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Long-term transmission and generation planning face numerous challenges to accommodate the integration of a high penetration of variable generation (VG). System flexibility, or the ability of a system to meet changes in demand and VG production, is one such issue receiving much attention. This paper presents a methodology to assess the flexibility of a power system while explicitly considering the limitations of the transmission network. By determining only the flexibility available from resources which could be realized in real-time operation due to transmission constraints, a more realistic assessment of a system's flexibility can be made. The flexibility of the IEEE Reliability Test System is assessed using two metrics. Transmission is shown to have a significant effect on the availability of flexibility and the risk posed to the system by net load ramps. The impact is seen to be strongly dependent on the variability of the net load and on the distribution of online resources. A nonlinear relationship between the installed VG and system flexibility is found, with regions of significant inflexibility occurring at certain VG penetrations.
Lannoye et al. (Mon,) studied this question.