A substation that utilizes the insulation medium in the form of open air gets a lot of threats from lightning strikes that inject impulse voltage so that it runs on the line. Equipment installed in the switchyard is usually designed and built to have a basic insulation level that can withstand the threat of lightning surges, with the exception of power transformers which must be paired with protection devices in the form of lightning arresters. In this research, a study of the rating of the insulation coordination of transformer protection devices is carried out. Then the Bewley Lattice Diagram method will be implemented to obtain comprehensive information on lightning surge waves traveling on the line. The Bewley Lattice Diagram method can represent the price, position, and direction of motion of impulse waves that experience reflection and refraction events. With regard to the research results, it is known that the installed protection device provides a margin protection for the transformer of 15.38 %. The impulse voltage traveling on the line is influenced by the inductance constant (L) and capacitance (C) of the conductor characteristics, so that the reflected and refracted residual voltage prices are also affected. The presence of lightning arresters causes the reflected and destroyed impulse voltage to reach the maximum voltage price of the system with a time of only 0.65 µs, 3 times faster than in the absence of an arrester which takes 2.09 µs. With this research, it is known that the transformer protection device installed is in accordance with the needs of the system and the characteristics of the lightning impulse wave traveling on the line will be reflected and forwarded repeatedly until it reaches the system voltage price
