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    Analysis of Shaft Voltage of Large Turbo

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    Analysis of Shaft Voltage of Large Turbo

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    34 views2 pages

    Analysis of Shaft Voltage of Large Turbo

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    Analysis of Shaft Voltage of Large Turbo-


    generators for rotor defect detection purposes
    Kévin DARQUES, Abdelmounaïm TOUNZI, Karim BEDDEK
    Yvonnick Le MENACH EDF Lab
    Univ. Lille, Arts et Metiers ParisTech, Centrale Lille, 7 Boulevard Gaspard Monje
    HEI, EA 2697 L2EP, Laboratoire d’Electrotechnique et F-91120 Palaiseau, France
    d’Electronique de Puissance karim.beddek@edf.com
    F-59000 Lille, France
    kevin.darques@univ-lille.fr

    Abstract—In the case of large turbo-generators, the analysis


    of the shaft voltage could be interesting to diagnose some
    defects such as eccentricities or field winding inter-turn short
    circuits. To this aim, a first step consists in determining in an
    accurate way the effect of these defects on the shaft voltage. In
    this paper, a 4–pole high-power non-salient pole synchronous
    generator is studied in a didactic way. The study is carried out
    gradually, with the aim to determine the effect of the parallel
    coupling of the armature windings.

    Keywords—Fault diagnosis, finite element model, shaft


    voltage, synchronous generator

    I. INTRODUCTION
    Fig. 1. Circumferential magnetic flux
    Monitoring of large turbo-alternators of power plants is
    of great interest in order to ensure a reliable operation along B. Numerical model
    with sustainability of the material [1]. Shaft voltage is
    generally an undesirable quantity that is due to numerous With the aim to determine the impact of the stator
    causes [2] such as slight inherent construction imperfections, winding, the shaft voltage of a large common turbo-
    air gaps introduced by the implementation of the sectors of generators of 1300 MW high power plants is investigated.
    steel sheets of the magnetic circuit or mechanical frictions.... This structure is a non-salient rotor machines with 4 poles.
    Thus, it induces currents that can damage the bearings of Using FE approach, a 2D model of this machine is built
    high-power machines. This voltage is also due to some up while taking a large air area around the machine in order
    defects that occur during the operation of the machine such to allow the leakage flux to be established. Furthermore, to
    as rotor short circuit or eccentricities and then, as it is mitigate any possible numerical error that can be introduced
    generally measured in high-power machines, its analysis
    by the quality of the mesh, the latter is carefully built in the
    could be useful to detect some of these defects. However, to
    be able to use it for reliable detection, it is first necessary to form of a regular mapped star mesh. Fig. 2 gives a view of
    quantify the contribution of each defect to be detected to this the mesh of the 4-pole structure which is composed of 82
    voltage. 000 prismatic elements. To take into account the static
    eccentricity the stator elements are moved by the value of
    This paper aims at analyzing shaft voltage of high power the eccentricity thickness towards the center of the machine
    synchronous generators by considering the effect of the
    [4].
    stator slots and armature windings when classical defects
    occur. An example of a 4-pole machine operating with a
    static eccentricity is investigated.

    A. Circumferential magnetic flux


    Shaft voltage is defined as the potential difference that
    appears between both ends of the rotor shaft. Several causes,
    either of capacitive or magnetic origins, can generate it [3].
    Furthermore, they may be internal to the machine or from
    external sources.
    In this paper, only magnetic asymmetries are concerned
    leading to a resultant circumferential magnetic flux which Fig. 2. View of the mesh
    induces an electric potential between the two ends of the To study and analyze in a didactic way the impact of each
    shaft. Magnetic asymmetries can be generalized to any cause that could generate a shaft voltage, the interaction of
    defect which results in an alteration of the magnetic flux the stator slots and windings with the considered defects is
    distribution in the machine. In faulty operation, this can be studied hereafter. Fig. 3 shows the coupling of the armature
    due to eccentricities or rotor short circuit. windings of the 4-pole machine studied.
    Each phase is constituted of four windings connected in
    parallel and two neutral points. R1 and Lf represent the

    978-1-7281-1560-3/19/$31.00 ©2019 IEEE

    Authorized licensed use limited to: Auckland University of Technology. Downloaded on June 05,2020 at 21:41:14 UTC from IEEE Xplore. Restrictions apply.
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    resistance and leakage flux inductance respectively, Rc is the


    phase load resistance and Rn is the resistance that connects
    the neutral points.

    Fig. 3. External circuit of the 4-pole turbo-generator

    Using the numerical model, different calculations are


    carried out at no load (RL =106 Ω), rated field current and
    speed 1500 rpm and with a 20 % static eccentricity. This b
    defect is obviously unrealistic but will allow understanding Fig. 4. Shaft voltage of 4-pole structure in the case of a static eccentricity
    the physical phenomena. The calculations are achieved (a) and its spectral decomposition (b)
    under magnetostatic hypothesis (no currents induced in the
    damper bars) while taking into account the nonlinear
    behavior of the magnetic material.
    In order to highlight the impact of the stator winding
    coupling on the shaft voltage when a defect occurs, three
    cases are investigated, i) Smooth stator; ii) Phase windings
    in series: This second case does not correspond to the real
    machine configuration but it can lead to the study of the
    only effect of the stator slots; iii) Phase windings in parallel.
    This last case constitutes the real coupling and then can give
    us the effect of the currents induced in the parallel windings. Fig. 5. Circulating currents in the stator windings of the 4-pole structure in
    the case of static eccentricity
    To emphasize the importance of these currents, the internal
    first phase current, i.e. IU1U2 and IU1U3 (see Fig. 3) are
    III. CONCLUSION
    presented below.
    In this paper, FEA of a 4-pole turbogenerator shows that
    II. APPLICATIONS a static eccentricity generates an inherent shaft voltage at a
    Fig. 4a and Fig. 4b show the time waveform of the shaft frequency of 50 Hz. The currents that are induced due to this
    voltage, and its harmonic content respectively, of the studied defect mitigate the latter while adding a significant
    component at 250 Hz. In the extended version, other defects
    alternator with a static eccentricity in the 3 cases introduced
    will be investigated such as dynamic eccentricity and an
    above. interturn rotor short circuit..
    For a smooth stator the shaft voltage has a quite
    sinusoidal 50 Hz waveform with a significant magnitude
    REFERENCES
    [5].
    [1] P. Nippes. Early Warning of Developing Problems in Rotating
    The variation of the air gap permeance, when the phase Machinery as Provided by Monitoring Shaft Voltages and Grounding
    windings are in series, strongly attenuates the magnitude of Currents. IEEE Transactions on Energy Conversion, 19(2):340-345,
    this voltage due to the increase of the average reluctance of 2004.
    the air gap while introducing a 5th order harmonic (250 Hz) [2] T. Plazenet, T. Boileau, C. Caironi, and B. Nahid-Mobarakeh. A
    Comprehensive Study on Shaft Voltages and Bearing Currents in
    along with a 2050 Hz harmonic due to the stator slots. Rotating Machines. IEEE Transactions on Industry Applications, Vol.
    Finally, when the real coupling of the alternator is 54, n. 4, pp: 3749-3759, 2018.
    considered, a static eccentricity induces circulating currents [3] C. Ammann, K. Reichert, R. Joho, and Z. Posedel. Shaft voltages in
    whose magnitude represents almost 3% of the rated currents generators with static excitation systems-problems and solution. IEEE
    Transactions on Energy Conversion, 3(2), 1988.
    in each circuit along with a 3rd order harmonic (Fig. 5.). [4] B. Iamamura, Y. Le Meanch, N. Sadowski, and E. Guillot. Study of
    The harmonic content of the shaft voltage shows then a static and dynamic eccentricities of a synchronous generator using 3d
    further decrease of the 50 Hz but these currents contribute to fem. IEEE Trans. Mag., Vol. 46, n. 8, pp: 3516-3519, 2010.
    the occurrence of non-negligible 5th order harmonic (250 [5] K. Darques, A. Tounzi, Y. Le Menach, and K. Beddek. Study of shaft
    voltage of a simplified synchronous generator. International
    Hz). Symposium on Applied Electromagnetics and Mechanics (ISEM),
    Chamonix, France, 2017.

    Authorized licensed use limited to: Auckland University of Technology. Downloaded on June 05,2020 at 21:41:14 UTC from IEEE Xplore. Restrictions apply.

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