Dynamic Voltage Restorer Based on Three-Phase Inverters Cascaded Through an Open-End Winding Transformer

IEEE Transactions on Power Electronics, 2015


This paper investigates a dynamic voltage restorer (DVR) composed of two conventional three-phase inverters series cascaded through an open-end winding (OEW) transformer, denominated here DVR-OEW. The DVR-OEW operating with either equal or different dc-link voltages are examined. The proposed topology aims to regulate the voltage at the load side in the case of voltage sags/swells, distortion, or unbalance at the grid voltage. A suitable control strategy is developed, including space-vector analysis, level-shifted PWM (LSPWM) and its equivalent optimized single-carrier PWM (SCPWM), as well as the operating principles and characteristics of the DVR. Comparisons among the DVR-OEW and conventional configurations, including a neutral-point clamped (NPC) converter based DVR, are furnished. The main advantages of the DVROEW compared to the conventional topologies lie on: i) reduced harmonic distortion, ii) reduced converter losses, and iii) reduced voltage rating of the power switches. Simulated and experimental results are presented to validate the theoretical studies.





Fig. 1 Example of a typical application of DVR in Medium-Voltage (MV) distribution system..



Fig. 2. System voltages for vca = vcb. (a) Grid voltages (egj). (b) DVR voltages at the secondary side of the injection transformers (vsj ). (c) Load voltages (vlj ). (d) Injected voltage (vp1) for one phase at the primary side of injection transformer.


Fig. 3. Pole voltages in one phase at inverters A (v1a0a) and B (v1b0b), respectively. (a) OEW inverter operates with alternatively leg of converter clamped in every half cycle. (b) OEW inverter operates by clamping inverter A.



In this paper a dynamic voltage restorer (DVR) obtained by means of the series connection of two three-phase inverters through an open-end winding transformer was presented. Two equivalent implementations with either level-shifted carrier PWM (LSPWM) or single-carrier PWM (SCPWM) strategy approaches were presented. The main advantages of the proposed topology, compared to conventional configurations with three legs (see Fig. 2(a)), six-leg (see Fig. 2(b)) and NPC (see Fig. 2(c)) lies on: (i) reduced harmonic distortion (operating at the same switching frequency), (ii) reduced converter losses (operating with the same harmonic distortion), (iii) reduced converter losses (with the same switching frequency), see Table III and (iv) reduced voltage rating of the power switches employed in the DVR. The operations with different dc-link voltages have been investigated and it is shown that much lower harmonic distortion can be obtained. The proposed DVR system is suitable for medium voltage application. Simulated and experimental results were also presented.



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