Investigation of Three-Phase AC–DC–AC Multilevel Nine-Leg Converter

Abstract

In this paper, a reversible multilevel nineleg converter for three-phase applications is investigated. This topology can be applied in line voltage regulators, power factor correction (PFC) devices and uninterrupted power supplies (UPS). It is composed of three singlephase three-leg converters with a leg shared by both load and grid sides. Suitable modelling, a pulse-width modulation (PWM) strategy based on vector approach and a control system are developed. The proposed PWM and control strategies are suitable to balance the DC-link voltages and to optimize the harmonic distortion, reducing switching stress and power losses. The investigated configuration is compared with two conventional topologies, including a neutral-point clamped (NPC) converter, and has demonstrated that it presents some advantages in terms of voltage rating of the power switches, harmonic distortion and semiconductor losses. Computer simulations and experimental results have been carried in the same operation conditions to verify the validity of theoretical considerations.

Single‐phase ac–dc–ac topology for grid overvoltage and voltage harmonic mitigation

“…The instantaneous reference current i g * is synchronised with the grid voltage e g via blocks GEN−i g and phase-locked loop to maximise the input power factor. Grid current i g is adjusted by controller R g (double-sequence PI controller [13]…”

“…The phase angle between and () needs to be considered in order to keep the required dc‐link voltages at minimum values [13]. Also according to Maia et al [13], load angle () can be set in order to decrease the shared‐leg current . In this case, angle is limited for highly inductive loads.…”

“…The instantaneous reference current is synchronised with the grid voltage via blocks − and phase‐locked loop to maximise the input power factor. Grid current is adjusted by controller (double‐sequence PI controller [13]) that set out reference input converter voltage . Reference load voltage is obtained by way of block GEN− from amplitude and phase angle .…”

“…Research in the industrial and power sectors related to ac-dc-ac converters has been conducted aiming at topologies that generate low harmonic distortion (HD), provide sinusoidal input currents with high-power factor and are effective to protect the load against line disturbances [13][14][15]. Owing to the dc-link energy storage element, there is an advantage that both converter stages (rectifier and inverter) are, to a large extent, decoupled for control purposes.…”

“…The ac-dc-ac configurations can provide bidirectional power flow and are mainly applied in unified power quality compensator [17,18], uninterrupted power supplies [19][20][21], generation systems [22,23], motor drive systems [24][25][26] and applications with fault tolerance [27,28]. Normally, the same input and output frequencies are observed in these applications, which allows the utilisation of shared leg between the converter stages, while maximising conversion efficiency [13]. Although it allows the reduction of the number of switches, the use of shared leg in ac-dc-ac converters couples both sides of the structure, increasing the complexity of the control system [29].…”