Multiphase variable-speed drives and generation systems (systems with more than three phases) have become one of the mainstream research areas during the last decade. The main driving forces are the specific applications, predominantly related to the green agenda, such as electric and hybrid electric vehicles (EVs), locomotive traction, ship propulsion, “more-electric” aircraft, remote offshore wind farms for electric energy generation, and general high-power industrial applications. As a result, produced body of significant work is substantial, making it impossible to review all the major developments in a single paper. This paper therefore surveys the recent progress in two specific areas associated with multiphase systems, namely power electronic supply control and innovative ways of using the additional degrees of freedom in multiphase machines for various nontraditional purposes.

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Advances in Converter Control and Innovative Exploitation of Additional Degrees of Freedom for Multiphase Machines

Multiphase drives are convenient for high power/high current applications as they allow the reduction of the phase current for given rated power and phase voltage. In addition, they possess inherent fault-tolerant capability due to their redundant structure. Therefore, the multiphase drives represent a promising solution for safety-critical applications, such electrical ship propulsion, railway traction, Hybrid/Electrical Vehicles (HEV/EV), wind power generation, More Electric Engine (MEE) and More Electric Aircraft (MEA) applications. Moreover, the additional “degrees of freedom” can be exploited for integrated battery chargers. Although the literature reports many machine design solutions and control techniques for multiphase drives, the penetration of the multiphase solutions in transportation electrification and energy production is still too limited. For this reason, the paper contains a brief overview of the multiphase solutions for transportation electrification and energy production. In addition, the paper includes a generic modeling approach of multi-three-phase machines that can be used when the multiphase drive is treated as modular independent three-phase units. Experimental results are provided for a reduced scale prototype of a twelve-phase multiphase induction motor drive using a control scheme based on the multi-three phase approach.

Multiphase electrical machines and drives: A viable solution for energy generation and transportation electrification

DC-link current ripple is one of the important factors for operating the current-source converter. In practice, the dc-link current ripple can be reduced by increasing the switching frequency or inductance. However, it may result in the higher switching losses, larger size, or higher cost. In order to deal with this problem, the inherent limitation of the conventional solution is discussed in this paper. The time-domain equation of the dc-link current is established for the first time. And then, a new space vector control strategy is proposed to reduce the ripple with no need of increasing the switching frequency or inductance. The space vector is divided into 12 sectors, where the dc-link current ripple can be effectively reduced by the optimal vector sequence. Finally, the proposed method is implemented on the TMS320F28335DSP + XC3400FPGA digital control hardware platform. The experimental results verify the effectiveness of the proposed strategy.

Optimal Space Vector Modulation of Current-Source Converter for DC-Link Current Ripple Reduction

The electric energy sector is moving toward extensive integration of renewable and clean energy resources, energy storage units, and modern loads via highly efficient and flexible multiterminal dc grids integrated within the traditional ac grid infrastructure. A voltage-source converter (VSC) is the main technology enabling the interconnection of dc and ac grids. In such demanding applications, dc-link voltage control is crucial to maintain system stability. However, the dynamics and control of VSCs considering the instantaneous power of both ac- and dc-side filters and dc grid uncertainties are not addressed in the current literature. Furthermore, as shown in this paper, the uncertainty in the effective dc grid parameters, including filter capacitance and dc-side inductance owing to connecting/disconnecting electric devices to/from the dc grid remarkably affects the converter stability and performance. To overcome these difficulties, this paper presents 1) a detailed small-signal model of the dc-link dynamics in grid-connected VSCs considering the instantaneous power of both ac- and dc-side energy storage components, and 2) a robust optimal dc-link voltage controller. The proposed controller ensures excellent tracking performance, robust disturbance rejection, and robust performance against operating point and parameter variation with a simple fixed-parameter controller. A theoretical analysis, comparative simulation studies, and experimental results are presented to show the effectiveness of the proposed control structure.

Dynamics and Robust Control of a Grid-Connected VSC in Multiterminal DC Grids Considering the Instantaneous Power of DC- and AC-Side Filters and DC Grid Uncertainty

In more electric aircraft (MEA), weight reduction and energy efficiency constitute the key figures. In addition, the safety and continuity of operation of its electrical power distribution system (EPDS) are of critical importance. These sets of desired features are in disagreement with each other because higher redundancy, needed to guarantee the safety of operation, implies additional weight. In fact, EPDS is usually divided into isolated sections that need to be sized for the worst case scenario. Several concepts of EPDS have been investigated, aiming at enabling the power exchange among separate sections, which allows better optimization for power and weight of the whole system. In this article, an approach based on the widespread use of multiport power converters for both dc/dc and dc/ac stages is proposed. System integration of these two is proposed as a multiport power conversion system (MPCS), which allows a ring power distribution, while galvanic isolation is still maintained, even in fault conditions. Thus, the redundancy of MEA is established by no significant weight increase. A machine design analysis shows how the segmented machine could offer superior performance to the traditional one with the same weight. Simulation and experimental verifications show the system feasibility in both normal and fault operations.

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A Multiport Power Conversion System for the More Electric Aircraft

Medium-voltage (MV) drives are increasingly used in high-power marine applications for running thrusters and main propulsion motors. In this paper, an MV drive solution employing active front-end current-source converters is proposed for a synchronous-motor-based propulsion system. The proposed solution includes two independent drives, each to control one of the two sets of three-phase windings of the synchronous motor. A dedicated communication link between the drives allows continuous load sharing and robust system operation in a master-follower drive configuration. Field-oriented control with the use of an absolute encoder is implemented for providing high starting torque and smooth speed control over a wide speed range including a 30% overspeed region. The major advantages of the proposed solution include simple structure, increased system power rating, redundant operation, low-harmonic input/output waveforms, improved reliability, elimination of a bulky input isolation transformer, and parallel drive control without the need of a complex coordinated inverter gating system. In addition, the system offers input power factor compensation and dynamic braking to allow operation on a generator-based supply system. Field test results obtained on a 14-MW ship propulsion system are provided to demonstrate the system performance.

Medium-Voltage Current-Source Converter Drives for Marine Propulsion System Using a Dual-Winding Synchronous Machine

Medium Voltage (MV) AC drives are increasingly used in high-power marine applications for running thrusters and main propulsion motors. In this paper, an MV drive solution employing active front-end current source converters is proposed for a synchronous motor based propulsion system. The proposed solution includes two independent drives, each to control one of the two sets of three-phase windings of the synchronous motor. A dedicated communication link between the drives allows continuous load sharing and robust system operation in a master-follower drive configuration. Field oriented control with the use of an absolute encoder is implemented for providing high starting torque and smooth speed control over a wide speed range including a 30% over-speed region. Major advantages of the proposed solution include simple structure, increased system power rating, redundant operation, low-harmonic input/output waveforms, improved reliability, elimination of a bulky input isolation transformer, and parallel drive control without the need of a complex coordinated inverter gating system. In addition, the system offers input power factor compensation and dynamic braking to allow operation on a generator based supply system. Field test results obtained on a 14 MW ship propulsion system are provided to demonstrate the system performance.

Ghodrat Esmaeili

Papers

Medium-Voltage Current-Source Converter Drives for Marine Propulsion System Using a Dual-Winding Synchronous Machine

Medium voltage current source converter drives for marine propulsion system using a dual-winding synchronous machine