Although the concept of variable-speed drives, based on utilization of multiphase machines, dates back to the late 1960s, it was not until the mid- to late 1990s that multiphase drives became serious contenders for various applications. These include electric ship propulsion, locomotive traction, electric and hybrid electric vehicles, ldquomore-electricrdquo aircraft, and high-power industrial applications. As a consequence, there has been a substantial increase in the interest for such drive systems worldwide, resulting in a huge volume of work published during the last ten years. An attempt is made in this paper to provide a brief review of the current state of the art in the area. After addressing the reasons for potential use of multiphase rather than three-phase drives and the available approaches to multiphase machine designs, various control schemes are surveyed. This is followed by a discussion of the multiphase voltage source inverter control. Various possibilities for the use of additional degrees of freedom that exist in multiphase machines are further elaborated. Finally, multiphase machine applications in electric energy generation are addressed.

Multiphase Electric Machines for Variable-Speed Applications

The area of multiphase variable-speed motor drives in general and multiphase induction motor drives in particular has experienced a substantial growth since the beginning of this century. Research has been conducted worldwide and numerous interesting developments have been reported in the literature. An attempt is made to provide a detailed overview of the current state-of-the-art in this area. The elaborated aspects include advantages of multiphase induction machines, modelling of multiphase induction machines, basic vector control and direct torque control schemes and PWM control of multiphase voltage source inverters. The authors also provide a detailed survey of the control strategies for five-phase and asymmetrical six-phase induction motor drives, as well as an overview of the approaches to the design of fault tolerant strategies for post-fault drive operation, and a discussion of multiphase multi-motor drives with single inverter supply. Experimental results, collected from various multiphase induction motor drive laboratory rigs, are also included to facilitate the understanding of the drive operation.

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Multiphase induction motor drives - : a technology status review

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Design Of Analog Cmos Integrated Circuits

The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system.

Review of Solid-State Transformer Technologies and Their Application in Power Distribution Systems

This paper investigates the capacitor-current-feedback active damping for the digitally controlled LCL-type grid-connected inverter. It turns out that proportional feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor due to the computation and pulse width modulation (PWM) delays. The LCL-filter resonance frequency is changed by this virtual impedance. If the actual resonance frequency is higher than one-sixth of the sampling frequency (fs/6), where the virtual impedance contains a negative resistor component, a pair of open-loop unstable poles will be generated. As a result, the LCL-type grid-connected inverter becomes much easier to be unstable if the resonance frequency is moved closer to fs/6 due to the variation of grid impedance. To address this issue, this paper proposes a capacitor-current-feedback active damping with reduced computation delay, which is achieved by shifting the capacitor current sampling instant towards the PWM reference update instant. With this method, the virtual impedance exhibits more like a resistor in a wider frequency range, and the open-loop unstable poles are removed; thus, high robustness against the grid-impedance variation is acquired. Experimental results from a 6-kW prototype confirm the theoretical expectations.

Capacitor-Current-Feedback Active Damping With Reduced Computation Delay for Improving Robustness of LCL-Type Grid-Connected Inverter

Recently, medium voltage SiC devices have been developed which can be used for grid tie applications at medium voltage. Two such devices - 15 kV SiC IGBT and 10 kV SiC MOSFET have opened up the possibility of looking into different converter topologies for medium voltage distribution grid interface. These can be used in medium voltage drives, active filter applications or as the active front end converter for Solid State Transformers (SST). Transformer-less Intelligent Power Substation (TIPS) is one such application for these devices. TIPS is proposed as a 3-phase SST interconnecting 13.8 kV distribution grid with 480 V utility grid. The Front End Converter (FEC) of TIPS is made up of 15 kV SiC IGBTs. This paper focuses on the advantages, design considerations and challenges associated with the operation of converters using these devices keeping TIPS as the topology of reference.

Solid State Transformer and MV grid tie applications enabled by 15 kV SiC IGBTs and 10 kV SiC MOSFETs based multilevel converters

Solid State Transformer (SST) is an alternative to the conventional distribution transformer for smart grid applications. By employing a compact Medium-Frequency (MF) transformer for isolation, the SST has merits on size and weight. It also provides flexible utilization as a FACTS component. The switching converters are a potential source of Common-Mode (CM) and HF EMI noises. These noises are more nuisance in a SiC device based SST which switches at a high dV/dT at the Medium-Voltage (MV) level resulting in high CM voltages. The SST floating metalic surfaces such as heatsink and the output must be grounded for safety and smooth operation. However there are various significant low impedance paths present, including the parasitics of the compact transformer, which may conduct CM noise to the grid. The generated CM noise may affect the controls. This paper presents the CM and grounding challenges in the multistage integration of a three-phase SST system based on 15kV SiC IGBTs termed as Transformerless Intelligent Power Substation (TIPS). The TIPS interfaces MV 13.8kV and LV 480V grids using MV ac-dc, MV to LV dc-dc dual active bridge and LV dc-ac inverter stages. A study on the CM noise in the TIPS and a passive filter solution for its attenuation is presented in this paper. A time domain simulation considering the passive filter specification is also presented. The experimental results for line to line 3.64kV MV grid integration are presented. A LV prototype is used to verify the complete grounding and the CM choke design at a scaled-down condition.

Grid connected CM noise considerations of a three-phase multi-stage SST

A protection circuit monitors the gate voltage of an insulated gate bipolar transistor (IGBT) or metal oxide semiconductor field effect transistor (MOSFET) to protect the transistor during a time when it is being turned on. In one embodiment, the circuit monitors a transient gate voltage of the transistor when it is turned on. A short or overcurrent condition is detected when the gate voltage exceeds a delayed reference signal.

Short circuit protection by gate voltage sensing

The Dual Active Bridge (DABC) dc-dc converter is an integral part of the recently popular Medium-Voltage (MV) dc micro-grid application due to its high-power density. The advent of 15kV SiC IGBT and 10kV SiC MOSFET, has enabled a non-cascaded MV and Medium-Frequency (MF) DABC converter which is expected to have higher MTBF than the cascaded H-bridge topology due to relatively small number of switches. A composite DABC three-level three-phase topology earlier proposed for MV-MF application, has dual secondary side bridges to meet the rated load conditions. The duty-ratio control of the primary and the independent operation of dual secondary bridges as a single active bridge, can be utilized to solve the light load ZVS problem. This paper presents flexible operating modes of this MV DABC for ZVS and higher efficiency. The MV DABC simulations are presented to bring out the advantages of this topology in wide range load and voltage-ratio conditions. This paper reports 8kV experimental validation of this DABC while using 15kV/40A SiC IGBTs on the MV side.

MVDC microgrids enabled by 15kV SiC IGBT based flexible three phase dual active bridge isolated DC-DC converter

Recently, with the emergence of Wide Bandgap semiconductor devices having higher blocking voltage capabilities and higher switching speed, ac-dc converters for Medium Voltage (MV) and Low Voltage (LV) dc micro-grid applications are becoming popular. In this paper, the first time experimental demonstration of such a 3-phase, isolated ac-dc power converter based on the newly developed 15 kV/40 A SiC IGBT is presented for 4.16 kV ac distribution grid interface. The presented converter consists of two bidirectional stages — the 4.16 kV ac to 8 kV dc front end converter followed by an 8 kV dc to 480 V dc dual active bridge converter with high frequency isolation. These stages are switched at 5 kHz and 10 kHz respectively. The converter design is presented along with experimental validation on a prototype at 9.6 kW.

Kamalesh Hatua

Papers

Solid State Transformer and MV grid tie applications enabled by 15 kV SiC IGBTs and 10 kV SiC MOSFETs based multilevel converters

Grid connected CM noise considerations of a three-phase multi-stage SST

Short circuit protection by gate voltage sensing

MVDC microgrids enabled by 15kV SiC IGBT based flexible three phase dual active bridge isolated DC-DC converter

Three-phase 4.16 kV medium voltage grid tied AC-DC converter based on 15 kV/40 a SiC IGBTs