High-frequency link matrix converter (HFLMC) is attractive for renewable grid-connection system because of the advantages of its high power density and reliability. Due to eliminate the intermediate DC link, the grid side fault of the HFLMC will directly reflect to the high-frequency link side, which will result in the severe voltage and current oscillations in the high-frequency transformer. This paper proposes a coordinate low voltage ride-through (LVRT) control for HFLMC. The proposed control strategy actively enables the HFLMC to inject reactive power into the grid to provide voltage support for the grid. In addition, the proposed control strategy coordinate controls the phase shift angles of the HFLMC with the objective of the minimal RMS value of the current on high-frequency transformer (HFT). Simulation studies with the time-domain professional tool PLECS are conducted to confirm the effectiveness of the proposed control strategy.

Thyristor branch loop (TBL)-based hybrid MMC (HMMC) is one of the promising technical solutions to handle the DC-line short-circuit faults in high voltage direct current (HVDC) systems. However, the submodule (SM) capacitor precharging characteristics of TBL-HMMC is quite different from conventional MMCs, due to the special configuration. During SM precharging and system startup process, the TBL-HMMC may suffer from surge current, SM capacitor under-voltage, unbalanced capacitor voltages between unipolar full-bridge (UFB)-SMs and half-bridge (HB)-SMs, and unbalanced capacitor voltages between upper and lower arms. A SM grouping-based capacitor precharging strategy is proposed in this paper, which could precharge both UFB-SMs and HB-SMs in upper and lower arms to their nominal voltage, and therefore achieving a smooth startup for the TBL-HMMC based HVDC system. Electromagnetic transient simulation studies using PSCAD/EMTDC tool are conducted to confirm the effectiveness of the proposed strategy.

In this work, the surge current ruggedness (the ability to withstand the surge current until failure happens) in the third-quadrant is investigated for evaluating the reliability of the SiC MOSFET. A dynamic surge test platform (DSTP) is developed, and the surge trajectories are observed in detail. It is found that the source-drain current is mainly carried by the body diode of SiC MOSFET during the surge event, regardless of the gate-source voltage is positive or negative. The gate-source terminals shorting and three-terminals shorting are two surge failure modes of the SiC MOSFET, which depend on the shocking strength of the surge power. Furthermore, the oscillation induced by dynamic avalanche effect (DAE) is observed in the surge test, which expands the understandings about the negative effect of surge event. This work is aimed to give the useful knowledge about the surge current ruggedness, and some practical suggestions based on the experimental results are given out to improve the system reliability.

In the bipolar HVDC system based on TBL-HMMC, a voltage sag can occur on the AC side during DC short-circuit fault clearing. In this article, the DC short-circuit fault clearing process for TBL-HMMC and its fault equivalent circuit are presented. This paper analyzes the underlying reasons for voltage sag resulting from thyristor triggering and submodule blocking and proposes a voltage optimization control strategy to suppress the transient voltage sag. In addition, compared with TBL-HMMC without proposed voltage optimization control, the proposed approach can effectively suppress AC side voltage sags in order to improve power quality, without adversely affecting the fault clearing process. Simulation studies using PSCAD are conducted to investigate the operation of the bipolar TBL-HMMC under DC short-circuit fault clearing and to verify the effectiveness of the proposed voltage optimization control method.

Low frequency transmission technologies can significantly enhance power transfer capacity and distance of alternating current transmission by reducing transmission frequency. This paper proposes a dual bipolar modular AC-AC converter (DBMAC) topology for the frequency conversion between low-frequency (15~20 Hz) and grid-frequency transmission (50 Hz). The operation principle is elucidated, showing the principal of the frequency decoupling. The mathematical models of DBMAC are developed and the control strategies are given for both input and output sides. The simulation is conducted and its results verify the effectiveness of the proposed DBMAC.

Continuous-control-set model predictive control (CCS-MPC) has been extensively adopted in dual active bridge (DAB) converters to achieve rapid and precise output voltage regulation. However, the voltage control performance of conventional CCS-MPC exhibits significant sensitivity to parameter variations in DAB system modeling. Discrepancies between model and actual parameters may degrade voltage tracking accuracy. To address this limitation, this paper proposes a continuous-control-set model-free predictive control (CCS-MPC), where an ultra-local data-model for the DAB converter is established and the real-time updates of this data-model is implemented through the Runge-Kutta method, thereby eliminating parameter dependency in output voltage regulation and enhancing system parametric robustness. Experimental results validate the effectiveness of the proposed approach through comprehensive verification.

Capacitance reduction is of great significance for economical and high power density operation of modular multilevel converters (MMCs). This paper proposes a low-capacitance MMC for high voltage direct current (HVDC) applications. The average capacitor voltage reduction control (ACVRC) is employed to decrease the average capacitor voltage, so that a high capacitor voltage ripple can be accommodated without exceeding the peak value of capacitor voltage. The capacitance reduction under ACVRC is analyzed. To prevent the increase of the arm references and avoid the decline of the linear modulation region, an optimal third-order harmonic voltage injection (THVI) method is proposed. The average capacitor voltage and capacitance are designed under ACVRC and THVI. Besides, the existing circulating current control-based method can also be used to further reduce capacitance. Simulations are conducted, which verify the correctness of the analysis and the feasibility of proposed low-capacitance MMC.

The modular multilevel converter (MMC) becomes attractive in the medium- and high-power application with high modularity. In this paper, the MMC and cycloconverter based machine drive system is proposed. The six-phase MMC is used as the grid-side converter, which produces six-phase medium-frequency ac voltage for the machine side converter. The six-phase to three-phase cycloconverter is used as the machine-side converter, which converts the medium-frequency ac voltage to the low-frequency ac voltage to drive the machine. In addition, the corresponding control for the grid-side converter and the machine-side converter are also proposed. The proposed MMC and cycloconverter based machine drive system can effectively realize the control for the high-voltage, high-power and low-speed machine. What is more, the SM capacitance is significantly reduced owing to the medium-frequency ac output voltage of the MMC. The simulation studies of the proposed MMC and cycloconverter based machine drive system and control are conducted with a detailed PSCAD/EMTDC model, and the results show their effectiveness.

Fault localization is one of the most important issues for the MMC consisting of large number of switches. This paper proposes a statistical multi-faults localization strategy for the MMC, where a feature extraction algorithm of capacitor voltage variations is proposed to extract the features of the MMC based on the exponential smoothing relationship among the capacitor voltages. Based on the extracted features, faults in the MMC can be easily located with the Grubbs Criterion according to the Grubbs Criterion Table. The proposed Grubbs Criterion-based fault localization strategy can construct concise simple features samples for the MMC containing all exponential smoothing global capacitor voltage variables, and accordingly it can locate faults with short time for the MMC. In addition, it not only does not require a large number of training data samples, but also does not require the creation of complex mathematical models and manual setting of empirical thresholds. It is compatible with both single and multiple switches open-circuit failures. The results of the simulation confirm the effectiveness of the proposed strategy.

Space vector pulse width modulation (SVPWM) technology is widely used in power converters as a modulation strategy that can effectively improve voltage utilization and reduce harmonics. However, as the number of levels increases, the number of space vectors suddenly increases, and the modulation algorithm design and implementation become more and more difficult. As a result, such application is hardly seen in three-phase multilevel current source rectifier (MCSR). This paper proposes a basic theory and control algorithm of SVPWM technology applied in 7-level rectifiers, which adopts a double closed-loop control strategy for inner current loop and outer voltage loop. System modeling and theoretical analysis with detailed vector selection method of the proposed control are provided. Simulation results show that both unit power factor and 7-level waveforms have been achieved, which has validated the proposed control.

Modular Multilevel Converter (MMC) is widely used in flexible DC transmission systems for its great performance in flexibility, stability and availability. Reliability is one of the most important challenges for the MMC consisting of a large number of submodules (SMs). The capacitance in each SM of the MMC is of great importance to the MMC's normal operation. This paper proposes a capacitance monitoring method for the full-bridge SM based MMCs, which can estimate the capacitance in each full-bridge SM with a simple algorithm. The simulation studies with the professional tool PSCAD/EMTDC are conducted with the proposed method. The study results show the effectiveness of the proposed method.

The modular multilevel converter (MMC) is attractive for medium-or high-power applications because of the advantages of its high modularity, availability, and high power quality. Reliability is one of the most important challenges for the MMC consisting of a large number of submodules (SMs). The capacitor monitoring in each SM of the MMC is an important issue, which would affect the performance of the MMC. This paper proposed an effective monitoring method for the capacitance in each SM of the MMC. The proposed method reveals the relationship between the arm average capacitance and the capacitance in each SM of the arm, which can be used for capacitance estimation with a simple algorithm. The simulation studies with the professional tool PSCAD/EMTDC are conducted with the proposed method. The study results show the effectiveness of the proposed method.

The input capacitors' voltages are unbalanced under the conventional control strategy in a dual half-bridge cascaded three-level (TL) DC/DC converter, which would affect the high voltage stresses on the capacitors. This paper proposes a pulse-wide modulation (PWM) strategy with two working modes for the dual half-bridge cascaded TL DC/DC converter, which can realize the zero-voltage switching (ZVS). More significantly, a capacitor voltage balance control is proposed by alternating the two working modes of the proposed ZVS PWM strategy, which can eliminate the voltage unbalance on the four input capacitors. Therefore, the proposed control strategy can improve the converter's performances in: 1) reducing the switching losses and noises of the power switches; and 2) reducing the voltage stresses on the input capacitors. Finally, the simulation results are conducted to verify the proposed control strategy.

A novel topology is proposed for three-phase electric spring (TPES) to achieve specific functionalities. With respect to the existing one, the novel topology contains an additional three-phase transformer with the primaries located at the position of the non-critical three-phase load (NCL) of the existing topology and its secondaries connected to the new three-phase NCL, thus forming a new three-phase smart load (SL). To control the novel topology, the so-called modified δ control utilized for the single-phase electric springs is extended to the three-phase case. Thanks to these solutions, TPES exhibits a behavior entirely different from the existing ones. For instance, under a fluctuation of the line voltages, it exhibits a change of the voltages across NCL that has the same trend as the line voltages. Moreover, it regulates the voltages across the three-phase critical load even under both imbalanced and distorted grid conditions. The effectiveness of the novel TPES topology and of its control is validated by simulation results.

A soft-switching solution implemented to the traditional bidirectional DC/DC converter is developed. The soft-switching cell, which composed of three auxiliary switches, one resonant capacitor and one resonant inductor, is equipped in the traditional bidirectional DC/DC converter to realize circuit ZCT or ZVT operations in both energy transmission directions. Two driving methods are presented to charging energy for different storage applications according to the storage materials' electrical characteristics. There are two methods in buck mode. The first method with low conduction losses suffers from a hard turning off process in one of the auxiliary switches, while the others experience the soft operation. As for the other method, the zero voltage or zero current transmissions in all switches are realized, however, the relative higher but fixed conduction losses are introduced by the resonant circle. And the proposed topology of bidirectional soft-switching dc-dc converter(TASBC) performs ideal soft switching at boost operations. The characteristics of the proposed converter has been verified by MATLAB simulations and experimental results.

In this paper, a double phase-shift control strategy is proposed for the full-bridge three-level (FBTL) DC/DC converter applied into DC distribution systems with the medium DC bus voltage. By utilizing the proposed control strategy, the voltage change rate dv/dt and voltage stress of the transformer can be effectively reduced, which means that the reliability and EMC of the FBTL DC/DC converter can be improved. The operation principle and performances of the proposed control strategy is analyzed in detail. Finally, the simulation results are presented to verify the proposed control strategy.

The modular multilevel converter (MMC) becomes attractive in the medium- and high-power application with high modularity. In this paper, the MMC is proposed to be applied in the variable speed wind turbine (VSWT) based on the full-scale back-to-back (BTB) power converter, where the generator-side converter is the three-level neutral-point-clamped (NPC) converter and the grid-side converter is the three-phase MMC. A control strategy is also presented for the MMC based VSWT for the second-order harmonic current elimination. The proposed MMC based VSWT and control not only mitigate the voltage ripple in the dc-link of the power converter to improve system performance, but also ensure the grid-side current balancing to increase the generated power of the wind turbine under the unbalanced grid fault, in comparison with the conventional VSWT based on BTB three-level NPC converters. The simulation studies of the proposed MMC based VSWT and control are conducted with a detailed PSCAD/EMTDC model, and the results show their effectiveness.

A control scheme is proposed for the three-phase electric springs (TPESs), based on the recently introduced δ control and an improved phase locked loop (PLL). The scheme utilizes three independent δ controls, one for each phase, to stabilize the voltages across the critical loads (CLs) while handing over imbalance and distortion of the microgrid voltages, in addition to their fluctuations, to the non-critical loads (NCLs). Instrumental in achieving this outcome is the operation of the improved PLL that separates the fundamental component of the positive sequence of the microgrid voltages from the other sequences and harmonics. The scheme is also able to stabilize the CL voltages under imbalance of CLs and NCLs. Simulation results obtained under various conditions of imbalance of the microgrid voltages and/ or the loads, and of distortion of the microgrid voltages, taken individually and together, show the effectiveness of the proposed control scheme of TPESs.

Constant power load converter will introduce unstable factor to the cascaded system. Exchanging the position of constant power load converter as the source converter can improve system stability. Active solutions to modify the negative impedance of constant power load converter can also make the system more stable. The concept of coordinative front-to-end impedance control is proposed in this paper, which can assist the cascaded system with better stability. This paper compares converter impedance behaviors between different control methods, and shows that the front-to-end impedance controller can make the system more stable. The conclusions have been validated by simulation results.