Diagnostic Tests on Transformers

Prepared by: SV Checked By: BK Approved By: MK Date: 15/07/2007

DIAGNOSTIC TEST ON

TRANSFORMER (METHOD STATEMENT)

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without prior written approval from AYT International LLC, Dubai


Diagnostic Test On Transformer Customer

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Client RASGAS Qatar Substation Name

Consultant Manufacturer

Contractor Circuit Reference Contract No. Date of test

Index

Sl No

Item Format No

1 General Safety

MS-DT-TR-GS-00-00

2 Insulation Resistance Test

MS-DT-TR-IR-01-00

3 Transformer Turns Ratio Test

MS-DT-TR-TR-02-00

4 Magnetic Balance Test

MS-DT-TR-MB-03-01

5 Short Circuit Test

MS-DT-TR-SC-04-00

6 Sweep Frequency Response Analysis

MS-DT-TR-SF-05-01

7 Tan Delta And Capacitance Test

MS-DT-TR-TC-06-00

8 Winding Resistance Measurement Test

MS-DT-TR-WR-07-00

9 Infrared Thermography Analysis Of Transformer

MS-DT-TR-IT-08-00

10 Frequency Dielectrospectroscopy (FDS)

MS-DT-TR-FD-09-00


Diagnostic Test On Transformer Customer

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Client RASGAS Qatar Substation Name

Consultant Manufacturer

Contractor Circuit Reference Contract No. Date of test AYT TP.: MS-DT-TR-GS-00-00 AYT Test sheet:

1.0 GENERAL SAFETY Safety is right at AYT INTERNATIONAL LLC (AYT), not a privilege! Comply with all the safety Regulations and Recommendations of the Equipment manufacturers and Plant operators. A LIFE LOST HAS UNRECOVERABLE DAMAGES! So take following Precautions;

1. Ensure that the EQUIPMENT UNDER TEST (EUT) is de-energized from all sources.

2. Ensure that Safety grounds are put before making any connections to the EUT.

3. Ensure that connections to the supply sources for tests are tight and FUSES/MCB of adequate ratings is used. The connection should preferably be taken from metal clad switches instead of plug points. Supply cables should not lie on ground but should be suitably supported,

4. Instrument should be arranged at height convenient for operation.

5. EUT and test area should be cordoned off. Only authorized persons should enter the area.

6. Ensure that necessary authorization for testing the equipment has been obtained as per Safety Tagging Procedure.

7. Ensure that all connections to the equipment are tight. Ground Connection should be neat if proper size. Cleaned and properly tightened.


Diagnostic Test On Transformer

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Client RASGAS Qatar Substation Name Consultant Manufacturer Contractor Circuit Reference Contract No. Date of test AYT TP.: MS-DT-TR-IR-01-00 AYT Test sheet: TS-DT-TR-IR-02-00

2.0 INSULATION RESISTANCE TEST 2.1 Scope: The insulation resistance test, also called Meggar test, is used to determine the leakage current resistance of the insulation. The resistance is a function of the moisture and impurity content of the insulation as well as the insulation temperature. At a constant voltage, the resistance also depends on the strength of the electric field across the insulation and therefore is a function of the size and construction of the transformer. This measurement gives information about the condition of the insulation and insures that the leakage current is adequately small.

2.2 Instruments: Battery/220V, 50HZ Insulation Tester with a capability to measure resistance 0 to 5T and Polarization Index (PI).

2.3 Preparation, Precautions & Warnings:

Precautions:- 1. Disconnect the jumpers so that the lightning arresters and associated equipments are not connected with the Transformers. 2. Clean the bushing porcelains by wiping with a piece of dry cloth. Any dirt on bushing may result in erroneous results.



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Warnings:- 1. The test should be discontinued immediately if the current begins to increase without stabilizing. 2. Under no conditions should the test be made while the Transformer is under vaccum. 3. After the test has been completed all the instruments should be grounded for a period of time sufficient to allow any trapped charges to decay to a negligible value.

2.4 Test Procedure: IR measurement shall be taken between collective (i.e. all the windings being connected together) and the earthed tank (Earth) and between each winding and the tank, the rest of the windings being earthed. Following combination of measurements relevant for Auto Transformer, Three winding transformer and Reactor. Figure 2.4-1 will give the connection diagram of instrument with Transformer

AT SR 2 WT 3 WT HV+IV to LV HV to E HV to LV HV+IV to E HV+IV to E HV+LV to IV LV to E HV+IV+LV to E AT Auto Transformer,SR Shunt Reactor, 2WT Two winding Transformer, 3WT Three winding Transformer



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Record date, time of measurement, Sl No, Meggar, Oil temperature and IR value at intervals of 15seconds, 1minute and 10minute in the format TS-DT-TR-IR-02-00

Figure 2.4-1



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2.5 Evaluation/Interpretation of test Results and Acceptance limit: The IEC standard 60076-1 and IEEE standard C57.12.90 provide no limit for IR. However the following guideline should be referred The condition of the insulation can be determined by comparing the measured resistance at 1 minute (R60) a minimum value for the voltage class of the winding. This comparison is performed only after all the measurements are converted to their 20C equivalent using the coefficients in table 2.5-1 (2). The minimum measured resistance corrected to 20C is given by the empirical relationship; R60 = CE/KVA,(1) Where, R60 = One minute minimum resistance in M KVA = is the rated transformer capacity of the winding under test E = Voltage rating in V, of one of the single-phase windings (Ph-Ph for delta connection and phase neutral for Wye connected transformers) For Testing without guard terminal C: 0.8 for oil filled Transformer at 20C



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C: 16 for dry compound filled or untanked oil filled Transformer. For Testing With Guard Terminal C: 1.5 for oil filled transformers at 20C C: 30 for dry, compound filled or untanked oil filled transformer If the transformer is the three phase type and the three windings are being tested together Temp(C)

Coefficient Temp(C)

Coefficient Temp(C)

Coefficient

0 0.25 24 1.33 41 4.20 5 0.36 25 1.40 42 4.50

10 0.50 26 1.50 43 4.80 11 0.54 27 1.60 44 5.10 12 0.59 28 1.74 45 5.60 13 0.62 29 1.85 46 5.95 14 0.66 30 1.98 47 6.20 15 0.70 31 2.10 48 6.80 16 0.76 32 2.30 49 7.20 17 0.82 33 2.45 50 7.85 18 0.86 34 2.60 55 11.20 19 0.96 35 2.80 60 15.85 20 1.00 36 3.00 65 22.40 21 1.08 37 3.20 70 31.75 22 1.15 38 3.40 75 44.70 23 1.25 39 3.70 80 63.50 24 1.33 40 3.95

Table 2.5-1(2)


IR Test results below this minimum value would indicate probable limitation breakdowns. A zero or very low value of ohms would indicate a grounded winding, winding to winding short or heavy carbon tracking. In case the recommendation from manufacturer is missing, the following IR values as per engineering practices may be considered as the minimum satisfactory values (R60). Note even if the limitation is dry, IR values will be low if the resistivity of oil is poor. Please find the table 2.5-2 (3)

Minimum desired IR value at 1minute (M) Transformer Winding Voltage(KV)

20C 30C 40C 50C 60C

Below 6.6 400 200 100 50 25 6.6 19 800 400 200 100 50 22 to 45 1,000 500 250 125 65

66 & Above 1,200 600 300 100 75 Table 2.5-2 (3) Polarization Index (PI) and Absorption Index (AI) The conduction process increases for an insulation system that is contaminated with moisture or impurities, the leakage current will increase at a greater rate than for a dry, clean insulation. Consequently under the same test configuration, the limitation resistance of a wet or contaminated insulation system will settle faster and at a lower value than that for a dry insulation. The result is that the Polarization Index (PI) which is ratio of (R600/R60) and


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Absorption ratio (R60/R15) for a wet insulation will be lower than the dry insulation. Refer to the following Table 2.5-3(1, 4) AI = 1.3 to 3 for Dry Insulation

PI Insulation condition Less than 1 Wet/ dangerous

1 1.1 Poor/Poor 1.1 1.25 Questionable 1.25 2.0 Good/fair

Above 2 Dry Table 2.5-3(1, 4)

2.6 References (1) Transformer maintenance guide by M. Horning (2) Service Handbook for Transformer by ABB (3) Electrical equipment testing & maintenance by A.S.Gill (4) Manual on EHV substation equipment maintenance by Central Board Of Irrigation & Power (CBIP)



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Client RASGAS Qatar Substation Name Consultant Manufacturer Contractor Circuit Reference Contract No. Date of test AYT TP.: MS-DT-TR-TR-02-00 AYT Test sheet: TS-DT-TR-TR-03-02

3.0 Transformer Turns Ratio Test

3.1 Scope: The function of a transformer is to transform power from one voltage level to another. The ratio test ensures that the transformer windings have the proper turns to produce the voltages required.

3.2 Instruments: 220V, 50HZ Turns ratio tester with a capability of measuring ratios from 0.8 to 15,000, three phase Excitation current, phase angle reading and can measure the turns ratio of PhaseShifting Transformer.(TRI- True turns ratio meter; make Vanguard Instruments USA)

3.3 Preparation, Precautions & Warnings: The voltage should be applied only in HV winding in order to avoid unsafe voltage.

3.4 Test Procedure: The turns ratio is a measure of the RMS voltage applied to the primary terminals to the RMS voltage measured at the secondary terminals: Figure 3.4-1 will give the connection diagram of instrument with Transformer



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r = Np = Ep Ns Es Where: r= voltage ratio E= open-circuit voltage N = number of turns p= primary s= secondary

Figure 3.4-1

3.5 Evaluation/Interpretation of test Results and Acceptance limit: The IEEE standard (IEEE Standard 62) states that when rated voltage is applied to one winding of the transformer, all other rated voltages at no load shall be correct within one half of one percent of the nameplate readings. It also states that all tap voltages shall be correct


to the nearest turn if the volts per turn exceed one half of one percent of the desired voltage. The ratio test verifies that these conditions are met. The IEC 60076-1 standard defines the permissible deviation of the actual to declared ratio as follows: Principal tapping for a specified first winding pair: the lesser 0.5 % of the declared voltage ratio or 0.1 times the actual short-circuits impedance. Other taps on the first winding pair and other winding pairs must be agreed upon, and must not be lower than the smaller of the two values stated above. Deviations in turns ratio readings indicate problems in one or both windings. In particular, the TTR test is useful for identifying shorted turns or open circuits in the windings, incorrect winding connections, and other internal faults or tap changer defects. If possible, the ratio at each tap setting should be checked against the nameplate ratio for each tap. Measurements are typically made by applying a known low voltage across the high-voltage winding (as a primary) so that the induced voltage on the secondary is lower, thereby reducing hazards while performing the test. For a three phase delta/wye or Wye/delta transformer, a three phase equivalency test is performed, i.e. the test is performed across corresponding single windings.


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3.6 References (2) Service Handbook for Transformer by ABB

(4) Manual on EHV substation equipment maintenance by Central Board Of Irrigation & Power (CBIP)


4.0 Magnetic Balance Test 4.1 Scope: Magnetic Balance Test is conducted only in three phase transformers to check the imbalance in the magnetic circuit and to evaluate its healthiness.

4.2 Instruments: 1. 220V supply with 50HZ 2. Voltmeter: High impedance/ Digital meter (Fluke 87V, USA) 3. 3Phase variac with current rating 10A (Ravistat, India)

4.3 Preparation, Precautions & Warnings: No winding terminal should be grounded; otherwise results would be erratic and confusing.

4.4 Test Procedure: Transformer tap to be kept in nominal tap positions, transformer neutral to be disconnected from ground. Single phase 230 V is applied across one phase of Intermediate Voltage (IV) winding terminal and neutral (call it v1) then voltage to be measured in other two IV terminals across neutral (call them v2 and v3 respectively). Test to be repeated for each of the three phases. Test to be repeated for the HV winding also. Figure 4.4-1 will give the connection diagram of instrument with Transformer


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Client RASGAS Qatar Substation Name Consultant Manufacturer Contractor Circuit Reference Contract No. Date of test AYT TP.: MS-DT-TR-MB-03-01 AYT Test sheet: TS-DT-TR-MB-04-01


Figure 4.4-1


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4.5 Evaluation/Interpretation of test Results and Acceptance limit: From the measurement carried out ensure that, v1 = v2+v3 respectively. Zero voltage or very negligible voltage induced in any winding should be investigated. Also the applied voltage may be expressed as 100% and the induced voltages as percentage of applied voltage. This will help in comparison of two results when applied voltages are different. Some of the benchmark references standard tables are mentioned below for the Y-Y transformer comparison Table 4.5-1(5) For Y-Y Transformer Supply voltage

AN in%

BN in %

CN in %

an in% bn in % cn in %

AN 100 8595 515 100/n (8595)/n (515)/n BN 4555 100 4555 (4555)/n 100/n (4555)/n CN 515 8595 100 (515)/n (8595)/n 100/n

Table 4.5-1(5)


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Some of the benchmark references standard tables are mentioned below for the -Y transformer comparison Table 4.5-2(5) -Y transformer comparison Supply voltage

AB in %

BC in %

CA i n %

an in %

bn in %

cn in %

ab in %

bc in %

Ca in %

AB 100 7580

2025

100/3n

(7580)

/3n

(2025)

/3n

100/n (3045)/n

(5570)/n

BC 4555

100 4555

(4555)

/3n

100/3n

(4555)

/3n

(8595)/n

(8595)/n

(515)/n

CA 2025

7580

100 (2025)

/3n

(7580)

/3n

100/3n

(3045)/n

100/n (5570)/n

Table 4.5-2(5)

4.6 References (4) Manual on EHV substation equipment maintenance by Central Board Of Irrigation

& Power (CBIP)

(5) 9th International conference on Transformer (TRAFOSEM 2005)


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Transformer Diagnostic Test Sheet Customer logo

Client RASGAS Qatar Substation Name Consultant Manufacturer Contractor Circuit Reference Contract No. Date of test AYT MS Ref.: MS-DT-TR-SC-04-00 AYT Test sheet: TS-DT-TR-SC-05-00

5.0 Short Circuit Impedance Test 5.1 Scope The test procedure is applicable to tests on two or three winding transformers rated at or above 100 V.

5.2Instruments: 1. Supplies: .1 230 V AC .2 440 V AC 2. 3-Phase Variac (Ravistat, India) 3. Voltmeter (Fluke 87V, USA) 4. Ammeter (Fluke 87V, USA) 5. Clamp meter (HT Italia 7012, Italy)

5.3 Preparation, Precautions & Warnings: .

1. Ensure that no parallel circuits are connected to the equipment under test. Disconnect ground connection to neutral of the transformer. Ensure there are no shorts on the secondary side.




2. In the case of ICTs in GIS, Tests have to be done at Earth switches after disconnecting the connections to enclosures and with earth switches closed. 3. From the transformer winding diagram, note the corresponding winding to be shorted.

4. From the computed values of impedance, 230 V a.c. can be applied

Through variac for windings with more than 5 ohms impedance. For impedance less than 5 ohms, pass the current through exciting transformer.

5. Use current leads of adequate size. 6. Size of the shorting lead should be equivalent to 0.1 Sq.in. lugged copper conductor. Use same shorting conductor for the current test. The LV impedance is usually low and the shorting lead should have very low impedance in comparison. 7. SC impedance test should be carried out phase by phase for Primary as well as secondary.




5.4 Procedure

1. Make the test connections as per circuit diagram. 2. Short the corresponding secondary winding. 3. Gradually increase current through the winding. 4. Record corresponding current and voltage reading for 3 current

values. Note: i) Voltage drop measurement leads should be connected to the Points nearest to the winding. ii) For two winding transformer, total six and for three winding Transformer, total eighteen values shall be recorded. iii) For transformers with zigzag winding, the test will be done with a three phase variac controlling 440 V, 3 phase source with all three secondary terminals short circuited.

Figure 5.4-1 will give the connection diagram of instrument with Transformer Impedance (%) = Rated current x Measured voltage/ Rated voltage x Measured current x 100 (At corresponding Tap)




Figure 5.4-1




5.5 Evaluation/Interpretation of test Results and Acceptance limit:

Variation between phases: not more than + 1% Variation with calculated value: not more than + 1%


& Power (CBIP)



Client RASGAS Qatar Substation Name Consultant Manufacturer Contractor Circuit Reference Contract No. Date of test AYT MS Ref.: MS-DT-TR-SF-05-01 AYT Test sheet: TS-DT-TR-SF-06-01

6.0 FREQUENCY RESPONSE ANALYSIS

6.1 Scope: Frequency response analysis (FRA) is made to access the mechanical integrity of the transformer. Transformers while experiencing severity of short circuit looses its mechanical property by way of deformation of the winding or core. These changes cannot be detected through conventional condition monitoring technique such as DGA, winding resistance measurement, Capacitance & Tan Delta measurement etc.Sometimes even transportation without proper precaution may cause some internal mechanical damages. FRA measurement, which is signature analysis, provides vital information of the internal condition of the equipment so that early corrective action could be initiated.

6.2 Instruments:

1. Supplies: 230 V AC/Battery operated 2. Sweep frequency Response analyzer (FRAX-101, Pax-Diagnostics Sweden)




6.3 Preparation, Precautions & Warnings: Sweep Frequency Response Analyzer designed to be used for measurements of power transformers in the field and at manufacturing sites. 1. Always follow local safety regulations. 2. Make certain that all personnel who work with SFRA instrument have been trained in its use and that all applicable safety precautions are taken. 3. Always connect the instrument to protective earth/ground using the separate earth/ground cable. 4. The earth/ground cable should be the first connection made and the last removed.

6.4 Procedure Short circuit forces can cause winding movement and changes in winding inductance or capacitance in power transformers. Recording the frequency response with these changes gives information regarding the internal condition of the equipment. FRA has proved to be an effective tool to detect such changes. Sinusoidal signal output of approximately 10V RMS from the FRA is applied and one measuring input (R1) is connected to the end of a winding and the other measuring input (T1) is connected to the other end of the winding. The voltage is applied and measured wrt the earthed transformer tank. The voltage transfer function T1/R1 is measured for each winding for frequency scans from 10Hz to 10MHz




and amplitude and phase shift results are recorded. While the low frequency analysis reveals the winding movements, the high frequency analysis reveals the condition of joints. It is ensured that winding which is not under test is terminated in open condition in order to avoid response difference among the three phases. The same procedure is followed on subsequent tests on the same or similar transformer, to ensure that measurements are entirely repeatable. The voltage transfer function T1/R1 is measured for each winding for frequency scan from 10Hz to 10MHz and amplitude and phase shift results are recorded for subsequent analysis. Figure 6.4-1 will give the connection diagram of instrument with Transformer




Figure 6.4-1

6.5 Evaluation/Interpretation of test Results and Acceptance limit: An SFRA analysis permits to predict faults such as

Core fault Low frequency variation

Shorted turn fault Low frequency variation




Open circuit or high impedance winding fault Low frequency

variation Overall radial shift of the winding Mid frequency variation

Axial shift Mid to High frequency variation

Core earthing related faults in transformer/tap changer leads &

bushing lead related faults High frequency variation As FRA is signature analysis, data of signature of the equipment when in healthy condition is required for proper analysis. Signatures could also be compared with unit of same internal design or with other phases of the same unit. Normally measured responses are analyzed for any of the following

Changes in the response of the winding with earlier signature

Variation in the responses of the three phases of the same transformer

Variation in the responses of transformers of the same design

In all the above cases the appearance of new features or major frequency shifts are causes for concern. The phase responses are also being recorded but normally it is sufficient to consider only amplitude responses.




Table 6.5-1(6) will give the relation between Relative factors and degree of transformer winding deformation Winding deformation degree Relative Factors R

Severe deformation RLFRLF0.6 or RMFRLF1.0 or 0.6RMF





& Power (CBIP)

(6) Seminar on Fault-Finding and Life-Assessment of Power Transformers

(V.K.Kanjlia, P.P.Wahi)



Client RASGAS Qatar Substation Name Consultant Manufacturer Contractor Circuit Reference Contract No. Date of test AYT MS Ref.: MS-DT-TR-TC-06-00 AYT Test sheet: TS-DT-TR-TC-07-00

7.0 TAN DELTA & CAPACITANCE OF WINDING

7.1 Scope: Dissipation factor/Loss factor and capacitance measurement of winding is carried out to ascertain the general healthiness of the ground and inter-winding insulation of transformer and reactors.

7.2 Instruments:

1. Supplies: 230 V AC, 50 HZ 2. Tan & Capacitance Measurement Instrument (ACTS12K Eltel, India) 3. Hygrometer (Eltel, India)

7.3 Preparation, Precautions & Warnings:

Isolation Required Ensure the isolation of Transformer from High Voltage to Low Voltage side with physical inspection of open condition of the concerned isolators/disconnectors. This testing shall be carried out during shut down period and all safety precautions to be undertaken. Test shall not be carried out when there is condensation on the porcelain. Preferably, tests shall not be carried out when the relative humidity is in excess of 75%.




Safety precautions as recommended by the manufacturer may be followed. The test set is a source of high voltage electrical energy and operator must use all practical safety precautions to prevent contact with energised parts of the test equipment and related circuits. Effect of Ambient Temperature At 20oC value of tan delta (Dissipation Factor) should not be more than 0.007. If tan delta measurement is carried out at temperature other than 20oC then correction factor furnished by manufacturer are to be applied. As temperature dependence could not be achieved to bring any common factor especially for Indian manufactured transformers, it is advisable to carryout the test almost between 20oC to 40oC of winding temperature as in this range the values are almost constant and do not vary much.

7.4 Procedure Measurement Combination The combination for tan measurement of winding is same as that of measurement of IR value. Combination Table 7.4-1(2) has given below




PF Measurement setup for Two-Winding Transformer Test Mode HV

Winding LV Winding Tank/Core Measured

Capacitance GST HV lead Meas.lead GND lead CH+CHL

GST/g HV lead Meas.lead (on guard)

GND lead CH

UST HV lead Meas.lead GND lead (on guard)

CHL

GST Meas.lead HV lead GND lead CL+CHL GST/g Meas.lead

(on guard) HV lead GND lead CL

UST Meas.lead HV lead GND lead (on guard)

CHL

Table 7.4-1(2) Test Voltage Site test could not be possible to be carried out at rated voltage. Normally C and Tan measurement can be performed at site about 10 kV to 12 kV. It is a good practice to have tan and capacitance tested at factory at these voltages to have reference. Figure 7.4-1 will give the connection diagram of instrument with Transformer




Figure 7.4-1

7.5 Evaluation/Interpretation of test Results and Acceptance limit:

An increase of DF accompanied by a marked increase in capacitance usually indicates excessive moisture in the insulation. Increase of DF alone may be caused by thermal deterioration or by contamination other than water. The changes in C and Tan values are caused by determination of the insulation, contamination or physical damage.




Maximum values of Dissipation Factor (Tan delta) of class A insulation, e.g., oil impregnated insulation is 0.007. Rate of change of tan delta and capacitance is very important. Comparison of test results with earlier results and that of pre-commissioning test results shall give desired information for trend analysis. Winding temperature correction to be applied only if indicated by the manufacturer. It is always desirable to test at about same temperature for better comparison. The Table 7.5-1(2) which given below will give the general guidelines in assessing PF results in oil filled power Transformer

PF Reading Possible Insulation condition 0.5% Good

>0.5% But 0.7% Deteriorated >0.7% But 1.0%(& increasing) Investigate

>1.0% Bad Table 7.5-1(2)




Power Factor Tip-Up Tests The PF Tip-up test is performed by applying voltage in equal steps from 0 to the maximum allowed voltage. The test is performed on the section of insulation with highest PF reading. For each applied voltage, the current and watts loss through the insulation is measured, and the PF is calculated. If moisture or other polar contaminants are the cause of high PF, the measured PF will be essentially the same for all applied voltages. If the PF increases with voltage, there is likely ionic contamination and / or carbonization of the oil or windings for oil-filled transformers. For dry type transformers, the problem may be due to ionic contaminants or the presence of voids in the winding insulation.

7.6 References (4) Manual on EHV substation equipment maintenance by Central Board Of Irrigation & Power (CBIP) (2) Service Handbook for Transformer by ABB



Client RASGAS Qatar Substation Name Consultant Manufacturer Contractor Circuit Reference Contract No. Date of test AYT MS Ref.: MS-DT-TR-WR-07-00 AYT Test sheet: TS-DT-TR-WR-08-00

8.0 WINDING RESISTANCE MEASUREMENT

8.1 Scope: Transformer winding resistance is measured in the field in order to check for any abnormalities due to loose connections, broken strands and high contact resistance in tap changers as pre-commissioning checks and compare the measured values with factory test values

8.2 Instruments:

1. Supplies: 230 V AC 2. Winding Resistance Measurement Instrument (RMO50T, DV power Sweden)

8.3 Preparation, Precautions & Warnings: As the transformer windings have low resistance, the measurement has to be carried out with the help of Kelvin Double Bridge / Transformer ohmmeter. To reduce the high inductive effect it is advisable to use a sufficiently high current to saturate the core. This will reduce the time required to get a stabilized reading. It is essential that a temperature of the windings be accurately noted because resistance is temperature sensitive. Care also must be taken to ensure that direct current circulating in the windings has settled down before the measurement is done. In some




Cases this may take several minutes depending upon the winding inductance.

8.4 Procedure Winding resistance measurements are performed with a low-resistance ohmmeter. For a three phase wye-connected transformer, the resistance is measured for each phase-to-neutral winding: if delta-connected, the resistance is measured for each phase-to-phase winding. Note that for delta-connected transformers, the measured resistance for each phase is composed of a parallel combination of the winding under test and the series combination of the remaining windings. It is therefore recommended to make three measurements for each phase-to-phase winding in order to obtain the most accurate results. It is also recommended to allow the transformer to sit de-energized until temperatures are equalized (difference between top and bottom temperatures does not exceed 5oC- ANSI/IEEE C57.12.90) before making resistance measurements. According to IEC 60076-1, in order to reduce measurement errors due to changes in temperature, some precautions should be taken before the measurement is made. For star connected winding with neutral brought out, the resistance shall be measured between the line and neutral terminal and average of three sets of reading shall be the tested value. For star connected auto-transformers the resistance of the HV side is measured between HV terminal and IV terminal, then between IV terminal and the neutral. For delta connected windings, connected windings, such as tertiary winding of auto-transformers, measurement shall be done




Between pairs of line terminals and resistance per winding shall be calculated as per the following formula: Resistance per winding = 1.5 x Measured value

For Dry type transformers, the transformer shall be at rest in a constant ambient temperature for at least 3 hours. For Oil immersed transformers, the transformer should be under oil and without excitation for at least 3 hours. In addition, it is important to ensure that the average oil temperature (average of the top and bottom oil temperatures) is approximately the same as the winding temperature. To avoid an inadmissible winding temperature rise during the measurement, it is also recommended that the measuring current should be limited to no more than 10 percent of the rated current of the winding. In order to diagnose possible problems, the measured results are compared to the factory values, values of other phases of the same transformer, or sister units, if available. Before making such comparisons, the resistance has to be converted to a common temperature base of 75oC or 85oC, depending on what is reported on the transformer factory test sheet. The correct resistance is calculated as: RCT= RM x (CF+ CI) CF+ Winding Temp(oC)




Where: RCT= Corrected resistance CF= 234.5 for copper windings; 225 for aluminium windings (IEEE

C57.12.90) CF= 235 for copper windings; 225 for aluminium windings (IEC 60076-

1) CT= 75 for 55oC rise transformers; 85 for 65oC rise transformers

RM= Measured winding resistance Consistency in measurements and record keeping are the keys to making the proper analysis using this test. If the unit has a tap changer, it is important to compare resistances for the same tap position. The contact resistance of other tap positions can be investigated by moving taps and repeating the measurements. Figure 8.4-1 will give the connection diagram of instrument with Transformer




Figure 8.4-1




8.5 Evaluation/Interpretation of test Results and Acceptance limit: Since it is difficult for precise winding temperature measurement at field, the expected deviation for this test with respect to the factory test result is about 2 to 5%. The values obtained also should not be more than 5% of other phases. As winding resistance are temperature sensitive, it is to be measured when equipment is stable in temperature and the values to be converted to 75oC. Changes greater than 5% may be indicative of bonded or bolted connections, short circuits or bad tap changer contacts.

8.6 References (4) Manual on EHV substation equipment maintenance by Central Board Of Irrigation & Power (CBIP) (2) Service Handbook for Transformer by ABB

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