Wide-area blackouts: why do they happen
and how can modelling help
Professor Janusz W. Bialek
Durham University
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Outline
Modelling of electrical networks
Overview of recent blackouts and their causes
How can modelling help in preventing blackouts
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Overview of recent blackouts
Only wide-area blackouts, not local ones
– Local ones a majority
Interconnected system blackouts in 2003:
US/Canada, Sweden/Denmark, Italy
UCTE “disturbance” 2006
May 2008 disturbance in the UK
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Modelling of electrical
networks
A network is a planar graph with
nodes (buses, vertices) and
branches (lines, edges)
GB high-voltage transmission
network is meshed and consists
of 810 nodes and 1194 branches
UCTE and US interconnected
networks consist of several
thousands nodes
For most analyses, the network
is described by algebraic
equation (Current and Voltage
Kirchhoff’s Laws)
Electromechanical stability of
rotating generators is described
by differential equations
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Preventing blackouts
We can’t live without electricity so the power system has to
be designed and operated in a robust manner
– Should ride through “credible” disturbances
– Trade-off between cost of keeping reserves and security
A proxy to probabilistic risk assessment: (N-1) contingency –
a deterministic criterion
N-1 contingency: a single disturbance (generation/line
outage) should not cause problems
– it is unlikely that 2 or more units will be lost simultaneously
– generation reserve: the loss of the largest infeed (a
nuclear reactor of 1320 MW at Sizewell B)
– Transmission reserve: loss of double-circuit line (N-D)
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When do blackouts happen?
... when (N-1) contingency analysis has not been
done properly (Italy 2003, UCTE 2006)
... or when more than 1 thing went wrong
(US/Canada 2003, Sweden 2003, GB 2008)
... or hidden mode of failure (London 2003)
The new world of renewables and Smart Grids may
require the use of probabilistic risk assessment
– Briefly today, more Thursday 3.30pm
“Mathematical modelling of future energy systems”
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Classification of blackouts
Transmission inadequacy: a failure in a transmission
network causes a cascading overloading of the
network (a majority)
Generation inadequacy: failures of power plant(s)
cause a deficit of generation (GB 2008 disturbance)
Usually a mixture: an initial network fault causes a
separation of the network into parts with
deficit/excess of generation
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Major transmission failures in late
summer/autumn 2003
7 blackouts affecting 112 million people in 5 countries
14 August 2003, USA/Canada
23 August 2003, Helsinki
28 August 2003, south London
5 September 2003, east Birmingham
23 September 2003, Sweden and Denmark
28 September 2003, whole Italy except Sardinia
22 October 2003, Cheltenham and Gloucester
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The Oregonian, 24 August 2003, after C. Taylor
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NE of USA/Canada: before
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NE of USA/Canada: after
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Where it all began: Ohio and surrounding areas
Source: US/Canada Power System Outage Force
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How it all started: tree flashover at 3.05 pm
Source: US/Canada Power System Outage Force
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Bad luck?
Alarm and logging system in FirstEnergy (FE) control room
failed 1 hour before the cascade started
Not only it failed, but control room engineers did not know about
it
When lines started to trip they could not take corrective action:
the system was not (N-1) secure after first trips
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Cascading tripping: an initial line trip casues
overloading on other parallel lines
Source: US/Canada Power System Outage Force
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Effect of line trips on voltages: depressed voltage
(Ohm’s Law)
Source: US/Canada Power System
Outage
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Bialek,Force
2010
Source: US/Canada Power System Outage Force
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Source: US/Canada Power System Outage Force
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Speed of cascading
Source: US/Canada Power System Outage Force
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Normal load, big margins
Denmark self-sufficient,
southern Sweden
supplied from
central/northern
Danish/Swedish blackout:
23/09/03, 5 M people
1.2 GW Oskarshamn
nuclear plant trips due
to a feed-water valve
problem
5 min later double busbar
fault trips 4 lines at
Horred substation
(N-5) contingency
1.8 GW Ringhals nuclear
plant shuts down
Southern Sweden and
western Denmark
blacks out
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Italy
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Frequency as real power balance indicator
Power generated must be equal to power consumed
Frequency is the same at any part of interconnected network
If there’s a sudden loss of generation, energy imbalance is made up
from kinetic energy of all rotating generators
The speed (frequency) drops triggering all turbine governors to
increase generation automatically (feedback control)
If frequency drops too much,
automatic load shedding is
activated
generation deficit =>
frequency drops,
generation surplus =>
frequency increases
Source: National Grid
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3 am: import 6.7 GW, 25% of total demand, 300 MW over agreed
CH operated close to (N-1) security limit but Italy didn’t know about it
86% loaded internal Swiss Lukmanier line trips on a tree flashover
3.11 am: ETRANS informs GRTN (disputed, no voice recordings)
GRTN reduces imports by 300 MW as requested
Source:
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Bialek,UCTE
2010
two more CH lines trip and Italy loses synchronism with UCTE
Island operation: import deficit leads to a frequency drop and load shedding
Until 47.5 Hz, 10.9 GW of load shed but 7.5 GW of generation lost
Frequency drops below 47.5 Hz and remaining units trip
Blackout 2.5 minute after separation: whole Italy, except of Sardinia.
Source:
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Bialek,UCTE
2010
UCTE disturbance in 2006
UCTE: Union for the Coordination of Transmission of
Electricity – association of TSOs
now renamed ENTSO-E
(European Network of
Transmission System Operators
for Electricity)
Source: UCTE
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Flows just before the blackout
Generation 274 GW including 15 GW of wind (5.5%)
Strong east-west power flows, i.e. the West depends on imports
strong wind generation in northern Germany
Source: UCTE
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Timeline
18 Sept: a shipyard request EON for a routine disconnection of double
circuit 380 kV line Diele-Conneferde in northern Germany on 5 Nov
3 Nov: the shipyard request to bring forward the disconnection by 3 hours.
Late announcement could not change exchange programs
4 Nov 9.30 pm: EON concludes empirically, without updated (N-1)
analysis, that the outage would be secure. Wrong!
9.38: EON switches off of the line
10.07: Alarms of high flows. EON decides, without simulations, to couple
a busbar to reduce the current
Result: the current increases and the line trips
As the system was not (N-1) secure, cascading line tripping follows
separation of UCTE into 3 regions with different frequencies
Image: http://www.cruise-ship-report.com/News/110506.htm
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10 GW surplus
51.4 Hz
0.8 GW deficit
49.7 Hz
8.9 GW deficit
49 Hz
Source: UCTE
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Western Europe: 8.9 GW deficit
Frequency drop to 49 Hz triggered automatic and
manual load shedding (17 GW) and automatic
tripping of pump storage units (1.6 GW)
However the frequency drop caused also tripping of
10.7 GW of generation – more load had to be shed
DC connection UK-France: continued export from
France despite the deficit!
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Resynchronisation
A number of uncoordinated unsuccessful attempts made without
knowledge of the overall UCTE situation
Full resynchronisation after 38 minutes
Source: UCTE
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GB May
2008
event: a
near miss
Source: National Grid
Cockenzie and Sizewell B were lost within 2 mins: (N-2) event, 1714 MW
Loss of Sizewell B is the largest infeed loss planned for (1320 MW)
Further 279 MW of wind tripped due to frequency drop (total 1993 MW)
Automatic load shedding of 546 MW triggered at 48.8 Hz
Voltage reduction caused reduction of demand by 1200 MW
More generation was connected and supply restored within 1 hour
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Will there be more blackouts?
People tend to learn from the past
... but generals are usually prepared to the last
(rather than future) war
Lessons learned from the blackouts - improvements
in communications and coordination in Europe and
USA
... but new challenges are looming ahead
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Generation adequacy issues
Possible problems after 2015 (Ofgem Discovery report)
Regulatory uncertainty
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Increased penetration of renewable generation
Wind already a contributing factor to UCTE 2003 and
GB 2008 disturbances
“Any feasible path to a 80% reduction of CO2
emissions by 2050 will require the almost total
decarbonisation of electricity generation by 2030”
(Climate Change Committee Building a Low Carbon Economy 2008)
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Smart Grids
Comms-enabled responsive demand, electric cars etc
Highly stochastic generation and demand: (N-1) contingency
criterion may become obsolete soon – new probabilistic risk
assessment tolls required
Dependence
on comms
networks is a
new mode of
failure
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Example of new modelling techniques:
preventive network splitting
EPSRC grant started January 2010 (Complexity
Science call)
Exciting collaboration between graph theorists from
Southampton (Brodzki, Niblo) , OR experts from
Edinburgh (Gondzio, McKinnon) and power
engineers from Durham (Bialek, Taylor)
Split the network in a controlled manner before it
partitions itself
Initial main challenge: speaking the same language,
mutual education
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Conclusions
(N-1) contingency criterion has served us well in the
past but there were a number of wide-area blackouts
in 2003, 2006 and 2008
New challenges of increased wind penetration and
Smart Grids
New mathematical modelling tools required to
prevent future blackouts
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