AC Voltage Sensorless-Based Natural Frame Control of Cascaded H-Bridge Converter Based on Virtual Flux Observer

Abstract

In this study, a novel natural frame control method without AC voltage sensors is proposed for single-phase cascaded H-bridge (CHB) multilevel converter and implemented in the three-cell CHB converter platform. An improved virtual flux observer (VFO) is introduced to convert the estimated AC voltage signal into the virtual flux (VF) signal and then to vectorize it. The relationship between the instantaneous power and flux linkage is used to achieve independent control of the active and reactive currents. Compared with the conventional single-phase dq control, phase-locked loop (PLL) and coordinate transformation are unnecessary for natural frame control. Furthermore, only orthogonal voltages are required in the proposed control scheme, thus eliminating the time delay caused by constructing orthogonal current signals to improve dynamic performance. The simplification of the entire system is achieved by the omission of AC voltage sensors, the PLL, and coordinate transformations without deteriorating the steadystate and dynamic performances of the entire system. Simulation and experimental results verify the effectiveness of proposed control method. INDEX TERMS Cascaded H-bridge (CHB), natural frame control, sensorless control, virtual flux observer (VFO).

Control strategy without phase‐locked loop based on power estimation and natural frame for single‐phase cascaded H‐bridge converter

“…The comparison of the steady‐state experimental results and current harmonic content of other control methods in grid‐connected mode for the same distorted grid is given in Figure 16. Figure 16A–D shows the steady‐state waveforms in grid‐connected mode for method 1 in Yang et al, 21 the method without PLL in Yang et al 26 (noted as method 2), the improved NFC method in Yang et al 32 (noted as method 3), and the voltage‐sensorless NFC method in Shao et al 33 (noted as method 4), respectively. Figure 16 gives a comparison of the steady‐state experimental results of other methods in grid‐connected mode.…”

“…The grid voltage estimation deviation, DC drift problem, and the difficulty in obtaining the fundamental grid voltage in the case of three-phase unbalance are respectively addressed in previous studies [23][24][25] to achieve grid voltage-sensorless control; however, they are based on complex observers that are not conducive to digital implementation. A direct current control method without PLL based on an improved virtual flux observer have been performed in Yang et al, 26 but the method does not validate the grid distortion condition.…”

Voltage-Sensorless Natural Frame Control for Single-Phase CHB Converter under Distorted Grid Conditions

“…However, the existing sensorless control methods have drawbacks, as follows. Firstly, the existing sensorless control methods are mainly based on ideal grid conditions without considering the distorted grid conditions [27][28][29]. The grid voltage is easily distorted because of the increase of nonlinear loads and distributed generators [30].…”