Edoc 12 Robles Edgar Field_diagnosti

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FIELD DIAGNOSTIC TESTING OF POWERGENERATORS AND TRANSFORMERS USING

MODERN TECHNIQUES

Edgar Robles

Instituto de Investigaciones ElctricasAv. Reforma 113, Temixco, Morelos, Mxico

[email protected]

Abstract

It is of great importance to define the operatingconditions of the main power electrical equipment.On-line and off-line partial dischargemeasurements are a powerful tool to achieve thisgoal. Some experiences on the evaluation of highvoltage generators are described in this paper,where discharge patterns can define the type ofdeterioration mechanisms. The actions taken aftersome critical generator evaluation are also

discussed.

Acoustic partial discharge has been recorded onpower transformers, the technique is brieflydescribed and some results associated with thedeterioration mechanisms are presented. Theapplication of some off-line techniques to findtransformer adverse conditions are alsopresented.

1. Introduction

The Electrical Equipment Department of theInstituto de Investigaciones Elctricas (Electrical

Research Institute) has concentrated its efforts onfield testing of power equipment. It has beenimplementing testing techniques to support themaintenance engineers to define deteriorationmechanisms and take corrective actions.

The work has been done for the Comisin Federalde Electricidad, CFE (the Mexican Electrical Utility)and for PEMEX (the Mexican Oil Industry). CFEoperates hydroelectric generators from 30 to 350

MW, and steam power generators from 30 to 600MW. The power electrical network operates at 230and 400 kV.

PEMEX generates its own electrical energy oneach main working center, oil refineries orpetrochemical plants. They generate at 13.8 kV onunits from 30 to 50 MW. They have also a 115 kVfeeder from CFE to attend emergency conditions.

This article presents some of the results on field-testing and techniques used to determine theconditions of power generators and transformers.

2. Generator Field Diagnosis

The reliability of power generators is of primeimportance to the electrical utilities. It is essential todefine the working conditions of an electricalgenerator operating under service stresses, withspecial reference to the insulation of the windings. Aseries of testing techniques has been developed toassess a generator at standstill. The Electra Review/1/ published a summary of the most commonprocedures for diagnostic testing.

Modern instrumentation, mainly digital partial

discharge detectors, allows performing on-linediagnostic testing of power generators. This hasthe advantage to have the normal electrical stressdistribution and the influence of mechanical andthermal stresses on the stator windings.

The insulation system that has been used in thestator windings is mica with a bonding compound.In the early years asphalt was used as a bondingmaterial with mica flakes. In the fifties the binder

Workshop 2000, Alexandria, Virginia, 13 & 14 September 2000

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changed to polyester resin. Modern materials usemica powder with an epoxy binder.

The main insulation is applied to a pre-shapedstranded conductor. Mica tape is half-lapped inlayers around the conductor strands to achieve therequired insulation thickness. Two types of insulationtapes can be used depending on the manufacturingprocess. If vacuum impregnation is available, then

the mica tape used is made on glass cloth with asmall amount of adhesive. Once the tape is applied,the bar is subjected to a vacuum to remove moistureand volatile materials contained in the mica tapelayers. The bar is then impregnated with an epoxyresin and pressurized to allow the resin to permeateinto all the small interstices. The resin-impregnatedbar is shaped as required and heated for curing.

If a vacuum impregnation process is not available,then the insulation tapes consist of a mica tape fullyimpregnated with epoxy resin. The resin content isabout 46% of the total weight. Woven glass cloth isnormally used as the supporting fabric. The tape is

applied either manually or with an automatic tapingmachine. The bar is then consolidated under heatand mechanical pressure to obtain the finaldimensions and shape. During manufacture, theresin is taken to the `B' stage resulting in a flexiblematerial with a volatile content of less than 1.0%.The resin flows under low consolidation pressures atelevated temperatures. This characteristic isparticularly important when good consolidation ofstator coil end-windings insulation is required andonly low external pressure can be applied.

3. Resonant transformer for AC testing

Typical capacitances of turbogenerators arebetween 0.1 F and 0.3 F per phase, for 30 MW to300 MW generators respectively. The capacitanceof a 400 MW hydro-electrical generator can be ashigh as 1.2 F per phase. A resonant transformer isrequired to excite a generator winding with an ACvoltage. There are two advantages when a resonanttransformer is used. As the inductor is tuned to be inresonance with the capacitance of the winding, therequired input power of the transformer and voltageregulator are significantly reduced. As aconsequence, the size and weight of the equipmentdiminish significantly. The equipment provides anoise free output voltage with greatly reducedharmonics, mainly when a series resonance circuitis implemented.

A variable frequency resonant transformer wasespecially designed and built for stator windingtesting. The fixed high voltage reactor is made oftwo coils encapsulated in epoxy resin. The coils canbe connected in series or in parallel, depending onthe type of generator under test. An output voltage

of 30 kV can be obtained with both windings inseries on a turbogenerator winding (300 nF load).An output voltage of 15 kV can be supplied to a load

of 2 F (large hydrogenerator).

The variable frequency supply was made of a three-phase rectifier with a filter and a single-phaseinverter. The output frequency can vary from 40 to120 Hz. The output voltage is controlled with the

width of the pulse of the variable frequencyelectronic source. This is an advantageous featurebecause it reduces considerably the weight and sizeof the resonant transformer. The variable frequencyresonant transformer is shown in Fig. 1.

4. Stator PD measurements

Partial discharge measurements and detection aredifficult to carry out in the field due to the high noiselevel normally present. All kinds of precautions toavoid noise should be taken into consideration whenperforming this test. When the generator useshydrogen as a cooling gas, measurements atdifferent pressures should be performed to clarifywhether the discharges are internal or external.When the partial discharge test is performed, theinception and extinction voltage should be recorded

at the circuit sensitivity.PD detection is a powerful tool to identify many ofthe deterioration mechanisms of electricalgenerators /2/. The oscilloscope display should becarefully studied to determine the type of dischargeoccurring in the sample. For example, if there is anasymmetry between the discharges in the positiveand the negative half cycle, then it is likely that slotdischarges might be occurring. The slot dischargesare very harmful to the machine insulation. It has

Fi . 1Variable fre uenc resonant transformer

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been observed during field testing that the possibilityof slot corona discharges can be detected at testvoltages close to the inception voltages. At voltageabove the inception the phenomenon tends to beoverridden by discharges at other sites.

A contaminated stator winding can be identifiedthrough partial discharge measurements.Contaminated generators produce external

discharge activity at the endwindings. Contaminationdeposits affect the electrical field distribution of thesilicon carbide coating. The discharges produceozone that attacks the insulation and in the longterm loosens the winding. This problem occurred inair-cooled generators operating in oil refineries andpetrochemical plants and on vertical hydroelectricgenerators that are affected with residues from thebrakes.

Direct cooled hydrogen generators use a resistor tokeep the cooling ducts at the same potential thanthe main conductor. A failure of this resistor caninduced external discharges that can generate a

catastrophic failure /3/. On-line and off-line partialdischarge measurements can detect defectiveresisitors and prevent this failure mechanism tooccur.

Damaged stator cores by abnormal operatingconditions give rise to hot spots that affect theinsulation and induced internal discharges. On-lineand off-line partial discharge measurements candetect this type of defect.

The use of a discharge probe to locate specificdefective areas is very efficient during off-lineinspection. Typical defects on the stator windingscan be (i) internal discharges within the insulation,(ii) external discharges at the endwindings producedby improper stress- relief at the endwindings or (iii)

slot discharges due to the loss of the conductivepaint.

A discharge probe with digital storage capability wasdesigned to ease the recording of the dischargelevel of each slot. The equipment stores thedischarge level of three generator positions, (theexciter side, the turbine side and one in the middlearea of the stator. Data is unloaded to a personal

computer trough a serial port. A view of thisinstrument is shown in Fig. 2.

5. Power Transformer Field Diagnosis

Oil impregnated paper, the insulation system ofthe power transformers, is quite susceptible topartial discharge activity. The discharge level thatis allowed in a transformer during the inducedvoltage test is limited to 250 pC. The inducedvoltage test is performed at two times the ratedvoltage. There is no doubt that the powertransformers are properly tested in the laboratory,

but it is necessary to take into account that thetransformer might be damaged duringtransportation and commissioning. The oil has tobe drained out from the tank and the bushings andaccessories are dismantled to move thetransformer to the field. It is possible that somemalfunctions may occur due to inadequatetransportation.

Power transformers are under intenseelectromechanical stresses in abnormal operatingconditions. If there is a failure in the transmissionline or other substation equipment, the transformerwindings can be mechanically deformed and

induced partial discharge activity. Power plant stepup transformers can be stressed during accidentalout of phase synchronization.

Another source of partial discharge activity onpower transformers is caused by the deteriorationand aging of the components. The wear and tearof the oil pumps can introduce metallic particleswithin the transformer that might be trapped in anintense electrical field zone and induced partialdischarge activity. Tracking of the insulatingstructure of the tap changer can also be a sourceof partial discharge activity.

Partial discharges can be detected with either

electrical or acoustical methods. Electricalmethods are more sensitive than the acoustical,but it is not easy to achieve a good sensitivity dueto corona discharges and other noise sources.Acoustical methods are less sensitive; they candetect mainly arcing processes within thetransformer. But it is easy to apply the techniqueand to correlate the type of damage that might beoccurring within the transformer. A goodknowledge of the construction of the transformer is

Fig. 2 Digital slot discharge meter with storage

capabilities


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required to correlate the results with thedeterioration mechanism.

6. PD Acoustic technique fundamentals

Partial discharge activity within the transformergenerates a pressure wave that propagates fromthe source to the tank walls. The ultrasonic

sensors, externally clamped can register them.The mechanical signals are converted intoelectrical signals that are amplified and plotted.The signals are proportional to the energycontained in the shock wave. In general, two typesof wave shapes are obtained. The arrow headshapeand the egg shapeenvelops. The type ofenvelope depends on the attenuation of the signaldue to the propagation path /4/.

The signal, with an arrow headenvelope, shownin Fig. 1, is obtained when there is a minimalattenuation; the propagation path is mainlyinsulating oil. The egg shapedenvelope, shown

in Fig. 2, is obtained when the ultrasonic signal ispropagated through different layers of solidmaterials (iron, copper, insulation, etc.) Theanalysis of the obtained wave shape permits todetermine the location and type of PD generatingsource.

PD ultrasonic signals have a duration between

50s and 2 ms, a frequency bandwidth from 100kHz to 200 kHz and characteristic frequency of160 kHz. The voltage amplitude depends on thedistance between the source and the sensor; thepath and the discharge intensity.

Fig. 3 Arrow head acoustic PD type of signal.

Fig. 4 Egg shaped acoustic PD type of signal.

The PD acoustic measuring system is made ofultrasonic sensors with internal pre-amplifiers, a

signal conditioning and an amplifier, a digitaloscilloscope with printer and the connecting wires.The sensor is a piezo electric transducer with anoperation frequency range (for longitudinal waves)from 70 to 200 kHz and a resonant frequency(maximum sensitivity) of 150 kHz. The sensorspre-amplifier gain is 40 dB and matching

impedance of 50 .

The conditioning stage has eight data acquisitioninputs that are protected against electromagneticinterference. All the system is interconnected bydouble shielded cable with characteristic

impedance of 50 . The maximum length of the

connecting cables is 30 m. A view of theinstrumentation is shown in Fig. 3.

Fig. 5 PD acoustic equipment set up.

The sensors are placed on each wall of thetransformer tank and PD acoustic activity isregistered on each wall. The signal with theshortest arriving time and greater amplitude will


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consequently have a shorter distance to the pointof the partial discharge source.

After locating the point of failure, it is necessary toanalyze the design drawings of the transformer todefine the severity of the problem. This informationpermits to take appropriate actions or carry outadditional tests or even to make an internalinspection of the transformer.

7. Field test results

Many transformers from the 400 kV MexicanElectrical Grid had been assessed with the PDacoustic technique. Floating core transformerswere found; high contact resistance in the lowvoltage side of the step up unit transformers werecorrected after being diagnosed; transformers withdeteriorated tap changers were found and properlymaintained. Some relevant results are presentedto show the benefits of the PD acoustic technique.

CASE 1. One of the transformers of a 75 MVA

400/115 kV bank, shell type, had a 2000 ppmhydrogen concentration. Ultrasonic detection wascarried out and found that heavy arcing wasoccurring inside the transformer, typical displays ofthe obtained signals are shown in Fig. 6. It wasdecided to take the transformer out of service,then the oil was drained to make an internalinspection.

A large amount of conducting particles (bronze)were found inside the transformer. The particlescame from worn down oil pump bearings. Theseparticles were concentrated in high electric fieldareas of the transformer and produced heavy

arcing that was detected trough the PD ultrasonicemission technique.

Fig. 6 Typical display obtained on the bronze particlescontaminated transformer.

CASE 2. It was programmed an ultrasonic test ona single-phase 100/125 MVA, 400/230/13.8 kVautotransformer, as part of the maintenanceprogram. High ultrasonic activity was detected onthe wall of the main tank adjacent to the On-LoadTap Changer (OLTC). Typical signals are shown inFig. 5. The PD activity came mainly from theregion of the OLTC diverter switch tank. As an

additional test, DGA was performed to the diverterswitch oil. Results indicated slightly abnormalvalues, considering the reduced number ofoperations made.

The OLTC was disassembled for inspection andthe pressboard insulated structure that supportsthe diverter switch mechanism was found verydeteriorated with carbon tracking. The degradationwas originated in the neutral connector. Fig. 8shows one of the deteriorated insulated supportswhere tracking can be observed.

Fig. 7 Oscillograms from a deteriorated diverter switch.Damaged insulated support is also shown.

CASE 3. A failure occurred in a step up unittransformers of a 107 MVA, 20/230 kV. It wasdecided to inspect the other three transformersincluding the spare one of the bank. Thetransformers are of a core type. Ultrasonic activitywas detected on the wall adjacent to the highvoltage bushing. The registered ultrasonic signals

are shown in Fig. 6.It was concluded that the PD activity came fromthe high voltage lead of the bushing. The unit wastaken out of service for an internal inspection. Amechanical problem between the high voltage leadand the equipotential ring was found. Thedischarges were occurring between the HV leadand the stress relief ring.

Carbon

tracking


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Fig. 8 Signals from a transformer between the HV leadand the stress relief ring.

CASE 4. During the commissioning of a230/138/13.8 kV, 25 MVA, single-phaseautotransformer, an audible noise coming from

inside the transformer was detected. Thecommissioning was temporarily suspended todetermine the noise source. A DGA analysis wasmade in oil samples from the main tank, the OLTCtank and the Buchholz relay deposit. The resultsshowed an abnormal amount of CO in the OLTC,indicating a process of burned cellulose. PDacoustic discharges were also recorded.

The autotransformer was energized through the13.8 kV tertiary winding with a distributiontransformer. Heavy arcing from the OLTC tankwas noticed. When the internal inspection wascarried out, a large amount of accumulated carbonwas found, mainly on the diverter switchresistance compartment (on the fixed contact areaand on the connector of the diverter switch lowerring). The OLTC insulated cylinder was alsotracked. Both defects are shown on Fig. 7.

8. Conclusions

Field testing of power equipment can be veryhelpful to avoid catastrophic failures and the lossof critical equipment. Techniques that werereserved for laboratory environments can be nowsuccessfully applied in the field. On-line testing isparticularly attractive to identify deterioratedequipment as it is under combined stresses.

References

/1/ Schuller, R. H. Report on diagnosis andmonitoring for evaluating the conditions of

windings of rotating electrical machines. ElectraReview, International Conference on Large HighVoltage Systems N 112, 9-16, May 1987.

/2/ Robles E. Garca A. Reyes O. Deteriorationmechanisms of electrical generators.

/3/ Robles Medina A, E. Lpez E. Description of aviolent failure of a 346 MVA generator. AnnualDoble Client Conference, 1995

/4/ IEEE PC57.127/D2, Trial use guide for thedetection of acoustic emissions from partialdischarges in oil-immersed transformers, 1989.

Trackedsurface

Carbondepositscontamination

Fig. 9 Damaged insulation support of the OLTCand carbon deposits contamination


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Edoc 12 Robles Edgar Field_diagnosti