4.7 Transformer Testing When the transformer is new before energizing and every 3 to 5 years, the transformer and bushings should be Doble tested. Transformer testing falls into three broad categories: Factory testing when the transformer is new or has been refurbished, acceptance testing upon delivery, and field testing for maintenance and diagnostic purposes. Some tests at the factory are common to most power transformers, but many of the factory tests are transformer- specific. Table 17 lists several tests. This test chart has been adapted from IEEE 62-1995 reference [19]. Not all of the listed tests are done at the factory, and not all of them are done in the field. Each transformer and each situation is different, requiring its own unique approach and tests. Details of how to run specific tests will not be addressed in this FIST. It would be impractical to repeat how to do Doble testing of a transformer when the information is readily available in Doble publications. With some exceptions, this is true for most of the tests. Specific information is readily available within the test instrument manufacturers literature. Another example is the transformer turns ratio test (TTR); specific test information is available with the instrument. However, information on some tests may not be available and will be covered briefly. 4.7.1 Winding Resistances. Winding resistances are tested in the field to check for loose connections, broken strands, and high contact resistance in tap changers. Key gases increasing in the DGA will be ethane and/or ethylene and possibly methane. Results are compared to other phases in wye connected transformers or between pairs of terminals on a delta-connected winding to determine if a resistance is too high. Resistances can also be compared to the original factory measurements. Agreement within 5% for any of the above comparisons is considered satisfactory. You may have to convert resistance measurements to the reference temperature used at the factory (usually 75 °C) to compare your resistance measurements to the factory results. To do this use the following formula: Ts + Tk Rs = Rm Tm + Tk Rs = Resistance at the factory reference temperature (found in the transformer manual) Rm = Resistance you actually measured Ts = Factory reference temperature (usually 75 °C) Tm = Temperature at which you took the measurements Tk = a constant for the particular metal the winding is made from: 234.5 °C for copper 225 °C for aluminum It is very difficult to determine actual winding temperature in the field, and, normally, this is not needed. You only need to do the above temperature corrections if you are going to compare resistances to factory values. Normally, only the phase resistances are compared to each other, and you do not need the winding temperature to compare individual windings. You can compare winding resistances to factory values; change in these values can reveal serious problems. A suggested method to obtain an accurate temperature is outlined below. If a transformer has just been de-energized for testing, the winding will be cooler on the bottom than the top, and the winding hot spot will be hotter than the top oil temperature. What is needed is the average winding temperature, and it is important to get the temperature as accurate as possible for comparisons. The most accurate method is to allow the transformer sit de-energized until temperatures are equalized. This test can reveal serious problems, so it’s worth the effort. Winding resistances are measured using a Wheatstone Bridge for values 1 ohm or above and using a micro-ohmmeter or Kelvin Bridge for values under 1 ohm. Multi-Amp (now AVO) makes a good instrument for these measurements which is quick and easy to use. Take readings from the top of each bushing to neutral for wye connected windings and across each pair of bushings for delta connected windings. If the neutral bushing is not available on wye connected windings, you can take each one to ground (if the neutral is grounded), or take readings between pairs of bushings as if it were a delta winding. Be consistent each time so that a proper comparison can be made. The tap changer can also be changed from contact to contact, and the contact resistance can be checked. Keep accurate records and connection diagrams so that later measurements can be compared. 4.7.2 Core Insulation Resistance and Inadvertent Core Ground Test. Core insulation resistance and inadvertent core ground test is used if an additional core ground is suspected; this may be indicated by the DGA. Key gases to look for are ethane and/or ethylene and possibly methane. These gases may also be present if there is a poor connection at the bottom of a bushing or a bad tap changer contact. Therefore, this test is only necessary if the winding resistance test above shows all the connections and if tap changer contacts are in good condition. The intentional core ground must be disconnected. This may be difficult, and some oil may have to be drained to accomplish this. On some transformers, core grounds are brought outside through insulated bushings and are easily accessed. A standard dc megohmmeter is then attached between the core ground lead (or the top of the core itself ) and the tank (ground). The megohmmeter is used to place a dc voltage between these points, and the resistance measured. A new transformer should read greater than 1,000 megohms. A service-aged transformer should read greater than 100 megohms. Ten to one-hundred megohms is indicative of deteriorating insulation between the core and ground. Less than 10 megohms is sufficient to cause destructive circulating currents and must be further investigated [19]. A solid core ground may read zero ohms; this, of course, causes destructive circulating currents also. Some limited success has been obtained in “burning off” unintentional core grounds using a dc or ac current source. This is a risky operation, and the current may cause additional damage. The current source is normally limited to 40 to 50 amps maximum and should be increased slowly so as to use as little current as possible to accomplish the task. This should only be used as a last resort and then only with consultation from the manufacturer, if possible, and with others experienced in this task. Part to be Tested Test to be Performed Windings Resistance Across Windings Turns Ratio/Polarity/Phase Excitation Current at All Tap Positions Short Circuit Impedance Insulation Resistance to Ground (megohmmeter) Capacitance (Doble) Power Factor/Dissipation Factor (Doble) Induced Voltage/Partial Discharge/Riv Bushings Capacitance (Doble) Dielectric Loss (Doble) Power Factor/Dissipation Factor (Doble) Partial Discharge (Doble) Temperature (Infrared) Oil Level (Sight Glass) Visual Inspection (Cracks and Cleanliness) DGA Insulating Oil Dissolved Gas Analysis Dielectric Strength Interfacial Tension Acid Number Visual Inspection Color Water Content Oxygen Inhibitor Power Factor/Dissipation Factor Tap Changers - Load Contact Pressure and Continuity Temperature (Infrared) Turns Ratio at All Positions Timing Motor Load Current Limit Switch Operation and Continuity Tap Changers - No Load Contact Pressure and Continuity Centering Turns Ratio at All Positions Visual Inspection Core Core Insulation Resistance to Tank Ground Test (megohmmeter) Tanks and Associated Devices Pressure/Vacuum/Temperature Gages - Calibration Temperature (Infrared) Visual Inspection (Leaks and Corrosion) Conservator Visual Inspection (Leaks and Corrosion) ir Drier Desiccant Proper Color Valves in Proper Position Sudden Pressure Relay Calibration and Continuity Buchholz Relay Proper Operation and Continuity Cooling System Temperature (Infrared) Heat Exchanger Radiators Clear Air Flow Visual (Leaks, Cleaning, and Corrosion) Fans Controls Visual Inspection and Unusual Noise Pumps Rotation and Flow Indicator Motor Load Current REFERENCES 1. IEEE Standard C57.12.01-1989 Standard General Requirements for Dry-Type Distribution, Power, and Regulating Transformers (ANSI). 2.IEEE Standard C57.12.00-1993 Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers (ANSI). 3.Power Transformer Maintenance and Testing, General Physics Corporation. 1990. 4.Guidelines for the Life Extension of Substations EPRI, TR-105070. April 1995. 5.Transformer Maintenance Guide, by J.J Kelly, S.D. Myers, R.H. Parrish, S.D. Meyers Co. 1981. 6.Transformer General Gasketing Procedures, by Alan Cote, S.D. Meyers Co. 1987. 7.NFPA 70B-1998 Recommended Practice for Electrical Equipment Maintenance. 8.Bushing Field Test Guide, Doble Engineering Company. 1966. 9.Testing and Maintenance of High-Voltage Bushings, FIST 3-2, Bureau of Reclamation. 1991. 10.IEEE Standard C57.19.00, 1991 General Requirements and Test Procedure for Outdoor Power Apparatus Bushings. 11.IEEE Standard C57.104-1991 Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers. 12.International Electrotechnical Commission (Draft IEC 60599 Edition 2), Mineral Oil-Impregnated Electrical Equipment in Service-Interpretation of Dissolved and Free Gas Analysis. 1997. 13.Dissolved Gas Analysis of Transformer Oil, by John C. Drotos, John W. Porter, Randy Stebbins, published by the S.D. Meyers Co. 1996. 14.IEEE Standard C57.94, 1982, Recommended Practice for Installation, Application, Operation and Maintenance of Dry-Type General Purpose Distribution and Power Transformers. 15.Criteria for the Interpretation of Data for Dissolved Gases in Oil from Transformers (A Review), by Paul Griffin, Doble Engineering Co. 1996. 16.Maintenance of High Voltage Transformers, by Martin Heath Cote Associates, London, England. 1989. 17.Thermal Monitors and Loading, by Harold Moore, from Transformer Performance Monitoring and Diagnostics EPRI. September 1997. 18.IEEE and IEC Codes to Interpret Incipient Faults in Transformers, Using Gas in Oil Analysis, by R.R. Rogers C.E.G.B, Transmission Division, Guilford, England. Circa 1995. 19.IEEE Standard 62-1995, IEEE Guide for Diagnostic Field Testing of Electrical Power Apparatus, Part 1: Oil Filled Power Transformers, Regulators, and Reactors. 20.FIST 3-5 Maintenance of Liquid Insulation: Mineral Oils and Askarels. 1992. 21.ANSI/ASTM D 971-91, Standard Test Method for Interfacial Tension of Oil Against Water by the Ring Method. 22.EPRI Substation Equipment Diagnostics Conference VII, Experience with In-Field Water Contamination of Large Power Transformers, by Victor V. Sokolov and Boris V. Vanin, Scientific and Engineering Center “ZTZ Service Co.,” Ukraine. 1999 23.Doble Engineering Company “Reference Book on Insulating Liquids and Gases” RBILG-391. 1993. 24.ANSI/IEEE C57.92-1981, Guide for Loading Mineral Oil Immersed Transformers. 25.Doble Engineering Company Client Conference Minutes 1998 Insulating Fluids No. 65PAIC98. 26.IEEE P1258, Trial-Use Guide for the Interpretation of Gases Generated in Silicone-Immersed Transformers. 1999. 27.ASTM D-1933-97. Standard Specification of Nitrogen Gas as an Electrical Insulating Material. 28.ASTM D-3487-88, Standard Specification for Mineral Insulating Oil Used in Electrical Apparatus. 29.ASTM D-5837-96, Standard Test Method for Furanic Compounds in Electrical Insulating Liquids by High Performance Liquid Chromatography. 30.ASTM F-36-99, Standard Test Method for Compressibility and Recovery of Gasket Materials. 31.ASTM D-2240-97, Standard Test Method for Rubber Property – Durometer Hardness.