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. This manual is in five parts:
AM18.2: Equipment explores in detail the equipment necessary to deliver medium voltage supplies to buildings - cables, switchgear and transformers.
The parts of this Applications Manual will not cover high voltage systems (that is, those using power supply voltages greater than 33 kV). This also means that we will not go into any detail regarding transmission and distribution at these voltages nor specialised 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 for 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 11 kV, 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 11 kV 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