ph: (865) 576-8401, fax: (865) 576-5728 email: reports@adonis.osti.gov Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161 ph: (800) 553-6847, fax: (703) 605-6900 email: orders@ntis.fedworld.gov online ordering: http://www.ntis.gov/ordering.htm This document was printed on recycled paper. ABSTRACT Interacting wind, solar, and load forecast errors can create significant unpredictable impacts on the

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... ission system, congestion, voltage and reactive power stability margins. These impacts will be increasing with the increasing penetration levels of variable renewable generation in the Western Interconnection and California power systems. To maintain a secure system reliability level, the probability and the magnitude of the impacts should be evaluated and communicated to the system operators. Based on this information, the system power flow limits, dispatches, voltage levels, and available reactive power margins could be adjusted to minimize the risk of system problems and failures to an acceptable level whenever it is required. To facilitate wider penetration of renewable resources without compromising system reliability, two tools intended for use by California Independent System Operator (CAISO) power grid operators have been developed for predicting and displaying the operational impacts of uncertainties in forecasts of loads and renewable generation. The first tool (so-called "ramping tool") addresses real-time (load following) capacity and ramping requirements, and the second ("transmission tool") addresses voltage, stability and transmission congestion caused by renewables resources (mainly wind power and solar power generation). The first tool is an industry-grade product connected to the California ISO systems and operated in real time in the California ISO Control Center. This tool is described in a separate report prepared and submitted by Pacific Northwest National Laboratory (PNNL). Based on a contract with the California Energy Commission (CEC) and California Institute for Energy and Environment (CIEE), PNNL is developing a second tool for predicting transmission uncertainties and related impacts ("transmission tool" -TT). The present report addresses this second tool -"transmission tool". vii