RMU
A Ring Main Unit (RMU) is a totally sealed, gas-insulated compact switchgear unit. The primary switching devices can be either switch disconnectors or fused switch disconnectors or circuit breakers.
Different combinations of these primary switching devices within the unit are commonly used.
In case a circuit breaker is the switching device, it is also equipped with protective relaying, either with a very basic self-powered type or a more advanced one with communication capabilities.
The rated voltage and current ranges for RMUs typically reach up to 24 kV and 630 A respectively. With many of the manufacturers of RMUs, the basic construction of the unit remains the same for the whole of the voltage range.
The increase in rated voltage is handled by an increase in the insulating gas pressure.
The figure above shows a typical RMU configuration where load disconnectors are the switching devices for the incoming cable feeders and circuit breaker works as the switching device for distribution transformer feeder.
Three-position design // Closing, Opening and Earthing
All of the switching devices are of three-position design, having the possibility to close or open or earth the feeder in question.
Closing
Closing the moving contact assembly is manipulated by means of a fast-acting operating mechanism. Outside these manipulations, no energy is stored. For the circuit breaker and the fuse-switch combination, the opening mechanism is charged in the same movement as the closing of the contacts.
Opening
Opening of the switch is carried out using the same fast-acting mechanism, manipulated in the opposite direction. For the circuit breaker and fuse-switch combination, opening is actuated by:
- A pushbutton
- A fault.
Earthing
A specific operating shaft closes and opens the earthing contacts. The hole providing access to the shaft is blocked by a cover which can be opened if the switch or circuit breaker is open, and remains locked when it is closed.
The figure below shows typical outlook of a three-feeder RMU. In the figure, the combination consists of load disconnectors for the incoming two feeders and a fused load disconnector for the distribution transformer feeder. The incoming and outgoing medium-voltage cables are attached using elbow-type plug-in cable ends.
Whereas the RMU type of units represents the very compact gas-insulated design for a dedicated purpose, the secondary medium-voltage switchgears represent an air-insulated, quite freely extendable and configurable solution.
References:
- Distribution Automation Handbook // Elements of power distribution systems – ABB
- RM6 Ring main Unit catalogue – Schneider Electric
- MV RMU SafeRing catalogue – ABB
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Sir I have an ABB (RMU) 11kv and is on service but for some days now if we on it is no supplying voltage out meaning the output is not transmitting out at closure
is it possible to merge two RMUs to synchronize between the power source and generator ?
Hello, Please i want to make enquiry for the following items
1. 11kv Siemens Extensible Panel comprising 1no incomer 1no BUS and 2no Outgoing Feeders
2. 11kv Sf6/Vacuum Sabre 200A RMU Circuit breaker extension
3. 11kv Sf6/Vacuum Sabre 630A RMU Circuit breaker Isolator.
I’ll appreciate your swift response.
Regards
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SIR I HAVE ONE RMU UNIT SCHNEIDER RM6, WITH 3WAY, WE USED ONE INCOMER 630 A AND AN O/G FOR TRANSFORMER 2000KVA. NOW WE HAVE PLAN TO INSTALL ANOTHER TRANSFORMER NEAR TO THIS RMU. CAN WE USE THIS UNUSED 3RD FEEDER AS INCOMER FOR NEW RMU/TRANSFORMER UNIT. THE MAIN CABLE IS ENOUGH TO SUPPORT TWO 2000KVA TRANSFORMERS (3CX120SQMM UNARMORED CABLE, 13.8KV)
3way RMU /CCT2
C – EB Power supply main Incoming cable connected to your RMU OD-1
C – OD-2 this device power supply out going to Another one RMU OD-1 or OD-2
incomer.,
T2- VL-1 device power supply cable connected your Transformer..
OD-1&2 power supply only incomer & Outgoing to one RMU to Other RMU there is no Relay & CT Protection…
VL-1 – Relay & CT protection Required for transformer only available for this devices.
OD-1 & 2 this device is CT & Relay Protection not available,
so this device power supply don’t given directly to Transformer..
120 sqmm size is low for 2X2000 KVA transformer.
Can anyone explain why it’s necessary to earth the load disconnector, please? As far as I know it’s for safety. Thạnk you very much.
Is R.M.U a switch device or a control device?
This RMU switchgear is utilized for medium-voltage power distribution, with voltages ranging from 7200 to 36000 volts. Whereas, a vacuum circuit breaker is a type of circuit breaker that uses a vacuum medium to quench the arc.