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Rapidly Responding to Changing Load Curves

Rapidly Responding to Changing Load Curves
June 22
08:30 2016

Since the advent of air conditioning in the US, the electric industry has had fairly predictable shapes to load curves. While system operators and T&D design engineers had uncertainty about year-to-year load growth, the shapes of the curves was predictable. Typical curves looked like the following for a large operating region:

load curves

winter load curve 2

Predictable shapes allowed system operators to consistently plan for load variability given weather forecasts. Similar curve shapes applied to the specific load on many distribution circuits serving residential and commercial loads giving transmission and distribution engineers a predictable pattern of demand to design for.

Design for new criteria

But as penetrations of renewables increase, system operators and engineers must design for new criteria – loads minus renewable output. On the system level operators know how much traditional generation must be available and/or operating at any point. At the distribution circuit level, it tells engineers what power flows they must design the circuit to reliably carry. As you might imagine, net load curves are fundamentally changing. At the system level, the shape of the curve has changed to a curve that looks like the back of a duck:

load curves

The above graph showing expected net loads in the California ISO (CAISO) systems demonstrates just how quickly things are changing. By 2020, mid-day net loads are expected to drop from 20,000 MW to 12,000 MW during high solar hours.

When net loads are observed on specific circuits with high penetration of solar, results are even more dramatic. Here are net loads on a representative circuit in Hawaii:

average transformer load

Note that for certain high solar hours the circuit has negative net load, which means that power is back loading into the distribution substation and back into the transmission line. In other hours, the circuit has a net load greater than 5 MW. In Hawaii, with rapidly growing solar installations, this curve has been referred to as the Loch Ness Monster since loads “disappear under the water” and are not visible to the system.

In an earlier blog post, we explored how the island of Kauai is dealing with this issue. As the above load curves show, system operators and system engineers around the country will need to learn from areas like Hawaii and California that have high early penetrations of renewable energy and prepare themselves for load curve changes when significant amounts of renewables come to their systems.

by Bob Shively, Enerdynamics President and Lead Instructor


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