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) MOSFETs. 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%.

Medium-voltage (e.g., 10 kV rated) silicon carbide (SiC) devices have great potentials in medium-voltage variable speed drives. But their high switching dv/dt can increase the voltage stress on motor windings and cause partial discharges. This paper presents a partial discharge study of a medium-voltage form-wound winding under two-level square-wave voltage pulses. A 10 kV SiC device-based test platform is built to generate voltage pulses with high dv/dt. A three-step test approach is proposed and employed to systematically investigate the effects of various voltage parameters on partial discharges. These include voltage rise/fall time, voltage pulse width, pulse repetitive rate, duty ratio, voltage polarity, fundamental frequency, and modulation index. Partial discharge inception voltages (PDIVs) and repetitive partial discharge inception voltages (RPDIVs) of the sample are measured with varied voltage parameters. Test results show that voltage rise/fall time is a major affecting factor which reduces PDIVs of the winding sample by 6.5% when it decreases from 800 ns to 100 ns. Based on test results, a hypothetical partial discharge mechanism is presented to explain the effects of fast voltage rise/fall edges. An empirical equation is also derived to estimate PDIVs of a winding sample under various voltage rise/fall time and pulse width conditions.

The brushless doubly-fed reluctance machine (BDFRM) with a multi-barrier rotor has shown great promise in applications where high reliability and low maintenance cost are required, such as offshore wind turbines. This paper presents a systematic analytical analysis and performance characterization of the BDFRM with a multi-barrier rotor. The airgap magnetomotive force (MMF) distributions in both open-circuit and loaded condition are detailed first, based on which a more practical rule than the widely-accepted |p1-p2|-1 is derived to effectively avoid the unbalanced magnetic pull (UMP) in such machines. Analytical formulas different from those for slip-ring doubly-fed induction machines are then obtained to estimate inductance parameters for circuit modeling and control purposes. Design equations for initial sizing and torque density comparison are also developed, with achievable torque density discussed. Both finite element analysis and experimental tests are conducted to validate the effectiveness of the presented analysis.

This work deals with the sizing of submodule capacitors for modular multilevel converters (MMCs) used for variable speed drives for medium voltage (MV) and high voltage (HV) 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 (< 30 Hz), a large current ripple flowing through the MMC submodule capacitors will cause significant capacitor voltage fluctuation. This paper proposes an optimal submodule capacitor selection method 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 experimental results show that the error between the actual and estimated values of the voltage fluctuation is within 3% using the proposed model.

This paper studies the voltage stress in random-wound machine windings under steep-fronted voltage excitations from inverter drives. The voltage stress in this paper includes overshooting voltages between any two conductors in the windings, and overshooting voltages between individual conductors and the stator ground wall. This stress may accelerate insulation degradation in inverter-fed machines compared to traditional AC machines. A high-fidelity finite element (FE) model was used to analyze individual conductors of random-wound windings and predict the voltage stress during switching transients. Experimental validation of the model was conducted using a purpose-made random-wound three-phase winding, designed to allow access to multiple coils and parallel strands. Firstly, the voltage propagation and the voltage stress for both single-phase and three-phase windings were simulated and tested under Pulse Width Modulation (PWM) voltage excitations without a rotor. Both voltage propagations among all the coils and individual turns were measured and compared with simulated results. The voltage stress on each part of the machine stator winding insulation system was tested and studied. Then, similar simulations and tests were conducted with a demagnetized rotor installed to study how the rotor saliency affects the voltage propagation and voltage stress. The measured winding voltage propagations for all scenarios generally agree with the FE simulated results.

This article investigates what parameters affect the voltage propagation pattern in inverter-driven electric machine windings and presents mathematical analysis of this pattern with experiment verification. This work provides a thorough understanding of the behavior of machine windings under surge voltage excitations. First, high-fidelity finite element (FE) models were constructed with various combinations of factors in random-wound stranded machine windings. These factors include various types of parasitic capacitances, parasitic inductances of parallel strands, and different wire layouts in slots. The surge voltage distribution along the windings was simulated and compared among different cases to identify the key factors that affect this voltage propagation pattern. Subsequently, machine windings with both single-phase and three-phase structures were analyzed via equivalent circuits that were constructed with those key parameters. Then, the expressions of the coil voltage were derived in the Laplace domain. These expressions explicate the uneven surge voltage propagation pattern at various frequencies. Finally, for verification, both simulations and tests were conducted to analyze the surge voltage propagation in single-phase and three-phase windings.