Ring Main Unit - RMU

We will discuss about the gas-insulated and air-insulated medium voltage Ring Main Units commonly known as the RMUs. RMUs are the metal enclosed switchgears which are used for medium voltage secondary distribution in the power system. In this article, we will try to develop understanding about the RMU design.

Ring Main Units are commonly used for power distribution in the residential complexes, in the small industrial plants, distribution of power for high rise buildings and utility network secondary distribution. IEC 62271-1 and IEC 62271-200 define the requirements for metal-enclosed high-voltage switchgear and control gear for rated voltage above 1kV and up to 52kV. Available rated voltages for RMUs are 12kV, 17.5kV, 24kV and 36kV.

Most RMUs are designed with rated currents from 400A to 1250A. Rated short-circuit currents are up to 25kA either 1-second or 3-seconds. Protection against internal arc faults is mostly available in the range of 12.5kA till 25kA for 1-second in both AFL and AFLR. Please verify your required ratings from manufacturer as these rating changes with RMU designs.

RMU designs are available for both indoor and outdoor type installations. Some designs without any additional housing are available for indoor and outdoor installations. Some designs are made for indoor installation, however can be installed outdoors with additional outdoor type housing as shown in the picture. Both AIS and GIS RMUs are designed free floor standing type and can be installed in the switchboard room as per manufacturer recommendations.

To develop further understanding about the RMU design now we go through the different compartments which form the RMU. We first take the example of GIS, first is the steel tank filled with SF6 gas and containing the switching device like circuit breaker or switch disconnector and the busbars. Second is the cable compartment where the incoming and outgoing power cables are connected. Cable compartment also contain components such as the current transformer for measurement purpose. Third is the user interface containing the measurement and protection devices. Fourth is the manual or motorized operating mechanism for the switching devices. Finally is the earthing circuit.

Similarly, for the AIS, the first component is the switching device like circuit breaker which can be available in both SF6 type and the vacuum type. Second is the cable compartment for the connection of incoming and outgoing power cables. Third is the user interface containing the measurements and protection devices. Fourth & fifth are the operating mechanism and disconnector switch along with earth connection. Sixth is the main bus bar for connection between different modular switchgears.

All compartments are segregated using the metallic partitions. If the partitions are metallic then the switchgear is classified as Partition Metallic or PM class under the IEC 62271-200.

Gas insulated RMUs are available in two designs, first design is called the compact design which is non-modular meaning that all the power circuits are enclosed in a single SF6 gas tank. The other design is the modular design, which is also called extensible design. In the modular design power circuits are enclosed in their own SF6 gas tanks.

The advantage of modular design is the increased reliability in case of fault as all the power circuits are sealed in tanks which are isolated from each other. The other advantage is the flexibility for future extension. In the modular design all the individual power circuits are coupled together. Therefore, in case of any required change or any required addition in the power circuit one needs to decouple the modules and again couple the new circuit as per requirement.

The modular RMU are available is three different configurations, first configurations allow extension at the right hand side, second available configuration allows extension at the left hand side and the third configuration allows extension at both right hand and left hand sides.

Extensible RMU are also available with top connected busbars as shown in the picture. However, it should be kept in mind that with top connection the height of the RMU will increase as compared to the side connection.

Mostly RMUs, either AIS or GIS, are installed with bottom cable entry as shown here in the pictures. However, if the top cable entry is required due to some installation constraint at site, top entry can easily be available mostly with AIS designs.

In the case of the GIS the incoming power cables from the field are terminated at the bushings present in the cable chamber of the RMU. These bushings form the link between the power circuit inside the SF6 tank and the power cable coming from the field. It is very important to follow manufacturer’s recommendation for the termination of power cables.

To enhance the safety of the people working in close vicinity of the installation RMU designs are available with the internal arc compatibility both AFL and AFLR.

AFL design provides safety to the people at front and lateral sides of the RMU. The AFLR design provides protection at all sides, that is front, lateral and rear sides of the RMU. The evacuation route for the gases is shown here for GIS with and without the evacuation duct option. Especially for the AIS design a duct can be installed on the top of the RMU to direct the hot gases outside the switchgear room. Downwards exhaust can also be made through trench as shown in the picture.

It is important to note that the selection of exhaust method is also dependent on the rated internal arc value and should be discussed with manufacturer during the design stage.