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The Automatic Generation Control (AGC) of the power system is examined in this paper and two earlier companion papers ,  from the vantage point of digital computer oriented estimation-optimization-control technology, and a new scheme is proposed which exploits solution techniques and experiences of the past and of the current practice. Economic Dispatch and Load Frequency Control are unified in this approach. The companion papers published earlier in the Transactions by the PES dealt with Load Estimation in Part I  and Dispatch Targeting in Part II . This current paper, Part III, deals with the problem of tracking the economic target curves established in Part II. In tracking the target curves, unit dynamics, including boiler dynamics, are taken into account. The problem is decomposed in such a way that the slower, bulk control is based on a fully nonlinear and dynamic model of the unit while faster load fluctuations are traced by fast, dynamic vernier control based on a linearized model. This results in fuel economy coupled with tight and smooth control which demands very moderate computational load. Constraints on turbinepower and on its rate of change are considered explicitly as well as reduced economy at valve points. Cogeneration and load management are readily introduced into this scheme.