A new approach being pioneered by Southern California Edison (SCE) may point the way for utilities to defer new generation and transmission. The concept is a new way to create and manage the remedial action schemes (RASs) that protect transmission lines.

The problem of reserve capacity

SCE supplies power to roughly 11 million people. The largest subsidiary of Edison International, it manages 16 utility interconnections, 1,200 transmission circuits spanning 12,600 miles and 4,150 distribution circuits spanning 85,000 miles.

Like all utilities, SCE must have reserve power capacity, which it can get by buying power or by maintaining surplus generation. SCE’s transmission system must also have reserve capacity. That reserve maintains stability during minor grid fluctuations. It can get that reserve by building extra transmission lines or more generation. It can also reduce the amount of extra capacity it needs through remedial action schemes (RASs).

The problems with traditional RAS solutions

A remedial action scheme detects abnormal conditions and takes automatic corrective action. Traditional RASs are localized, quick-acting algorithms individually installed at substations or control centers. They are labor-intensive because they require engineers and technicians to visit and re-visit far-flung sites to tweak the algorithms until they work right.

They are also time-intensive. In SCE’s territory, the Western Electric Coordinating Council (WECC) must approve each RAS, typically taking about a year.  After approval, SCE can implement, test, and go live – but that testing takes another six months and puts a severe strain on an already overstretched work force.

As of May 2008, SCE had 16 RASs approved, with another 64 pending. But SCE has realized it cannot test and implement another 64 RASs using the old paradigm of engineers crisscrossing the territory to make adjustments and re-adjustments.  Furthermore, such an oppressive workload creates a morale issue.

SCE’s Patricia Arons works in transmission and interconnection planning. She says traditional RASs have other problems, too. One issue is the “one-size-fits-all” dilemma. Most RASs are not fine-tuned enough to most efficiently mitigate a particular event.  Result: Utilities shed more load than they really need to. What’s more, because RASs are often localized, they only consider nearby load shedding. They fail to consider shedding more distant load, even if it would be a better solution.

SCE’s new C-RAS approach

Now SCE is applying its system engineering expertise to explore what it calls Centralized Remedial Action Schemes (C-RAS). SCE proposes to develop and manage RASs centrally, in a structured environment that integrates all the analysis, testing, and simulation. The process is built on the Unified Analytic Platform (UAP) from Systems Integration Specialists Company (SISCO).

Centralized development avoids the expense of high-paid engineers traveling long distances to work and re-work localized RASs. Based on “pre-pilot” tests, SCE believes C-RAS can improve transmission capacity ratings 5-10% while dramatically shrinking the time to create and fine-tune a new RAS.

No more wasteful load shedding

In addition, C-RAS would allow utilities to determine the optimal loads to shed, even if the loads are not in the transmission corridor experiencing the problem. Expected benefits include:

·         Simplified design and maintenance

·         Improved testing and data gathering (more data to use in improving effectiveness of RASs)

·         More precise mitigation – just-in-time and just-enough load-shedding by considering all the utility’s assets, not just those at certain locations

By reducing the surplus capacity required of reserve transmission lines, SCE may be able to defer building an 800MW generating facility.

Next Steps

WECC is now considering whether C-RAS testing is as good as traditional methods. SCE and SISCO believe they have developed intensive, overlapping test procedures for all systems and components that make C-RAS better than traditional RAS.

By taking a holistic, systems engineering approach, SCE expects the future C-RAS will provide a bird’s eye view of multiple events affecting the larger grid.  These Smart Grid developments will open the door to new automation and control philosophies that can serve as broad safety nets to protect system reliability well beyond today’s capabilities. C-RAS provides an excellent early example of the centralized, wide-area command and control that will be possible with tomorrow’s Smart Grid.   

   SCE Presentation on CRAS (pdf)        

   SISCO Home Page