Which test provides a better evaluation of changes that may occur in the oil from transformers and load tap changers?

The test that provides a better evaluation of changes that may occur in the oil from transformers and load tap changers is ASTM D1816. This test is specifically designed to measure the dielectric breakdown voltage of insulating oils, which is critical in assessing the condition of transformer oils and their ability to function safely under electrical stress.

As transformers operate, they can introduce contaminants and degrade the insulating properties of the oil. ASTM D1816 helps in determining the oil's quality and its effectiveness as an insulator, allowing for an evaluation of its performance over time. An increase in breakdown voltage is desirable, while a decrease can indicate potential problems.

Other tests, such as ASTM D971, focus on different properties like interfacial tension, which relates more to the presence of contaminants than the overall insulation quality. IEC 60247 involves testing for dielectric strength as well but is not as commonly referenced for transformer oils compared to ASTM D1816. IEEE 62 pertains to the specific evaluation of insulating liquids but does not provide as direct a method for assessing changes in transformer oil as ASTM D1816 does. Thus, ASTM D1816 is the most pertinent choice for evaluating the condition of oil in transformers and load tap changers.