Secondary frequency control with on-off load side participation in power networks

We study the problem of secondary frequency regulation where ancillary services are provided via load-side participation. In particular, we consider on-off loads that switch when prescribed frequency thresholds are exceeded in order to assist existing secondary frequency control mechanisms. We show that system stability is not compromised despite the switching nature of the loads. However, such control policies are prone to Zeno-like behavior, which limits the practicality of these schemes. As a remedy to this problem, we propose a hysteretic on-off policy where loads switch on and off at different frequency thresholds. Moreover, we show that by exploiting appropriate decentralized passivity conditions, stability guarantees for a power network with hysteretic loads can be provided. Several relevant examples are discussed to demonstrate the applicability of the proposed results. Furthermore, numerical investigations of the results are provided on the Northeast Power Coordinating Council (NPCC) 140-bus system.