Understanding and Maintaining the Power Grid Reliability

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.

2. The U.S. Has Three Power Grids Western Grid Eastern Grid ERCOT Grid

4. 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

5. Generation Mix on Typical Winter Day

6. Generation Mix on Typical Summer Day

8. ERCOT Eastern Western 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. 1-Minute Window

9. 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) Thump the grid (i.e., unit trip) and it rings

10. 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.

11. 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

12. 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

13. 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!