We study the problem of secondary frequency regulation where ancillaryservices are provided via load-side participation. In particular, we consideron-off loads that switch when prescribed frequency thresholds are exceeded inorder to assist existing secondary frequency control mechanisms. We show thatsystem stability is not compromised despite the switching nature of the loads.However, such control policies are prone to Zeno-like behavior, which limitsthe practicality of these schemes. As a remedy to this problem, we propose ahysteretic on-off policy where loads switch on and off at different frequencythresholds. Moreover, we show that by exploiting appropriate decentralizedpassivity conditions, stability guarantees for a power network with hystereticloads can be provided. Several relevant examples are discussed to demonstratethe applicability of the proposed results. Furthermore, numericalinvestigations of the results are provided on the Northeast Power CoordinatingCouncil (NPCC) 140-bus system.
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