Large oil-filled electrical power equipment is a critical element in an electrical power system. Power transformer is utilized to raise or lower the voltage level as needed to serve the transmission or distributino networks. Through the use of device such as the transformer, AC electric power is generated at one central location. Its voltage stepped up for transmission over long distances at very low losses, and then stepped down for final use. Once it steps up the voltage level of a circuit, it decreases the current to keep a constant power balance. Since the transmission losses in transmission lines are proportional to the square of the current, raising the transmission voltage and reducing the resulting transmission current by a factor of 10, it reduces power transmission losses by a factor of 100. Power transformer plays the key role in preserving efficiency and in maintaining continuous transmission of power in the grids. Power transformers rate from few kilowatts to hundred megawatts, and substantially weight over hundreds of tons. The price of a single power transformer ranges from 50 thousand to five million of dollars. It is one of the most expensive asset in power transmission and distribution networks that is crucial to the profitability of electrical utilities company. Million of dollars are spent each in operating, maintaining and repairing this equipment. Without the transformer, it would be impossible for the modern society to use electric power in many of the ways it is used today.