Ring Main Unit (RMU)
The RMU is intended to be used in medium voltage (11KV to 36KV) Ring distribution system in any power supply/distribution utilities. RMU offers greater flexibility in supply system in maintaining continuity and offers practically maintenance free operation. The RMUs are very compact in size compared to MV switchgear panels.
RMU cubicles form a system of small size modular equipment for Medium Voltage (11KV & 22KV for India ), each module having a specific function. Every function has its own metal enclosure, which contains a stainless steel/Epoxy tank filled with SF6 gas, in which the switching devices and busbar are kept. Full SF6 makes equipment pollution resistant and able to withstand even the flooding of MV sub-station. SF6 gas being inert, electro-negative and non-inflamable gas has many physical & chemical advantages.
The RMU gas tank is a permanently sealed system where the integrity of the seal is effective for 30 years in conformance with IEC-298. The tank charged with SF6 gas to the pressure of 0.03 Mpa which allows switch to break loads of upto 2600 amps at 12KV or 1300 Amps at 24KV (which under normal atmospheric conditions 630 amps at 12KV, that is, without SF6 gas).
The RMU switching system consists of 3 phase, 3 position load breaking & fault making, the three positions being "close", "open", and "earth". The actuating is linked with the external earth switch as well as the panel door so as to provide safety interlocks which ensures that the operator may only access the panel when the system is isolated and earthed.
The most common versions of RMU are Extensible & Non-extensible types - 3 Function & 4 Functions. A typical 3 function RMU shall comprise of 2 x 630 Amps Load Break Switch + 1 x 200/400/630 Amps Circuit Breaker (SF-6 or Vacuum type). Similarly, a typical 4 Function RMU will comprise of 2 x 630 Amps Load Break switch + 2 x 200/400/630 Amps Circuit-Breaker. The busbar is capable of carrying a load current of 630 Amps and the short circuit fault current of 20 KA for 3 secs. The system of mounting the bus bars conform to IEC 298. The RMU generally conforms to the following standards
Load Break Switch - IEC 265/BS 3804
Load Break Sw. + Fuses - IEC 420/BS 16926
Circuit Breaker - IEC 56/BS 5311/IS 13118
Earthing Switch - IEC 129/BS 1985
Monitoring & Control - IEC 801
Degree of Protection - IEC 529
Metal enclosed switchgear - IEC 298/BS 5311/BS 5227
The standard feature to have voltage indicator lamps for the cables (capacitive dividers shall provide low voltage power to the lamps) and the Circuit Breaker provided with self powered O/C + E/F relays. Electronic Fault Passage Indicator for each Isolator Switches may be provided as Optional feature. It is desirable to have Fault Passage Indicator fully field programmable having atleast 16 settings for E/F and 4 settings for Phase - Phase. Also should have a SCADA output contact. The spring charged drive mechanism can be operated manually or driven by motor system (optional) to energise the charging spring for closing & opening operations. The RMU as compact, metal enclosed indoor switchgear shall meet in accordance with IEC 60298.