Peak Demand And The Arizona Power Grid 101

By Nicholas Gerbis
Published: Thursday, August 20, 2020 - 6:06pm
Updated: Friday, August 21, 2020 - 7:53am

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Palo Verde nuclear generating station
Nicholas Gerbis/KJZZ
A containment building (rounded top, center) and surrounding support buildings at Palo Verde Nuclear Generating Station.

Temperatures in Arizona are so high the weather maps are running out of colors, and the Salt River Project recently lost a key Phoenix transmission line to the Salt Fire.

It's no surprise, then, that utility companies are urging customers to back off on peak energy consumption. But how does the grid make such adjustments?

Arizona generates electricity via a mix of nuclear, hydroelectric, coal and natural gas, plus a smattering of solar.

Palo Verde Nuclear Generating Station provides the lion's share as base load, and rarely cycles its output up or down.

Some natural gas and coal plants lie dormant unless needed to meet extra demand, although they operate under different constraints, as Vijay Vittal, an engineering professor at Arizona State University and expert in power systems, explained.

"You cannot simply shut down a coal plant and start it whenever you need it, because there are certain thermal characteristics. You have to give a certain cool-down period," Vittal said.

A natural gas plant offers more flexibility.

"You can start it very fast, and you can also shut down very fast. And that is essentially what they use quite a bit for maneuvering when generation is required immediately, or when you want to lower generation," Vittal said. 

To ensure the system produces as much energy as required — no more, no less — providers use special prediction systems and trade electricity with neighboring states. Trade with California requires submitting bids 24 hours in advance to its energy market.

Unlike some areas, Arizona does not have an electricity market.

"We have primarily vertically integrated utilities. That means they own generation, transmission and distribution," said Vittal.

Although transformers can step up voltage for transmission over long distances, the alternating current must maintain a constant frequency of 60 Hertz.

"You can take the analogy of a set of horses driving a chariot. If they are not in sync, the chariot really won't move," said Vittal.

A variety of built-in systems ensure power generation remains within safety margins, whether during Arizona's peak energy demand in July and August, or throughout the winter-to-spring lull utilities use for maintenance.

But some things — like wildfires and pandemics — lie somewhat out of their control.

Vittal said heat from summer wildfires can cause transmission lines to sag and conductors to snap.

"If they sag too much then, at the points at which they are strung — the two end points — tension increases and the stress there becomes too much," he said.

Although the COVID-19 pandemic has increased residential power use, reduced draw from the largest users, industrial and commercial customers, has somewhat offset the shift.

"The residential loads, rather than looking like weekday loads, now look more like weekend residential loads," said Vittal.

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