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.
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:
Feeder | Distributor | Service main |
---|---|---|
Conductor that connects the substation with distributor. No tapping take place within a feeder. | Conductor through which tapping take place in order to provide supply to consumer. | Cable connecting distributor and consumer utility. |
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:
Characteristics | Radial system | Ring Main System | Inter-connected system |
---|---|---|---|
Substation quantity | A single substation feeds each feeder | Consist of single substation | Consist of more than one substation |
Feeder connection | No connection of any feeder with its adjacent feeder | Feeders are connected in form of loop | Feeder forms a ring |
Supply to distributor | Each feeder is connected with a single distributor | Due to loop formation each distributor will be fed by 2 feeders | Ring formation and supply from more than 2 substation leads in an uninterrupted power supply |
Radial System Schematic
The given figure shows a radial system. Here OA and AC represents feeder and AB represents distributor. It could be easily seen if a fault on OA and AC feeder is not supplied with any substation there would be no supply at AB.
Ring Main System Schematic
The figure shows a ring main network. Here SRQPONML are connected to form a loop and taped from Q, O and M. The figure shows that a fault at LM will not affect supply at point M distributor, it will still have its supply from OM feeder. Also, it can be easily seen if the substation gets faulty, all feeders get out of supply.
Interconnected System Schematic
The given figure shows an interconnected system. There are two substations available S1 and S2, the feeders are connected to form loop QARBOCPD and four distributors at point R, O, P and Q. It is clear from the figure that if any of a substation gets faulty or cut off for the purpose of maintenance for example: if S1 cuts off form system, the distributor can still have the supply from S2.
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’.
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’
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
Easy installation of RMU leads in saving commissioning and installation time and cost.
Less Maintenance Requirement
RMU are less effected to climatic change and can be easily installed indoor and outdoor irrespective of environmental variation which makes it a maintenance free system.
Small Size and Compact Design
Due to its compact design and small size, RMU are easy to install and operate at any place with no trouble.
Safe and Reliable Operation
With enhancement in design and use of gas insulated system, RMU ensures an easy to understand by operators, safe, efficient and reliable operation and uninterrupted supply at load end all time.
Smart Skills
with the use of smart electronic circuits and communication abilities, it can be easily integrated with distribution management system and in building a smart grid solution.
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
Difference between RMU and Conventional VCB Switch Gear
Characteristics | RMU | Conventional VCB Switch Gear |
Arc Quenching Technology | SF6 gas | Vacuum bottle |
Maintenance Requirements | Less | more |
Space Requirements | 50% less as compare to VCB switch gear | 50% more as compare to RMU. |
Costs | Due to compact design and less civil work, shipment, installation and maintenance cost becomes less. | Trolley size and cable length addition increases its shipment, installation and maintenance cost |
Switching and operational life cycle | Due to less switching cycle (2,000 – 3,000), its operational life becomes less. | Due to high switching cycle (>10,000), its operational life becomes more. |
Future modification | Can be done under supervision of professional and skilled labors | Can be done by local facility electrician and workers |
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.