Ring Main Distribution with RMU:

Nowadays Ring Main units are widely used in Metro cities and smart cities for providing reliable supply to the consumers. It is essential for Electrical Engineers to know the working of Ring Main Networks and Ring Main Units.

A Ring Main Network is formed by connecting feeders in a fashion to form a loop, and is fed at one or several points.

Objective of Ring Main Distribution Network:

The main aim of this ring main network is to provide consumers uninterrupted supply. This is possible by feeding consumers with another healthy feeder in case of loss of supply due to working feeder. If a fault occurs in any branch of sub transmission circuit, that branch is removed from service and power continues from the remaining loop without discontinuity in the supply. Every consumer has redundant supply in Ring main distribution system.

Operation of Ring Main Network:

The figure shows the circuit diagram of ring main network used for distribution (Secondary) of power to houses and small industries.

In the figure shown above the ring main network is feeding with two incoming
supplies, via two incoming circuit breakers and a Normally Open bus-section in the primary Substation.

Each distribution Substation supplied with two or more Incomers (In case more than one primary substation feeding ring main). The ring is open at one of the distribution substation with one switch is in Normally Open condition. Each feeder in the ring main normally runs as radial feeder.

The fault is likely to occur in two sides.

1. Fault is at Primary Substation side:

If the fault occurs on any incomer circuit that respective Circuit breaker would open and Bus coupler closed. Hence power is continued with another incomer.

2. Fault is at Distribution Substation side:

If the fault occurs on any part of the interconnecting cable the Load Break Switch on either side of the faulty cable opened and NO switch of the ring main is closed to continue power supply to all consumers.

Now the repairs to the faulty cable can be carried out safely with one side isolated and in earthed condition.

Let’s see about Ring Main Unit.

What is Ring Main Unit [RMU]?

Ring Main Unit is a compact Gas Insulated Switchgear cubicle equipped with SF6 Switch disconnector and SF6 circuit breakers. The below figure shows the RMU with 3-Incomers and 1-Outgoing feeder to distribution transformer.

Ring Main Units are the major part of Ring Main Network. The power is distributed through these RMUs for the end consumers. Ring Main Unit distribution is costly compared to other distribution networks, so this type of distribution can find in urban cities where supply reliability is major issue.

Ring main units are available in different voltage ratings ranging from 11KV to 33KV. These units allow the connection of transformers through Under Ground cables and also provide protection to the transformers.

Advantages of Ring Main Units:

• The major advantage of Ring Main Units is the safety they provide to the operators. Like the operation of switching devices with interlocking system requires less knowledge and effort.
• Working with IEDs allows remote operation. SCADA implementation is easy with smart Ring main units.
• The space occupied by RMUs is less as they are Gas Insulated Switchgear.
• The time taken for installation and commissioning of RMUs is very less. RMUs require less maintenance.

Monitoring of RMUs:

All RMUs are connected through SCADA system using Remote Terminal Unit [RTU] and Modem for providing communication.

RTU: The RTU consists of Analog and Digital cards for converting Analog signals to Digital and Vice versa. RTUs receive and send information about RMU devices information. So that all the information about RMU devices display in HMI system which is located at Data centre. The commands sending from control centre are received by RTU and respective action takes place without help of any operator.

Modem: Modem is used for communication purpose. It contains a SIM card with a unique device address. Modem sends and receives information using Antenna. Modem communication works just like our mobile phone network.

Major Components of RMU:

1. Isolators with earthing switches [Load Break Switches]

Incoming cables are connected to these Isolators. These are load breaking spring assisted ring switches, each with integral load breaking earth switches. Each ring switches having three positions ON, OFF and EARTH as shown in figure below.

2. Circuit breaker (CB) with earthing switches

Outgoing cable is connected to the transformer through circuit breaker. Circuit Breaker also operated in three positions ON, OFF and EARTH.

3. Capacitor voltage dividers serving live-line cable indicators

These are used for for verification of safe isolation from supply.

4. Fault Passage Indicator -FPI

FPIs are used to detect faulty cable. Fault indication on the basis of monitoring fault current through the device. FPIs can sense short circuit and earth fault currents separately. Current sensors placed on cables and earth which are used to detect phase faults (L1, L2,L3) and earth fault (E).

5. Numerical relays for O/C and E/F protection

6. Power Supply Unit

A power supply unit, including auxiliary power transformer and battery backup, to
provide stable 24 V DC sources of power for the RMU’s spring-charge motors, relays etc.

7.Current Transformers, Potential Transformers and HRC Fuses

8. Solid-insulated busbars