As you might understand before, RMU or ring main unit is a gas-insulated, sealed compact switchgear unit.
The main switching devices could be either:
- fused switch disconnector
- switch disconnector
Ring Main Units is utilized in a secondary distribution system.
It’s utilized basically for an uninterrupted power supply.
It also safeguards your secondary side transformer from the recurrent transient currents.
Depending on your loading conditions and applications, you can utilize a:
- a circuit breaker and;
- switch fuse combination to safeguard the transformer.
That transformer linked to the circuit breaker or switch fuse is referred to as Tee off.
In a typical arrangement, you got load break switches on both sides of your Tee off.
RMUs are composed of incomer feeder and outgoing feeder.
The incomer feeder is either SF6 or vacuum circuit breaker with associated earth switch and disconnect switch.
You see;
The incomer feeder is provided with the delivery protruding from the enclosure of RMU for supply cable connection.
On the other hand;
The outgoing feeder is composed of load break switches along with:
- Earth switch and
- Disconnect switch
Provision for cable connection is given in outgoing feeder.
Aside from that:
The load break switch is only an isolator that can break the circuit through on load situation.
The incomer and outgoing feeders are covered in an SF6 setting to make the entire design compact.
Its compatible instrument for tracking of SF6 gas pressure is installed on the RMU.
The scale of this device is categorized into three or more subscales for various temperatures.
What’s more, every subscale has a red and green colored zone.
For a specific temperature, you must refer the necessary subscale and gas pressure.
That’s because it should be in the green colored zone.
Mechanical indicators are presented on the panel for the status of:
- earth switch
- disconnect switch and;
- panel for breaker
Current transformer of the desired ratio is set up in the incomer feeder.
The secondary of that is linked to a self-powered relay for needed protection.
You see;
A self-powered connection device is also installed on the RMU.
That relay is microprocessor-based numerical rely on upon along with user interface.
That relay offers limited protection like:
- earth fault protection and;
- overcurrent protection
Even though the various manufacturer provides a various set of protection, some manufacturer might provide protection according to the specification of the user.
Within the distribution system, Ring Main Unit is widely utilized.
That’s because it is a total package.
It only requires a cable connection and installation.
Everything else is in the package.
That massively lowers the commissioning period.
RMUs are utilized in underground cable distribution network at the:
- junction points and;
- load connection points of a ring-type distribution network.
Furthermore,
RMUs are also ideal for use in the following applications:
- small industries
- solar power plants
- wind power plants
- compact secondary substations
- airports, hospitals, business centers, shopping centers, residential housing complex, office building and hotels
There’s no doubt, the ring main unit is the only solution that is filled with wide-ranging capabilities.