HV: high voltage, Generally, it is above 110kV, such as 110kV, 220kV, 500kV, 750kV, etc
MV: medium voltage, Generally refers to the voltage range between 3kV and 100kV, such as 3kV, 6kV, 10kV, 20kV, etc
LV: low voltage, It generally refers to below 3kV, and for distribution, it generally refers to 400V, which is commonly referred to as 380V
The function of these substations is to convert AC voltage into appropriate voltage levels. HV is for long-distance transmission, MV is for close range distribution, and LV is for power supply to users. MV/MV is an AC voltage conversion with equivalent or identical voltage levels, which can serve as electrical isolation or can be converted into multiple outputs.