low-voltage distribution grids while integrating distributed energy storage elements, such as batteries and super-capacitors. The multiport energy gateway concept is based on a modular input-series output-parallel topology includes medium frequency transformer for the galvanic isolation, and combines open-loop and closed-loop operated converter stages. Comprehensive converter design aspects and operating principles are presented and discussed considering representative cases of studies. The validity of the approaches is proved by simulation and experimental results obtained on a small scale prototype.

To improve the efficiency, power quality and reliability for large-scale renewable energy sources (RESs), a transformerless energy storage system (ESS) based on cascaded H-bridge converter (CHB) is presented in this study. Furthermore, the virtual synchronous generator (VSG) control is investigated to make ESS integrated to grid friendly. This study points out and discusses that the primary frequency regulation (PFR) of ESS will be deteriorated by the damping link in VSG due to the coupling problem, and to this end, the proposed virtual transient damping link (TDL) is introduced to simulate the transient and steady-state characteristics of synchronous generators (SGs) accurately without affecting the frequency regulation of ESS. In addition, the small-signal dynamic model of VSG with TDL is established, besides, the influence of VSG control parameters on system stability is analysed by the generalised root locus method and the parameters are quantitatively designed. Furthermore, a direct voltage-phase discrimination-based pre-synchronising control strategy is proposed to realise the seamless transfer between islanded and grid-connected modes for ESS. The simulation and 10 kW prototype experimental results verify the effectiveness of the proposed control strategies and the theoretical analysis.

Switched capacitors (SCs)-based modules are being increasingly used for multilevel DC to AC power conversion, especially for low input voltage applications. Many of these topologies operate in two stages involving H-bridge switches, which endure high-voltage stress. The SC 9-level module (SC9LM) presented here operates in a single stage with one DC source and two capacitors. In addition, the peak inverse voltage of all power switches is confined to the voltage of the input DC source. The proposed 9-level module ensures a reduced number of power switches. In addition, with appropriate utilisation of states, two of the eleven switches in the module operate at fundamental switching frequency, thereby minimising the switching losses. A single SC9LM achieves a voltage gain of two. The proposed module is validated through circuit analysis followed by simulation and experimental results.

The fault diagnosis methods and remediation strategies (RSs) are critical for high-reliability operation of power converters in switched reluctance motor drive (SRD). This study presents an online sensorless fault diagnosis method and effectively selects the RSs from the reliability view. To begin with, the equations of neutral voltage are derived for each phase leg in popular asymmetric half-bridge converter under normal and faulty operation conditions. Next, the diagnosis criterion is proposed with the combination of drive signals and neutral voltage signatures. Besides, the low-cost circuits are designed to save the diagnosis cost, especially for the control strategies without any current or voltage sensor. Moreover, the diagnosis time can be decreased to 7 µs and not affected by the speed, load torque, control strategies and topologies. Moreover, the proposed method can also identify the multiple faults of transistors. In addition, to further enhance the reliability of power converter and then for SRD, the reliability-oriented selection of RSs is carried out by means of the dynamic Markov model. Finally, the experiments of fault diagnosis and thermal stress measurement are conducted to confirm the effectiveness of the proposed diagnosis method and RSs selection.

The unbalanced magnetic pull (UMP) in electrical machines is due to the existence of additional magnetic flux waves. When squirrel cage induction machine (SCIM) running below its rated slip, most of the additional magnetic flux waves can be damped by the counteracting flux waves produced by its parallel-connected rotor bars. Here, this UMP damping effect is comprehensively discussed by comparing the UMP and the air-gap flux of SCIMs and wound rotor induction machines. Subsequently, a UMP damping coefficient is proposed to be included in the conventional UMP analytical model, in which the damping of UMP caused by the counteracting flux waves can be taken into account. Lastly, the UMP of SCIM at different excitation frequencies and rotor resistance are investigated to further verify the UMP damping effect.

A Halbach permanent magnet (PM) tubular linear voice coil motor (LVCM) with a semi-closed structure is developed to enhance the thrust density in this study. Generally, the end of conventional LVCM is open and suffers from the considerable end reluctance. The usage of Halbach array in LVCM makes this problem more prominent although it improves the air-gap flux density greatly. The flux at the end of LVCM can be guided to improve the air-gap flux density. Therefore, this study aims to constrain and direct the magnetic field at the end of a Halbach PM LVCM by using a ferromagnetic auxiliary yoke to construct a semi-closed structure. Magnetic equivalent circuit method is utilised to analyse the effect of the auxiliary yoke on air-gap flux density. Besides, finite element analysis is used to analyse the magnetic circuit and design the parameters of LVCM. The semi-closed tubular LVCM topology is manufactured and the effectiveness of this topology is validated by experimental measurements. Results show that the thrust density of LVCM is improved effectively by using an auxiliary yoke at the end.

This study presents design and analysis of a pulse capacitor charge power supply (CCPS) system by employing a novel brushless field assisted induction generator (BFAIG). Unlike the conventional induction generators, in the proposed configuration of this study, in addition to the phase windings, the stator utilises an assisted DC field coil, which is responsible for production of controllable flux inside the machine. As the main salient feature of including the DC field coil which increases controllability of the flux, providing a regulated terminal voltage in a very wide range appropriate for CCPS applications can be pointed out. In addition, due to the lack of brush and winding on the rotor in the proposed BFAIG, stability, long life and high reliability can be addressed as appropriate characteristics of this structure. Moreover, in this study, analytical design of the proposed BFAIG is presented and two-dimensional magnetic-field distribution is performed by using MagNet CAD package to verify the design procedure. On the basis of obtained results from electromagnetic-field analysis, a CCPS is simulated and studied. The simulation results confirm the performance and efficiency of the presented CCPS.

Although the cores made with amorphous metal alloy have lowered no-load losses compared with the orientation silicon steel cores, a high cost and noise level are inevitable because the magnetostriction for amorphous metal alloy strip is larger than the ordinary one. This study mainly focused on the experimental systems and methods for magnetic properties measurement, vibration characteristics and noise level of amorphous alloy strip and combinations. The vibration characteristics and noise level of amorphous alloy composite strip were also studied. The influence of the annealing process on magnetic energy, vibration characteristics and noise level of amorphous alloy core were analysed in detail as well. All the testing results and analysis above are helpful for the transformer manufacturers, and they can replace the expensive amorphous alloy metal strip with the combinations to reduce the noise and cost of the cores.

In this study, a turn-to-turn short circuit of motor stator fault diagnosis system based on deep auto-encoder and soft-max classifier is proposed. It is also considered in the proposed system that an unbalanced power supply has similar current characteristics and is possible to be misidentified. The determination of neural network parameters and their effect in training are given systematically, which improves the performance compared with traditional auto-encoder. The proposed fault diagnosis system can map the preprocessed three-phase motor current signals to a two-dimensional vector, corresponding to certain area of the feature plane to identify different fault types. The proposed system is verified via simulation experiment using MATLAB and physical experiment using a motor in laboratory. The conclusion illustrates that when signals originating from discrete motor speed, load, and fault severity are used in training, the fault diagnosis is still effective in continuous state. The accuracy is 89% for speed verification and 95% for load verification in simulation and is above 99% in the physical experiment.

Accurate and fast calculation of the self and mutual inductance of coils is an important factor in system design and optimization for many applications. The methods of calculation mutual inductance between two spiral disk coils include elliptic integral solutions and coefficients in literature. In this study, a new semi-analytical method is proposed to calculate the mutual inductance between two thin disk coils without any coefficient and complex integral solutions. For this purpose, the circular winding is treated as polygonal winding with multiple edges. Unlike Grover's average diameter approach, the gaps between the turns of the coils and the diameter of the wire are included in the calculation. To observe the reliability of the method developed here, the mutual inductance between identical disk coils with an inner diameter of 10 and 20 cm were calculated using both methods. To confirm the accuracy of the proposed method, the experimental results are compared with two methods using the same coils. It is observed that the new method gives acceptable results (maximum error of 2.83%) especially at the distances equals to the inner radius. Consequently; this method is useful for the design of loosely coupled systems, such as wireless power transfer and pressure sensors.

As it has been widely and wrongly believed that noises at any frequency of permanent magnet synchronous motor (PMSM) are positively correlated with subjective annoyance, reducing sound pressure level or sound power level of PMSM is mostly regarded as the only optimisation goal of sound quality (SQ). This study presents a sensitive critical band (SCB) diagnostic method for SQ of PMSM for electric vehicles (EV). First, a new acquisition method of near-field noises without sound attenuation of PMSM was proposed and a neural network model for SQ evaluation was established to determine the specific operating condition with the worst SQ. Second, the band-pass filter of CB (BPFCB) and band-stop filter of CB (BSFCB) were designed to diagnose positive SCBs or negative SCBs in which noises were positively or negatively correlated with the subjective annoyance, respectively. Finally, the major excitation sources of abnormal noises in SCBs were diagnosed by using the analytical calculation. An 8-pole-48-slot PMSM was considered as a case study to testify to the effectiveness and practicability of the proposed method. It may provide a new route for the precise SQ optimisation of PMSM.