Stability properties of certain thyristor-bridge control systems. Part 2: The interrelationships of discrete- and continuous-design methods

Part 1 of the paper developed discrete models of thyristor-bridge control systems, and introduced some simple design roles for ensuring stability. The present paper investigates the common features of continuous-system design methods for stable thyristor-bridge control systems, and the implications of applying these techniques to a discrete control system are discussed. This is followed by a detailed discussion of the existing describing-function method. Results for describing functions obtained by using crosscorrelation techniques are given. The critical conditions for 1st- and 2nd-order systems are used to show that the Nyquist conditions for stability must take into account critical points other than the point (¿ 1, 0). A derivation of the equation for the general translation of the critical point is given using discrete-system methods, and a discussion on the translation for low- and high-bandwidth systems follows. An example shows the use of the equation in synthetising the minimum sampling frequency for a given continuous system. An example of a halfsection filter is investigated using discrete-system and root-locus techniques, and the results are used to show that unstable systems exhibit some interesting random characteristics.