Alleviating constraints on California’s Path 15: A more immediate solution than building power lines
Thanks to progressive utilities like Pacific Gas & Electric (PG&E) and the PIER projects of California Energy Commission, real time ratings are already providing “virtual power lines” in California - in the sense that optimizing the use of current power transmission lines to their actual capacities is mitigating and delaying the need for new lines. This is no hocus pocus. Real time ratings are simply a matter of accurately measuring key parameters of overhead lines that directly affect power transfer capacity, simply a matter of going from often conservative seasonal estimates to knowing the actual state of the lines at all times. Clearly, new lines may not be necessary if existing power lines can safely and reliably be run at a 10-30% higher rating.
During high load conditions, California’s transmission grid can become severely constrained. These constraints prevent moving electric power between different regions and have been causes for significant price differences within the state. Similarly, preventing the optimal use of generation facilities causes overall electricity price increases in the market. Finally, transmission constraints can make application of environmentally friendly generation sources less attractive. For example, wind generation often cannot reach distant consumption centers due to bottlenecks.
In the past decade there has been essentially no transmission line construction in California, while the load of the system has continued to increase. While several new transmission expansion projects are now in various planning stages, the process of line planning, permitting, and construction typically takes several years. Thus, the existing grid will be operating under increasing loads and increasing stress for the foreseeable future.
A highly stressed part of California’s transmission system is PG&E’s Path 15, consisting of two 500 kV lines and four 230 kV lines south of Fresno. This path limits the power transfer from southern to northern California to a maximum of 3,400 MW. This causes a major problem in the winter, when loads are high in Northern California. Thus, not surprisingly, in 4Q2000, the cost of Path 15 constraints exceeded $150 million.
Path 15 presents a very complicated rating problem, because of its sensitive role in California’s network. Reliability considerations require that the limiting conductor temperatures are not violated even if the most critical circuits of the path are lost. Thus, system operators need to have a forecast of what the line temperatures would be during the first ten minutes following a transmission outage and they need software to allow calculation of the maximum allowed power transfer considering such a limitation.
Through the Energy Commission’s PIER program, sophisticated rating software specific to Path 15 was developed by The Valley Group, Inc. in cooperation with Power Delivery Consultants, Inc., Niskayuna Power Consultants, BEST Systems, Inc., and Stanfield Systems, Inc., to reconcile line rating information with grid complexities and procedural requirements. Its real time rating inputs are based on existing TVG CAT-1TM systems at PG&E, with data transmitted to PG&E’s substations and control center in San Francisco. Related algorithms at the control center calculate the safe load levels of each line. Because of the critical nature of Path 15, the path rating software will be extensively tested by PG&E, CAISO, and the developers before going into full operational use. The initial results have been encouraging and indicate that additional path transfer capability will be available most of the time.
A related PIER software project, to increase the real time loading capability of the next limiting element, the 500/230 kV transformer at Gates substation, will likely further increase Path 15 capabilities. For more information, see “Dynamic Ratings of California’s Path 15” at Reference/IEEE at www.cat-1.com - presented at the 2002 IEEE Summer Power Meeting.