Stanford Summer Energy SchoolFundamentals of the Electric Grid

Kevin TomsovicCTI Professor and EECS Department Head

tomsovic@tennessee.edu

Overview – Part 1• Broad overview of power grid fundamentals

• DC vs. AC • Edison and Westinghouse (Edison may yet win)

• Three phase systems

• Power concepts

• Traditional generation technologies• Synchronism as the foundation of operation

• Load – Frequency control• Load following – frequency control

• The power system in the steady-state• Transmission system and power flows

• Reactive power/voltage and real power/phase

• Power system reliability• Security vs. adequacy

Components of the Grid

• Generation

• Transmission– 115 kVolts 765 kVolts

– Networked

• Distribution– 4 kVolts to 69 kVolts

– Radial

• Load

http://www.nerc.com/page.php?cid=1|15

DC vs. AC

• Direct current (DC)– DC machines– Batteries– Fuel cells– Photovoltaic

• Alternating current (AC)– AC machines– Power electronic converters– 60 Hertz in the US

)2sin()( ftIti

Iti )(

Edison vs. Tesla/Westinghouse

• DC– Pushed by Thomas Edison (GE)– Could not change voltage levels (no transformer) so cannot transmit over long distances– DC generator (high maintenance)– Difficult to interrupt high currents (no zero crossing)

• AC– Nikola Tesla (moved from Edison to Westinghouse)– Can efficiently change voltage levels (transformer) and so transmit over long distances (high voltage)– Induction and synchronous machines – Easier to interrupt high currents

DC actually has many advantages

Frequency and Phase• The number of “cycles” per second (Hertz)

– Zero for DC– Many options for AC

– Grid - 60 in the US, 50 in Europe, both in Japan

– Aircraft – 400, Some trains in Europe – 16.67

• Phase – relative relationship between two signals– Usually measured in degrees (easy to translate to time)

– 30 degrees or 1.4 msec

)6/2sin()(

)2sin()(

2

1

ftIti

ftIti

Phasors

• Need a simpler notation and all that matters is magnitude and relative phase if single frequency

• Define

• We properly do this using Euler’s identity

• with j an imaginary number or 90O

Iti )(

)2sin()2cos(2 ftjfte jftj

Three phase poweran extremely useful trick

• All large scale power applications

• No need for return line to carry current

0)()()(

)3/42sin()(

)3/22sin()(

)2sin()(

tititi

ftIti

ftIti

ftIti

cba

c

b

a

Electric power

• By definition

• Average power is what does useful work

• P often called real power

)()()( titvtp

)6/2sin()(

)2sin()(

ftVtv

ftIti

T

dttpT

P0

)(1

Three phase electric poweranother major benefit

• Assume balanced in all three phases

• Constant power output – far more efficient

)()()()( tptptptp cba

Apparent Power Scalculation using phasors

• By definition

P – real part of S

Q – imaginary part of S, reactive power

jQPVIS *

S

I

V

P

Q

What is reactive power?And why should we worry about it?

• The part of p(t) which does no work on average (but it may be needed to get work done)

• Analogies– Pressure in a water hose

– Foam on the beer (just takes up room in glass)

• Physically– Primarily line charging (magnetic fields) associated with

transmission lines and motor windings

• Practically– Needed to maintain voltage for long distance transmission and to

supply induction machines

P (Watts – you pay for this)

Energy ConversionThree phase synchronous Machines

• DC supplied to rotor which is driven at some constant speed of rotation (say 3600 RPM for two pole machine resulting in 60 Hz)

• Three phase windings spaced by 120 degrees• Power is produced only at this frequency (else p(t)=0)• Relative angle between fields determines real power output

N S

ROTOR

STATOR

a a'

b

b'c

c'

Steam Turbine/Generator Hydro Units at Coulee

Generator mix80% Thermal (nuclear, coal, gas, etc.)

20% Hydro

Essentially all synchronous

Since most generation is from synchronous machines, the interconnected power system swings together.

Synchronism

Frequency

• To maintain frequency, load and generation (minus losses) must balance• An increase in load decreases frequency so generators respond to frequency dip

by increasing output• Coordination from control centers results in a simple but very effective means of

load following

• Load frequency control• Inputs – scheduled and actual tie line flows (difference is area control errors), frequency

deviation (also frequency response characteristic)• Output – generator set point adjustments around once every 4 seconds

North American Control Areas

Frequency Monitoring(FNET – Yilu Liu, UT)

Troy, NY

Holyoke, MA

Danbury, CT

Toronto, Canada

Vienna, VA

Roanoke, VA

Blacksburg, VA

Winnipeg, Canada

Alberta, Canada

Duluth, MN

Grand Rapids, MI

Tiffin, OH

Raleigh, NC

Seattle, WA

Pullman, WA

Pasadena, CA

Tempe, AZ

Sugar Land, TXSan Antonio, TX

Bismarck, ND

St. Paul, MN

Ames, IA

Rolla, MOLexington, KY

Hopkinsville, KY

Gainesville, FL

Starkville, MS

Orland Park, IL

Carmel, IN

Chattanooga, TN

Nashville, TNJackson, TN

Knoxville, TN

Huntsville, ALTupelo, MSMuscle Shoals, AL

Tallahassee, FL

Detroit, MI

Morgantown, WV

Le Roy, NY

College Park, PA

Simpsonville, SC

Princeton, NJ

Cookeville, TN

NewPortNews, VALouisville, KY

Birmingham, AL

Oak Ridge, TN

Atlanta, GA

Pensacola, FLGulfport, MS

New Orleans, LA

Cleveland, OH

Cincinnati, OH

Philadelphia, PA

Wichita, KS

Pittsburgh, PA

Oklahoma City, OK

Omaha, NE

Lincoln, NE

Shreveport, LA

Charleston, SC

Springfield, IL

Dallas, TXEl Paso, TX

Salt Lake City, UT

Denver, CO

Colorado Springs, CO

Las Vegas, NV

San Diego, CA

Portland, OR

Vancouver, Canada

Montreal, Canada

Chillicothe, OH

Norristown, PA

Palo Alto, CA

Idaho Falls, ID

Bangor, ME

Gillam, Canada

Plant City, FL

Lees Summit, MO

Montgomery, AL

Greensboro, NC

Boise, ID

Augusta, ME

New York, NY

Madison, WI

Disturbance location

Effect FDR in the case

Effect FDR in the mode

0.23~0.27Hz

Frequency Monitoring(FNET – Yilu Liu, UT)

Frequency EventNigeria – Shows system dependence

Summary Comments and Opinions

• Electricity grid is central to solving energy problems• Wind has perhaps the greatest potential – difficulty

of variability may have been overstated by media and utilities

• Appropriate control methods need to be developed with greater demand side response and new storage

• Shifting of greater load to grid has benefits both for reduced emissions and for easier control

References

• A few useful websiteshttp://tcip.mste.uiuc.edu/applet1.htmlhttp://tcip.mste.uiuc.edu/applet2.html

http://www.eia.doe.gov/

• Some general introductory power textsBergen and Vittal, Power Systems Analysis, Prentice Hall, 2000.

El-Sharkawi, Electric Energy: An Introduction, CRC Press, 2005.

•Energy Information Administration, Electric Power Annual, online at: http://www.eia.doe.gov/fuelelectric.html. •F. Giraud and Z.M. Salemeh, “Steady-state performance of a grid-connected rooftop hybridwind-photovoltaic power system with battery storage,” IEEE Transactions on Energy Conversion, Vol. 16, No. 1, pp. 1-7. •C.W. Gellings, M. Samotyj, and B. Howe, “The future's smart delivery system ,” IEEE Power and Energy Magazine, Vol. 2, No. 5, Sept.-Oct. 2004, pp. 40-48. •K. Tomsovic and M. Venkatasubramanian, “Power System Operation and Control,” Electrical Engineering Handbook, Elsevier Academic Press, 2005, pp. 761-778. •K. Tomsovic, D. Bakken, V. Venkatasubramanian and A. Bose, “Designing the Next Generation of Real-Time Control, Communication and Computations for Large Power Systems,” Proceedings of the IEEE, Vol. 93, No. 5, May 2005, pp. 965- 979. •H. Sira-Ramirez, S.K. Agrawal, Differentially Flat Systems, CRC, 2004. •V.N. Chetverikov, “Controllability of Flat Systems,” Differential Equations, Vol. 43, No. 11, 2007, pp. 1558-1568.• X. Yu and K. Tomsovic, “Application of Linear Matrix Inequalities for Load Frequency Control with Communication Delays,”  IEEE Transactions on Power Systems, Vol. 19, No. 3, Aug. 2004, pp. 1508-1515.•D. Logue, P. T. Krein, “The power buffer concept for utility load decoupling,” Proc. IEEE Power Electronics Specialists Conference, 2000, pp. 973-978.•R. S. Balog, W. W. Weaver, P. T. Krein, “The load as an energy asset in a distributed architecture,” Proc. IEEE Electric Ship Technologies Symposium, 2005, pp. 261-267.•H. Qi, W. Zhang, L. M. Tolbert, "A Resilient Real-Time Agent-Based System for a Reconfigurable Power Grid," International Conference on Intelligent Systems Application to Power Systems, November 6-10, 2005, Arlington, Virginia. •J. Sun, “AC Power Electronic Systems: Stability and Power Quality,” Proceedings of IEEE 2008 COMPEL (Control and Modeling for Power Electronics) Workshop, August 2008, Zurich, Switzerland.•R. Mookherjee, B.F. Hobbs, T. Friesz and M.A. Rigdon, “Dynamic Oligopolistic Competition on an Electric Power Network with Ramping Costs and Joint Sales Constraints,” Journal of Industrial and Management Optimization, Vol. 4, No. 3, Aug. 2008. •D. Shawhan, D. Mitarotonda, and R. Zimmerman. "A regional incentive-based carbon dioxide emission regulation in the power sector: Impacts predicted using an alternating-current model." Presented at Agricultural and Applied Economics Association annual meeting, July 28, 2008. Available from shawhd@rpi.edu. •C. Taylor, Power System Voltage Stability, IEEE Press, 1993. •B.A. Renz, A.J.F Keri, A.S. Mehraban, J.P. Kessinger, C.D. Schauder, L. Gyugyi, L.J. Kovalsky and A.-A. Edris, “World’s First Unified Power Controller on the AEP System,” CIGRE Meeting, Paper 14-107, Paris, 1998.•B. Fardanesh, B. Shperling, E. Uzunovic, and S. Zelingher, “Multi-Converter FACTS Devices: the Generalized Unified Power Flow Controller (GUPFC),” Proceedings of the 2000 IEEE PES Winter Power Meeting, Vol. 2, pp. 1020-1025, July 2000. •E. V. Larsen and J. H. Chow, “SVC Control Design Concepts for System Dynamic Performance,” in IEEE Power Engineering Society Publication 87TH0187-5-PWR “Application of Static Var Systems for System Dynamic Performance,” 1987.•E. V. Larsen, J. J. Sanchez-Gasca, and Joe H. Chow, “Concepts for Design of FACTS Controllers to Damp Power Swings,” IEEE Transactions on Power Systems, Vol. 10, pp. 948-956, 1995.

Reference List

•N. G. Hingorani and L. Gyugyi, Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems, IEEE Press, 2000. •X. Wei, J. H. Chow, B. Fardanesh, and A.-A. Edris, “A Common Modeling Framework of Voltage-Sourced Converters for Loadflow, Sensitivity, and Dispatch Analysis,” IEEE Transactions on Power Systems, vol. 19, pp. 934-941, May 2004.•X. Jiang, X. Fang, J. H. Chow, A.-A. Edris, E. Uzunovic, M. Parisi, and L. Hopkins, “A Novel Approach for Modeling Voltage-Sourced Converter Based FACTS Controllers,” IEEE Transactions on Power Delivery, vol. 23, no. 4, pp. 2591-2598, Oct. 2008. •W. D. Jones, “Blackout a Turn-on for Long Island Cable,” IEEE Spectrum, Oct. 2003. •X. Fang, Rated-Capacity Dispatch, Sensitivity Analysis, and Controller Design of VSC-Based FACTS Controllers, PhD dissertation, Rensselaer Polytechnic Institute, Troy, NY, March 2008.•X. Jiang, J. H. Chow, A-A. Edris, B. Fardanesh, and E. Uzunovic, “Transfer Path Stability Enhancement by Voltage-Sourced Converter-Based FACTS Controllers,” submitted to IEEE Trans. on Power Delivery.•J. H. Chow, R. de Mello, and K.-W. Cheung, “Reliability in Deregulated Power Systems,” Proceedings of IEEE, vol. 93, no. 11, pp. 1956-1969, November 2005. •J. F. Hauer, D. J. Trudnowski, G. J. Rogers, W. A. Mittelstadt, W. H. Litzenberger, and J. M. Johnson, “Keeping an Eye on Power System Dynamics,” IEEE Computer Applications in Power, pp. 50-54, October 1997.•J. E. Dagle, "North American SynchroPhasor Initiative," in Proceedings of the 41st Annual Hawaii International Conference on System Sciences (HICSS 2008), pp. 165-168, 2008.•J. H. Chow, et al, “Preliminary Synchronized Phasor Data Analysis of Disturbance Events in the US Eastern Interconnection,” to be presented at IEEE PES Power System Conference and Exposition, Seattle, March 2009. •J. H. Chow, A. Chakrabortty, L. Vanfretti, and M. Arcak, “Estimation of Radial Power System Transfer Path Dynamic Parameters using Synchronized Phasor Data,” IEEE Transactions on Power Systems, vol. 23, no. 2, pp. 564-571, May 2008. •L. Zhao and A. Abur, “Multiarea State Estimation using Synchronized Phasor Measurements,” IEEE Transactions on Power Systems, vol. 20, no. 2, pp. 611-617, May 2005. •M. Parniani, J. H. Chow, L. Vanfretti, B. Bhargava, and A. Salazar, “Voltage Stability Analysis of a Multiple-Infeed Load Center Using Phasor Measurement Data,” Proceedings of IEEE Power System Conference and Exposition, October 2006.• G. E. Boukarim, S. Wang, J. H. Chow, G. N. Taranto, and N. Martins, "A Comparison of Classical, Robust, and Decentralized Control Designs for Multiple Power System Stabilizers," IEEE Trans. Power System, vol. 15, no. 4, pp. 1287-1292, Nov. 2000. •R. Piwko et al, The Effects of Integrating Wind Power on Transmission System Planning, Realiability and Operation, Report prepared for the New York State Energy Research and Development Authority, 2004. Available from http://www.nyserda.org/publications/wind_integration_report.pdf. •L. Freeman, Western Wind and Solar Integration Study: Statistical Analysis, Presentations for NREL Stakeholder Meeting 8-14-08. Available fromhttp://www.nyserda.org/publications/wind_integration_report.pdf. •PSCAD Transient Simulation Software by Manitoba HVDC Research Center. Available from http://www.pqsoft.com/pscad/index.htm. •Y. Xu, F. Li, J. D. Kueck, and D. Tom Rizy, “Experiment and Simulation of Dynamic Voltage Regulation with Multiple Distributed Energy Resources,” IREP Symposium 2007 - Bulk Power System Dynamics and Control, Charleston, SC, August 2007.•Y. Xu, D. Tom Rizy, F. Li, and J. D. Kueck, "Dynamic Voltage Regulation Using Distributed Energy Resources," Proceedings of CIRED 2007, Vienna, Austria, May 20-24, 2007.

Reference List