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
Related electrical guides & articles
Is there any difference between an RMU and a compact switchgear unit ?
i just want to know how much to refill a SF6 gas on the RMU.
I always enjoy articles from EEP. please, I wish to get daily updates from this portal. Many thanks.
Good day,
Please is it possible to interface an RMU with SCADA?
if yes is it in operatin anywhere
Yes, we can Interface an RMU with SCADA by using any Make of FRTU.Also we can operate from Any where by Connecting 2/3/4G Modem.
i want buy RMU , who can help em . thank you and best regards.
Hello there,
I am looking forward to do a revamp for various of older version of RMUs. Really seek an expertise from you that can do a modification on the RMUs from mechanical to a scada ready that can be monitored the status, alarm and measurement to send to RTU and Scada.
Some of the RMU brands are from:
– ABB
-Lucy
-Merlin Gerin
-Etc
Highly appreciate your response. Kindly please advise.
For further clarification, please do feel free to contact me.
Thank you.
Looking forward.
Best regards,
Hasnizam
Sales Engineer
Progressive Impact Technology Sdn. Bhd. (Malaysia)
The concept of the article about RMU is perfect since it gives clear understanding on how you can benefit with RMU in ring distribution network
I didn’t like the way the article is presented.No clear data on advantages,applications and differentiation w.r.t other insulation equipment’s like VCB,SF6 etc
I am really interested in total RMU unit Schematic line diagram to understand , as well as RMU v/S AMU I am interested in understanding all different relay protection s for RMU. It’s operation & site testing & Commissioning of unit etc.
Ring main Unit is very much in use in middle east; in fact for U/ground distribution network it is safe and effeicent electrical equipment and catagorized of its’ number of Incomer switches and Tee Off; like : 2+1, 2+2, 3+1, 3+2, etc.
I recently quote a RM6. It is expensive. 40K! But in the other hand they are reliable and save tons of money in case of shor circuit failure..
In our cabling project , there are two switching stations proposed , my doubt is , instead of switching station ,any other alternate way is there to lay cable . whether Switching staion can be replaced by RMU.
Can an RMU be used on the incomer side of 11KV transformer instead of only circuit breaker?
Yes offcourse..But generally maximum current rating of R.M.U is upto 630 amps. Only where as panels available with higher rating also. Apart from this most of the incomer panel designed with line P.T…But RMU’s built with only live line indicators, for P.T it requires some sort of extension units..
Can a RMU made of three LBS ? If so what are the advantages?
Please I need help. Can anyone email me a PDF of a primary and secondary sub
stations that explains the it’s constituents .thank You