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

Dual Active Bridge (DAB) is the heart of upcoming solid state transformer technology which is intended to replace bulky 50Hz/60Hz transformer. DAB ensures bidirectional power flow and magnetic isolation between HVDC and LVDC sources. Single phase DAB topologies are widely used for their simplistic structure. Single phase DABs are generally adopts scalar control. In this paper a single phase DAB is controlled by vector control technique which is widely used in grid connected converter and motor drive for quick dynamic performance and independent control of active and reactive power. The proposed control improves dynamic response, optimizes losses by appropriate control of reactive and active power flow in the DAB. Though the vector control approach is common for AC-DC conversion, surprisingly it is not explored in DC-DC conversion. In this paper a three-level single phase converter is used at the HVDC side and a two-level single phase converter is used at the LVDC side. The proposed DAB is controlled by vector control technique. Simulation results verify the effectiveness of the proposed control.

Vector control adopted for single phase Dual Active Bridge

Dual Active Bridge (DAB) converter became very popular among the isolated bidirectional DC-DC converters for high voltage and high power applications due to its high power density and higher efficiency. The high power density is achieved as the isolation is provided with a high frequency transformer (HFT). Apart from the isolation, the HFT participates in the active power transfer through its leakage inductance. In this paper a design methodology is proposed for an HFT, which is suitable for DAB application. A step-by-step procedure is given in a flow chart to design the HFT. The process involves the selection of core, winding, number of turns and the winding arrangements. As a specific leakage inductance is required for a given active power transfer for a DAB, a suitable winding arrangement is proposed in this paper. The detailed loss analysis is carried out in Ansys Maxwell simulation. The proposed design is verified in a hardware prototype of a 48/400V, 1kW HFT with an operating frequency of 50kHz.

Design of High Frequency Transformer for a Dual Active Bridge (DAB) Converter

Induction motor drives operating on the sensorless vector control mode are widespread in the industrial sector, owing to the better dynamic performances it provides over the other methods and the significant reduction in cost, mounting space and maintenance it provides due to the lack of encoder. But the method's performance is hugely dependent on the accurate measurement or estimation of motor parameters. This paper deals with the estimation of essential motor parameters for the proper operation of the control mode. It is a combination of standstill and rotational modes to estimate parameters by developing a self-commissioning method for industrial drives using the already existing current sensors and DC voltage sensor such that the estimated parameters can be used for both star and delta connections without any further recalculations and minimal motor name plate data. The paper deals with the verification of sensorless vector control method with the estimated parameters by using simulation results for a 15kW motor. Further hardware results on 5.5kW motor load setup are detailed which further validate the method's effectiveness.

Parameter Estimation in Sensorless Vector controlled Induction motor Drives

This article proposes a closed-loop control architecture for a three-stage [medium-voltage (MV), isolated dc/dc, low-voltage (LV)] solid-state transformer (SST) system and a modulation technique for the MV stage that shows better performance over existing solutions in terms of control simplicity, semiconductor loss, and cost. A simple and effective closed-loop control architecture is proposed for the entire SST system using the natural cell balancing ability of the series resonant dual-active bridge converters in the dc/dc stage. In order to reduce semiconductor losses and cost, Si insulated-gate bipolar transistor and SiC mosfet devices are introduced properly in the MV stage of the SST power architecture, and the proposed modulation technique switches these devices appropriately according to their merits. The proposed modulation scheme ensures more than 50% semiconductor loss reduction in the MV stage. A circulating logic is proposed to achieve equal power sharing among cells in the MV stage and associated components. Detailed dynamic modeling, closed-loop control architecture, and modulation scheme are discussed in this article. The proposed control technique and modulation scheme are verified experimentally in a 600-/100-V, 5-kW SST system.

Solid-State Transformer Based on Naturally Cell Balanced Series Resonant Converter With Cascaded H-Bridge Cells Switched at Grid Frequency

Solid State Transformer (SST) is expected to be a key component in future smart grid systems, which integrates MVAC grid with LV DC/AC grid. For a three stage SST, resonant converter is a potential candidate as the DC-DC stage due to its high conversion efficiency. However, it exhibits an oscillation of 100 Hz (double line frequency) in the high frequency (HF) link current, which increases device current stress and conduction loss. This paper proposes an analytical model which determines the magnitude of this oscillation at different power factor operations. The obvious solution to minimize this oscillation is to keep a high value of capacitor at each MVDC bus, but it reduces the power density of the system. Therefore, this paper proposes an improved solution with a combination of inductor and capacitor at the LVDC bus. The proposed filter reduces the RMS value of the HF link current by 29% at unity power factor operation. The dynamics of the proposed filter is also included in the control system design by deriving a dynamic equivalent circuit in this paper. The design is verified in a laboratory prototype of 8 kVA, 1.65 kV/300 V solid state transformer.

Kamalesh Hatua

Papers

Vector control adopted for single phase Dual Active Bridge

Design of High Frequency Transformer for a Dual Active Bridge (DAB) Converter

Parameter Estimation in Sensorless Vector controlled Induction motor Drives

Solid-State Transformer Based on Naturally Cell Balanced Series Resonant Converter With Cascaded H-Bridge Cells Switched at Grid Frequency

Minimization of Low Frequency Current Oscillation in Resonant Link of a Solid State Transformer by Passive Filters