Electrical effects in winding of large electrical ac machines application to advanced large size DFIM

To meet the electrical grid's demand on pump power variation, GE Renewable Energy develops since 10 years variable speed machines (also called Doubly Fed Induction Generators or DFIG). Within turbogenerators, the phenomenon of circulating currents in Roebel bars is well known, while is it has not yet been studied for DFIGs. The main goal of this study is to calculate the circulating currents in the stator and rotor bars of DFIG under different operating points with a theoretical and practical precision of around 1%.

This study starts with an overview of the current situation in circulating current calculation and presentation of the characteristic circulating current curves for a hydrogenerator, study that led to a patent application. After a presentation of the possible calculation methods and models based on a deep and broad literature review, this study performs with a deep review of the slot inductance model analysing its precision and limitations. Based on these finds, two novel analytical models are proposed to enhance the taking into account of the strand dimensions. Only the last slot inductance model developed, based on a slot differential inductance model, permits to take the strand dimensions and the saturation into account. This model is validated experimentally using a small-scale slot/strand-model, while all slots models are compared to each other to highlight their differences.

The winding overhang model and novel analytical expressions are presented in a later chapter as well as the analytical treatment of the rotor overhang made of non-linear steel. The winding overhang model uses analytical expressions to determine the magnetic field and vector potential in the winding overhang, which have the advantage of additional knowledge compared to the results of a finite-element computation.

\In another chapter, novel exact transient current and torques expressions are derived for a DFIG experiencing a 3-phase and a 2-phase short-circuit. Then the winding overhang force computation and the circulating current calculation results are presented in two crowing chapters. In these chapters, the influence of the approximation and boundary on the end winding forces as well as the origin of the end winding forces are shown. The last result chapter is dedicated to the circulating current calculation, where several original results are presented to detail the circulating current losses reduction potential and the impact of well-known classical special transitions on the circulating currents in the case of a DFIG. The influence of the operating point and the boundary are also shown for the stator and the rotor.

This study presents many original contributions on several domains. It presented a novel slot inductance model, which was validated using a specially designed small-scale model of a slot. This small-scale model concept can certainly be extended to other parts of an electrical machine, which could help to study these effects in a laboratory instead of a power plant. This study could quantify the circulating current losses in the stator winding of a DFIG, losses that can easily be reduced to increase the efficiency of this machine. This study also presented several original fundamental contributions in the field of analytical expressions for the transient expression of current and torque in the case of a 3-phase and 2-phase short-circuit.

Contribution to the active generator principle for high power electric supply

Massimiliano Visintin

In recent years the use of solid state frequency converters is rapidly increasing in the industrial and power plants where electric machines are installed, since it allows variable speed operations for the electric system, thus becoming a key factor which is capable of increasing the overall efficiency, as well as the global plant flexibility. The benefits that can be reached when electric motors are under investigation can be so enticing, in particular from the perspective of the operating machine coupled to the motor, that drawbacks take a back seat and can be overcome. On the contrary, power electronic for variable speed is generally not used in power generation, since the gains brought out by the present technology are not sufficient to attract anyone's interest. There are some exceptions in the wind- and the hydro power plant businesses, where there are some cases of variable speed already introduced. However in the former applications the output power is limited to some MW, while in the latter the converter is sized only to a fraction of the whole generating power, since it feeds the machine's rotor.This dissertation investigates on a possible electronic converter for power generation in the 40 MW range and above, which is also attracting from both the cost's and the efficiency's point of view, as well as from the operation reliability perspective and thus may be proposed as a breakthrough in this field. Among the possible frequency converters, those ones have been selected which perform natural commutations only, i.e. those that employ thyristors as the semiconductor devices, since the efficiency, the robustness and the relatively high performances are actually the key success factors to be addressed from the beginning. Well known matrix converter topologies will be referred to, as well as new matrix converter arrangements will be brought out and analyzed: their behaviours will be targeted by the dissertation, in particular when different strategies are adopted for controlling the operations. Specifically comparisons will be carried out when the same converter topology is used, but driven by either the well-known cycloconverter strategy or the newly brought out "active generator" one. In addition several generator winding arrangements will be proposed and analyzed, with the aim of increasing the actual feasibility of the proposed solution. Finally some test results on a sized down experimental rig will be analyzed and compared to the simulation outcomes.

EPFL2006

A contribution to energy saving in induction motors

Electric motors consume over half of the electrical energy produced by power stations, almost the three-quarters of the electrical consumption in industry and almost the half of commercial electrical consumption in developed countries. Motors are by far the most important type of electric charges, and so constitute the main targets to achieve energy saving. Owing to their simple and robust construction, the asynchronous motors and especially those of squirrel-cage types, represent about 90-95% of the electrical energy consumption of electric motors, which is equivalent to about 53% of total electrical energy consumption. They are widely used as electrical drives in industrial, commercial, public service, traction and domestic applications. Owing to the importance of induction motors, this thesis is aimed at contributing to energy saving efforts, more specifically in the field of low power induction motors. A contribution is kept in perspective by taking into consideration the energy saving potential during the motor design stage as well as during its operation. Every effort to save energy in motor application can be made by always attempting to use energy only as much as what needed during its operation. The best way is to exploit the saving potential during motor design, while at the same time taking into account its intended application. It can be achieved either through the improvement of motor design or through the reduction of its input electrical energy when the motor has already been built. These two efforts are studied, elaborated and worked out thoroughly in this thesis. To attain this objective, a synthesis has been started with the description of how to model an induction motor. To obtain a better model, an improvement is proposed by using the Schwarz-Christoffel mapping to calculate the slot leakage inductance in induction motor. With such method, slot-leakage inductance can be determined more precisely, resulting in more accurate prediction of motor characteristics. It is based on the stored magnetic energy calculation using two-directional field distribution in the slot. The air gap influence can be observed easily, so that a reasonable slot leakage definition can be adopted. Unlike the conventional method, which is only suitable for rectangular slots (otherwise empirical corrections are required), the proposed general slot form can be extended to any desired polygonal slot form. Consideration of saturation is also indispensable because ignoring it could result in inaccuracy in motor performance prediction. Considering the saturation is essential owing to its important role in self-excitation phenomenon to establish voltage build-up in induction generator. However, the self-excitation phenomenon is undesirable in certain group of capacitor motors as it may hinder the switching-off process and mechanical braking at a desired moment. The undesirable switching-off failure condition is to be avoided by properly designing the capacitor motor. Like in this capacitor motor special application, where a proper design is useful from the point of view of operation safety, designing properly a motor is also very important in energy saving efforts. Motor design and optimization to minimize losses as well as to make possible wide speed-range motor operation are some of the efforts. However, when induction motor has already been built, saving energy is only possible by managing its supplying electrical energy. Various strategies are possible and a particular emphasis on the use of triac to reduce motor input voltage is presented. Besides, a brief economic saving evaluation is given to draw attention to the energy saving potential.

EPFL2005

A Numerical Test Platform for Large Synchronous Machines also useful as a Design Optimization Tool.

Jean-Jacques Simond, Mai Tu Xuan

This paper deals with the determination of large synchronous machine characteristic quantities using an extended 2D-finite element method and based on the knowledge of the geometry and of the physical properties of the electrical and magnetic circuits of the machine. In this respect sudden no-load three phase short-circuits and short-circuits in the quadrature axis under load conditions at cosphi = 0 are simulated in rotation using a numerical, virtual test platform. The saturation effect, the eddy currents in the damper circuit and in the rotor solid iron parts are taken into account. The results of these two tests lead to all characteristic quantities in both axis of the machine. Other tests can be performed on this numerical test platform (standstill frequency response, DC decay, low speed, symmetrical and asymmetrical load tests), which can also be used as a design optimization tool. Comparisons between calculations and measurements for three large existing units (turbo-generators and hydro-generators) are presented

2006

A contribution to energy saving in induction motors

EPFL2005

Contribution to the active generator principle for high power electric supply

Massimiliano Visintin

EPFL2006