This study presents a non-isolated high step-up DC/DC converter based on a high degree of freedom voltage gain cell (HDF-VGC). Both voltage stress of components and conversion ratio of the proposed converter could be adjusted by the number of the HDF-VGC, the current stress of the semiconductor devices can also be adjusted by the input phase numbers of the converter. Moreover, the input current of each phase are automatically balanced. These characteristics make the proposed converter very suitable for high-power and high-voltage applications, such as offshore wind power high-voltage DC transmission system. In this study, the topology construction of the proposed converter has been discussed. The operation principle and performance characteristics of the converter are analysed with four input phases and five HDF-VGCs, then the converter is simulated and a 400 W experimental prototype has been built to verify the correctness of the theoretical analysis.

The new emerging topologies in multilevel inverter field with reduced dc voltage sources and device counts, offer high power capability with less commutation losses and less harmonics in output voltage. However, these topologies have some disadvantages such as voltage balancing problem, increased number of electronic components, larger in size and complex control techniques. This study proposes a new asymmetrical multilevel inverter topology which requires less number of switching devices and driver circuits as compared to conventional multilevel inverter topologies. In the proposed topology, eight switches are required for generation of 15-level single phase output voltage. The proposed topology is simple and can be extended easily to get more number of levels in the output voltage. Therefore, there is a significant reduction in size, cost and complexity for higher number of levels in output voltage. All positive and negative levels as well as performance parameter in term of Total Harmonic Distortion in the output voltage generated by proposed MLI have been evaluated using simulation in MATLAB environment. Various simulation and experimental results are presented to verify the operational accuracy of the proposed topology for a single phase 15-level inverter.

In this study, two quadratic boost derived hybrid multi-output converter topologies are proposed. The proposed converters are capable of giving n-number of simultaneous ac and one dc outputs. The proposed converters are developed from a single switch quadratic boost converter by replacing its main switch by either n-number of series connected H-bridge or n-number of parallel connected H-bridge inverter topologies. The converter developed from n-series connected H-bridge inverters give series mode of the proposed converter and is capable of giving n-number of ac outputs with variable voltages and same currents (to all the ac loads) along with one dc output. Further, the converter developed from n-parallel connected H-bridge inverters give parallel mode of the proposed converter and can give n-number of ac outputs with same voltages (to all the ac loads) and variable currents along with one dc output. Due to the quadratic behaviour of the proposed hybrid multi-output converters, high voltage gain can be achieved from small shoot-through duty cycle. The proposed converter topologies can be applied for simultaneous dc/ac and dc/dc power conversion in a hybrid microgrid. Steady state and dynamic modelling have been carried out for analysing the steady state and transient behaviour of the proposed hybrid converters for two ac and one dc outputs. Simulation and experimental results are presented and efficiency analysis is carried out to validate the performance of the proposed converter topologies for two ac and one dc outputs.

This study presents a high-performance controller based on the Lyapunov stability criterion that enhances the dynamic performance and disturbance rejection capability of resonant DC/DC converters when compared with classical PI control. The series–parallel resonant converter (SPRC) is used as the candidate converter to which this controller design is applied but the design can be generalised to other types of resonant DC/DC converters. By using a multiple module approach, low-power modules of this resonant converter are stacked to enable operation at medium-voltage DC (MVDC). The proposed controller design is applied to modular structure of the SPRC to verify its high-performance output in conjunction with active sharing control loops that ensure uniform current/voltage distribution across the multiple interconnected modules. Detailed controller design, closed-loop stability criteria, robustness and parameter sensitivity are investigated and controller performance is compared and verified against the classical PI control in simulation and low-scaled experimental prototype. Operations in single-module and two-module input-series output-parallel modes are both studied. The study affirms the selection of the modular DC/DC converter architecture and its associated proposed controls for high-performance MVDC applications.

An asymptotically stable controller for solid-state transformers (SSTs) based on Lyapunov direct stability (LDS) method is presented in this study. The proposed controller has four control objectives for the SST application, which includes unity power factor at medium-voltage AC (VAC) side of the SST, constant DC-link voltage and constant output voltage magnitude and frequency at low-VAC side of the SST. To fulfil the above-mentioned objectives four control laws are derived from the Lyapunov function, directly. The proposed LDS-based controller is simulated using MATLAB/Simulink software. The obtained results indicate the fast and superior dynamic characteristics of the proposed controller. The LDS-based controller is comprehensive and can be adopted for the SST applications.

A segmented power-distribution control system based on a hybrid cascaded multilevel converter with parts of energy storage is proposed in this study. The energy storage cells and ordinary cells are connected in series to form a hybrid cascaded topology, which reduces the number of isolated supplies. So, the front-ends and secondary windings of phase-shifting transformer are simplified. According to the different operation modes of the motor, a novel three-segmented control strategy based on active and reactive power control is proposed to realise the energy flow among the ordinary cells, storage cells, and the motor. Thus, the regenerated energy can be stored and reutilised automatically, and then the DC-link voltages of two types of cells will be kept within the appropriate scope. A hybrid cascaded multilevel converter with two ordinary cells and two energy storage cells in each phase is taken as an example conducted on a 380 V scale-down laboratory prototype. Experimental results demonstrate the effectiveness of the proposed converter system.

Rapid rise of current and sudden drop of voltage at the DC side caused by a pole-to-pole DC fault in the modular multi-level converter medium voltage direct current (MMC-MVDC) transmission system seriously threaten the normal operation of the system. This study analyses the development process of the fault and studies the analytical method of the fault transient process. The study is structured as follows: the pre-blocking equivalent circuit model is first proposed, involving the impact of the AC system and the dynamic switching of MMC submodules. Then the solving method of the state equations of fault current in each part of the converter is studied. Combined with the equivalent circuit model after the MMC is blocked, the solving method of state equations of fault current is proposed, with the non-linearity of diodes considered. Finally, an MMC-MVDC system model is built in PSCAD/EMTDC and fault simulations are carried out, verifying the correctness of the equivalent circuit and the accuracy of the fault current solving method.

High-power converters for high-voltage direct current transmission systems and collecting networks are attracting increasing interest for application in large offshore wind farms. Offshore wind farms are capable of generating more electric energy at lower cost when compared with onshore wind systems. In this study, DC/DC voltage conversion should be achieved with a power converter that uses readily available semiconductor devices. A modular DC/DC converter can achieve the required system currents and voltages without exceeding semiconductor ratings. In this study, the operation and control strategy for an input-series–input-parallel–output-series (ISIPOS) energy conversion system for wind systems are presented. The ISIPOS system allows the direct connection of wind turbines to the DC grid. In this research, the design process to control the input and output currents and voltages is explained. In addition, a new method to ensure voltage and current sharing between the different modules is presented and explained. The basic structure, control design, and system performance are tested using MATLAB/SIMULINK. Practical results validate the control design flexibility of the ISIPOS topology when controlled by a TMSF280335 DSP.

Traditionally, the network composition of offshore wind farms consists of alternating current (AC) 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 (WT), a generator and a power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus. The transformer in each WECU is replaced by a converter which is more compact and smaller in size compared with the transformer, thus simplifying the wind farm structure. The use of AC offshore grids instead of DC offshore grids is mainly motivated by the availability of protection devices. Efficient solutions to protect DC grids have already been addressed. Presently, there are no operational DC wind-farms, only 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 study discussed some of the main components required for a DC grid including: the WT-generator models, the control and protection methods, the platform structure, and the feeder configurations. The key component of a DC grid is the converter; therefore, this study also reviews some topologies of converter suitable for DC-grid applications.

With the integration of large-scale electric vehicles (EVs), more capacities of medium voltage distribution networks need to be released. However, there are limited spaces to build new lines, and voltage violation may happen due to the power fluctuation when EVs charge/discharge in a random way. This study proposes to convert some existing AC medium voltage distribution lines to DC and forms a hybrid AC/DC medium voltage distribution network through connecting existing AC lines with a voltage source converter (VSC). Transfer capacity of lines can be increased through DC distribution and flexible power shift between the AC and DC lines can be achieved, based on which more EVs can be accommodated. Configurations of hybrid AC/DC distribution networks are developed, and the capacities released are quantified. A control scheme, which includes a loss minimisation mode and a voltage regulation mode, is proposed in order to optimise real and reactive power outputs of the VSC. Simulation studies are performed to verify the proposed method.

The variable impedance can be applied in many fields of power system. A novel four-quadrant variable impedance based on voltage-sourced inverter control is proposed in the study. The schematic diagram of the novel variable impedance is presented, and its principle and transient characteristic are addressed. The system stability of the variable impedance when it is connected in series with a resistive and inductive load is described. Based on the developed variable impedance, a novel shunt power quality controller (SPQC) is proposed. In the SPQC, the fundamental equivalent impedance of the primary winding is a variable impedance, which can be used to compensate the fundamental reactive power. The equivalent impedance of the primary winding for nth order harmonics is short-circuit impedance, which is very low impedance and the harmonics produced by loads will be short-circuited in primary winding. The control strategy is also analysed. Two sets of single-phase prototypes have been constructed. One is for the demonstration of the variable impedance, the other is for the demonstration of the SPQC. The validity of the novel variable impedance and the SPQC has also been verified through experimental results.

Normally for IGBTs five stages can be identified during Turn-Off transient, among which stage 3 and 4 are most complicated and time consuming in calculation, due to its intrinsically complicated physics process. In this paper, through extensive experiments, we found an interesting phenomenon that in stage 3 the slope of the driving gate voltage is linearly related to the FS-IGBT voltage and current, which greatly simplifies modeling and calculation. In stage 4, we further simplify the model by assuming (i) no interactive effects between the redistribution current and combination current; (ii) the base excess carrier density at depletion boundary far less than that at FSL boundary; (iii) The total charge in the base only relating to carrier life time. With these simplifications, equivalent circuit for each stage with less calculation efforts is developed, and the running time of the simulation under Matlab is less than one second. A double-pulse testing platform was built. The experimental results for both test beds with single IGBT and two IGBTs in series connection match the proposed model developed in Matlab pretty well, under quite different operating conditions, with the maximum error on both I _L–E _off and V _DC–E _off<17%.

The proposed study is about the design and experimental results of a power supply for gate drivers that can sustain 40 kV of insulation voltage with a parasitic capacitance of 4 pF. The first section focuses on insulation materials suitable for medium voltage capabilities and therefore to justify the use of polyesterimide and Teflon polytetrafluoroethylene materials. In a second step, experimental results are provided to guaranty that with a thickness of 1 mm of the insulation material, an insulation voltage of 40 kV is provided. Then, a DC–DC converter based on a series-series topology is designed and lead to a physical prototype of 5 W. The measured parasitic capacitance is 4 pF between the primary and the secondary sides.

Solid-state circuit breakers (SSCBs) inherently have excellent protection performance, thus they are popular for DC distribution protection. However, the safety and reliability of the SSCBs are limited by the rapid response ability of their own gate drivers. In this study, a self-powered SSCB with a normally-on silicon carbide (SiC) junction gate field-effect transistor (JFET) is proposed as the solid-state switch, whose gate drivers are optimised as well. First, both an optimised fault detection circuit and a designed forward–flyback DC/DC converter of the SSCB gate driver have been presented to achieve fast protection during the course of fault isolation. Then, the detailed analyses of the gate driver circuit parameters effect on protection speed are further investigated based on circuit theory and MATLAB calculation. In order to compare and analyse the dynamic responses of the SSCB with and without optimisation, the actual interruption tests of the fabricated SiC JFET circuit breaker prototype and the employed prototype 400 V DC distribution system have been carried out. The results show that the implementation approach is able to improve protection speed for the SSCBs within the order of a few microseconds.

A DC/DC converter is a vital component for offshore DC grids, in which a medium frequency transformer (MFT) is the main element. Therefore, it is necessary to optimise the core material and the design method so that the loss of the MFT would be reduced. In this study, the multi-objective genetic algorithm is proposed, by which two MFTs with different core materials are both optimised. The magnetic flux density of the transformer core and the current density of the windings are introduced as the optimised variables. Also, the key equations for transformer design are put forward as the objective functions. Moreover, with constant iteration of the process of segmentation, parallel selection, mergence, recombination and variation, the Pareto optimal solution is determined. Furthermore, to verify the optimisation results, the temperature field and electric strength of the amorphous alloy MFT are calculated and simulated by finite-element analysis. Finally, a MFT prototype with an amorphous alloy core is built, and the experiment is carried out.

A new soft switching high step-up boost converter is proposed in this study. Coupled inductors technique is used along with voltage multiplier in order to provide high-voltage gain and reduce voltage stress across semiconducting elements. Using an active clamp circuit, soft switching condition is provided for both the main and auxiliary switch which reduces the switching losses and improves the circuit efficiency as well as power density. The reverse recovery problem of diodes is alleviated due to the existence of the leakage inductances of the coupled inductors in series with diodes. The circuit performance is investigated and theoretical analysis is provided. A 200 W 40 V input to 400 V output laboratory prototype of the proposed converter is implemented, and the experimental results show the proper performance of the proposed converter.

This study proposes a MHz isolated resonant converter, which achieves buck–boost DC/DC conversion, zero-voltage switching and the resonant components absorb the parasitic parameters. The DC analysis method and MATLAB computational algorithm are proposed for acquiring the specific circuit characteristics, including the voltage conversion ratio, the rms value of switch current and peak voltage of the semiconductors. The active power and apparent power are also analysed to provide design basis of optimising efficiency. The proposed converter shows greater advantages on voltage stress, current stress and efficiency over traditional isolated Sepic multi-resonant converter. A wide-output-voltage prototype based on the proposed converter was built in the lab and the experimental results verify the theoretical analysis well.

State-of-the-art microprocessors require very low DC-voltages at sub-1 V levels. Many processors draw high-current at low voltages and require low-noise DC-power rails. Switched-mode power supply (SMPS) topologies are the common approach to design voltage regulator modules (VRMs). Fast operations of switches in SMPSs allow the use of smaller inductors but ultimately result in radio frequency and electromagnetic interference issues. Compared to SMPSs, linear regulators have lower noise, high quality DC output, and faster response to the load high-current slew rates; however, with the serious disadvantage of low efficiency. Supercapacitor assisted low-dropout (SCALDO) regulator is a technique to achieve high end-to-end efficiency (ETEE) for linear regulator-based converters. Though, the switch-operation frequency is extremely low, number of switches required for SCALDO configuration is three times larger than that of supercapacitors. Reduced-switch SCALDO (RS-SCALDO) is a topological variation of SCALDO that requires fewer switches. By designing an alternately operated high-current LDO pair, RS-SCALDO can handle high load currents, allowing development of a linear VRM. This study presents a proof of concept prototype of 3.5-to-1.5 V RS-SCALDO for a maximum 5 A load with digitally adjustable output voltages. The prototype achieved an ETEE better than 80%, and required half the switch count of an equivalent SCALDO circuit.

The conventional hysteresis current control (C-HCC) method has a fast transient response and good robust performance. However, due to interphases dependency among the three-phase currents, several shortcomings, for example, a high-switching frequency and large cycle current oscillation will arise inevitably when C-HCC is applied in a three-phase pulse width modulation (PWM) voltage-source converter (VSC). First, this study gives a review on the space vector-based HCC (SV-HCC) which is researched to overcome the shortcomings of C-HCC. The SV-HCC can avoid the most shortcomings of C-HCC. However, SV-HCC has more hysteresis comparator and the adjustment process of hysteresis band (HB) is fussy. In addition, the switching numbers of the three-phase bridge-legs are unbalanced. Then, a switching pattern (SP)-based HCC method is presented in this study which refers to the concept of zero voltage vector in space vector PWM (SVPWM). The presented method set an HB which depends on the system parameters. In the presented method, zero-vector SP will be selected when the three-phase current errors are all within the given HB, and conversely, the specified switching logic will be selected when the current error exceeds the HB. Finally, the effectiveness and superiority of the presented method are verified by simulation and experiment tests.

An accelerated switching function model (SFM) of the hybrid modular multilevel converter comprising both full-bridge (FB) and half-bridge (HB) submodules (SMs) in each arm is presented for HVDC system simulation, where auxiliary circuits are adopted to represent all possible current paths during normal and fault conditions. The proposed SFM can represent the negative voltage generating capability of the FB SMs and the equivalent switching functions in the blocking states of the FB and HB SMs are also introduced in the proposed model to accurately replicate the potential charging of the SM capacitors, yielding improved simulation accuracy compared to other alternatives. In addition to the faster simulation speed, the proposed model accurately reproduces the converter behaviour during various operating conditions, including normal operation, AC fault, DC fault and so on. The proposed SFMs are assessed in MATLAB/Simulink environment using both down- and full-scale HVDC links and the simulation results confirm the validity of the proposed model in terms of model accuracy and improved simulation speed.

This study proposes a novel isolated bidirectional DC/DC converter for micro-grid system, which can fulfil battery charging and discharging. Even though the proposed converter only employs four active switches and a coupled inductor, it can achieve high-voltage ratio without excessive duty ratio or high transformer turns ratio. The power stage of the converter is mainly developed by integrating a three-winding coupled inductor, three-switched capacitors, and a flyback-behaviour converter into a novel structure. The energy stored in the leakage inductance can be totally recycled for efficiency improvement. The operation principle, steady-state analysis, and design considerations of the proposed converter are described in detail. Finally, a laboratory prototype is built to validate the converter. The measured results have verified the correctness and the theoretical analysis.

This study extends a recently proposed dc auto-transformer to a true multi-port converter that can function simultaneously as dc auto-transformer and a dc/ac converter for dc voltage matching and tapping, and power control in highly meshed multi-terminal dc grids. The presented multi-port converter can generate multiple adjustable dc voltages from a fixed or variable input dc voltage, or from an active ac grid. Theoretical discussions and simulations indicate that when the proposed multi-port converter is used as a hybrid dc and ac hub to manage congestions and resolve loop flow issues in a highly meshed multi-terminal dc network, large dc power can be fed to the lower dc terminal, without exposing the switching devices of the lower subconverter to excessive current stress. Its increased control flexibility and response to dc faults at high and low dc terminals are validated using simulations and experiments.

In this study, a hyper-plane multi-input–multi-output (MIMO) sliding-mode controller (SMC) is presented for control of the grid-connected Z-source inverter (ZSI). The presented controller can simultaneously control all of the system state variables including grid-side AC current and DC-link capacitors voltage. These state variables are directly regulated by amplitude modulation index and shoot-through interval. The non-minimum phase problem of the capacitors voltage is solved by indirect regulation of the DC-side inductor current. The proposed controller is developed using non-linear MIMO model of the converter; hence, it is possible to apply the proposed controller in a wide operating range. Controller coefficients are designed using Jacobian linearisation approach to ensure stability of the system. According to application of the Lyapunov approach, it is proved that the proposed controller is asymptotically stable against changes of the system state variables. Some simulations are presented to verify the effectiveness and stability of the developed controllers by MATLAB/Simulink toolbox. Also, a laboratory prototype is implemented using a digital signal processor TMS320F28335. Experimental results are given for the presented controller in the single-phase ZSI. It is seen that the experimental and simulation results are in good agreement.

To solve the capacitor voltage balancing problem of modular multilevel converter (MMC) under carrier phase-shifted modulation, MMC is modelled by generalised averaging approach. The quantitative relation between the capacitor voltage of submodule and other operating parameters is derived, and the corresponding balancing method is proposed which can make each capacitor voltage to track the reference value. According to the average model of MMC, a current controller is designed, which is suited to the voltage balancing method and can decouple the active and reactive currents. The effectiveness of the voltage balancing method and the current control method is verified by simulation results.

Research concerns have grown towards precede diagnosis and prognosis of faults in power converters that result from thermal overheating problems of semiconductor devices. These problems directly affect the overall system availability and reliability as well. Many papers in the literature have addressed these problems. However, almost all of them suffer from major challenges such as increased components count, reduced output current ratings, reduced output voltage levels, and unbalanced voltages over the DC-link capacitors. These challenges impede the maximum energy harvesting, especially in renewable energy systems. A novel space vector modulation (SVM) algorithm has been proposed for lifetime prolongation of thermally aged power semiconductor devices in multilevel inverters. The proposed SVM algorithm functions to alleviate the affected device and to prevent the harmful consequences such as short-open circuit faults. The feasibility of the proposed method has been verified by simulation and experimental results on the three-phase three-level T-type inverter and compared with the previously addressed approaches. It can be concluded that the proposed algorithm provides a significant reduction of thermal stresses on the thermally aged power devices in addition to maintaining the same components count, the same output ratings, the same output levels, and balanced capacitors’ voltages as well.