Fig.1 __Rewinding Furnace Transformer 43000 Amps
Fig. 4 __Reapair and Replacement Switching Contact V / I / 250 such as :
1. Techniques for Interpretation of Data for DGA From Transformers, Lance Lewand, Doble Engineering
1. FIST 3-30 Transformer Maintenance, United State Department of the Interior Bureau of Reclamation, Denver, Colorado, June 2003
2. FIST Volume 3-31, Transformer Diagnostics, Facilities Instructions, Standard and Techniques, Bureau of Reclamation, Denver, Colorado, June 2003
3. Power Transformer Maintenance And Acceptance Testing, Technical Manual, TM 5-686, Headquarters, Department Of The Army, 16 November 1998
4. Power Transformer Maintenance, field Testing, Andres Tabemero Garcia, Hardware Project Manager, Unitronics, S.A
5. Electrical Maintenance Workshop, Transformer maintenance & Testing, Progress Energy, November 6, 2003Guide to Transformer Maintenance (Insitu-invivo), J.J.Kelly, SD Myers. RH Parish , Transformer Maintenance Institute, Divison, SD Myers Inc. Akron. Ohio
6. IEEE C57 - 104-1991 7. Private experiences
8. much more... INSPECTIONS ITEMS
Table 1. __Inspections items
Winding
DC Resistance Turn Ratio
Percent Impedance / leakage reactance Sweep Frequency Response Analyzer (SFRA) Doble test for winding and oil
- Capacitance - Excitation current
- Power Factor Capacitance Tan Delta
Bushing and Arrester
Capacitance (Doble Test) Dielectric Loss Watt Power Factor
Temperature (Infra red camera) Oil level on bushing
Visual inspection for Percelin crack and chip
Insulation Oil
DGA (Dissolve Gas Analysis) Dielectric Strength
Moisture
Power Factor / Dissipation Interfacial Tension
Furance
Tank and Auxiliaries
Fault pressure relay (function test) Pressure relieve devices (visual test) Bucholze Relay (visual check for gas) Top Oil Temperature Indicator
Winding temperature indicator Infra Red temperature Scan Fault Analyzer (Ultrasonic) Sound Analysis
Conservator
Visual (oil leake and leake in diagpragm) Inert Air Systems ( desiccant color) Level gauge calibration
Cooling Systems
Clean ( fan blade and radiator) Fan and control (check fan rotation)
Oil Pump (check flow indicator , check rotation) Pump bearings ( vibration, sound and temperature) Check Radiator ( valve open)
Check Cooling System with infra red camera Core Insulation Resistance
Ground Test
TDCG Limit
Table 2 : Dissolved Key Gas Concentration Limits in Parts Per Million (ppm) Status Hydrogen (H2) Methane (CH4) Acetylene (C2 H2) Ethylene (C2H4) Ethane (C2H6) Carbon Monoxide
(CO) Carbon Dioxide (CO2) TDCG Condition 1 100 120 35 50 65 350 2,500 720 Condition 2 101-700 121-400 36-50 51-100 66-100 351-570 2,500-4,000 721-1,920 Condition 3 701-1,800 401-1,000 51-80 101-200 101-150 571-1,400 4,001-10,000 1,921-4,630 Condition 4 > 1,800 > 1,000 > 80 > 200 > 150 > 1,400 > 10,000 > 4,630
Table 3 : Actions Based on Dissolved Combustible Gas (Adapted from [4])
Conditions
TDCG Level or highest Individual Gass (see table "A")
TDCG Generation Rates (ppm per day)
Sampling Intervals and Operating Actions for Gas Generation Rates Sampling
Interval Operating Procedures Condition
1
< 720 ppm of TDCG or highest condition based on individual
< 10 Annually: 6 months for extra high
Continue normal operation.
combustible gas
from table A. voltage transformer 10-30 Quarterly
> 30 Monthly
Exercise caution. Analyze individual gases to find cause. Determine load dependence. Condition 2 721–1,920 ppm of TDCG or highest condition based on individual combustible gas from table A. < 10 Quarterly
Exercise caution. Analyze individual gases to find cause. Determine load dependence. 10-30 Monthly > 30 Monthly Condition 3 1,941–2,630 ppm of TDCG or highest condition based on individual combustible gas from table A.
< 10 Monthly Exercise extreme caution. Analyze individual gases to find cause. Plan outage. Call manufacturer and other consultants for advice
10-30 Weekly > 30 Weekly Condition 4 > 4,639 ppm of TDCG or highest condition based on individual combustible gas from table A.
< 10 Weekly Exercise extreme caution. Analyze individual gases to find cause. Plan outage. Call manufacturer and other consultants for advice.
10-30 Daily
> 30 Daily
Consider removal from service. Call
manufacturer and other consultants for advice.
The most important indicators are the individual and total combustible gas (TCG) generation rates based on IEC 60599 and IEEE C 57104™ standards.
A four-condition DGA guide to classify risks to transformers with no previous problems has been published in the Standard IEEE Standard (Std). C57-104™. The guide uses
combinations of individual gases and total combustible gas concentration as indicators. It is not universally accepted and is only one of the tools used to evaluate dissolved gas in transformers. The four IEEE® conditions are defined immediately below, and gas levels are in table A following the definitions.
Condition 1: Total dissolved combustible gas (TDCG) below this level indicates the transformer is operating satisfactorily. Any individual combustible gas exceeding specified levels in table B should have additional investigation.
Condition 2: TDCG within this range indicates greater than normal combustible gas level. Any individual combustible gas exceeding specified levels in table B should have additional investigation. A fault may be present. Take DGA samples at least often enough to calculate the amount of gas generation per day for each gas. (See table 2 for recommended sampling frequency and actions.)
Condition 3: TDCG within this range indicates a high level of decomposition of cellulose insulation and/or oil. Any individual combustible gas exceeding specified levels in table A should have additional investigation. A fault or faults are probably present. Take DGA samples at least often enough to calculate the amount of gas generation per day for each gas.
Condition 4: TDCG within this range indicates excessive decomposition of cellulose insulation and/or oil. Continued operation could result in failure of the transformer. Table.4__Doble Limits for In Service Oil
Voltage Class
69 kV > 69 # 288 kV > 288 kV
Dielectric Breakdown Voltage, D 877, kV min 26 30 Dielectric Breakdown Voltage
D 1816, .04-inch gap, kV, min.
20 20 25
Power Factor at 25 °C, D 924, max. 0.5 0.5 0.5
Water Content, D 1533, ppm, max. 235 225 220
Interfacial Tension, D 971, dynes/cm, min. 25 25 25 Neutralization Number, D 974, mg KOH/gm, max 0.2 0.15 0.15
Visual Exam clear and bright clear and bright clear
Soluble Sludge 3ND 3ND 3ND
Table 5.—Typical Faults in Power Transformers adapted from IEC 60599 Appendix A.1.1
Fault Examples Partial
discharges
Discharges in gas-filled cavities in insulation, resulting from incomplete impregnation, high moisture in paper, gas in oil supersaturation or cavitation, (gas bubbles in oil) leading to X wax formation on paper.
Discharges of low energy
Sparking or arcing between bad connections of different floating potential, from shielding rings, toroids, adjacent discs or conductors of different windings, broken brazing, closed loops in the core. Additional core grounds. Discharges between clamping parts, bushing and tank, high voltage and ground, within windings. Tracking in wood blocks, glue of insulating beam, winding spacers. Dielectric breakdown of oil, load tap changer breaking contact.
Discharges of high energy
Flashover, tracking or arcing of high local energy or with power follow-through. Short circuits between low voltage and ground, connectors, windings, bushings, and tank, windings and core, copper bus and tank, in oil duct. Closed loops between two adjacent conductors around the main magnetic flux, insulated bolts of core, metal rings holding core legs. Overheating
less than 300 °C
Overloading the transformer in emergency situations. Blocked or restricted oil flow in windings. Other cooling problem, pumps valves, etc. See the "Cooling" section in this document. Stray flux in damping beams of yoke. Overheating Defective contacts at bolted connections (especially busbar), contacts within
300 to 700 °C tap changer, connections between cable and draw-rod of bushings. Circulating currents between yoke clamps and bolts, clamps and
laminations, in ground wiring, bad welds or clamps in magnetic shields. Abraded insulation between adjacent parallel conductors in windings. Overheating
over 700 °C Large circulating currents in tank and core. Minor currents in tank walls created by high uncompensated magnetic field. Shorted core laminations.
Table. 6__RECOMMENDED TEST VALUES CONTINUED SERVICE OF TRANSFORMER INSULATION OIL Authority NN Mg KOH/g Maximum IFT Dynes/Cm Minimum Dielectric D-877 KV Min. P,F %at25oC Max Moisture Content PPM. Max Westinghouse 0.15 21 28 1.0 - GE 0,20 24 26 0,65 - Kemper 0,36 21 24 1,0 25 Factory Mutual 0,25 18 23,5 - -
American Nuclear Insures 0,20 22 23,5-31 - 50
EPRI Utility Survey 0,32 23 27 0,80 34
NFPA 0,40 - 22 - -
IEC 0,50 15 30-50 - 20-30 mg/l
Table 7__Suggested test limit for service – aged oil by voltage class IEEE Insulating Fluids Committee, Projects 637, April 1980
Property / Test Limits
Voltage Class 69 KV and Below Above 69 KV through 288 KV 345 KV and Above ASTM Test Method Dielectric Breakdown Voltage 60
Hz, KV Minimum 26 26 26 D – 877
Dielectric Breakdown Voltage KV Minimum
[0,040 inch gap thin ]
23 26 26 D – 1816
Neutralization Number, Mg KOH / gm, oil maximum
0,2 0,2 0,1 D – 974
Interfacial Tension Test Dynes/cm, minimum
24 26 30 D – 971
oC
Max. Percent Moisture Contents PPM Maximum
35 25 20 D – 1533
Table 8 ___SUGGESTED PROPERTY REQUIRMENTS OF RECLAMED OIL FOR TRANSFORMER
PROPERTY / TEST LIMIT ASTM TEST METHOD
Physical
Flash point, minimum,
oC 140 oC D – 92 Specific gravity 150C / 15oC, Maximum 0,91 D – 1298 Viscosity Maximum At 40oC [mm2 / sec ] 12,0 D – 88 or D -445
Visual examination Clear D – 1524
IFT minimum [dyne / cm] 35 D – 971
Electrical
Dielectric Breakdown Voltage 60 Hz, Min. KV 30 D – 877 Power Factor at 60 Hz, 100 oC Max. % 1 D – 924 Chemical Neutralization Number [NN] Max. mg KOH/g 0,05 D – 974 Oxidation Inhibitor Max. % by weight 0,3 D – 2668 Water Maximum, ppm 35 D - 1533
References : IEEE Insulating Fluids Subcommittee (proposed, April, 1980] Project 637. Table 9.__Gas Qualitative and Quantitative Interpretation
TYPES OF PROBABLE FAULTS
Nitrogen + 5 % or less of Oxygen Normal operation of sealed transformer Nitrogen + more 5% of Oxygen Check for tightness of sealed transformer
Nitrogen, Carbon dioxide, or Carbon monoxide, or all
Transformer overloaded or operating hot, causing some cellulose breakdown, check operating conditions
Nitrogen and Hydrogen Corona discharge, electrolysis of water, or rusting
Nitrogen, Hydrogen, Carbon dioxide, and carbon monoxide
Corona discharge involving cellulose or severe overloading of transformer Nitrogen, Hydrogen, Methane, with small amount
of ethane, and ethylene
Sparking or other minor fault causing some breakdown of oil
Nitrogen, Hydrogen, methane, with carbon dioxide, carbon monoxide, and small amount of other hydrocarbons, acetylene is usually not present
Sparking or other minor fault in presence of cellulose
Nitrogen with high hydrogen and other
hydrocarbons including acetylene High energy arc causing rapid deterioration of oil Nitrogen with high hydrogen methane, high
ethylene, and some acetylene
High temperature arcing of oil but in confined area, poor connection or turn to turn shorts
Electrical Test
1. Across Winding Resistance Measurements
Winding resistance measurements in transformers are of fundamental importance for the following purposes:
Calculations of the I2R component of conductor losses.
Calculation of winding temperature at the end of a temperature test cycle. As a diagnostic tool for assessing possible damage in the field.
Transformers are subject to vibration. Problems or faults occur due to poor design, assembly, handing, poor environments, overloading or poor maintenance. Measuring the resistance of the windings assures that the connections are correct and the resistance measurements indicate that there are no severe mismatches or opens. Many transformers have taps built into them. These taps allow ratio to be increased or decreased by fractions of a percent. Any of the ratio changes involve a mechanical movement of a contact from one position to another. These tap changes should also be checked during a winding resistance test.
Agreement within 5% for any of the above comparisons is considered satisfactory. If winding resistances are to be compared to factory values, resistance measurements will have to be converted to the reference temperature used at the factory (usually 75 °C ). To do this, use the following formula:
Rm = Resistance at the factory reference temperature ( found in the transformer ) Rs = Resistace actually measured
Ts = Factory reference temperature ( usually 750C)
Tm = Temperature at which you took the measurements
Tk = A constant for the particular metal the winding is made from 234.50C for copper,
2250C for Aluminum
2. Winding DC Resistance to Ground (Megger ® )
The insulation resistance is generally accepted as reliable indication of the presence of absence of harmful contamination or degradation.
The temperature of the equipment under test has marked influence on the results obtained insulation resistance values decrease with increasing temperature. If the reading are taken at the different temperature, all readings should be corrected to a common base of 200C . A
short listing of temperature correction factor is shown in the table below.
The test is relatively quick and easy to make and is normally performed with a 500 V, or 1000 V, or 2500 V, or 5000 V DC.
Table 10.__PF Correction to temperature
TEMPERATURE CORRECTION FACTOR WITH BASE 20OC
FOR OIL-FILLED TRANSFORMER
OC OF CORRECTION FACTOR 0 32 0.25 5 41 0.36 10 50 0.50 15 59 0.720 20 68 1.00 30 86 1.98 40 104 3.95 50 122 7.85
R = Insulation Resistance of Winding to Ground at 20oC
C = 0.8 for oil filled transformer at 20oC
C = 16 for dry compound filled or untanked oil filled transformer E = Voltage rating of winding under test
KVA = Rated Capacity of winding under test PI / Polarization Index
Polarization Index can be determined by the ratio of 10 minutes resistance to the 1 minute resistance value. Since the leakage current is increases at a faster rate with moisture present that does the absorption current, the polarization index is lowered for insulation in poorer condition.
Polarization Index guide for evaluation of transformer condition Table 11.__PI Value
Condition PI (Polarization Index) Dangerous Less than 1.0
Poor 1.0 to 1.1 Questionable 1.1 to 1.25 Fair 1.25 to 2.0 Good Above 2.0
3. Capacitance Power Factor (Doble Test )
Power factor insulation testing is important to determining the condition of the
transformer because it can detect winding and bushing insulation integrity. Power factor and excitation current tests are conducted in the field on de-energized, isolated, and properly grounded transformers. Excitation current tests measure the single-phase voltage, current, and phase angle between them, typically on the high-voltage side with the terminals of the other winding left floating (with the exception of a grounded neutral). The measurements are performed at rated frequency and usually at test voltages up to 10 kilovolts (kV). The test detects shorted turns, poor tap changer contacts, and core problems
The purpose of this test is to determine the state of dryness of the windings and insulation system and to determine a power factor for the overall insulation, including bushings, oil, and windings. It is a measure of the ratio of the power (I2R) losses to the volt-amperes
applied during the test. The power factor obtained is a measure of watts lost in the total transformer insulation system including the bushings. The power factor should not exceed
0.5% at 200C. Temperature correction of test results can be done automatically on the
Doble test set. The watts loss should not exceed one-half of one percent of the total power input (volt-amps) from the test. The values obtained at each test are compared to previous tests and baseline factory tests, and a trend can be established as the insulation system ages.
Capacitance Tests
This test measures and records the capacitance (including bushings) between the high and low voltage windings, between the high voltage winding and the tank (ground), and
between the low voltage winding and the tank (ground). Changes in these values as the transformer ages and events occur, such as nearby lightning strikes or through faults, indicate winding deformation and structural problems such as displaced wedging and winding support.
4. Transformer Turn Ratio (TTR) Automatic
Transformer Turn Ratio (TTR)
Turns Ratio Test . The transformer turns ratio (TTR) test detects shorts between turns of the same coil, which indicates insulation failure between the turns. These tests are
performed with the transformer de-energized and may show the necessity for an internal inspection or removal from service.
Measurements are made by applying a known low voltage across one winding and
measuring the induced voltage on the corresponding winding. The low voltage is normally applied across a high voltage winding so that the induced voltage is lower, reducing hazards while performing the test. The voltage ratio obtained by the test is compared to the
nameplate voltage ratio. The ratio obtained from the field test should agree with the factory within 0.5%. New transformers of good quality normally compare to the nameplate within 0.1%.
Bushing Maintenance Tests.-
Common maintenance tests are power factor, RIV (radio-in-fluence-voltage), dc insulation resistance, and testing oil or compound for moisture. Descriptions of these various tests follow:
1. Power-factor Doble Tests.- The power-factor test is the most effective known field test procedure for the early detection of bushing contamination and deterioration. This test also provides measurement of ac test current which is directly proportional to bushing
capacitance.
Bushings may be tested by one or more of four methods depending upon the type of bushing and the power-factor test set available. For more complete detailed instructions on the method of test and test procedure, please see the appropriate power-factor test set instruction book. The four test methods are as follows:
a. The GST (grounded specimen test).-This test measures the insulating qualities of the
insulation between the current carrying or center conductor and the mounting flange of a bushing. The application of such a test is necessarily limited to bushings out of the
apparatus such as spare bushings, or bushings which have been isolated from connected windings and interrupters. The test is performed by energizing the bushing conductor and grounding the flange.
Large variations in temperature have a significant effect on power-factor readings on certain types of bushings. For comparative purposes, readings should be taken at the same
temperature, or corrections should be applied before comparing readings taken at different temperatures.
The hot-guard test.-This test measures the insulation between the cur-rent-carrying or
center conductor and the mounting flange of a bushing. The test was designed specifically for "draw-lead" type bushings but is applicable to any bushing in apparatus which can be isolated from connecting windings and bus, but not sufficiently to withstand test potential. Both the bushing and the draw-lead, winding, and bus are energized at the same test potential, but only the current and losses of the bushing are measured.
The UST (ungrounded-specimentest).-This test measures the insulation between the
current-carrying or center conductor and the capacitance tap, power-factor tap, and/or ungrounded flange of a bushing. This test may be applied to any bushing in or out of apparatus which is either equipped with capacitance or power-factor taps or the flange of which can be isolated from the grounded tank in which the bushing is installed. The
insulation resistance between the taps or insulated flanges and ground should be 0.5 meg-ohm or better. While in this case, anything that is attached to the bushing (such as contact assemblies or transformer windings) would also be energized; only the insulation of the bushing betweenthe center conductor and the ungrounded tap or flange would be
measured. In the case of bushings equipped with capacitance taps, a supplementary test should always be made on the insulation between the tap and the flange.
d. The hot-collar test.
This test measures the condition of a specific small section of bushing insulation between an area of the upper porcelain rainshed and the current-carrying or center conductor. It is performed by energizing one or more electrodes (collars) placed around the bushing porcelain with the bushing center conductor grounded. This test is used to supplement the three tests described above or to test bushings in apparatus when the above-mentioned three tests are either inapplicable or impractical. Hot-collar tests are effective in locating cracks in porcelain, deterioration or contamination of insulation in the upper section of a bushing, low compound or liquid level, or voids in compound,
Table 2. - Manufacturer's P. F. (power factor) limits for bushings Table. 12.__ Bushing test
Manufacturer Bushing type or class Initial P.F. for new bushings, at Dangerous P. F. value at 20 EC (%)
General electric A 6.0 8.0 B 10.0 12.0 F 1.5 2.0 L 3.0 4.0 LC 2.5 3.5 OF 2.6 6.0 S 3.5 6.0 U 1.0 1.5
Lapp bushings POC 0.5
PRC 0.7-1.2
Ohio Brass manufactured prior to 1926 and after
1938 ODOF G L 1-10 Initial P.F. = 22
Ohio Brass manufactured
1926 to 1938, inclusive ODOF G L 2-4 Initial P.F. = 16
Ohio Brass Class GK type C 04.-0.6
Class LK type A 0.6-0.7
Pennsylvania Transformer P PA PB 0.5 1.0
Westinghouse
D 6.0
O 1.4
OCB & Inst. Trans.
69-kV and Below 3.5 OCB & Inst. Trans.
92-kV to 138-kV 2.8 Power & Dist. Trans. OCB & 161-kV to
Types of Bushings
High-voltage bushings for use on transformers and breakers are made in several principal types, as follows:
A. Composite Bushing.- A bushing in which insulation consists of two or more coaxial layers of different insulating materials.
B. Compound-Filled Bushing.-A bushing in which the space between the major insulation (or conductor where no major insulation is used) and the inside surface of a protective weather casing (usually porcelain) is filled with a compound having insulating properties. C. Condenser Bushing.- A bushing in which cylindrical conducting layers are arranged coaxially with the conductor within the insulating material. The length and diameter of the cylinders are designed to control the distribution of the electric field in and over the outer surface of the bushing. Condenser bushings may be one of several types:
1. Resin-bonded paper insulation; 2. Oil-impregnated paper insulation; or 3. Other.
D. Dry or Unfilled Type Bushing.-
Consists of porcelain tube with no filler in the space between the shell and conductor. These are usually rated 25 kV and below.
E. Oil-Filled Bushing.- A bushing in which the space between the major insulation (or the conductor where no major insulation is used) and the inside surface of a protective weather casing (usually porcelain) is filled with insulating oil.
F. Oil Immersed Bushing.-A bushing composed of a system of major insulations totally immersed in a bath of insulating oil.
G. Oil-Impregnated Paper-Insulated Bushing.- A bushing in which the internal structure is made of cellulose material impregnated with oil.
H. Resin-Bonded, Paper-Insulated Bushing.- A bushing in which the major insulation is provided by cellulose material bonded with resin.
I. Solid (Ceramic) Bushing.-A bushing in which the major insulation Is provided by a ceramic or analogous material
