Cristian Demian

Offline and Online Methods for Stator Core Fault Detection in Large Generators

Fault diagnosis techniques play a crucial role in large turbogenerators. Due to the fact that it can cause catastrophic damage to the machine in a very short time, the development of new and more efficient methods represents a huge challenge for the researchers. Following this, the aim of this paper is to present two real-scale experimental devices manufactured to study interlaminar short-circuit faults in the stator of large generators. The first one is based on a stack of laminations in which faulty sheets can be inserted. In the second device, a special attention is given to the development of a partial exciter of a real 125-MW turbogenerator. Experimental online and offline results are presented and show that the measurement of flux density at the level of the external key bar can provide information about a short circuit between laminations.

AC Magnetic Circuits Using Nonsegmented Shifted Grain Oriented Electrical Steel Sheets: Impact on Induction Machine Magnetic Noise

The main topic of this paper is to describe the impact on the reduction of the magnetic noise generated by induction machine by using a new stator magnetic circuit design made from non segmented shifted grain oriented electrical steel sheets. This design was developed first in order to reduce the machine iron losses.

Contribution to the Study of Losses Generated by Interlaminar Short-Circuits

The paper proposes an improved analytical model, valid for a wide frequency range, of interlaminar short-circuits on the edges of the sheets of high power transformers working at 50-60 Hz. This model is able to determine the current flowing in the microscopic interlaminar contacts formed by burrs, in order to estimate the maximum power dissipated in such hot spots. With this theoretical approach, it is also possible to get an interpretation of impedance measurements on a test coil wound around the core, in order to define a new index able to characterize the part of the shearing burrs in the manufacturing factor of large transformer cores. This new index will help the transformer manufacturers to improve their processes.

Real scale experimental devices for stator core fault analysis

That paper presents two real scale experimental devices manufactured to study the stator interlaminar short circuit fault in large generators. The first one is based on a stack of lamination in which faulty sheets can be inserted. The second one is a partial exciter of a real 125MW turbo generator. These devices are useful to validate experimental procedures for detection of stator core faults.

An improvement of a diagnosis procedure for AC machines using two external flux sensors based on a fusion process with belief functions

In this paper, a method for diagnosis of AC machines using the spectrum of the near magnetic field is presented. The method is associated to a fusion process based on belief functions which analyze the measurements. In previous works, it has been shown that it is possible to detect the inter-turns short circuit in the stator windings of electrical machines using a noninvasive method. It is based on the analysis of the variation of sensitive harmonics when the load varies, and eliminates the main drawback presented by other diagnostic methods which use the comparison with a healthy state assumed known. Several measurements around the machine are necessary to increase the probability of the fault detection because the fault position relatively to the sensor can strongly influence the results. So in this paper it is proposed to exploit conjointly the whole measurements in order to obtain a more robust and reliable diagnostic and to increase the probability of detecting the fault. The merging of the different estimations being realized through the belief functions framework, this approach is tested on real measurements. Experimental tests are performed on a special rewound induction machine in order to validate the theoretical approach.

High-Frequency Harmonic Effects on Low-Frequency Iron Losses

The study of iron losses in electrical machines represents a major interest in academic and industrial environment, especially in case of systems with high-frequency (HF) harmonic content in the supply voltage. In this context, this paper presents an analysis concerning the impact of HF injection on low-frequency (LF) iron losses. A new phenomenon is observed in a toroidal magnetic circuit with nonoriented electrical steel laminations, supplied by the superposition of two sinusoidal voltages at low and high frequencies. The experimental results show that the injection of HF harmonics (0-10 kHz) in the toroid affects the distribution of low frequencies in the steel in a way that the LF iron losses decrease of about 25%. An explanation for this phenomenon might be attributable to motion mechanism of Bloch walls.

Experimental Model for Study of Electromagnetic Phenomena in Stator Core-End Laminations of Large Generators

This paper describes an experimental model for the characterization of electromagnetic phenomena that occur within the end regions of large turbo-generators. The study is based on a test bench that contains a stack of steel laminations from a 900MW turbo-generator stator and two exciting circuits in order to combine a transverse magnetic ∞ux with the in-plane ∞ux. In order to explain the ∞ux penetration within the magnetic sheet stack, accurate experimental measurements are performed. Results are compared with flnite-element simulations using code Carmel3D. In the same time, theoretical and experimental results are analyzed with a view to examining the in∞uence of transverse ∞ux on additional losses.

Information Fusion With Belief Functions for Detection of Interturn Short-Circuit Faults in Electrical Machines Using External Flux Sensors

This paper proposes a diagnosis method that exploits the information delivered by external flux sensors placed in the vicinity of rotating electrical machines, in order to detect a stator interturn short circuit. This fault induces a dissymmetry in the external magnetic field that can be measured by the sensors. Sensitive harmonics are extracted from the signals delivered by a pair of sensors placed at 180° from each other around the machine, and data obtained for several sensor positions are analyzed by fusion techniques using the belief function theory. The diagnosis method is applied on induction and synchronous machines with artificial stator faults. It will be shown that one can obtain high probability to detect the fault using the proposed fusion technique: on various series of measurements, the proposed approach has obtained a 90% detection rate on a considered machine.

Increasing the energy-efficiency of induction machines by the use of grain oriented magnetic materials and die-casting copper squirrel cage in the rotor

This paper deals with an induction machine whose squirrel cage is made of copper obtained by a die casting process. The major issue of this process is the thermal shock suffered by the laminations, leading to the deterioration of insulation coating as well as the generation of interbar currents, those currents being responsible for addition stray losses. This issue can be counteracted by using grain-oriented (GO) laminations on the rotor. In particular, the paper highlights that due to the specificity of the insulation coating of that kind of electrical steel, it does not suffer from the thermal shock. It is shown that using a squirrel cage made of copper combined to GO laminations in the rotor leads to a decrease of both core losses and stray load losses and an increase of the efficiency of 1.6%.

Original article: Induction machine magnetic noise: Impact of a new stator magnetic circuit design

The main topic of this paper is to examine the impact on induction machine magnetic noise of a new stator magnetic circuit design. This new design was developed in order to increase AC machine energetic performance by assembling non-segmented shifted grain-oriented steel sheets which make it possible to reduce iron losses. Experimental results on induction machine efficiency increase are presented and justified using reluctance network simulations. An analysis, finally, shows and explains how this structure behaves considering acoustic magnetic noise in a Pulse Width Modulation supply case.