Ring Main Unit,RMU is a compact, enclosed and sealed type of Switchgear used for medium voltage power distribution. It is a complete Switchgear in itself. A complete Switchgear means, assembly of required switching devices, protection device as well as metering device. RMU of different voltage (mostly 12 kV and 24 kV) and current ratings are available. Indoor as well as outdoor types of Ring Main Units are available.
Ring Main Unit comprises of one incomer feeder and one or more outgoing feeder. The incomer feeder is either Vacuum Circuit Breaker or SF6 Circuit Breaker with associated Disconnect Switch and Earth Switch. This incomer feeder is given with the provision of bushing protruding from the enclosure of RMU for supply cable connection. A typical Ring Main Unit is shown in figure below.
Outgoing feeder comprises of Load Break Switches with associated Disconnect Switch and Earth Switch. Provision for cable connection is provided in outgoing feeder. The Load Break Switch is nothing but an isolator having the capability of breaking the circuit during on load condition. A simplified schematic diagram of RMU is shown in figure below.
The incomer as well as outgoing feeders are enclosed in an SF6 environment as to make the design compact. Compatible instrument for monitoring of SF6 gas pressure is mounted on the Ring Main Unit, RMU to monitor the gas pressure. The scale of this instrument is divided into three or more sub scales for different temperature. Further, each sub scale is having green and red colored zone. For a particular temperature, one should refer the appropriate sub scale and the gas pressure should be within the green colored zone. Mechanical indicators are provided on the panel for Breaker, Disconnect Switch and Earth Switch status.
Current Transformer of suitable ratio (as demanded by the user) is installed in the incomer feeder. The secondary of this CT is connected to a self-powered relay for necessary protection.
A self-powered protection device is also mounted on the Ring Main Unit, RMU. This relay is microprossor based Numerical Relay with user interface (different manufacturer have different design). This relay provides a limited protection such as Over Current and Earth Fault protection. Though different manufacturer offer different set of protection or manufacturer may offer protection based on the user’s specification.
In distribution system, RMU is widely used as it is a complete package and only needs installation and cable connection. Everything else is within the package. This greatly reduces the commissioning time.