Transcript Fundamentals of the Electric Grid

Stanford Summer Energy School
Fundamentals of the Electric Grid
Kevin Tomsovic
CTI Professor and EECS Department Head
[email protected]
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
i(t )  I
• Alternating current (AC)
– AC machines
– Power electronic converters
– 60 Hertz in the US
i(t )  I sin(2ft)
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)
i1 (t )  I sin(2ft )
i2 (t )  I sin(2ft   / 6)
– 30 degrees or 1.4 msec
Phasors
• Need a simpler notation and all that matters
is magnitude and relative phase if single
frequency

i(t )  I
• Define
• We properly do this using Euler’s identity
e j 2ft j  cos(2ft   )  j sin(2ft   )
• with j an imaginary number or 90O
Three phase power
an extremely useful trick
• All large scale power applications
ia (t )  I sin(2ft )
ib (t )  I sin(2ft  2 / 3)
ic (t )  I sin(2ft  4 / 3)
ia (t )  ib (t )  ic (t )  0
• No need for return line to carry current
Electric power
• By definition
p(t )  v(t )i(t )
i(t )  I sin(2ft )
v(t )  V sin(2ft   / 6)
• Average power is what does useful work
T
1
P   p(t )dt
T0
• P often called real power
Three phase electric power
another major benefit
• Assume balanced in all three phases
p(t )  pa (t )  pb (t )  pc (t )
• Constant power output – far more efficient
Apparent Power S
calculation using phasors
• By definition
S  VI *  P  jQ
P – real part of S
Q – imaginary part of S, reactive power
S
Q
V
I
P
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 Conversion
Three 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
STATOR
b'
c
ROTOR
a
N
c'
S
a'
b
Generator mix
80% Thermal (nuclear, coal, gas,
etc.)
20% Hydro
Essentially all synchronous
Steam Turbine/Generator
Hydro Units at Coulee
Synchronism
Since most generation is from synchronous machines, the
interconnected power system swings together.
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)
Gillam,
Canada
Alberta,
Canada
Vancouver,
Canada
0.23~0.27Hz
Winnipeg,
Canada
Seattle, WA
Pullman, WA
Portland, OR
Bismarck, ND
Montreal,
Canada
Duluth, MN
Toronto,
Canada
Boise, ID
Idaho Falls, ID
St. Paul, MN
Bangor, ME
Augusta, ME
Troy, NY
Le Roy, NY
Holyoke, MA
Madison, WI Grand Rapids, MI
Detroit, MI
Ames, IA
Salt Lake City, UT
Omaha, NE
Palo Alto, CA
Lincoln, NE
Denver, CO
Colorado Springs, CO
Pasadena, CA
Oklahoma City, OK
Tempe, AZ
El Paso, TX
Tiffin, OH
Lees Summit, MO
Rolla, MO
Wichita, KS
Las Vegas, NV
San Diego, CA
Danbury, CT
New York, NY
Cleveland, OH College Park, PA
Norristown, PA
Princeton,
NJ
Pittsburgh, PA
Philadelphia, PA
Carmel, IN
Morgantown, WVVienna, VA
Springfield, IL
Chillicothe, OH
Orland Park, IL
Cincinnati, OH
NewPortNews, VA
Louisville, KY
Lexington, KY Roanoke, VA
Blacksburg, VA
Hopkinsville, KY
Greensboro, NC
Oak Ridge, TN
Raleigh, NC
Nashville, TN
Knoxville, TN
Jackson, TN
Cookeville, TN
Chattanooga, TN
Simpsonville, SC
Muscle Shoals, AL
Huntsville, AL
Tupelo, MS
Birmingham, AL Atlanta, GA Charleston, SC
Dallas, TX
Montgomery, AL
Shreveport, LA Starkville, MS
Pensacola, FL
Gulfport, MS
Sugar Land, TX
San Antonio, TX
Disturbance location
Effect FDR in the case
Effect FDR in the mode
New Orleans, LA
Tallahassee, FL
Gainesville, FL
Plant City, FL
Frequency Monitoring
(FNET – Yilu Liu, UT)
Frequency Event
Nigeria – 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 websites
http://tcip.mste.uiuc.edu/applet1.html
http://tcip.mste.uiuc.edu/applet2.html
http://www.eia.doe.gov/
• Some general introductory power texts
Bergen and Vittal, Power Systems Analysis, Prentice Hall, 2000.
El-Sharkawi, Electric Energy: An Introduction, CRC Press, 2005.
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