Experience with On-Line Diagnostics for Current Transformers.pdf

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Fall 2002 1

Special Feature

The industry has always sought better tools to assess the general

condition of high-voltage equipment and to identify potential prob-

lems. Today’s microprocessor technology provides a sophisti-

cated means to capture information. While much of this technology

has already been integrated into off-line techniques, it also adapts

well to on-line measurement.

One of the primary challenges to the on-line approach is the manage-ment of a tremendous amount of data. Fundamentally, the desired resultof the diagnostic test is to:

1. Determine the status of the apparatus,2. Determine how critical any detected anomalies are, and

3. Determine how soon those anomalies will require attention.

This level of information analysis can not be achieved with a simplemonitoring system; it requires an expert system that can digest all of thedata being generated. It is this expert system which differentiates an on-line monitoring system from an on-line diagnostic approach. This articleoutlines how on-line diagnostics can be applied to evaluating bushingsand also shares some applications experiences.

Off-line diagnostics have served the industry well, and they continueto play an important role in the reliability of the power system. How-ever, the off-line application has some inherent flaws — for example, the

need to remove the apparatus from service and the inability to identifyserious changes taking place between test intervals. If the gestation pe-riod (time from normal condition to failure) is shorter than the test inter-val, then the test result will never exhibit the problem. The frequency of the on-line measurements provides the ability to determine the rate of change, knowing if the change was gradual over a long period of time orsudden, as well as when the change took place. This is all informationthat can be used to determine the appropriate course of action. An appa-ratus owner may decide to tolerate an abnormality that has stabilizedand let the expert system determine when the situation becomes morecritical, or plan corrective action at a more optimal time.

by Robert Brusetti, PE

Product Manager Doble Engineering

To achieve this level of confi-

dence, one must place a great dealof trust in the expert system. Theexpert system must distinguish between noise and actual change,identify a wide array of problemsat the incipient state, and provideevidence as to why an alarm is be-ing issued as well as some type of traceability. An expert systemshould not rely solely on thresh-olds set from conventional off-linemethods, which traditionally have been conservative due to the limi-

tations of the approach. The diag-nostics should be capable of learn-ing the specific characteristics of the individual apparatus and notrely solely on user limits or aver-age values from other similar ap-paratus. The expert system should be capable of learning the normal behavior of the apparatus beingevaluated and apply this to theanalysis.

E xperience with On-line Diagnostics for Bushings

and Current Transformers



2 NETA WORLD

Why apply on-line diagnostics to bush-ings? Bushings are certainly not the mostexpensive piece of equipment in a substa-tion, so the financial loss of a bushing fail-ure is not the driving force. However, thedamage that a failed bushing can inflict onits affiliated apparatus could indeed be cata-strophic. Specifically, bushing failures lead-ing to a damaged power transformer have

been well documented. The role of the bush-ing subjects them to high dielectric, thermal,and mechanical stresses, which tends tomake bushings one of the most vulnerablecomponents of major apparatus. Most bush-ing failures can be attributed to internal de-terioration or contamination, so being ableto detect these irregularities is essential tomaintaining a stable system. Many of thesame dynamics affect the performance of current transformers. For stand-alone cur-rent transformers the insulation system isvery similar to bushings, making it feasible

to apply the same diagnostic tools.The industry has accepted the conven-

tional off-line power factor/capacitance testas the most reliable tool for identifying prob-lem bushings and current transformers. Itis the success of the power factor/ capaci-tance measurement in bushing diagnosticsand the awareness of the catastrophic resultsof bushing failure that have led industryexperts and insurance providers to recom-mend more frequent testing of bushings.These guidelines are in conflict with the cur-rent philosophy of the industry, which is to

minimize downtime. The concept of bush-ing/current transformer on-line diagnosticscombines the advantage of the power fac-tor/ capacitance test with the ability to per-form the measurements on-line, without in-terruption of service.

One approach to determining in-servicecondition of bushings is to calculate the imbalance current measured at their tap for athree-phase set, Figure 1A. The sum currentmethod is based on the principal that in asymmetrical three-phase system, the sum of the voltage and current vectors is zero, Fig-ure 1B. This allows the condition of bush-ings to be determined by vectorially add-ing the currents measured at the bushingtaps. If the bushings are identical and sys-tem voltages are perfectly balanced, then thesum current will equal zero. In this situa-tion, the expert system would only need torely on the most recent recordings to deter-mine the condition of the bushings. Since

Case Story

The high power factor associated with the 138 kV

bushings was realized during the commissioning

of the on-line diagnostic system. The bushings were

General Electric, Type U, manufactured in 1972, in-stalled on a 224 MVA transformer. Bushings that ex-

hibited this level of power factor would typically be

removed from service, especially the phase C bush-

ing which registered a power factor greater than three

times the nameplate. In this situation, spare bushings

were not available; thus, it was decided to rely on the

expert system to monitor the situation. This would

provide an opportunity to prove that this technology

was not only capable of detecting a problem but also

would permit the user to continue operating with a

known problem. As long as the situation remainedstable, a high power factor bushing could remain in

service. In order to be successful, the expert system

would have to overcome the thermal fluctuation to

be able to track the rate of change. The on-line diag-

nostic system was installed in April 1997, and the sys-

tem continued to monitor the bushing for more than

a year without issuing any alerts. In May the follow-

ing year, the transformer began running hotter than

usual, thus triggering the expert system to issue an

alert based on a significant change in sum current. The

expert system identified the power factor on the phaseC bushing to be the cause; it also noted there was no

significant change in the bushing capacitance. This

suggested that the problem with the phase C bushing

was contamination. With spare bushings available, the

utility elected to replace the bushing. The plan is for

removed phase C bushing to be energized again in a

protected environment with an on-line diagnostic sys-

tem installed and run to failure, in order to learn more

about the behavior of bushings just prior to failing.

This example provides an opportunity to point out

the importance of the rate of change. If the expert sys-tem relied solely on a limit, it is very likely that it would

have triggered alarms soon after installation, requir-

ing the asset owner to make a decision based solely

on absolute values. In order to arrive at an intelligent

conclusion about the bushing’s condition, the power

factor and capacitance values along with rate of change

needed to be available.



Fall 2002 3

bushings/current transformersare never identical and systemvoltages are never perfectly bal-anced, the sum current is a non-zero value, which is unique to the bushing/current transformer set.As a result, the sum current is avector unique to that bushing set,Figure 1C. The expert system es-

tablishes a benchmark sum currentduring an initial learning periodthat is then compared to the con-figuration data, which consists of the last off-line measurement and/or nameplate data. This configu-ration is also used in the analysisto determine the present powerfactor and capacitance of the prob-lem bushing/current transformer.

This benchmark value of sumcurrent is compared to subsequentmeasurements. Subtracting the

benchmark from the latest mea-sured sum current provides a thirdphasor, which is referred to as the‘change in sum current,’ Figure 1D.The angle of this third vector withrespect to the reference bushing isused to identify which bushing iscausing the change. Once the de-teriorated bushing is known, themagnitude and phase of thechange in sum current vector isused to calculate the change in ca-pacitance and power/dissipation

factor of this bushing. The quadra-ture component of the change insum current is used to calculate thechange in capacitance while the in-phase element can be attributed toa change in power/dissipation fac-tor, Figure 1E.

This exercise calculates the ab-solute values of the power factorand capacitance of the dominant bushing (the bushing experiencingthe greatest degree of degrada-tion). This essentially replicates theoff-line tests, provided the test con-ditions are the same. The advan-tage of the on-line approach is thefrequency of data points, whichprovides the means to determinethe rate of change. The expert sys-tem arrives at this information byperforming a least-squared fittingon a subset of consecutive powerfactor and capacitance values ac-

cumulated over a specified period of time. This exercise will produce aquadratic polynomial equation that, if plotted, would generate a curvethat provides a “best fit” through a series of points (in this case powerfactor and capacitance). By representing a series of power factor and ca-pacitance values as a polynomial, the analysis can use applied math-ematical tools to determine the stability of the situation. From this infor-mation, the expert system can reach a more informed conclusion on thecriticality of the incident.

This technique can also be applied to stand-alone current transform-

ers with and without taps. If the current transformers are equipped withtaps the imbalance current is calculated using the tap current, similar to bushings. To apply the sum current approach to current transformerswithout taps typically requires electrically isolating the current trans-former from the ground grid with the exception of one grounding point.The current measured at the grounding point is used in place of the tapcurrent to calculate the imbalance current.

Previously, it was suggested that the absolute power-factor measure-ment using the on-line sum current approach would duplicate the off-line measurements, provided the test conditions were similar. The keypoint in this statement is the test condition. The experience from off-linediagnostics indicates that bushing problems affecting power-factor mea-surements are accentuated at elevated voltage and temperature. The in-

fluences of voltage and temperature on good and deteriorated insula-tion are illustrated in Figure 2. The plots track the power factor using theconventional off-line C1 (center conductor to tapped layer) insulation of two bushings as test conditions (voltage and temperature) are varied.The two specimens are both 115 kV bushings of the same manufacturer,type, and vintages. One is considered to have good, low power factorwhile the other has deteriorated, high power factor. Figure 2(A) showsno change in power factor for the good bushing as the voltage was rampedup to rated voltage, while the deteriorated bushing exhibited an increase.A similar behavior can be observed in Figure 2(B), when the power fac-

A

B

C

A

B

C

C

C

CC

C

C

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I

I

I ∑

1

1

12

2

2

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A B

∑= 0

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Figure 1

Tap Current Measurement and Summation

(A)

Sum of Phasors in a Symmetrical Three-Phase System

(B)

Real Three-Phase SystemInitial Sum Current is a Non-Zero Value

(C)

Change in Sum Current is Equalto a Change in Busing Current

(D)

Change in Sum Current Vector used toDetermine Change in Power Factor,

Capacitance and Identify Problem Busing(E)



4 NETA WORLD

Figure 3

tor of the two bushings was measured at increasingtemperatures. This phenomenon was realized in anon-line situation when two sets of three bushings, oneset containing a degraded bushing, were correlatedto the top oil temperature of the transformer on whichthey were installed. Figure 3 shows seven days of topoil temperature plotted with sum current data fromtwo sets of Type U bushings in the same transformer.The middle plot is for the high-side (230 kV) bush-ings demonstrated “good”: 0.3 percent, 0.32 percent,

and 0.29 percent. The top plot is for the low-side (138kV) bushings with elevated power factor values: 0.73percent, 0.9 percent, and 1.08 percent (phases A, B,and C respectively). The influence of temperature(lower plot) on the sum current is clearly evident, pro-viding on-line confirmation that deteriorated bush-ings experience greater fluctuations caused by changein the operating environment.

The intent of this discussion is not to lobby for theexclusion of the traditional method of apparatus test-ing, but to show that in certain applications on-linediagnostics can be a better alternative. The conven-tional off-line power factor test for bushings and cur-rent transformers continues to be an excellent tool toevaluate their condition; however, there are situationswhere its inherent constraints can not be tolerated.Bushings which are tied to critical system apparatusor which can not be readily removed from service arecandidates for on-line diagnostics. The on-line diag-nostics should offer the possibility of duplicating thetraditional tests as well as taking advantage of thelarge sample size to perform trending and projectionof potential problems.

Figure 2

Test Voltage (KV)(A)

Temperature (C)(B)

C1%P

owerFactor

References:Mark F. Lachman, Stephen Skinner and Wolf Walter

“Experience with On-Line Diagnostics and Life Management of High Voltage Bushings,” Proceeding of thSixty-Six Annual International Conference of Doble Clients, 1999, Sec 3-4

Mark F. Lachman, Wolf Walter, and Philip A.VanGuggenberg, “Experience with Application of SumCurrent Methods to On-Line Diagnostics of High Volt

age Bushings and Current Transformers,” Proceedinof the Sixty-Fifth Annual International Conference of DoblClients, 1998, Sec 3-5

Donald T. Angell, Reneé B. Kringel, Stephen Skinner, “Report of On-Line Diagnostics at Idaho Power,Proceeding of the Sixty-Fifth Annual International Con ference of Doble Clients, 1998, Sec 3-6

Paul J. Griffin, Dennis J. Kopaczynski and Mark HRivers, “Continuous Diagnosis: The Ultimate DefensAgainst Failures” Proceeding of the Sixty-Fourth AnnuaInternational Conference of Doble Clients, 1997, Sec 1-2

Robert Brusetti, PE, received his Bachelor of Science degrefrom the University of Vermont in 1984 and a Masters in BusinesAdministration from Boston College in 1988. He has been employed at Doble Engineering Company for the past twelve yearand is currently Product Manager. Prior to his present position hworked as a field engineer and assisted in the development of thexpert system for the Insite on-line diagnostic system. Mr. Brusetis a licensed Professional Engineer in the state of Massachusett

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Experience with On-Line Diagnostics for Current Transformers.pdf