Finite-State Predictive Current Control of a Simplified Three-Level Neutral-Point Clamped Inverter

Abstract

Multilevel inverter is one of the most important parts in renewable energy based power generating section as well as in motor drive applications. The quality of an inverter system depends on current total harmonic distortion (THD), switching loss, fault tolerant ability, dynamic responses, voltage stress, common mode voltage etc. Multilevel inverter yields low current THD, less voltage stress across the semiconductor switches and low switching frequency and thus less switching loss. However, using more number of semiconductor devices and neutral point voltage variation are the common problems for a neutral point clamped inverter. This is why different topologies of multilevel inverter are available in the literature in order to solve the aforementioned problems. The control scheme of a multilevel inverter also plays an important role to guarantee system’s performance. Recently, model predictive control (MPC) draws much attention to the researchers for its intuitive features and easy handling of nonlinearities of a system. The controller uses system model to predict the future behavior of the system over a prediction horizon. The control objectives are met by minimizing a predefined cost function that represents the expected behavior of the system. The objective of the proposed research work is to control the output load current of a three level simplified NPC (3L-SNPC) inverter topology using MPC. The simplified NPC inverter is considered, because less number of semiconductor devices used in the topology, even though further investigation is required on different factors such as voltage stress, common mode voltage, losses and switching frequency. MPC is used as controller because it can handle the dc link capacitors voltages balancing problem in a very intuitive way. Moreover, the average switching frequency reduction and over current protection can be easily implemented. Simulation results show that the proposed 3L-SNPC yields similar current THD, transient and steady state responses, voltage stress on the switches at the load side and over current protection capability as the conventional diode clamped based NPC inverter system. The two dc-link capacitor voltages are balanced properly with a neutral point voltage variation of close to zero. However, in comparison with the conventional NPC inverter, the proposed system is 15.25% computationally expensive which yields long execution time and thus less sampling frequency. In this study, two simplified MPC strategies are proposed for the 3L-SNPC inverter system in order to reduce the computational burden: single voltage vector prediction based MPC and selective voltage vector prediction based MPC. Both simplified strategies yield similar performance as the conventional MPC. The required execution times for the simplified MPC strategies are tested on hardware dSPACE 1104 platform. It is found that the single voltage vector prediction based MPC and the selective voltage vector prediction based MPC are computationally efficient by 8.28% and 62.9%, respectively, in comparison with the conventional MPC strategy. However, the average switching frequency and the overall loss in the proposed 3L-SNPC inverter are higher by 83.33% and 46.3%, respectively, than the conventional NPC inverter for a specified load current.