NREL prints on paper that contains recycled content. iii 2 Develop Study Databases and Establish Initial Conditions This section contains definitions and context for the study. Specifically, Section 2.1 discusses the selection and improvement of the study cases. Refinement and documentation of the study cases are reported in Section 2.2. Performance and monitoring metrics are described in Section 2.3. The development of the Hi-Mix (33% annual energy penetration from wind and solar) cases is reported in Section 2.4 through Section 2.6. 2.1 Selection of Study Cases This section covers the selection of the WECC cases from which the WWSIS-3 study cases were developed. The investigation and improvement of the selected cases are discussed in detail. The initial condition metrics for new base cases are presented. 2.1.1 Key Considerations The study databases were selected based on the following requirements: • The study case should be credible and meaningful. • The study case should have a longer horizon, e.g., extend beyond the year 2020. • The load flow in the study cases should include major system generation changesadditions and retirements-for which there is some degree of consensus in WECC. • The study case should have a transmission build-out that is realistic and not overly optimistic.

This paper describes the development of a variable time step algorithm for the numerical simulation of power system phenomena spanning time periods from several seconds to several hours.The algorithm automatically reduces the time step of integration to capture fast transients and increases it when the system variables are varying slowly. The solution algorithm is based on a numerically stable, implicit, low order integration similar to the trapezoidal rule.Simulation results for two scenarios are presented. These scenarios are: a severe system load ramp leading to voltage collapse, and a six-hour operational cycle.

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