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AM18.2 Medium voltage distribution: Equipment (2022)
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AM18.2 Medium voltage distribution: Equipment (2022)

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This Applications Manual on medium voltage distribution has been produced to help fill the gap in electrical knowledge in respect of how to safely employ medium voltage for the distribution of electrical power.

Our aim is to provide an understanding of a number of key aspects at different design and construction stages of a medium voltage (11 kV) power distribution system to buildings. Consideration is given to system design and to the selection and erection of equipment, including the associated practical aspects.

It is intended that this Applications Manual will be used by practitioners in conjunction with established international wiring standards and relevant codes of practice. It will also be of interest to designers and authorities who, while not directly concerned with the design or installation of electrical systems, must understand the advice offered to them by specialists.

Further, the manual should be of value to those who wish to enhance their knowledge of electrical power and building services engineering.

Medium voltage distribution is a specialised area of power engineering.The first part, AM18.1: Methods, introduced the basic principles and the common methods of medium voltage distribution. This second part, Equipment, explores in more detail the equipment necessary to deliver medium voltage supplies to buildings – cables, switchgear and transformers. Note: protective relays associated with the operation of circuit breakers and other protection equipment will be covered in the next part, AM18.3: Protection.

Parts already published:

  • AM18.1: Methods

Upcoming parts will include:

  • AM18.3: Protection
  • AM18.4: Fault calculations
  • AM18.5: Grading

These will explore protection devices and equipment in greater detail. They will provide typical application calculations and solutions for the protection of transformers, distribution cables, busbars, generators and motors, and will provide examples so that these calculations are clear.

This Applications Manual will not cover high voltage systems (that is, those using power supplies greater than 33 kV). This means that we will not go into any detail regarding transmission and distribution, or specialist power applications within the industry (such as the protection of power factor correction capacitors, harmonic snubber circuits, high-voltage direct current, high-power static conversion, battery storage or smart grids). For information regarding possible solutions to these applications, it is best to refer to manufacturers’ literature.

Table of Contents

1             Introduction      

1.1         Cable types

1.1.1     Overhead lines

1.1.2     Underground cables

1.2         Cable short-circuit ratings

1.2.1     Electromagnetic forces and cable bursting

2             MV switchgear

2.1         Circuit breakers

2.1.1     The interrupting capacity of the breaker

2.1.2     Non-manual closure

2.1.3     Arc control

2.2         Circuit breaker designs

2.3         Forms of isolation – withdrawable (vertical/horizontal) or fixed

2.3.1     Vertical isolation

2.3.2     Horizontal isolation

2.3.3     Fixed equipment

2.3.4     Terminals and cable boxes

2.4         Oil circuit breaker

2.5         Gas-insulated circuit breaker

2.6         Available alternatives to SF6

2.6.1     Novec™ dielectric

2.6.2     g3

2.6.3     Nuventura – synthentic air

2.6.4     Vacuum switchgear

2.7         Vacuum circuit breaker

2.7.1     An arc in a vacuum and restrike voltage

2.8         Current chopping

2.9         Gas-filled versus vacuum switchgear – a comparison of features

2.10       Switchgear tripping battery units

2.11       Switchgear control circuits

3             Transformers

3.1         Fundamental transformer principles

3.2         Basis of construction and operation

3.2.1     Core and winding losses

3.3         In-rush current

3.4         Load harmonic currents

3.4.1     Factor K and K-factor

3.4.2     K-rated or de-rated?

3.4.3     Harmonic diversity         

3.5         Harmonic limits G5/5

3.6         Transformer specification

3.6.1     Construction standards

3.6.2     Types of transformer

3.6.3     Insulation and cooling

3.6.4     Rating

3.6.5     Vector groups

3.6.6     Primary and secondary electrical supplies

3.6.7     Transformer duty

3.6.8     Location

3.6.9     Impedance

3.6.10   Neutral and earth arrangements

3.6.11   Voltage tappings

3.6.12   Terminals and cable boxes

3.6.13   Accessories and tank fittings

3.6.14   Enclosures

3.6.15   Testing

3.7         Transportation, installation and commissioning

3.8         Electromagnetic compatibility of distribution transformers

3.9         Noise levels

4             The maintenance of MV networks

4.1         Periodic inspections

4.2         Planned intrusive maintenance

Annex A: Cable selection and sample MV cable data

A1          MV cable selection: example flowchart

A2          Sample cable data sheets            

A2.1      BS 6622 XLPE PVC SWA 6.35/11 (12) kV cable

A2.2      BS 7835 XLPE LSZH SWA 6.35/11 (12) kV cable

A2.3      BS 6622 XLPE MDPE SWA 6.35/11 (12) kV cable

A2.4      BS 60228 XLPE PVC 6.35/11 (12) kV cable

Annex B: Electric arcs

Annex C: SF6 circuit breaker operation   

Annex D: Gas-filled and vacuum circuit breaker selection              

Annex E: Switchgear battery maintenance guide for Planté (lead-acid) battery types

Annex F: Factor K and K-factor example calculation          

Annex G: Liquid-filled and dry-type transformers

G1         General information

G1.1      Maintenance

G1.2      Efficiency

G1.3      Operation

G2         Information specific to categories

G2.1      Dry-type transformers

G2.2      Cast resin transformers

G2.3      Liquid-filled transformers            

Annex H: Example transformer data sheet(s)

Annex I: Sample typical transformer test certificates

Annex J: Further suggestions for MV maintenance procedures

J1           11kV, SF6 and gas-filled circuit breakers and RMUs          

J1.1       Equipment and materials required

J1.2       Pre-maintenance requirements

J1.3       Procedure

J1.4       Post-maintenance requirements

J2           11kV distribution transformer inspection and maintenance

J2.1       Equipment and materials required

J2.2       Pre-maintenance requirements

J2.3       Procedure

J2.4       Post-maintenance requirements

Authors: Les Norman, Brunel University London; Adam Rawlinson, PCS Consulting Services Ltd; Phil Reed, RPS Group PLC

Peer Reviewers Derek Elliott, Insight PFM Ltd/CIBSE Electrical Services Group; Neil Hitchman, Vinci Construction UK/CIBSE Electrical Services Group; Tony Sung, Energy Reduction Management Ltd/CIBSE Electrical Services Group chair