This paper describes the requirements for the generator's excitation system. Different parts of the excitation system are also presented. Two totally different types of excitation systems are installed in generators of both Eesti and Balti power plants. The one - fast static excitation system UNIITROL5000 produced by ABB - was installed in 2005. The other is a rather slow excitation system - high-frequency AC machine-produced in Russia in the middle of the 70s. The paper presents main structures of those excitation systems and their control systems and also proposes models of control systems for dynamic calculations of those systems.