  We propose a method for determining the most likely cause, in terms of
conventional generator outages and renewable fluctuations, of power system
frequency reaching a predetermined level that is deemed unacceptable to the
system operator. Our parsimonious model of system frequency incorporates
primary and secondary control mechanisms, and supposes that conventional
outages occur according to a Poisson process and renewable fluctuations follow
a diffusion process. We utilize a large deviations theory based approach that
outputs the most likely cause of a large excursion of frequency from its
desired level. These results yield the insight that current levels of renewable
power generation do not significantly increase system vulnerability in terms of
frequency deviations relative to conventional failures. However, for a large
range of model parameters it is possible that such vulnerabilities may arise as
renewable penetration increases.
