ON-LINE DIAGNOSTIC CASE STUDY INVOLVING A GENERAL ELECTRIC TYPE U BUSHING

Transcription

1 ON-LINE DIAGNOSTIC CASE STUDY INVOLVING A GENERAL ELECTRIC TYPE U BUSHING Pamelyn Bahr and Jon Christensen Intermountain Power Service Corp. Robert C. Brusetti, P.E. Doble Engineering Company ABSTRACT If your on-line bushing monitoring system indicated that a bushing had gone from good to a state of serious deterioration, 5% power factor, in the matter of a few hours how would you respond? Complicate the matter by placing the bushing on the 345 kv winding of a 968 MVA GSU that feeds the base load of Los Angeles. This was the situation facing Intermountain Power Service Corporation the morning of September 13 th, After an exhaustive analysis of the data, which attempted to eliminate all peripheral factors, the decision was made to take the unit off-line. This paper will evaluate the on-line data leading to the decision to remove the GSU from service, and correlate the data with the field test results obtained once the unit was removed from service. The finding of the subsequent factory tear down will illustrate how changes in the capacitance and power factor are manifested in a bushing insulation system. BACKGROUND The on-line monitoring system was installed on the high side bushings of a generator step up transformer rated 968 MVA, 345/26 kv. The transformer was manufactured by GE in The unit feeds a DC line that transmits to the Los Angeles area. The bushing nameplate information is listed in Table 1. TABLE 1 Bushing Nameplate Information Dsg Serial Mfr Type C1 C1 %PF Cap kv Amps Year H GE U H GE U H GE U H GE U X GE T X GE T X GE T The last off-line measurements were performed in March of 2003 (see Table 2), and the on-line bushing diagnostics were installed in June Over the next two years the on-line system showed only a slight capacitance increase in the H3 (phase A) bushing (less the 5pF), which represents less than a 2% increase, with no noticeable change in the power factor. The other bushings did not manifest any changes in either capacitance or power factor. Note these values are extracted from the on-line leakage measurement using a modified sum current approach. On June 10 th, 2005 the transformer was de-energized as part of a plant outage, the unit went back into service on July 14 th, Once back in service in the leakage current of 1

2 the H3 (phase A) bushing exhibited a slight increasing trend. Over a three month period this change represented only a few picofarads increase in capacitance. TABLE 2 March 2003 Off-Line C1 Test Results ID Serial NP %PF NP Cap Test kv ma Watts %PF corr Corr Fctr Cap(pF) IR auto IR man H G H G H G H G X G X G X G RAPID DEGRADATION OF BUSHING INSULATION On September 9 th, 2005 the relative angle associated with the phase A bushing began to decrease. Over the next three days the measured angle decreased by almost half a degree. On September 12 t,h, 2005 at 21:00 the current began to increase,. In the span of seven hours the H3 (phase A) current magnitude went from approximately 24.8mA to 27.2mA and the angle decreased by 2.5. The expert system, which on this system used the sum current, issued an alert based on the drastic change in the sum current. The diagnostics calculated the power factor of the phase A bushing at 5.6% and the capacitance at 350pF. After a comprehensive review, which eliminated the typically external factors that could influence the bushing leakage current, the plant manager was advised of the findings and the decision was made to take the transformer out of service. Once removed from service the bushing was immediately tested; the offline test measurement for the phase A bushing was 333 pf and 5.5% power factor; which is a dramatic increase in the C1 power factor and capacitance from the March 2003 value of 0.27%, and pf. DISCUSSION OF RESULTS Figure 1 plots the hourly current magnitude of the three bushings and attempts to identify the gaps in the data. Figure 2 displays the same information normalized to the reference bushing (which is Phase B). Both Figures 1 and 2 exhibit an increase in the Phase A bushing (H3) current magnitude which becomes more evident following the unit returning to service in July. However, the total change in current magnitude through September 12, 2005 is only 2%. Changes in current magnitude reflect a change in the bushing capacitance. Traditionally a change in capacitance of 2% would not warrant corrective action. Figure 3 is the capacitance calculation for the H3 bushing using the analysis of the on-line bushing diagnostic system. The analysis used by the instrumentation monitoring the bushing employed three trending cycles, one day, one week and one month. Trending over various periods is required to reducing the influence of external factors, such as power system fluctuations and imbalances. The information plotted in Figure 3 suggests the capacitance between January 2005 and the outage in June was essentially stable. Once the unit was put back in service (July 2005) the capacitance of Phase A showed a tendency toward increasing. However, over a two month period the increase was less than 1%, and the total change in capacitance from the start of the on-line monitoring until September 12, 2005 was less than 10 pf. Traditionally these are not levels that would trigger an investigation or a bushing replacement. Over a seven hour period on September 12 th the capacitance of the H3 bushing increased significantly, by more than 10%, based on the daily trend. The rapidly changing condition of this bushing illustrates the need 2

3 for trending over both short and long periods. Long trending cycles employ more data and therefore are less sensitive to external influence; however the longer trending cycles also require more time to accumulate the information and consequently will take longer to register the anomaly 1. Figure 4 plots the phase angle deviation. Note that phase B will always be zero degrees, since it is being used as the reference. There is essentially no change in the angle of the leakage current of the H3 bushing through September 9, From September 9 th to the 12 th the phase angle of the H3 bushing decreased by approximately half a degree. Figure 5 which trends the power factor of the H3 bushing translated the degree in angle to approximately 0.5% power factor increase based on the daily trend. There is a direct correlation between the leakage current angle and the power factor of a bushing. Over the same period of September 12 th when the magnitude of the leakage current of H3 was increasing the leakage current angle decreased by 3 O. Referencing the daily power factor trend on Figure 5 this translated to a power factor of approximately 5.5%. Based on the on-line data the H3 bushing had experienced significant physical change (capacitance) and degradation of the insulation (power factor). Change in capacitance associated with a bushing insulation system is usually associated with short circuiting of condenser layers. June 10, 2005 July 14, 2005 March 31, 2004 January 31, 2005 Hourly Phase Current Magnitude FIGURE 1 3

4 September 12, 2005 Normalized Hourly Current Magnitude FIGURE 2 Capacitance Trend Phase A Bushing (On-line) FIGURE 3 4

5 September 9, 2005 Change in Phase Angle FIGURE 4 % Power Factor Trend of Phase A Bushing (On-line) FIGURE 5 Table 3 represents the bushing C1 test results after the transformer had been removed from service. The temperature compensated H3 power factor was 4.97% and the capacitance had increased to 333.5pF, essentially verifying what the expert system was observing on-line. There were no significant changes in any of the other bushings. 5

6 TABLE 3 September 15, 2005 C1 Test Results ID Serial NP %PF NP Cap Test kv ma Watts %PF corr Corr Fctr Cap(pF) IR auto H G H G H B H G X G X G X G Table 4 contains the dissolve gas analysis results (in ppm) of a sample drawn from the H3 bushing following the September 15 th tests. The presence of high temperature gases, specifically acetylene, and the high quantities measured indicate severe arcing taking place. TABLE 4 DGA Results, Sampled September 15, 2005 Hydrogen H Oxygen O Nitrogen N Methane CH Carbon Monoxide CO 668 Ethane C 2 H Carbon Dioxide CO Ethylene C 2 H Acetylene C 2 H BUSHING TEARDOWN Since both the on-line and off-line test indicated the bushing should not be placed back into service, the bushing was sent to the ABB factory in Alamo TN, were it was dismantled. With the oil drained and the porcelain weather shed removed, dark areas on the lower half of the paper core extending to the exposed conductor were immediately evident (Figure 6). The report from ABB attributed this sooty appears to an indication of partial axial failure. The compositions of these discolorations were later verified, through analysis of the paper, to be carbon deposits. With the outer cellulose removed, overheating was observed on the edges of the paper (Figure 7). Signs of overheating were also observed on the seam of the foil that makes up the grounded condenser layer (Figure 8). The bushing condenser design employed the Rescon Ink condenser (Figure 9). The Rescon (conductive ink) is graphite suspended in a resin solution. The Rescon ink is applied to kraft paper in a herringbone pattern. The bushing core is made up of two electrical grade papers wound onto a single conductor. An ink-lined paper forms the capacitive layers while plain kraft paper is used to establish the insulation between the conductive layers. One of the more common failure mechanisms of the GE Type U bushing is ink migration to the insulating layer, which leads to partial discharge and/or overheating of paper 2. This phenomenon was observed as the core was unwrapped. Figure 10 exhibits how the fault was traveling across multiple layers and Figure 11 shows a similar event where the puncture damage was severe. 6

7 However, no fault location punctured the entire condenser insulation; most only traveled a few layers. The ABB report noted five areas where the burned damage penetrated multiple layers. An interesting observation made by the ABB representative Lonnie C. Elder: It is interesting to note that all the areas of localized burning were actually connected electrically to each other, either through the arcing that burned through adjacent layers of paper, or via the herringbone shaped printed conducting lines. Some of the burned areas were spaced axially quite far apart, but if one followed along the affected printed lines, one of these lines would connect through one of the other burned areas The degree of polymerization of various paper samples ranged between 900 and 1000, suggesting there had not been significant paper aging. Carbon Deposit Bottom Portion FIGURE 6 Indication of Overheat of Paper Edges Portion FIGURE 7 7

8 Discoloration on the Seam of the Grounded Condenser Layer FIGURE 8 Herringbone Conducting Paint Design FIGURE 9 8

9 Overheat Across Multiple Layers FIGURE 10 Insulation Breakdown across Multiple Layer FIGURE 11 9

10 CONCLUSION What is unique about this bushing problem is how quickly the insulation medium deteriorated. Prior online experience involving degraded bushings suggests a long gestation period, with the end user opting to replace the bushing before the condition becomes critical. In this scenario the quality of the bushing insulation deteriorated in a period of three days to the point that the bushing should be removed from service, with no prior indication of degradation. The plotted results of the H3 bushing capacitance seem to suggest a very gradual increase, approximately 2% over a two year period, a rate that typically would not warrant corrective action. It has always been theorized that prior to dielectric breakdown (failure) the insulation medium experiences rapid degradation. The behavior of the H3 bushing may appear to support this theory, however the lack of any interval when the condition is worsening presents a different scenario. While it is impossible to predict when the bushing would have failed, both the on-line and off-line measurements indicated the bushing had to be removed from service. The subsequent tear down of the bushing substantiated the decision to remove the bushing from service, as it revealed burning within the paper insulation. The ABB report attributed the change in power factor to increase real losses in the bushing core. The change was the result of shorted condenser layers. While 3.8% change in capacitance typically does not warrant investigation, the report brings up a valid point. There are thousands of individual herringboneshaped conductive lines, each representing a capacitive value. Even if a few are short circuited, this is still a small percentage of the total number of condenser lines. Therefore, one should not expect the capacitance to increase significantly in Type U bushings with the herringbone design of condenser, even though a significant problem may exist. REFERENCES: 1.Brusetti, R.C., "Update on On-Line Bushings Diagnostic," Seventy-Third Annual International Conference of Doble Clients, 2006, page 3. 2.Duarte, E. M., "A Review of Past Bushing Problems," Seventy-Third Annual International Conference of Doble Clients, 2006, page 10. BIOGRAPHY: Pamelyn J. Bahr received her Bachelor of Science in Electrical Engineering degree from Utah State University in 1986, and a Masters in Business Administration from Brigham Young University in She has been employed at Intermountain Power Service Corporation for the past 19 years, and is currently an Electrical Engineer in the Instrumentation and Electrical Engineering Group. Jon P. Christensen, P.E., received his Bachelor of Science in Electrical Engineering degree from Brigham Young University in He has been employed at Intermountain Power Service Corporation for the past twenty years, and is currently the Engineering Supervisor for the Instrumentation and Electrical Engineering Group. Prior to working for Intermountain Power he worked for the Los Angeles Department of Water and Power in their Quality Assurance Group. Mr. Christensen is a licensed Professional Engineer in the state of Utah. 10

11 Robert C. Brusetti, P.E., received his Bachelor of Science in Electrical Engineer degree from the University of Vermont in 1984 and a Masters in Business Administration from Boston College in He has been employed at Doble Engineering Company for the past sixteen years and currently works in the Client Service department as Product Marketing Manager. Prior to his present responsibility he has held positions as Product Manager and Principal Engineer. Mr. Brusetti is a licensed Professional Engineer in the state of Massachusetts. 11