Ring Main Units (RMUs), Construction, Working and Comparison With Switch-Gears

Author:

The author is a professional electrical engineer and has supervised installation and commissioning of multiple RMUs in his facility.

The Ring Main Unit (RMU)

The ring main unit, with its new compact design and SF₆ arc quenching technology, has been a relatively recent entrant in the distribution infrastructures industry. It has challenged the long undisputed dominance of conventional VCB switch-gears in the medium voltage distribution equipment market.

Here we will explore:

How RMUs are constructed and how they work.
Their utilization in distribution feeder schemes.
Lastly, we will see if they are really viable alternatives to conventional switch-gears which their manufacturers promise.

Construction of a Ring Main Unit

A typical ring main unit is essentially an encapsulated medium voltage (11kV - 66kV) bus bar, that has provision to either terminate any number of incoming feeders or rise outgoing load feeders, each in a separate modular compartment.

Schematic Diagram of a Typical Ring Main Unit

A typical 5 section RMU can have the schematic as shown in the picture below: This scheme is used to operate two different AC sources in a tie bus bar scheme.

Schematic diagram of a typical 5-section Ring Main Unit.

(from left to right)

Section 1: Outgoing to 11kV/380V transformer, with protection relay, circuit breaker and metering (all three options).
Section 2: Incoming from either 132kV/11kV transformer or another 11kV feeder, with isolator only.
Section 3: Bus Coupler, with isolator only.
Section 4: Incoming from either 132kV/11kV transformer or another 11kV feeder, with isolator only.
Section 5: Outgoing to 11kV/380V transformer, with protection relay, circuit breaker and metering (all three options).

The Common Busbar

Throughout the length of the RMU structure, runs a common copper bus bar, concealed behind the SF₆ compartments and covers. Each cable termination terminates on this common bus bar.

The Schneider Electric's RM6 RMU pictured from backside with covers removed. The main Common bus bar is visible exposed at the bottom, whereas the earth bus bar is at the top.
Schneider Electric Industries SAS. Adapted from RM6 Official Catalouge

Modular Compartmentalized Design

RMUs are ordered according to the number of sections that are required for e.g., engineers may order either a 3 - section, 5 - section, 7 - section or even higher sections unit, depending on their distribution scheme.

Each section/modular compartment may either have, depending on the requirement:

Customizable options for each section/compartment of a Ring Main Unit.
Option 1: Circuit Breaker / Outgoing Feeder	Option 2: Isolator / Incomming Feeder	Option 3: BusCoupler / Riser
Microprocessor protection relay	Sealed SF6 Isolator / switch disconnect	Sealed SF6 Isolator / switch disconnect
Sealed SF6 circuit breaker	Earth switch
Earth switch	Metering (Optional)
Metering

RMUs are extendable; it is possible to join an existing installation with additional sections for expansion.

Sealed Cable Terminations

MV cables are terminated onto the common bus bar in separate compartments. Each cable termination joint is completely enclosed in a special insulating plastic cladding (bootkits) that eliminates the possibility of flash-over with the nearest conductor. After the cables are bolted onto the common bus bar, a vacuum pump is used to seal the plastic cladding on the cable termination bolts.

Sealed MV cable termination kits commonly referred to as 'boots' inside an an RMU compartment.
Ali Khan's Photography

What Makes an RMU Smaller in Size?

The fact that an RMU occupies 40% less space compared to traditional vacuum circuit breaker trolleys is enabled by:

Sealed termination boot kits:

They completely encapsulate the termination points of MV cables and preventing the possibility of arc flash-over even if the cables are spaced closely. In other words, they significantly reduce the spacing requirement between cables and enable them to terminated more closely to each other.

Hermetically Sealed SF₆ Tanks:

They are usually smaller in size as compared to traditional vacuum circuit breakers, due to superior arc quenching properties of inert Sulphur Hexaflouride (SF₆) gas.

Distribution Philosophy of Ring Main Units

RMUs are commonly employed in ring main distribution schemes, where their compact size and modular installation have made them quickly replace switch-gears.

A standard ring main feeder with RMUs is illustrated below:

A typical ring main distribution scheme making use of RMUs
Ali Khan

In this scheme, a typical downstream substation has:

An RMU, having an incoming and outgoing feeder
Two transformers
LT tie-bus bar

RMU vs. Conventional Switch-Gear, Which One Is a Better Choice?

Should you opt for the RMU or go with the traditional Vacuum Circuit Breaker trolley switch-gear in a new installation?

This is the question operational engineers, facility designers and consultants face when they are designing new electric installations or overhauling existing installations. When faced with this question, engineers can evaluate their choice on the basis of the below factors and make an informed decision based on their particular needs.

Comparing RMUs with conventional switchgears.
Factor	RMU	Conventional VCB Switch-Gear
Arc Quenching Technology	Latest SF6 gas insulated hermetically sealed chamber	Conventional vacuum bottles
Switching Cycles	2,000 - 3,000	>10,000
Maintenance Requirements	Almost nil, external cleaning and check for SF6 pressure only	Numerous health tests, cleaning and greasing of mechanical parts is required pereodically.
Space Requirements	Approximatly 50% less then conventional VCB trolley.	Approximatly 50% more then RMU.
Installation costs	Less as compared to switchgear.	More as compared to RMUs, due to increased cable lengths and trunkings.
Equipment capital cost	Variable according to region.	Variable according to region.
Future customization and modifications	Possible but require skilled personal due to compact new design.	Generally local facility electricians can modify the control wiring
Operational life cycle	Lower as compared to VCB due to less mechanical operational cycles	Higher due to more switching cycles, with responsible maintenance.
Shipment cost, logistics and storage	Shipment and logistics costs are less due to compact design	Standard logistics and shipement costs according to trolley size
Manufacturing lead times	Higher, since RMUs are not manufactured locally	Lower, since most countries have local switchgear assembly plants.
Combined Project Costs	Significantly reduced, because less civil works and real estate are required to house an RMU installation.	Civil works must give due allownce for walking distances and trolley rack-outs in the substation housing a switchgear installation, thereby increasing costs.

Conclusion

Space constraints and installation costs will make RMU the preferred choice over switch-gears.
Operational life cycle and switching requirements will make switch-gear the preferred choice.
Costs and logistic intervals vary according to location but should be considered in the decision-making process.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.