Offline and Online Methods for Stator Core Fault ... - IEEE Xplore

4084

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 9, SEPTEMBER 2013

Offline and Online Methods for Stator Core Fault Detection in Large Generators Raphael Romary, Member, IEEE, Cristian Demian, Pierre Schlupp, and Jean-Yves Roger

Abstract—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. Index Terms—Alternators, fault diagnosis, fault location, insulation testing, magnetic field measurement, maintenance, shortcircuit currents.

N OMENCLATURE p β f n k1 ˆ B m MMF EMF S L Ieff ec Kc e Nr D R μ0

Number of pole pairs. Opening angle. Supply frequency. Number of turns in series for a whole phase. Winding coefficient. Magnitude of the flux density in the air gap. Number of slots per pole and per phase. Magnetomotive force in the air gap. Electromotive force. Section. Thickness of the exciter. Current in a single coil. Width of the equivalent air gap. Carter coefficient. Minimal air gap. Number of slots. Distance of assigned slots. Inner radius of the stator. Permeability of free space.

Manuscript received April 21, 2012; revised July 13, 2012, August 16, 2012, and September 14, 2012; accepted September 21, 2012. Date of publication October 11, 2012; date of current version May 2, 2013. This work was supported in part by the Motors and Electrical Devices for Energy Efficiency (MEDEE) program, by Elecricite De France (EDF) R&D, by the “Nord-Pasde-Calais” region, by Fond Europeeen de Developpement Regional (FEDER), and by the French Ministry for Research. R. Romary and C. Demian are with Univ Lille Nord de France, F-59000 Lille, France, and UArtois, LSEE, F-62400 Béthune, France (e-mail: [email protected]; [email protected]). P. Schlupp and J.-Y. Roger are with EDF R&D, 92141 Clamart, France (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2012.2224077

I. I NTRODUCTION

L

ARGE turbogenerators, designed to operate for more than 40 years, are usually subject to several failures like interlaminar core faults, rotor winding interturn short circuits, and core and region heating. Among all of these problems, stator interlaminar short circuit is one of the most common faults that occur in the stator core of turbogenerators. Its importance is even greater as it can cause catastrophic damage to the machine in a very short time. This is particularly true since repairing or rebuilding a turbogenerator is usually very expensive and it generally takes several months. In this way, interest in the detection of the incipient stator interlaminar short circuits has grown to improve maintenance of alternators in power plants, to estimate the machine life span, to schedule stator replacement operations, and also to ensure exploitation in good working conditions until the next maintenance period. In the literature, methods have been proposed to evaluate the presence of stator core faults in turbogenerators [1]–[3]. First, one can find “offline” methods performing during the scheduled maintenance periods. They are generally based on the manual and visual inspection of the inner surface of the stator in order to detect looseness of bars in slots, wear and residues on surfaces due to repeated partial discharges, signs of overheating, and more. One of the oldest methods is the ring test, but the use of a power source of several megavoltamperes to excite the core close to the rated flux involves special safety procedures [4]. In the last time, a low-power test electromagnetic core insulation detection (ELCID) was also developed and widely used [3], [5]–[8]. The application of this method meets problems when applied to the end stepping portion of the core [9]. A stator interlaminar fault generally occurs near the edge of laminations, at the surface of a tooth, but in some cases, it can occur deeply in the core. In that case, the detection hardly can be done, and some faults may be missed during inspection. In [10], a method based on special flux injection system through adjacent teeth which include an excitation coil and a measurement one is presented. This permits detecting interlaminar faults located in depth of the stator core. In [2], a special sensor is designed to be placed at the edge depression area of the stator slot. The online methods consist in instrumentation of the electrical machine and perform data collection from measurements. At the beginning, such methods were developed in induction machines for the detection of stator and rotor faults by analysis of electrical variables, and they are constantly improved [11]– [20]. More recently, methods based on stray flux analysis were developed [21]–[24]. In [25], the detection of stator core faults is investigated in induction machines. In large generators,

0278-0046/$31.00 © 2012 IEEE

ROMARY et al.: OFFLINE AND ONLINE METHODS FOR STATOR CORE FAULT DETECTION IN LARGE GENERATORS

online analysis concerns essentially the thermal [26] or mechanical [27] variables, but recent investigations [28], [29] show the possibility to identify the presence of surface currents by measuring the flux density at the back of the stator core that make it possible to detect stator core faults. Apart from this, in recent years, computer simulations using the machine models facilitate the study and the understanding of interlaminar fault phenomena. In [30]–[32], the finiteelement method allows the study of rotating electromagnetic systems connected to electrical circuits. This method gives very interesting results, but a turbogenerator simulation constitutes a complex numerical problem and requires a large computation time and a huge computational resources. Generally, all the presented methods work quite well, but an improvement of the results is necessary taking into account the necessity of the detection of incipient faults. The major problem is that these studies are demanding because they require an experimental bench that represents a real stator, which is rather unusual. With all those considerations in mind, the aim of this paper is to present two experimental devices devoted to study core faults in large generators and which can bring some improvement to the actual offline and online methods. This paper is organized as follows. The second section of this paper starts with a recall of some principles of interlaminar short circuits in large generators, and two experimental devices are presented in detail. The first one is a bench able to excite stacks of laminations in large generators, and the second one is a partial exciter of a full 125-MW generator. In order to develop fault-detection methods, offline and online tests are presented in the third and the fourth sections. The offline method is based on the ELCID detector, while the online tests are based on flux density measurements at the back of the core of the 125-MW generator.

II. E XPERIMENTAL D EVICES A. Short Circuits Between Laminations Stator interlaminar short circuits concern essentially large electrical machines not forgetting that some papers are dedicated to study the influence of these faults in large transformers [33], [34]. During the machine service life, interlaminar shortcircuit faults in a stator generally start with a local degradation of the insulation between sheets due to the following external causes [9], [35]: 1) mechanical damages during assembly, rewinding, or rewedging; 2) foreign particles introduced during assembly; 3) vibrations, arcing, and heat. If a contact occurs between laminations for one of the aforementioned reasons, then a circulating current may appear between the contact and the external key bar (Fig. 1). This current originates from the induced EMF excited by the flux which flows in the stator core. The current may lead to local overheating that can make the fault spread to the adjacent laminations [32], [36]–[38]. More serious damage, such as burning or melting of laminations, may reach the insulation of the stator

4085

Fig. 1. Stator core fault.

Fig. 2. Experimental device.

conductors, causing the complete failure of the machine, most often progressing at an exponential rate. In order to study and to improve procedures of detection of stator-lamination short circuits at an early stage, two different experimental prototypes are presented next. B. Test Bench for Lamination Stacks The stator of large generators is not made of single laminations as for small machines. It consists of small-size elementary sheets with only three or four stator teeth. A special experimental device has been built to reproduce the magnetic excitation in real conditions, as shown in Fig. 2. This device is composed of two stacks of 250 laminations, separated by wedges. The entire sheet package is put under a 15-bar pressure. In order to avoid irregularities at the junction between the inductors and the lamination stacks, air gaps made of 0.5-mm-thin wedges are put between both, with wound flux sensors to control the magnetic flux. Therefore, the most of the MMF generated by the inductors is used to magnetize the air gaps. The difference with the device shown in [10] is that these stacks are magnetically excited by two inductors which can bring 90% of the rated flux. The main advantage of this device is that it offers the possibility to perform a wide range of tests to develop procedures for the detection of stator core faults. Different fault configurations can be studied according to the following: 1) different locations of the fault within the laminations;

4086

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 9, SEPTEMBER 2013

Fig. 3. Welded laminations.

Fig. 5.

One-hundred-twenty-five-megawatt alternator.

Fig. 6.

Stator and exciter prototype.

Fig. 4. Stack back core.

2) the number of faulty laminations; 3) the level of the magnetic excitation. The short circuit between laminations is achieved by a local welding, as shown in Fig. 3, where five laminations are shortcircuited. This is only an example; the fault can be placed at various positions, and it is possible to short-circuit up to 40 laminations by welding. The faulty laminations are inserted in the upper stack. The contact at the back of the core that closes the electric circuit for the current circulation is made of copper tape welded on the external faulty laminations as shown in Fig. 4. It allows the short-circuit current measurement, but it also enables a current injection. Consequently, two kinds of test can be performed: tests with injected current (inductor nonsupplied) and test with induced current issued from the core magnetization. Several sensors are used to perform a wide analysis of the phenomena by measuring various signals: 1) ELCID detector; 2) Rogowski coil; 3) flux density probes (hall effect and wound sensors); 4) infrared camera; 5) temperature probes (thermocouple). C. Partial Exciter of a Full Stator The second real-scale device consists of an experimental platform prototype that comprises a real 125-MW turbogenerator shown in Fig. 5. This turbogenerator, whose rotor has been removed, belongs to our laboratory and enables us to carry out static tests. The main generator data are as follows: two poles, rated voltage of 15 500 V, rated frequency of 50 Hz, rated current of 5820 A, and rated power factor of 0.8. In order to test online detection methods, the experimental bench requires the reproduction of both rotor and stator excitations. As the rotor is removed from the stator, the idea is to reproduce locally the excitation of the stator magnetic circuit by

creating a static sliding field. The solution consists of making an exciter based on the linear motor structure which takes the shape of the inner surface of the stator (Fig. 6). This way, the exciter will be able to reproduce the normal operation of the machine in order to develop online fault detectors. The calculation of the parameters of the exciter is detailed in the following section, requiring defining the device specifications. D. Parameter Calculation The first step is to define the constraints according to the tests that have to be performed. These give a starting point to calculate all the parameters and enable us to choose the most appropriate design. The main features of the prototype are as follows: ˆ = 0.8 T. 1) Maximum value of flux density in the air gap: B 2) Angular velocity of the poles: 3000 r/min. 3) Supply voltage = 230 V (imposed by variable speed drive). 4) Exciter length must be limited in order to excite only one stack of stator laminations. 5) Opening angle must be large enough to ignore the edge effects. 6) Maximum reduction of the saturation and the losses in the device. 7) Use of an industrial variable speed drive to supply the device.

ROMARY et al.: OFFLINE AND ONLINE METHODS FOR STATOR CORE FAULT DETECTION IN LARGE GENERATORS

4087

An opening angle β = 150◦ is chosen, which enables us to neglect the edge effects and to cover almost half the lower part of the stator. By using (1) with the number of pole pairs p = 4, one calculates the maximum supply frequency f = 480 Hz, which corresponds to the angular velocity of 3000 r/min of the poles: p f = 100π . β

(1)

To obtain the desired frequency of 480 Hz, an industrial inverter is used. The voltage upper limit is then imposed at 230 V. This will enable us to determine the other parameters through ˆ (2), which gives E the rms value of the stator EMF versus B 4π ˆ E = √ nk1 f S B 2

Fig. 7. Exciter winding scheme.

(2)

where n is the number of turns in series for a whole phase and k1 is the winding coefficient defined as: k1 =

sin π6 π m sin 6m

(3)

m corresponds to the number of slots per pole and per phase. In this equation, one chooses m = 2 not only to have a good sinusoidal MMF in the air gap but also to limit the number of slots; consequently, we obtain k1 = 0.97. The number of winding turns is chosen n = 24 (three coils per slot). The surface S is deduced from (2), which enables us to determine the thickness of the exciter: L = 100 mm. Subsequently, the current in a single coil calculated by: Ieff =

ec B μ0 pn

Fig. 8. Mesh of the 2-D model.

(4)

where ec is the width of the equivalent air gap, taking into account the slotting effect: ec = eKc .

(5)

Kc is the Carter coefficient, and e is the minimal air gap imposed owing to thin wedges. According to (4) and (5) and by choosing e = 0.5 mm, the rms current in the winding is Ieff = 60 A. With two slots per pole and per phase and four pole pairs, the number of slots is N r = 6mp = 48. The slots are assigned to a distance of D = Rβ = 1517.13 mm (where R is the inner radius of the stator). For the calculated value of the current, we obtain a 22-mm height for the slots. The exciter windings are shown in Fig. 7. E. Finite-Element Analysis A finite-element simulation enables us to verify that the exciter satisfies the requirements when it is placed within the stator. The advantage is that adjustments of the parameters are possible in order to optimize the design. The numerical analysis is based on a 2-D plane-parallel finite-element approach by using Flux 2-D. Fig. 8 shows the mesh of the geometry which contains about 45 000 elements and 30 000 nodes.

Fig. 9. Flux density distribution.

By solving the 2-D magnetostatic field problem, one obtains the magnetic field lines and the chart of the flux density level distributed over the entire surface of the exciter (Fig. 9). The curve of the evolution of flux density B along the air gap is shown in Fig. 10. This curve takes into account the normal and the tangential components of the flux density. According to Fig. 10, the fundamental value of the flux density is approximately 0.75 T, which is close to the imposed value used to design the exciter. F. Building the Prototype The production of the structure is made according to the previously calculated dimensions. The sheets used to build the magnetic circuit of the prototype are nonoriented laminations, with low thickness (0.5 mm) to reduce iron losses. The rigidity

4088

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 9, SEPTEMBER 2013

Fig. 12.

Voltage inside the exploration coil.

Fig. 13.

Evolution of the flux density in the air gap.

Fig. 14.

ELCID probe.

Fig. 10. Induction B along the air gap.

Fig. 11. Realization of the exciter.

of the whole structure is ensured by adding thicker sheets (0.60 mm) at the edges. This way, for a 100-mm thickness, the whole structure contains 200 sheets. Fig. 11 shows the exciter placed inside the stator, ready to excite the stator magnetic circuit by creating a sliding field. G. Experimental Results on Testing the Prototype The design of the exciter requires the use of a three-phase system supply with a voltage frequency varying between 50 and 500 Hz. The results presented here are made at a 50-Hz frequency and with the current in the three phases of the exciter imposed at 50-A rms. Of course, the supply voltage in this case is reduced: V = 23 V. The instrumentation of the stator consists of a search coil made of two turns placed in the air gap, between the exciter and the stator. Fig. 12 shows the voltage delivered by the search coil. Fig. 13 corresponds to the flux density in the air gap calculated after introducing an integrator. It can be observed that the induction level is about 0.8 T which is very close to the result given by the finite-element simulation. III. O FFLINE T ESTS : ELCID A. ELCID Detector The most commonly used method for the detection of statorlamination faults is based on the ELCID detector [5]–[8]. This technique is an offline method that needs the rotor to be removed for scanning the inner surface of the stator with a probe.

Two probe sizes are available: a wide probe with extremities placed on two teeth and a small one, shown in Fig. 14, that scans only one tooth for a more accurate localization of the fault. The probe is made of a Rogowski coil which is not closed and which delivers a voltage signal that is proportional to the difference of scalar magnetic potential at the probe extremities: Δε = ε1 − ε2 . This quantity is also equal to the circulation of H along the probe (Fig. 15). The voltage signal is treated by filters tuned in order to provide the actual value of Δε in amperes. Δε is a sine signal which is in phase with the main core flux and with the excitation current in healthy conditions. The presence of a faulty current modifies Δε, changing the phase. As the impedance that limits the faulty current is essentially resistive, it appears that this faulty current has a quadratic phase with the excitation current. The principle of the ELCID is based on the extraction of the quadratic component (“QUAD” information) of the signal for a reference taken as the excitation current. The device also provides the component in phase with the reference (“PHASE” information).

ROMARY et al.: OFFLINE AND ONLINE METHODS FOR STATOR CORE FAULT DETECTION IN LARGE GENERATORS

4089

Fig. 15. Principle of ELCID measurement. Fig. 18. Stator core excitation.

Fig. 16. Upper stack with faulty laminations.

Fig. 19. ELCID and field measurements on the 125-MW turbogenerator.

in the short circuit. With five faulty laminations, the ratio falls to 10%. It can also be noted that, obviously, the signal is maximum when the probe is placed at the center of the faulty laminations. C. Experimental Results—125-MW Turbogenerator Fig. 17. Experimental results.

B. Experimental Results—Sheet Package At this stage, the results of a basic test carried out on the sheet package test bench with the small probe ELCID detector are presented. The fault is located at the top of the central tooth of a lamination. The faulty laminations are inserted at the center of the upper stack of the system. This test gives the sensitivity of the ELCID detector versus the number of faulty laminations and versus the position of the ELCID probe (Fig. 16). In this test, a 2-A rms current is injected at the back of the core. The ELCID reference is taken on the injected current so that the analysis concerns only the components in phase. Fig. 17 shows this component of Δε in amperes, delivered by the ELCID detector for different numbers of laminations concerned by the fault, where x represents the position of the probe relative to the center of the faulty stack. It can be noted that, in our worst case scenario (40 faulty laminations), the detector gives only 50% of the current flowing

The use of the ELCID probe for large turbogenerators implies the fact that the whole stator is excited by a long coil wrapped around the stator, supplied by the grid (Fig. 18). This way, the yoke of the stator core is excited at 3%–4% of the rated flux level. Fig. 19 shows the small area of the stator where measurements take place. The faulty mode is obtained by welding an entire group of laminations for one tooth at the inner surface of the stator [Fig. 19(a)]. The way the short circuit is realized does not enable the measurement of the short-circuit current. At the same time, the access to the outer part of the stator core is allowed in order to perform some measurements at the level of the key bar, covering up to eleven stacks of laminations. The excitation of the yoke of the stator core with 3%–4% of rated flux is enough to induce fault currents. The most of the magnetic field generated by fault occurs in the air above the surface of the core due to the high permeability of the iron core and its laminated structure. The ELCID probe (large probe) making it possible to measure this magnetic potential difference

4090

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 9, SEPTEMBER 2013

TABLE I ELCID R ESULTS—125-MW T URBOGENERATOR

Fig. 21.

Field measurements—Healthy stator core.

Fig. 22.

Field measurements—Short circuit.

Fig. 20. Results of the field measurements on the bars.

between its two ends is placed in the inner part of the stator core near the short circuit. Six ELCID measurement points are defined, as shown in Fig. 19(b). The PHASE and QUAD components of the faulty current for the healthy stator core and for the stator with interlaminar short circuits are given (Table I). The existence of a short circuit can be observed by an increase of the QUAD component when the default is placed between the probe extremities (points 3 and 4).

IV. F IELD M EASUREMENT ON THE BARS A. Offline Tests It has been shown in [28] that the back current in the key bar modifies the flux density repartition in this area. This enables the detection of deep faults. According to the direction of the current, the azimuthal component of the magnetic field should be measured. Here, the whole stator core is still excited at 4%. Measurements have been performed with a commercial teslameter based on the Hall effect. This device is convenient for this kind of measurement as the magnitude range goes up to 200 μT and the frequency range goes down to dc. Moreover, it provides information related to the flux density in only one direction. This sensor is placed in the outer part of the stator core [Fig. 19(c)] along the three adjacent bars (lower, central, and upper keys bars) that can be flowed through by the faulty current according to the position of the fault. Results are shown in Fig. 20. It can be observed that the magnetic field near the external key bar is sensitive to a fault on the inner surface of the stator. The default can be detected in the sixth lamination, but also, it can be visible in the two adjacent bars.

B. Online Tests Online monitoring is clearly highly desirable if results can be used to keep a machine running as long as possible between maintenance shutdowns while continuing to detect problems before they cause severe damage. When the machine operates, the change of magnetic field at the level of a key bar can be an interesting means of fault detection. Here, we use the partial exciter that makes it possible to provide the rated flux density and makes the fault current circulate. Figs. 21 and 22 show the flux density measured at the level of two external key bars concerned in the circulation of the faulty current [Fig. 19(c)]. The results are related to the tangential flux density. Results are given versus the axial position of the sensor along the key bar. An increase of the tangential flux density in faulty conditions can be observed. The partial exciter generates eight poles at the level of the air gap, but it does not cover the whole inner surface of the stator. That corresponds to 20 fictitious poles along the periphery of the full air gap. Therefore, for the same peak value of the air-gap flux density, the core flux will be ten times less than that in the case of a real two-pole rotor. Thus, the fault current will also be ten times less than what it would be in real operating conditions. However, the partial exciter makes it possible to display the phenomena concerning the change of key-bar flux density that is actually underevaluated compared to real operating conditions.

ROMARY et al.: OFFLINE AND ONLINE METHODS FOR STATOR CORE FAULT DETECTION IN LARGE GENERATORS

V. C ONCLUSION In large generators, the state of the stator core is unknown in operating conditions, and existing methods for stator core fault detection are based on the inspection of the inner stator surface when the rotor is removed, during scheduled maintenance periods. Then, if major damages are detected, it can be decided to replace the stator laminations, thus delaying the restart of the turbogenerator. In this context, the development of online methods can be of interest in order to increase the availability rate of the machine, but it requires experimental benches to test, validate, and refine the methods. Thus, the experimental devices presented in this paper can bring a strong contribution for this kind of studies. One example of online methods has been presented in this paper. It exploits the flux density at the level of the external key bar that can be measured even in real operating condition. It is shown that this variable can provide information about a short circuit between laminations. The short-circuit current that flows through by the key bar does not depend very much on the location of the fault, and then, deeper faults can be detected, whereas conventional offline methods such as those based on the ELCID are not sensitive to deep faults.

[13]

[14]

[15]

[16]

[17] [18] [19]

[20]

ACKNOWLEDGMENT This study enters within the MEDEE program, supported in part by EDF R&D, by the “Nord-Pas-de-Calais” region, by European funds (FEDER), and by the French Ministry for Research. R EFERENCES [1] P. J. Tavner and A. F. Anderson, “Core faults in large generators,” Proc. Inst. Elect. Eng.—Elect. Power Appl., vol. 152, no. 6, pp. 1427–1439, Nov. 2005. [2] S. B. Lee, G. B. Kliman, M. R. Shah, N. K. Nair, and R. M. Lusted, “An iron core probe based inter-laminar core fault detection technique for generator stator cores,” IEEE Trans. Energy Convers., vol. 20, no. 2, pp. 344–351, Jun. 2005. [3] D. R. Bertenshaw, A. C. Smith, C. W. Ho, T. Chan, and M. Sasic, “Detection of stator core faults in large electrical machines,” IET Elect. Power Appl., vol. 6, no. 6, pp. 295–301, Jul. 2012. [4] S. B. Lee, G. B. Kliman, M. R. Shah, D. W. Kim, W. T. Mall, N. K. Nair, and R. M. Lusted, “Experimental study of inter-laminar core fault detection techniques based on low flux core excitation,” IEEE Trans. Energy Convers., vol. 21, no. 1, pp. 85–94, Mar. 2006. [5] D. Bertenshaw, “Analysis of stator core faults—A fresh look at the EL CID vector diagram,” in Proc. Hydropower Dams, Porto Carras, Greece, Sep. 2006. [6] J. Sutton, M. Sasic, and D. R. Bertenshaw, “30 years experience with EL CID stator core testing,” in Proc. Iris Rotating Mach. Conf., Long Beach, CA, 2008. [7] D. R. Bertenshaw, J. F. Lau, and D. J. Conley, “Evaluation of EL CID indications not associated with stator core inter-laminar insulation faults,” in Proc. EIC, 2011, pp. 254–260. [8] M. Sasic and D. Bertenshaw, “EL CID testing of turbo-generators at higher frequency,” in Proc. EPRI, Aug. 2010, pp. 1–11. [9] M. M. Znidarich, “Hydro generator stator cores part 2—Core losses, degradation mechanisms, testing and specification,” in Proc. AUPEC, Dec. 2008, pp. 1–9. [10] G. B. Kliman, S. B. Lee, M. R. Shah, R. M. Lusted, and N. K. Nair, “A new method for synchronous generator core quality evaluation,” IEEE Trans. Energy Convers., vol. 19, no. 3, pp. 576–582, Sep. 2004. [11] A. Bellini, F. Filippetti, C. Tassoni, and G.-A. Capolino, “Advances in diagnostic techniques for induction machines,” IEEE Trans. Ind. Electron., vol. 55, no. 12, pp. 4109–4126, Dec. 2008. [12] A. Garcia-Perez, R. de Jesus Romero-Troncoso, E. Cabal-Yepez, and R. A. Osornio-Rios, “The application of high-resolution spectral

[21]

[22] [23] [24] [25] [26]

[27] [28] [29] [30]

[31] [32] [33] [34]

4091

analysis for identifying multiple combined faults in induction motors,” IEEE Trans. Ind. Electron., vol. 58, no. 5, pp. 2002–2010, May 2011. G. R. Bossio, C. H. De Angelo, P. M. de la Barrera, J. A. Solsona, G. O. Garcıa, and M. I. Valla, “Stator winding fault detection in induction motor drives using signal injection,” in Proc. 8th IEEE SDEMPED, Sep. 2011, pp. 92–97. C. De Angelo, G. Bossio, S. Giaccone, M. Valla, J. Solsona, and G. Garcia, “Online model-based stator-fault detection and identification in induction motors,” IEEE Trans. Ind. Electron., vol. 56, no. 11, pp. 4671– 4680, Nov. 2009. C. Seungdeog, B. Akin, M. M. Rahimian, and H. A. Toliyat, “Performance-oriented electric motors diagnostics in modern energy conversion systems,” IEEE Trans. Ind. Electron., vol. 59, no. 2, pp. 1266– 1277, Feb. 2012. R. M. Tallam, S. B. Lee, G. S. Stone, G. B. Kliman, J. Yoo, T. G. Habetler, and R. G. Harley, “A survey of methods for detection of stator related faults in induction machine,” IEEE Trans. Ind. Appl., vol. 43, no. 4, pp. 920–928, Jul./Aug. 2007. A. Gandhi, T. Corrigan, and L. Parsa, “Recent advances in modeling and online detection of stator interturn faults in electrical motors,” IEEE Trans. Ind. Electron., vol. 58, no. 5, pp. 1564–1575, May 2011. D. C. Patel and M. C. Chandorkar, “On-line load test for induction machine stator inter-turn fault detection under stator electrical asymmetries,” in Proc. 36th IEEE IECON, 2010, pp. 933–938. M. A. Samonig, P. Nussbaumer, G. Stojicic, and T. M. Wolbank, “Analysis of rotor fault detection in inverter fed induction machines at no load by means of finite element method,” in Proc. 37th IEEE IECON, 2011, pp. 1758–1763. S. H. Kia, H. Henao, and G.-A. Capolino, “Windings monitoring of wound rotor induction machines under fluctuating load conditions,” in Proc. 37th IEEE IECON, 2011, pp. 3459–3465. H. Henao, C. Demian, and G. A. Capolino, “A frequency-domain detection of stator winding faults in induction machines using an external flux sensor,” IEEE Trans. Ind. Appl., vol. 39, no. 5, pp. 1272–1279, Sep./Oct. 2003. A. Ceban, R. Pusca, and R. Romary, “Study of rotor faults in induction motors using external magnetic field analysis,” IEEE Trans. Ind. Electron., vol. 59, no. 5, pp. 2082–2093, May 2012. O. Poncelas, J. A. Rosero, J. Cusido, J. A. Ortega, and L. Romeral, “Motor fault detection using a Rogowski sensor without an integrator,” IEEE Trans. Ind. Electron., vol. 56, no. 10, pp. 4062–4070, Oct. 2009. L. Frosini, A. Borin, L. Girometta, and G. Venchi, “A novel approach to detect short circuits in low voltage induction motor by stray flux measurement,” in Proc. 20th ICEM, Sep. 2012, pp. 1536–1542. P. M. de la Barrera and G. R. Bossio, “Stator core faults detection on induction motor drives using signal injection,” in Proc. 8th IEEE SDEMPED, Sep. 2011, pp. 98–104. S. Stipetic, M. Kovacic, Z. Hanic, and M. Vrazic, “Measurement of excitation winding temperature on synchronous generator in rotation using infrared thermography,” IEEE Trans. Ind. Electron., vol. 59, no. 5, pp. 2288–2298, May 2012. W. Q. Lim, D. H. Zhang, J. H. Zhou, P. H. Belgi, and H. L. Chan, “Vibration-based fault diagnostic platform for rotary machines,” in Proc. 36th IECON, 2010, pp. 1404–1409. R. Romary, S. Jelassi, and J. F. Brudny, “Stator interlaminar fault detection using an external flux density sensor,” IEEE Trans. Ind. Electron., vol. 57, no. 1, pp. 237–243, Jan. 2010. R. Romary, C. Demian, P. Schlupp, J. Y. Roger, and T. Szkudlapski, “Real scale experimental devices for stator core fault analysis,” in Proc. 8th IEEE SDEMPED, Sep. 2011, pp. 71–76. J.-L. Müller, R. Romary, A. Benabou, T. Henneron, F. Piriou, J. P. A. Bastos, and J.-Y. Roger, “Interlaminar short circuit detection: Modeling and measurement,” Int. J. Comput. Math. Elect. Electron. Eng., vol. 31, no. 5, pp. 1448–1457, 2012. W. Yixuan, W. Ying, Q. Haifei, and L. Xin, “Dynamic design and simulation analysis of 1000 MW large turbo-generator,” in Proc. ICMA, 2009, pp. 1650–1655. S. J. Murray, J. Edmonds, J. R. Foulds, R. A. Sire, and W.-M. Chi, “Modeling fault propagation in an electric generator stator core,” in Proc. EPRI Gener. Workshop, Jul. 2003. C. A. Schulz, S. Duchesne, D. Roger, and J. N. Vincent, “Short-circuit current measurements between transformer sheets,” IEEE Trans. Magn., vol. 46, no. 2, pp. 536–539, Feb. 2010. J. P. Bielawski, S. Duschesne, D. Roger, C. Demian, and T. Belgrand, “Contribution to the study of losses generated by inter-laminar shortcircuit,” IEEE Trans. Magn., vol. 48, no. 4, pp. 1397–1400, Apr. 2012.

4092

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 9, SEPTEMBER 2013

[35] V. I. J. Kokko, “Electro-thermal ageing in lifetime estimation of hydroelectric generator stator windings,” in Proc. 8th IEEE SDEMPED, Sep. 2011, pp. 294–299. [36] K. Yamazaki, Y. Yamato, H. Mogi, C. Kaido, A. Nakahara, K. Takahashi, K. Ide, and K. Hattori, “In-plane eddy current analysis for end and interior stator core packets of turbine generators,” in Proc. ICEM, 2008, pp. 1–6. [37] K. Yamazaki, S. Tada, H. Mogi, Y. Mishima, C. Kaido, S. Kanao, K. Takahashi, K. Ide, K. Hattori, and A. Nakahara, “Eddy current analysis considering lamination for stator core ends of turbine generators,” IEEE Trans. Magn., vol. 44, no. 6, pp. 1502–1505, Jun. 2008. [38] B. Cougo, A. Tuysuz, J. Muhlethaler, and J. W. Kolar, “Increase of tape wound core losses due to interlamination short-circuits and orthogonal flux components,” in Proc. 37th IEEE IECON, 2011, pp. 1372–1377.

Raphael Romary (M’10) received the Ph.D. degree from Lille University, Lille, France, in 1995 and the D.Sc. degree from Artois University, Béthune, France, in 2007. He is currently a Full Professor with Artois University and a Researcher with the “Laboratoire Systemes Electrotechniques et Environnement.” His research interest concerns the analytical modeling of electrical machines with applications to noise and vibration, losses, electromagnetic emissions, and diagnosis.

Cristian Demian received the Ph.D. degree from the University of Picardie Jules Verne Amiens, Amiens, France, in 2005. He is currently an Associate Professor with Artois University, Béthune, France, where he joined the Laboratoire Systemes Electrotechniques et Environnement. His research interests focus on diagnosis, modeling, efficiency, analysis, and the reduction of noises and vibrations of electrical machines.

Pierre Schlupp received the Engineer Diploma from the Ecole Nationale Supérieure de Techniques Avançées, Paris, France, in 1991. Since 1992, he has been with EDF R&D, Clamart, France, and since 2004, he has been in charge of the research activity on insulation systems of electrical machines.

Jean-Yves Roger received the B.S. degree in electrical engineering from the University of Technology of Compiegne, Compiegne, France, in 2003. Since 1995, he has been with EDF R&D, Clamart, France, where he started working on large transformer and has been a Research Engineer since 2004. His search interest is turbogenerators both for thermal and nuclear power plants.