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Stability and Security of Power Networks G. T. Heydt Arizona State University

Stability and Security of Power Networks G. T. Heydt Arizona State University ECEDHA 2004 Annual Meeting. March, 2004 Orlando, Florida. Outline. Stability and security: a general discussion Weaknesses and strengths of the North American grid Some theoretical considerations

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Stability and Security of Power Networks G. T. Heydt Arizona State University

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  1. Stability and Security of Power Networks G. T. Heydt Arizona State University ECEDHA 2004 Annual Meeting March, 2004 Orlando, Florida

  2. Outline • Stability and security: a general discussion • Weaknesses and strengths of the North American grid • Some theoretical considerations • Solutions: short range and long range • Propaganda: power engineering education • Conclusions

  3. Stability Power system stability basically refers to the ability of operating an AC network with all generators in synchronism, retaining synchronism even after a large disturbance

  4. Stability • Each synchronous generator has a ‘Newton’s law’ second order nonlinear differential equation that describes the machine angle – and control systems (e.g., power system stabilizers) also contribute a higher order nonlinear controller to the dynamics • A large interconnection (WECC, e.g.) may have about 200 generators + 150 PSSs = about 1000 to 10000 order nonlinear differential equations

  5. Stability The basic analysis technique is state space analysis / eigenvalues for the linearized system, or simulation for the nonlinear system. Typically, the dimension is very high – in the 1000 – 10,000 range. The interconnection is modeled as Ibus = Ybus Vbus which is reduced to eliminate the non-dynamic nodes (i.e., remove the non-generation nodes).

  6. Power system stabilizers A PSS is a controller that uses (usually local) measurements to provide a signal to one generator so that damping torque is produced by the machine field winding. The basic concept is that a linear controller is used with standard feedback control technology to place the poles of the linearized system solidly in the LHP. Virtually all large generating units in North America are fitted with PSSs.

  7. Power system stabilizers The main weaknesses of this approach are that the nonlinear system may respond poorly, and also dynamics external to the generator + PSS are not modeled (nor included in the measurements). Therefore modes that result from inter area dynamics may not be damped. x x x x x x x x x

  8. By injecting the appropriate signals from distant measurements in the system, transmitted through LEOS, the controller is able to obtain superior performance in terms of damping interarea oscillations compared to use of conventional local signals. The main concept is to use interarea signals for interarea controls Wide area robust power system stability control Low Earth Orbit Satellites LEOS REGIONAL MEASUREMENTS LOCAL SPSS MEASUREMENTS

  9. Hierarchical robust power system controller • Execution Level Signal pre-processor Actuator / Distributor • Operation Level System modal identifier • SPSS damping loop • Management Level Fuzzy logic based parameter tuner Management Level Operation Level Execution Level Control Input Data Power System

  10. Voltage Regulator With PSS and SPSS Remote Signals SPSS Vt PSS ,f, or Pa + Generator Excitation System  Ref Voltage Regulator + Gen - Generator Field

  11. Key issues • Full scale nonlinear solution (transient stability study) • Eigenvalues of the linearized system near the operating point (small signal stability) • Line and component ratings • Voltage ratings (maximum and minimum) • Coherency - groups of generators swinging together • Synchronizing torque, PSSs • Acceptable operating conditions (including operation within about 50 mHz of 60 Hz)

  12. Security Market Network Communication systems Internal Sources Information & decisions Natural calamities Intentional human acts External Sources Security refers to the ability of the system to respond only to intended operator commands, blocking all unintended operations

  13. Electric power system is vulnerable to failure due to Natural disasters Deliberate attack Equipment failures Operator error Accidents Tree-related events High load periods Software failures

  14. Monitoring of electric power networks Reduce Vulnerability Underground Transmission Lines Substations Advanced PMU Transformers EMS Sensor Systems Overhead Transmission Lines

  15. Energy management systems Archiving Sensory information E M S Generator controls Command and control Operator interaction State estimator

  16. Network vulnerability reduction through virtual sensor utilization No Data! Network Data Lost EMS EMS EMS Virtual Data Virtual Sensor Present

  17. Tradeoffs betweenvirtual and physical sensors $ $ $ $ $ $ $ $ $ $ Low Cost Less Accurate High Cost Greater Accuracy Z = [H] X V I Physical Sensors Virtual Sensors

  18. What is needed to enhance both security and stability • Ability to acquire and interpret extensive real-time information from diverse sources, ranging from sensors to satellites. Sensory data used in Hx = z state estimators to enhance system performance. • Ability to quickly evaluate system vulnerability with respect to catastrophic events in a market environment involving competing, self-serving agents • Ability to adapt protective device performance based on system-wide and external system assessment • Ability to reconfigure the power network to minimize system vulnerability • Ability to develop system restoration plans to minimize the impact of disruption

  19. Strategic Power Infrastructure Defense System

  20. Communication system for strategic power infrastructure defense Time synchronization (GPS) / Self healing / Info. Exchange (LEO) GPS or LEO satellite communication Internet based communication channel Internet based or more direct and faster communication channel Strategic power infrastructure main system Intranet Ethernet or model based network is used in the Intranet. Each Intranet can have a “gateway” that handles IP addresses in the Intranet Internet or any other communication channel for a number of Intranets

  21. The North American grid NERC: policies, rules, reliability, plans, synchronous interconnections

  22. North American Electric Reliability Council • Sets standards for the reliable operation and planning • Monitors, assesses and enforces compliance with standards • Provides education and training • Assesses, analyzes and reports on bulk electric system adequacy • Coordinates with Regional Reliability Councils • Coordinates the provision of applications, data and services • Certifies reliability service organizations and personnel • Coordinates critical infrastructure protection • Enables the reliable operation by facilitating information exchange and coordination among reliability service organizations • Administers procedures for appeals and conflict resolution

  23. Weaknesses and strengths of the North American grid • Basic transmission design is over 40 years old. Some basic distribution circuits are over 60 years old. • Never designed to handle high levels of bulk power • Both transmission and generation constrained • The impact of market driven exchange of power has stressed the transmission grid • The transition to market based infrastructure has stressed the newly created control entities (e.g., ISOs) – in an industry that is rapidly loosing corporate memory

  24. The Northeast blackout of 2003 Time 8/14/2003 4:09:57 PM EDT:  The first significant events were initially recorded in Michigan and Ohio

  25. The Northeast blackout of 2003 Time: 8/14/03 04:10:39 PM EDT: The disturbance was then recorded all over Michigan , Ohio , and the city of Buffalo, NY

  26. The Northeast blackout of 2003 Time: 8/14/03 04:10:58 PM EDT: 19 seconds later, the disturbance had propagated to the eastern seaboard.

  27. The Northeast blackout of 2003 Main causes • Failure of state estimator in MISO to model ‘external’ system changes • Combination of heavy power exchanges, high reactive power flows, planned outages of transmission circuits and planned outage of a main generating facility (none of which are unusual) • Operator error / training of MISO operators / imprudent operation of an Ohio utility (generation outages) • Unplanned unit and line outages

  28. The Northeast blackout of 2003

  29. The Northeast blackout of 2003

  30. Generation building boom of the past 200 180 160 140 120 GW Installed in Five Year Period 100 80 60 40 20 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Coal Oil Gas Nuclear Other

  31. A hindsight view of the past building boom 29.41 19.23 17.93 11.93 11.69 Generation Building Boom Follows the Baby Boom Labor Force Entry 35 200 180 30 160 25 140 120 20 Percent Change in Labor Force 100 GW Installed in Five Year Period 15 80 60 10 40 5 20 0 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Coal Oil Gas Nuclear Other

  32. Generation building boom of the future 1400 Total System Generation Capacity 1200 1000 By 2020, the U.S. will need 1300 new power plants at 300 MW each GW 800 600 Cumulative Additions 400 200 0 2000 2005 2010 2015 2020 2025 2030

  33. Employment at major IOU’s

  34. TRANSMISSION DISTRIBUTION

  35. The N9s problem • Electric power quality • Extreme bus voltage reliability, for example 'five nines' (i.e., 0.99999 availability), or six nines or even higher • Utilization of new transmission and distribution technologies for improvement of reliability • Utilization of distributed energy sources (DERs) to improve reliability • Working with manufacturers of information technology equipment to reduce load vulnerability

  36. 24/7 UTILIZATION OF POWER SYSTEM ULTRA HIGH RELIABILITY INFORMATION PROCESSING, FINANCIAL SERVICES, AIRLINES, POLICE, MILITARY

  37. Reliability enhancement • Distributed rather than concentrated loads • Loop circuits for distribution systems • Information Technology and sensitive manufacturing loads • Independence of energy sources • Environmental issues

  38. AS A RESPONSE TO THE 1993 TERRORIST BOMBING OF THE WTC, THE PRIMARY DISTRIBUTION SYSTEM IN THE BUILDING WAS IMPROVED TO KEEP THE POWER ON IN THE CASE OF SEVERE DISRUPTION OF THE SUPPLY / INTERRUPTION OF THE IN-BUILDING PRIMARY DISTRIBUTION. THERE WERE TEN SUBSTATIONS IN EACH WTC TOWER, ON FLOORS 7, 41, 75, AND 108, AND THE SOUTH TOWER HAD AN ADDITIONAL TENANT OWNED DOUBLY FED SUBSTATION ON FLOOR 43

  39. THE USE OF MULTIPLE FEEDS, MULTIPLE SUBSTATIONS, AND ISOLATED POWER SUPPLIES KEPT THE POWER ON IN MOST OF THE WTC FOR 102 MINUTES AFTER THE INITIAL STRIKE. IT IS BELIEVED THAT THIS WAS THE MAIN FACTOR IN SAVING THE LIVES OF AS MANY AS 18,000 PEOPLE WHO ESCAPED FROM THE TOWERS BEFORE COLLAPSE

  40. Independence of sources LOAD 1-P = (1-P1)(1-P2) TWO FEEDERS RELIABLE LOAD BUS The dependence of the sources will result in a much higher outage rate than (1-P1)(1-P2)

  41. Modeling dependence of sources The dependence effect of multiple sources can be modeled using a difference equation of the form qn+1 = Cqn+(1-C)(q1)1/n qn whereqn = 1-pn = outage rate of circuit upon addition of nth feeder, C is a correlation coefficient The(q1)1/nterm is called a discounting term and it accounts for increased potential for dependence for cases of large n (large numbers of feeders)

  42. Discounted model C = 0 indicates no correlation between multiple feeders C = 1 indicates the feeder outages among several feeders are ‘common mode’

  43. Reliability of multiple feeds The addition of feeders to improve reliability has a diminishing effect. For practical cases, use of more than three ‘independent’ feeders of 100% capacity is counter- productive.

  44. Probabilities of uncommon events COMMON (?) Event_______ Loosing at roulette in Las Vegas – bet on 00 Loosing the PowerBall lottery FAA design criteria for aircraft LIFE Probability, N 97.368, 1.6 99.99995, 6.3 0.999999999 0.999999999999, 9 to 12 POWER SYSTEM RELIABILITY Reliability N Outage time 99.9 3 8h 45 min / yr 99.998631 4.9 1 day / 200 yrs 99.999 5 5 min 15 s / yr 99.99999 7 3.2 s / yr 99.999999 8 18.9 cycles / yr 99.9999999 9 1.8 cycles / yr

  45. Solutions: short range • Distributed generation • Added small generation units at all levels • Conservation / electronic control of loads • Investment in distribution systems • Sharp increase in research in both transmission and distribution engineering • Recruiting of students to the power area at all levels • Improvement of software tools

  46. PHOSPHORIC ACID 250 kVA FUEL CELL PROTON EXCHANGE MEMBRANE FUEL CELL - 7.5 kVA

  47. Microturbines • Low capacity, high speed units with electronic interface with 60 Hz bus • Alternative fuel sources (e.g., biogas, gasifier, pyrolysis, fuels that have less than 10% of heat content compared to fossil fuels) • Catalytic combustor to reduce nitrous oxide production • Heat recovery • Lower capacities -- e.g., 5 - 300 kVA • High efficiency small units • New IEEE standard requires disconnection from the distribution system within a few cycles during low voltage or outage events

  48. Solutions: long term • Added generation in larger units • Local solutions for high reliability requirements • Added capacity in distribution systems • Adaptive islanding of interconnected systems • Coordinate national energy policy with system realities

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