15/04/2014

EEEN60301 Power System Modelling

Introduction to Power Systems
Monday 23 September 2013

Introduction to
Electrical Power Systems

Prof. Peter Crossley

p.crossley@manchester.ac.uk

Function of a Power System:

• Generate electrical energy economically and with

minimum ecological disturbance

• Transfer this energy over transmission lines and

distribution networks with maximum efficiency and
reliability

• Deliver electrical energy to consumers at virtually fixed

voltage & frequency

1
 15/04/2014

Electrical Power Systems:

When compared with other man-made networks (e.g.

communications, gas, water, sewage) electrical power
systems are the most expensive in terms of capital invested,
most influential in terms of disruption to our mode of life in
case of breakdown, most visually intrusive in terms of
impact on the landscape and the most ecologically intrusive
in terms of thermal, chemical and potential radiological
pollution.

Structure of a Power System

2
 15/04/2014

Boiler + turbines + Exhaust gases

generators Coal store

Cooling towers

Electrical substation
Coal fired power station

Generation:

• Large synchronous generator

driven by a steam turbine
produces three phase electrical
power at a typical voltage of
22kV line-line.

Q1. What is the output current per phase if a 500MW turbine +generator is
used and is operating at its rated output power ?
500MW/3 ÷ (22kV/√3) = 13kA at 22kV
Q2. Why is the generator operating at 22kV rather than 400kV or 400V ?

3
 15/04/2014

Stator Winding of a 660MW Generator

Repairing a 200MW generator rotor

Stator of a hydro generator

4
 15/04/2014

Future:- Thermal Power Station burning domestic waste,

river water cooling and gas/particle emission filtering

What is a Power System?

Load
Generator Load

Generator

5
 15/04/2014

Transmission:

• Generator Voltage of 22kV is too low for 1960’s

transmission over long distance, hence 275kV
step up to 275kV or 400kV (UK) using transformer
transformers.
– 500MW = 13kA at 22kV = 1kA at 275kV

• At 275/400kV the power is transmitted via

an overhead line to a bulk power
substation for connection to the 400kV
transmission network.

Transmission, Sub-transmission & Distribution Network

transformer
Generating

6
 15/04/2014

= circuit breaker
400kV Substation
= disconnector
= transformer

Transmission feeders
Substation:
interconnecting 4
transmission feeders

Miss July

Transmission
Pylon of the Month!
Mr
January

it was the wrong sort of snow!

7
 15/04/2014

Transmission Line after a Hurricane (India)

= circuit breaker
Bulk Substation
= disconnector
= transformer

Transmission
network

Sub-transmission
network

8
 15/04/2014

busbar

isolator
circuit breaker CT’s

Electrical Substation

Sub-transmission &
distribution line on
same tower (USA)

Mixed voltage lines not

used in UK

9
 15/04/2014

Distribution Network

• In a UK distribution substation, the transmission or sub-transmission

voltage is reduced to 33kV or 11kV
• Large industrial consumers are supplied at 33kV or 11kV
• Large commercial are supplied at 11kV
• Generally 33kV is then reduced to 11kV
• Finally, 11kV is stepped down to 415V 3-phase supply
• 3-phase 415V supply consists of three phases plus neutral
• Most consumers supplied via underground cable and fed from one-phase
(line) and a neutral (plus an earth)

3.3kV feeder supplying 400V 3-phase load

10
 15/04/2014

Conversion from overhead line to cable

Distribution substation

11
 15/04/2014

11kV Radial rural distribution feeder with wind generator

11 kV overhead line
V1 V2 V3 SW3
33/11 kV
Transformer SW1 SW2 680V/11 kV
Transformer

680V wind generator

Load-1 Load-2

LV Distribution Network (400V)

• 3-phase 400V supply consists of three phases plus neutral
• Normally supplied by 4-core underground cable
• Most consumers connected to one-phase (line) and neutral
• An earth is supplied to consumers for protection purposes
• The earth is connected to the outer metal casing of electrical appliances, if an
insulation breakdown occurs, the casing becomes live, the fuse blows and the
consumer is protected.
• Fuses in a domestic distribution box are now being replaced by over-current
operated miniature circuit breakers (mcb’s). Operate at ten’s of amperes.
• Residual current tripping devices (RCD) are also used to protect consumers.
Operate when difference between line and neutral current is greater than 50mA
(reduces risk of electrocution).

12
 15/04/2014

Domestic 240V electrical system

3-phase supply

kW & kWH meter

Domestic 240V single-phase supply

Electric Electric Water

shower cooker heater

Revision:- What is an electrical power system?

Sources of electrical energy:-

generators convert mechanical energy into electrical energy
mechanical energy derived from steam in thermal power station
mechanical energy also extracted from a wind-turbine or a gas-turbine.

Transmission network:-
overhead lines are used across the country and cables in urban areas
generated power = consumed power, demand increases must be matched by increases in generation
system frequency (50Hz) increases if generated power > demanded power
system frequency (50Hz) decreases if generated power < demanded power
system frequency is controlled to be 50Hz.

Distribution network:-
transfer power from transmission network to regions, towns and homes
discuss reasons for different voltage levels.
Environmental impact:-
impact of fossil fuels on air quality, CO2, SOX, NOX emissions
advantages/disadvantages of renewable energy sources and nuclear
Economic cost of different sources of energy and need for reliability

13
 15/04/2014

Revision

• Why 50Hz or 60Hz?

– lower frequencies cannot be used due to flicker in filament lamps
– iron losses increase in proportion to frequency
– leakage reactances increase in proportion to frequency
– capacitive reactance between lines reduce with freq.
– interference with telephone lines increase with freq.
– higher frequencies enable smaller motors, generators and
transformers.

• Remember electrical networks were first designed in the

1890’s:- hence 50 or 60Hz was sensible choice.

Revision

• Why 3-phase?
– Increases the rating of a.c. generators
– a.c. allows conversion of electrical to mechanical energy
– Comparative capacity of machines operating with different phases:
• 1-phase = two connecting wires = 1.00 capacity
• 2-phase = three connecting wires = 1.41 capacity
• 3-phase = three connecting wires = 1.50 capacity
• 4-phase = four connecting wires = 1.53 capacity
• -phase =  connecting wires = 1.57 capacity

– Hence power gains beyond 3-phase are small,

– 3-phase only needs three wires and
– polyphase systems = balanced rotating magnetic field in machines

14
 15/04/2014

Revision

• Why such a variety of voltages?

– Power received by the load is: P = V.I.cos
– Percentage transmission loss is Ploss = I2R x 100% / P
– For efficient transmission Ploss should be minimised.
– Hence reducing I reduces power loss, but if P is to be held
constant V must be increased proportionally
– Implies high transmission voltage is desirable
– Problem:- higher voltages = taller towers + longer insulator strings
+ broader rights of way + more expensive plant & construction
costs
– Compromise between rising capital costs as voltage increases
and greater losses as voltage reduces.

What is distributed, dispersed or embedded generation?

Generation connected to a circuit from which other customers are

supplied directly.
(2 kW – 100MW connected at 415 V - 132 kV)
e.g. Combined Heat & Power
Co-generation: Diesel generators, micro-turbines (gas)
Wind turbines/Biomass/Photovoltaics
Fuel cells /Storage

CIGRE definition of Dispersed Generation: not centrally planned

not centrally dispatched and connected to distribution network

15
 15/04/2014

Diesel Generators:

Wind, Photo-Voltaic & Tidal

16
 15/04/2014

Power
company
Photo Voltaic Wind power Hydraulic Fuel cell

Gas
company Measure
ment

Information
network

Buildings,
hospitals, Hydrogen
schools, etc. Control Supply side
system

Sewage Biomass
Co-generationDemand side treatment power

Electricity
Heat
Residential Public Energy storage
load Refuse power
load (NAS battery)

Features of distributed generation

• Renewable - resource led:

– technology = induction generator, electronic interface (AC-DC-AC, DC-
AC), DFIG (double fed induction generator).
• CHP - heat led:
– technology = synchronous generator

• Widely varying output and technical characteristics

• Many owners and operators
• Output does not usually respond to network conditions

17
 15/04/2014

Biomass バイオマスプラント
牛糞尿 plant (Night soil)

PV power
太陽光発電 Bio Gas

Gen
← Engine

Load Generation system

発電設備

Load
負荷

Historical perspective

• Before the construction of the UK National Grid all generation was

embedded in distribution networks

• Objective of the CEGB was to generate and transmit electricity in the

most economic manner (statement true in most countries)

• Some countries (e.g. Sweden) maintained town and city authorities and
constructed CHP plants (provides district heating & local electricity generation)

• Objective of CHP:- minimise heating and electricity costs

18
 15/04/2014

Distributed Generation

Coal, gas, oil, nuclear stations

Central
Generators
Interconnected
Transmission Network

Distribution
Network

Loads CHP Loads

19
 15/04/2014

Could we replace coal

stations with wind
Past generators?

Future ??

Reasons for Distributed Generation

• Reduce environmental impact of generation

– harness dispersed renewable energy resources
– increase overall energy efficiencies (Combined Heat & Power)
– reduce need for bulk fossil fuel generators
(closure of “old” coal-fired stations)

• Future
– DG provides system reinforcement
(i.e. local real & reactive support)
– DG provides fuel security (depends on energy source)

20
 15/04/2014

Issues:

• Conventional energy sources:

• coal, oil, gas, hydro, nuclear, impact on environment and society
• Future of conventional energy sources:
• clean coal, combined cycle gas turbines, fission, fusion, C0 2 storage
• Renewable energy sources:
• renewable fuels, wind, wave & tidal power, PV, energy networks
• Energy storage
• bulk and domestic mechanical, chemical, water and heat storage
• Energy flexibility and conservation
• domestic, vehicles, commercial, industry, shops, public attitude etc
• Education of society & strategies for sustainability
• acceptance of need for change, revision of planning laws, true costs etc
• energy usage, subsidies and taxation, energy audits, efficiency

Sustainable Energy Sources:-

• Operating life:- energy produced > energy consumed

• High conversion efficiency
• Low energy construction
• Low environmental impact
• Economic and ergonomically acceptable designs
• Integration into existing energy networks
• Public & political acceptance
• Expansion of UK manufacturing base = employment
• Decommissioning:- Recovery of energy in materials

21
 15/04/2014

Diversified energy networks

• Replacement of large fossil fuelled power stations with multitude of small

renewable sources
• Requires radical re-think of how energy network operates
• Immediate:- conventional generation/storage technology
• Future:- fuel-cells, micro-generators, electronic interfaces, superconductors,
chemical storage etc
• Loosely controlled hierarchy of generation, storage, load management
• Complex, active distribution networks with non-deterministic sources of
economical low carbon energy

S Tr. S Tr.

Main Centralized
Generation Plant

TL TL
Transmission System

DSS DSS
Gas Turbine

Fuel Cell
DSS Micro Turbine
DSS

PV Gas Turbine
Flywheel
Batteries
Commercial Load Industrial Load
Fuel Cell
Distribution System
Residential Loads
Distribution Network Is No More Vertically Operated

22
 15/04/2014

Passive Distribution Network

P,Q P,Q

Load

Dispersed Generation

P,Q? P,Q?

P,-Q
pv
P+/-Q
P,+/-Q
CHP S

23
 15/04/2014

Benefits of DG:- Economics

a) Save world’s fuels reservoirs

b) Save T&D Expansions Costs

c) Increase the electric power equipment lifetime

d) Can Be Installed incrementally

e) Provide Combined Heat and Power (CHP)

f) Not Restricted by Geographically Limitations

g) Reduce wholesale power price (Location Margin Pricing)

Benefits of DG:- Operation

• Positive Impact on Network Voltage profile

• Reduce System’s Power Losses
• Satisfy the Thermal Constraints of T&D feeders
• Help for “Peak Load Shaving”
• Maintain System’s Continuity, Stability & Reliability
• Provide Local Reliability (in case of emergency & main source outage as standby generation)
• Maintain System’s Security and reinforce the Critical Electric Power
Infrastructure (Islanding)
• Positive Impact on Environment & eliminate/reduce emissions

24
 15/04/2014

new-build estates with micro-generators, PV & storage

Power
Load curve
Supply from the
utility grid
(constant)

0
Charge Charge
energy DG output energy
storage storage
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
hour

Fuel cell

Hydrogen

Sewage treatment Biomass power

25
 15/04/2014

CIRED Data
Dispersed generation/Total installed generating capacity
EnergieNed Data
CIGRE Data
45
40
35
30
25
%

20
15
10
5
0

Netherlands
Germany

Ireland
Belgium

Italy
Denmark

Finland

Poland

Sweden
Australia

Portugal
Greece
Austria

France

Spain

UK
Distributed generation & consumption in
Denmark (source Eltra)

7000

6000

5000
Consumption

4000
MW

Offshore wind
3000 CHP
Onshore wind
2000

1000

0
1990 1998 2005 2015

26
 15/04/2014

Q & A:- Options for Electrical Energy in 2020

• Starting point:- generation in 2006

– Peak demand = 60,000MW, minimum demand = 18,000MW, average =
35,000MW.
– Gas = 43%, coal = 30%, nuclear = 18%, wind = 4%, water = 2%, oil = 2%,
other = 1%.
• Will average demand in 2020 reduce, increase or stay constant?
• Estimate 2020 average demand?
• What will cause the change?
• Should the UK build new nuclear power stations? (remember if no nuclear station built,
nuclear contribution by 2020 = approx 5%)
• Should we build more gas fired power stations? (uses imported gas)
• Should we build more wind-generators? (remember wind is not continuous, average
output from a 3MW machine is 1MW)
• Should we build more conventional coal stations? (what about Kyoto)
• Will we have the technology to build economic clean coal power stations?

• Estimate the average output in MW from gas, coal, nuclear, wind, water, oil and other
power stations by 2020?

Q&A

Prof. Peter Crossley

p.crossley@manchester.ac.uk

27