Advantages of the modular multilevel converter (MMC) such as modularity, scalability, fault-tolerant ability, low device switching frequency, transformerless grid connection etc. have made it the most promising converter topology for medium-to-high-voltage, high-power applications. However, in MMC-based medium-voltage drives, the excessive fundamental-frequency submodule capacitor voltage ripple at low speeds or start-up poses a major technological challenge. A lot of low-frequency ripple suppression methods have been proposed to address this problem. This study summarises the state of the art in this area and classifies these methods into three categories. The pros and cons of them are discussed. An in-depth analysis of these methods from a practical viewpoint is given. Some future research trends are also discussed.

A control strategy to achieve fault ride-through capability and to provide high performance of the output voltage of a single-phase uninterruptible power supply (UPS) inverter is proposed in this study. This strategy consists in controlling the voltage and current waveforms measured at the inverter output filter, with an inner current control loop and an outer voltage control loop, using a plug-in structure based on multiple resonant stages in addition with proportional controllers. In order to achieve stability from no load to short-circuit conditions, the implementation of the multiple resonant controllers includes a compensation of the system phase lag. Moreover, it presents a comparative analysis between two controller structures, the proposed plug-in and the classical proportional + resonant. From this analysis, it can be concluded that the plug-in structure presents improved characteristics of closed-loop output impedance and output voltage dynamic response during fault ride-through events. A controller design methodology to achieve robustness to parametric uncertainties and UPS standard compliance, is detailed. Experimental results from a single-phase 2 kVA inverter prototype are presented to validate the feasibility of the proposal.

The current stress of three-level dual active bridge (3L-DAB) converter under the conventional phase shift (CPS) control becomes much high, especially when the voltage conversion ratio is unequal to one, which may bring low efficiency and damage of power devices. An optimised phase shift (OPS) control strategy is presented to address the issue. The OPS control is realised by calculating the optimal operation point with real-time detection of voltages and current. The principle of 3L-DAB under CPS control is presented. The mathematical models of transmission power and current stress are then established. The minimum current stress and the corresponding optimal phase shift ratios are deduced within different ranges of transmission power and voltage conversion ratio, the comparative analysis of current stress and reflux power under OPS and CPS controls have been carried out. The current stress is minimised with any given transmission power and voltage conversion ratio in the whole power range after adopting the OPS control strategy. Moreover, the reflux power is reduced as well within the boundary of voltage conversion range. An experimental prototype was developed to verify the effectiveness of the proposed control strategy and the correctness of the theoretical analysis.

Light emitting diode (LED) lighting is widely used in current. However, lighting robustness is low in a series loop since the lighting shuts down as one LED fails. This study proposes a power chip for fault detection and isolation for enhancement of LED's fault-withstanding capability. The chip can automatically detect the failed LEDs and bypass them, and let others continue to work. The authors implemented four bypass loops using a power metal oxide semiconductor (MOS) on the single chip. This chip can drive 1 A with 4-bit current dimming, which includes four bypass power P-type MOSs, one power N-type MOS, schedule controller, level shifter and over voltage protection. The schedule controller with a state machine can quickly find which one LED fail and recover it soon. The prototyping chip was designed using TSMC 0.25 μm, which is the second generator high-voltage technology, and the resulting chip area with pads is only 1.3 × 1.7 mm². The chip had been successfully implemented and measured to drive and bypass four LED components or modules.

Transformerless inverter for grid-tied photovoltaic (PV) system has been widely used due to lower cost, higher efficiency and lighter weight. Various transformerless inverter topologies have been proposed to meet the safety requirement of low leakage current and obtain the reactive power capability. To get better performance, a novel transformerless hybrid-H6 inverter with an improved modulation technique is proposed in this study. By adopting the improved modulation technique, two symmetry paths are realised to share the current during the freewheeling mode. Thus, without paralleling any additional capacitors to the switch, the inverter can reduce the influence of junction capacitance on common mode voltage naturally which results in mitigating the leakage current issue. Thanks to the dead time reduction through the improved modulation, the qualities of output waveforms are improved. Moreover, reactive power control is achieved without any modification of the inverter structure. Finally, a 1 kW prototype is simulated and tested to verify the theoretical analysis of this study. Not only the reactive power capability is obtained for the proposed inverter, but also the small common mode voltage fluctuation is achieved at the same time. In addition, the total harmonic distortion of the current is decreased by more than 1.7%.

On control of the grid-connected inverter (GCI) with LCL filter, the inverter-side current model predictive control is adopted conventionally. The ultimate grid-side current is controlled indirectly by control of inverter-side current. The ideal scenario is that grid-side current is directly controlled. However, the conventional control model is complicated and the calculation is heavy. This study proposed a novel direct grid-side current model predictive control (GSC-MPC) for GCI with LCL filter. Based on timing coordination of both forward and backward difference methods, a direct connection is established between the grid-side output current and the inverter-side output voltage. Meanwhile, a proper mathematical model is built for an improved model predictive control. The exact required voltage vector in the next sampling interval is predicted and calculated by this model. The output optimal voltage vector is modulated by space vector pulse width modulation technique to control the inverter. Furthermore, the proposed GSC-MPC is rather robust to parameter variation. Simulation and experimental results present that the proposed GSC-MPC can improve the current control performance effectively for GCI with LCL filter.

There are two common-mode voltages (CMVs) in a six-phase machine with two Y-connected windings shifted by 30°, which bring adverse effects on the six-phase system. In this study, a method of six-phase space vector pulse width modulation (SVPWM) is proposed, which can suppress the magnitude and frequency of the two CMVs. First, all basic vectors are divided into 16 classes, from which two classes of basic vectors are selected. With these two classes of basic vectors, some auxiliary vectors are constructed according to an optimisation model. Then a reference vector is synthesised by using these auxiliary vectors. There are two kinds of synthesis schemes, single-class synthesis and two-class synthesis. Finally, optimal sequences of the basic vectors that play a role in each switching period are obtained. The experimental results show that the peak-to-valley value of CMV is only one-third of the DC-bus voltage of the converter in the two-class synthesis, and the frequency of CMV is only three times of the line voltage fundamental frequency, which is far below the switching frequency of the converter.

The dc/dc converter with interleaving and coupling technique is preferred in high-power applications for its excellent steady-state and transient-state performances. In this study, the principle of three-phase interleaved dc/dc converters with a coupled inductor is first analysed. Also, merits are proved theoretically based on the concepts of equivalent steady-state and transient-state inductance. As most of the proposed single-core coupled inductor may suffer several issues, such as asymmetrical coupling parameters, high manufacturing cost, and large dc flux, an equivalent model is then deduced to use two-wingding coupled inductors instead of the single-core three-phase coupled inductor. With the proposed model, the manufacturing cost can be reduced, the parameters can be symmetrical and there will be nearly no dc flux. In real applications, however, the proposed model may also be faced with current unbalance among phases, which may lead to core saturation and even system failure. A phase-decoupled current sharing strategy is therefore proposed and used in the proposed topology. The superiority and practicability of proposed topology and current sharing method are verified by simulation in PSIM and prototype experiment results.

Parallel-connected power converters have been widely used in the application scenarios required high power to deal with the high load current. In this study, the pulse train (PT) controller with the advantages of fast response speed and simple structure is newly applied to the parallel-connected buck converters with current-sharing ability. However, as the converters operated in continuous conduction mode, undesirable low-frequency voltage oscillation will be brought about by the PT control method. To resolve this problem, a novel capacitor-current-feedback-based PT (CCF-PT) control method is proposed to eliminate the low-frequency oscillation of the parallel-connected buck converters. The approximate discrete time model of the CCF-PT-controlled buck converters is established, and the bifurcation diagrams of output voltage and inductor current changing along with the current scale factor, the capacitor-current-feedback coefficient and the load resistance are drawn to show the operation stability of the converters. Simulation and experimental results are obtained, which indicate that the proposed parallel-connected buck converters with CCF-PT control have excellent performance such as fast response, wide load range and effective suppression of low-frequency oscillation.

In order to constantly improve the output characteristics of wind turbines, the HIL (Hardware In the Loop) experimental platform of wind turbines is built to optimize control. According to the product parameters of 1.5 MW direct-drive PMSG (Permanent Magnet Synchronous wind Generator) of Dongfang Electric Corporation (DEC), the simulation model of wind turbine is built in RTDS (Real Time Digital Simulator), and optimised by comparing with the wind speed–power curve of products, the simulation model of wind turbine with engineering accuracy can be obtained. Base on the engineering model and actual controller of products, the HIL experimental platform of wind turbines is built to optimise control. In the case of unbalanced or asymmetrical grid voltage, in order to make the wind turbine output be stable, the DDSOGI-PLL (direct Decoupling Double Second-Order Generalized Integral Phase-Locked Loop) is proposed to improve the performance of PLL, the active/reactive current decoupling algorithm based on positive and negative sequence voltage orientation respectively is proposed to eliminate the negative sequence current to maintain a constant output power. The effectiveness of algorithm is verified by the HIL experimental platform, the active power output of wind turbine remains constant, the grid-connected adaptability of wind turbines is improved significantly.

Magnetic resonance-based wireless power transfer (WPT) system offers a promising solution to provide power for implantable spinal cord stimulator (SCS). To meet with the clinical requirements, the SCS-WPT system should have high transfer efficiency and long transfer distance. As the topology of the resonant network will directly affect the transfer efficiency and output power of the WPT system, a new resonant network, series-parallel and series (S-PS), is selected in this work. The performance of WPT system with S-PS resonant network has been compared with those with the classic resonance networks, it is concluded that S-PS is the suitable resonant network for the WPT system in a wide range of load. Finally, an SCS-WPT prototype with S-PS resonant network is built with detailed parameter design to verify the correctness of theoretical analysis. The system efficiency is up to 68.4% at a coil separation of 50 mm.

This study proposes a controllable regenerative braking process for hybrid battery–ultracapacitor electric drive systems. The motor in the system is controlled to brake with a constant torque. To this end, a control framework is proposed which includes a circuit topology, decentralised active disturbance rejection controllers (ADRCs) and an operational modes switch controller (OMSC). The motor brakes with ultracapacitor when its speed is fast, and brakes with a dissipative resistor when its speed is slow. Decentralised ADRCs guarantee that ultracapacitor-based braking mode and dissipative resistor-based braking mode can be controlled individually. OMSC coordinates the decentralised ADRCs working cooperatively. Modified ADRC is proposed to implement bumpless transfer when the operational mode or braking mode is switched. The advantages of the proposed control system are as follows: (i) the control of the regenerative braking process based on ultracapacitor and dissipative resistor is linked with the control of the motor; (ii) speed of the motor in the electric drive system is controllable during the regenerative braking process; and (iii) bumpless transfer is guaranteed when braking mode changes from ultracapacitor-based braking mode to dissipative resistor-based braking mode. The following experimental results validate the proposed control framework for the controllable regenerative braking process.

Due to the different application prospects, the coils of inductive power transfer (IPT) system varies. The coils on printed circuit boards (PCB) have drawn considerable attention gradually. However, the applications mainly involve portable equipment and implantable medical devices operating at a high frequency, not fit for IPT system. In this paper, an IPT system based on PCB coils were designed. All the characteristics of single turn coils on a single-layer, multi turns coils on a single-layer and multi turns coils on multi-layers of printed spiral coils (PSCs) were analysed and calculated. The self-inductance and mutual inductance of the series windings on PSCs were modeled and calculated firstly. An equivalent circuit of coils were proposed to simplify the calculation and facilitate the analysis. Furthermore, in order to increase mutual inductance and transfer more power, the multi-layer coils connecting in series were designed for the first time and the inductance parameters can be acquired by the means of proposed method. The analyses on coils model were verified by simulations using finite element method and measuring. Finally, the coil characteristics on IPT system were verified by the circuit simulation and experiment results and the designed coils can meet the desired requirements.

In wireless power transfer technology, power control technique is required to realise constant current (CC) and constant voltage (CV) modes for battery charging. CC and CV modes are needed to effectively charge lithium (Li)-ion batteries to ensure long life span and maximum capacity utilisation. The use of additional battery charging circuitry reduces efficiency, increases volume, and increases circuit complexity. The frequency modulation method is a method that can control power without an additional charging circuit, but a high-resolution controller is needed to control the voltage and current for charging the battery by frequency modulation. This study proposes a system that controls current and voltage for CC and CV modes through a step-charging method using a battery hysteresis with a low-cost control system. The in-band communication circuit is analysed and used to link the primary and secondary sides of the inductive power transfer (IPT) coils, which enables effective feedback control without an additional wireless communication module. This method enables high-efficiency charging while minimising the overall size and cost of mobile IPT applications such as power tools. The proposed system shows 90.3% transfer efficiency with a 20.75 V, 4 Ah Li-ion battery at 7 mm distance.

Traditional three-level (TL) DC−DC converters are suitable for the power conversion with high-input voltage, but the performance of the converter is difficult to be optimized within a wide-input-voltage range. A combined three-phase TL (C-TPTL) DC−DC converter suitable for high and wide input-voltage applications was investigated, and the topology consists of a TL clamping cell and a three-phase full-bridge (TPFB) sub-converter cell. The TL clamping cell ensures that the voltage stress on switches is half of the input voltage, and the TPFB cell reduces the current rating of switches and the output filter requirement. Meanwhile, the C-TPTL converter can operate in the half-input-voltage mode and full-input-voltage mode by controlling the switching sequence of the TL clamping cell. The relationship between the input and output voltages, and the choice of the boundary between different operation modes were discussed. Also, the loss analysis to achieve the optimal efficiency was included. A 200–600 V input and 48 V–20 A output prototype was fabricated and tested in the lab, and the experimental results can effectively verify the theoretical analysis and the performance of the C-TPTL converter.

A novel dual-transformer DC–DC converter with multiple resonant elements is proposed. Owing to the multi-resonant feature, the converter is able to achieve a wide DC voltage gain range through the proper selection of resonant parameters. The operating frequency is restricted within a narrow scope. The potential high-frequency losses are avoided, which apparently contributes to efficiency improvement. Besides, since the proposed converter contains more resonant frequencies, it is possible to additionally transmit the third-order active power to the load. All the power switches have realised the turn-on soft-switching, while the diodes obtain the soft-switching or quasi-soft-switching for both the turn-on and turn-off periods. To verify the theoretical analyses, a 500 W prototype is established in the laboratory. The experimental results demonstrate that the proposed converter can maintain relatively high efficiency (∼95%) among a wide load range. The highest efficiency reaches 95.4% at 300 W.

This study employs a novel smart solid state ferroresonance limiter (SSFL) for stabilising the chaotic behaviour of a voltage transformer (VT). It is shownthat the ferroresonance overvoltage in VTs includes some resonance modes that can be classified as a fundamental, sub-harmonic and chaotic oscillations. The effects of the suggested SSFL on decreasing the amplitude of the ferroresonance over-voltages are examined. To confirm the simulation results, a laboratory prototype is implemented and tested. The performance of the suggested SSFL is simulated using MATLAB software and experimental results are compared with the simulation results. The measurement and experimental results are in agreement and clearly show the ability of the suggested SSFL on ferroresonance cancellation and VT protection against the occurred overvoltage.

In this article, a novel DC–DC converter with high voltage gain capability is presented. The proposed converter is synthesised from (i) a basic two-phase interleaved boost converter (IBC) which uses coupled inductors (CIs) instead of discrete inductors and (ii) diode-capacitor multiplier (DCM) cells to achieve higher voltage conversion ratio. The outputs obtained from the interleaved and the DCM stages are judiciously cascaded with the outputs obtained from the secondary winding of the CIs to enhance the voltage gain. The input current is almost ripple free due to the adopted interleaving mechanism. As voltage gain is extended using CIs and DCMs, the voltage stress on the semiconductor devices is minimal and only a fraction (10.5%) of the output voltage. Experimental results obtained from the 18 V/380 V, 150 W prototype converter, operating at a maximum efficiency of 94% under full-load condition, validate the proposed concept. Further, practical results obtained under closed-loop condition confirm that the converter yields a constant output of 380 V DC which is suitable for microgrid application.

This study presents a new orthogonal signal generation (OSG) technique for the control system of a single-phase grid-connected voltage-source converter (VSC). The existing methods mostly suffer from delay, rely on the system parameters, or require multiple inputs. Among them, the second-order generalised integrator (SOGI) excludes the drawbacks of the other methods and shows better steady-state and dynamic response. The proposed OSG method here includes the advantages of the SOGI technique while demonstrating superior dynamic response and higher disturbance rejection capability. It is structurally simple without adding complexity to the control system. The proposed method is implemented in a DQ-frame current controller and its feasibility and reliability are verified through mathematical analysis, simulation, and experimental results.

The configuration of modular paralleled three-level T-type inverters (3LT²Is) has been widely utilised to extend the system power rating. However, zero sequence circulating currents (ZSCCs) are generated when sharing the common DC-link and the AC side without isolated transformers. This study presents a novel model predictive control (MPC) based strategy to eliminate the ZSCC in parallel operating 3LT²Is. Both the differences of common-mode voltages (CMVs) and neutral point potentials (NPPs) between paralleled 3LT²Is are investigated to be the potential exciting sources of the ZSCC based on the developed model of the ZSCC. According to this conclusion, an MPC-based zero CMV method which adds an additional term to the cost function is used to reduce the differences of CMVs between paralleled 3LT²Is. Besides, a closed-loop ZSCC feedback control method is further introduced to compensate the ZSCC by modifying the cost function. With the proposed MPC-based ZSCC elimination strategy, the ZSCC between the paralleled 3LT²Is can be effectively eliminated in both steady and dynamic states. Besides, good performances can be achieved for both load current tracking and NPP balance control. Simulation and experimental results validate the effectiveness of the proposed ZSCC elimination strategy.