IEEE C57.121-1998 pdf download IEEE Guide for Acceptance and Maintenance of Less Flammable Hydrocarbon Fluid in Transformers
Care must be taken when filling the test cell with LFH fluids to guard against trapping air bubbles, which may result in misleading, low breakdown voltages.
The following two methods are recognized for measuring the power frequency dielectric breakdown voltage of an oil:
— Method D877 is recommended for the routine acceptance of new fluid as received in bulk tankers from a vendor. Method D877 uses flat-faced cylindrical electrodes with a 0.1 inch gap.
— Method D1816 prescribes the use of spherically capped VDE electrodes (as described in Method IEC 60156). It is sensitive to small amounts of contaminants. This method is not recommended for the acceptance testing of new, unreprocessed fluids as received in bulk tankers from a vendor. Method D1816 is the preferred method for use with bulk fluid after processing by the transformer manufacturer. It is also recommended for testing new fluid in new equipment, particularly for high- voltage apparatus. Its significance for evaluating service-aged oils is being determined. While the suitability of Method D1816 has not been determined for oils having viscosities of more than 1.9 × 10 –5 m 2 /s at 40 ° C, several manufacturers and users have reported satisfactory experiences using 2 mm (0.08 in) electrode spacing.
4.3 Dielectric breakdown voltage (impulse conditions) (D3300) Insulating fluids used in high-voltage apparatus are subjected to transient voltage stresses arising from such causes as nearby lightning strikes and high-voltage switching operations as well as being subjected to steady-state voltage stresses associated with continuous operation of the apparatus at commercial power fre- quencies. The ability of the insulating fluid to withstand such transient voltage stresses has become increas- ingly important to determine, as the use of oil-filled apparatus has been extended to operating voltages of 345 kV and higher (extra-high and ultra-high voltage). Breakdown voltage is of primary importance to the designers of such equipment. Its importance to other types of apparatus has yet to be determined.
Transient voltages caused by lightning and switching surge phenomena may be either negative or positive in polarity. Although polarity of the voltage wave has little or no effect on the breakdown strength of an oil in uniform fields, polarity does have a marked effect on the breakdown voltage of an oil in nonuniform electric fields.
The transient voltages already referred to may also vary over a wide range in both the time to reach crest value and the time to decay to half crest or to zero magnitude. The IEEE standard impulse test specifies a 1.2-by-50-µs negative polarity wave. The standard wave shape for switching surge tests on transformers is 100 µs to crest and greater than 1000 µs to zero.