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

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.
A 7-section ABB Ring Main Unit

A 7-section ABB Ring Main Unit

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.

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.

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 FeederOption 2: Isolator / Incomming FeederOption 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 on to 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 on to 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.

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

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 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 has 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

A typical ring main distribution scheme making use of RMUs

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 below factors and make an informed decision based on their particular needs.

Comparing RMUs with conventional switchgears.

FactorRMUConventional 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.

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