Integrating Renewables, Optimizing Transmission

Phil Carson | Aug 04, 2010

 

Economics have driven smarts into the transmission system, but challenges loom that require more smarts: siting new lines, integrating renewable energy and optimizing existing lines. We discussed the background, drivers and possible means of obtaining new lines in yesterday's column.

The public has demanded greater integration of renewable energy in many states. Greater grid complexity—and public opposition to new transmission lines—has driven the quest for optimizing the capacity of existing lines.

That's all well and good—except if you're on the front lines, attempting to solve daunting technical challenges to accomplish these goals.

Merwin Brown, director of electric grid research at the California Institute for Energy and Environment (CIEE), University of California, Sacramento, is one researcher among many making that effort. (The CIEE's work is often made possible through funding by the California Energy Commission.)

Four qualities of renewable energy make its integration into the grid a major focus of research, according to Brown: intermittency, ramp rates, over-supply and inertia. "Ramp rates" refer to the rapid on, rapid off nature of solid-state technologies used in wind and solar photovoltaic generation—fast changes in power flow that must be mitigated. Inertia refers to the tendency of central power generators' turbines to continue to spin (due to their mass) and produce power as they are shut down; the solid-state circuitry in wind and solar PV doesn't exhibit inertia.

Among technologies that soften RE's challenging characteristics, Brown said, "the holy grail is storage."

The Pacific Northwest National Laboratory, CEC, CIEE, Bonneville Power and the California Independent System Operator are working on a project (due for completion this fall) that would give multiple control areas the use of fast-acting, short-term storage technologies to address intermittency and ramp rates (though not over-supply or inertia). The project seeks to combine a limited number of flywheels, which are fast-acting but expensive, and hydro power, which is cheap but slow to ramp.

The third category drawing R&D attention, in Brown's view, is the optimization of existing transmission lines.

"The existing grid is strained as it is," Brown observed. "Now we're talking about more power flow across it. It's not just difficult to add new transmission. It's also difficult to upgrade existing transmission, if you have to go in and change out towers and wires. This is a problem regardless of whether you're talking about integrating renewables or adding conventional power plants. It doesn't matter whether it's a nuke plant, a coal plant or a solar plant. The fact is, the existing grid cannot handle much more capacity. Turn that statement around and, in California, we cannot meet our renewable portfolio goals without increasing the capacity of the existing system."

With "deterministic planning" that fed known power flows across the transmission system—with capacity estimated rather than measured—"you're leaving a lot of capacity on the table," Brown said.

A study in the Sacramento area found that increasing the capacity of one transmission line allowed a 14-fold increase in related lines, though other issues crop up, he acknowledged. Various technologies allow operators to see how close a transmission line is to its thermal limits to utilize unused capacity, so that's a promising area for advances, Brown said.

In terms of stability, it's those great white sharks again: the "poster child" for stability limits being low-frequency oscillations. "Under-damped," low-frequency oscillations caused the 1996 blackout on the Western Interconnection. This may be a greater problem in the West than the East due to long distances between power plants and the lengths of transmission lines, according to Brown.

The old solution was de-rating to a conservative capacity that left lines under-utilized. For example, a "gateway" between California and Oregon has a 7,200 MW capacity, technically, but has been de-rated to 4,800 MW due to instabilities in the system. Research focused on phasor measurement units and synchrophasor data, however, will increase the "damping ratio" and keep those instabilities from overwhelming the system.

Brown said that one lesson he learned from leading the R&D effort at Pacific Gas and Electric earlier in his career was that value and ROI had to encompass the big picture.

"Where's the value proposition?" Brown asked rhetorically. "You have to capture the full benefits of any one of these technologies to add up the value."

Phil Carson
Editor-in-chief
Intelligent Utility Daily
pcarson@energycentral.com
303-228-4757