Power grid frequency fluctuations and smart devices with dynamic demand control

Abstract

Power grid frequency control is a demanding task requiring expensive idle power plants to adapt the supply to the fluctuating demand. An alternative approach is controlling the demand side in such a way that certain appliances modify their operation to adapt to the power availability. This is especially important to achieve a high penetration of renewable energy sources. A number of methods to manage the demand side have been proposed. In this work we focus on dynamic demand control (DDC), where smart appliances can delay their switchings depending on the frequency of the system. We introduce a simple model to study the effects of DDC on the frequency of the power grid. This includes the power plant equations, a stochastic model for the demand that reproduces, adjusting a single parameter, the statistical properties of frequency fluctuations measured experimentally, and a generic DDC protocol. We find that DDC can reduce small and medium-size fluctuations but it can also increase the probability of observing large frequency peaks due to the necessity of recovering pending task. Although these events are very rare they can potentially trigger a failure of the system and therefore strategies to avoid them have to be addressed. Therefore we also introduce a protocol for communication among DDC devices belonging to a given group, such that they can coordinate opposite actions to keep the group demand more stable. We show that this protocol reduces the amount of pending tasks by a factor 10 while large frequency fluctuations are significantly reduced or even completely avoided. We also study the influence of the grid topology in the shape of the power spectrum of the electrical frequency fluctuations.