Which of the following is NOT one of the five types of underground transmission?

High Pressure Air Filled (HPAF) is not recognized as one of the standard types of underground transmission systems. The commonly accepted types include High Pressure Fluid Filled (HPFF), which utilizes a fluid for insulation and cooling; Gas Insulated Lines (GIL), which use gas for insulation to reduce space requirements and improve safety; and Self Contained Fluid Filled (SCFF), which maintains a fluid within a pipe for optimal heat transfer and insulation.

While air insulation systems exist in other contexts, the HPAF does not have the same level of application or standardization in underground transmission systems. Therefore, identifying HPAF as not one of the five types highlights a nuance in the classification used for underground transmission technology.