Wojciech Koltunowicz - Nagoya University · Corresponding author: W. Koltunowicz, ... i .f om5k Hz t2M F h ... has a total length of 340000-km (data 2000). 34.8% of

Proceedings of International Symposium on EcoTopia Science 2007, ISETS07 (2007)

Corresponding author: W. Koltunowicz, [email protected]

On-site Testing and Monitoring of Electrical Components

Wojciech Koltunowicz

IPH Institut „Prüffeld für elektrische Hochleistungstechnik” (CESI Group), Berlin, Germany

Abstract: The paper describes some of the recent fields of activity in CESI Group aimed at the determi-nation and application of the most effective diagnostic indicators and on the selection of the best diagnos-tic techniques and on-site test procedures for major electrical components, with particular reference totransformers, power MV and HV cables and HV GIS equipment.

Keywords: Electrical Component, Diagnostic Indicator, Diagnostic Method, On-site Tests

1. INTRODUCTIONMaintenance policies have evolved in the last years

more rapidly than ever before. The evolution from cor-rective maintenance to preventive maintenance was basedon the availability of diagnostic tools and monitoringsystems able to predict the progressive degradation of theconditions of components before the inception of failure.Since many years, CESI is involved in the optimisation ofmaintenance of electrical components, starting from thedetermination of the best diagnostic indicators, setting upcondition assessment techniques and applying optimiseddiagnostic tests on-site. The present paper describes someof the recent fields of activity in CESI aimed at the de-termination and application of diagnostic indicators andtechniques for power transformers, power MV and HVcables and HV gas insulated switchgear.

2. CONDITION ASSESSMENTS OF POWERTRANSFORMERS

2.1. GeneralWhen a transformer has undergone abnormal stresses

its ability to withstand further short circuits can be re-duced. It is therefore important to monitor two factors,which can affect power transformers reliability: insula-tion deterioration and winding geometrical deformation.Condition of the insulation can be derived from Dis-solved Gas Analysis (DGA) and from Recovery VoltageMeasurements. To evaluate winding mechanical integritythe Sweep Frequency Response Analysis method (SFRA)is very promising. The SFRA method was experimentedin CESI and the results were compared to those obtainedfrom conventional, bridge-method (usually SheringMaxwell bridge method), leakage reactance measure-ments, nowadays the biggest data source.

2.2. Sweep frequency response methodThe classical method adopted for checking the presenceof displacement and deformation of the windings is themeasurement of the leakage inductance, however, practi-cal experience has shown that only radial deformationscan be seen with this method, while axial deformationslinked with localised mechanical stresses on the windingextremities may not be detected. The SFRA method wasdeveloped to cope with this lack of sensitivity [1]. It isbased on the assumption that any mechanical deformation

is associated with a change in the capacitive-inductiveequivalent circuit and therefore is detectable through atransfer function. In the SFRA method, a sinusoidal sig-nal characterised by a frequency variable from 10Hz to5MHz is injected into one winding terminal, recordingboth the applied signal and the transferred one to theother terminal. The two signals ratio in the frequencydomain represents the transfer function. The experiencehas shown that in the evaluation of the test results, thebehaviour in three different ranges of frequencies has tobe considered:Frequency < 10kHz: in this range phenomena linked withthe transformer core and magnetic circuits are evidencedlike coil faults, winding interruptions, magnetic circuit’sproblems;Frequency in the range 5kHz to 500kHz: in this rangephenomena linked with radial relative geometricalmovements between windings are evidenced;Frequency > 200kHz; in this frequency range axial de-formations of each single winding are evidenced.The sensitivity of the SFRA method was checked on a400kVA MV/LV distribution transformer submitted toshort-circuit withstand tests. Checks with SFRA methodwere performed after each short-circuits test and werecompared to the results obtained with bridge inductancemeasurements. The measurements relevant to the threephases of the transformers before and after the short cir-cuit test series (6 tests) focused on the most significantfrequency range: i.e. from 5kHz to 2MHz. From theanalysis of diagram (measurements made on the delta-connected primary windings leaving floating the secon-dary terminals) three frequency ranges were chosen toevaluate the winding displacements: 8 kHz - 12 kHz, 45kHz - 75 kHz and 500 kHz - 800 kHz.The chart in Fig.1 (measurements within 8 kHz - 12 kHzband) focuses the attention on phase VU with referenceto the first resonance point at middle-low frequency, aftereach short-circuit test performed on the transformer.The chart in Fig. 2 (measurements performed in the range45 kHz - 75 kHz) focuses the attention on phase VU withreference to the second resonance point at middle-highfrequency, after each short-circuit test performed on thetransformer.

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-62 .0

-60 .0

-58 .0

-56 .0

-54 .0

-52 .0

-50 .0

8 0 00 8 50 0 9 00 0 9 5 00 1 00 00 1 05 00 1 1 00 0 1 1 50 0 1 20 00

F re q ue nza (H z)

Am

piez

za (

dB)

0 0 -1V U 1 01 -1 VU 1 0 2-1 V U 1 03 -1V U 1 0 4-1 V U 1 0 5 -1 V U 1 06 -1V U 1

Fig.1. Measurements in the range 8 kHz – 12 kHz

-3 1 .0

-3 0 .5

-3 0 .0

-2 9 .5

-2 9 .0

-2 8 .5

-2 8 .0

-2 7 .5

-2 7 .0

-2 6 .5

4 5 0 0 0 5 0 0 0 0 5 5 0 0 0 6 0 0 0 0 6 5 0 0 0 7 0 0 0 0 7 5 0 0 0

F re q u e n z a (H z )

Am

piez

za (

dB)

0 0 -1 V U 2 0 1 -1 V U 2 0 2 -1 V U 2 0 3 -1 V U 2 0 4 -1 V U 2 0 5 -1 V U 2 0 6 -1 V U 2

Fig.2. Measurements in the range 45 kHz –75 kHz

The chart in Fig. 3 (measurements in the range 500 kHz -850 kHz) focuses the attention on phase VU with refer-ence to the third resonance point at high frequency, aftereach short-circuit test performed on the transformer..

-1 9 .0

-1 8 .0

-1 7 .0

-1 6 .0

-1 5 .0

-1 4 .0

-1 3 .0

-1 2 .0

-1 1 .0

-1 0 .0

5 0 0 0 0 0 5 5 0 0 0 0 6 0 0 0 0 0 6 5 0 0 0 0 7 0 0 0 0 0 7 5 0 0 0 0 8 0 0 0 0 0 8 5 0 0 0 0

F re q u e n z a (H z )

Am

piez

za (

dB)

0 0 -1 V U 3 0 1 -1 V U 3 0 2 -1 V U 3 0 3 - 1 V U 3 0 4 -1 V U 3 0 5 -1 V U 3 0 6 -1 V U 3

Fig.3. Measurements in the range 500 kHz – 850 kHz

In parallel, short-circuit inductance measurements withbridge method were also made to compare and evaluatethe sensitivity of the SFRA method. The results of thesemeasurements are shown in Table 1. The three first testcycles didn’t show significant variations in terms of in-ductance, while greater deviations were seen from thefourth test cycle on. The analysis of the transfer functionsdetected at middle-high frequencies (higher than 200kHz) confirms this behaviour, as shown in the previousdiagrams. The percentage difference among values ob-tained during subsequent checks reveals the presence ofgeometrical deformations in the windings. A 2% induc-tance variation was thought to be a sure index of wind-ings deformation/damage. The last edition of IEC 60076-5 lowers that threshold to 1%, considering this value areliable index of possibly dangerous mechanical defor-mations.

Table 1. Results of bridge method short-circuitinductance measurements

Short-circuit inductance[mH]

measured at terminals

Variation[%]

with respect to startvalues

Testno.

U-V U-W V-W U-V U-W V-WStart 147,37 148,13 148,19 - - -

Test 1 147,32 148,07 148,15 -0,03 -0,04 -0,03Test 2 147,39 148,07 148,14 0,01 -0,04 -0,03Test 3 147,44 148,09 148,17 0,05 -0,03 -0,01Test 4 149,74 149,79 149,26 1,61 1,12 0,72Test 5 150,56 152,15 150,30 2,16 2,71 1,42Test 6 151,24 125,54 151,90 2,63 -2,98 2,50

3. DIAGNOSTIC OF MEDIUM VOLTAGE CABLES3.1. General

The medium voltage network of Italian Utility – ENELhas a total length of 340000-km (data 2000). 34.8% ofthe network is made of underground power cables. Thenet failure rate (i.e. the failure rate linked with internaldefects of cables and accessories) ranges from 2.45 fail-ures per 100 km of underground cable lines per year inthe northern regions to 7 in southern Italy. To improve thequality of the electrical supply to customers and to com-ply with the increasing severity of the regulations im-posed by the National Authority, a wide condition as-sessment program were performed that foreseen in theperiod 2002-2003, the condition a of about 20000 km ofunderground cables (i.e. 6500 km of lines) correspondingto about 5.5% of the underground network of ENEL.Three companies participated in this program, amongthem CESI had about 40% of the work.

3.2. Selection of suitable diagnostic systemFour medium voltage cable diagnostic systems were

considered in the preliminary selection (Table 2).

Table 2. Diagnostic systems considered

System Voltage sourceA Sinusoidal voltage at power frequencyB Sinusoidal voltage with very low frequency

(0.1 Hz)C Oscillating wave with variable frequency and

low dumpingD Oscillating wave with variable frequency and

high dumping

They were selected among the most modern, advancedand representative on the market. All of them are basedon an off-line approach that means, the cable beingchecked has to be de-energised. All systems are equippedwith their own voltage generator producing a specificvoltage wave shape and all systems base the diagnosticprocess on the measurement of a PD signal consideringboth the amplitude and the location of the source alongthe cable route. From the technical point of view, the as-sessment of the different systems under multi-criteria ap-proach is reported in Fig. 4.

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Fig. 4. Technical evaluation of different systems

The better is the performance of the system consideredthe higher is the score shown in the figure. The score ofthe voltage source was determined in the lab on the baseof the PD inception voltage obtained using the standardIEC procedure. The immunity to interference waschecked in terms of capability to screen or reject externalinterference. The systems were also compared from eco-nomical and management points of view; transportability,necessary investments, operation and maintenance costs[2] were analysed. The systems were compared in the laband in the field. In lab, systematic tests were carried outon five medium voltage cables rated 12/20 kV and insu-lated with different materials: impregnated paper, EPRand XLPE. Defects were created artificially to check thecapabilities of the diagnostic systems to point out, locateand recognise the different types of possible defects. Inthe field, more than 60 cables were tested.System C was selected for regular measurements as itproved to offer the best compromise between the techni-cal performances, the transportability and especially theintegrability with the existing systems in use in CESI.

3.3. Failure analysisIn the period 2001-2002 CESI carried out nearly 130

failure analysis on joints and about 60 on terminations.As considered time to failure, the typical bath-tub shapewas evident, showing that after an initial period (about 5years) where “childhood” defects were brought to evi-dence, a long period (15-20 years) with a reduced failurerate appeared; the failure rate increased again when age-ing phenomena started to prevail on defects.As concerns defect type, the inspections revealed thepresence of more than 700 defects, out of which morethan 400 were directly linked with the failure (fatal de-fects). Most of the defects in joints occurred during thelatest parts of the mounting sequence i.e. from the recon-struction of the insulation to the sealing and its conse-quences; several damages were linked with the manipu-lation of the joint and cable during assembly and duringlaydown. The most critical parts of the termination as-sembly seemed to be the inner ones, linked with the ma-nipulation and preparation of the conductor and the con-nectors; the insulation is very often damaged during this

manipulation. It is interesting to observe that in more than10% of the terminations showing fatal defects, the pri-mary cause of failure was linked with a production defectin the material. The contribution of the fatal failures ofthe different phenomena driven by the defects pointed outduring inspections is shown in Fig. 5.

Fig.5. Contribution of the different types of failure ori-gins to fatal failures

3.4. Condition assessment method used by CESIThe system was based on off-line approach; the systemwas equipped with its own generator to energise the cablewith damped oscillatory voltage having a frequency inthe range from 150 to 250 Hz; the diagnostic process wasbased on the measurement of a PD signal consideringboth the amplitude, pattern and the location of the sourcealong the cable route. In order to optimise the work effi-ciency, the equipment is organised into a van, which wasspecifically designed and assembled for the purpose. Twomeasuring aspects were important for the Client:• determination of the position of joints. It was per-

formed with the use of joint locator system based onthe traditional but versatile reflectometric technique.Software of the system allows powerful noise rejectionfeatures and exports the joint position directly to thePD location map;

• determination of the position of defected part on theterrain. It was done by means of commercial systemcomposed of a generator and an antenna. In 95% of thecases, the component indicated by diagnostic meas-urement was found when digging.

An example of PD location map of the cable section withimportant PD activity concentrated in correspondence ofa joint is presented in Fig. 6.

0

5291

10582

15873

21164

26455

0 145 290 435 580 725

P D l oc a t i on ( m)

Umax (1,19 p.u.) Uo = Ui Giunto

Fig. 6. Example of PD location map with importantactivity on a joint

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Fig. 7 shows the distribution of critical defects among thedifferent line components. About 90% of the defects wererecorded in the cable accessories, confirming the critical-ity of in-field manipulation. More than half of the totaldefects was located in joints.

Fig. 7. Distribution of the defects over the different linecomponents

The critical points were ranked in four levels; for everycable section classified at level 4 or 3 a repetition of themeasurement after 5 and 3 years respectively was ad-vised, while for levels 2 and 1 a repair action was indi-cated as necessary within 1 year and few months respec-tively. The critical points ranked 1 &2 were removed andanalysed to verify the presence of any reason justifyingthe removal of the cable portions. The analysis was per-formed on the base of visual inspections disassemblingthe removed components. No reason for PD activity wasfound only in 18% of the cases. All the other cases can beconsidered as avoided failures.Economical aspects; to determine the advantages of thecondition based maintenance (CBM) against correctivemaintenance (CM) option, the following costs have to beconsidered: the cost of the test (the cost of the measure-ments, the cost of the utility personnel necessary to oper-ate the network during the measurements, the cost for thephysical location of the defected points and the cost forthe repair), the costs of failure and avoided failure.Table 3 reports the different phases contributing to theoverall cost in the two cases considered.

Table 3. Different phases of CBM and CM overall costs

CBM (Diagnostic) CM (Run-to-failure)Diagnostic measurement(including the costs of the

service and utility personnel)

Possible penalty

Location of the Defected point

Location of thefailed point

Repair Repair

The costs for the failure/defect location and repair can beconsidered equivalent for both options. The advantage inthe application of condition based maintenance policy isin the possible reduction of the measurement costs that

can be achieved improving productivity, limiting the costof the test and especially performing the accurate selec-tion of the section to be tested. When the number of thecritical points detected during diagnostics tests is re-duced, the cost of the test is higher than the benefit ob-tained reducing the cumulated time per year of unavail-ability of energy (indicator of the quality of service).

4. ON-SITE TESTING AND MONITORING OFEHV/HV POLYMERIC CABLES

4.1. GeneralWeight, limited bending radius as well as transport and

laying restrictions limit the maximum production lengthof high-voltage and extra-high voltage cable segments toapprox. 800 - 1000 m. The segments are assembled on-site to form long cable lines using straight joints. Instal-lation work on-site has a risk of introducing faults, asconditions are not ideal for an installation of HV accesso-ries; small particles, dust, moisture, droplets might lead todefects in electrically critical parts of the accessories,possibly leading to a reduced lifetime of the cable systemor to failure. Dielectric tests performed on site do not re-place type tests or routine tests. They are supplementaryto routine tests and aim at checking the dielectric integrityof the fully assembled cable line in order to eliminate thementioned above defects.

4.2. On-site test proceduresThe most important stress situations for high-voltage

AC apparatus during normal operation arise from the op-erational alternating voltage and from switching over-voltages. Consequently, the preferred voltage for on-sitetests is AC voltage. As voltage testing delivers only bi-nary results (withstand or breakdown), it is recommendedto combine AC voltage testing with sensitive on-site PDmeasurements. PD measurements offer the chance for aneffective HV on-site testing and for early detection offault spots within power cable systems [3,4].On-site test procedures usually have to be negotiatedbetween the manufacturer and the user on the basis ofinternational and national standards. Two IEC standardscover after installation tests of extruded cable systems:IEC 60840:2004 (for cables of rated voltages fromUm = 36 kV up to Um =170 kV) and IEC 62067:2001 (forrated voltages above 150 kV up to Um = 550 kV) [5,6].Both standards define equal requirements for the shape ofAC test voltage and for the time of its application:

• substantially sinusoidal waveform• frequency between 20 and 300 Hz• time of voltage application equal to 1 hour (at

U0: 2 4 h)The AC test voltage level of new cable systems dependson the cable rated voltage and is between 1.7U0 and2.0 U0 for rated voltages between 30 kV and 150 kV [5].At higher voltages the test voltage levels decrease from:1.4 U0 (220-230 kV) to 1.3 U0 (275-345 kV), 1.2U0 (380-500kV) and 1.1U0 for cables of 500 kV rated voltage [6].

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4.3. Alternating Current TestingHigh test power, especially for long cable lines, can

only be efficiently generated by mobile resonant testsystems, where the weight-to-power ratio and feedingpower demand is relatively low and the transport volumeis acceptable. The most commonly used test system gen-erate the test voltage by a reactor with fixed inductanceand frequency-tuned voltage excitation (ACRF test sys-tems) [7] (Fig.8).

Fig.8. Four mobile resonant test set-ups of ACRF typeconnected together (3 in parallel and 1in series) to reachtest voltage of 280 kV and current value of 240 A, [8]

In a series resonant circuit, the test voltage is a pure sinu-soid, but its frequency increases with decreasing load ca-pacitance. The whole test should be performed as a steptest. PD measurements should be taken at every step. Byincreasing the test voltage in steps of e.g. 20% of themaximum test voltage, critical defects are usually identi-fied before breakdown. On-line PD diagnostics and HVtesting on-site complete each other in an excellent way.

4.4. On-site PD measurementsAs the damping of a PD signal along the long cable

line makes PD detection at the end of the cables line ex-tremely difficult because of the poor sensitivity, a selec-tive PD measurement on all accessories overcomes thisproblem. PD sensors are directly put inside the accesso-ries. Different kinds of unconventional PD couplingmethods based on e.g. capacitive or inductive sensors ledto increased PD sensitivity at the accessories (joints)compared to conventional PD detection. However, thesetypes of sensors have to be implemented during the lay-ing of the cable, retrofitting a direct buried cable systemis often not possible. To avoid all these drawbacks, PDsensors have to be placed in easily accessible and uncriti-cal parts of the HV cable system. A promising approachis the use of inductive PD sensors at cross bonding linksof long HV cable systems [3, 4] (Fig.9).As the total cable line is energised, a PD test must be car-ried out simultaneously at all accessories per phase. Thisrequirement leaded to the development of a new synchro-nous multi channel PD measurement system. SelectivePD measurements need a potential-free connection fromthe accessories to the storage and visualisation unit; thisis achieved using optical fibres (Fig.10).

Fig.9. Inductive sensor on cross bonding links

Fig. 10. Example of multi-channel PD measurement sys-tem with optical communication system

The PD measuring system on each accessory consists ofPD sensors build into the joints and terminations, whichdetect the apparent charge (Fig. 11). The PD units havefully digital PD signal processing capabilities. Speciallydeveloped digital bandwidth filters enable filtering of thePD signal within a frequency range of up to 20 MHz. Thebandwidth can be remote-controlled and also the centrefrequency can be varied separately. This leads to an opti-mised signal-to-noise ratio and increased sensitivity ofthe PD measurement system, as the frequency range canbe moved to a section of the spectrum with a low noiselevel. Via optical fibers the PD data is transferred to thecentral storage and visualisation unit (PC), and the PDdetection units are controlled [9].

Fig. 11 PD detection system1-PD detection unit, 2-quadruple 3-battery power supply,

4-PD sensor outlet, 5-optical fibre

4.5. Check of the system performance on-siteOn site, damping, dispersion and refraction of calibra-

tion pulses due to long cable length require the modifica-tion of the standard calibration procedure. In this case, the

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performance of PD equipment should be checked. An ex-ample of such verifications performed during testing of400 kV XLPE cable system in London is presented in thefollowing.The internal test signal generators of each PD measuringunit were activated to verify full functional readiness ofthe system. Then calibration pulses of 500 pC were fed invia one cable outdoor termination. The 500 pC calibrationsignal and its part-reflection at the next joint can clearlybe distinguished from the background noise level evenover a distance of 20 km (Fig. 12).

Fig.12. Propagation of calibration pulses, [8]

5. Diagnostic of Gas Insulated Substation5.1. General

Effective on-site tests are necessary to verify that theGIS is still defect free after transportation, final assemblyand mechanical testing on site. According to the new IECstandard [10], the best procedure to check electrical with-stand of the GIS insulation is application of power fre-quency voltage together with very sensitive PD meas-urements (the maximum permissible PD value in GISshould not exceed 5 pC). As the standard method (IEC60270) is very difficult to apply on-site, the use of alter-native methods is considered. The ultra high frequencymethod (UHF), optical and acoustic methods, to detectand recognise different GIS defects were tested.

5.2. Comparison of diagnostic methods for PDmeasurements

The comparison of PD methods was performed on the420 kV GIS module filled with SF6 gas at a pressure of0.45 MPa and supplied with power frequency voltage[11]. Three sensors, one for each method, were located onthe GIS enclosure. Optical and UHF sensors were in-serted into optical accesses already present on the enclo-sure for arc location. Acoustic sensor was externally at-tached to the bottom part of the enclosure (Fig.13).Two defects were simulated taking into account thecommon presence of the defect after assembling on-site,critical size and low apparent charge value generated bythe defect (below 5 pC):� free moving metallic particles (copper and alumin-

ium), as the most common defects being critical atpower frequency voltage [7,8];

� fixed defects as critical under LI voltage [12]. Parti-cle attached to the surface of the insulator and sharpprotrusions on HV conductor were tested [11].

The following measuring PD signals were considered:• UHF signal captured by internally located sensor; time

domain analyses were performed and the signal am-plitude over several consecutive voltages cycles, andcorrelated to the phase of the voltage was 3-D dis-played in per-cent of the full-scale;

• acoustic signal measured in time with ultrasonic sensoroperating in the 20-100kHz range, commonly used forthe measurements on GIS [12]; the accumulation pro-cedure was used to analyse the data;

• optical signal measured in time domain by photomulti-plier tube, using photon-counting technique [13]; thesignal was obtained by storing signals coming from theoptical system; the resulting pattern was an histogramwith amplitude related to the total number of photons(counts per seconds) collected by the optical detector.

Fig. 13. Test set-up

PD measurements were performed simultaneously withall the methods. The sensitivity of the methods wasevaluated at PD inception voltage level and was indicatedas a ratio between the amplitude of the PD signal andbackground noise level (Table 4).

Table 4. Signal to noise ratio at PD inception level fordifferent method and defect type

Defect type UHF% of full scale

Opticalc.p.s.

AcousticsmV/mV

Free particle 55-60 4-6 8-10Particle onInsulator

55-60 4-6 1.5

HV (2mm)protrusion

50-60 5-6 1.5

Free moving particles; all the techniques detected freeparticles movements at the same voltage level, which wasthe “ lift-off” voltage for the particles considered. Thepattern produced by moving particles was easy to recog-nise and similar for all the methods (Fig.14).When accumulating technique is applied, as for acousticand optical techniques, the correlation with the phase ofthe voltage is evident on both polarities. At inceptionlevel PD signals have the same amplitudes on the positiveand negative semi-cycles. The signal to noise ratio of themeasurements was very high for acoustic technique as inthis case the direct mechanical impacts of the particles onthe aluminium enclosure were measured. A lower signalto noise ratio was obtained for UHF and optical methods.

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Fixed defects; optical technique turns out to be the mostsensitive one and the inception voltages for both type offixed defects results even lower than for the standardmethod. Even the 1 mm HV protrusion was detected withoptical and UHF methods below this value. This defectlength is critical as it can still lower the dielectric strengthof the insulation below the LI level during on-site tests.The acoustic measurements were not performed as beingstrongly disturbed by corona effect on HV connections.As concerns the defect pattern, the inception of the defecttook place at the negative polarity of the voltage applied.At higher voltage levels a positive defect activity wasalso observed. The signal to noise ratio, when comparedto free particles, was lower for acoustics, the same forUHF and much higher for optical method. The most sen-sitive PD optical signal to noise ratio at inception levelwas about 8 times higher in the case of particle on spacerthan with free moving particles.

Fig. 14. UHF (a), acoustic (b) and optical (c) PD signalsrelated to free particle activity

5.3. PD testing of GIS in serviceIn general, all diagnostic methods have good sensitivity

to critical defects [12,13] but have some practical limitswhen applied in the field. Among various PD methods,the acoustic technique has been the most commonly ap-plied one under field conditions. This method has goodsensitivity to detect, locate and recognise of most defecttypes and, at the same time, has good immunity to envi-ronmental noise. For free particles, considered the singlelargest cause of insulation failure in GIS, the sensitivityof the acoustic method is much better than any othertechnique. Acoustics is a low cost and non-invasive tech-nique, but as the acoustic signal coming from the defect isstrongly attenuated; the acoustic measurements should be

performed on at least one point for each compartment.Measurements of medium sized substations do not exceedtwo working days and are performed without outages ofthe voltage.The results of the acoustic measurements performed onGIS in-service in Malaysian network are presented in Ta-ble 5. Fifty-five GIS substations of thirteen manufactur-ers consisting of 500 bays of 145 kV and 300 of 13 dif-ferent manufacturers were installed in Malaysia up to2000.

Table 5. Results of acoustic measurements

Defect type No. of locations/rated voltage

Componentinvolved

Mobile particlesMech. Vibration

Mech. Vibration

Loose element

Loose element

1/ 300 kV2/ 300 kV6/ 145 kV8/ 145 kV3/ 300 kV3/ 300 kV7/ 145 kV2/ 300 kV

Bus ductBus duct

Voltage trans.

Voltage trans.

Disconnector

The detected problems can be divided as mechanical andelectrical ones. The mechanical vibrations can lead tofailures and should not been neglected. Two types of me-chanical vibrations were detected. One type was charac-terised by very low frequencies modulated by the powerfrequency. Sometimes this kind of signal is difficult todetect with ultrasonic sensors and in this case the acceler-ometers working in the frequency range from some tothousands of kHz is more appropriate. This kind of vi-bration was found in two substations of the same designand was related to wrong design of support structure. Inone case this vibration lead to the loose of electric shield.In this case the high signal was easy to locate and syn-chronous to the zeros of the voltage applied (Fig.15).

Fig. 15. Acoustic signal related to loose element

Mobile particles were detected only once in one of thebus ducts. The amplitude of the signal was low and theflight time much below the critical value. This defect wasestimated as a harmless one and no immediate action wasundertaken. The PD activity in this point will be periodi-cally checked in the future.PD signal of loose element was detected in two discon-nector switch compartments and an immediate action wasundertaken.The characteristics of the acoustic signal in relation to thedefect type are presented in Table 6.

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Table 6. Characteristics of acoustic signal fordifferent type of defect

Defect Type Acoustic signalAmpl. (*) correl. to 50Hz

Action

Free particlesMech.Vibrationon VTLoose element

3-4 NO 2-4 NO

8-12 YES

NONO

YES

(*) - Amplitude is indicated in signal to backgroundnoise ratio (mV/mV)

On the basis of the field experience the following proce-dure was proposed for in-service measurements.For a new substation the acoustic measurements will beperformed at the end of guarantee period, in general oneyear after commissioning. Reference will be done to themeasurements performed with this method during on-sitetests. Further checks will depend on strategic importanceof the GIS, good or bad experience with the same design.The measurements should be repeated approximatelyevery two years. In “dirty” points the measurementsshould be performed more frequently and the measure-ments interval should be based on the acoustic signal pre-viously measured in particular point. Acoustic measure-ments can be used to anticipate periodic maintenance orcan help to predict the maintenance work.In the case of potential problems the manufacturers has tobe contacted and consulted for their point of view. Riskassessment is performed taking into account criticality ofthe defect location, value of the local electric field andprevious experience with this particular GIS design. Theresults of acoustic measurements should be comparedwith earlier experience and knowledge from on-sitemeasurements. It is very useful to correlate the acousticsignal to the voltage applied. This is not possible in threephase-enclosed designs. It was decided that the GISwould only be opened if it was sure that this kind of thedefect could lead very fast to the failure. Opening inservice can lead to risk that new problems might be intro-duced.

6. CONCLUSIONSPower transformers; transfer function analyses per-formed within different frequency bands confirmed thegood sensitivity of SFRA method to detect winding geo-metrical deformation, residual magnetism and turn-to-turn faults. Present experimental activities are aimed atfinding the correlation between the differences in themeasurement with SFRA and the actual amount of thegeometrical deformations.Power cables; the condition assessment of MV powercables can be obtained by a combination of informationderived from the insulation integral methods such as thetan δ and location of defective points derived from thePD activity measurements. The methods used are sensi-tive to the majority of the defects. In 95% of the cases,the component indicated by diagnostic measurements wasfound when digging. About 90% of the defects were re-corded on the cable accessories. With the accurate selec-

tion of the cable section to be tested, the benefit of diag-nostic method appears evident to reduce the cumulatedtime per year of unavailability of energy.HV cables; the combination of AC voltage testing withPD measurements on all accessories give the chance foreffective on-site testing of cable lines.GIS switchgear; UHF, optical and acoustic methods canbe recommended for PD measurements on-site. Thesemethods are able to detect GIS defects of low apparentcharge values what fulfils the requirements of a new IECstandard. The optical method for on-line PD monitoring,developed in CESI, was successfully tested and confirmshigh sensitivity for detection of critical defects in GIS.Acoustic, non-invasive, low cost method proved to havegood sensitivity to detection, location and recognition formost defect types and, at the same time, has good immu-nity to environmental noise and can be proposed for in-service measurements.

REFERENCES1. A. Lapworth, T. J. McGrail, Transformer Winding Move-

ment Detection by Frequency Response Analysis (FRA), inproceedings of Sixty-sixth Annual International Conferenceof Doble Clients, April, 12-16, 1999, Boston, USA.

2. V. Colloca, A. Fara, M. de. Nigris, G. Rizzi, Comparisonamong Different Diagnostic systems for Medium VoltageCable Lines, CIRED 2001, Amsterdam, the Netherlands.

3. CIGRE WG 21.09: “After Laying Tests”, Electra 166, 1996.4. Chan, Duffy, Hiivala, Wasik: “PD Testing of Solid Dielec-

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5. IEC 60840 Ed3. 2004-4, Power cables with extruded insu-lation and their accessories for rated voltages above 30 kV(Um = 36 kV) up to 150 kV (Um = 170 kV).

6. IEC 62067 Ed.1 2001-10, Power cables with extruded in-sulation and their accessories for rated voltages above 150kV (Um = 170 kV) up to 500 kV (Um = 550 kV).

7. W. Hauschild, W. Schufft, W. Spiegelberg, Alternating volt-age on-site testing of XLPE cables: The parameter selectionof frequency-tuned resonant test systems, 10th ISH Mont-real (1997), Vol. 4, pp 75-78.

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10. IEC Standard 62271-203, Gas-insulated metal-enclosedswitchgear - Highest voltage of equipment of 72,5 kV andabove.

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Wojciech Koltunowicz - Nagoya University · Corresponding author: W. Koltunowicz, ... i .f om5k Hz t2M F h ... has a total length of 340000-km (data 2000). 34.8% of