DC & AC Voltage Testing of Electrical Equipments
Vivek Jha 1
Presentation outline y Introduction y DC testing of Insulation : y DC Testing Methods y DC Testing Applications y AC Testing : y Power Factor and Dissipation Factor Test y AC Testing Applications y Conclusion
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Introduction
Need of Maintenance & Testing y To lengthen the mean time between failures (MTBF) of the electrical equipment. y Reduced cost of repairs y Reduced downtime of equipment y Improved safety of personnel and property.
ROLE OF MAINTENANCE MANAGEMENT TO BALANCE BETWEEN COST OF MAINTENANCE
PLANT AVAILABILITY
MAINTENANCE MANAGEMENT 22 November 2010
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Different approaches to maintenance y y y y y
Run to failure or Reactive maintenance: Inspect and service as necessary Time Based Scheduled preventive maintenance Condition Based Preventive Maintenance Reliability centered maintenance (RCM)
22 November 2010
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22 November 2010
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Rules of maintenance y y y y
Keep it dry Keep it cool Keep it clean Keep it tight
22 November 2010
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Causes of Insulation Degradation and Failure Modes of Electrical Equipment y y y y
Mechanical stress Thermal stress , hot spots Environmental (moisture, chemicals, dirt, and oils) Electrical stresses (corona, surges, and partial discharges)
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TEAM Stresses • Temperature • Electrical • Ambient • Mechanical
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Insulation Strength no TEAM transients Insulation Strength
Good design, manufacture & O&M: significant margin
Poorer design & manufacture- faster rate of decline Insulation Spare Margin
Insulation Stress New
Increasing Age
Old
Insulation Strength with TEAM transients Reducing Strength with time and after incidents
Insulation Strength Insulation Spare Margin
Failure Insulation Stress New
Incidents
Increasing Age
Old
Types of tests y y y y
Factory Tests Acceptance tests Routine maintenance tests Special maintenance tests
Insulation as a Capacitor
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Insulation as a Capacitor The dielectric constant of an insulator is an indication of how much dielectric flux the insulation will allow through it. Under identical conditions insulation with a higher dielectric constant will pass more dielectric flux through it than another insulation having a lower dielectric constant
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Insulation as a Capacitor
Perfect Insulator
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Insulation as a Capacitor
Practical Insulator
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Insulation Test : AC or DC?
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Insulation Test : AC or DC?
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Insulation Test : AC or DC?
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Insulation Test : AC or DC?
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DC testing of Insulation
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Introduction : DC tests Information provided by DC Tests will be basis for : y Decision making about corrective maintenance / replacement needed y Decision making about energizing the new equipment y Recording trend of gradual deterioration over time
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DC voltage Application to Insulation When DC voltage is applied to insulation, current drawn by insulation can be analyzed 3 components: y Capacitance charging current y Dielectric absorption current y Leakage current
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DC voltage Application to Insulation …
Capacitance charging current Charging current E ic= --e-t/RC R
• • • •
E = Voltage in KV R = Resistance in MΩ C= capacitance in μF t = Time in sec.
• It is initial charging current when voltage is applied • It is a function of time, and decreases as time of application increases • Test readings should not be taken until this charging current decreases to quite low value
Dielectric Absorption current Absorption current ia= K ECt-n
• • • • •
E = Voltage in KV K = proportionality constant C= capacitance in μF t = Time in sec. n = constant
• Initially high (not as high as capacitance charging current) • Decreases with time (at a slower rate than capacitance charging current) • charging current when voltage is applied • It is a function of time, and decreases as time of application increases • Test readings should not be taken until this absorption current decreases to quite low value
Leakage current • It is the current which flows through volume of insulation • this is the current that is used to evaluate the status of insulation system. • charging current when voltage is applied • Test readings should be taken only after stabilization of this leakage current.
Advantages of DC Voltage Testing y Preferred for equipments having high charging current, such as cables y DC voltage stress is much less damaging as compared to AC voltages y Time of voltage application is not as critical as AC voltage y Historical data can be compiled accurately for comparison y Size and weight of DC test equipment is significantly reduced as compared to AC test equipment 29
Disadvantages of DC Voltage Testing y Stress distribution for electrical equipments is different under AC voltage y Defects, untraceable with DC, can sometimes cause failure under AC y DC Test results are affected by temperature and humidity. y Residual charge after DC test should be carefully discharged
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DC Testing Methods
DC Testing Methods Two types of tests are done with DC voltage application: • Insulation resistance testing (IR) • High potential testing (Hi‐Pot)
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DC Testing Methods Insulation Resistance Testing • Test voltage application : 100 – 15000 v • Instrument used: Megohmmeter
(hand/motor/electronic) • All readings to be corrected to standard temp, by correction factor table • MΩ value is inversely proportional to volume of insulation under test • IR value by themselves do not indicate weakness or strength, they can indicate trouble if downward trend continues further
DC Testing Methods Insulation Resistance Testing Four common IR test methods: y Short Time Readings or Spot checking y Dielectric Absorption Ratio test y Polarization Index Test y Step Voltage Readings
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Short‐Time Readings y This test simply measures the insulation resistance value for a short
duration of time, through a spot reading. y The reading only allows a rough check of the insulation condition. y Readings can be used for comparison of this value with previous values . A continued downward trend is indicative of insulation deterioration ahead. y For interpreting the results, the values used for comparison should all be normalized to 20°C.
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DC Testing Methods Time–Resistance Readings: Dielectric Absorption Ratio test y A good insulation system shows a continued increase in its resistance value over
the period of time. On the other hand, an insulation system that is contaminated with moisture, dirt, etc will show a low resistance value. y In good insulation, the effects of absorption current decreases as time increases. In bad insulation, the absorption effect is shown by persisting high leakage current. y The time‐resistance method is independent of temperature and equipment size. The ratio of time‐resistance readings can be used to indicate the condition of the insulation system. y The ratio of a 60 s reading to a 30 s reading is called the DAR (Dielectric Absorption Ratio) DAR
Resistance reading at 60 s Resistance reading at 30 s:
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DC Testing Methods PI (Polarization Index) Test y The PI test is a specialized application of the dielectric absorption test. This test is used for dry insulation.
PI =
insulation resistance at 10 min insulation resistance at 1 min
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LEAKAGE CURRENT in μA
DC Testing Methods Step Voltage Test
Breakdown of insulation system
VOLTAGE IN KV
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DC Testing Methods High potential testing (Hi‐Pot) y DC voltage applied across the insulation at or above DC equivalent of power frequency AC voltage y Test can be applied as ‘gradual’ test or step‐voltage test y In gradual test, max voltage is applied slowly, gradually and max voltage is held for a period. Leakage current readings are recorded. y In step voltage method, max voltage is applied in steps and readings taken at each step.
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DC Testing Applications
DC Testing Applications: Transformers Test procedure y Do not disconnect ground connection to tank y Disconnect all HV,LV, neutral connections, cooling system, meters, LA and other LV control system y HV/LV jumpers should not touch metal or grounded parts
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Test connections for 3 ph transformer HV to LV & Earth
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Test connections for 3 ph transformer HV to LV & Earth Megohmmeter
Guard
Line
Earth
Transformer
• • X1 H2 • • X2 H3 • • X3 • X0
H1
Test connections for 3 ph transformer HV to Earth, LV Guarded Megohmmeter
Guard
Line
Earth
Transformer
• • X1 H2 • • X2 H3 • • X3 • X0
H1
Test connections for 3 ph transformer LV to Earth, HV Guarded Megohmmeter
Guard
Line
Earth
Transformer
• • X1 H2 • • X2 H3 • • X3 • X0
H1
Megohmmeter
Guard
Line
Earth
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Megohmmeter
Guard
Line
Earth
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Megohmmeter
Guard
Line
Earth
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Interpretation of DC test readings
Interpretation of DC test readings Dc test results can serve as guide to decide one of the following actions: Put the equipment into service till next scheduled inspection y Put the equipment into service now, but plan to repair/replace as soon as possible y Put the equipment out of service y
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Interpretation of DC test readings How to determine whether insulation is good or bad: y Manufacturer’s information y Comparison with values obtained during installation y Comparison with values from previous routine tests y Comparison with values of similar equipment y Rule of thumb: y Min 1 MΩ / rated KV + 1 MΩ y Never < 1 MΩ in any case
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Interpretation of DC test readings More rules of thumb: y For transformers, Min IR = (CE)/ √ kVA IR = wdg to Earth MΩ, E= voltage rating, kVA = rated kVA C = constant
y For cables, Min IR = K log10 (D/d) IR = cond.to Earth MΩ per 100 ft of cable D= outside dia of cond insulation k = constant for insulating material, d= dia of conductor
value of C Trafo type value of C at 20 deg C oil filled 1.5 Dry type 30
Min value of K Impreg paper
2640
polyethylene
2000
Synthetic rubber
2000
XLPE
20000
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Acceptance criteria for judging insulation Nominal voltage class
Typical system voltage Min acceptable IR value at 20 0 C (MΩ)
600 v
120/240/440 v
1.5
7.2 kv
6.9 kv
8.2
15 kv
11 kv / 13‐8 kv
14.8
33 kv
33 kv
35
72 kv
69 kv
70
242 kv
220 kv
231
550 kv
500 kv
501
Resistances above do not necessarily indicate sound insulation condition, but only that the equipment may be energized without significant risk of failure
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Interpretation of DC test readings y A clean, dry insulation in excellent condition should have resistance several times the min. value y IR value has little significance on one time, absolute value basis. Long term trend indicates progressive deterioration y To allow meaningful trending, influence of temp, humidity should be minimized
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AC Voltage Testing of Electrical Equipments: PF Test
Vivek Jha 55
Introduction y Power factor (PF) and dissipation factor (DF) tests are conducted in the field for acceptance and field maintenance testing of insulation of electrical equipments. y Purpose of AC Tests: y Identify if the equipment has been installed correctly y Determine need for corrective maint/ repair y Track the gradual deterioration of the equipment
y Why AC tests: Gives best info about equipment condition
(near true operating conditions)
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Introduction y Various types of AC voltage tests:
Power Factor (PF) and Dissipation Factor (DF) Test y AC High Potential Test y 0.1 Hz Voltage Test Power Factor (PF) and Dissipation Factor (DF) Test is the most widely used test. It is considered non destructive test because test voltages normally do not exceed designed voltage of equipment. AC Hi‐pot is considered destructive test and should not be repeated frequently y
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Power Factor and Dissipation Factor Test y Used since early 1900s y Based on Schering Bridge y The test uses AC and pursues to know loss angle of the tested equipment, to y y y y y y
know the condition of insulation. The test provides information on overall condition of insulation in the form of a ratio (PF or DF) which is independent of the volume of insulation being tested. Provide assessment of insulation under normal frequency working condition, which are not time dependent like DC tests The test does not overstress the insulation Provide handy tool for comparison with similar equipments It is important to take note of transformer temperature and environmental moisture (surface leakage). This test is sometimes referred as ‘Doble Test’ because of historical significance of Doble test kit. 59
Power Factor and Dissipation Factor Test : Principle of PF/DF Test PF and DF y The PF of insulation is defined as the ratio of watt loss to total charging volt‐amperes, or the cosine of the angle θ between total current vector (IT)and the impressed voltage vector. It is a measure of the energy component (resistive component) of the charging current. y The DF is defined as the ratio of the watt loss to charging amperes, or the tangent of the angle δ between the total current vector and the capacitive current vector. The angle δ is the complementary angle of the PF angle θ.
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Power Factor and Dissipation Factor Test y Ic = V/ Xc = VωC
y PF = cos θ , DF = tan δ For very small δ, PF = DF
y Changes in capacitive current indicate insulation degradation due to moisture, shorted layers, changed geometry etc y Changes in resistive current indicates carbon tracking, volumetric leakage, surface conduction, corona etc
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Example y insulation of PF = 1.0%.
y PF is approximately equal to DF when PF and DF <10.0%, that is y Cos θ=tanδ=cotan θ y cos(89.43)=tan(0.57)=0.01(1%)
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Factors that influence PF measurement y Changes in insulation quality result in measurable changes in some of the
basic electrical characteristics of the insulation, such as capacitance, dielectric loss, or PF. Therefore, by measuring these electrical characteristics over time, changes in the integrity of the insulation can be assessed, The main factors influencing PF measurement are: Temperature y Humidity y Surface leakage y
y It is necessary to normalize the results to a common base temperature y Equipment should be retested if ambient temp is too high or too low y PF tests should not be performed for detection of presence of moisture in
the insulation when the temperatures are much below freezing,
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PF Test Equipment Cs = standard reference capacitor Cx = insulation under test A voltage is applied to both CS and CX. The ratio arms, Ns and Nx are adjusted to balance capacitive current, and the variable resistor Rs is adjusted to balance resistive current. The null indicator is used to determine when the bridge circuit is balanced. The values of Ns and Nx are used to determine capacitance and the value of RS correlates to power ( dissipation) factor of the test insulation. Safety features, self diagnostic and calibration check facility is provided in the kit 64
General procedure for the Operation of the PF Test Set 1. Assemble the test set in accordance with the operating instruction manual. 2. Connect ground lead from the test set to a station ground. Caution: PF tests are performed only on de‐energized and isolated apparatus. Verify the equipment is cleared before attempting to connect leads. 3. Prepare the specimen for testing. This may include removing external connections, shorting winding terminals, etc. 4. Connect test leads: first to the test set, then to the apparatus to be tested following the instructions in the operating manual. 5. Check operation of safety and ground interlocks if supplied on the test set. 6. Select the proper test configuration for the insulation to be measured. 7. Initiate voltage output from the test set. Raise output voltage to the desired level. 8. Continue operation of the test set to obtain test readings, following the specific instructions in the operating manual. Balance the bridge for capacitance and PF. 9. Reduce voltage to zero, or lowest setting and cut off voltage output. 10. Record all values as provided by the test set: test voltage, current, watts‐loss, capacitance, and PF.
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Basic Test Connections (Test Modes) for PF Testing y Grounded‐Specimen Test Mode (GST) y GST Mode with Guard (GST‐G) y Ungrounded‐Specimen Test Mode (UST)
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Basic Test Connections (Test Modes) for PF Testing Grounded‐Specimen Test Mode (GST)
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Basic Test Connections (Test Modes) for PF Testing GST Mode with Guard (GST‐G)
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Basic Test Connections (Test Modes) for PF Testing Ungrounded‐Specimen Test Mode (UST)
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Safety Precautions with PF Testing y All PF tests are performed with the apparatus to be tested completely de‐
energized and isolated from the power system. In addition, the apparatus housing or tank must be properly grounded. There is no substitute for a visual check to ensure that the apparatus terminals are isolated from the power source y Safety grounds should be applied to all apparatus terminals before doing any work on them, and before connecting and disconnecting the PF test leads y operator should have an unobstructed view of the apparatus under test and of various personnel assisting in the tests. y proper clearance should be maintained between the test set and apparatus; it should be recognized that damaged or defective apparatus may fail during test.
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Safety Precautions with PF Testing ….. y Test sets should have ground‐relay that prevents test voltage being applied
until : 1. A heavy‐duty safety (station ground) has been applied to the ground receptacle of the test set. 2. The test case has been grounded through the power supply cord. 3. The voltage control is at the fully counterclockwise zero voltage position
y Do not connect test leads to the apparatus terminals unless the leads are
already connected to the PF test set. y Before making the first test, both safety switch operation should be checked y Never short circuit safety switch y The heavy‐duty test set ground is the last lead to be removed from the test set.
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Applications of PF Test
Applications of PF Test y PF testing is normally used for acceptance testing, preventing maintenance, and post maintenance insulation assessment, and for condition trending. y The test voltage used for PF testing should be sufficient to detect any latent weaknesses in the insulation, but since the test is intended to be non‐destructive, the voltage should not exceed normal line‐to‐neutral or line‐to‐ground operating voltage of the apparatus under test.
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Applications of PF Test : Transformers
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Applications of PF Test : Transformers
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Applications of PF Test : Transformers Calculation of results : Test 1 minus test 2 1. Subtract charging current of test 2 from test 1 2. Subtract watt loss of test 2 from test 1 3. Then calculate the CHL and PF, that is [CH+CHL]−[CH]=CHL
Test 3 minus test 4 1. Subtract charging current of test 4 from test 3 2. Subtract watt loss of test 4 from test 3 3. Then calculate the CHL and PF, that is [CL+CHL]−[CL]=CHL
y The calculated value of CHL from the above calculation should be same. If it
is not then there is either an error in the test results or the calculations
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Applications of PF Test Bushings
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Applications of PF Test: Bushings
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Applications of PF Test Rotating Machinery: PF measurement of Phase to ground insulation.
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Applications of PF Test Rotating Machinery: PF measurement of Interphase (End‐turn) insulation.
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PF test : Shielded cables
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Evaluation of PF and DF Test Results y Four categories:
1 Good Insulation : condition is good and suitable for continued service 2 Deteriorated Insulation : condition is satisfactory for service but should be checked within six months to see if the condition has further degraded 3 Marginal Insulation : condition is not satisfactory for service—immediate investigation of the degraded conditions should be begun and if this is not possible then it should be begun as soon as possible 4 Bad Insulation: Remove from service and recondition to restore insulation to good condition, if not possible, then replace The
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Evaluation of PF and DF Test Results y References for evaluating test results: Manufacturer’s recommendations y year‐to‐year test results y Results of similar equipments y
y Whenever the test results are questionable or marginal, it is generally
recommended to perform tests on more frequent basis y A gradual and consistent increase in PF may be due to contamination,
deterioration, or normal aging, where as a sudden increase in the PF is a cause of immediate concern even when the absolute PF value is not considered excessive 87
Typical of maximum allowable PF values * Equipment
Typical max allowable PF values at 20 Deg C
Oil‐Filled Power and Distribution Transformers
0.5% may be up to 1% for old transformers
Dry‐Type Power and Distribution Transformers Transformer Oil Bushings Rotating Machines Cable insulation: Paper XLPE Ethylene/propylene rubber Rubber (older type) Varnished cambric
*
up to 2% may be up to 5% for old transformers 0.05% 0.50% 1% 0.50% 0.05 ‐ 1.0 % 0.5 ‐ 1.0 % 3 to 5 % 4 to 8 %
These are general guidelines. Please Refer Manufacturer’s recommendations and equipment history
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AC High Potential Test AC Hi‐pot is used as acceptance test (go/no go type) Thumb rule: Acceptance :75 % of factory test voltage, as certified by manufacturer Maintenance : 60 % of factory test voltage, as certified by manufacturer
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Conclusion
Conclusion For optimum benefit : y Essential to record all test data in proper formats for future reference y Test Equipment y Proper selection y Regular maintenance and y Regular calibration
y Trained persons with knowledge, experience and cool mind
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Testing Information The information on testing can be obtained from several different sources such as Power equipment manufacturer’s manuals (BHEL etc), y Test equipment manufacturer’s manuals such as y
y
AVO, International: http://www.avointl.com
y
Doble Engineering: http://www.doble.com AEMC: http://www.aemc.com
y
y
industry standards such as y Indian Standards (IS) y Institute of Electrical and Electronic Engineers (IEEE), y American National Standard Institute (ANSI), y National Electrical Manufacturers Association (NEMA), y National Fire Protection Association (NFPA) y International Electrical Testing Association (NETA), y Insulated Cable Engineering Association (ICEA), y And of course our best friend, INTERNET
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THANK YOU
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Temp Correction factors Deg C
Rotating Equipment
Transformer
Class B
oil filled
Temp correction factor of 2 for
0
0.4
0.25
5
0.5
0.36
10
0.63
0.5
15
0.81
0.74
20
1
1
25
1.25
1.4
30
1.58
1.98
35
2
2.8
40
2.5
3.95
45
3.15
5.6
50
3.98
7.85
55
5
11.2
60
6.3
15.85
65
7.9
22.4
70
10
31.75
75
12.6
44.7
Reproduced from NETA MTS – 1997 Table 10.14
•Approx every 10 deg increase in Temp for oil filled Transformers
•Approx every 15 deg increase in Temp for Rotating equipments having class B insulation
