Cascaded Open-End Winding Transformer based DVR

2016, IEEE

ABSTRACT

This paper introduces and generalizes a class of multilevel dynamic voltage restorer (DVR) for voltage sags/swells compensation of high-power sensitive loads. Such a device can improve the power quality of sensitive loads located in stiff systems. The proposed DVR is based on three-phase bridge converters series-connected by means of cascaded transformers using the concept of open-end winding (OEW). Hence, two DC links can provide either symmetrical (i.e., equal DC-link voltages) or asymmetrical (i.e., different DC-link voltages) operation of the DVR converters. Generalization for K-stages is presented as well. The proposed configuration is named as DVR-COEW (i.e., cascaded open-end winding). Such a topology permits to generate a maximized number of voltage levels per converter leg. The multilevel waveforms at the output voltages of the converter are generated by using a suitable PWM strategy associated with both: i) DC-link voltages ratio and ii) transformers turns ratio. Modularity and simple maintenance make the proposed DVRCOEW an attractive solution compared to some conventional configurations. The model and PWM control are addressed in this paper. Simulation and experimental results are presented.

KEYWORDS:

1. Dynamic voltage restorer (DVR)
2. Open-end winding (OEW)
3. DC-link voltages
4. Pulse Width Modulation (PWM)

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Example of a DVR-based system

EXPECTED SIMULATION RESULTS:

Figure 2. Phase-voltage of the resultant converter in phase-a (vra) for conventional and proposed DVRs considering an operation with 1-stage (i.e.,k = 1 with N1 = 1) having converters with a total of 6 legs. (a) Conventional DVR. (b) Proposed COEW-DVR and symmetrical DC-link voltages (i.e., DC link voltage ratio 1:1). (c) Proposed COEW-DVR and asymmetrical DC-link voltages (i.e., DC-link voltages ratio 2.:1)

Fig. 3. Figure 7. Phase-voltage of the resultant converter in phase-a (vra) for conventional and proposed DVRs considering an operation with 2-stages (i.e., k = 2) having converters with a total of 12 legs. (a) Conventional DVR with N1 = 1 and N2 = 3. (b) Proposed COEW-DVR and asymmetrical DC-link voltages (i.e., DC-link voltage ratio 2:1) N1 = 1 and N2 = 2. (c) Proposed COEW-DVR and asymmetrical DC-link voltages (i.e., DC-link voltages ratio 4:1) N1 = 1 and N2 = 2.

CONCLUSION

This paper has presented a cascaded open-end winding (COEW) transformer based DVR. The COEW-DVR configuration is generalized for k-stages. A comparison between the proposed and conventional (using HB [6]) configurations (operating with 1-stage) is summarized in Table IV. It can be seen that the proposed COEW-DVR has a better quality of output voltages when compared to the conventional one with HB [6]. This is observed by means of WTHD of the output resultant voltages vrj of the COEW-converter. Additionally, the lower values of WTHD at the output voltages permits the proposed COEW-DVR to reduce its switching frequency to match the same harmonic distortion value obtained for the conventional one. In this way, semiconductor losses would give a fair comparison. The semiconductor losses estimation was done by using the thermal module of PSIM, in which both configurations have operated under the same vrj magnitude and WTHD value. The power of the three-phase load was 6 kW. The conditions and more details for these comparisons under 1-stage operation are found in [19]. Hence, the semiconductor losses estimation for COEW-DVR with 1-stage operation are reduced up to 48.1% compared to the values obtained with conventional one. The losses comparison for 2 stages operation has been done as well. In that case, for the same switching frequency (i.e., fsw =10kHz) the proposed COEW-DVR has presented a reduction of 25% compared to losses obtained for conventional one.

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