Duty-Cycle Predictive Control of Quasi-Z-Source Modular Cascaded Converter Based Photovoltaic Power System

Yingjun Guo,Yi Zhang,Yaosuo Xue,Yushan Liu,Hexu Sun,Xiao Li

doi:10.1109/access.2020.3024935

Abstract

A duty-cycle predictive control is proposed for dc grid integration of front-end isolated quasi-Z-source modular cascaded converter (qZS-MCC) photovoltaic (PV) power system. The post-stage qZS half-bridge dc-dc converter deals with PV maximum power point tracking, dc grid integration, and dc-link voltage balance; whereas, the front-end isolation converters operate at a constant duty cycle of 50%. Thus, it saves control hardware resources while overcoming challenges from PV-panel voltage variations and dc-bus voltage limit. The proposed control uses the derived circuit model to predict the global active-state duty cycle for grid-connected current control and predict the shoot-through duty cycles for dc-link voltage balance, achieving a fast and accurate tracking target. The proposed control method has advantages of: i) eliminating weighting factors that exist in conventional model predictive control (MPC), ii) no sophisticated loop parameters design that exists in proportional-integral (PI) control, iii) operating at constant switching frequency that is different from the conventional MPC with variable switching frequency. Simulation and experimental tests are carried out to verify the effectiveness of the proposed control method and compare with the PI-based control system.

Highlights

Nowadays, the ever-increasing installed capacity of the largescale photovoltaic (PV) power plants locate in remote areas, which usually have abundant irradiation
(a), it can be seen that each SM has the time duration of Mk Ts in active state, and (1−Mk )Ts in zero and ST states during one control cycle Ts, k∈{1, 2}. When both SMs are in the active state, the quasi-Z-source modular cascaded converter (qZS-MCC) output voltage vo is the sum of two SMs’ dc-link peak voltage VDC1 + VDC2, and the filter inductor is charged leading to an increasing of the grid-connected current io
It can be seen that the overshoot of SM1’s dc-link voltage vDC1 and qZS-MCC output voltage vo are ignorable, and the currents and voltages become stable within 50 ms, when applying the proposed predictive control

Summary

INTRODUCTION

The ever-increasing installed capacity of the largescale photovoltaic (PV) power plants locate in remote areas, which usually have abundant irradiation. Each SM needs a dual-loop PI regulatorbased shoot-through (ST) duty cycle control to balance the dc-link voltages, and one extra PI regulator-based PV panel voltage control to track the MPPT, while a PI regulator controlling the dc grid-connected current. The qZS-MCC PV power system has to consider the system-level control functions, such as PV MPPT, dc grid integration, dc-link voltage balance, and cascaded voltage output. There are vink = vPVk D0 = 2vPVk , iink = D0iPVk = iPVk 2 This design can achieve benefits as follows: i) isolation between PV panel and high voltage grid; ii) PV voltage boost; iii) the maximum utilization of the HB isolated converter [11]; iv) low size of transformer; v) simple control implementation due to constant duty cycle of 0.5. The n+1 predictive loops and n PI regulators fulfill the control targets

MPPT AND GRID-CONNECTED CURRENT CONTROL

IMPLEMENTATION AND ADVANTAGES

1) RESULTS OF PROPOSED PREDICTIVE CONTROL

CONCLUSION