Bushing C2 Paper
THE DOBLE TAP-INSULATION TEST FOR BUSHINGS (A REVIEW) D. J. Kopaczynski and S. J. Manifase Doble Engineering Company * GENERAL This report reviews the Doble test as applied to the tap-to-ground insulation of bushings. However, much of the material presented also applies generally to the tap insulation of certain designs of current transforners1,2 and cable potheads equipped with taps.3,4 The following are the major topics covered: 1. Historical Review of the Doble Tap-Insulation Test 2. Comparison of Test (Power-Factor) Taps and Potential (Capacitance) Taps 3. Recommended Voltages to be Applied to Tap Insulation 4. Tap-Insulation Test Techniques 5. Analysis of Tap-Insulation Power Factor and Capacitance 6. Field Experiences Material from a variety of sources has been assembled with the aim of providing a comprehensive review. However, the main message of this report can be stated briefly, as follows: ALWAYS MEASURE THE CONDITION OF THE TAP-TO-GROUND INSULATION WHEN PERFORMING DOBLE TESTS ON BUSHINGS EQUIPPED WITH TEST TAPS AND POTENTIAL TAPS In bushings the tap-to-ground insulation (or, tap insulation) is designated as C 2, whereas the main insulation between the center conductor and tap is referred to as C 1. HISTORICAL REVIEW OF THE DOBLE TAP-INSULATION TEST In a 1956 Doble Conference paper5 a manufacturer made reference to limiting to 5 kV the test voltage that could be applied to the "capacitance" taps of their bushings. This indicates that some Doble clients were performing tap-insulation tests at that time and perhaps also doing inverted UST measurements (Ungrounded-Specimen Tests).6,7 A 1960 paper by the Doble Engineering Company provides the first direct reference to the tapinsulation tests.8 In that paper the following statement appears with regard to capacitance (potential) taps: "experience has indicated that a supplementary test on the tap-to-ground insulation (C 2) is desirable to check the condition of the core portion of C 2 and of the tap bushing." In 1961 a paper was presented by the Doble Company dealing exclusively with the tap-to-ground insulation of bushings,9 particularly those bushings equipped with power-factor test taps. In that paper, in addition to reaffirming that Doble tests should be performed routinely on capacitance-tap insulation, there was a recommendation that the power-factor tap of General Electric Company Type "LC" bushings should also be tested routinely because, it was discovered, moisture could enter the main chamber of these bushings through deteriorated gaskets on the tap plugs. 9,10,11 It is appropriate at this time to reaffirm our previous recommendations, and to emphasize now that the tap-to-ground
insulation of ALL bushings with taps should be considered as an integral component to be included in all routine Doble test programs; again, not only for bushings with potential taps, but also for those bushings equipped with power-factor test taps. Supplementing the UST measurement of the main C1 insulation with a check of the tap insulation, C2, provides a more complete picture of the total condition of bushings (and, current transformers). With emphasis by today's power system operators to extend the interval between routine tests, maintenance and test specialists must use all tools available for finding not only serious defects that can have immediate consequences, but also incipient defects that could get gradually worse and ultimately result in failure before the next test period. Based on the replies to Doble technical questionnaires in 1985, it might well be three years or more before the next test is scheduled to be made. 12,13 COMPARISON OF TEST (POWER-FACTOR) TAPS AND POTENTIAL (CAPACITANCE) TAPS The following definitions are taken from a proposed revision of an IEEE (The Institute of Electrical and Electronics Engineers, Inc.) Standard:14 BUSHING TEST TAP A connection to one of the conducting layers of a capacitance graded bushing for measurement of partial discharge,power factor, and capacitance values. BUSHING POTENTIAL TAP A connection to one of the conducting layers of a capacitance graded bushing providing a capacitance voltage divider. Note: Additional equipment can be designed, connected to this tap and calibrated to indicate the voltage applied to the bushing. This tap can also be used for measurement of partial discharge, power factor and capacitance values. The "Bushing Test Tap" is commonly referred to as a power-factor tap; however, the term "test tap" is preferred. Modern "condenser-type" bushings (now referred to as "capacitance-graded" bushings) rated 15 to 69 kV are usually equipped with a test tap, whose primary purpose is to provide the capability of making a separate test on the main insulating core of the bushing without the need of isolating the bushing from the apparatus. Certain designs of high-Voltage (HV) and extra-high-Voltage (EHV) cable potheads are equipped with test taps. 3,4 The "Bushing Potential Tap," formerly referred to as a capacitance tap, is found generally on bushings rated above 69 kV, and is also included in some designs of HV and EHV condenser paper oil-filled current transformers (CTs).1,2 The potential tap is like the test tap in that it can be used for test purposes; however, unlike the test tap, which has a relatively low-Voltage rating to ground, the potential tap is designed to provide an appreciable Voltage (at least several kV typically) for operating a potential device. There are special cases whereby some bushings rated 69 kV have been provided with potential taps while certain bushings rated above 69 kV have been equipped with test taps. Also, there are instances of bushings having both a potential tap and a test tap; for example, certain bushings manufactured by The English Electric Company Limited and Passoni & Villa (Italy). Besides the fact that the tap electrodes and housings of test taps are generally smaller than their potential tap counterparts, the way in which the tap-to-ground insulation is made up inside the bushing is also different, as noted in the following: GENERAL CONSTRUCTION FEATURES OF BUSHING TAPS Test Taps The tap connection is made to the last conducting layer of the main capacitance-graded insulating core. The tap insulation includes, besides the tap insulator*, the insulation between the last
con ducting layer of the core and the bushing ground sleeve and mounting flange. See Figure 1 (a). Potential Taps The tap connection is made several conducting layers in from the outer diameter of the capacitance graded insulating core. A permanent ground connection is made between the outer metallic ground sleeve and the outermost conducting layer of the core. The tap insulation includes, besides the tap insulator*, the insulation between the internal conducting layer to which the potential tap is connected, and the permanently grounded outer conducting layer of the core. See Figure 1(b). *The Tap Insulator refers to the assembly that brings out from the main chamber of the bushing the lead which connects to the internal conducting layer of the main insulating core. Figure 1 illustrates the basic differences between bushings with test taps and those with potential taps. 15
The capacitance of the tap-to-ground insulation, C2, differs among the various manufacturers' designs. However, the following generalizations can be made: COMPARISON OF CI AND C2 CAPACITANCES FOR BUSHINGS WITH TEST TAPS VERSUS BUSHINGS WITH POTENTIAL TAPS
The C2 capacitance of bushings with potential taps may be of the order of ten times the C 2 capacitance of bushings with test taps.
For bushings with test taps, the C1 and C2 capacitances are often approximately equal.
For bushings with potential taps, the C2 capacitance is often approximately ten times the C 1 capacitance.
RECOMMENDED VOLTAGES TO BE APPLIED TO TAP INSULATION The following are Doble's recommended test voltages to be applied to the tap insulation of bushings:6,16 DOBLE RECOMMENDED TEST VOLTAGES TO BE APPLIED TO THE TAP INSULATION OF BUSHINGS Test Taps - 500 Volts (Except for the Ohio Brass Class L bushing tap, to which no more than 250 Volts should be applied) Potential Taps - 2000 Volts (A higher Voltage, up to 5000 Volts maximum, may be applied in instances where a higher tap-Voltage rating is known) TAP-INSULATION TEST TECHNIQUES To test the tap insulation alone, the technique shown in Figure 2 is used. 6,7,9
Standard Tap-Insulation Test Method Measurement Of C2 Only (Bushing Center Conductor Guarded) FIGURE 2 An alternative method of checking the tap insulation is shown in Figure 3.
Test Mode: GST-GROUND Alternative Method Tap-Insulation Test Measurement of the Parallel Combination C1 + C2 (Bushing Center Conductor Grounded) FIGURE 3 The technique illustrated in Figure 2 is the preferred method since it measures the condition of the tap insulation (C2) alone. In some cases the technique shown in Figure 3 may be necessary, particularly for bushings with test taps, which usually have relatively low capacitance and are tested at low voltage. That is, in the presence of strong electrostatic interference, the technique shown in Figure 2 can result in large reversals of the Watts/Milliwatts readings since a guard connection is made at the tops of bushings, which may be exposed to stong electric fields due to their proximity to energized overhead lines. While Doble field power-factor test sets are specially designed for coping with electrostatic interference, Doble test engineers may elect to use the alternative method for checking the tap insulation. Note that grounding the bushing center conductor (Figure 3) essentially eliminates the possibility that electrostatic interference current will be introduced into the test set measuring circuit.6,7,9 For bushings that are tested using the technique shown in Figure 3 (i.e. C 1 + C2) the Equivalent 10kv or 2.5-kv Values of current and Watts/Milliwatts are measured and, from these values, the current and loss obtained for the Ungrounded-Specimen Test (UST) on C1 may he subtracted in order to
obtain the current and loss Of C2 alone. Refer to the Instruction Manuals6,7 for comments on the use of Doble 10 10-KV and 2.5-kV test sets at reduced test voltage, including determining Equivalent values of current and loss. This subject was also discuss- discussed in our 1961 paper.9 For bushings equipped with potential taps the C2 capacitance is usually much higher than the C1 capacitance. Accordingly, even when C2 is tested in parallel with C1 using the method shown in Figure 3, the current and loss will be primarily the contribution of C 2 alone. Thus, defects in the C2 insulation of these bushings will be obvious without subtracting the current and loss recorded for the C 1 UST measurement For bushings that operate a potential device, the initial tap test is performed with the potential device cable disconnected between the bushing tap and the potential device. The potential device cable is also tested separately. 17 Then, a test is performed with the potential device cable connected to the bushing tap but isolated from the device network. This third test becomes the benchmark against which subsequent tap tests are compared since the potential device cable is not normally disconnected from the bushing tap for routine tests. If questionable results are obtained on future tests, the potential device cable is then isolated at the bushing tap, and separate tests are performed both on the bushing tap, and on the potential device cable. 17APPENDIX A includes, specific details with reference to making Doble tests on bushings with connected potential devices. ANALYSIS OF TAP-INSULATION POWER FACTOR AND CAPACITANCE Tests on the tap insulation, C2, of bushings are analyzed on the basis of the percent power factor and capacitance(or, total charging Current). Percent Power Factor Tap insulations are expected to have measured power factor of order of 1%, or less. This is especially true in the case of potential taps. Bushings with test taps tend, on average, to have slightly higher C 2 power factor than bushings with potential taps. In fact, some have exceptionally high power factor; for example, Lapp Insulator Company Type "PRC" bushings.18 The use of high power-factor materials in the tap circuit should be discouraged since, as explained later, the tap-insulation power factor can be an indicator of serious bushing problems not revealed by other tests. Thus, the use of high-loss materials in one area of the tap circuit (e.g. in the tap insulator as shown in Figure 1) can mask unintentional high-loss conditions else where in the tap-Insulation circuit (see Figures 1 and 4d). The tap-insulation power factor is compared with the factory value, if any ,recorded on the nameplate; however, most bushings and current transformer do not have a nameplate power-factor value for C2. The results of initial and follow-up tests for similar bushings should be compared. Once a field benchmark C2 power-factor value has been established for a given bushing, then note any significant changes (higher and lower) relative to the benchmark and if other similar bushings tested at the same time exhibit a similar change. In general, like all low power-factor insulation systems, a doubling of the measured C 2 power factor is noteworthy. In the case of bushings with relatively high benchmark power factor, a smaller percentage change can indicate a problem. In the latter case it is important to compare similar bushings tested at the same time. The power factor of tap insulations are not corrected for temperature. However, that is not to say that they do not vary with temperature. It may well be that in some instances the power factor varies significantly with temperature. In certain cases the cap insulation may behave with temperature similar to the main (C1) insulation. For example, in the case of bushings with potential tap (Figure lb), the major capacitance for the tap insulation is drawn from the core; that is, that portion of the core between the tapped layer and the permanently grounded foil layer. For this type of bushing the tap-
insulation power factor versus temperature characteristic may possibly be similar to that of the main C1 insulation. However, specific data is lacking at this time . Whenever a significant power factor difference is obtained for tests performed at widely different temperatures, repeat tests should be performed at higher or lower temperatures as the case may be. Generally, however, comparing the C2 power factors of similar bushings in the same apparatus is sufficient to resolve any questions There should be concern whenever one of several similar bushing, does not correlate with the others. Capacitance (Total Charging Current) For the initial test the C2 capacitance should he compared with the factory-measured nameplate value, if any, and with the capacitance obtained for similar bushings tested at the same time. The measured capacitance is typically ±2% of nameplate. For follow-up tests look for changes in capacitance relative to the benchmark value; however, compare the results for similar bushings in the same apparatus and note if the C2 capacitance of each bushing has changed similarly compared to their respective benchmark values. In the case of significantly higher-than-nameplate capacitance, check the test procedure; if the tap test was performed with the center conductor grounded, then the measured C 2 capacitance will be higher than the nameplate value by an amount approximately equal to the C 1 capacitance (see Figure 3). For bushings used with potential devices, benchmark data is obtained with the potential device cable disconnected in order to compare the measured C2 capacitance with nameplate. An additional test is performed with the potential device cable connected in order to establish a benchmark value against which subsequent tests are compared. It is not often that the C2 capacitance increases: however, as for any insulation system generally, an increase in capacitance can be the result of short circuiting of a portion of the insulation. An increase in capacitance could also be the result of some unusual condition, perhaps similar to that reported by two clients at the 1987 Doble Conference, 19,20 whereby the capacitance of bushings changed due to cracks in the varnish coating of the lower bushing section, allowing transformer oil to migrate into the bushing core insulation. Change in capacitance of the C2 insulation is more apt to occur in the direction of lower-thannormal value, Some possible reasons for lower-than-normal C 2 capacitance are as follows: CAUSES OF LOWER-THAN-NORMAL CAPACITANCE FOR THE TAP INSULATION (C2) OF BUSHINGS
High resistance or open circuit between the tap electrode and tapped layer.
For potential taps, occurrence of high resistance or open circuit between the normally permanently grounded foil layer of the core and the metallic ground sleeve. See Figure 1 (b).
Loss of bond between the metallic ground sleeve and the mounting flange.
Mounting flange becoming ungrounded.
Refer to the APPENDIX B for a summary of important factors relating to Doble testing the tap insulation (C2) of bushings. FIELD EXPERIENCES It is not obvious why tap-insulation tests should be performed routinely, After all, isn't the tap (in most cases) grounded during service and, furthermore, isn't the Ungrounded-Specimen Test (UST) on C1 the definitive measurement? While the UST measurement of C 1 checks directly the most important
insulation of the bushing, it is not, of itself, a complete and absolute measure of the total insulation system. Actually, it takes at least three tests to check completely the insulation integrity of oil-filled capacitance-graded bushings with taps; a fourth test is required in the case of compound-filled bushings, and oil-filled bushings without liquid-level gauges (or, with gauges which may be thought defective). See Figure 4.
FIGURES 4(a) & 4(b)
FIGURES 4(c) & 4(d)
Single Hot-Collar Test Configuration Highlighting the Stressed Insulation (Shown Darkened) (e) Typical Capacitance-Graded Bushing Showing Stressed Areas of Insulation for the various Doble Test Methods FIGURE 4 TESTS REQUIRED TO CHECK TOTALLY THE INSULATION INTEGRITY OF CAPACITANCE-GRADED BUSHINGS EQUIPPED WITH TEST TAPS OR POTENTIAL TAPS (REFER TO FIGURE 4) 1. Overall Test (Figure 4b) - Checks the overall insulation between the center conductor and ground including: main insulating core; liquid filler ; upper porcelain/epoxy weathershed (and sight glass, if any); lower insulator; continuity of bushing ground circuit comprising the following: the mounting flange, metallic sleeve below the flange, and connection between the tapped capacitance-graded conducting layer of the core and the tap cover. 2. Ungrounded-Specimen Test (UST), C1(Figure 4c) - Checks the main insulation of the capacitance-graded core, including the connection to the tapped layer. Note: While moderate surface leakage on the weathershed and lower insulator do not normally contribute to the UST measurement on the C 1 insulation, it is known that excessive surface leakage, particularly on the lower insulator, can influence this test. 8
3. Tap-Insulation Test,C2 (Figure 4d) - Checks the insulation of the main core between the tapped layer and ground; the tap insulator; the insulating fluid or compound in the vicinity of the tapped layer; and the integrity of the ground circuit comprising the following: the permanently grounded internal conducting layer of the capacitance-graded core in the case of bushings with potential taps, the mounting flange, and metallic sleeve below the flange. Note: A contaminated weathershed could also affect this measurement. Accordingly, if the surface of the weathershed is contaminated, repeat the test with a Guard-Collar placed beneath the bottom skirt. 4. Single Hot-Collar Test (Figure 4e) - A Single Hot-Collar test around the top skirt of compoundfilled bushings will detect the presence of water that has entered through the top of the bushing and settled on top of the heavier-than-water compound. This test can also detect low compound level (and low oil level, in the case of oil-filled bushings). For spare bushings equipped with taps, Doble recommends that Overall, UST (C 1), and TapInsulation (C2) tests be performed. For all compound-filled bushings, and oil-filled bushings without liquid-level gauges (or, with gauges which maybe thought defective), add the Single Hot-Collar test under the top skirt. Bushings in apparatus cannot be conveniently tested overall because other connected parts of the apparatus add current and loss to ground. However, in some cases it may be possible to isolate the bushing mounting flange from ground in order to make a conductor-to-flange measurement (i.e. an Overall measurement) using the UST technique.6,7,8 Unfortunately, this technique is usually not practical and, therefore, it is not recommended as a routine test method. For bushings in circuit breakers the Breaker Overall Tests include the overall current and loss of the bushings. Furthermore, for most dead-tank breakers without line-to-ground capacitors, the bushings comprise the major capacitance of the breaker ground insulation. In the case of power transformers, however, the overall current and loss of the individual bushings is but a small percentage (usually) of the contribution made by the winding insulation to ground. Fortunately, the UST measurement, which is performed on all bushings with taps, is sensitive to incipient conditions developing in the main C1 insulation, where most problems in bushings occur. Certain situations requiring the tap-insulation measurement are apparent; these are summarized in the following: OBVIOUS SITUATIONS REQUIRING TAP-INSULATION MEASUREMENTS
New Bushings - As part of users acceptance program, the Tap-Insulation Test complements the Overall and UST measurements. This trio of tests provides a comprehensive check of the total electrical and mechanical integrity. For compound-filled bushings, and oil-filled bushings without liquid-level gauges (or, with gauges which maybe thought defective), also perform a Single Hot-Collar test.
Bushings with taps that provide a source of voltage to operate a potential device.
Bushings that operate with voltage across the tap insulation, even when the bushing is not used with a potential device (e.g. certain Westinghouse Electric Corporation Type "O" and earlier bushing designs).
General Electric Company Type "LC" Bushings - Problems with this type were reported at the 1961 Conference.9,10,11 Moisture can enter the main chamber of these bushings through deteriorated gaskets on the tap plugs.
To determine if abnormal current and loss for UST measurements may be the result of low
impedance between the tap and ground.
When fluctuating Watts/Milliwatts for UST indicates possible poor connection to the tapped capacitance-graded conducting layer.
As investigate tool when other tests, or visual evidence (e.g., oil leak, physical damage), indicate a possible problem.
Yet, other serious problems can develop which might not be revealed unless the tap-insulation test is performed. These are summarized as follows: SERIOUS DEFECTS REQUIRING THAT TAP-INSULATION TESTS BE PERFORMED ROUTINELY
An improperly grounded mounting flange or floating metallic sleeve below the flange.
Cracked tap insulator.22,23
Contaminated or deteriorated liquid filler in the main bushing chamber.
If the metallic sleeve and mounting flange are not properly grounded, this would be revealed by a lowerthan-normal capacitance and charging current. The exact consequences of having an improperly grounded flange or metallic sleeve are uncertain, but potentially disastrous; accordingly, such a condition, when detected, must he corrected immediately. No bushing that normally operates line-to-ground should ever be energized at operating voltage with the flange or metallic ground sleeve floating. A cracked tap insulator would probably not be revealed by the Overall or UST methods. Yet, if a tap insulator were to fail completely the insulating oil could flow into the tap compartment and, later, out of the bushing entirely when the tap cover is removed. 22,23 Bushings with contaminated or deteriorated insulating fluid might be revealed by the Overall test when the apparatus is a relatively low-capacitance specimen such as an oil circuit breaker. However, except in extreme cases, contaminated or deteriorated fluid in a bushing would not be revealed by the Overall apparatus test if the bushing is in a relatively high-capacitance apparatus such as a power transformer. Contaminated or deteriorated fluid in a bushing could be the result of a chemical reaction between incompatible materials. If the dielectric-breakdown strength of the fluid is unaffected, then the bushing might not be in imminent danger. However, this would further depend on the nature of the reaction; for example, if it results in mechanical or structural weakening of the core, then an in-service failure might occur. Contamination of the fluid with moisture and/or particles, or a chemical reaction that reduces the dielectric-breakdown strength of the fluid would, on the other hand, constitute a definite failure hazard since flashover might occur line-to-ground longitudinally through the fluid. 18 For this reason, the ability to detect contaminated or deteriorated insulating fluid in a bushing must be within the capability of the test program. The first step in the investigation of the possibility of such a condition would be to draw a sample of fluid for measurement of its power factor, dielectric-breakdown strength, presence of particulate matter, etc.24 Two actual cases of contaminated or deteriorated fluid in bushings are given in the following: Example 1 - Contaminated or Deteriorated Fluid of Bushings in an Oil Circuit Breaker The open-breaker, UST, and tap-insulation power factors for six General Electric Company 138-kV Type "U" bushings in an oil circuit breaker were as follows: EXAMPLE OF CONTAMINATED OR DETERIORATED FLUID
IN 138-KV BUSHINGS IN OIL CIRCUIT BREAKER
Bushing No 1 2 3 4 5 6
% POWER FACTOR* Overall(OpenUST Breaker) 0.89 0.34 0.82 0.33 0.87 0.46 1.00 0.34 1.01 0.49 1.10 0.81
Tap 0.34 0.75 4.38 1.93 3.84 4.45
*The Overall and UST values listed were corrected to 20°C; the Tap power factors are as measured. Bushing No. 6 had high power factor for both the UST and tap-insulation tests. Bushings Nos. 3 and 5 had noticeably high tap-insulation power factor, but their UST power factors were only slightly higher-than-normal. Bushings Nos. 2 and 4 had acceptable UST power factors; however, Bushing No. 4 had a high tap-insulation power factor, whereas the tap-insulation power factor for Bushing No. 2 was not high in an absolute sense although it was noticeably higher than Bushing No. 1. Bushings Nos. 3, 4, 5, and 6 were subsequently reconditioned by draining the old oil and refilling under vacuum with new oil. Unfortunately, the manufacturer was unable to identify the type and source of the contaminant in the original oil. The subject Type "U" bushings were manufactured around 1956, and the condition noted was detected in 1967. As a matter of interest, though not directly related to this case to our knowledge, General Electric Company, presented a report at the 1957 Doble Conference with reference to high UST power factors reported for their Type "F" bushings design in which "the high power factor was traced to hydrated calcium sulfate in the oil which had been leached out of the cylinder paper". 25 Example 2 - Contaminated or Deteriorated Fluid in a Transformer Bushing The following power-factor test results were obtained for three McGraw-Edison Company 1981 vintage Type "PA" 23-kV 10,000-Ampere low-side bushings on a generator step-up transformer tested in 1989: HIGH TAP-INSULATION POWER FACTOR FOR A 23-kV LOW-SIDE BUSHING IN A GENERATOR STEP-UP TRANSFORMER % POWER FACTOR Bushing No.
X1 X2 X3
0.55 0.52 0.56
0.46 0.60 0.45
0.50 2.78 0.50
*Air Temperature 4°C, Top Oil Temperature 10°C The C1 power factor of Bushing X2 was slightly though noticeably higher than X1 and X3. (The difference in the three UST C1 power factors is accentuated slightly when taking into account the lower nameplate value for X2.) In any event, while the C1 power factor for X2 is higher than X1 and X3, the difference is not great. However the tap-insulation (C2) power factor showed very clearly that X2 had a problem. Interestingly, Single Hot-Collar tests at the top, middle, and bottom sections of the upper porcelain were performed as part of the client's investigation. The loss for each Single-Collar test was consistently higher for Bushing X2 by about 0.04 Watt at 10 kV.
Bushing X2 was subsequently removed from service and torn down. The capacitance-graded core was found to be in excellent condition and samples of the fluid were sent to Doble for analysis. The fluid was found to be "loaded" with particles: black granular particles; white cellulosic fibers; brown/red fibrous material; small shiny shards (some nonmetallic and some probable metals); dirt; and brown particles (probably a varnish type material). The power factor of the fluid was 0.15% at 25°C, but jumped to 6.4% at 100°C. The dielectric breakdown was 20 kV by ASTM Method D877, but only 6 kV by D1816. Note that the air and top oil temperatures were low when the subject bushing was tested in the transformer as shown by the foregoing data and, furthermore, Doble's laboratory-measured fluid power factor was only 0.15% at 25 °C. From this we conclude that the high tap-insulation power factor measured in the field was not so much affected by the power factor of the fluid proper, but was the result of contamination that had settled on the insulated capacitance-graded core and other solid parts inside the bushing. The extremely low dielectric breakdown value of the fluid leads us to the conclusion that the detection of this condition quite possibly prevented an in-service failure. SUMMARY AND CONCLUSIONS The tap insulation of bushings is easy to check for power factor and capacitance; furthermore, the tap-insulation test (C2) can reveal problems not evident by the Overall measurement and Ungrounded-Specimen Test (UST) on the main (C1) insulation. For this reason the Doble Engineering Company recommends that the tap insulation ALWAYS be checked whenever Doble tests are performed on bushings (and, current transformers) equipped with test taps and potential taps. This applies to Acceptance Tests, Routine Tests, and whenever tests are performed as part of Special Investigations. APPENDIX B of this report summarizes important factors relating to Doble testing the tap insulation of bushings. REFERENCES 1. Alsthom Savoisienne Descriptive Bulletins for Type IHC Current Transformers. 2. Westinghouse Bulletin I.L. 44-060-3B -Instructions for Oil Insulated EHV Current Transformers, Types ACT-1300, ACT-1550, & ACT-1800," Effective September, 1972. 3. Morelli, A. R. "O-B Capacitor Graded Potheads," Minutes of the Twenty-Seventh Annual Conference of Doble Clients, 1960, Sec. 8-401. 4. Lichtenberg, J. and Webb, R. S. "Electrical Tests on 230-kV High-Pressure Pipe Cable," Minutes of the Fortieth Annual International Conference of Doble Clients. 1973, Sec. 8-101. 5. DeBlieux, E.V "General Electric High-Voltage Bushings," Minutes of the Twenty-Third Annual Conference of Doble Clients, 1956, Sec. 4-301. 6. Doble Engineering Company, "The Doble Type M2H 10-kV Portable Insulation Test Set," Instruction Manual, Copyright, 1988. 7. Doble Engineering Company, "The Doble Type MEU 2500-Volt Portable Insulation Test Set," Instruction Manual, Copyright, 1987. 8. Rickley, A. L. and Clark, R. E. "Application and Significance of Ungrounded-Specimen Tests," Minutes of the Twenty-Seventh Annual Conference of Doble Clients, 1960, Sec. 3-201. 9. Rickley, A. L. and Clark, R. E. "Tap-Insulation Tests on Bushings," Minutes of the TwentyEighth Annual Conference of Doble Clients, 1961, Sec. 4-501.
10. Upperman, J. M. "Experiences with Type LC Bushings," Minutes of the Twenty-Eighth Annual Conference of Doble Clients, 1961, Sec. 4-301. 11. Albright, J. W. "General Electric Bushings," Minutes of the Twenty-Eighth Annual Conference of Doble Clients, 1961, Sec. 4-401. 12. Doble Technical Questionnaire, Circuit Breaker Question No. 10 "Doble Test Interval," Minutes of the FiftySecond Annual International Conference of Doble Clients, 1985, Page Sec. 5-SSS. 13. Doble Technical Questionnaire, Transformer Question No. 7 "Doble Test Interval" Minutes of the FiftySecond Annual International Conference of Doble Clients, 1985, Page Sec. 6-ZZZ. 14. Project - PC57.19.00/d10 November, 1989, Proposed Revision of IEEE General Requirements and Test Procedure for Outdoor Power Apparatus Bushings, IEEE 21-1976 (ANSI C76.1-1976). 15. Moran, J. H. "High Voltage Bushings," Book, Hodgins Printing Company, Inc., Copyright, 1989. 16. Doble Client Committee on Circuit Breakers and Bushings, "Doble Bushing Field-Test Guide." 17. Doble Technical Questionnaire, Bushing Question No. 5 "Bushing Potential Device Cables," Minutes of the Fifty-Third Annual International Conference of Doble Clients, 1986, Page Sec. 4-YY. 18. Atkinson, G. L., Lapp Insulator Division, Interpace Corporation, Letter to Doble Engineering Company, July 26, 1973. 19. Marquez, A. "Investigation of Increased Capacitance in Westinghouse Type OS Bushings," Minutes of the Fifty-Fourth Annual International Conference of Doble Clients, 1987, Sec. 4101. 20. Learn, W. R. "Discussion of the A. Marquez Paper, 'Investigation of Increased Capacitance in Westinghouse Type OS Bushings,' " Minutes of the Fifty-Fourth Annual International Conference of Doble Clients, 1987, Sec. 4-101A. 21. Smith, L. W. "Use of Ground Sleeves for Bushings", Minutes of the Twelfth Annual Conference of Doble Clients, 1945, Sec. 4-101. 22. O'Leary, R. E. "Replacing Capacitance-Tap Insulators in 345-kV Bushings in Place," Minutes of the. Thirty-First Annual Conference of Doble Clients, 1964, Sec. 4-101. 23. Manger, H. C. "Replacement of a Capacitance Tap Receptacle in a Westinghouse 500-kV Bushing Without Removing the Bushing from the Transformer," Minutes of the Forty-Seventh Annual International Conference of Doble Clients, 1980, Sec. 4-301. 24. Harman, G., Parkes, B. and Proskurnicki, J. A. "Improvements to Doble Test Tools and Bushing Oil Extraction Methods," Minutes of the Fifty-Third Annual International Conference of Doble Clients, 1986, Sec. 5-601. 25. VanLund, J. A. "General Electric Bushings," Minutes of the Twenty-Fourth Annual Conference of Doble Clients. 1957. Sec. 4-701. APPENDIX A RECOMMENDED PRACTICE FOR MAKING DOBLE TESTS ON APPARATUS HAVING POTENTIAL DEICES CONNECTED TO BUSHING TAPS
* When making Overall tests on apparatus that have a potential devices connected to the bushing(s), the ground switch of each potential device is put in the closed position. This grounds the tap of the associated bushing and prevents the device network from influencing the Overall results. To perform the Ungrounded-Specimen Test (UST) on the main insulation of the bushing (C 1), and tap test (C2 ), the internal potential device transformer and related network are then isolated from the cable that connects the potential device to the bushing. To perform a UST measurement on the C 1 insulation of the bushing, the Low-Voltage Lead of the Doble set is brought into the potential device housing where the connection to the bushing tap electrode is made through the center conductor of the potential device cable. To perform the tap-insulation test the High-Voltage Test Cable of the Doble set is brought to the device housing where, usually, a clip connection is made between the Doble High-Voltage Cable and the center conductor of the potential device cable. After completing all tests the potential device is returned to its operating mode; that is, the device is reconnected and the ground switch is subsequently opened.
APPENDIX B SUMMARY OF IMPORTANT FACTORS RELATING TO DOBLE TAP-INSULATION (C2) TESTS ON BUSHINGS * PERCENT POWER FACTOR
Most taps have measured insulation power factors of 1% and less.
Tap-insulation test results are not corrected for temperature.
Compare the measured tap insulation power factor with the factory measured C 2 power factor, if any, recorded on the nameplate.
Compare the measured C2 power factor of similar bushings tested at the same time.
In the case of normally low power-factor tap insulation, a doubling of the power factor versus benchmark (or, double the value of other similar units tested at the same time) is cause for concern. A smaller increase may be of concern in the case of aged bushings, which have had stable tap-insulation power factor for a long time only to show a sudden noticeable change compared with previous data and with other similar bushings associated with the same apparatus.
High C2 power factor may indicate contamination or deterioration of the insulating fluid in the main chamber of the bushing. Contaminated or deteriorated fluid per se might not necessarily constitute an imminent failure hazard; however, such a condition could signal a reduced dielectric breakdown strength of the fluid. Insulating fluid with low dielectric-breakdown strength in a bushing is a failure hazard.
High C2 power factor or fluctuating Watts/Milliwatts may indicate a defective tap insulator which, if it breaks apart, could cause the fluid in the main chamber to leak out.
Whenever questionable C2 power factors are obtained, clean and dry the tap housing then perform repeat tests. (If heat is applied to dry the tap, allow the tap to cool before performing
a retest since the elevated temperature of the tap insulator could increase the power factor.)
In the case of potential taps, if questionable power factors are obtained, make supplementary tests at several voltages, up to the maximum allowed, in order to determine if a voltagesensitive condition exists.
Fluctuating Watts/Milliwatts may be an indication of a poor connection to the tapped foil layer.
High tap-insulation power factor for General Electric Company Type "LC" bushings is of concern because no seal exists between the tap housing and the bushing main chamber. Thus, moisture which enters the tap housing can migrate to the bushing core.
Some Westinghouse Electric Corporation Type "O" bushings operate in service with the tap insulation energized. For many of these bushings the connection to the tap is made using a special test probe without having to remove the tap cover. If high power factor is obtained for the tap-insulation test of these bushings, remove the tap cover and completely clean and dry the tap chamber. Perform a test to confirm that cleaning and drying the chamber has improved the condition. Then, refill the chamber with insulating fluid prescribed by the manufacturer, and retest again.
For bushings that operate a potential device, perform the initial C 2 test with and without the potential device cable connected to the tap. (Also, for the initial test, check the potential device cable separately.) If questionable power factor is obtained for a subsequent test with the cable connected, then make separate tests on the bushing tap alone, and on the potential device cable alone. CAPACITANCE
The measured C2 capacitance should be compared with the value (if any) stamped on the bushing nameplate.
Compare the measured C2 capacitance with values obtained for similar bushings tested at the same time.
Higher-than-nameplate C2 capacitance will result if the tap insulation is tested with the center conductor grounded. Either subtract the C1 capacitance or retest with the center conductor connected to guard.
Higher-than-nameplate C2 capacitance will result if the tap insulation is tested with a potential device cable connected. Initial tests should be performed with and without the potential device cable connected in order to establish benchmark values both ways. (Initially, the potential device cable is also tested separately.) Follow-up tests are then performed with the cable connected. If questionable capacitance is obtained for a subsequent test with cable connected, then disconnect the potential device cable from the bushing tap and make separate tests on the bushing tap, and on the device cable.
For bushings not used with potential device, an increase in the tap-insulation capacitance is unusual. An increase in capacitance is generally associated with short circuiting of insulation layer; however this might also be the result of some unusual or fundamental change in the insulation system.
A lower-than-nameplate value or decrease in C 2 capacitance versus the benchmark value may indicate, in the case of bushings with potential tap, that the normally permanently grounded foil layer has become ungrounded. Lower-than-normal C 2 capacitance for bushings may also indicate that the outer metallic sleeve below the flange has become ungrounded, or
that the mounting flange has become ungrounded.
If abnormal C2 capacitance is obtained, check or confirm the connection to the bushing center conductor. Make certain that the center conductor is not floating, and that it is not connected to UST. Usually, the tap test is made with the center conductor connected to guard. If the center conductor is Grounded, the measurement will include C 1 and C2 in parallel.
©1990 Doble Engineering Company All rights reserved 90 BUSHINGS 4-3. 1