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Wide-area blackouts: why do they happen and how can modelling help

Wide-area blackouts: why do they happen and how can modelling help. Professor Janusz W. Bialek Durham University. Outline. Modelling of electrical networks Overview of recent blackouts and their causes How can modelling help in preventing blackouts. Overview of recent blackouts.

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Wide-area blackouts: why do they happen and how can modelling help

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  1. Wide-area blackouts: why do they happen and how can modelling help Professor Janusz W. Bialek Durham University

  2. Outline • Modelling of electrical networks • Overview of recent blackouts and their causes • How can modelling help in preventing blackouts

  3. 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

  4. 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

  5. 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)

  6. 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”

  7. 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

  8. 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

  9. The Oregonian, 24 August 2003, after C. Taylor

  10. NE of USA/Canada: before

  11. NE of USA/Canada: after

  12. Where it all began: Ohio and surrounding areas Source: US/Canada Power System Outage Force

  13. How it all started: tree flashover at 3.05 pm Source: US/Canada Power System Outage Force

  14. 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

  15. Cascading tripping: an initial line trip casues overloading on other parallel lines Source: US/Canada Power System Outage Force

  16. Effect of line trips on voltages: depressed voltage (Ohm’s Law) Source: US/Canada Power System Outage Force

  17. Source: US/Canada Power System Outage Force

  18. Source: US/Canada Power System Outage Force

  19. Speed of cascading Source: US/Canada Power System Outage Force

  20. Normal load, big margins Denmark self-sufficient, southern Sweden supplied from central/northern 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 Danish/Swedish blackout: 23/09/03, 5 M people

  21. Italy

  22. 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 isactivated • generation deficit => frequency drops, generation surplus => frequency increases Source: National Grid

  23. 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: UCTE

  24. 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: UCTE

  25. UCTE disturbance in 2006 • UCTE: Union for the Co-ordination of Transmission of Electricity – association of TSOs now renamed ENTSO-E (European Network of Transmission System Operators for Electricity) Source: UCTE

  26. 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

  27. 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

  28. 10 GW surplus 51.4 Hz 0.8 GW deficit 49.7 Hz 8.9 GW deficit 49 Hz Source: UCTE

  29. 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!

  30. Resynchronisation • A number of uncoordinated unsuccessful attempts made without knowledge of the overall UCTE situation • Full resynchronisation after 38 minutes Source: UCTE

  31. GB May 2008 event: a near miss • 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 Source: National Grid

  32. 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

  33. Generation adequacy issues • Possible problems after 2015 (Ofgem Discovery report) • Regulatory uncertainty

  34. 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)

  35. 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

  36. 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

  37. 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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