Ring Main Unit in Electrical Distribution System
What is a Ring Main Unit?
In an electrical power distribution system, a ring main unit (RMU) is a factory assembled, metal enclosed set of switchgear at the load connection points of a ring-type distribution network. It contains different types of switches for different purposes for example some switches connect with load, some feed a distribution transformer etc.
The metal enclosed unit connects to the transformer either through a bus throat of standardized dimensions or through cables and is usually installed outdoors. Ring main cables enter and leave the cabinet. This type of switchgear is used for medium-voltage power distribution.
RMU is an extensible and non-extensible ring main unit for the secondary distribution network. RMU can be supplied in various configurations suitable for most switching applications in 24 kV distribution networks. RMU is a completely sealed system with a stainless-steel tank, gas tight metal enclosure, containing all the live parts, switching-disconnector, earth switch, fuse switch, the circuit breaker.
This blog describes the importance of Ring Main Unit in power distribution and its comparison with other types of switch gear and in which type of power distribution application using ring main unit can be cost effective.
Stay Sharp & Join our Mailing List!
Our surveys show that 90% of the people who join our blog have found the content valuable! Join today
Power Distribution System
Power system is classified into three main portions which include:
- Generation
- Transmission
- Distribution
The area between substation that are fed through transmission up till user meter are part of distribution system. It consists of:
Connection Schemes in Distribution Scheme
Connection schemes shows the arrangement of feeder, distributor, and service main.
According to connection scheme, distribution system can be classified into three main types:
- Radial System Schematic
- Ring Main System Schematic
- Interconnected System Schematic
Click here to read the detailed explanation of each type of distribution system.
Ring Main Unit
Connections in a Distribution Network
Ring main unit are used in ring main network for the purpose of connecting feeders to form a loop and monitors fault or any malfunction at feeders and distribution transformer end and make sure that power is provided to the utility without any disturbance and with least possible voltage drop.
Ring Main Unit Design
Features
Some features of RMU switch gears are as follows:
- Small in size and compact insulation filled sealed switch gear.
- Used for 11KV/33KV systems. Normally rated for 630 A and 21KA interrupting rating.
- Comprises of three types of insulation: oil, air and SF6 gas. SF6 gas insulation is widely used.
- Normally has 2 incoming and one outgoing and vice versa.
- Have separate protective and measuring devices.
- Can be used in indoor and outdoor
- Reduce hazards related to fire
Modular Compartmentalized Design
Each RMU consist of at least 3 section/ modular compartment
- Incoming feeder
- Outgoing feeder
- Bus coupler
Depending on type of distribution scheme, section can also increase in odd number form like 5, 7, 9 and even more in each unit. A 5 section RMU may consist of two incoming and outgoing feeder each and a bus coupler. Similarly, a 7 section RMU may consist of three incoming and outgoing feeder each and a bus coupler and so on.
Construction
- Cable Test Compartment
- Gas Tank
- Interlocking Latch for Cable Test Compartment
- Padlocking Facilities for Load Break Switch
- Load Break Switch Operating Mechanism
- Padlocking Facilities for Earthing Switch
- Vacuum Circuit Breaker
- Ring Switch Cable Compartment
- MV Cable Fixing Support
- Tee-Off Switch Cable Compartment
- Voltage Presence Indicator
- Switch-disconnector Operation (Motor Optional*)
- Operation Counter
- Circuit Breaker Operation (Motor Optional*)
- Operating Latch for Vacuum Circuit Breaker
- Protection Relay
- Lifting Lugs
- Padlocking Facilities for Vacuum Circuit Breaker
- Vacuum Circuit Breaker Operating Mechanism
- Gas Pressure Indicator
Components
Following are the main components used in ring main unit:
Load Break Switch
Load break switch or isolators are used for breaking contact of any feeder isolated due to the reason of fault or maintenance purpose with the load. There is an earth protection switch also available which is used when the feeder is de-energize due to fault condition. Standard followed for load break switch ‘IEC 265/BS 3804’.
Circuit Breaker
Circuit breaker can be used:
- Either for switching
- Or if used as medium voltage protection then it will be issued with trip signal using relays.
Standard for circuit breaker involves ‘IEC 56/BS 5311/IS 13118’.
👉🏼 We also have written a blog titled Selecting the Right Circuit Breaker and Its Type. Have a look at it to grab more information about circuit breakers.
CT/PT
For the purpose of monitoring current and voltages, current and potential transformer are used. CTs can be used for giving the input of current to relays which then senses a normal or fault condition according to the current.
Numerical Relays
Relays used for over current, under and over voltage protection.
Cable Testing Compartment
Ring main units offer a safe way of cable testing from the easily accessible cable testing compartment on the front panel. Operators can simply conduct the cable testing through plug-in type connectors after opening the cover. The cable test cover is safely interlocked and cannot be opened unless the load break switch is earthed.
Gas Pressure Indicator
Gas density is an important operating parameter for SF6 insulated MV equipment. If the required gas density is not sufficient, safe operation cannot be guaranteed. On ring main units, a gas pressure indicator is fitted to the tank for providing a reliable warning indication against low gas levels. The gas pressure indicator has sections to show the minimum pressure for safe operation.
Voltage Presence Indication System
All ring main units are integrated with a voltage presence indication system. A voltage signal comes from the VPIS through the voltage divider positioned into the cable entrance of bushings. The VPIS can be used to check whether a voltage is present across the cables.
SCADA System
SCADA systems are used for the trip settings of fault current, over loading and time current curves of the protective devices. There is a connection available for C.T which is used for monitoring purpose. Also there is a communication cable available.
Standard followed for monitoring and control system involves ‘IEC 801’
👉🏼 We previously wrote a blog about SCADA and Its Application in Electrical Power Systems. If this finds your interest, check it out and let us know what you think.
RMU Operation in case of a Fault
For example: if a fault occurs at feeder 1 side then first of all the relay will issue a trip signal at load side circuit breaker to open first and then both isolators disconnect. Now for energizing the load again isolator 2 first connects and then breaker is closed.
The earth switch is available for the protection of human from getting electrical shock while maintenance.
Advantages and Disadvantages of RMU
Advantages of RMU
- Cost and Time Saving
- Less Maintenance Requirement
- Small Size and Compact Design
- Safe and Reliable Operation
- Smart Skills
Click here to read the details of each advantage of RMU.
Disadvantages of RMU
- Have high capital cost.
- It is less robust as compared to other switch gears. It may have an operation cycle of 2000 which is less as compared to other switch gear.
- Can fail due to leakage of SF6 gas and with moisture entry.
Application in Industry
As ring main unit are very high in cost due to the use of smart monitoring and SF6 circuit breaker and reliable protective device. They can be cost effective for the application where load requires a reliable and continuous power supply which may include:
- Industries
- Solar panel
- Wind turbine
- Airports, hospital and metro etc.
Typical Position of RMU in Network
Above figure explains the location of RMU in a Power system. It can be distributed in 3 levels as shown above. Here the core setting is that every load is fed by 2 sources. In case one source fails other one is connected to load with the help of RMU.
Difference between RMU and Conventional VCB Switch Gear
Conclusively, RMUs are being widely used all around the world so as to proficiently encounter the increasing requirements of reliable energy. We hope that this blog was helpful and gave you valuable information on the topic. Feel free to suggest or ask any questions you might have in the comments section below. Thank you.
