Describing Function Analysis and Experimental Investigation of a High Power Utility Friendly AC-DC Converter System

In this paper, a utility friendly synchronous link three level diode clamped AC-DC converter having rectification and regeneration capability is presented. The three-level AC-DC converter draws sinusoidal AC current with low harmonics from the supply and keeps the DC-link voltage constant. Hysteresis current controller is used for current control because of its simplicity, easy implementation and good performance. The paper presents an analysis for limit cycle control of switching logic for the three-level synchronous link converter. The ideal hysteresis current control method leads to infinite frequency and that results in chattering phenomenon. In medium and high voltage converter applications, the switching elements, such as IGBT and GTO, have finite switching frequency. To bring the switching frequency into the limit of practical devices, hysteresis logic with a suitable band width (h) is required. The hysteresis logic being non-linear, describing function (DF) analysis is applied with non-linearity replaced by its DF. Thus, by using DF analysis, the maximum possible switching frequency for a given hysteresis band can be predicted. The closed-loop control scheme with hysteresis current control is implemented for the three-phase three-level synchronous link converter (SLC). Simulation and laboratory prototype test results demonstrate the validity of the proposed hysteresis current control method. The converter is found to be stable for a large range of hysteresis band