Capacitor Voltage Ripple Minimization of a Modular Three-Phase AC/DC Power Electronics Transformer With Four-Winding Power Channel

Abstract

Power electronic transformer (PET) with modular cascaded structure has become a flexible interface between AC and DC grids. Nonetheless, the notable 2 nd-order ripple power introduced from the AC source requires considerable energy storage in the module capacitors, which is likely to degrade the overall power density and increase system cost. In this paper, a power channel based four-winding PET (PC-PET) is proposed to address such an issue. Owing to the phase-crossing power channels, the low-frequency ripples can cancel each other through magnetic flux counteracting. To improve the accuracy of power decoupling, the effect compromised by inconsistent windings is considered. Accordingly, a proportional-resonant (PR) control based power decoupling strategy for PC-PET incorporated with power balance is proposed to decouple the three-phase low-frequency ripple powers, and minimize the voltage ripple in module capacitors. Additionally, the improvement of power density is discussed based on analyses on power losses, capacitor volume and transformer volume. Simulation and experimental results verified the validity of proposed control strategies.

An AC Voltage Balancer and its Improved Modulation Strategy for CHB Based PV Inverters

“…In order not to lose generality, an inverter system consisting of three submodule converters is chosen here for analysis. By linearizing the model shown in (4) and combining it with the mathematical model of the ACVB controller shown in (14), the equivalent impedance small signal model of the output side of the rear stage inverter can be derived as shown in (15).…”

“…Existing solutions include zerosequence current injection [5] and zero-sequence voltage injection [6], but these schemes have limited compensation capabilities [7]. In order to address the issue of interphase power imbalance thoroughly, researchers proposed a twostage converter combining quadruple active bridge (QAB) converters and CHB inverters [8][9][10][11][12][13][14][15][16]. Attractive features of this two-stage converter include: 1) the output power of a single PV module is evenly distributed among the three phases, thereby achieving natural power balance between phases; 2) the double-line-frequency fluctuating power of the H-bridge inverter unit is offset in the four-winding transformer to avoid the use of bulky capacitors; 3) the use of high-frequency isolation transformers completely solves the common mode leakage current problem of PV panels.…”