subject: Know More About Rmu (ring Main Unit) [print this page] Ring Main Unit is proposed to be used with medium voltage range (11 KV-36 KV) with ring distribution method in any other power distribution or supply utilities. The RMU provides superior flexibility in the supply method in maintaining stability as well as provides nearly maintenance free operations. RMU is very compacted in size as compared with MV switchgear panel.
RMU stalls form any small size system for modular equipments for the Medium Voltage range 11KV-22KV in India), every module has precise functions. Each function is having individual metal enclosures that contain Epoxy or stainless steel tank, completely filled of SF6 gas through which switching devices as well as bus bar is kept. The complete RMU SF6 makes the equipment pollution challenging as well as capable of withstanding even flooding of the MV sub-station. SF6 gas is electro-negative, inert, and non-inflammable gas with many other chemical and physical advantages.
The gas tank of RMU is eternally sealed system in which integrity of seal is efficient for around 30 years with conformance of IEC-298. This tank is charged with the SF6 gas for pressure of the 0.03 Mpa that permits switch of breaking loads of equal to 2600 amps with 12KV/1300 Amps on 24KV (that under standard atmospheric conditions should be 630 amps with 12KV and that also without the SF6 gas).
RMU switching method contains 3 position load, 3 phases, and fault and breaking making, three positions are "open", "close", and "earth". The starting is associated with external earth switching and panel doors therefore with regard to offer safety interlocks that make sure that operator can only use panel while system is earthed and isolated.
The most frequent RMU versions are Extensible and Non-extensible type - 3 and Function with 4 Functions. A usual 3 functions RMU will comprise of the 2 x 630 Amps Loading Break Switches + 1 x 200/400/630 Amps Circuit Breaker (Vacuum type or SF-6). In the same way, the usual 4 Functioning RMU may comprise of the 2 x 630 Amps Load Break switches + 2 x 200/400/630 Amps Circuit-Breakers. Bus bar is skilled of carrying the load current having 630 Amps as well as short circuit faults current for 20 KA of 3 secs. This system of increasing bus bars matches to the IEC 298. This RMU usually conforms for following standards:
Circuit Breaker: IEC 56/BS 5311/IS 13118
Control and Monitoring: IEC 801
Degree for Protection: IEC 529
Earthing Switching: IEC 129/BS 1985
Load Breaking Switch: IEC 265/BS 3804
Load Breaking Sw. + Fuses: IEC 420/BS 16926
Metal enclosure switchgear: IEC 298/BS 5311/BS 5227
It is required to have the Fault Passage Indicators completely field programmable with minimum 16 settings of E/F as well as 4 settings of Phase - Phase. In addition should have the SCADA output contacts. It should have spring charged driving mechanism, which will be operated physically or determined by the motor system, which is optional to energize charging spring of opening and closing operations. RMU is compact and metal enclosed switchgear, which should meet compliant with the IEC 60298.
