Transcript Slide 1

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
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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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