Abstract

۱٫ Introduction

۲٫ PUC inverters

۳٫ Microgrid

۴٫ proportional–integral (PI) control of PUC inverters

۵٫ MPC of PUC inverters

۶٫ Simulation results

۷٫ Hardware-in-loop (HIL) validations

۸٫ Conclusions

Declaration of competing interest

References

Abstract

     In this paper, an outline of microgrid arrangements and control methods at various hierarchical levels of Packed U-Cell (PUC) converters are provided. The paper discusses the control of these topologies using various techniques. The goal of these control strategies is to maintain a small THD, superior steady-state, fast-dynamic response, and high-power factor while balancing capacitor voltages under different operating conditions. The PI current controller is modelled on five and seven-level PUC inverters. The PI voltage and current controllers have also been modelled on a seven-level PUC inverter. Thereafter, model predictive control of five, seven and fifteen level inverters are also formulated and then simulated using MATLAB/Simulink. Finally, HIL validation of different PUC inverters is performed.

Introduction

     The electrical power generated from renewable energy resources (solar, wind and hydro, etc.) are either used for domestic purposes directly or they are supplied to the grid [1]. For example, the power generated by a solar plant is of dc nature which is first conditioned and then inverted to get an alternating output. This ac power is then synchronized with the grid according to the grid’s characteristics (frequency and phase angles) [2]. In each of the steps, there are many parameters to be controlled in an accurate manner so that power quality is maintained. For this purpose, the most suitable consideration is the multi-level family converters.

     Due to improved power quality compared to their two-level counterparts in recent times, multilevel inverter (MLI) topology has become increasingly accepted in commercial applications. The reduced harmonic deformation, improved waveform like a sinusoidal waveform and enhanced use of switches due to diminished voltage stress. MLI have their uses in various fields of renewable energy sources including, HVDC, distributed generation (DG) system, application of industrial drive systems, uninterruptible power supplies, etc. [3], [4], [5], [6]. Based on a few semiconductor devices coupled with different dc links, MLIs generate sinusoidal waveforms at their outputs through staircase waveforms. MLI has traditionally been used in three major topologies in commercial applications within the last few decades: Cascade H-Bridge (CHB) inverters, Flying Capacitor (FC) inverters and Neutral Point Clamped (NPC) inverters. Each MLI topology, however, have some drawbacks, such as the increasing count of source necessities, the balancing of capacitor voltage, and the high necessity for switches counting in the CHB, FC, and NPC network topologies, respectively [3].

Conclusions

     The main purpose of this work is to present that MPC can be a better alternative to the classical PI control when applying such control techniques on different PUC topologies. Although MPC is known for its limitations in terms of the steady-state error, MPC can be one of the greatest alternatives to the classical PI due to the simplicity of designing this control, the ability to achieve multiple objectives, and the high accuracy in transient conditions.

     In this paper, MPC was successfully applied on five, seven and fifteen level PUC inverters and compared with PI control when applied on the same topologies. Although the converters’ voltages usually have higher THD when MPC is applied, the easy implementation of MPC and the capability to control more than one control variables are the some of the advantages which increase the chances of using MPC in such applications.

     In this work, using MPC, the capacitor voltages of the different PUC inverters were well regulated to follow their reference values using a single controller. However, in order to achieve similar performance using the classical PI control, two control loops, voltage and current control loops, were required to be designed which significantly increase the complexity of this control.