Hang Seng Che

Postfault Operation of an Asymmetrical Six-Phase Induction Machine With Single and Two Isolated Neutral Points

The paper presents a study of postfault control for an asymmetrical six-phase induction machine with single and two isolated neutral points, during single open-phase fault. Postfault control is based on the normal decoupling (Clarke) transformation, so that reconfiguration of the controller is minimized. Effect of the single open-phase fault on the machine equations under this control structure is discussed. Different modes of postfault operation are analyzed and are further compared in terms of the achievable torque and stator winding losses. Validity of the analysis is verified using experimental results obtained from a six-phase induction motor drive prototype.

Current Control Methods for an Asymmetrical Six-Phase Induction Motor Drive

Using the vector space decomposition approach, the currents in a multiphase machine with distributed winding can be decoupled into the flux and torque producing α-β components, and the loss-producing x-y and zero-sequence components. While the control of α-β currents is crucial for flux and torque regulation, control of x-y currents is important for machine/converter asymmetry and dead-time effect compensation. In this paper, an attempt is made to provide a physically meaningful insight into current control of a six-phase machine, by showing that the fictitious x-y currents can be physically interpreted as the circulating currents between the two three-phase windings. Using this interpretation, the characteristics of x-y currents due to the machine/converter asymmetry can be analyzed. The use of different types of x-y current controllers for asymmetry compensation and suppression of dead-time-induced harmonics is then discussed. Experimental results are provided throughout the paper, to underpin the theoretical considerations, using tests on a prototype asymmetrical six-phase induction machine.

Operation of a Six-Phase Induction Machine Using Series-Connected Machine-Side Converters

This paper discusses the operation of a multiphase system, which is aimed at both variable-speed drive and generating (e.g., wind energy) applications, using back-to-back converter structure with dual three-phase machine-side converters. In the studied topology, an asymmetrical six-phase induction machine is controlled using two three-phase two-level voltage source converters connected in series to form a cascaded dc link. The suggested configuration is analyzed, and a method for dc-link midpoint voltage balancing is developed. Voltage balancing is based on the use of additional degrees of freedom that exist in multiphase machines and represents entirely new utilization of these degrees. The validity of the topology and its control is verified by simulation and experimental results on a laboratory-scale prototype, thus proving that it is possible to achieve satisfactory dc-link voltage control under various operating scenarios.

Modulation techniques to reduce leakage current in three-phase transformerless H7 photovoltaic inverter

Recently, reduced common-mode voltage (CMV) pulsewidth modulation (RCMV-PWM) methods have been proposed to reduce the leakage current in three-phase transformerless photovoltaic (PV) systems. However, most of these studies only focus on leakage current elimination and neglect the overall performance of the PV systems on issues such as cost, voltage linearity, dc-link current ripples, and harmonic distortion. In this paper, a three-phase transformerless inverter, adapted from the single-phase H5 topology, is investigated. Since the H5 topology has been conventionally developed for a single-phase system, its adaptation to the three-phase system requires the development of corresponding three-phase modulation techniques. Hence, modulation techniques are proposed based on conventional PWM. The performances of the proposed PWM, in terms of CMV, leakage current, voltage linearity, output current ripples, dc-link current ripples, and harmonic distortion are studied and discussed via simulation and experiment. It is proven that the proposed topology is able reduce the leakage current without sacrificing the overall performance of the system.

Fault-Tolerant Operation of Six-Phase Energy Conversion Systems With Parallel Machine-Side Converters

The fault tolerance provided by multiphase machines is one of the most attractive features for industry applications where a high degree of reliability is required. Aiming to take advantage of such postfault operating capability, some newly designed full-power energy conversion systems are selecting machines with more than three phases. Although the use of parallel converters is usual in high-power three-phase electrical drives, the fault tolerance of multiphase machines has been mainly considered with single supply from a multiphase converter. This study addresses the fault-tolerant capability of six-phase energy conversion systems supplied with parallel converters, deriving the current references and control strategy that need to be utilized to maximize torque/power production. Experimental results show that it is possible to increase the postfault rating of the system if some degree of imbalance in the current sharing between the two sets of three-phase windings is permitted.

A six-phase wind energy induction generator system with series-connected DC-links

The paper presents a study of a six-phase wind energy conversion system (WECS) with series-connected dc-links. The structure of the generation system requires the dc-link voltages to be balanced at the generators side. To achieve this, it is shown that a dc-link voltage balancing controller can be realised by exploiting the extra degrees of freedom provided by the xy plane of the six-phase machine. To facilitate the controllers implementation, an alternative modified transformation matrix is also suggested. The feasibility of the studied system, including the operation of the dc-link voltage balancing controller, is verified using Matlab/Simulink simulations.

A Unified Analysis of the Fault Tolerance Capability in Six-Phase Induction Motor Drives

The fault tolerance of electric drives is highly appreciated at industry for security and economic reasons, and the inherent redundancy of six-phase machines provides the desired fault-tolerant capability with no extra hardware. For this reason some recent research efforts have been focused on the fault-tolerant design, modeling, and control of six-phase machines. Nevertheless, a unified and conclusive analysis of the postfault capability of six-phase machine is still missing. This paper provides a full picture of the postfault derating in generic six-phase machines and a specific analysis of the fault-tolerant capability of the three mainstream six-phase induction machines (asymmetrical, symmetrical, and dual three phase). Experimental results confirm the theoretical post fault current limits and allow concluding, which is the best six-phase machine for each fault scenario and neutral arrangement.

Fault-tolerant control of six-phase induction generators in wind energy conversion systems with series-parallel machine-side converters

Multiphase generators in multi-MW wind energy applications can be realized with a variety of possible topologies. Series connection of machine-side converters elevates the dc-link voltage for the same voltage rating of IGBTs, allowing medium voltage generation on the grid-side. On the other hand, parallel connection of machine-side converters provides fault-tolerant capability, enhancing the system reliability. The combination of series and parallel connection of machine-side converters simultaneously elevates the dc-link voltage and provides some fault tolerance to the system. This work discusses the series-parallel topology for six-phase induction generators and analyzes the fault tolerance capability of the topology. Theoretical analysis and simulations confirm that it is possible to obtain additional fault tolerance at the expense of some unbalance on the individual dc-link voltages.

Current Control of a Six-Phase Induction Generator for Wind Energy Plants

A wind energy conversion system (WECS), using a six-phase asymmetrical induction generator, is elaborated in this paper. The current control strategy for the system, including the method of suppressing imbalance caused by the asymmetrical conditions in the six-phase generator, is discussed. A new modified transformation matrix, which allows a more effective suppression of the asymmetries in the six-phase machine, is suggested. Simulation results, obtained using Matlab/Simulink, are used to verify the effectiveness of the suggested method.

Experimental magnetizing inductance identification in five-phase induction machines

Parameter identification of multiphase machines is a new and interesting topic in the development of multiphase drive systems. Regardless of the applied control technique, an accurate knowledge of the parameters is required to ensure high-performance operation of the machine. This also applies to the magnetizing inductance of the machine. Available identification schemes for multiphase induction machines utilize AC and time-domain methods, some of which require non-conventional winding arrangement or a combination of different procedures that need tests in the non-flux/torque producing plane(s). This paper introduces a simple magnetizing inductance identification technique, which relies on an induced DC voltage test. It is an extension of a procedure previously proposed for the three-phase case to the five-phase induction machine. Experimental results illustrate the reliability and validity of the technique using two different five-phase induction motor drives.