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This work deals with the sizing of submodule capacitors for modular multilevel converters (MMCs) used for variable speed drives for medium voltage and high voltage electric machines. Variable speed drives are required to run over a wide frequency range from zero to a few hundred Hz or even over 1 kHz. Under low frequency operation (

This article presents the design and development of a 7-kV dc bus, 1-MVA, 0–1000 Hz modular multilevel converter (MMC) prototype for high speed medium-voltage variable frequency drives. The system is designed based on 1.7-kV silicon carbide (SiC) mosfet s. First, the full in-depth design of the prototype is provided, including the submodules, arm inductors, and system integration. Then, the unique control scheme and the control hardware design are presented to achieve the full-frequency range operation. The power loss based on Si and SiC devices is analyzed. Finally, experimental results are presented to demonstrate the performance of both the SiC-based submodule and the full-scale MMC system. Compared with the traditional Si-based solution, the established SiC MMC can reduce the drive system volume by 75% and achieve 325% power density, and the peak energy efficiency reaches around 99.35%.

An optimized model predictive control (MPC) method is proposed based on the selected switching states to control the four-level hybrid-clamped converter. The proposed method utilizes 64, instead of 512, switching states, compared with a conventional MPC, which greatly eases the calculation burden while achieving the high-quality regulation of output currents and the balance of capacitor voltages. The steady-state performance at both high and low frequencies is investigated, and results are compared with those by the traditional carrier phase-shifted pulsewidth modulation method. The dynamic response to the current or frequency step change is also evaluated. Both simulation and experimental results validate the feasibility and effectiveness of the optimized MPC method.

Errors in the injected high-frequency circulating currents are originally investigated for modular multilevel converters (MMCs) in low-speed drive operation. The mathematical analysis reveals that the magnitude and phase errors in the injected circulating currents cause serious energy imbalance in the MMC arms, resulting in large voltage fluctuations in the submodules. Furthermore, these fluctuations get larger when the circulating currents are injected at higher frequencies, which reduces the drive system stability and damages the submodule capacitors. A remedy approach is proposed to minimise the error effects by adjusting the circulating current command. A full-scale system comprising a 1 MVA MMC and a 1 MW induction machine is simulated to verify the theoretical analysis. The experimental investigation of a down-scale MMC feeding an inductor is also conducted to validate the theoretical results.

To develop an assessment tool to assess the severity and healing of skin impairment with the central venous access device.Delphi technique.The instrument domain list was developed through a systematic literature review and semi-structured interviews. Experts from China evaluated the relevance and significance of these items in assessing the degree of skin impairment surrounding central venous access device sites through two Delphi rounds. The APA Style JARS checklist for this article was used.For the systematic literature review, 28 articles were included to develop the wound assessment instrument. From the articles and interview contents, 15 criteria were selected based on reporting frequency. After further screening via in-depth discussion, the central venous access devices associated with the skin impairment assessment tool were refined to include 14 major domains. Through a two-phase Delphi process, 71 items in 12 domains were ultimately retained.

In this article, a double-vector-based model predictive control (MPC) method with efficient computation for single-phase five-level active neutral-point clamped (5L-ANPC) converters is proposed. It uses double voltage vectors per control cycle rather than the single in the conventional MPC method, resulting in reduced output current ripples and fixed switching frequency. First, the MPC method based on the Lyapunov function is optimized to select two voltage sectors rather than 25, reducing the calculation complexity significantly. Then, two current tracking algorithms based on double voltage vectors are presented, which are using the current cost function and current slope, respectively. Finally, a fast voltage balancing scheme for the dc-link and flying capacitors is developed and embedded in the MPC method. Both simulated and experimental results are used to validate the correctness and feasibility of the proposed MPC method.

Modular multilevel converters (MMC) are popular for high-voltage, high-power electric energy conversion. To improve the reliability of MMCs, fault diagnosis and tolerant control have been studied by many researchers. Fault reconfiguration, one important step of the fault tolerant control, has become a challenge, especially under high-voltage operation. Arm current overshoot occurs during the reconfiguration if the MMC does not provide redundant submodules (SMs). Existing publications focus on the fault detection and localization. This work investigates the reconfiguration and proposes a suppression control strategy for the overcurrent. The strategy does not require redundant SMs. The proposed method is simulated in Matlab/Simulink and validated by experiments based on a three-phase MMC using an RL load.

In this paper, a dv/dt filter is designed for the modular multilevel converters (MMCs) based variable frequency motor drives. This concept utilizes the inherent arm inductors of the MMC as a part of the output low-pass filter, thus eliminating the need for an add-on inductor. Particularly, a mathematical model is proposed to optimize the filter loss and volume for a given dv/dt requirements. It turns out that with SiC-MOSFET-based high switching frequency operation, the dv/dt filter improves the output power quality and alleviates the voltage stress in motor drive applications. For applications with various requirements on filter volume and loss, weighted-sum optimal solutions are presented. Simulation results based on a seven-level MMC are provided to validate the theoretical analysis.

Modular Multilevel Converters have been proposed to be a viable topology for medium voltage and high power motor drives due to the stackable structure of the topology. This allows for a transformer-less drive, better output current THD and reduces input and output filter sizes compared to traditional topologies. Replacing the traditional Si IGBT based submodules with SiC MOSFETs has been shown to improve the overall efficiency of the MMC. However, the lower switching losses of SiC MOSFETs have not been fully utilized to realize further system level improvements in the MMC. In this paper, the impact of the SiC MOSFETs on the efficiency, power density and output current THD of the drive is compared against Si IGBTs using analytical and simulation. A 7 kV dc bus, 4.16 kV ac, 1 MVA, 0-1,000 Hz medium voltage variable speed drive based on 1.7 kV devices based is used for the comparison.

With a growing number of silicon carbide (SiC) devices used in the modular multilevel converter (MMC) based medium voltage motor drive systems, the reflected wave phenomenon (RWP) under high dv/dt needs to be studied. In this paper, the voltage reflection in the MMC based motor drives is analyzed with a focus on the impact of the leakage inductance of the arm inductors. A seven-level MMC built with 1.7 kV SiC MOSFETs is modeled and simulated. The simulation results show that the leakage inductance of the arm inductors helps reduce the dv/dt at the MMC output but increases the voltage stress on the arm inductor windings and the load. A simple method is proposed to mitigate dv/dt and voltage overshoots applied to the machine windings. Presented simulation and experimental results validate the proposed method.

We study the numerical Adomian decomposition method for the pricing of European options under the well-known Black–Scholes model. However, because of the nondifferentiability of the pay-off function for such options, applying the Adomian decomposition method to the Black–Scholes model is not straightforward. Previous works on this assume that the pay-off function is differentiable or is approximated by a continuous estimation. Upon showing that these approximations lead to incorrect results, we provide a proper approach, in which the singular point is relocated to infinity through a coordinate transformation. Further, we show that our technique can be extended to pricing digital options and European options under the Vasicek interest rate model, in both of which the pay-off functions are singular. Numerical results show that our approach overcomes the difficulty of directly dealing with the singularity within the Adomian decomposition method and gives very accurate results. doi:10.1017/S1446181117000438

This paper investigates a control method to significantly reduce the low-frequency current oscillation for six-step operation of a three-phase inverter. The six-step operation of an ac electric machine drive system under high speeds has a few advantages compared with pulse width modulation (PWM) operation including reduced switching losses, a better utilization of the dc bus voltage, and an enhanced speed capability. In digital implementation of the six-step control, power electronics may not be able to switch on and off exactly at the zero crossing points of reference voltages, which will result in a dc offset in phase voltages and cause a low-frequency oscillation in phase currents. The low-frequency oscillation will create issues such as higher power losses, lower efficiency, and excessive heat that may demagnetize magnets on the rotor for a permanent magnet synchronous machine. This research presents a control algorithm using asymmetric PWM to ensure that power electronics switch on and off exactly at the reference voltage zero crossing points. Both simulation and experimental results demonstrate the algorithm can reduce the low-frequency oscillation in phase currents by more than 90%.

This paper investigates control methods to quickly and safely discharge the high-voltage dc bus capacitor for the permanent-magnet synchronous machine drive systems of hybrid electric vehicles during emergency situations such as a crash event. The published dc bus capacitor discharge strategies either need impractically heavy and bulky hardware or are only effective for low machine speeds. Applying the existing methods under high speeds will lead to loss of the current control and a system shutdown, followed by the bus voltage recharging again. The proposed new control method applies a modulation index controller together with a bus voltage regulator to quickly discharge the bus voltage and avoid any bus recharge regardless of the electric machine speed. Simulation and experimental results are presented to verify the effectiveness of the proposed control algorithm.

This paper proposes a simple control strategy for the dv/dt reduction in the modular multilevel converters (MMCs) based motor drives. In particular, the control strategy aims at the reduction of the MMC output voltage slew rate by coordinating the switching transients of several submodules. It is realized via utilizing extra submodules or simply applying a delay time to the switching transients. Both methods are implemented without affecting the switching speed of the power devices. Thus, the methods could mitigate the drawbacks caused by the high dv/dt while maintaining the advantages brought about by the fast switching speed of the silicon carbide (SiC) devices. The simulation results and test results based on a SiC MMC are provided. It is shown that the proposed control methods bring down the output dv/dt by around 50 %.

In this paper, a system-level analysis method is provided to evaluate the factors that affect the switching transients of modular multilevel converters (MMCs) based motor drives. In particular, the voltage transients at the device terminal are compared to the arm voltages as well as the MMC output voltage. The differences are identified and in detail evaluated via frequency domain approach and simulation. Experimental results based on a seven-level silicon carbide (SiC) MMC are provided to validate the mathematical analysis and the model-based simulation results. The study suggests that a dv/dt reduction is observed from the switching devices to the MMC output terminal, and both the system parameters and the operation principles affect the system transient performance.

As a learning step to gain experience in designing high specific power electric machines for demanding applications such as aircraft propulsion, this paper presents research results in the design, fabrication, and experimental testing of a scaled-down 300-kW electric machine. With advanced structure and materials, this machine achieves an active-material specific power of 14.4 kW/kg and an electromagnetic efficiency of 99.2% under full load condition using finite element analysis. The mechanical integrity of the rotor is fully validated at the rated speed of 17, 000 rpm in finite element analysis. The fabricated machine prototype is mounted onto a 350-hp dynamometer for comprehensive performance characterization. The efficiency of the machine prototype is mapped based on the experimental measurements under various operating conditions. In this paper, the electromagnetic analysis, trade-offs at the design stage, mechanical stability considerations, and experimental testing results are summarized and presented in detail.

This paper presents a high-speed and high-dynamic variable frequency drive (VFD) based on the modular multilevel converter (MMC) and silicon carbide (SiC) devices. A compact submodule design with the straightforward extension feature is introduced. An integrated control scheme including the low- and high- speed control is developed. Based on the designed prototype and the advanced control scheme, the dynamic response including speed acceleration and torque step-change capabilities are investigated. Besides, the steady-state performance in the high-speed range is also examined in terms of speed control accuracy and current distortion. Validated by the experimental results, the developed VFD prototype with the proposed control scheme can successfully reach the high speed (6000 rpm). It has accurate speed control (±1%) and low current distortion (THD%=2.54%) in high-speed operation, as well as the robust dynamic response.

Modular multi-level converters (MMCs) are one of the promising topologies in recent years for medium or high voltage applications. They are considered as the next generation DC/AC converters for medium/high voltage (MV/HV) motor drive applications due to their transform-less structures, high efficiency and modularity. Reliability is one of the most important challenges in MMCs, since a large number of power switching devices are used and each of these devices can be considered as a potential failure point. It is significant to detect and locate the fault accurately and apply appropriate protections in a timely manner. This paper investigates the behavior of the failure submodules and healthy submodules and use artificial neural network (ANN) classification algorithms for single-submodule open-circuit fault diagnosis. The ANN algorithm is implemented in field programmable gate array (FPGA) and the ANN parameters training is completed offline in Matlab. Experimental results of the proposed fault diagnosis for a single-module open-circuit fault are presented in this paper.

Advanced protection and operation of power systems, especially microgrids, requires rapid detection and estimation of system states. Microgrids, however, are a challenge due to the high variability of distributed generation and modern load profiles. Setting of relays and establishing Remedial Action Schemes on these systems requires rapid detection and state estimation and modeling of complex loads. This work demonstrates the design and operation of a high-speed method for estimating states and load composition across a 286-bus campus test-bed utilizing only a few PMUs. The method is based on training data that is analyzed offline via Singular Value Decomposition to determine clustering vectors that can be utilized for state estimation in real-time. This is combined with a priori bus studies to estimate load composition in terms of residential, industrial, and commercial in real-time. A case-study using the Oregon State University campus grid is presented, and it is shown that the estimation method is accurate (to within 1% in the test shown) while requiring less than 0.2 ms computation time.

A novel discharging control strategy is proposed and implemented for submodules in modular multilevel converters (MMCs). By sequentially controlling the arm currents of a MMC to the arm resistors, all submodule capacitors are discharged safely and rapidly without adding additional devices and circuits, resulting in improved performance and reduced cost. Both open- and closed-loop control methods are evaluated for effectiveness of the new control strategy. Simulation and experimental results are provided to validate the new control strategy.

The errors between the reference and actual injected circulating currents are investigated for modular multilevel converters (MMCs) in low frequency operation. These errors cause low frequency oscillatory power components in the MMC arms and large voltage fluctuations in the submodule capacitors. By adjusting the circulating current reference with a single parameter, it is possible to compensate the errors and suppress the undesired voltage fluctuations. Computer simulations and experimental results are provided to verify the theoretical derivations.

This paper studies the phenomenon of resonant oscillation in Modular Multilevel Converters (MMCs) for variable frequency operation. Without proper control, it may easily induce huge MMC inner circulating currents and damage major power devices. A Laplace model is established to clarify the mechanism of resonant oscillation in MMCs, and Bode diagrams are used to study the frequency response over a wide frequency range. Meanwhile, a straightforward resonant suppression method based on circulating current control is used to damp out the resonant condition, and a cross comparison is conducted in terms of system losses, capacitor voltage ripples, and load voltage harmonics with and without the resonant oscillation. Both simulation and experimental results are provided to validate the MMC resonant oscillation theory and its suppression control method.

Modular multilevel converters (MMCs), with the filter-less and transformer-less features, are considered as a promising topology for high-efficiency medium voltage (MV) high voltage (HV) drive systems. However, a large current ripple flowing through the capacitors of each submodule will cause significant capacitor voltage fluctuation. This paper proposes a new compensation method in common mode voltage injection to further reduce the magnitude of the fundamental component in capacitor voltages. The capacitor voltage ripples are reduced significantly by using the proposed control method, which means that smaller size of capacitors can be used for same voltage ripple requirements. Simulation and experimental test results verify the proposed compensation method using a three-phase permanent magnet synchronous machine (PMSM).

Modular multilevel converters (MMCs), with the filter-less and transformer-less features, are considered as a promising topology for high-efficiency medium voltage (MV) high voltage (HV) drive systems. However, at low frequencies (< 30 Hz), a large current ripple flowing through the capacitors of each submodule will cause significant capacitor voltage fluctuation. This paper proposes an optimal submodule capacitor selection based on the common mode voltage injection. The proposed sizing model includes two functions: a) accurately estimating the capacitor voltage ripple based on the drive system parameters; and b) identifying the minimum capacitance of the submodule capacitors based on the allowed maximum capacitor voltage fluctuations. Simulation and hardware-in-the-loop (HIL) test results show that the error between the actual and estimated values of the voltage fluctuation is within 3% using the proposed model.

We present a new American-style option whereby on the event of exercise before expiry, the holder pays the writer a fee (which will be referred to as a ‘penalty’). The valuation of the option is not straightforward as it involves determining when it is optimal for the holder to exercise the option, leading to a free boundary problem. As most options in the traded markets have short maturities, accurate and fast valuations of such options are important. We derive analytic approximations for the value of the option with short times to expiry (up to [Formula: see text] months) and its optimal exercise boundary. Some properties of the option, such as the put–call relationship, are explored as well. Numerical experiments suggest that our solutions both for the optimal exercise boundary and option value provide very accurate results.

The six step operation of an AC electric machine drive system under high speeds has a few advantages compared with the pulse width modulation (PWM) operation including the reduced switching loss, better utilization of DC bus voltage, and enhanced speed capability. The six step operation is generally applied when the fundamental electrical frequency is high. However, the low carrier-fundamental frequency ratio will make power electronics not able to switch on and off exactly at the zero crossing points of the phase voltage. This can result in a DC offset in the phase voltage and cause the subharmonic oscillation in the phase current. The subharmonic oscillation will create issues such as higher power loss, lower efficiency, and excessive heat that may permanently demagnetize permanent magnets on the rotor for a permanent magnet synchronous machine. This paper investigates a control method to significantly reduce the subharmonic current for the six step operation of a three-phase inverter. Simulation and experimental results demonstrate the algorithm can reduce the low frequency oscillation in the phase current by more than 90%.