2nd lesson faults in electrical machines - aalborg...

Introduction to monitoring and Introduction to monitoring and diagnosis of electrical machines and

apparatusapparatus

2nd lessonFaults in Electrical Machines

Ewen Ritchie Krisztina Leban

1. Introduction to course

2. Introduction to diagnosisg

3 O i f f l i id 3. Overview of fault incidence

4. Detection Methods

Lesson 1 5. Monitoring MethodsLesson 1

6 S & T d

g

6. Sensors & Transducers2

Lesson 2 ObjectivesLesson 2- Objectives• In Lesson 2, the student should learn about:

– Faults to be diagnosed on electrical machinesMeasurements to be made– Measurements to be made

– Diagnostic methods– Diagnostic IndicatorsDiagnostic Indicators– Examples of Results– Towards Reliable Diagnosisg

• A closer look at the different faults that mayA closer look at the different faults that may arise and the influence they may have on the signals collected.

2010-10-20 Ewen Ritchie and Krisztina Leban 3

Agenda – Lesson 2Agenda – Lesson 2• Induction machines

– Stator– Stator– Rotor

• Synchronous machines– StatorRotor– Rotor

• Mechanical faults

• Diagnosis by observing only one variable for instance:variable, for instance:– Current– Flux linkagesg


Induction Machines

Invented by Nikola Teslay1856‐1943

Induction machines – Stator faults arisingInduction machines – Stator faults arising• In the traditional induction machine, the

stator comprises a laminated iron core, with l tslots– An insulated winding is fitted in the slots– The individual turns of the winding are

insulated from each other• There is a winding for each phase

– Coils belonging to each phase winding are insulated from those of the other phase windingswindings

• All three phase windings are insulated from Earth

• Usually, the first sign of an insulation failure i h d d bl fis a shut‐down due to a blown fuse or a tripped earth fault protection relay

• This is a catastrophic failure as it causes unplanned downtimep

• Tesla’s version shown here, was a two phase system


What is a Short Circuit Fault?What is a Short Circuit Fault?

• One fault that occurs regularly is a short circuit in the stator• One fault that occurs regularly is a short circuit in the stator winding

• It is important to understand the mechanism of a "short‐circuit fault "circuit fault.

• A short‐circuit fault in an electrical machine winding is an abnormal condition where one or more coils unintentionally makes contact with another coil or earthmakes contact with another coil or earth.

• Due to the abnormal conditions, excessive current may flow in the short circuit fault causing local heating and insulation damagedamage

• An internally generated or induced EMF drives current in the short circuited portion of the winding and the impedance of the short circuitthe short circuit

• To understand the mechanism, we need to evaluate the EMF and the Impedance


Induction machines – Stator faults arisingg• A model may help us to

visualise and consider thevisualise and consider the problem

• At IET we considered a short circuit arising in a statorcircuit arising in a stator winding

• iDwas flowing in the short D circuited portion of the winding

• i’Dwas flowing in the shorti D was flowing in the short circuit

• We wanted to model the fault and to verify thefault and to verify the model


Induction machines – Stator faults arisingg

• We modelled the fault as an i di lextra winding electro‐

magnetically linked with the actual phase windings

iB

actual phase windings

• The short circuit is electro‐magnetically linked with ALL

i’D

magnetically linked with ALL other windings in the machine

iC

iA


Sample of simulation results – 1.5% of 1 stator phase shorted – No Load Load

• The 3 phase currents are f i l i il i li dfairly similar in amplitude– Around 5 Amp

• The fault current amplitude• The fault current amplitude is much larger– Around 50 AmpAround 50 Amp

• It is difficult to diagnose a stator winding short circuit gfault by measuring the supply current


Sample Experimental Results – 1.5% of 1 stator phase shorted

– No Load • Experimental verification gives a

similar resultsimilar result– Except the phase current is

much smaller

• The measured amplitude of theThe measured amplitude of the fault current is about 50 Amp

• But the unbalance in amplitude of the line current is about 0 5 Aof the line current is about 0.5 A

• Unbalance in the line current may be caused by many different problems external to the motorproblems external to the motor– e.g. unbalanced supply voltage

• Read [03‐01] Extract from Report St d f th Eff t f Sh tStudy of the Effects of Short Circuits in Stator Winding of Induction Machine


Experiment to study the effect of external faults on doubly fed induction machines

• Read reference [03‐2 and 03‐3]• Many wind generators employ doubly fed

i d ti hiinduction machines• Generators are subject to external faults

e.g. on distribution and transmission lines• Under normal duty, power is taken both

f h d d hfrom the stator winding and the rotor winding

• However, an external fault may cause unbalanced loading

• Rotor current supplied to the fault may be controlled by the power electronics converter

• It may be difficult to control the stator current

• Therefore it becomes interesting to study the effect of external faults in the doubly fed induction generator

• Some researchers propose that current signature analysis may be useful for detecting faults in the stator circuit


At IET we studied current signal analysis to diagnose faults in wind generator systems. wind generator systems.

– Experimental arrangementsL =(15 & 50) mHV ( )

Switch on / off

B

A

DFIGB

C

One stator phase inductive unbalance

A

Simulated turn-to-turn fault in one stator phase

DFIG

Switch on / off

B

one stator phase

One stator phase resistive unbalance

COne rotor phase resistive unbalance


Study of current signal analysis to diagnose faults in DFIG wind generator systems.

• Experimental Results, stator inductive and resistive unbalance

The stator currents acquired by TDS 540 under Stator currents acquired by TDS 540 under one stator The stator currents acquired by TDS-540 under inductive unbalance-Lv=50 mH at P=2 kW. The

unbalance stator current is on channel 2. Scaling factor is 4 [mV / A].

Stator currents acquired by TDS-540 under one stator phase resistive unbalance (Rv=10 ) at P=2kW and n=1475 rpm. The unbalance stator current is configured on channel 2. Scaling factor is 4 [mV / A].



The degree of unbalance exhibited in one stator phase, obtained by Introducing

th i d ti b l f 50 H (10 ti th th t t h i d t ) the inductive unbalance of 50 mH (10 times more than the stator phase inductance),

may be observed very clearly in the stator current.



– Experimental results, turn‐to‐turn fault

The rotor currents acquired by TDS-540 under turn-to-turn fault in one stator phase (Lv=50 mH in parallel ) at P=2 kW and n=1475 rpm. Scaling

factor is 4 [mV / A].

The stator currents acquired by TDS-540 under turn-to-turn fault in one stator phase (Lv=50 mH in parallel ) at P=2 kW and n=1475 rpm. Scaling

factor is 4 [mV / A].



The rotor current observed during this type of turn to turn fault exhibits almost balanced amplitude ,while the unbalance created in the stator currents is greater This indicates that the stator current may be a good indicator for the detection of rotor faults.



– Experimental results, rotor unbalance

The rotor speed and stator currents of DFIG acquired by ICS-645 under the resistive unbalance of 1.2 Ω in one rotor phase at 2kW.

The rotor currents of DFIG acquired by TDS 540 under the resistive unbalance of 1.2 Ω in one rotor phase at 2 kW. Scaling factor is 4 mV/A



MCSA to detect DFIG faults– Machine current signature analysis (MCSA) is a non‐invasive, online or offline monitoring technique for the

diagnosis of problems in induction machines, such as turn‐to‐turn fault, broken rotor bars, static or / and dynamic eccentricity;

– Due to its powerful technique merits in diagnosis and detection of faults the MCSA monitoring technique was chosen as a fault detection methodchosen as a fault detection method.

– The stator and rotor currents monitoring system that has been developed consists of three main sub‐systems. These include: signal conditioning, data acquisition and data analysis.

– Data acquisition and data analysis sections are accessed by Matlab software package run from a PC. The controlling software allows the operator of the system to store the data in data files for future processing.controlling software allows the operator of the system to store the data in data files for future processing.



– MCSA to Diagnose Stator Turn‐to‐Turn Faults

n=1,2,3,..;k=1,3,5,..



– The obvious change between balance and turn‐to‐turn fault is that completely new current components exist around 125 Hz (k=1, n=3) and 375 Hz (k=1, n=13) . In addition, if we follow the trajectory of the current phase angle (Is1), we may observe that it has changed around 125 Hz, where a new faulty component exist as well.


At IET we studied current signal analysis to diagnose faults in wind generator systems.

– MCSA to Diagnose Stator Winding Unbalance


At IET we studied current signal analysis to diagnose faults in wind generator systems.

• Comparison of the stator current spectra for resistive unbalance in one stator phase (R=10 Ω) and for inductive unbalance in one stator phase (L=50 mH) reveals that the inductiveand for inductive unbalance in one stator phase (L=50 mH), reveals that the inductive unbalance causes new components to appear in the stator current spectrum at around 175 Hz and 425 Hz, while the resistive unbalance introduces new components at around 125 Hz, 325 Hz and around 375 Hz



– MCSA to Diagnose Rotor Winding Unbalance

Frequency spectrum of stator currents and rotor currents of DFIG for rotor unbalance at 2 kW. FFT was computed and current spectrum were plotted using MATLAB software and getting via ICS 645.



The rotor line current spectrum, for unbalance in one rotor phase, contains more information about the effect of this fault than the stator line current spectrum does Either may be useful as an the effect of this fault than the stator line current spectrum does. Either may be useful as an indicator for detection of rotor winding unbalance. For resistive and inductive unbalance in one stator phase and simulated turn-to-turn fault in one stator phase, the stator line current spectrum holds more information.



– MCSA versus Instantaneous Power

Frequency spectrum of partial and total instantaneous power and stator current of DFIG for stator unbalance (a) and rotor unbalance (b) at 2 kW. FFT was computed and power

and current spectrum were plotted using MATLAB software and getting via ICS 645.



• Conclusion– The experimental results indicate that monitoring line currents in stator and

rotor ma be useful in identifying the presence of unbalance in one stator and rotor phase and exhibit the presence of harmonics related to a specific type of fault;

– The article indicates that the detection of different kind of faults may be possible using time and frequency domain analysis

– The spectrum of stator and rotor line currents gave promising results.Th i t l lt d t t th t MCSA b f l t di– The experimental results demonstrate that MCSA may be useful to diagnose turn‐to‐turn faults

– MCSA may also be useful to diagnose other problems in induction generators such as inductive and resistive unbalance in one stator and rotor phase.

– The instantaneous power seems to be an alternative diagnostic medium for the machine signature analysis.As this study was limited to simulating turn to turn fault placing an– As this study was limited to simulating turn‐to‐turn fault, placing an inductance in parallel to one phase winding, research intothe effect of real short circuits is certainly needed.


Model of Stator Inter-turn Short Circuit fault in Doubly-fed Induction Generators for Wind Turbine Read reference [03-2]

M h i l d l f h d bl f d i d i– Mathematical model of the doubly fed induction generator

asi'di di

asi

did d

bsi

csibsi

csi


Model of Stator Inter-turn Short Circuit fault in Doubly-fed Induction Generators for Wind Turbine

– Mathematical model of the doubly fed induction generator

LL

lTT

ULILdRLI

1

)( 11

rrrs

srss

LLLL

L

ADACABAA

MMLMMMML

le TTJ

rsrT

se IdLdIT

DDDCDBDA

CDssCA

BDssBAss

LMMMMLMMMMLM

L

Tcrbrardcsbsas iiiiiiiI

rrr

rrr

rrr

rr

LMMMLMMML

L

Tcrbrarcsbsas uuuuuuU 0

Trs

CcCbCa

BcBbBa

AcAbAa

sr LMMMMMMMMM

L

DcDbDa

CcCbCa

MMM

rrrDssA rrrrrrrdiagR ,,,,, ,



– Calculation of circuit element values

NN s

DA r

NNNr

1

1

AsD rrr

2MMMM sBAAB

3MMMM sCAAC

,3,

2,

1

210

2

22

1

2

pppk

ykDs

DDD k

kp

lNL

NN

L

14 kklNN

DDDAAD LMMM 1

DDDBBD LMMM 2

,5,3,1

12

1102

1

21

cos4

1k

wkykD jq

kkkk

plNN

M

12

1102

1

32

21

cos4

2 wkykD jq

kkkklNN

M

DDDCCD LMMM 3

22

11

2210

221

21

21cos2 wkwk

CbBbqqrk

kkk

plNNMM

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22 32k kp

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12

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cos4

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wkykD jq

kkkk

plNN

M

,5,3,1 22k kp

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22

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22

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k

wkykDDa

qjrkkkk

plNNM

DDsmsD

AA LMLLN

NNL

1

2

1

1



– Generator parameters

Parameter Value

Rated Power 11 kW

Rated Voltage 400 V

Frequency 50 Hz

Stator resistance 1.1

Rotor resistance 0.908

St t l k i d t 3 82 HStator leakage inductance 3.82 mH

Rotor leakage inductance 4.593 mHRotor leakage inductance 4.593 mH

Mutual inductance 99.77 mH



SIMULATION RESULT OF INTER TURN SHORT CIRCUIT FAULT

di

– SIMULATION RESULT OF INTER‐TURN SHORT CIRCUIT FAULTi d

i a s

i (A) di

as i

t(s) a) Three phase current and short circuit current id a)Three phase currents and short circuit current

st

dB

f(Hz) b) Sp ectrum curve of phase A current

b) Spectrum curve of phase A current

Hzf

Simulation result of inter-turn

short circuit fault ( α = 3.125%)

b) Spectrum curve of phase A currentSimulation result of inter-turn

Short circuit fault( )%25.6



SIMULATION RESULT OF INTER TURN SHORT CIRCUIT FAULT– SIMULATION RESULT OF INTER-TURN SHORT CIRCUIT FAULT• Phase angle difference analysis under inter-turn fault

Phase angle

Condition

AB BC CA

Condition

Healthy 120° 120° 120°

3 125 % 130 8° 113 6° 115 6°3.125 % 130.8 113.6 115.6

6.25 % 131.6° 110.6° 117.7°

9.375% 134.8° 108.7° 116.4°

12.5 % 135.6° 108.6° 115.8°

1. Short circuit current id is very large, and decreases along with fault percentage increasing.1. Short circuit current id is very large, and decreases along with fault percentage increasing.2. 150Hz and 450Hz current components appear in spectrum curve.3. The phase angle difference becomes unbalance.



• EMULATED STATOR INTER-TURN SHORT CIRCUIT FAULT

Phase A stator winding is parallel with a resistance R=25Ω.

) Th h ta) Three phase currents ia R

ib

ic

b) Phase A current spectrumSimulation result of emulated inter-turn

short circuit fault (R=25Ω)

Diagram of emulated inter-turn short circuit fault



• EKSPERIMENTAL SETUP

Freq.conv22kW

Grid

I d ti motor

Frequency converter

Control box of DFIG

DC link

22kWBack-to-backconverter

IG

Testsystem

u,i

ω

ω

Gear box

Gear boxInduction cage motorControl PC

IM Gear Gear

Model of windModel of wind generator

DFIG

Simple diagram of experimental

turbine Model of wind generator

Speed transducer

p g psystem

Fig.8 Laboratory Experimental system



• EKSPERIMENTAL RESULTSDFIG healthy

DFIG phase A parallel with R=25

i(A)

y

a) Three phase currentst(s)

a) Three phase currentst(s)

a) Three phase currents a) Three phase currents

dB

Frequency (Hz)Frequency (Hz)

b) Phase A line current spectrum analysis

Frequency (Hz)b) Phase A line current spectrum analysis



– CONCLUSION– A complete model to simulate inter‐turn faults have been set up

Emulation of inter turn faults are done by setting a resistance in parallel with– Emulation of inter‐turn faults are done by setting a resistance in parallel with phase A

• This is shown both by simulations and experimental results– Both simulation result and experimental result have two changes:

• 150Hz and 450Hz current components appear in spectrum curve.

• The phase angle difference becomes unbalance• The phase angle difference becomes unbalance.


Asynchronous machines – Rotor faults yarising

• In the squirrel‐cage rotor of an asynchronous machine the current links the magnetic polesg p

• A cracked or broken rotor bar or end‐ring will disturb this patternTh t b d ll d• The rotor may be modelled as a mesh of current paths

• These current paths induce magnetic fields that link with gall other windings on the machine

• It has been very popular to conduct research to detectconduct research to detect rotor faults in squirrel cage induction machines


Asynchronous machines – Rotor faults arisingarising

• In the squirrel‐cage rotor of an asynchronous machine the current links the magnetic polesthe magnetic poles

• A cracked or broken rotor bar or end‐ring will disturb this pattern

• The rotor may be modelled as a mesh of current pathscurrent paths

• The mesh may be rearranged to reflect local changes regarded as faults

• The magnitude and frequency of the rotor t d d th t l l fcurrents depend on the actual value of

the slip• The rotor mesh currents link and interact

with the air‐gap main flux



• Fault currents in the rotor cause characteristic frequencies to appear in both the current and the flux linkageto appear in both the current and the flux linkage waveforms

• Experimental flux spectra are shown for a motor with and without broken rotor bars s 0 05 f 50 Hzwithout broken rotor bars, s = 0.05, f = 50 Hz

• The amplitude of the harmonic of order (1‐2s)f exhibits a marked increase with the quantity of rotor faults.



• As the order of the new frequency caused by the fault is governed by the expression (1‐2s)f the possibility of detection is dependent on thethe expression (1 2s)f the possibility of detection is dependent on the actual value of slip

• For a small slip, the new frequency may be masked by the fundamental frequency if the discrimination of the FFT is insufficientlyfundamental frequency if the discrimination of the FFT is insufficiently fine

• Read [03‐4] Dynamical Model of Three‐Phase Squirrel‐Cage Induction Motor with Rotor FaultsInduction Motor with Rotor Faults


Synchronous machines

Synchronous machines – Stator faults arisingarising

• Stator faults arising in a synchronous machine are similarsynchronous machine are similar to those arising in an induction machine

• However the current excited

iBHowever, the current excited rotor will supply energy to the fault– As long as the field current flows

i’D

As long as the field current flows– As long as there is kinetic energy

stored in the rotor

• This is a different situation fromThis is a different situation from the Induction machine where the magnetising energy is supplied from the faulted stator system

iCfrom the faulted stator system– Here the fault may contribute to

dissipation of the magnetisationiA


Synchronous machines – Rotor faults yarising

• Faults arising on the rotor of a synchronous machine may includey– Loss of magnetising current

Short circuits arising in the magnetising winding– Short circuits arising in the magnetising winding

– Short circuits arising in the damper winding


Mechanical faults• Mechanical faults arising in

rotating electrical machinesrotating electrical machines are chiefly related to:– Bearings– Eccentricity

• Rolling bearings may become damaged or worn

C2

become damaged or worn• Eccentricity may arise

because of manufacturing C1gfaults, external accidental damage, or bending of shafts due to unsuitableshafts due to unsuitable mounting arrangements


Mechanical faults - eccentricityy• The eccentricity must be below

5% of the air gap in order to5% of the air gap in order to eliminate vibration problems due to magnetic pull.

• Eccentric machine– C1 is the centre of the rotor, and

C2 is the centre of the statorC2

– With static eccentricity, C1 is the centre of rotation

– With dynamic eccentricity, C2 is C1With dynamic eccentricity, C2 is the centre of rotation

– With mixed eccentricity, centre of rotation may be anywhereof rotation may be anywhere between C1 and C2.


Mechanical Faults - bearingsg

• Bearings may be plain b i lli b ibearings or rolling bearings

• Rolling bearings are widely used in industrialused in industrial applications

• We will consider only faults• We will consider only faults that may arise in rolling bearingsg


Mechanical faults - bearingsg• Many rotating electrical machines

are provided with rolling bearingsare provided with rolling bearings

• Rolling bearings are a complicated structure

• The outer ring or race is supported in a bearing housing

• The rolling elements are• The rolling elements are supported in a cage

• The inner guide ring or race is fitted to the shaft

• The seals keep dirt out and lubrication grease ing


Mechanical faults - bearingsg

• Any or all components of a lli b i ffrolling bearing may suffer

damage

• This damage may progress• This damage may progress from being incipient to being serious, and further,being serious, and further, to being catastrophic

• The photos show spherical p proller bearings with damage to the inner race and rollers


Diagnosis by observing only one Diagnosis by observing only one variable

As an example, we take the caseAs an example, we take the case of a short circuit in the stator winding of an induction motor g[03‐01]

Schematic diagram of the experimental systemsystem

• To observe any variable we need an experimental systemT i ll d t k th ti f ti

Phase 3

Frequency (Hz) 50• Typically we need to know the time function

of the machine supply variables, voltage current, torque, speed, etc.

• A typical experimental arrangement is as shown below

Output power [kW)

1.5

Efficiency 82%

Voltage (V)220-240shown below

• In this case we wanted to monitor what happened if a short circuit arose in the stator winding of an induction motor

Voltage (V)380-415 Y

Current (A)3.4-3.75Y5.9-6.5

Speed (r/min) 2860-2890

Power factor 0.85-0.79

Motor details


Schematic diagram of the experimental systemsystem

• An induction motor was provided with taps to enable partial short circuit of the

Phase 3

Frequency (Hz) 50p pstator winding to be applied at will

• All accessible signals were measured• The data acquisition system was with

Output power [kW)

1.5

Efficiency 82%

Voltage (V)220-240

simultaneous sampling on all channels– A signal conditioning box was used to

match the probes to the D/A converter

Voltage (V)380-415 Y

Current (A)3.4-3.75Y5.9-6.5

Speed (r/min) 2860-2890

Power factor 0.85-0.79

Motor details


Schematic diagram of the experimental psystem

P5200 i diff ti l lt

• All values were measured as functions of time for various fault• P5200 is a differential voltage

probe• PR30 is a current probe

functions of time for various fault values and motor loads

• This enabled a relationship to be drawn between the severity of

• RGF 75 is a current probey

the fault and any one of the various measured values

• Or a combination of these


Diagnosis by observing only one variable, e g Currente.g. - Current

• It was found that a short circuit producing a violently large short circuit current gave rise to a modest increase in the amplituderise to a modest increase in the amplitude of the shorted phase

• The current unbalance produced could easily come from other, comparatively harmless sourcesharmless sources

• It is clear that the winding cannot tolerate continuous operation when carrying the large fault current

• It would be interesting to reliably detect• It would be interesting to reliably detect even the smallest short circuit by observing the terminal values of current and voltage

• In this way catastrophic failures causingIn this way catastrophic failures causing loss of not only the motor itself, but also costly product, may be avoided

• Reliable detection is required to prevent spurious operation of the alarm/spurious operation of the alarm/ shutdown system as spurious operation would also cost loss of valuable product


Diagnostic Indicators

A diagnostic indicator is the time function of i bl l d f i ia variable selected for monitoring

This may be the current, the voltage, the flux linkages etcflux linkages, etc.

Current Signal Analysis (CSA) and Symmetrical Component Analysis

• Many researchers have ychosen to monitor the current as the diagnostic indicator

• Current is directlyCurrent is directly measurable thus simplifying the p y gmathematical calculations necessary yto obtain a diagnosis

• E.g. ref. [03‐5]g [ ]


Diagnosis by observing a derived variable, e g Fluxe.g. - Flux

• Researchers have also considered that derived variables makingthat derived variables, making use of two or more measured variables may carry more i f ti th b f d iinformation than may be found in a single variable

• One example of this is the flux ( )v i R dt linkages

• Here coherent measurements of both the voltage and the current

( )v i R dt both the voltage and the current are required in order to estimate the flux‐linkages

• This technique was used by Xiaolan Deng in ref. [03‐8]


ReviewReview

• Many different faults may arise in rotating electrical machines

• These may be electrical faults such as short circuits and open i it i th i di th b h i l f lt hcircuits in the windings, or they may be mechanical faults such as eccentricity or damaged bearings

• Research shows that many faults may be detected and• Research shows that many faults may be detected and identified by interpretation of measurements of the current and voltage, etc. made during normal operation of the g , g pmachine

• Later in this course you will consider some of the methods used to interpret the information held in the current or voltage measurement signals


2nd lesson faults in electrical machines - aalborg...

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