  Generation expansion planning (GEP) models have been useful aids for
long-term planning. Recent growth in intermittent renewable generation has
increased the need to represent the capability for non-renewables to respond to
rapid changes in daily loads, leading research to bring unit commitment (UC)
features into GEPs. Such GEP+UC models usually contain discrete variables
which, along with many details, make computation times impractically long for
analysts who need to develop, debug, modify and use the GEP for many
alternative runs. We propose a GEP with generation aggregated by technology
type, and with the minimal UC content necessary to represent the limitations on
generation to respond to rapid changes in demand, i.e., ramp-up and ramp-down
constraints, with ramp limits estimated from historical data on maximum rates
of change of each generation type. We illustrate with data for the province of
Ontario in Canada; the GEP is a large scale linear program that solves in less
than one hour on modest computing equipment, with credible solutions.
