Diagnosis of Static Eccentricity in 3-Phase Synchronous Machines using a Pseudo Zero-Sequence Current

Abstract

Large synchronous generators are the heart of the modern world, while producing the vast majority of the electric power consumed globally. Although they are robust devices, they are prone to degradation and failure. If such failures are not detected at an early stage, then the negative impact may be catastrophic in terms of financial costs, repair times, human lives and quality of life. This is the reason for continuous research in the field of condition monitoring aiming towards the reliable operation of synchronous generators. This paper proposes a novel technique for the diagnosis of the static eccentricity in synchronous generators. The proposed approach is off-line and non-intrusive, allowing the estimation of the fault severity with stator current measurements only. The performed work has been carried out with Finite Element Simulations and extensive experimental testing. Energies 2019, 12, 2476 2 of 16where the electrical machine operates under an asymmetrical air-gap due to displacement of the rotor. The eccentricity fault may be either static when the rotor's dislocation is fixed in space and time, or dynamic when the rotor geometric center changes continuously over time. If the two conditions co-exist, then this is known as the mixed eccentricity condition [5].The static eccentricity fault usually appears at the commissioning stages and starts as a manufacturing fault. It can be caused by improper alignment of the shaft or the stator inner ovality. Some level of inherent eccentricity is expected, however, and this is due to the very small air-gap between the rotor poles and the stator teeth [6]. The impact of the rotor weight affects the level of the static eccentricity in large electrical machines over time.The impact of eccentricity on synchronous machines has been studied in the past [6][7][8][9]. The radial and tangential forces are known to increase in the presence of eccentricity conditions [7]. It has been presented in reference [8] that variation of the air-gap magnetic reluctance will generate eddy currents in the rotor damping cage, even at unloaded operation [8]. A paper on the Unbalanced Magnetic Pull (UMP) caused by eccentricity faults in synchronous machines presented the independence between the static and dynamic eccentricity [6]. Finally, it was shown that the impact of the damper cage in case of eccentric conditions is very weak, an attribute associated with its high ohmic resistance [9]. However, the damper cage's impact on the forces due to the eccentricity fault is small, an attribute due to the cage's high ohmic resistance.Moreover, focusing on the actual diagnosis of the eccentricity fault in synchronous machines, it is clear that most past works focus on either permanent magnet [10][11][12][13] or reluctance machines [14-16], while literature is very limited for salient pole ones. Despite this unequal research effort, some contributions have been identified and are listed here. Intrusive techniques such as the installation of flux sensors to capture the air-gap magnetic flux give...

Utilization of Two Sensors in Offline Diagnosis of Squirrel-Cage Rotors of Asynchronous Motors

“…When evaluating a rotor defect, the operator often needs additional information, e.g., a view of the defect type. Therefore, we decided to use a technique known, e.g., in ovality or eccentricity measurements [34][35][36][37].…”

Online Model-Based Fault Detection of Synchronous Generators Using Residual Analysis

“…In these methods, instead of the model of SG, a database of all possible operating points in healthy and faulty conditions is required. Eccentricity fault, inter-turn short circuit fault, ground-fault, field winding, phase-phase/3-phase fault, and phase-ground faults are some consideration faults to present fault detection techniques [21][22][23][24][25][26][27][28][29][30][31]. Biet [21] has introduced a signal-based method based on the signals analysis of flux probes and classical electric measurements for rotor faults diagnosis.…”

“…Bruzzese [22] employed a combined space-vector and fast Fourier transformation analysis for eccentricity fault diagnosis. Gyftakis et al [23] introduced a method for static eccentricity fault diagnosis by using stator currents. In [24] and [25], two methods for detecting inter-turn short circuits of rotor windings were proposed.…”