Insulating Oil for Circuit Breaker
General
The requirements of insulating oil used in circuit breakers, as distinguished from insulating oil used in transformers, are uniquely different. Modern oil circuit breakers require low viscosity and low pour-point oil because a large percentage of them are used outdoors and, in many cases, at low temperatures. It should be noted that all circuit breakers are "free breathing" (open to the atmosphere through a breathing device). This does not prevent the admittance of humid air to the device. In the case of older oil circuit breakers where the use of higher viscosity oil is deemed necessary, caution and judgment must be exercised. The resultant effects of oil mixing and the addition of oxidation inhibitors upon thermal characteristics must be considered.
Reconditioning of circuit breaker oil is conducted in the same manner as transformer oil. The presence of dissolved heat and arc by-products is a significant problem in circuit breakers. In most transformers these products should not be present. Oil circuit breakers are usually designed with higher dielectric margins and thermal capacities compared with a transformer. In oil circuit breakers, by-products are created when the circuit breaker operates. These by-products, combining with moisture and oxygen in the breaker oil, will contribute to reduced dielectric strength in the circuit breaker. The accumulation of these substances results in the need for cleaning the breaker and reconditioning of the oil. For guidance in the application of circuit breakers, see IEEE Std C37.010.
Testing
Testing methods for circuit breaker oils are the same as those used for insulating oils. Samples are taken in the same manner as for insulating oil, at specified intervals, varying from a few months to annually. Some of the established ASTM tests applicable to mineral insulating oils are more significant to transformers than to circuit breakers, because a circuit breaker is essentially a free-breathing device that operates at ambient temperature.
New oil properties - as refined
New mineral insulating oils as received from the supplier must conform to certain qualifying functional requirements listed in ASTM D3487.
Test limits - shipments of new mineral insulating oil
When mineral insulating oil meeting the property requirements of ASTM D3487 is received in the field for installation in circuit breakers, it should be checked for certain key values that may be affected by shipment and storage. These values are shown in Table 6. Some users may wish to perform additional tests outlined in ASTM D3487; however, as a minimum, the tests listed in Table 6 should be performed.
Viscosity [12 cSt (12 mm2/s) maximum at 40 °C - see ASTM D445] and pour point (-40 °C, maximum - see ASTM D97) should be checked in cold climates to ensure that the oil does not interfere with the free operation of the equipment.
New oil properties - oil shipped in new equipment
Oil circuit breakers are normally shipped from the factory without oil in the tank(s). New oil is processed on-site, and the equipment is filled with the oil. In cases such as these, refer to Table 7.
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 manufacturer and then introduced into the equipment. Table 7 gives test limits for mineral circuit breaker insulating oil after being processed, placed in equipment, and allowing for standing times before energizing.
Service-aged oil properties
Suggested limits for continued use of service-aged circuit breaker oils are shown in Table 8.
If additional tests are desired, they should be made in accordance with the guidelines given for insulating oil testing earlier in this guide. Some users of circuit breakers recondition or change their oils without testing them after a preset number of operations.
Reconditioning
The chief problem in circuit breaker oil maintenance is to keep the fluid free of water, arc decomposition products, and other contaminants. If visual examination shows the presence of these materials and the dielectric strength of the oil drops below an acceptable value, the oil can be reconditioned, provided it is free of chemical contaminants. The accepted means of reconditioning is by using blotter papers or paper cartridge filters. This subject is discussed in detail in IEEE Std 637.