GB2157005A - Detecting rotor winding faults - Google Patents

Detecting rotor winding faults Download PDF

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Publication number
GB2157005A
GB2157005A GB08408904A GB8408904A GB2157005A GB 2157005 A GB2157005 A GB 2157005A GB 08408904 A GB08408904 A GB 08408904A GB 8408904 A GB8408904 A GB 8408904A GB 2157005 A GB2157005 A GB 2157005A
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United Kingdom
Prior art keywords
current
phase windings
stator
phase
circulating
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GB8408904D0 (en
Inventor
John Muhlhaus
David Michael Ward
Ian Lodge
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Central Electricity Generating Board
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Central Electricity Generating Board
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Priority to GB08408904A priority Critical patent/GB2157005A/en
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Publication of GB2157005A publication Critical patent/GB2157005A/en
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/34—Testing dynamo-electric machines
    • G01R31/343—Testing dynamo-electric machines in operation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/34—Testing dynamo-electric machines
    • G01R31/346—Testing of armature or field windings

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Synchronous Machinery (AREA)

Abstract

Apparatus for detecting a rotor winding fault in a two-pole alternator having each phase of its stator divided into two parallel connected half-phase windings (10,11), comprises two current transducers (14,15) for sensing the respective currents in the two half-phase windings (10,11) of one of the phases of the stator and means (16,17,21) for comparing the two respective currents to detect equal and opposite currents in the two half-phase windings (10,11) corresponding to current circulating around between the two half-phase windings, whereby spatial harmonics in the flux density distribution can be detected. <IMAGE>

Description

SPECIFICATION Detecting rotor winding faults in two-pole alternators The present invention is concerned with the detection of rotor winding faults in two-pole alternators used for generating electric power.
The field winding on the rotor of a modern two-pole alternator consists of a number of series connected coils. Each coil has a number of turns and is located in a pair of slots running the length of the rotor body. The coils are grouped in two pole windings so that there is a precise symmetry about the interpolar plane. If a low resistance short circuit occurs between adjacent conductors within one slot, then one or more complete turns of the winding can be bypassed so that the current flowing in these turns is reduced. Thus, the short circuit will cause an asymmetry I between the current distributions on the two poles of the rotor. This may cause increased vibration, since the asymmetry in the ohmic heating of the rotor may cause the rotor to bend, and there will also be an unbalanced magnetic force on the rotor.Shorted turns can also lead to shaft magnetisation and increased shaft voltages.
It is accordingly useful to the operators of such alternators to know when there are short circuits present in the rotor winding. Prior Art techniques of determining this include the "recurrent surge oscillograph technique" which is used when the alternator is off-load. This technique can give an indication of the existence of anomolies in the rotor winding insulation, which may be shorted turns, but cannot distinguish between very low resistance shorts, and shorts with a resistance of several ohms. In an alternative Prior Art technique, the shorted turns can be detected when the alternator is on load, using a small search coil which is specially installed in the air-gap of the machine. The installation of such coils, however, usually requires the alternator in question to be out of commission for an extended period with removal of the rotor.
In an article entitled "A new Principle for Synchronous machine protection from rotor winding interturn and double earth faults" by S.S.Khryukin published in English in 1972 in Elect. Technol. USSR 2, 47 to 59, it is pointed out that a perfectly symmetrical and unfaulted alternator should have a perfectly symmetrical flux density distribution over the two-pole faces. The flux density distribution is not in fact sinusoidal but the symmetry implies that Fourier analysis of the distribution would contain only odd spatial harmonics. The presence of any asymmetry in the machine results in asymmetry in the flux distribution, implying the presence of even spatial harmonics. In the above referred article, Khryukin suggests that asymmetry in the machine could be determined by observing such even harmonic content in the stator voltage.He observes that such even harmonics would normally cancei and not appear in an ideal machine with a diametral full pitch stator winding. He observes, however, that machines in fact have slightly less than full pitch stator windings so that some even harmonic component of e.m.f. should be observable. However, it is apparent that any second or other even harmonic content of the alternator output voltage will be extremely small compared to the fundamental voltage and odd harmonics.
In nearly all large two-pole alternators, each phase of the stator winding is divided into two half-phases which are connected in parallel. The present invention makes use of these separate half-phase windings to greatly facilitate the detection of even spatial harmonics in the flux density distribution.
According to the present invention, apparatus for detecting a rotor winding fault in a two-pole alternator having each phase of its stator divided into two parallel connected half-phase windings comprises two current transducers for sensing the respective currents in the two half-phase windings of one of the phases of the stator and means for comparing the two respective currents to detect equal and opposite currents in the two half-phase windings corresponding to current circulating around between the two half-phase windings.
It will be appreciated that, whereas even spatial harmonics in the flux density distribution would normally cause equal and opposite e.m.f. to be generated in a full-phase winding stator, cancelling each other out, the use of two parallel connected half-phase windings result in the generation of equal and opposite even harmonic currents in respective half-phase windings, so that the resultant even harmonic current circulates around between the two half-phase windings. The two current transducers can readily sense the respective currents in the two half-phase windings and by comparing these detect the circulating current. In this way the presence of even spatial harmonics can be readily detected.
Preferably, said means for comparing includes means indicating the magnitude of the circulating current component at at least one even harmonic of the generated supply frequency. By indicating the magnitude of the current component in this way, an estimation can be made of the position and seriousness of the core winding fault causing the even harmonic current.
Normally the indicating means is arranged to indicate the magnitude of the second harmonic current component. The second harmonic is usually the largest.
In one arrangement, the indicating means comprises a spectrum analyser.
Preferably, each current transducer is formed as a coil wound on a non-magnetic former. Such current transducers are especially useful since it will normally be necessary to mount the transducer within the alternator casing. Conventional iron cored current transformers would not then be suitable since they cannot be used where there is a strong magnetic field due to sources other than the current to be measured.
Converriently, each current transformer comprises a helical coil on a non-magnetic former arranged for encircling a conductor of the respective half-phase winding. The former may be toroidal. In one arrangement, the former is flexible.
The present invention further envisages a method of detecting a rotor winding fault in a two-pole alternator having each phase of its stator divided into two parallel connected half-phase windings, the method comprising sensing the respective currents in the two half-phase windings of one of the phases of the stator and comparing the two respective currents to detect equal and opposite currents in the two half-phase windings corresponding to current circulating around between the two half-phase windings.
Preferably the method includes the further step of indicating the magnitude of the circulating current component at at least one even harmonic of the generated supply frequency. Conveniently the magnitude of the second harmonic is indicated. Further, the frequency spectrum of the circulating current may be analysed. Normally, the method also includes the step of estimating from said indicated magnitude the location of the rotor winding fault.
An example of the present invention will now be described with reference to the accompanying drawing which is a schematic block diagram illustrating an embodiment of the invention.
As explained above, the present invention is applicable to two-pole alternators of the type which have each phase of their stator divided into two parallel connected half-phase windings. The half-phase windings of one phase of the stator of such an alternator are illustrated schematically in the diagram by windings 10 and 11, which are shown connected in parallel between a line end 12 and a neutral end 13. It is not considered necessary to an understanding of the present invention to illustrate the structure of the alternator in further detail.
In a perfect fault free alternator, equal currents I flow in each of the two half-phase windings 10 and 11.
However, as explained above, if there is a fault resulting in asymmetry in the flux density distribution in the air gap of the alternator, this has the effect of introducing a differential between the currents 1, and 12 flowing in the two half-phase windings. This differential in the currents is constituted by equal current components flowing in opposite directions in the two half-phase windings, thereby producing in effect a circulating current component travelling around between the two half-phase windings.
The stator circulating current in the two half-phase windings is 1/2 (l,-12).
A current transducer 14 and 15 is illustrated associated with each of the half-phase windings 10 and 11 to sense the respective currents 11 and 12 flowing in each. For most alternators, the half-phase windings are connected together within the casing of the alternator and thus, the current transducers 14 and 15 must normally be located also within the casing. Because of the substantial magnetic fields present within the alternator casing corresponding to the various currents flowing in other windings of the aiternator, the current transducers must then be of the kind comprising coils wound on non-magnetic formers. Such current transducers are known in the art and give an output from the coil of the transducer dependent on the rate of change of current in the conductor being monitored.Examples of current transducer which could be used in the present invention are shown in the specifications of our patent No.
2034487 and patent application No. 2088568. These are essentially "air-cored" mutual inductors, in the form of toroidal coils arranged to encircle the current carrying conductor. When installing these in on the stator winding conductors of an alternator, care must be taken to provide suitable high voltage insulation and electrostatic screening between the coil of the transducer and the stator conductor.
Referring again to the figure, the coils of the two current transducers 14 and 15 are each connected across respective potentiometers 16 and 17. Corresponding terminals of the coils of the transducers are connected together as by line 18.
Voltages indicative of the current sensed by the current transducers are tapped off from the potentiometers 16 and 17 by wipers 19 and 20 resepectively and fed to the input terminals of a differential amplifier 21.
The potentiometers 16 and 17 can be adjusted to balance the output signals from the two transducers so as precisely to cancel out the components in the currents 1, and 12 which are of the same polarity, corresponding to the stator output current. It can be seen, then, that the output of the differential amplifier can represent only the current circulating around the two half-phase windings of the phase of the stator.
In the example illustrated, a spectrum analyser 22 provides an analysis of the circulating current to indicate the magnitude of the various components of this current at the harmonics of the generated supply frequency.
Considering an idealised two-pole alternator, with infinitely permeable iron, the effect of a single perfectly shorted turn in the rotor on the flux distribution can be represented by superposing onto the flux distribution of a fault free alternator, the flux distribution of a single turn, located in the same position as the shorted turn, and carrying an excitation current in the reverse direction. It can be shown that the e.m.f.E induced across a half-phase winding due to this single turn will be of the following form
where I is the rotor current o is the rotational frequency T is the number of stator turns in series R is the mean radius of the air-gap L is the active length of the rotor C is the Carter factor to account for slotting g is the width of the air-gap k is the stator winding factor for the nth harmonic ss is half the angle subtended by the single turn o IJo have their usual meaning.
The e.m.f. induced across the other half-phase winding of the particular phase of the stator will be of the same form but the term sin(not) will be replaced by one of the form sin(nO)t-nqr). Thus, the odd harmonic components of the e.m.f. will be of the same sign in the two half-phases and so will add in the parallel output of the stator. The even harmonic components will be of opposite signs in the two halfphase windings and so will produce circulating currents.
To calculate the circulating currents it is necessary to know the impedence of the stator winding to circulating currents. For an ideal unsaturated machine, the reactants of the stator winding for circulating second harmonic currents, X2, can be shown to be given by
where Xd is the stator magnetising reactance k is the stator winding factor for the fundamental k is the stator winding factor for the nth harmonic and the summation is taken over only even values of n.
In a real machine, the effect of saturation will be to reduce both E in equation 1 and X2 in equation 2.
In addition, the above analysis for an unsaturated machine implies that the ratio of the magnitude of the second harmonic current (100 Hz in the U.K.) circulating current to the rotor current will be constant, for a given shorted turn. Saturation in the machine may cause this ratio to vary with machine load.
It can be deduced from the above that the second harmonic circulating current will be larger than any of the higher even harmonic circulating currents. In particular, the sixth harmonic (300Hz in the U.K.) circulating current should be zero, since kW6 is zero. The magnitude of the circulating second harmonic current depends strongly on the location of the shorted turn because of the sin(2,13) term that appears in equation 1, and so will be smailest for shorted turns in the slots farthest away from the pole face, (ie, near the quadrature axis, where ss is nearly 90". Using numerical values typical of large alternators, the above equations lead to a value for the rms second harmonic circulating current that varies between 2.5% and 15% of the rotor current, for various locations of a single shorted turn. Expressed differently this amounts to somewhere between 0.5% and 2.5% of the fundamental stator current (between a few tens and a few hundred amps in a typical machine).
In tests, current transducers to monitor circulating currents have been installed in three alternators, here called A, B and C. Alternator A was known to have a fault-free rotor, since it also had an air-gap search coil installed which gave no indication of a fault. Both alternators B and C were suspected to have shorted turns on their rotors, from the results of recurrent surge oscillograph tests, and their history of bearing vibration levels. In both cases the recurrent surge oscillograph test indicated a possible fault in the windings in the slot farthest from the pole face, where the circulating current detection technique is least sensitive.
For generator A, the rms circulating 100 Hz current, measured both at open circuit, and near full load, was less than 0.12% of the rotor current. For alternator C the rms circulating 100 Hz current was about 2.2% of the rotor current, while for alternator B it was about 3.0% on short-circuit, and 2.2% on load.
These results are consistent with alternators B and C each having a single shorted turn on one pole of the rotor in the slot farthest from the pole face. The measured circulating 100 Hz current in alternator A was very much less than would be caused by a single shorted turn.
Table 1 shows the first four even harmonic circulating currents for alternator B at one load condition.
These figures confirm the statements made earlier that the 100Hz circulating current should be the largest, and the 300Hz current should ideally be zero.
TABLE 1: CIRCULATING EVEN HARMONIC CURRENTS IN ALTERNATOR B, NEAR FULL LOAD Frequency Fraction of Rotor Current 100 hertz 2.2% 200 hertz 1.2% 300 hertz 0.005% 400 hertz 0.15% Circulating currents in the stator winding may arise from causes other than shorted turns in the rotor windings.
The alternator rotor may have a static eccentricity due partly to a small relative misalignment of the rotor and stator, and partly to the self-weight bending of the rotor. This static eccentricity causes an assymmetry in the flux density that rotates at half rotor speed and thus induces a circulating current at 50 Hz. Rotor vibration (or dynamic eccentricity) causes an asymmetry in the air-gap flux density which can be represented as a second harmonic component of flux density rotating at rotor speed. Thus it gives rise to a 100Hz circulating current, which will be proportional to vibration amplitude, but virtually independent of rotor current, at fixed terminal voltage. Simple estimates indicate that the magnitude of this current at full load conditions is likely to be small compared with the circulating current due to a shorted turn.
Core faults (ie, electrical breakdown between stator core laminations) and shorted turns in the stator winding will give rise to circulating currents at 50Hz. A simple calculation suggests that the circulating 50Hz current for any credible size of core fault is small compared with the 50Hz current due to typical residual static eccentricity.
If the current transducers used in the two half-phase windings do not have identical gains, then there may appear to be a circulating current when none actually exists, particularly at 50Hz. There is a 100Hz component of current in the stator output current, due mainly to the television load on the power system, which would give rise to an apparent 100Hz circulating current if the transducer gains are not matched. If the gains are matched then this "straight through" 100Hz current will give no indication of a circulating current.

Claims (15)

1. Apparatus for detecting a rotor winding fault in a two-pole alternator having each phase of its stator divided into two parallel connected half-phase windings, the apparatus comprising two current transducers for sensing the respective currents in the two half-phase windings of one of the phases of the stator and means for comparing the two respective currents to detect equal and opposite currents in the two half-phase windings corresponding to current circulating around between the two half-phase windings.
2. Apparatus as claimed in Claim 1 wherein said means for comparing includes means indicating the magnitude of the circulating current component at at least one even harmonic of the generated supply frequency.
3. Apparatus as claimed in Claim 2 wherein the indicating means is arranged to indicate the magnitude of the second harmonic current component.
4. Apparatus as claimed in Claim 2 or Claim 3 wherein the indicating means comprises a spectrum analyser.
5. Apparatus as claimed in any preceding claim wherein each current transducer is formed as a coil wound on a non-magnetic former.
6. Apparatus as claimed in Claim 5 wherein each current transducer comprises a helical coil on a nonmagnetic former arranged for encircling a conductor of the respective half-phase winding.
7. Apparatus as claimed in Claim 6 wherein the former is toroidal.
8. Apparatus as claimed in Claim 6 wherein the former is flexible.
9. A method of detecting a rotor winding fault in a two-pole alternator having each phase of its stator divided into two parallel connected half-phase windings, the method comprising sensing the respective currents in the two half-phase windings of one of the phases of the stator and comparing the two respective currents to detect equal and opposite currents in the two half-phase windings corresponding to current circulating around between the two half-phase windings.
10. A method as claimed in Claim 9, and including the further step of indicating the magnitude of the circulating current component at at least one even harmonic of the generated supply frequency.
11. A method as claimed in Claim 10, wherein the magnitude of the second harmonic is indicated.
12. A method as claimed in either Claim 10 or Claim 11, wherein the frequency spectrum of the circulating current is analysed.
13. A method as claimed in any of Claims 10 to 12, and including the further step of estimating from said indicated magnitude the location of the rotor winding fault.
14. Apparatus for detecting rotor winding faults in two-pole alternators, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
15. A method of detecting rotor winding faults in two-pole alternators, substantially as hereinbefore described.
GB08408904A 1984-04-06 1984-04-06 Detecting rotor winding faults Withdrawn GB2157005A (en)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0258023A2 (en) * 1986-08-26 1988-03-02 Era Patents Limited Method and apparatus for testing electrical equipment
EP2728367A1 (en) * 2012-11-01 2014-05-07 ABB Research Ltd. A method for detecting a fault condition in an electrical machine

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN112557901A (en) * 2020-12-01 2021-03-26 重庆邮电大学 Precise micro-motor detection device and method based on multiphase magnetoelectric induction

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0258023A2 (en) * 1986-08-26 1988-03-02 Era Patents Limited Method and apparatus for testing electrical equipment
EP0258023A3 (en) * 1986-08-26 1989-02-08 Era Patents Limited Method and apparatus for testing electrical equipment
EP2728367A1 (en) * 2012-11-01 2014-05-07 ABB Research Ltd. A method for detecting a fault condition in an electrical machine
WO2014067742A1 (en) * 2012-11-01 2014-05-08 Abb Research Ltd A method for detecting a fault condition in an electrical machine
CN104823064A (en) * 2012-11-01 2015-08-05 Abb研究有限公司 Method for detecting fault condition in electrical machine
US10088506B2 (en) 2012-11-01 2018-10-02 Abb Research Ltd. Method for detecting a fault condition in an electrical machine

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