In this article, a novel point of view on grid-forming control with current controlled power electronic converters is presented. A control concept based on the emulation of the combined effect of a virtual synchronous condenser and a parallel-connected virtual current source is proposed. It combines the positive effects of virtual synchronous generator control, such as the provision of inertia, voltage support, the possibility of islanded operation, and good control performance in weak grids, with the benefits of vector current control, such as fast current limitation, improved synchronization properties, and fast setpoint tracking. The proposed control concept is simply a modification of standard vector current control. Therefore, it can be implemented easily on all control platforms with minor software changes. The control method is verified with analytical models, time-domain simulations, and experimental results.

In grid-forming control, the swing equation of a synchronous machine is emulated by a power controller. Thereby, frequency droop, power-oscillation damping, and/or virtual inertia can be obtained. In this letter it is shown that a phase-locked loop (PLL) - which is normally grid following - can be designed so as to, in turn, emulate a generic power controller, thus, becoming grid forming.

With today's demand for increased industrial process automation a trend towards Integrated Motor Drives (IMDs) has evolved allowing a low complexity and compact installation of the drive system. Especially servo applications with high short-term overload requirements (e.g., three times the nominal current for several seconds) are a thermal challenge for the power electronics. Consequently, high efficiencies and power densities are key requirements of these motor-integrated Variable Speed Drives (VSDs). Multi-Level (ML) inverter topologies allow small LC output filter designs and benefit from utilizing low-voltage semiconductors with superior conduction and switching performance, and thus represent an interesting approach for future IMDs. In this work an experimental comparison between three different 800 V DC link supplied drive systems is presented, namely between a 3L Flying Capacitor Converter (3L-FCC) (employing 650 V GaN HEMTs), a 7L Flying Capacitor Converter (7L-FCC) (using 200 V SI MOSFETs) and its promising alternative, a 7L Hybrid Active Neutral-Point Clamped Converter (7L-HANPC) (using both, 650 V GaN HEMTs and 200 V SI MOSFETs). All three systems are realized as hardware demonstrators for the same specifications, i.e., for integration into a Permanent Magnet Synchronous Motor (PMSM) with a case temperature of 90°C, 7.5 kW nominal output power at >99%efficiency and a short-term overload capability of three times the nominal current for 3s. Thereby, the efficiencies and the thermally critical overload capability are experimentally verified. Overall, the 3L-FCC shows the best performance trade-off with lowest complexity and/or highest reliability and minimal control effort.

Integrated Motor Drives (IMDs) are gaining popularity in industrial Variable Speed Drive (VSD) applications, thanks to their more compact realization and simpler installation. However, mission profiles of, e.g., servo applications, demand overload torques of two to three times the nominal value during several seconds, which is thermally challenging for the power electronics. Accordingly, high efficiency and power density of the inverter are of paramount importance for motor integration. Multi-Level Flying Capacitor inverters (ML-FCis) benefit from a reduced output filter volume and improved switching and on-state performance of low-voltage devices for increasing number of levels $N$ , whereas the PCB overhead and gate drive volume increases. In this paper, the most power dense solution between a seven-level (7L) FCi with Si semiconductors and a three-level (3L) FCi with GaN semiconductors is evaluated. Thereby, a straightforward design procedure allows to dimension both FCis for 99% efficiency at nominal operation, while providing a high short-term overload torque (three times the nominal torque) also for low inverter output frequencies. Due to its slightly higher power density and greatly reduced complexity, a phase module of the 3L GaN FCi is realized as an IMD hardware demonstrator. A transient thermal model is employed to specify the feasible overload operating range, considering the limited heat spreading in the baseplate and parameter variations, e.g., from manufacturing tolerances. The experimental analysis of the demonstrator verifies an efficiency of 98.94% and the practically required overload capability.

Brushless DC operation with a trapezoidal waveform of permanent magnet machines provides a simple, robust and cost effective method to drive the motor. In this paper, the proven technology is applied to a motor drive system with series-connected multiphase windings and converter cells. The proposed topology provides a modular approach and enables MOSFETs, which are more efficient and cost competitive, to replace the IGBTs which are typically used in 3-phase motor drives. Control schemes of this topology are also studied and the results show that for voltage control mode, the cell voltages have a potential to self-balance. However, when current control mode is implemented, a closed loop control is required to balance the cell voltages. A 2 kW prototype has been built and tested to validate the conclusions.