I. Introduction
Frequency regulation service involves the injection or with-drawal of active power from the power grid to maintain the system frequency [1]. In the United States, frequency regulation is equivalent to secondary frequency control, while primary frequency control is more commonly known as frequency response. The most common mean for a unit to provide frequency regulation is following a system operator's automatic generation control (AGC) signal, which computes the area control error (ACE) from frequency deviations and interchange power imbalances.