Some Diagnosis Methods for Voltage Source InvertersIn Variable Speed Drives with Induction Machines

A Survey

Friedrich W. FuchsUniversity of Kiel

Kaiserstr. 2D-24143 KielGERMANY

fwf@tf.uni-kiel.de

Abstract – Status monitoring and performance diagnosisfor variable speed ac drives today is a need, more or less,depending on their application. Diagnosis can help toavoid unplanned standstill, to make possible to run anemergency operation in case of a fault or to keep the timeto repair short in case of a fault. For the voltage sourceinverter several faults are possible. In this paper, thesefaults and their diagnosis are covered. Possible faults andremedial stategies are listed. It is enumerated, whichfaults in today’s standard protection systems are diag-nosed. The main faults, in general not covered in thesesystems, are the transistor open circuit fault and the dcbus capacitor fault. In these fields diagnosis methods areunder reseach. Some of the various reports of researchgroups in this field are outlined here as a survey withrespect to function and properties.

I. INTRODUCTION

Automation of industrial processes has led to comprehen-sive electromechanical systems, where the electrical drive isimportant and in parts is the central component. Their pro-duction and operation costs are high. Therefore, the costs ofplanned standstill of these automation systems can be high,for unplanned standstill often much higher than for plannedstandstill. Breakdown or standstill of a single of dozens ofdrives often leads to the complete malfunction of the system.

Thus, for an economic operation a high utilization overtime and short standstill times are required. This leads to theconsequences, that in case of inevitable maintainance onlyshort interruption times are allowed, unplanned standstill andfaults have to be kept to a minimum and in case of possiblefaults short time to repair is necessary. Monitoring and diag-nosis as well as preventive maintainance are valuable meansto achieve this. All this depends on the economical condi-tions for the given system and its application.

The most common drive in industry is that with voltagesource inverter and squirrel cage induction machine. Diagno-sis of constant speed induction machines has been investi-gated for a long time and is well documented [1] and iscommonly employed, generally by spectral analysis of me-chanical and electrical signals. Applying these methods toconverter fed machines seems possible, but there are only afew reports on results. Regarding diagnosis of converters,

especially the voltage source pwm inverters, in comparisonto electrical machines, there are a lot of components andfaults to be diagnosed. Some of them are easy to diagnoseand their diagnosis today is included in standard protectionschemes. For others, diagnosis is difficult and not included inindustrial converters. Investigations, resulting in lots of dif-ferent methods presented in papers, have been carried out.

This paper gives a survey on function and properties ofthe diagnosis methods for voltage source inverters for induc-tion machine drives with squirrel cage rotor. Typical faultsare enumerated and the general diagnosis procedure is ex-plained in section II. Section III comprises the diagnosismethods covered by the converter standard protection sys-tem. In section IV the transistor open circuit fault diagnosis isoutlined, in section V the diagnosis of open circuit diodefaults in the rectifier is presented. Section VI contents thediagnosis of growing faults and operation in fault mode. Inchapter VII the dc bus capacitor diagnosis is outlined, chap-ter VIII contents the conclusion.

II. FAULTS AND DIAGNOSIS IN VOLTAGE SOURCEPWM POWER CONVERTERS

A. Faults

The basic drive with voltage source inverter and squirrelcage induction machine can be subdivided into the powerconverter, consisting of the machine side pwm inverter, thedc bus capacitor and the mains side diode rectifier, inductionmachine with squirrel-cage rotor and the control for thepower converter and the drive.

Fig. 1: Variable speed ac drive with induction machine andvoltage source inverter

IM

T1 T2 T3

T4 T5 T6

D1 D2 D3

D4 D5 D6

Converter + Drive Control

The mains side can equally be equipped with a pwm rectifier,of which the diagnosis is quite similar to that of the pwminverter.

The possible most important faults in voltage-fed invert-ers, either with mains side diode or transistor rectifier, forinduction motor drives can be classified as following [2, 3]:- ac line fault, single line to ground, line to line, missing

line;- dc bus fault, earth fault, capacitor short circuit fault,

voltage limiting transistor fault;- power semiconductor fault (transistor or diode in

inverter or rectifier) short circuit or open circuit;- dc link capacitor short-circuit fault;- sensor faults, ac current sensor, dc bus voltage sensor

etc.;- faults in the control equipment.

The percentage of these faults for a switchmode powersupply calculated on the base of a military reliability hand-book are given here, they can give some idea about the pos-sible distribution for the power converter: dc link capacitor72 (60) %, power transistors 10 (31) %, inductive elements 4(19) %, diodes 10 (3) % [8] (values in brackets for differentcircuit). Probably the high percentage for the capacitor iscaused by the circuit design and the high frequency applica-tion, but in general this indicates, that the capacitor is to betaken into account concerning faults. Another previous report[9] on faults of variable speed ac drives in industry from1995, with a rougher classification shows 38 % faults inconverter power part, 53,1 % in control circuits and 7,7 % inexternal auxiliaries. The high control circuit fault rate may becaused by limited reliability of early days pwm control. Aninvestigation of the effects of these faults on complete drivesystems is given in [2].

Faults in the drive can be classified into three differentgroups:- faults with total drive break down,- faults with emergency operation possible,- beginning fault with only small effects on the operation .Besides diagnosis there is another way to improve the faultbehaviour. To avoid faults or their consequences, accordingto the required reliability in the perspective application, forexample higher reliability in space applications, the designconcept has to be adapted, for higher reliability in general athigher costs:- a „fault free“ design, for example with redundancies,- a fault reduced design, for example with overdimen-

sioning, for an increased mean time to repair,- a fault tolerant design of the drive, for example with

possible emergency operation,- a design with increased protection measures, or- a standard design related to faults and protection, for a

required standard mean time to repair.

B. General Diagnosis Procedure

Diagnosis and monitoring in electrical variable speed acdrives usually is done by means of measuring, adapting andstoring several measured signals from the drive as therecould be voltage, current, flux, torque, vibration, speed or

similar signals. Some of these signals are usually measuredin a drive as they are necessary for control tasks. It would bevery economical, if this number would be enough for diagno-sis and monitoring. These signals are adapted to the applica-tion, i.e. filtered and often they are transformed as for exam-ple with Fourier Transform, Space Vector technique orSymmetrical Components. The next step is fault detectionand the last step is fault localization and defining actions tobe taken. For this, several techniques can be applied, mostspread are knowledge-based ones as pattern recognition[3,4], but also average modelling, expert systems [5], Neuralnetworks [6], Fuzzy logic or other kinds of Artificial Intelli-gence are used, often combined with fault classification orfault tree techniques [7].

Diagnosis here in general can be defined to run in threesteps:- data acquisition- fault detection- fault classification and determination of consequences.For modern power converters and for power converters inelectrical drives several reports concerning single aspects ofstatus monitoring and fault diagnosis have been published.There are only few reports on total inverter or drive moni-toring systems, as for example [Raison]. The field has beeninvestigated more deeply since about 1990, thus results arerelatively new.

III. DIAGNOSIS BY MEANS OF THE STANDARDPROTECTION SYSTEM

Today power converters in electrical drives designed forindustrial use are equipped with more or less comprehensiveprotection systems. They enable protection against distur-bances and execute a shut down in case of severe faults toavoid greater damage The quantity of functions depends onthe application. Basic protection functions which give diag-nosis results are:- fuses and fuse monitoring and drive shut down if a fuse

is blown, via sensors and central control; for fuses inmains supply line and for control eqipment as well asbase drive power supply;

- power transistor overcurrent or short circuit detectionwith transistor switch off and drive shut down or con-trolled transistor restart, via desaturation detection cir-cuits in the base drive and the main control;

- power transistor overvoltage detection and reduction, viacircuits in the base drive ;

- power converter overcurrent detection and reduction, viamain control;

- mains supply voltage monitoring and shut down in caseof undervoltage or missing line;

- power supply voltage monitoring and drive shut down incase of undervoltage.

This diagnosis functions of the standard protection sytem arepartly carried out in the main mikrocontroller control system,partly in control subsystems. Nevertheless, a modern powerconverter is equipped with a lot of additional protection andthus also diagnosis functions as for example fan diagnosis or

maximum current limitation according to the ambient tem-perature, which will not be presented here in detail. They cancount up to 50 or more functions.

Thus, a todays industrial drive is well equipped with di-agnosis functions, but there are still functions that in generalare not covered because of several reasons as necessary ad-ditional sensors or necessary higher computing power ormissing diagnosis methods. Regarding the fault distributionin a power converter given before as an example, there aretwo important diagnosis functions that have not been coveredby the ones listed above as there are power transistor opencircuit faults and dc bus capacitor faults.

IV. POWER TRANSISTOR OPEN CIRCUIT FAULTDIAGNOSIS

Power semiconductor faults in power converters are sub-divided into short cicuit and open circuit. Short circuit ofthese elements in most cases causes an overcurrent, detectedby the standard protection system, and a shut down of thedrive is carried out. Further operation is not possible, a repairis neccessary. The standard protection system in power con-verters today can locate the defective element, thus givinginformation for the repair.

A. Behaviour with Transistor Open Circuit

Voltage Source PWM power converters with an opentransistor fault show a typical current wave form in the timedomain, see figure 2 for steady state fault condition. Thecurrent in one phase becomes zero during a part of the periodand it is unsymmetrical. The converter continues to operate.Because of the irregular current waveform, the operation isaffected. The fault has to be detected and as a consequence,stop of operation or emergency operation, has to be decided.

Fig. 2: Converter ac current in case of transistor T1 open

circuit fault, variation with time [3]

If the three phase current signal is transformed into thetwo phase α,ß-space vector representation,

))t(ia)t(ia)t(i(32)t(ij)t(i)t(i R

2SR ⋅+⋅+⋅=⋅+= βα (1)

the current space vector trajectory is a circle in faultfreeoperation, figure 3.a. In case of an open transistor fault the accurrent space vector trajectory shows a typical fault pattern,

too, it becomes a semicircle, figure 3.b. The relative positionof the of the semicircle is characterising the defect transistor[3, 10].

is

isα

isβ

ρ

a.

b.

Fig. 3: Converter ac current space vector trajectory;a. fault free;b. transistor open circuit faults [10]

Fig. 4: Converter ac current space vector trajectory;transistor T2 open fault mode after fault free opera-tion, measurement [3]

For power semiconductor with open circuit fault, thereforethe diagnosis approach of ac current signature analysis(ASCA), either in time domain or in space vector representa-tion, is an appropriate one.

B. AC Current Space Vector Trajectory Slope Analysis

Localization of the fault can be done with the ASCAmethod by calculation of the slope Ψ of the diameter of thecurrent space vector trajectory [3]. With iαk, iβk... the sampledα,β-components of the current the slope can be calculatedfrom:

)1k(k

)1k(k

iiii

−ββ

−αα

−

−=Ψ (2)

This gives for a faulty transistor a constant value during thattime when the current vector takes the course on the diame-ter, the quantity, three values are possible ( ∞− ,3,3 ),

defines the faulty leg. The defect transistor in the leg, upperor lower one, can be detected by determining which alterna-tion of the current is missing in the faulty phase.

There is a simple calculation formula and a direct faultdetection to be stated.

C. AC Current Space Vector Average Quantity Approach

The fault signature in figure 3, middle, shows a charac-teristic waveform with an uncentered midpoint of the currenttrajectory, which is typical for the fault case. This is equiva-lent to dc components in the ac currents in case of fault,which are obvious from figure 2.

Thus, the fault occurrence detection and defect transistorlocalization can be done by calculating and analysing theposition of the current trajectory’s midpoint [11], whichmeans the mean value of the ac current space vector (Park´svector) over one period. This is done by at first calculatingthe mean value of the phase currents( ))t(i),t(i),t(i( TSR and then transforming the result into

an average value space vector acI according to eq. (1). Itsamplitude and phase angle characterise the fault. The meas-ured average space vector current data in table 2, left, wellconfirm the considerations on figure 3. Out of that theknowledge base for fault detection and localization can be setup, table 2, right.

TABLE IAVERAGE CURRENT SPACE VECTOR FOR TRANSISTOR

OPEN CIRCUIT ANALYSIS [11]

Experimental Results Derived Knowledge baseTran-sistor acNac I/I Θ/ °

acNac I/I Θ/ °

T1 1,59 181,04 >Ilim/IacN 150...210T2 1,68 240,01 >Ilim/IacN 210...270T3 1,75 298,25 >Ilim/IacN 270...330T4 1,75 0,41 >Ilim/IacN 330...30T5 1,52 58,9 >Ilim/IacN 30...90T6 1,46 114,54 >Ilim/IacN 90...150

The current limit for fault detection has to be well definedbetween zero and the minimum steady state fault current (0<Ilim/IacN <( acNac I/I )min) to enable early detection as well as

robust operation. For this method, the calculation is simple.Nevertheless, the use of a mean value could effect the timeresponse negatively.

D. AC Current Space Vector Instantaneous Frequencymethod

Another diagnosis method for open circuit transistors is theinstantaneous frequency method [3]. As the frequency of thecurrent space vector is zero when it is moving on the diame-ter of the semicircle at open circuit fault, see figure 3, thedetection of faults can be done by this feature. The instanta-

neous frequency fi of the ac current space vector can easily becalculated from the sampled currrent data by:

))k(i)1k(i)1k(i)k(i()1k(i)k(i

1T2

1fi αβαβ −−−⋅−⋅π

= (3)

For a test system, with this method a failure detection time of20 ms was achieved.

The formula is somewhat less simple compared to theones presented before. A direct detection is possible.

E. AC Current Space Vector versus Rotation Angle Fun-damental Method

Diagnose of an transistor open circuit fault and its loca-tion can also be done by analysing the difference of the am-plitude of actual and reference ac current space vector versusrotation angle[10]. The diagramm in figure 5 shows thiscurve for steady state fault. The irregularities in case of afault are detected by performing a Discrete Fourier Trans-form (DFT). The amplitude of the fundamental, which incase of a fault becomes greater than zero, indicates the faultcase. The phase angle of the fundamental indicates the posi-tion of the defect transistor.

Fig. 5: Deviation of ac current space vector versus rotationangle for open transistor fault[10] (ck amplitude ofcurrent space vector deviation value; k proportionalto the angle of current vector rotation, k = 32 equalto 360 degree)

This method with DFT is more complex. The fault detec-tion is not done in a direct way, but via the DFT for which asignal of minimum length (1 period) is necessary.

F. Comparison of AC Voltage Actual and Reference Quan-tity Approach in ‘Time Domain

Most methods take about one fundamental period to detectthe fault occurence. There have been also special methodsdeveloped to minimize the time between fault occurence anddetection [12]. One possibility is to analyse the ac side ormotor side voltages, terminal to star point (vkn, phase k, neu-tral n) or terminal to midpoint (vk0, 0 midpoint) of the dc linkcapacitors. They show irregularities at open circuit faults (vn0voltage machine neutral to midpoint of the dc link).

Fig. 6: Voltage waveform ac terminal to dc link midpointand detected fault flag [12]

These voltages can be monitored and these irregularitiescan be detected, which gives a direct location of the faultytransistor position as has been derived. A fault detection tablecan be derived, see table 2 (∆vin measured voltage ac termi-nal i to star point of load; ∆vi0 voltage ac terminal to midpoint dc link as result of open transistor).

TABLE 2VOLTAGE ERROR AND CORRESPONDING DEFECT

TRANSISTOR FOR OPEN CIRCUIT FAULT [12]

∆v1n ∆v2n ∆v3n DefectTransistor

2/3 ∆v10 -1/3 ∆v10 -1/3 ∆v10 T1-1/3 ∆v20 2/3 ∆v20 -1/3 ∆v20 T2-1/3 ∆v30 -1/3 ∆v30 2/3 ∆v30 T3-2/3 ∆v10 1/3 ∆v10 1/3 ∆v10 T41/3 ∆v20 -2/3 ∆v20 1/3 ∆v20 T51/3 ∆v30 1/3 ∆v30 -2/3 ∆v30 T6

This is also a kind of knowledge based procedure. Detec-tion times of one fourth of a period have been attained. Thereare several similar ways possible, based on other voltages.This method can also be transferred to a current based ap-proach [12]. As there is no current reference signal, in thatcase this can be obtained from a parallel running machinemodel. However, the analysed signal has conceptionally wellto be chosen to avoid incorrect detection in case of machineunsymmetry.

The detection is quite simple and direct. For this fast de-tection in some cases extra sensors are necessary, for exam-ple for the load star voltage.

G. Current Pattern Recognition Method in Time Domain

The fault modes of the complete converter system in figure 1can be characterized by patterns derived from the three phasecurrent waveform in time domain [4]. The pattern character-istic is defined by three parameters: the dc offset value, thepolarity of the average current value in the first quarter of a

complete period and in the second quarter. Each one can beequal to zero, greater than or lower than zero. In case of asemiconductor fault, nine of the total 27 parameters, calledflags, are set in a special way and enable the detection of afault. The distribution of the flags characterises the defectsemiconductor.

TABLE 3TYPICAL CURRENT PATTERN FLAG SET UP FOR OPEN

CIRCUIT FAULT IN TRANSISTOR 1 [4]

Here, many signals have to be processed. Additional

equipment as for example current zero crossing detection isnecessary. On the other hand, direct detection is performed.

H. Converter Behaviour Rules Diagnosis in Time Domain

For a VSI commutation cell, a phase leg of the converter,considered as a black box, the switching state can be deter-mined by means of obvious behaviour rules. For example seefigures 1 and 2: positive ac voltage means upper branchopen, positive current means transistor, negative means diodeis conducting. This determination is based on the measureddc bus current in each leg, the ac currents and ac voltages.This concept can be transferred to a three phase converter.

As the dc current for each leg, base for applying thismethod, is difficult to measure, its virtual quantity can becalculated by checking Kirchhoff’s laws for the currents forall possible switching states. This knowledge base of behav-iour rules has been implemented into an expert system andbeen proven to run in an off-line mode [13].

Fig. 7: Behaviour and fault detection scheme with beha-viour rules [13]

Though well defining the total switching behaviour of thepower converter, there is remarkable calculation effort andextra sensors are necessary for this method. An implementa-

tion into an online expert sytem should prove the function.The detection is direct.

I. Spectrum Analysis Method

Some remarks concerning spectrum analysis methods, forconverters, widely used for diagnosis in electrical machines[1], are given in [14]. For an open transistor a characteristiccurrent spectrum is given and can be analysed for detecting afault. Therefore Fast Fourier Transformation with high com-puting power is necessary and thus the detection is not adirect one.

VI. DIODE RECTIFIER OPEN CIRCUIT FAULTSDIAGNOSIS

A. Spectrum Analysis Method

Some remarks concerning AC Current Analysis: Mains sidediode rectifier faults, short or open circuit, result in typicalirregular space vector current trajectories in a way equivalentto the transistor inverter. These can be analysed for the faultdiagnosis [15, 16], in some way equivalent to the methodsfor transistor inverters or rectifiers. The properties are similarto that shown in the previous chapter. Therefore, this fieldwill not be deepened.

B. Symmetrical Space Phasor Components Approach

In fault free operation of a cyclical operating diode bridgeconverter circuit, voltages and currents are symmetrical. Incase of faults they become asymmetrical. By means of repre-sentation of the electrical quantities in symmetrical spacephasor components, a combination of space phasors andsymmetrical components, the symmetrical and asymmetricalcomponents can be separated [17]. Representation of electrialquantities in symmetrical space phasor components leads viathe space vector, see equation (3), to homopolar or zerocomponent )t(v )0( , positive sequence component

)t(v )1( and negative sequence component )t(v )2( :

)3/T2t(v)3/Tt(v

)t(v

aa1aa1111

31

)t(v)t(v)t(v

2

2

)2(

)1(

)0(

+

+⋅= (4)

For a six pulse bridge configuration, a diode bridge isbeeing investigated, it is shown to be useful to extend thisrepresentation to six components. The trajectory of eachsymmetrical component space phasor has to be calculatedand analysed.

The trajectories show special patterns, nevertheless, de-tection of faults will be difficult and a high processing poweris necessary. The detection is not done directly, as patternsfor some periods are necessary for detection .

Fig. 8: Symmetrical Space Phasor Components Approach,current trajectories of a bridge with diode fault [17]

VI. DIAGNOSIS OF GROWING FAULTS ANDOPERATION IN FAULT MODE

To increase the reliability of power converters, not onlypost fault diagnosis is sufficient, but also diagnosis of grow-ing faults, especially in power semiconductors, is necessary.Growing faults arise for example from loosing of the bondwires. This results in higher collector emitter saturation volt-age. However, results presented in [18] show, that it is adifficult task to measure and detect this irregularities and it isopen if it will lead to reliable results. Therefore this field willnot be presented here more deeply.

In case of faults often it is neccessary to keep the driveoperating, maybe with decreased load capacity. Some re-search work on this field has been reported to enable andoptimize motor operation when fed from an inverter with onephase missing because of a fault, i.e. with a two phase feed-ing. The starting has been analysed and otpimised [19] aswell as a method to run the drive with a smoothed torquedespite of two phase feeding [20].

VII. DIAGNOSIS METHODS FOR DC BUSCAPACITORS

Dc bus electrolytic capacitors have a limited lifetime,which is often shorter than that of the power semiconductors.So, there is a big nee to includ them into the diagnosis. Dcbus capacitor failures can be subdivided into total breakdowns and growing faults. In the case of a total break down,which corresponds to a blown fuse of the capacitor, the con-verter will usually be shut down because of an insufficientsmoothing effect in the dc link. In case of a growing faultbeyond a limit to be defined, a preventive maintenance isnecessary to avoid further growing and following total breakdown.

The aging in the capacitor can be characterised on thebase of its increasing internal series resistance ESR(t),

))K273T

K4700exp(tk1()0(ESR

1)t(ESR

1−

−⋅⋅−⋅= (5)

(T: aging temperature, t aging time, ESR(0) resistance ESRat time t = 0; k constant depending on design and construc-tion of the capacitor)

which can be taken for diagnosis of the remaining lifetime[21]. The internal resistance increases remarkably duringlifetime of the capacitor. For high frequency operation aswith VSI pulse width modulation, the capacitors impedance

Z = ESR + jω⋅ESL + Cj

1ω

≈ ESR (6)

is mainly constituted by the internal resistance component.The stray inductance and capacitor component can be ne-glected.

uC,ac = ESR⋅iC (7)

Thus, the capacitor current, an ac current, causes an ac volt-age mainly at the resistive component of the capacitor.

Fig. 9: Current and rectified fundamental voltage in aswitch mode power supply at transient for new andaged capacitors [21]

By means of measurement over time of both the currentand the ac component of the dc voltage at the capacitor, theresistance ESR(t) and so its increase and capacitor aging canbe determined.

The influence of the ambient temperature and the caseheating with the additional influence of current and voltagehas to be regarded, too. To get the exact determination andavoid the disturbing influence of transient conditions, thefundamental of the ac voltage and current should be taken forcalculation.

Fig. 10: DC bus capacitor lifetime prediction via internalresistance [ 22];Calculation scheme for internal resistance by meansof voltage fundamental with influence of ambienttemperature, voltage and current

In a prediction model the future growing of the internalresistance ESR and the remaining lifetime, represented by alimit value for ESR, can be evaluated. Data acquisition anddiagnosing must be accurate, but must because of the slowaging process no fast computing is necessary.

VIII. CONCLUSION

A survey on some diagnosis methods for voltage sourceconverters in electrical drives with induction machines ispresented with respect to function and properties. The diag-nosis methods of the total converter are outlined shortly. Thefocus is put on the diagnosis of these converter components,where there is still a high activity of investigation, thus thestatus of technique still is not fixed. These are the open cir-cuit semiconductor faults and the dc bus capacitor faults. Inthis field, many different methods have been presented withpromising properties suitable for future applications.

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