Structure and the space vector modulation for a medium‐voltage power‐electronic‐transformer based on two seven‐level cascade H‐bridge inverters

Abstract

This study presents the structure and the space vector pulse-width modulation (SVPWM) for power electronic transformer (PET) based on two seven-level cascade H-bridge (CHB) inverters. The DC links of CHB inverters are coupled with nine dual-active bridge (DAB) converters with medium-frequency transformers. The DC-link voltages are equalised with two methods-through the control of DAB voltages and through the modulation strategy applied to both CHB inverters. In the proposed SVPWM, the influence of vector sequences on predicted DC-link voltages is analysed, and the optimum vector sequence is selected to equalise them. Regardless of this, the proposed SVPWM strategy enables the proper generation of output voltage vector also in the case of DC-link voltage imbalance-the calculation of the space-vector area takes into consideration the inequality of the DC-link voltages and its influence on the lengths and positions of active vectors. To simplify the modulation algorithm, the multilevel CHB inverter is considered as a set of three-level inverters connected in series. Each of them is controlled using the same SVPWM algorithm. The proposed modulation method reuses the H-bridges with zero duty cycles determined in the initial stages of the output voltage generation process. This enables the optimal management of the DC-link voltage distribution. The experimental research was carried out on a 600 kW/3.3 kV PET. The results are presented in this study.

Space Vector Pulsewidth Modulation Strategy for Multilevel Cascaded H-Bridge Inverter With DC-Link Voltage Balancing Ability

“…The phase power is obtained as the product of the phase current and the reference phase voltage. The respective reference phase voltages can be obtained from the reference voltage vector using inverse Clark's transformation, [1]. The limitations of this method are related to the necessity to adopt the directions of the voltages generated by the H-bridges in each of the inverter phases.…”

“…As this configurational concept proceeds, the inverter output voltage waveform nears a sinusoid; with less harmonics content. CHB inverters are popularly deployed in power electronic transformer applications, [1], [2], [3], static synchronous compensator (STATCOM) [4], [5] energy storage system, [6], [7], [8]. They also have wide application in renewable generation systems, [9], [10], [11]; as well as in electric drive applications, [12], [13] and charging stations [14].…”

“…The developed SVPWM solutions in the literature mainly include methods of identifying the active vectors that can be used to generate the output voltage vector [18]. Some solutions rely on dividing the multilevel space vector diagram (SVD) into several lower level SVDs [19], [1], [20], [21]. Due to the difficulty of developing an effective SVPWM algorithm, the most popular modulation strategies used in CHB MLIs are carrier-based sinusoidal pulse width modulations (SPWM) [22], [23], [24], [25]; and selective harmonic-elimination pulse width modulation (SHE-PWM)…”

SiC-Based Power Electronic Traction Transformer (PETT) for 3 kV DC Rail Traction

“…If the obtained output voltage is different from the reference motor voltage, the next H-bridge is activated in each phase. At each switching sequence only one H-bridge per phase provides a modulated output voltage, while the others are negatively/positively connected or bypassed [37]. Since their transistors do not switch, they do not generate commutation loses.…”

“…For the above reason, the control system can consider the topology of the seven-level CHB converter as a set of 3 three-level CHB converters connected in series, which simplifies the control strategy. Each of them is then composed using three H-bridges (one H-bridge in each phase of the inverter) and can be controlled using simple SVPWM patterns [37].…”