Transmission and Distribution

Lecture Outline

• The electricity supply chain• Components of an electrical power system• Power Lines

– Standard voltages– Equivalent circuit, voltage regulation and

power transmission capability– Choosing the line voltage

• Substation Equipment

The Electricity Supply Chain

Generation

Transmission

Distribution

Retail

National Electricity Market

Principal components of an electrical power system

Power Station Substation

System Loads

SubstationTransmission Lines

Distribution Lines

• Step-up transformers & switchgear

• Step-down transformers & switchgear

• lighting

• heating

• appliances

• motors

Power Lines

Power LinesConsiderations in the design of a power line:• The amount of active power it has to transmit• The distance over which the power must be

carried• The cost of the power line• Aesthetic considerations, urban congestion,

ease of installation, expected load growth

Types of Power LinesPower Station Substation

System Loads

Transformer

Substation

High voltage (HV)

Medium Voltage (MV)

Low Voltage (LV)

e.g. Swanbank

e.g. Tennyson

e.g. Pole-mount

e.g. House

Standard Voltages

132kV, 275kV, 400kV, 750kV

> 100kVHV

3.3kV, 6.6kV, 11kV, 33kV, 66kV

1kV – 100kVMV

415V (3-phase)240V (1-phase)

< 1kVLV

Standard levelsRangeClassification

HV Power Line ComponentsConductors• Normally bare• Stranded copper or steel-reinforced aluminium cable (ACSR)

Insulators• Support and anchor conductors and insulate them from ground• Ceramic or glass• Must withstand mechanical and electrical stress

Support Structures (towers etc.)• Wooden poles / H-frames for lower voltages, galvanized steel

towers for higher voltages

Equivalent circuit of a line

Distributed impedance of a line

Lumped model

Inductive impedance is normally low

Capacitive impedance in normally high

Resistance varies with transmission voltage and power…

Voltage Regulation and Power Transmission Capability of Transmission Lines:

Resistive Line

0%10%20%30%40%50%60%70%80%90%

100%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Vload / Vsupply

P /

Pm

ax

Load

R

Supply

RV

P ply

4

2sup

max =

Load has unity power factor

Voltage Regulation and Power Transmission Capability of Transmission Lines:

Inductive Line

Load

L

Supply

L

ply

XV

P2

2sup

max =

Load has unity power factor

0%10%20%30%40%50%60%70%80%90%

100%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Vload / Vsupply

P /

Pm

ax

Choosing the Line Voltage• The voltage should remain as constant as

possible over the entire length of the line – from source to load – for all loads between zero and rated load.

• The line losses must be small so as to attain a high transmission efficiency and not overheat the conductorsThere is a trade-off between capital cost and

transmission losses in the choice of line voltage for a given power transmission capability

Substations

Substation Equipment• Transformers• Circuit breakers• Switches (horn-gap, disconnect & grounding)• Surge arrestors• Current-limiting reactors• Relays and protective devices• Instrumentation• See text for more info and examples