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.

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

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

1. Incoming feeder
2. Outgoing feeder
3. 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

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

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.

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  About The Author

  Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.