Phase Voltage Harmonic Imbalance in Asymmetrical Multiphase Machines with Single Neutral Point

Multiphase (n-phase) machines are often designed with l sub-windings on stator, each having k phases, and the machine is typically operated with l isolated neutral points. However, such a machine can also operate with a single neutral point, which improves the fault-tolerant properties. When a machine is inverter supplied, low order harmonics may be present due to the low switching frequency and nonlinear inverter properties. Moreover, low order zero-sequence harmonics can be deliberately injected to increase dc bus voltage utilisation. This paper investigates a phenomenon that has not been reported so far in relation to asymmetrical multiphase machines with a single neutral point, namely that the presence of balanced low order harmonics in leg voltages produces unbalanced phase voltage harmonics and consequently unbalanced phase current harmonics. By analysing the neutral point (common mode) voltage harmonics, imbalance in the phase voltage harmonics is explained. Analytical expressions for neutral point voltage harmonics and phase voltage harmonics are provided for asymmetrical machine configurations with a single neutral point having arbitrary numbers of sub-windings and sub-winding phases. Theoretical considerations are verified using simulations and experiments with asymmetrical twelve- and nine-phase loads with a single neutral point, respectively

Multiple Three-Phase Induction Generators for Wind Energy Conversion Systems

During the past decade, there has been a considerable increase in the number of published works on multiphase machines and drives. This increased interest has been largely driven by a need for the so-called green energy, i.e. energy generated from renewable sources such as wind, and also an increased emphasis on greener means for transportation. Some of the advantages multiphase machines offer over three-phase counterparts are better fault tolerance, smaller current and power per phase, and higher frequency torque ripple. This thesis examines use of a multiphase induction generator in wind energy conversion systems (WECS). In particular, multiphase generators that comprise multiple 3-phase winding sets, where each winding set is supplied using an independent 3-phase voltage source inverter (VSI), are studied. It is claimed that these topologies offer advantages in cases where a WECS is connected to a multitude of independent ac or dc microgrids, systems where a single high-voltage dc link is needed or where a simple fault tolerance is achieved when a complete winding set is switched off. All of these examples require an arbitrary power or current sharing between winding sets. In order to achieve arbitrary current and power sharing, the control can be implemented using multi stator (MS) variables, so that the flux and torque producing currents of each winding set can be arbitrarily set. As an alternative, this thesis uses vector space decomposition (VSD) to implement the control, while individual winding set flux/torque producing currents are governed by finding the relationships between MS and VSD variables. This approach has all the advantages of both MS and VSD, i.e. access to individual winding set variables of MS and the ability to implement control in the multiple decoupled two dimensional subspaces of VSD, while heavy cross coupling between winding set variables, a weakness of MS, is avoided. Since the goal of the thesis is to present use of multiphase machines in WECS, modelling and simulation of a simple multiphase WECS in back-to-back configuration has been performed at first. All systems relevant to machine control where considered, such as grid and machine side VSIs, grid filter, indirect rotor field oriented control, current control in both flux/torque producing and non-producing subspaces, low order harmonic elimination, maximum power point tracking control, and voltage oriented control of the grid side VSI. Moreover, various WECS supply topologies were considered where developed current and power sharing would be a necessary requirement. Development of the proposed current sharing control commences with an analysis of multiple 3-phase machine modelling in terms of both MS and VSD variables. Since the actual control is implemented using decoupled VSD variables, VSD modelling has been studied in detail, resulting in an algorithm for creation of the VSD matrix applicable to any symmetrical or asymmetrical multiphase machine with single or multiple neutral points. Developed algorithm always decouples the machine into orthogonal two-dimensional subspaces and zero sequence components while making sure that all odd-order harmonics are uniquely mapped. Harmonic mapping analysis is offered as well. Next, relationship between MS and VSD variables has been developed by mapping MS variables into VSD subspaces. Since VSD matrix creation algorithm is valid for any multiphase machine, relationship between MS and VSD variables is applicable to any multiple 3-phase machine regardless of the configuration (symmetrical/asymmetrical), number of neutral points or machine type (synchronous or induction). Established relationship between MS and VSD has been used to implement current sharing control in decoupled VSD subspaces of the machine. It is shown that in order to achieve arbitrary current sharing it is only necessary to impose currents in flux/torque non-producing subspaces. Hence, total machine’s flux and torque are not affected at all. Besides verification by Matlab simulations, two topologies are experimentally investigated, a parallel machine side converter configuration and the case when a single high voltage dc link is created by cascading dc-links of the machine side VSIs. In the first case the ability of arbitrary current sharing between winding sets is validated, while the second tested topology demonstrates use of the developed control for the purpose of voltage balancing of the cascaded dc links

Direct Torque and Predictive Control Strategies in Nine-phase Electric Drives Using Virtual Voltage Vectors

One of the main distinctive features of multiphase machines is the appearance of new degrees of freedom ( - voltages/currents) that do not exist in their three-phase counterparts. As a direct consequence, control approaches that apply a single switching state during the sampling period cannot achieve zero average - voltage production. In direct torque control (DTC) this implies that - currents are not regulated, whereas in finite-control-set model predictive control (FCS-MPC) an enhanced - current regulation is feasible only at the expense of disturbing the flux/torque production. Aiming to avoid these shortcomings, this work makes use of the concept of synthetic/virtual voltage vectors (VVs) to nullify/limit the - voltage production in order to improve the current regulation in the secondary planes. Two strategies using two and four virtual voltage vectors (2-VV and 4-VV, respectively) are proposed and compared with the standard case that applies a single switching state. Since standard MPC has the capability to indirectly regulate - currents, the improvements with the inclusion of VVs are expected to be more significant in DTC strategies. Experimental results validate the proposed VVs and confirm the expectations through a detailed performance comparison of standard, 2-VV and 4-VV approaches for DTC and MPC strategies

Arbitrary Power Sharing Among Three-Phase Winding Sets of Multiphase Machines

The paper develops a technique for arbitrary power sharing among three-phase winding sets of a multiphase generator. Multiple d-q modelling is commonly used when independent control of the winding sets is required. This work utilises instead the vector space decomposition modelling as the starting point and combines it with multiple d-q approach to preserve the advantages of the vector space decomposition, while still enabling independent control over each winding set. The power sharing is achieved by imposing appropriate x-y currents at the fundamental frequency, so that flux and average torque are not affected. The theory is developed initially for the nine-phase machine. A general expression for arbitrary current sharing is derived further for any multiphase machine with multiple three-phase windings. The obtained equations are valid for any possible machine topology (asymmetrical/ symmetrical, with single or multiple neutral points). The theory is validated experimentally using an asymmetrical nine-phase induction generator with indirect rotor field oriented control

Constrained Model Predictive Control in Nine-phase Induction Motor Drives

The advent of powerful digital signal processors (DSPs) has recently permitted the real-time implementation of model predictive control (MPC) in high-performance electric drives. Nevertheless, the use of MPC together with multiphase systems is increasingly challenging as the number of phases gets higher. On the positive side, the redundancy provided by the extra phases also opens the possibility to further optimize the control action. This work describes the implementation of MPC for nine-phase drives using a three-step approach with an initial discarding of the switching states, a dynamic selection of the voltage vectors using hard constraints (HCs), and an improved performance including soft constraints (SCs). Experimental results confirm the ability of the proposed MPC to highly reduce the computational burden and switching frequency, while maintaining satisfactory steady-state and dynamic performance

Dissociative adsorption of NO upon AI(111): Orientation dependent charge transfer and chemisorption reaction dynamics.

In order to clarify the underlying mechanism of the initial oxidation of aluminum, the reaction between a heteronuclear diatomic molecule, nitric oxide, and the Al(111) surface was studied. It was shown that the reaction of NO with aluminum is a two-step process including a change of the orientation of the molecule with respect to the surface

The ASY-EOS experiment at GSI: investigating the symmetry energy at supra-saturation densities

The elliptic-flow ratio of neutrons with respect to protons in reactions of neutron rich heavy-ions systems at intermediate energies has been proposed as an observable sensitive to the strength of the symmetry term in the nuclear Equation Of State (EOS) at supra-saturation densities. The recent results obtained from the existing FOPI/LAND data for $^{197}$Au+$^{197}$Au collisions at 400 MeV/nucleon in comparison with the UrQMD model allowed a first estimate of the symmetry term of the EOS but suffer from a considerable statistical uncertainty. In order to obtain an improved data set for Au+Au collisions and to extend the study to other systems, a new experiment was carried out at the GSI laboratory by the ASY-EOS collaboration in May 2011.Comment: Talk given by P. Russotto at the 11th International Conference on Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1, 2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference Series (JPCS

Coulomb breakup of neutron-rich 29,30^{29,30}Na isotopes near the island of inversion

First results are reported on the ground state configurations of the neutron-rich $^{29,30}$Na isotopes, obtained via Coulomb dissociation (CD) measurements as a method of the direct probe. The invariant mass spectra of those nuclei have been obtained through measurement of the four-momentum of all decay products after Coulomb excitation on a $^{208}Pb$ target at energies of 400-430 MeV/nucleon using FRS-ALADIN-LAND setup at GSI, Darmstadt. Integrated Coulomb-dissociation cross-sections (CD) of 89 $(7)$ mb and 167 $(13)$ mb up to excitation energy of 10 MeV for one neutron removal from $^{29}$Na and $^{30}$Na respectively, have been extracted. The major part of one neutron removal, CD cross-sections of those nuclei populate core, in its' ground state. A comparison with the direct breakup model, suggests the predominant occupation of the valence neutron in the ground state of $^{29}$Na${(3/2^+)}$ and $^{30}$Na${(2^+)}$ is the $d$ orbital with small contribution in the $s$-orbital which are coupled with ground state of the core. The ground state configurations of these nuclei are as $^{28}$Na_{gs (1^+)\otimes\nu_{s,d} and $^{29}$Na$_{gs}(3/2^+)\otimes\nu_{ s,d}$, respectively. The ground state spin and parity of these nuclei, obtained from this experiment are in agreement with earlier reported values. The spectroscopic factors for the valence neutron occupying the $s$ and $d$ orbitals for these nuclei in the ground state have been extracted and reported for the first time. A comparison of the experimental findings with the shell model calculation using MCSM suggests a lower limit of around 4.3 MeV of the sd-pf shell gap in $^{30}$Na.Comment: Modified version of the manuscript is accepted for publication in Journal of Physics G, Jan., 201

Interplane cross-saturation in multiphase machines

The use of electrical machines in electric vehicles and high-power drives frequently requires multiphase machines and multiphase inverters. While appropriate mathematical models under the linear magnetic conditions are readily available for multiphase machines, the same cannot be said for the models of the saturated multiphase machines. This paper examines the saturation in an asymmetrical six-phase induction machine under different supply conditions and addresses the applicability of the existing saturated three-phase machine models for representation of saturated multiphase machines. Specifically, the mutual coupling between different sequence planes in the vector space decomposed model under saturated conditions is analyzed. The paper relies on analytical considerations, finite element analysis and experimental results. It is shown that the saturation of the main flux path is influenced by the current components in the orthogonal (non-fundamental) sequence plane. This implies the need to develop new multiphase machine models which take this effect into account

A space vector PWM algorithm for a three-level asymmetrical six-phase motor drive

A space vector pulse-width modulation (SVPWM) algorithm for a three-level asymmetrical six-phase drive based on vector space decomposition (VSD) approach is presented in this paper. A modification in zero plane of the transformation matrix is proposed in order to meet the requirement that the realisation of sinusoidal output phase voltages can be obtained through the chosen output leg voltage space vectors. Furthermore, a method of choosing the switching sequences based on all possible one-level transitions of the leg voltages, i.e. a permutation method, is introduced. The algorithm is then validated experimentally and obtained results show that the developed method successfully achieves the desired fundamental phase voltage, although low order harmonics are present due to uncompensated inverter dead time. Last but not least, the performance of the proposed SVPWM algorithm is compared to several carrier-based PWM algorithms including in-phase disposition with ‘double min-max injection’ (PD-DI). This is a little known type of injection, which is verified to obtain identical performance as the presented multilevel algorithm