I have seen Ring Main Units in various single line diagrams of electric substations.
Can you help me in understanding the role and types of RMU and its essentiality in power distribution.
Also I have seen in single line diagrams OLU and HFU units shown with RMU. What is this?
Can you please refer to me a site where I can find detail about it?

Do a Google search for "Ring Main" and you will find several sites that will explain the Ring Main system popular in Britain homes.

Do a Google search for "Ring Main Power Distribution" and you will find sites that explain how it is used in substations.

The required components are frequently available in an integrated package known as RMU