2005 EIC - Bushing Paper

8/12/2019 2005 EIC - Bushing Paper

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ON LINE MONITORING OF BUSHINGS ON LARGE POWERTRANSFORMERS

Claude Kane

Predictive Diagnostics of Eaton Electrical5421 Feltl RoadSuite 190

Minnetonka, MN [email protected]

Alexander Golubev

Predictive Diagnostics of Eaton Electrical5421 Feltl RoadSuite 190

Minnetonka, MN [email protected]

Abstract: Industry statistics suggest that 80% of all plant andequipment failures occur on a random basis and only 20% of thefailures are age related. This means that 80% of failures havenot been detected with common test and maintenance practicesand therefore these failures have not been prevented. Based ondifferent sources up to 30-35% of large power transformerfailures are attributed to bushing insulation failures. About halfof these bushing failures result in an explosion and fire.In today's competitive environment, increasing demands arebeing placed on the management of physical assets. Advances intechnology are allowing new approaches to maintenance. Theseinclude reliability-centered maintenance, predictive maintenance,condition monitoring, and expert systems. Trend settingorganizations are increasingly taking advantage of theconvergence of these new technologie s to implement proactivemaintenance programs.

INTRODUCTION

During the late sixties and early seventies the former SovietUnion experienced a high rate of catastrophic HV bushingfailures on their 500kV power transmission systems. Rootcause of the failures was a combination of design,manufacturing and technological problems. The existingmaintenance strategy based on periodical off line insulationtests were not effective in preventing failures due to the fastrate of defect development. On line bushing monitoringmethods and technology were developed and introduced atthat time by P.M. Svi and his colleagues [2,3].Implementation of the technology quickly reduced the bushingfailure rate by timely detecting developing insulation defectsand replacing failing bushings. The instrumentation withsome modifications is still in use in most of the 500kV and750kV apparatus in the former Soviet Union republics.

The basis of this on line monitoring method is to compareinsulation characteristics of three-phase bushing system.Technology for several three-phase bushing sets have beendeveloped and tested but has not been widely used due tomore complexity despite of its better noise immunity.

Figure 1 Basic Block Diagram of Bushing Monitoring System

During commissioning the null-meter is balanced to zero. Asdefect develops the complex conductivity of the bushinginsulation changes and the current and its phase angle in oneof phases also changes. Therefore, the null-meter will nolonger be null. The amplitude of the change reflects theseverity of a problem and the phase angle indicates whichphase that is experiences the change.

The change can be approximately represented by the equation(1) under the assumption of a single defective phase:

2

0

2

0

)tan(

+= C

C I

I (1)

Where:

- Parameter GAMMA,

tan - Tangent delta change,

0C C - Relative change in bushing capacitance.

The monitor discussed in this paper has the ability to makephase measurements and temperature correlations thatprovides for a new knowledge base for on line bushinginsulation monitors.


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BUSHING MONITORING SYSTEM

The system continuously monitors the power frequencycurrent through the insulation of a 3-phase set of bushings aswell as top oil temperature. Ports are provided that allow theattachment of measurement equipment for making periodicalpartial discharge (PD) measurements [4]. The sensors areconnected to the bushing capacitor taps, and an additionalneutral PD sensor is installed on the available groundedneutral of the transformer or on the transformer tank groundfor noise cancellation purposes when PD tests are performed.

Each bushing sensor has internal protection for the test tapinsulation which limits the residual voltage at the tap to a levelof not more then 135 Volts RMS even if open circuited. Innormal operation, the test tap voltage does not exceed 10 VoltsRMS.

Figure 2 - Bushing Monitor and bushing sensors installedFactors Affecting Accuracy

Factors affecting accuracy of this on line technology aredivided into three groups:

Device accuracyNoiseSystem voltage variations.

Device Accuracy

Accuracy of the device was specified to timely detectdangerous changes in power factor or capacitance that maylead to bushing insulation failure. Experience indicates thatthe change in Gamma is of several percent

1. Therefore, the

accuracy of 0.1% for relative magnitudes and 0.1 O in phaseangle were specified and achieved.

1 Decades of experience have demonstrated warning and alarmsetting were 3% and 5% respectively.

Noise

The device monitors power frequency currents from thebushing potential or test taps. Higher power frequencyharmonics are considered as noise, especially the oddharmonics. The odd harmonics from the three phases are notbalanced in the balancing unit but are summarized. In order to

suppress this noise, the device must have high quality low passfilters. These filters allow for the application on DC/ACconverter substations where signals have a significant highcontent of power frequency harmonics.

Other noise that exists to some extent is a common mode noiseresulting from two grounding points in a measuring circuit.There are two protection issues requiring two grounds. Thefirst is to provide absolute safety for personnel that mayoccasionally control the monitor from its keypad or touch itsenclosure. Reliable grounding of the device and the enclosureto a local ground and isolating all interfaces provides forpersonnel safety.

The second requirement is to protect the bushing itself nomatter what even if all connections to the device are brokenand device protection is not functional. The sensor must berugged and have adequate protection built into the sensor.Normally a sensor has impedance connected in parallel to theground and a surge-protecting device. The impedance passespower frequency test tap current to the ground, keeping avoltage on the tap to an acceptable range in case the measuringcircuit is open circuited. The internal surge protectorsuppresses surges and lightning strike currents. Therefore, thesecond path (normally relatively high resistance path) to theground must be built into the sensor

2.

Installing the device enclosure next to a transformer tank andgrounding it to the transformer ground would normally resolvethe issue due to a small AC voltage drop across thetransformer tank. The only problematic connection is on threesingle-phase transformer banks. The banks have some ACvoltage between their tanks and this will produce commonmode noise for the summation circuit. Good groundingpractice will keep the AC voltage between the tanks within afew hundred milli-volts is sufficient.

2 The concept of only having surge protection in a sensor doesnot provide for reliable protection. Open circuit protection isrequired. In the case of disconnecting the measuring device at750kV and 60Hz power frequency, surge protection shouldcarry up to 150mA rms continuously at its residual voltage.Several different types of commercially available dischargegaps carrying such a current have been destroyed in ourlaboratory within weeks.


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For example, if the monitor is installed on the middle phase ofthree single-phase 500kV transformers. Figure 3 shows theschematic of common noise influence.

RO

Ua

Ya

Rd

Ub

Yb

Rd

Uc

Yc

Rd

Vb

Vab Vcb

I a +V a b / R d I c

+ V

c b

/ R d

I b

Figure 3 Schematic for common mode noise

Common mode noise sources are applied between a sensorground at test tap and measuring circuit ground in the deviceenclosure. These sources are labeled as V ab, V b, Vcb since thedevice installed on the phase B bank. V b voltage is negligiblysmall, but the other two were measured as 100 200 mVRMS. Attempt to provide additional low impedanceconnections between banks did not significantly changecommon mode voltage. This voltage creates additional noisecurrent that is measured by the device. Taking 200mV rmsand bushing capacitance of 500 pF additional noise currentimpact can be estimated as below 0.1%, which is acceptable.

System Voltage Variations

System voltage behavior is one of the main contributors tomethod accuracy as a whole. This issue becomes very criticalwhen the precision of 0.1% is required. A variation in systemvoltage (magnitude or relative phase shift between phases)creates an unbalance and may be interpreted as capacitance orpower factor change. Magnitude variation may be interpretedas capacitance change and phase shift variation as powerfactor.

System voltage (phase) variations have been observed at alllocations where the monitor is installed regardless of region orcountry. Figure 4 below shows the A-B and A-C phase

angles3

from four units in three locations across NorthAmerica for a time period of several weeks.

Figures 4a and 4b are for standalone units and 4c and 4d fortwo identical units connected to the same HV buses. Behaviorof phase angle variations is very similar in the last two cases

3 Phase angles of 120 0 and 240 0 subtracted from A-B and A-Cangles respectively to scale data to the same origin.

despite the two units were not fully synchronized in takingtheir readings (time difference between units taking readingswas within 10 minutes).

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

11/1 3/03 11/23/ 03 12/ 3/03 12/ 13/03 12/23/03 1/2/ 04 1/12/04 1/22/04

Time

V a r i a

t i o n s

[ D

e g . ]

a-b a-c

Fig. 4a Unit #1, Location #1

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

11/20/03 11/30/03 12/10/03 12/20/03 12/30/03 1/9/04 1/19/04 1/29/04

Time

V a r i a

t i o n s

[ D e g . ]

a-b a-c

Fig. 4b Unit #2, Location #2

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

11/23/03 12/3/03 12/13/03 12/23/03 1/2/04 1/12/04

Time

V a r i a

t i o n s

[ D e g . ]

a-b a-c

Fig. 4c Unit #3, Location #3

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

11/23/03 12/3/03 12/13/03 12/23/03 1/2/04 1/12/04

Time

V a r i a

t i o n s

[ D e g . ]

a-b a-c

Fig. 4d Unit #4, Location #3

Figure 4 A-B and A-C Phase variations due to system voltageasymmetry of four transformers

For changes in all phases the formulas should be changed tovector summations. Gamma will react on asymmetric changesin system voltage only. All symmetric voltage changes willcompensate each other (the same increase of all voltagemagnitudes, for example, will not disturb a balance).

Therefore, accuracy of the method depends upon the statisticsof the asymmetric voltage variations in the particular locationand statistical data processing procedure.


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Diagnostics

The technology as originally introduced and implemented wasfocused on producing timely alarms and then suspectedbushing should be further evaluated with additional off linetests. This part remains unchanged and the Gamma-parameter

is a very reliable indicator of a dangerous trend in bushinginsulation system. In addition, modern microprocessor basedinstrumentation allows for additional diagnostics performedon line while a unit is running. On line diagnostics givesadditional valuable information and therefore advantages inmaintenance strategy and as a result saves money.

The main goal of on line diagnostics is to locate defectivebushing, determine the predominant failure mode and finallypredict timely critical insulation triggering shut down andbushing replacement.

The diagnostics includes three parts: time trend, temperaturedependencies and defect identification. Defect identificationrequires determining the tangent delta and capacitance of allthree bushings.

In the worst case of a single stand-alone unit installed on onethree-phase transformer (or three single-phase transformers)five independent quantities can be obtained: three currentmagnitudes from the test tap and two independent phaseangles between the currents. The number of variables istwelve, three of each: tangent deltas, capacitances, systemvoltage magnitudes, and phase angle between system voltagevectors. The situation partially improves by learning the

statistical behavior of the system voltage at the particular

location for a period of time and assuming that the tangentdeltas and capacitances are known at the time and equal totheir off line values. Based on the voltage behavior statisticswe can then compensate for the change in the variousquantities over time.

Practical Results

Four units installed in North America have been selected forthis paper. This selection provides a good representation ofrated voltages, apparatus, and field situations. The nameplateinformation is shown in Table 1 and the phasor gamma graphsand gamma trend are shown in Figure 5. Top oil temperatureis recorded along with other data and is also available foranalysis. A 100 Ohm Pt RTD is installed on the top of thetransformer radiator or cooler header next to the transformertank.

The most challenging is the installation on Unit #1 that hasthree single-phase transformers with all three having differentbushings. The transformer operates in a peaking mode andgenerally has either full load or no load. Two clusters areobserved in the phasor graph (Figure. 5a) reflecting differentload modes: left loaded and right unloaded. Temperaturevariations during the observation period are from 15 0C to64 0C.The second unit is most quiet. Gamma variations are verysmall over the recorded temperature variations of 20 0C to49 0C. The unit is base loaded.Gamma variations in units #3 and 4 are moderate withtemperature ranging of 20 0C and 30 0C.

Table 1Nameplate Data for Four Transformers Discussed in this Paper

Unit Descript. Phase MVA /Imp. %

RatedVoltage kV

BushingPow. Fact.

BushingCapac.

Bushing Manuf. /Year

Tr-r Manuf.

A 105.3/16.56%

512.5/13.8 0.34 470.1 GE U-type/ 1986 CanadianGE

B 96.7/12.93%

512.5/13.8 0.46 507.9 Bushing / 1971 PioneerElectric

Unit #1Three single-phase step uptransformers

C 105.3/17.1%

512.5/13.8 0.4 543.1 Trench / 2001 Pauwels

A 0.41 377 GE U-type

B 0.47 389 GE U-type

Unit #2

Three-phasestep up tr-r C

100/ 230/13.8

0.43 386 GE U-type

ABB

A 0.26 439 ABB O+CB 0.26 441 ABB O+C

Unit #3Three-phaseAuto-tr-r C

400/ 230/115/12.47

0.26 439 ABB O+C

GE

A 0.26 604 Haefely COTAB 0.26 606 Haefely COTA

Unit #4Three-phaseAuto-tr-r C

400/ 230/115/12.47

0.33 640 Haefely COTA

GE


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Trend

All units were base lined for a month and then used theinformation on system voltage variations for furtherdiagnostics. Diagnostics results for tangent delta andcapacitance trend are shown in Figure 6. Note that the scales

in the graphs representing trend of capacitance are differentfor different units.

Trends of both the capacitance and tan delta do not indicateany essential insulation deterioration in all four units. Even inthe worst case of system voltage instability on units #3 and #4the variations in tangent delta are very small of about 0.1%. Itis noticeable that the parameters variations on units #3 and #4are very similar, which reflects system behavior rather than achange in insulation condition.

As expected, unit #2 shows the best stability in both tangentdelta and capacitance with overall variations within 0.05%.

Bushing Tan Delta Trend

0

0.1

0.2

0.3

0.4

0.5

0.6

25-Nov-03 25-Dec-03 24-Jan-04 23-Feb-04

Time

T a n

D e l t a

Tan A Tan B Tan C

Bushing Capacitance Trend

460

480

500

520

540

560

25-Nov-03 25-Dec-03 24-Jan-04 23-Feb-04

Time

C a p a c i t a n c e p

CapA CapB CapC


0

0.1

0.2

0.3

0.4

0.5

0.6

25-Nov-03 25-Dec-03 24-Jan-04 23-Feb-04

Time

T a n

D e l t a

Tan A Tan B Tan C


375

380

385

390

25-Nov-03 25-Dec-03 24-Jan-04 23-Feb-04

Time


CapA CapB CapC


0

0.1

0.2

0.3

0.4

0.5

0.6

25 -N ov -03 10 -D ec- 03 25- Dec -0 3 0 9- Jan- 04 24 -Ja n- 04

Time

T a n

D e l t a

Tan A Tan B Tan C


438

440

442

25-Nov-03 10-Dec-03 25- Dec-03 9-Jan-04 24-Jan- 04

Time


CapA CapB CapC


0

0.1

0.2

0.3

0.4

0.5

0.6

25 -N ov -03 10 -D ec- 03 25- Dec -0 3 0 9- Jan- 04 24 -Ja n- 04

Time

T a n

D e l t a

Tan A Tan B Tan C


600

610

620

630

640

650

25-Nov-03 10-Dec-03 25- Dec-03 9-Jan-04 24-Jan- 04

Time


CapA CapB CapC

Figure 6 Power Factor and Capacitance Trends for the bushings on the four transformers discussed in paper

Fig. 6bUnit #2

Fig. 6cUnit #3

Fig. 6dUnit #4

Fig. 6aUnit #1


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Temperature Dependency

Another very important diagnostic characteristic istemperature dependency, primarily in tangent delta and also incapacitance. During the learning period of 30 days thischaracteristic is also determined.

Correlation to the temperature has been detected in unit #1with correlation coefficient of 0.8. Temperature variationrange is sufficient. Some temperature dependency has beenobserved in phase A tangent delta which may explain the 0.5%Gamma variation over the temperature range.

Correlation in units #2 and #3 is very low with a correlationcoefficient of about 0.1. In unit #4 the correlation coefficientapproaches 0.3, but both the linear approximation andtemperature range are very questionable.

Correlation with Dissolved Gas AnalysisTable 2 shows the Dissolved Gas Analysis (DGA) on two ofthe bushings from Unit # 1. The Phase A bushing is 35 yearsold and Phase B is around d 25 years old. The Phase Cbushing is only a few years old and no DGA data exists. TheDGA analysis indicates some slight overheating in the PhaseA bushing and supports the change in the Gamma Parameter.

Table 2 DGA on the A and B Phase BushingsUnit 1

Phase C2H2 C2H4 C2H6 CH4 CO CO2 H2

A < 2 56 66 238 2393 3B < 13 6 3 106 1598 25

CONCLUSIONS

High voltage bushing is one of the primary failure componentsof large power transformers. For over thirty years, technologyhas been in place to monitor the insulation condition ofbushings on-line but it is not wide spread in North America.Many bushing defects occur very quickly and performances ofperiodic off-line tests may not be the answer. Recentadvances in on-line monitoring technology have improved theaccuracy, reliability, and the diagnostics capability of suchdevices.The described device not only monitors changes in Gammaparameter and provides timely alarm signal on a defectgrowth, it also performs diagnostics based on bushingtemperature and provides trending of the power factor andcapacitance of each bushing. It is a viable system to monitorcritical and important units.Factors affecting the accuracy, such as noise (harmonics),voltage and phase variations of operating systems are

addressed. The achieved accuracy allows for reliableinformation in planning and implementing predictivemaintenance strategy. Monitoring a unit load current alongwith monitored parameters may allow for further improvementin diagnostics accuracy. Data is presented from fourtransformers installed in North America.

REFERENCES

Lau, M. Y. et All On Line Monitoring Systems For Bushings:2004 Doble International Client Conference.[1] Lau, M. Y.; 500KV Bushing Failures and Bushing OilSampling Program; 2002 Doble International ClientConference.[2] Svi P. M., Diagnostics of Insulation of High VoltageEquipment; 2-nd edition, EnergoAtomIzdat, 1988 (InRussian);[3] Svi P. M., Methods and Techniques of Diagnostics ofHigh Voltage Equipment; 2-nd edition, EnergoAtomIzdat,

1992, 240pp, (In Russian);[4] Golubev A. A., Kane C. F., Seliber A. B., Blokhintsev I.D., On-Line Predictive Diagnostics Technologies for PowerTransformers, The Proceedings of TechCon 2003 NorthAmerica, pp.263-279, February 5-6, 2003, St Petersburg, FL.

AUTHORS

Dr. Alexander Golubev got his MS in Experimental Physicsand Ph.D. in Physics and Mathematics from the MoscowPhysical Technical Institute (Russia) in 1978 and 1985,respectively. He has an extensive experience in research anddesign in Laser and Electron Beam Generation, PlasmaCoatings, High Frequency Measurements. Since 1995 he is aManager of R&D Engineering of the IPDD. This Company,is now a subsidiary of Cutler-Hammer. He develops newtechnologies for on-line monitoring and diagnostics of highvoltage electrical equipment, produces monitoring equipmentand provides on-site expert evaluations of equipmentcondition for electric utilities and industrial customers.

Claude Kane has nearly 35 years of experience in theinstallation and preventive and predictive maintenance

practices on a large variety of power distribution, transmissionand generation equipment. He started with Westinghouse in1972 as a field service engineer in Kansas City, MO and hasheld a number of technical and management positionsthroughout his career. Claude is currently the Engineering andProduct Line Manager for the Cutler-Hammer PredictiveDiagnostics Group, based in Minneapolis, MN.

Documents

2005 EIC - Bushing Paper