We Depend on the Grid for Much of our Energy Needs
U.S. Energy Information Administration, Annual Energy Review 2010.
All energy input US average = 90.3 MWh per person, per year.
Corresponds to 10.3 kW average energy input per person.
1 kWh = 3,412 BTU
How Much of the Total US Energy
Above Flows Through the Grid?
10.3 kW avg. input per person total
energy, multiplied by 40% of total =
4.12 kW avg. electrical energy input
per person.
4.12 kW avg. input, multiplied by 35%
avg. power plant efficiency = 1.4 kW
avg. electrical consumption per
person.
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The U.S. Has Three Power Grids
Western Grid
Eastern Grid
ERCOT Grid
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ERCOT
• 23 million customers
• 550 generating units
• 70,000 MW peak (about 3 kW
per person)
• 40,000 miles of high voltage
transmission lines
• 335 million MWhr annual
energy (about 15 MWhr per
person)
• Average annual load is 1.7 kW
per person
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Generation Mix on Typical Winter Day
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Generation Mix on Typical Summer Day
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Basics of Grid Design and Operation
• Planning number, 3 MW peak load per 1000 people
• Typical generator is about 500 MW, nuclear generator about 1000 MW
• Grid frequency is typically within 0.1 Hz of 60 Hz, and voltage is
typically within 0.90 to 1.05 per unit of nominal.
ERCOT
Western
Eastern
1-Minute Window
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Basics of Grid Design and Operation, cont.
• The grid is low loss, lightly damped for oscillations, and very efficient (about
3% net losses in transmission, and 3% in distribution)
Unit Trip 2011/05/30 03:03:00 GMT. UT Pan Am Relative to U.T. Austin.
Measured
Total Curve Fit
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Thump the grid (i.e., unit trip) and it rings
Degrees
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Second
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Basics of Grid Design and Operation, cont.
• The grid is designed so there is no load is lost and no lines/transformers are
overloaded when a contingency occurs (such as the loss of a transmission line
or generator).
• The grid has inertia, but not as much as you might think. The rotating kinetic
energy of a generator is about 6 seconds worth of rated power output.
• The sudden trip of a large generator causes the frequency to fall rapidly, until
the governor response of the other generators halts the fall. The less grid
inertia, the faster the frequency fall.
On 01/21/12, a sudden loss of generation occurred at 13:34 totaling 584
MW. Frequency declined to 59.752 HZ, ERCOT load was 31,275 MW.
60.1
60.0
59.9
59.8
59.7
0
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Minutes
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Grid Protection Must be Fast and Sure. But You Don’t Want
False Trips!
• Problems such as lightning-induced faults are isolated within 0.1 second
• Events such as faults cause generators to speed up because their power
output drops but their mechanical power input is unchanged
• 0.2 seconds is a very long time in grids, in fact it is so long that generators
can trip on overspeed and cause a blackout
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Grid Protection Must be Fast and Sure, cont.
• Relays monitor voltages and currents, and when a grid problem
occurs, relays send trip signals to circuit breakers to isolate
problems.
• Relays (i.e., computers) and their signal processing algorithms are no
better than their sensing devices. Current transformers (CTs) isolate
and scale transmission line currents down to the 0-5 Amp range.
Voltage transformers (PTs) isolate and scale grid voltages down to the
120 Volt range.
• Today, most new relays are actually computers that use advanced
signal processing techniques on voltage and current waveforms to
quickly detect and deal with grid events while at the same time
avoiding false trips
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Grid Protection Must be Fast and Sure, cont.
• Directional relays communicate with their neighbors through fiber
optics, microwave, or low-frequency power line carrier signals, much
like baseball players signal for the catch. Neighbors, one or two steps
back, act in case the player drops the ball!
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High-Voltage Grid Transformers, 100’s of MW
Normal Magnetizing Current
“Smart” Protection Uses Waveform Analysis
Amperes
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No DC
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With DC
0
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-6
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Feds cite issues behind massive Southwest blackout
(Source: www.ksl.com, May 1, 2012)
• Federal energy regulators blamed inadequate planning and lack of
grid coordination for a massive blackout last year that started with
the loss of a single transmission line at an Arizona substation and
quickly affected millions of people in Southern California and
Mexico's Baja California
• The outage knocked out traffic lights, causing gridlock on roads in
the San Diego area. Two reactors at a nuclear power plant along the
California coast went offline after losing electricity, and nearly 3.5
million gallons of sewage spilled into the ocean, closing beaches in
San Diego.
• It was another reminder that U.S. transmission lines remain
vulnerable to cascading power failures despite the safeguards that
are in place.
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• Federal regulators found if the emergency settings had allowed for a
higher level of power flows, it could have given operators time to
mitigate the problem and the outage could have affected a much
smaller area.
• Similar relay problems contributed to the 2003 blackout, said NERC
Senior Vice President.
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Blackstart – The term used for grid recovery from a widespread or
total blackout
Almost all generators require grid power to start. Few generators can start
themselves. Hydro generators are sometimes capable of self-starting, and
then energize the grid in sections so that other generators can start, and so
on.
• FERC requires grids to have a blackstart plan, i.e., reestablishing the
grid from a total blackout. ERCOT has a blackstart training school.
• Some questions from grid engineers are: Gas-fired generators have no
local fuel, and depend on pipeline network. If gas pipeline compressors
are not operational, gas-fired generators cannot start.
• Communications are essential for blackstart – will communications be
functioning after a blackout?
• Recovery from a total blackout can take 1 week. No credit cards, no gas
pumps, no ATM machines, …
• How much cash to you carry???
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