Insulating Oil for Load Tap Changer (LTC)
General
See IEEE Std C57.131. The requirements of insulating oils used in load tap changers (LTCs) are comparable with those of insulating oils used in power transformers. For LTCs where oil is used for arc-quenching, the arcing at the arcing switch or arcing tap switch contacts causes contact erosion and carbonization of the arcing switch oil. The degree of contamination depends on the operating current and step-voltage of the LTC, the number of operations, and to some degree, on the quality of the insulating oil (not relevant in the case of LTCs with vacuum interrupters).
If the LTC is a "free-breathing" design, the desiccant should be checked periodically to prevent the admittance of humid air.
Maintenance and inspection intervals depend on the type of LTC, the LTC rated through-current, the field experience, and the individual operating conditions. They are suggested as periodic measures with respect to a certain number of operations or after a certain operating time, whichever comes first. The recommended maintenance intervals for an individual LTC type are given in the operating and inspection manuals available for each LTC type. In general, at inspections the oil will be reconditioned or changed.
Testing methods
Testing methods for LTC oils are the same as those used for insulating oils of power transformers. Samples are taken in the same manner and at the same intervals as for insulating oils used in power transformers.
New oil properties - as supplied
New mineral insulating oils as received from the supplier must conform to certain qualifying functional requirements listed in ASTM D3487.
Test limits - shipment of new mineral insulating oil
When mineral insulating oil meeting the qualifications of ASTM D3487 is received in the field for installation in LTCs, it should be checked for certain key values that may be affected by shipment and storage (Table 1). Viscosity [12 cSt (12 mm2/s) maximum at 40 °C] and pour point (-40 °C, maximum) should be checked in cold climates (below -25 °C and above -40 °C) to ensure that the oil does not interfere with the free operation of the equipment.
New oil properties - oil shipped in new equipment
LTCs are normally shipped from the factory without oil. If an LTC is filled with oil prior to shipping, refer to Table 9.
New oil properties - prior to energizing
When mineral insulating oil is received in bulk shipping containers, it is processed according to the instructions of the transformer manufacturer and then introduced into the equipment. Table 9 gives test limits for LTC mineral insulating oil after being processed, placed in equipment, and allowing for standing time before energizing.
Service-aged oil properties
Suggested limits for continued use of service-aged LTC oils are shown in Table 10.
Reconditioning
If the dielectric strength of the oil drops below the values given in Table 10, or the water content exceeds the values given in Table 10, the oil should be reconditioned or changed.
