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The leakage inductance of medium-frequency transformers (MFTs) significantly affects the operation and efficiency of isolated converters, that are one of the most important components in solid state transformers, and should be accurately controlled or be minimized. This paper proposes a general analytical model of leakage inductance based on the superposition theorem and Rogowski coefficients, taking non-ideal geometric factors, such as the different heights of the primary and secondary winding in practical implementations, into account, with only less than 4% error compared to most 3D finite element simulations and 7% to all experiments. Based on the proposed model, an optimization scheme to minimize the leakage inductance for typical winding structures through adjustment of the winding arrangement is proposed and mathematically proved, which can reduce the leakage inductance by up to over 40% compared with the traditional concentrated arrangement, also validated in experiments. In addition, the optimization method does not need to redesign other magnetic parameters, basically free to use at no cost.

Multiphase drives with high reliability are receiving increasing attention by the research community in safety-critical applications. Condition monitoring is an effective way to improve drive reliability, because it can detect the development of faults and offer the possibility of predictive maintenance. Among the different condition monitoring methods, online estimation of stator resistance is a promising technique for its ability to detect high-resistance connections, overheating of stator windings and other defects. In this paper, an online estimation method of stator resistance based on dc-signal injection is proposed for multiphase drives. The DC currents are injected in the subspace with the smallest main inductance to minimize the induced torque ripple. Phase voltages are measured and the DC components are extracted to calculate the resistance in all the phases. Compared with existing methods, the proposed method can estimate the per-phase stator resistance with higher accuracy and higher robustness, which provides additional benefits in condition monitoring applications. In addition, the proposed method is derived in a general way, which makes it applicable to most kinds multiphase drive systems. Experimental results on a nine-phase induction machine and a five-phase induction machine validates the high accuracy, high robustness and high generality of the proposed technique.

A high efficiency dual active bridge with coupled inductor (DABCI) for fuel cell vehicles is proposed in this paper and its operation principle is analyzed to reveal the optimal operation modes of this converter, which provides the basis for circuit parameters design. Compared with dual active bridge (DAB), the DABCI is more suitable for high voltage gain applications because its low voltage bridge works like interleaved boost circuit and it is easy to get high voltage gain by adjusting the duty cycle D. In order to reveal the best operation mode of the DABCI, an optimal current stress modulation strategy is proposed, which can realize zero voltage switching (ZVS) operation in wide load range for all switches. Furthermore, the input current ripple is suppressed by interleaving two boost circuits in low voltage bridge, which can extend the life span of fuel cell vehicles. In addition, advanced magnetic integration technology is adopted to improve the system power density. Finally, a 4kW rated prototype is made and experimental results are presented to verify the previous analysis.

As the increase of power and voltage level of back-to-back three-level NPC converters, the high-amplitude and high-frequency common-mode voltage (CMV) has become an urgent problem. To address this, a CMV elimination method based on PWM pulse alignment for back-to-back three-level NPC converter is proposed. On this basis, a neutral-point potential balance method based on zero-sequence voltage (ZSV) is also proposed. In order to decouple the CMV elimination and the neutral-point potential balance, a same optimal ZSV is injected into both the rectifier and inverter sides and the CMV elimination will not be affected. Simulation and experiments are presented to verify the effectiveness of the proposed method.

High-voltage and fast-switching silicon carbide (SiC) power modules are needed to construct high-voltage and high power-density converters. Stacking multiple low-voltage SiC devices to increase the equivalent blocking voltage shows advantages in on -state resistance, current capacity, and cost over one single high-voltage device. An indirect series-connected SiC MOSFET power module using quasi-two-level hybrid-clamp topology is proposed in this article. The voltages across the devices are automatically balanced with an open-loop modulation strategy to avoid sensors and control algorithm. Moreover, only ceramic capacitors are needed besides the MOSFETs and diodes in the topology. Thanks to this, the topology was highly integrated into a power module using SiC dies, which is equivalent to a general-purpose two-level medium-voltage SiC MOSFET power module. The switching losses of the power module show advantages over other stacking methods and even a single high-voltage switch and it can be evaluated with a two-level half-bridge (HB). To achieve this, the automatic voltage-balancing performance is analyzed in detail and the parasitic output capacitance in the topology is investigated, based on which the design considerations are presented. The simulation results from LTspice confirm the voltage-balancing and a 3600V/20A HB power module is built in the lab and the experimental results verify the design.

Interleaving is widely used in paralleled converters to enhance the output current quality, but subsequent circulating currents could lead to harmonic distortion and extra power losses. This article proposes an improved interleaved discontinuous pulsewidth modulation (IIDPWM) method to reduce zero-sequence circulating currents (ZSCC) in paralleled converters with common dc and ac bus. For two-paralleled converters, optimal vector sequences are selected considering minimal common-mode voltage (CMV) differences, the NTVs principle, and power losses. Furthermore, a novel carrier-based scheme is presented to implement the proposed strategy in a general case with any number of paralleled units. The 180 $^\circ$ carrier phase shift between two unclamped phase legs in discontinuous PWM is first applied for ZSCC reduction. Compared to existing works, the proposed strategy shows a practical value achieving good overall performance in ZSCC, output currents, switching losses, ease of use and scalability. Simulation and experimental results verified the effectiveness of the proposed IIDPWM strategy.

Four-level neutral-point-clamped (NPC) inverter suffers from the neutral-point (NP) voltage-balancing problem, which seriously hinders its application in the industry. Carrier-overlapped pulsewidth modulation (COPWM) is a newly proposed carrier-based modulation method which can solve this problem. However, the total switching transitions under this modulation method are doubled, and hence the switching losses are increased. In order to alleviate this problem, a composite modulation and voltage-balancing method is proposed in this article. For a low modulation index range under 0.5, phase-disposition pulsewidth modulation (PDPWM) is utilized and the relationship between the NP currents and the reference phase voltage is derived. A zero-sequence voltage injection-based NP voltage balancing method is proposed. For a high modulation index range more than 0.5, a decoupled voltage-balancing method based on COPWM is used. By combining the two modulations and voltage-balancing methods, the NP voltages can be balanced under the full modulation index range with reduced switching losses. Simulation and experimental results are presented to confirm the validity of this method.

The triple-phase shift (TPS) modulation has been widely used in dual-active-bridge (DAB) converters to improve system efficiency but still has much greater potentials to be explored. Specifically, pulsewidth modulation (PWM) generation techniques are often not discussed, which, however, are found to be crucial to enhance the converter performances. First, in conventional TPS modulations, switches in the same H-bridge turn on and off under different currents, which leads to imbalances in switching losses and, thus, different temperature rises. This will result in different heat stresses on the switches and eventually impact reliability. Second, transient dc biases occur when phase shifts are updated, which is hard to be addressed by dc-blocking capacitors. The abovementioned two issues can be simultaneously resolved by properly utilizing PWM generation techniques, without any extra components or sensors. To this end, this article presents a comprehensive analysis of loss imbalances and transient dc biases and then proposes mitigation methods. To fully exploit the proposed methods, a generalized PWM generation technique that combines the advantages of switching loss balance and transient dc-bias mitigation is proposed for enhanced performances and fast dynamic responses of DAB converters. The proposed modulation and the PWM generation strategy were verified with a laboratory prototype.

Neutral-point voltage imbalance is one of the main problems of five-level neutral-point clamped inverters that impedes their industrial application. In order to solve this problem, a closed-loop neutral-point voltage balancing method based on carrier-overlapped pulsewidth modulation (COPWM) is proposed in this article. The four dc-link capacitor voltages are balanced by three steps: voltage balancing between the two outer capacitors, voltage balancing between the two inner capacitors, and voltage balancing between the two inner and the two outer capacitors. The relationship between the neutral-point currents and the reference voltage is studied, and the top and the bottom dc-link capacitor voltages are balanced by zero-sequence voltage injection. The two inner capacitor voltages are balanced by adjusting the width of switching signals. Simulation and experimental results are presented to confirm the validity of this method.

With the development of renewable energy generation, medium-voltage dc collection in large-scale photovoltaic (PV) plants has become a research focus with less losses and less system complexity. Maximum power point tracking control of PV panels requires converters with a large input voltage range and a stable output, among which active-clamp isolated boost converters qualify for PV applications, due to their high and variable step-up ratio. In this paper, an improved dual-active-clamp quasi-resonant isolated boost converter is proposed to increase the efficiency and decrease the voltage pressure across switches. The proposed topology has an additional resonant branch with a small resonant capacitor on top of the conventional active-clamp isolated boost converters, where the capacitor resonates with the leakage inductor of the transformer to improve the voltage and current waveforms during switching transients. The proposed converter achieves soft switching of all the switches at both turn-on and turn-off transients, while retaining good voltage regulation ability. Additionally, the induced voltage spikes due to the parasitic inductors in the conventional topology are suppressed well. Discussions on detailed steady-state operation, zero-voltage switching (ZVS) regions, and design considerations are given to show the advantages of the proposed converter. Simulation and experimental results are presented to verify the expected performances.

Five-level active neutral-point clamped (5L-ANPC) converter is an attractive topology, which is suitable for medium-voltage high-power conversions. Both phase-disposition pulsewidth modulation (PDPWM) and phase-shifted PWM (PSPWM) can be used to control this converter. PDPWM has the best harmonic characteristics, but the conduction and switching losses are not uniformly distributed. Although PSPWM has the natural voltage balance ability for flying-capacitor cells, its quality of line voltage is much worse than PDPWM. In order to combine the advantages of these two modulation methods, an optimized carrier-based PWM method for 5L-ANPC converters is proposed in this article, which features the same harmonic performance with PDPWM and the same control characteristics with PSPWM. An optimized neutral-point potential balance method is also proposed, which can make a tradeoff between the neutral-point potential balancing and the harmonic performance. Simulation and experimental results demonstrated the feasibility of this method.

The neutral-point voltage unbalance problem holds back the extensive application of neutral-point-clamped (NPC) multilevel converters with more than three voltage levels in industry. Traditional phase-disposition pulsewidth modulation (PDPWM) or nearest-three-vector (NTV) modulation cannot achieve the voltage balance over the full modulation indexes and load power factors when the voltage level is higher than three. To solve this problem, a novel generalized carrier-based PWM method, named carrier-overlapped PWM (COPWM), for NPC multilevel converters is proposed in this article, which is proven to satisfy the volt–second balance principle. With this modulation method, the average values of all the neutral-point currents are equal to zero in a fundamental period, and therefore, all the dc-link capacitor voltages can be naturally balanced in the ideal and steady-state conditions. In order to simplify its implementation, an equivalent multireference modulation method with only one triangular carrier is also derived. Simulation and experimental results on a five-level diode-clamped inverter are presented to verify the proposed modulation method.

Because the root of pole-frequency vibration is the fundamental component of magnetic field, which is the basis of electromechanical energy conversion, by now no effective reduction method of pole-frequency vibration has been proposed, even few literatures has touched on this. In this article, one method by adding some permanent magnet (PM) interpoles to fill the force valley and then reduce the pole-frequency vibration for surface-mounted PM synchronous machines, is proposed and verified by experiments on prototype motors. First, a general expression of radial force density, including the amplitude, frequency, and spatial order, of a PM motor is investigated in detail. Based on this, the deep reason of pole-frequency vibration is ascribed as the valley of force density and the zero-crossing area of magnetic flux density. To fill the force valley and reduce the vibration, some PM interpoles are arranged between original main PM poles and its principle of vibration reduction is presented. A three-dimensional weak-coupling electromagnetic- mechanical finite-element model is built to simulate the radial force and the vibration of the PM motors. Finally, to validate the proposed method and finite-element method results, two 6 poles/36 slots PM motors are manufactured and the experiments are conducted.

A comprehensive study of common-mode voltage (CMV) reduction and neutral point (NP) potential balance for a back-to-back three-level NPC converter is proposed in this article. Based on phase-disposition PWM, a zero-sequence voltage (ZSV) injection method is adopted. The ZSV is divided into two parts: ZSV for CMV reduction, and ZSV for NP potential balance. The specific selection method is discussed in detail in this article. After using the proposed ZSV injection method, the CMV can be reduced to 1/6 of the dc bus voltage, and the NP potential fluctuations can also be suppressed. Simulation and experimental results verify the effectiveness of the proposed method.

The fault-tolerant ability is a significant advantage of multiphase machines over the three-phase ones. Parallel connections of stator windings or converter legs are preferred to enhance the power rating or drive performance. Open-circuit faults on parallel branches are reflected as the derating of current limits for the corresponding phases, and fault-tolerant strategies are required to keep the phase currents within asymmetrical current limits. This article analyzes the optimal phase current trajectories of a five-phase induction machine under three different faulty scenarios in parallel situations and then proposes the offline optimized current trajectory tracking (OCTT) fault-tolerant strategy, which can reach the maximum achievable fault-tolerant power while guaranteeing optimized ohmic losses over the full power range. Furthermore, based on the generalized symmetrical components theory, an extended rotor field-oriented control (RFOC) is proposed to realize the OCTT mode in field-oriented control. Experimental results confirm the efficiency improvement of the OCTT mode in the full-power-range operation and verify the asymmetrical current trajectory tracking ability of the extended RFOC.

Four-level hybrid-clamped inverter is a newly proposed topology that can operate under a wide voltage range without switches connected in series. However, when it is applied in medium voltage high power conversions, the flying capacitors in each phase will occupy a huge volume and a high switching frequency is required to restrain the voltage ripples. In order to overcome this drawback, a four-level active neutral-point clamped inverter is discussed in this paper, which consists of only six switches and no diodes or flying capacitors are required. In order to balance the neutral-point voltages under the full power factor and modulation index range, a capacitor voltage balance method based on carrier-overlapped pulsewidth modulation is proposed in this paper. The upper and lower dc-link capacitor voltages are balanced by zero-sequence voltage injection and the central dc-link capacitor voltage is balanced by adjusting the duty cycles of switching signals slightly. Simulation and experimental results are presented to confirm the validity of this method.

The high fault-tolerant ability of multiphase drives is favored in safety-critical applications. Under open-phase faults, to guarantee ripple-free torque, stator current references should be revised. In this paper, a global fault-tolerant control strategy based on an online current optimization algorithm (OCOA) is proposed for symmetrical multiphase machines (SMMs) to achieve both the maximal torque production range (TPR) and minimal stator winding losses under all possible OPFs. The OCOA can calculate the minimum losses current references covering the full TPR online, regardless of the number and locations of faulty phases. With the online-optimized references, ripple-free torque and minimum losses in full TPR can be achieved under faulty conditions. Additionally, the proposed method can be adopted to SMMs with arbitrary phase numbers. Simulation and experiments demonstrate that the online optimization can be completed in a short time, meanwhile, achieves the same TPR and loss reduction compared with an existing offline strategy based on prestored look-up tables. Due to the flexible online calculation ability and excellent extensibility, the proposed method is especially favored in modular multiphase converters and machines with high phase numbers, where converters can adapt to machines with different phase numbers or the number of possible faulty conditions can be extremely large.

The development of wide band-gap semiconductor devices is rapidly promoting the development of power electronics technology, especially in high frequency (HF) power converter region. A category of HF power converter is switch-mode power amplifier (PA). This paper focuses on the implementation of a multiphase dc–dc PA with high bandwidth and high power rate. When naturally sampled phase-shifted carrier modulation is implemented in the practical digital control system to control this HF power converter, the pulsewidth error problem caused by limited domain frequency of control chip becomes much severer. As a result, dc voltage component of each phase becomes different under several conditions, leading to current imbalance among phase inductors. This paper analyzes this problem and proposes a sensorless control method to solve it, based on the injection of random sequence in reference wave. A four-phase prototype employing GaN devices is fabricated, switching at 1 MHz. Current imbalance and effectiveness of the proposed control method are verified by experimental results. The prototype is capable of uniformly amplifying sine signal within 200 kHz and a 350 kHz bandwidth random signal, with peak output power up to 810 W.

Five-level hybrid-clamped converter is a newly proposed topology, which is suitable for medium-voltage high-power conversions without switches directly connected in series. Phase shifted pulsewidth modulation (PWM) or (PSPWM) is commonly used to control this converter. However, the line voltages of PSPWM include many zones in which the synthetic voltage jumps two voltage levels continuously, which will deteriorate the harmonic performance and increase the current ripples. This article presents an improved PSPWM method for the hybrid five-level converter, which can imitate the effect of phase disposition PWM and improve the quality of the line voltage and current. A decoupled capacitor voltage balancing control is also implemented based on this modulation method. Simulation and experimental results are presented to demonstrate the feasibility of this method.

The voltage balancing problem of dc-link capacitors in multilevel diode-clamped converters limits their applications in industry under some operational conditions. Virtual vector PWM (VVPWM) can attain the capacitor voltage balancing under all operational conditions but it increases the complexity and computation burden by increasing the number of triangular regions for higher levels as compared to conventional space vector PWM (CSVPWM). In order to solve the dc-link voltage balancing problem in four-level diode-clamped converter, a simplified virtual vector PWM (SVVPWM) algorithm is proposed in this paper. The pair of new virtual vectors is defined in such a way that the space vector diagram of four level converter is transformed into three level space vector diagram that lessens the complexity and computation burden by decreasing the number of triangular regions. The balancing of capacitor voltages can be achieved by using proposed SVVPWM algorithm for the whole range of modulation index load power factor. Steady state and dynamic performance of the SVVPWM algorithm is verified through the simulations and experiments.

Modular multilevel converter (MMC) is an attractive topology for medium- and high-voltage applications due to the merits of high modularity, easy scalability, high-quality output voltage, and fault tolerance of submodules. This letter focuses on the control of a three-phase MMC under the single arm fault condition, which can further improve the fault tolerance ability of the MMC. According to three essential operating constraints of the MMC, this letter proposes a feasible arm voltage and current configuration method for the single arm failed MMC, which can guarantee its stable operation with reduced output power. Several operating limitations are also discussed. Experiment results based on a prototype with four half-bridge submodules per arm are presented to verify the proposed arm fault tolerance method.

This letter discusses the reactive components and the power factor of dual-active-bridge converters under triple-phase-shift modulation. Different from previous methods based on backflow power integral or Fourier series expansion, this letter presents a straightforward analysis and categories of reactive current through time domain analysis, and then gives a unified time domain definition of reactive current, reactive power, and power factor. Compared with all the previous definitions, the expressions of reactive components and power factor are much more concise and of a profound physical meaning. On this basis, specific calculations of reactive power and power factor are derived for all practical voltage patterns and an optimal global power factor phase shift modulation is proposed to improve the power factor on both bridges. At last, the effectiveness of analysis and proposed modulation were validated using a 500-W laboratory prototype. The results showed efficiency improvements compared to other optimal power factor modulation strategies.

This paper proposes a fast repetitive control (FRC) scheme with harmonic correction loops for the three-phase three-wire shunt active power filter (APF) applied in weak power grid. The FRC scheme consists of a repetitive control loop which is designed in the synchronous rotational frame and a fractional delay (FD) filter for approximating the FD caused by the fixed sampling rate. It can significantly improve the dynamic performance for the harmonic compensation. In weak grid situation, the grid frequency, voltage and then harmonic currents would vary rapidly with disturbances. A cumulative error cancellation loop is introduced into the FRC to improve the harmonic detection accuracy when grid frequency drifts. The harmonic correction loops are proposed to correct the harmonic references with selected orders when they vary rapidly with the grid voltage. With such loops, the compensation precision of the shunt APF can be highly improved. Simulation and experiment results verified the effectiveness of the proposed scheme.

In order to reduce the size of high-voltage flying capacitors in a five-level hybrid-clamped inverter, a modified phase-shifted pulsewidth modulation (PWM) method is proposed in this paper. The four carrier waves are redistributed and the charge/discharge time of the high-voltage capacitor can be reduced. The only disadvantage of this PWM method is that the harmonic characteristic will become worse. However, no low-order harmonics is imported and most of the harmonics are around carrier frequency and multitimes the carrier frequency, which are very easy to be filtered. Simulation and experimental results are presented to demonstrate the effectiveness of this method.

Neutral-point clamped (NPC) multilevel converters with more than three voltage levels are hard to balance the dc-link capacitor voltages, which limits their application in the industry. In order to solve the neutral-point potential balancing problem of four-level NPC converters, a novel carrier-overlapped pulsewidth modulation (PWM) method is proposed in this letter, which satisfies the volt-second balance principle and can achieve the natural voltage balance of three dc-link capacitors under ideal and steady states. Experimental results are presented to confirm the validity of this PWM method.

This paper presents system-level efficiency evaluation of the suitable isolated dc/dc converters in input-series-output-parallel power electronics transformers (PET) applied for medium-voltage dc systems and provides engineering insights into the design of PETs from a system perspective. To this end, three candidates-a single-stage dual active bridge (DAB) converter, a boost converter plus a DAB with a constant boost output voltage, and a boost converter plus a DAB with a constant DAB voltage gain-are selected and evaluated from a system-level aspect. Comprehensive system-level efficiency comparisons are done with the chosen candidates, considering numbers of normal and hot standby submodules, different working conditions, rated voltages and power of the PET, switching frequencies, and commercial devices. Analysis with detailed loss breakdowns clearly shows the distribution of losses and elaborates the profound reasons behind the efficiency differences. In the comparison, the boost and DAB with a constant boost output voltage have higher or similar efficiency than the other two candidates, verified by simulation and experimental results. Considering its advantages in redundancy in the PET, the boost and DAB with constant boost output voltage are a preferable choice for submodules in input-series-output-parallel PETs.

In urban rail transit, there is a large number of harmonics brought by diode rectifiers, and shunt active power filters (APFs) are an effective method of harmonics rejection. Traditional shunt APFs work with a dedicated DC link leading to complexity, while those with a common DC bus but using non-isolated topologies bring serious problems of zero-sequence circulating current (ZSCC), which introduce losses and provoke poor quality. Consequently, this paper first analyzes the limitations of traditional non-isolated APFs on modulation radio. Based on the analysis, this paper put forward a novel isolated APF with a common DC link based on existing diode rectifiers in urban rail transit, which realizes braking energy recovery as an additional function. To this end, harmonics brought by diode rectifiers are reduced while rejecting ZSCC. Meanwhile, braking energy can feedback to the city grid with lower harmonics. Finally, simulation and experiment using a 1-kW prototype converter verify the feasibility and validity of the proposed converter on harmonics suppression and braking energy recovery.

A sawtooth carrier-based sinusoidal pulse width modulation (SC-SPWM) technique for asymmetrical six-phase two-level inverters is proposed in this paper, which could reduce both the peak-to-peak amplitude and the changing frequency of the common-mode voltage (CMV). The intersection-plotting method reveals that the mirror symmetrical sawtooth carriers for two three-phase groups can help optimize the distribution of overlapping areas, which is the CMV reduction mechanism of SC-SPWM. Additionally, the Fourier analysis is employed to quantitatively evaluate the modulation index and harmonic performance of SC-SPWM compared with the conventional SPWM and two phase-shifted SPWM methods. Also, further analysis demonstrates that dead time does not affect the reduction of CMV in a noticeable way. Finally, both simulation and experimental results coincide with the theoretical analysis that the proposed SC-SPWM shows the best performance in reducing the CMV amplitude and changing frequency among these four SPWM methods, though it leads to a higher phase current distortion.

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage high-power applications. It features easy scalability, high-quality input and output waveforms, superior availability, etc. This letter aims to further increase the availability of the M3C when one or more of the nine branches are lost. A general nonlinear multivariable optimization model is set up to optimize the branch current configuration after the branch lost. The proposed method reduces system power capability loss after the branch failure. It achieves a smooth transition from a full M3C to a reduced topology without shutdown of the system. A M3C prototype with 27 cells is designed, and experiment results are presented to confirm the validity of this method.

Five-level hybrid-clamped (5L-HC) converter is a newly proposed topology which is suitable for high-performance medium-voltage high-power conversions without switches directly connected in series. The critical issue of this converter is that two flying capacitor voltages of each phase and two neutral-point potentials of the dc-link need to be balanced. This paper presents a decoupled voltage balancing method for this 5L-HC converter based on modified phase-shifted pulse width modulation (PS-PWM) and optimal zero-sequence voltage injection. The voltages across the central dc-link capacitor and two flying capacitors are balanced first by adjusting the width of four PWM signals. Second, the relationship between the neutral-point currents and the output phase voltage is studied and the upper and lower dc-link capacitor voltages are balanced by zero-sequence voltage injection. Both steady-state and dynamic-state simulation and experimental results are presented to confirm the validity of this method.

In order to increase the output voltage levels and reduce the isolated dc sources, a symmetrical hybrid nine-level inverter for high-speed motor drive applications is presented in this paper. Each phase of this inverter is composed of a five-level dc/dc converter cell and an H-bridge cell. The dc/dc converter is operated at high frequency with low-voltage devices and the H-bridge is operated at fundamental frequency with high-voltage devices. The three phases are connected to a common dc-link and each drives an isolated winding of an open-winding motor. The operating principles and modulation method are introduced. A detailed analysis of the average currents through the flying capacitor and neutral point of the dc-link is presented and a capacitor voltage balancing method based on phase-shifted pulse-width modulation is proposed. Simulation and experimental results are presented to demonstrate the feasibility of this topology and control method.

With distinct advantages over three-phase machines, multiphase machines are gaining increasing popularity in ship electric propulsion applications. Many fault-tolerant control strategies have been proposed for multiphase drive to suppress the torque ripples under faulty conditions; however, most of them depend heavily on the mathematical models and the need of the prefault to postfault transitions, which would increase the complexity of the control system. Because of high robustness, Fuzzy Logic controllers are suggested to substitute the proportion integration (PI) controllers in the current loops of rotor-flux oriented control (RFOC) for multiphase induction motors in this paper. The experimental results from a multiphase drive platform demonstrate that compared with the conventional RFOC with PI current controllers, Fuzzy logic-RFOC has significantly improved the fault-tolerant ability of the nine-phase drive system in various kinds of faulty conditions.

The matrix converter (MC) is a direct ac-ac power converter without an intermediate dc link. It has several attractive features, such as high power density, four-quadrant operation, and bidirectional power flow. However, the application of MCs in high-voltage and high-power systems is still limited by the device voltage levels. It is well known that the multilevel power converter technology can be a solution to enhance the voltage rating and improve harmonic performance in many applications. Therefore, an application of a multilevel topology based on a direct ac-ac power converter is proposed in this paper. This paper first introduce the topology structure, the switching pattern grouping method, and the proposed output voltage modulation method for a five-level capacitor clamped multilevel matrix converter (5LMC). Amplitude coefficient adjustment is also introduced as a part of modulation procedure to simplify the duty cycles calculation. Simulation from MATLAB/Simulink is given to demonstrate the theoretical work. Experimental results from a five-level matrix converter prototype are presented to validate the theoretical findings.

Modular multilevel converter (MMC) is an emerging and highly attractive multilevel topology for medium- and high-voltage applications. This paper proposes an MMC-based power electronic transformer (PET) for dc distribution grid. The high-voltage side is an H-bridge MMC to generate a high-frequency adjustable-magnitude sinusoidal voltage, and the low-voltage side is paralleled multiwindings and rectifiers to allow high-load currents. As the number of MMC voltage levels increases, the complexity of the control system as well as the control algorithm increases largely. In order to simplify the control system and reduce the computation time, a hierarchical control system is designed for the MMC-based PET, and the relevant hierarchical control algorithm is also presented in this paper. The modulation method, capacitor voltage balancing method, and communication solution under the hierarchical control system are all discussed in detail. Simulation and experimental results are presented to demonstrate this system and control method.

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage and high-power direct ac-to-ac power conversion applications. Fault tolerance ability is one of the advantages of the M3C. To further enhance the reliability of the M3C, this article proposes a novel branch current configuration method for branch fault conditions, which is available either one or two branches are failed. By deriving basic branch current configurations and analyzing branch dc power equations under branch fault conditions, feasible branch current configurations can be directly derived. In terms of minimizing the maximum peak branch current, the derived configuration is also the optimal one for the single branch fault condition. Compared with the existing method, the proposed method does not need to solve configuration coefficients of branch currents offline, which is automatically adaptive to different load conditions. An M3C prototype with three submodules each branch is built, and experimental results are presented to validate the proposed branch fault tolerance method.

The increasing interest in multiphase drive systems has led to the extension of inverter topologies and pulse width modulation (PWM) methods from three-phase to multiphase occasions. Carrier-based PWM (CPWM) dominates space vector PWM when the phase number increases, because of its simple computation and modular implementation. Although intensive work has been done on modifying PWM methods to reduce the common-mode voltage (CMV), not enough work has been done on CPWM methods with CMV reduction for multiphase drives. This paper extends the phase-shifted sinusoidal PWM (PS-SPWM) method for five-phase and six-phase two-level voltage-source inverters (VSI) and employs the intersection-plotting method and Fourier analysis to reveal the nature of the CMV. Both experiment and simulation results comply with theoretical analysis that compared with the conventional SPWM, the PS-SPWM can effectively reduce the CMV in peak-to-peak value and RMS value, though it leads to a higher phase current distortion. In addition, the hardware realization of PS-SPWM for multiphase inverters by a distributed control system is presented in this paper.

Four-level hybrid-clamped (4L-HC) inverter is a newly proposed topology which is suitable for high-performance medium-voltage drives. Each phase of this topology consists of eight switches and a flying capacitor and the dc-link is split into three parts by two neutral points. In order to ensure the topology operating properly, the voltages across the flying capacitors and three dc-link capacitors should be controlled and maintain balanced at their nominal values. This paper presents a decoupled voltage balancing method for this 4L-HC inverter, including the voltage balancing of dc-link capacitors and flying capacitors. A modified phase-shifted pulse-width modulation method is used to regulate the voltages of flying capacitors and the central dc-link capacitor. The relationship between the neutral-point currents and the output voltage is studied and the upper and lower dc-link capacitor voltages are balanced by injecting an optimum zero-sequence voltage. Simulation and experimental results are presented to verify the validity of this method.

The current imbalance between stator phase windings could shorten the motor life and affect the drive performance, whereas in case of multiphase drive applications, there is not a general current balance control strategy that can apply to symmetrical inverters with any phase number m . Based on the generalized symmetrical component theory, this letter analyzed the values of relative symmetrical components of the multiphase systems in balanced and unbalanced cases. Then, developed from the conventional rotor-flux oriented control (RFOC), the current-balance RFOC is proposed to compensate the current imbalance for symmetrical m -phase inverters. Finally, the experimental results from the five-phase and nine-phase drive systems confirm the validity of the control strategy in eliminating current imbalance.

This paper proposes a multiport power electronic transformer (PET) topology with multiwinding medium-frequency transformer (MW-MFT) isolation along with the associated modeling analysis and control scheme. The power balance at different ports can be controlled using the multiwinding transformer's common flux linkage. The potential applications of the proposed multiport PET are high-power traction systems for locomotives and electric multiple units, marine propulsion, wind power generation, and utility grid distribution applications. The complementary polygon equivalent circuit modeling of an MW-MFT is presented. The current and power characteristics of the virtual circuit branches and the multiports with general-phase-shift control are described. The general current and power analysis for the multiple active bridge (MAB) isolation units is investigated. Power decoupling methods, including nonlinear solution for power balancing are proposed. The zero-voltage-switching conditions for the MAB are discussed. Control strategies including soft-switching-phase-shift control and voltage balancing control based on the power decoupling calculations are described. Simulations and experiments are presented to verify the performance of the proposed topology and control algorithms.

Five-level active neutral-point clamped (5L-ANPC) converter is an attractive topology for high-power medium-voltage motor drives. This paper presents a capacitor voltage-balancing method for the 5L-ANPC converter, including the voltage balancing of dc-link capacitors and flying capacitors. In order to ensure that the series-connected or high-voltage switches of the 5L-ANPC converter are operated at fundamental frequency and the other switches are operated at a constant switching frequency, phase-shifted pulse width modulation is used to control this converter. The relationship between the average neutral-point current and zero-sequence voltage is investigated, and an optimum zero-sequence voltage is calculated to regulate the neutral-point potential. The voltage across the flying capacitor is also regulated by adjusting the switching duty cycles of two PWM signals, which varies the operation time of redundant switching states in each switching period. Simulation and experimental results are presented to verify the validity of this method.

In this paper, a novel modular cascaded multi-level converter with multiwinding high-frequency (MWHF) transformer is proposed for medium- or high-voltage applications. In the proposed converter, a cascaded H-bridge rectifier (CHBR) is connected directly with the input ac source while the traditional step-down transformer is no longer necessary. Then, an isolated dc-dc converter composed by a group of H-bridge converters and a MWHF transformer with high power density is used to isolate the dc buses produced by the CHBR. The equivalent circuit and mathematical model of the MWHF transformer and the high frequency (HF) converter are obtained firstly in this paper. Then, the voltage stability under unbalanced loads is analyzed and the naturally balance ability is verified. To accelerate the dc bus voltage balancing process, a voltage balance control algorithm based on energy exchange between different transformer windings is proposed that is realized by accomplishing the phase shift adjustment of the terminal voltages on different windings. Simulations and experiments are implemented to verify the performance of the proposed converter and voltage balance algorithm.