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This paper presents the internal energy balance of a modular multilevel cascade converter with photovoltaic arrays as distributed energy resources. This converter is characterized by a modular arm structure composed of bidirectional cascaded chopper cells with floating DC capacitors, where each cell is connected to a photovoltaic array through a DC/DC converter. A continuous model and a control method are proposed to maintain the internal energy balance and achieve independent voltage control of all the floating DC capacitors, allowing the tracking of the maximum power point for each photovoltaic array. The operation with asymmetric energy generation at different levels of irradiance is discussed to maintain the internal energy balance without distortions in the DC link and AC output currents, focusing on the operation with individual cells with zero energy generated from PV array. This topology and control method is suitable for hybrid multi-terminal high voltage DC systems and hybrid generation systems, such as wind-solar energy systems. Experiments are presented to validate the proposed model and control method.

This paper presents a ground breaking traction drive for fuel cell powered light rail vehicles based on a multilevel cascade converter with H-bridge cells. The converter provides dc-ac power conversion with unique buck-boost capability to compensate for the variation of fuel cell terminal voltage with the load power. In comparison to a conventional boost inverter, based on the cascade connection of a boost dc-dc converter and a voltage source inverter, the proposed converter has the significant benefits of much smaller inductor size, lower dc side current ripple, and multilevel voltage waveforms at the ac output. This paper presents the converter modelling and design of the boost inductor and sub-module capacitors, taking into account the full speed range of the motor. The concept has been fully validated on a bespoke experimental prototype driving a small induction motor, showing the suitability of the proposed converter as a traction drive for hydrogen-powered rail vehicles.

A modular multilevel cascade converter based on single-delta bridge cells, hereinafter referred to as the single-delta bridge-cell (SDBC) converter, is a prospective power converter for a static synchronous compensator intended for voltage regulation and stability improvement of electric power systems. However, the SDBC converter cannot exchange nonzero active power with the three-phase ac mains unless energy storage elements such as batteries are connected to the dc side of each bridge cell. This article presents a dc/1 $\phi$ /3 $\phi$ power converter that combines the SDBC converter with a single-phase converter via a single-phase medium-frequency transformer with an operating frequency of 150 Hz. It can control the active power of 37% of the rated reactive power between the dc source and three-phase ac mains, thus enhancing voltage regulation and stability improvement. The operating principles and control method are first discussed, followed by a description of experiments using a downscaled 110-V, 5-kVA model.

An integrated motor-drive and battery- (IMB) charging system based on the split-field-winding doubly salient electromagnetic machine (DSEM) is proposed in this article, where field winding of DSEM is integrated as the filter inductance of the front-end dc–dc converter. In the driving mode, the coupling relationship between the front-end dc–dc converter and rear-end inverter for DSEM is deduced, and the corresponding basic control method is given to achieve independent control of the front-end dc–dc converter and rear-end inverter based on the power balance of the cascade converter. Due to the constraints analysis of the IMB system in the dynamic procedure, an optimized triloop control strategy of speed, power, and dual-threshold voltage is proposed to improve output performance. Finally, the simulation model and experimental platform are established to verify the effectiveness and the steady and dynamic performance of the IMB system with the proposed driving control strategy.

In order to obtain the compact and low-cost on-board battery-charging system for the electric vehicles, the doubly salient electromagnetic (DSEM) with the cascade converter is employed as the driving system, the reactance of the phase windings and split-field-winding is reused as the filter inductance for the battery-side DC/DC converter and grid-side pulse-width modulation (PWM) rectifier. The corresponding charging control schemes for the DC/DC converter and the double-loop decoupling control for the PWM rectifier are given, and the demagnetisation control of DSEM is achieved to eliminate the output torque based on the in-series split-field-winding control. Due to the coupled magnetic path of the flux linkage of field and phase windings of DSEM, the properties of the charging current fluctuation with respect to the mutual inductance and output torque ripple rated to the rotor position are given to achieve the best rotor location setting of DSEM in the charging mode. Finally, the co-simulation model and experiment platform are built to verify the feasibility and coupling properties of the proposed system by the results in the steady and dynamic procedure.

In this article, a novel passive lossless clamped circuit composed of two capacitors and one diode is proposed. One of the two capacitors is used to construct a flowing path for the energy of leakage inductor and the other is used to store the leakage energy. Compared with a classical passive lossless clamped circuit, the novel circuit structure is more flexible and versatile, and the number of the extra capacitor and diode is consistent to the classical circuit for recycling the energy. Moreover, an idea of using more capacitors in series instead of a single output capacitor to reduce voltage stress and volume is presented. In order to extend their application scope and demonstrate their practical value, the novel clamped circuits and the idea are integrated into a family of cascade dc-dc converters based on the popular boosting cell, such as voltage-lift cell, symmetric voltage multiplier cell and asymmetric voltage multiplier cell. As a representative, a novel cascade converter, which integrates the novel passive lossless clamped circuit and the idea is analyzed in detail. The proposed converter has a high voltage gain and low ripple input current. The voltage stresses of an output diode and output capacitor can be effectively reduced. Its output capacitor adopts a 20-uF polypropylene (CBB) capacitor rated at 250 V to replace the higher rated electrolytic capacitor. A prototype circuit in the laboratory is established to verify its performances.

This paper presents a novel online technique, which simultaneously exploits the global search ability of differential evolution (DE) and rapid convergence of Newton Raphson (NR) methods (named as DE-NR) to solve intricate simultaneous transcendental trigonometric set of harmonic elimination pulse width modulation equations for modular multilevel cascade converters based power systems. Major contribution of this paper is a harmonically efficient online algorithm with rapid convergence. Switching angles for an extensive range of modulation index ( M , 0.86 ⩽ M ≤ 9.97 ) are computed online to demonstrate the harmonically efficient working of proposed method. Total harmonic distortion values of the filtered line-to-line output voltages during online operation are restricted to 0.35% of the fundamental component, which depict successful removal of unwanted harmonics and are significantly better than the values allowed by IEEE standard 519-2014. Comparison between DE-NR, differential evolution and Newton Raphson methods demonstrates the rapid convergence behavior of DE-NR as it requires only (on average) 4 iterations compared to 490 and 182 iterations respectively. It has also shown superior harmonic control than the recently devised online Middle-Level Selective Harmonic Elimination Pulse-Amplitude Modulation method. Successful simulation and experimental validations of DE-NR method have been done by developing three-phase modular multilevel cascade converter in MATLAB-Simulink and single-phase eleven-level modular multilevel cascade converter hardware prototype.

DC Transformer (DCX) is widely used because of its high efficiency and tight conversion ratio. Although the traditional cascade converter can regulated output voltage, it causes a huge loss (over 10% efficiency). The proposed topology outputs regulated voltage with high efficiency and wide input voltage. It consists of a push-pull DCX converter in parallel with a voltage compensation circuit. The compensation circuit only processes part of input power with low stress and low loss; thus, the total efficiency is improved. Two compensation circuits are compared and discussed, then the small-signal model and the control-to-output transfer function are derived. A design example is presented, and the simulation and calculation results verify the theory. The results show that the secondary-side compensation circuit has high efficiency (92.5%), wide input voltage range (±43%), and the efficiency penalty caused by voltage compensation is less than 2.5%.

The Modular Multilevel Matrix Converter (M3C), also known as Triple Star Bridge Cells (TSBC) converter from the family of Modular Multilevel Cascade Converter (MMCC) has attracted attention in medium/high voltage, high power applications. Recent research has identified its potential for various direct high power AC/AC conversion applications due to its modularity, redundancy and high power quality. However, it suffers from cross-coupling while utilising low/fundamental frequency switching techniques such as Nearest Level Control (NLC) along with the double αβo transformation. This article proposes a decoupling approach to eliminate the coupling effect and low frequency oscillation at the output port for implementation of NLC in an MMCC-TSBC converter.

This paper presents the internal energy balance of a direct AC/AC modular multilevel cascade converter (MMCC) with photovoltaic (PV) arrays as distributed energy resources. This converter is characterized by a modular arm structure composed of bidirectional cascaded bridge cells with floating DC capacitors connected to the photovoltaic array through a DC/DC converter. Based on the analytically developed power flow relations in the arms, a control methodology is proposed to maintain internal energy balance and fully independent control of the floating DC capacitor voltage in each cell, allowing the operation with asymmetric power generation at different levels of irradiance. The operation with individual cells with zero power generated from photovoltaic array is discussed to maintain the internal energy balance without distortions in single-phase and three-phase currents. This topology and control methodology is suitable for hybrid power generation systems, such as wind-solar power generation. Comprehensive experimental results are presented to validate operating principles and control methodology.

© 2021 IEEE.Due to the limited reserves of fossil fuels and the devastating damage they cause to the environment, the interest in renewable energy sources is increasing all over the world. As a natural consequence of this trend, the traditional electricity grid has lost popularity, and DC microgrid, which employs many power converters and facilitates the grid integration of renewable energy sources, comes into prominence. The most of renewable energy sources produce DC voltage and they need DC microgrid. It could be possible that renewable energy sources are more increasingly located in traditional grid with using power electronic devices like converters. Despite the many advantages of DC microgrid, the most important shortcoming is the instability problem. In this study, the characteristic behavior of constant power load (CPL) encountered in cascade-connected DC converter system were firstly analyzed under the guidance of literature review. Then the instability effect on the feed-side converter caused by the CPL load was solved using the classical passive and active damping methods. The results obtained from the sample DC-DC cascade converter structure created in MATLAB software were compared with each other and their advantages over each other were emphasized. Thus, the comprehensive study on the instability problem and CPL load behavior on DC microgrids, which is a current research topic, has been performed.

© 2021 IEEE.Increasing DC distribution systems and the share of renewable energy sources in energy production allows the widespread use of multiple DC/DC converter systems. Cascade converter structure, which is frequently used in multi-converter systems, exhibits constant power load behavior. Constant power load leads to the instability effect on the open-loop system due to its negative incremental impedance. However, these systems can be stable as a closed-loop by using different control techniques. Compared to conventional control methods such as PI and PID, sliding-mode control, which is one of the non-linear control methods, is a robust stance against external disturbances. In this paper, a sliding-mode controller based on a super twisting algorithm has been designed for a DC/DC buck converter feeding constant power load. The super twisting algorithm is a technique added to the sliding mode controller to suppress the chattering effect. The proposed controller ensures the stability of the system during steady state as well as provides that the output voltage remains constant whenever there is an alteration in the input voltage or constant load power. Moreover, the chattering effect, which is the most important weakness of the sliding mode control, has been significantly reduced by means of developed controller. The performance of the proposed controller has been verified by performing simulations in a MATLAB/Simulink environment.

The boost integrated LLC (BI-LLC) resonant converter has less switches and higher conversion efficiency compared with the traditional two-stage cascade converter. By employing fixed-frequency pulse width modulation (PWM) to control output voltage, the gain range and optimal operation area can be easily regulated. However, small-signal modeling of this topology is rarely discussed, and the design of control system is not specific. In this paper, the small-signal model of BI-LLC resonant converter is derived using both state space averaging (SSA) method and extended describing function (EDF) method. According to the obtained small-signal model, a feasible PI controller is designed and applied. Finally, the accuracy of the small-signal model and the performance of the controller is verified by simulation.

This paper proposes two hybrid asymmetric cascaded multilevel inverters for driving medium-voltage open-end winding motors. Each phase of the proposed systems consist of a three-level (3L) H-bridge cell and a nine-level (9L) H-bridge cell connected in series. An unequal dc-voltage ratio of 1:8 between cells is used, providing 19 phase voltage levels. The topologies of the 9L H-bridge cell are based on modular multilevel cascade converter (MMCC) with double-star chopper cells, while the 3L H-bridge cell is fed by a floating capacitor. Compared to conventional MMCC for open-end stator winding motor drives, the proposed inverters can produce more phase voltage levels. In addition, they require a smaller number of power semiconductor switches, drivers and capacitors. Simulations and experimental results are presented in order to demonstrate the performance and feasibility of the proposed topologies.

This article proposes a capacitor voltage ripple reduction method used for a modular multilevel cascade converter (MMCC) with single delta bridge cells (SDBC), by applying a third harmonic zero-sequence current. A practical case study on an 80 MVar/33 kV MMCC-SDBC-based STATCOM is used to demonstrate the method. The impact of the third harmonic zero-sequence current level of the capacitor ripple reduction and the electrothermal stresses on insulated gate bipolar transistor modules are analyzed. An optimal parameter of the current level is obtained by compromising the above-mentioned performance factors. The obtained result shows that the required capacitance, as well as the capacitor bank volume, is reduced by 20% without increasing the total power semiconductor losses by using the proposed method.

This paper analyses the use of a simplified ac-side current control algorithm strategy, independent of the number of submodules, applied to a Modular Multilevel Cascade Converter acting as an Active Power Filter (MMCC-APF) with a reduced hardware structure. Here, Model Predictive Control (MPC) with a reduced number of states is used for both ac-side current control and submodules' capacitor voltage balancing algorithm. Furthermore, analytical criteria are developed, to define the minimum number of states required for current control. This approach improves the control strategy, thus reducing computational effort for MMCC in APF applications. For the calculation of APF compensating currents, two main strategies are addressed, in order to assess ac-side currents control limitations: (i) total and (ii) specific frequencies harmonic compensation. The system analyzed in this paper consists of a nine-level MMCC connected to a medium voltage distribution grid, compensating the harmonics drained by a diode rectifier. All the analyses are conducted in the environment of circuit simulation PSIM.

This chapter starts with three‐level neutral‐point‐clamped and neutral‐point‐piloted inverters, and three‐level and four‐level flying capacitor inverters, followed by the modular multilevel cascade converter family. It provides an intensive discussion on circuits and systems of the following two grid‐tied applications. The first is a three‐phase transformerless static synchronous compensator based on one family member for industrial and utility distribution systems and the other is a back‐to‐back system based on another family member for an asynchronous inter‐tie between two transmission power systems. The chapter also provides a general description of a single‐star bridge‐cell (SSBC)‐based static synchronous compensator (STATCOM) characterized by phase‐shifted‐carrier pulse width modulation (PWM), with a focus on capacitor‐voltage balancing. It presents a downscaled SSBC‐based STATCOM with phase‐shifted‐carrier PWM. The chapter also presents a three‐phase downscaled back‐to‐back system without DC‐link capacitor between the two identical 17‐level double‐star chopper‐cell converters.

In this study, the authors propose a model predictive control method for modular multilevel cascade converter-single star bridge cells (MMCC-SSBC) with an improved zero-sequence voltage injection algorithm under unbalanced conditions. First, a model predictive controller is designed to optimise the control process, and a novelty cost function is proposed to control the current, voltage and switching frequency. Second, an improved dc voltage balancing algorithm based on zero-sequence voltage injection is proposed. New unknown variables are selected to avoid the inverse trigonometric calculation and convert the non-linear problem to a linear problem. Third, the regulation capability is analysed to avoid the saturation and non-linear characteristic caused by the zero-sequence voltage injection. Simulation results indicate that the proposed algorithm shows a shorter calculation time and achieves better performance when single-phase voltage sag occurs. The experimental results further demonstrate that MMCC-SSBC combined with the proposed controller has excellent steady state and dynamic performance; at the same time, the switching frequency can be controlled by the new cost function.

Modular multilevel cascade converters (MMCCs) are becoming attractive solutions as high-voltage Static Synchronous Compensators (STATCOMs) for power plants in renewable energy generation, in order to satisfy the strict grid codes under both normal and grid fault conditions. This paper investigates the performances of four potentially used configurations of the MMCC family for the STATCOM in large-scale offshore wind power plants, with special focus on asymmetrical low-voltage ride through capability under grid faults. The specifications and the sizing of components of each type of practical 80-MVar/33-kV-scaled MMCC-STATCOM are carefully designed and compared. The total cost and volume are compared based on the total power semiconductor chip area and the total energy stored in the passive components. Asymmetrical reactive power delivering operation of the MMCC family considering the dc-link capacitor voltage-balancing method is solved mathematically in order to quantitatively understand the performance limitations and behaviors. The electrothermal stress of the power modules used in each type of the MMCC for a practical 80-MVar/33-kV-scaled STATCOM is analyzed. The asymmetrical reactive power capability of the MMCC solutions is compared under different scenarios of grid faults, while considering the device temperature limits as well as voltage saturation. It is found that the MMCC configuration with double-star bridge cells becomes the most attractive circuit configuration for the STATCOM application based on the obtained results.

A Buck-Boost LLC cascade converter is proposed in this paper. In virtue of the Zero-Voltage-Switching (ZVS) modulation strategy for Buck-Boost circuit, all the switches can be soft switched with wi...

In this paper, a high step-up coupled-inductor cascade boost dc–dc converter with lossless passive snubber is proposed. Although a conventional cascade boost converter has larger voltage gain compared to a boost converter, it is still not suitable for high step-up voltage conversion. In the proposed converter, a coupled inductor is adopted for the cascaded boost converter to further increase the voltage gain. However, a leakage inductance of the coupled inductor causes a high voltage spike at a main switch. A resistor–capacitor–diode snubber is commonly used to simply solve this problem, but it is also the cause of additional power loss. Therefore, the lossless passive snubber is suggested in this paper in order to prevent efficiency drop by a snubber circuit. In conclusion, the proposed converter has high voltage gain and improved power efficiency. Experimental results from an output 400-V 200-W prototype at a constant switching frequency of 50 kHz are presented to verify the performance of the proposed converter.

This paper proposes a newly developed single-delta bridge-cell, modular multilevel cascade converter (SDBC-MMCC)-based distribution-level static synchronous compensator (DSTATCOM) for single-phase load compensation in three-phase, three-wire electric power distribution systems. Each main circuit arm of the DSTATCOM uses a modular multilevel cascade converter based on full-H-bridge (FHB) cells. The three main DSTATCOM arms are delta-connected to allow phase-independent operations for phase balancing and unity power factor correction of the single-phase load in three-phase, three-wire electric power distribution systems. By using the symmetrical components method, a feedforward compensation algorithm was employed for the DSTATCOM. A simulation of the DSTATCOM was performed for functioning verification. Finally, a hardware test system was built by using a multi-DSP-based control system. The test results verified the effectiveness of the proposed SDBC-MMCC-based DSTATCOM in single-phase load compensation.

This paper proposes two hybrid topologies based on a single ac–ac modular multilevel cascade cascade converter (MMCC) with double-star chopper cells (DSCC), in each of which a voltage reference technique (VRT) is associated for generating the output modulating references. In doing so, either the grid-connected leg or the shared leg can be switched at the line frequency, thus reducing the switching losses. For proper operation of the VRTs in real-time, the input and output converter voltages must be in sync, hence a synchronization technique is proposed. Furthermore, when no dc-link capacitor is used in a DSCC converter, the dc-link is subjected to voltage ripples derived from the pulsewidth modulation (PWM) technique. In that case, high-frequency PWM techniques can produce high-frequency components across the arm inductors, which are reflected in the dc-link. Therefore, this paper focuses on the operability of the hybrid modular multilevel DSCC converters, but also on the investigation of the proposed VRTs in reducing the dc-link voltage ripple on the conventional DSCC converter using PWM with phase-shifted carriers (PSCs). Simulated and experimental results are presented to validate the proposed techniques and converters.

This study reports a new slow-scale bifurcation phenomenon in linear frequency of power factor correction (PFC) cascade converters, which will offer a useful design reference for PFC power application. Compared with the resistive loads analysed before, the stability of PFC cascade converters is studied, which are more complex but practical. This study has introduced the research method that the secondary efficient point-of-load converter can be equivalent to a constant power load. Based on this method, the small-signal model of the whole system is constructed and the characteristic equations are thus derived. Subsequently, the stable boundary is determined when the coefficients of the characteristic equations are positive according to Routh–Hurwitz stability criterion. Furthermore, the time-domain waveforms and phase portraits between the output voltage and the inductor current are given according to numerical simulation and experimental results at different points. Meanwhile, the total harmonic distortion and the fast Fourier transform diagrams are compared to reveal the influence of the bifurcation phenomenon on power quality of the system. The results provide useful information of parameter space for the design and operation of the converter in the desired fundamental stable regime.

The multi-level cascade converter is composed of H-bridge module in series. An improved 15-level hybrid cascade converter based on DC energy storage of electric vehicle can be obtained by changing the ratio of DC energy storage to supply voltage from 1:1:1 to 4:2:1 of the traditional cascaded 7-level converter cascaded by three H-bridge modules. In this paper, the modulation strategy of the 15 level hybrid cascade converter is studied, and a new algorithm for the switching angle of the multilevel converter powered by DC energy storage device for electric vehicle is proposed. Based on Saber simulation software, three different modulation strategies, stepped-wave modulation, PWM and partial PWM, are applied to verify and compare. Finally, the simulation results of total harmonic distortion, low-order harmonic component and switching loss of the converter based on the output voltage without any filter are compared, and the related modulation strategies are discussed.

This paper proposes a new modular multilevel cascade converter (MMCC) topology for active power filter application, which presents a reduction in the number of power switches and consequently reduction in costs and losses. A model predictive control (MPC) strategy, which considers a reduced number of states, is used to regulate the ac-side currents. Moreover, a simplified algorithm also based on sorting and MPC concepts, implements the task of maintaining the submodules’ capacitor voltage regulated. These control strategies are applied to a nine-level MMCC connected to a medium voltage distribution grid. Its performance is analyzed based on simulation results conducted in the environment of circuit simulations PSIM.

In this paper, a design review and control algorithm of a solid state transformer (SST)model intended to replace the existing 60Hz power transformer for the railway application is presented. The proposed smart transformer is a high-frequency power electronic transformer. It is made of multiple cascade AC-DC converter modules, linked by a high voltage DC-link to the multiple parallel dual active bridges (DAB)DC-DC converters. This arrangement of modules is susceptible to the unbalanced voltage of the DC-link capacitors, and the voltages of modules as well. However, if the control system is well designed, the voltage balance can be maintained. The main advantage of the presented SST is that it is capable of controlling the instantaneous voltage and improving the power quality by compensating the harmonics. The proposed control algorithm includes the design analysis of the output DC voltage control, the voltage balancing capability across the DC-link capacitors and the cascaded modules. It guarantees the great performance during the verification process. The validity of the proposed algorithm is verified by PSIM simulation and scale-downed experimental results.

Capacitor voltage balancing is indispensable in normal operation for modular multilevel cascade converter- single star bridge cell (MMCC-SSBC) based static synchronous compensator (STATCOM). This paper proposes a novel individual voltage balancing scheme with dramatically reduced voltage sensors. First, zero-sequence voltage injection is enabled to regulate the cluster voltage based on only one voltage sensor in each phase; Then, through the additional inserted hardware units, individual voltage balancing can be achieved. Compared with conventional software voltage balancing method, the proposed balancing scheme has the advantage of simple implementation, lower transportation delay, fast balancing speed, and dramatically reduced voltage sensors. In addition, only some popular power modules are added to balance the individual capacitor voltage. The effectiveness of the proposed individual voltage balancing scheme is verified by simulation results in MMCC-SSBC based STATCOM. With the proposed scheme, the individual voltage balancing and reactive power compensation can be realized simultaneously.

Multilevel converters are widely considered to be the most suitable configurations for renewable energy sources. Their high-power quality, efficiency and performance make them interesting for PV applications. In low-power applications such as rooftop grid-connected PV systems, power converters with high efficiency and reliability are required. For this reason, multilevel converters based on parallel and cascaded configurations have been proposed and commercialized in the industry. Motivated by the features of multilevel converters based on cascaded configurations, this work presents the modulation and control of a rooftop single-phase grid-connected photovoltaic multilevel system. The configuration has a symmetrical cascade connection of two three-level T-type neutral point clamped power legs, which creates a five-level converter with two independent string connections. The proposed topology merges the benefits of multi-string PV and symmetrical cascade multilevel inverters. The switching operation principle, modulation technique and control scheme under an unbalanced power operation among the cell are addressed. Simulation and experimental validation results in a reduced-scale power single-phase converter prototype under variable conditions at different set points for both PV strings are presented. Finally, a comparative numerical analysis between other T-type configurations to highlight the advantages of the studied configuration is included.

It is difficult to obtain the accurate small signal modeling by traditional modeling method especially for the complex converter. System identification is a new method to construct the high voltage cascade converter small signal modeling. The first-stage and the second-stage can be analyzed on the whole. The improved least-square parameter estimation is used to analyze the sampling data of input and output. Furthermore, the system parameter and accurate system modeling can be acquired. Comparing the step response of system model with the Pspice stimulating result, the validity of the method is verified.

SUMMARY This paper provides an experiment-based discussion on a modular multilevel cascade converter based on triple-star bridge cells (MMCC-TSBC) for a low-speed high-torque motor drive. The TSBC is a direct ac-to-ac power converter capable of achieving bidirectional power flow as well as drawing and feeding three-phase sinusoidal input (supply-side) and output (motor-side) currents with any power factor at both sides. This paper discusses active dc-capacitor-voltage control applied to the low-speed, high-torque motor drive. A specially designed downscaled system combining a 320-V, 38-Hz, 6-pole, 15-kW induction motor with a 400-V, 15-kW TSBC is constructed and tested to confirm the validity of the motor drive. Experimental waveforms obtained from the downscaled system confirm stable operation with the rated load torque across a range from a standstill to the rated speed, including satisfactory start-up performance.

The generator voltage of a cutting-edge wind turbine generation system (WTGS) is expected to increase from the conventional low-voltage level of 690 V to a medium-voltage level of 6.6 kV with a power level up to 10 MW without the application of a parallel connection of multiple power units. On the other hand, it is difficult to use a conventional two-level or three-level converter in a WTGS with a dc-link voltage of 10–13 kV owing to the limitations on the voltage ratings of power devices. This paper presents a 10-MW WTGS consisting of a three-phase open-winding synchronous generator equipped with six lead terminals and a modular multilevel cascade converter (MMCC) formed by three identical subconverters. The MMCC carries out direct ac/ac power conversion from three single-phase voltages with a time-varying frequency (from the generator) to a three-phase voltage with a line frequency of 50 Hz. This paper discusses the operating principles and control methods for the WTGS, followed by simulations using the software package PSCAD/EMTDC. The experimental result obtained from a 200 V 6-kW downscaled system confirms the reliability of simulation results of the complete three-phase system.

This paper provides theoretical and experimental discussions on an optimal carrier frequency for a phase-shifted pulsewidth modulation modular multilevel cascade converter for grid connections. The theoretical discussion concludes that a ratio of the optimal carrier frequency with respect to the supply (ac) frequency is 5/2. A three-phase downscaled front-to-front system is designed, constructed, and tested to confirm validity of the theoretical discussion. It consists of a pair of three-phase modular multilevel cascade converters rated at 400 Vdc and 10 kW, where the carrier frequency is set to 450 Hz and the ac frequency to 180 Hz, that is, their ratio is equal to 5/2. Moreover, simulated waveforms are compared to experimental ones under the same operating conditions. Both experimental and simulated waveforms agree well with each other not only in steady states but also in transient states.