The system frequency of a power systems is a good indicator of the networks resilience to major disturbances. The frequency control is generally a multi-layered control structure with primary, secondary and tertiary control. In a completely deregulated setting, for example in the Nordic power system, the system operator controls the system frequency manually by calling-off bids handed in to a market, called the regulating market. In this paper we formulate the problem of optimal bid call-off on the regulating market, that the system operator is faced with each operating period, as a multi-period optimal switching problem with execution delays. (C) 2018, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved.