Current and DC-Link Voltage-Drift Active Hysteresis Control of a Four Level Quasi Nested Converter

Currently, most multi-level converters are controlled through the use of voltage source based control techniques, such as space vector modulation or multi -carrier based modulation. However, in many applications such as field oriented drives, a high bandwidth current control is more desirable. In this paper, the concept of active hysteresis control is set forth. Hysteresis current control (H CC) based voltage source converters (VSCs) have several advantages like fast dynamic response, over-current protection, insensitive to load parameter variation and ease of implementation. The application of HCC technique for two-level VSCs is well established and few modifications of the same are also reported to operate with variable or constant switching frequency in case of multilevel converters. However, control of the dc-link voltage drift is commonly implemented using classical linear control techniques in a cascaded control loop, thus limiting the dynamic response. In this work, an active hysteresis control technique is proposed and implemented in a four level Quasi Nested Converter, for current tracking and including dc-link volt-age drift compensation control. This approach has the advantage of a reduced implementation complexity compared to classical linear techniques and Model Predictive Control (MPC) strategies. The proposed control technique is explained and implemented for the four level Quasi Nested Converter topology, considering the voltage drift phenomena. The topology and control strategy were implemented using PLECS simulation software and verified by Hardware-in-the-loop (HIL) results; its performance was evaluated under different operational conditions. The proposed control strategy is capable to mitigate the dc-link voltage drift, while keeping track of the load currents.