Dynamic Voltage Restorer Using Switching Cell Structured Multilevel AC-AC Converter

IEEE Transactions on Power Electronics, 2016

ABSTRACT:

Dynamic voltage restorer (DVR) technology has become a mature power quality product. In high-power applications, DVR using a multilevel converter is commonly used. However, DVR using a multilevel direct pulse width modulation (PWM) ac-ac converter has not been well studied. This paper presents a new DVR topology using a cascaded multilevel direct PWM ac-ac converter. In the proposed scheme, the unit cell of the multilevel converter consists of a single-phase PWM ac-ac converter using switching cell (SC) structure with coupled inductors. Therefore, the multilevel converter can be short- and open-circuited without damaging the switching devices. Neither lossy RC snubber nor a dedicated soft commutation strategy is required in the proposed DVR. This improves the reliability of the DVR system. The output voltage levels of the multilevel converter increase with the number of cascaded unit cells, and a high ac output voltage is obtained by using low-voltage-rating switching devices. Furthermore, a phase-shifted PWM technique is applied to significantly reduce the size of the output filter inductor. A 1-kW prototype of single-phase DVR is developed, and its performance is experimentally verified. Finally, the simulation results are shown for a three-phase DVR system.

KEYWORDS:

1.      Commutation problem

2.      coupled inductor

3.      direct PWM AC-AC converter

4.       dynamic voltage restorer (DVR)

5.      multilevel converter

6.      pulse width modulation (PWM)

7.      switching cell (SC)

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Three-phase DVR systems using VSI [2]. (a) DVR with energy storage. (b) DVR with no energy storage.

EXPECTED SIMULATION RESULTS:

(a)

(b)

(c)

Fig. 2. Simulated waveforms of the three-phase DVR ( v_oa=v_ob=v_oc=220 V_rms,P_o=3kW, )

CONCLUSION:

In this paper, a new DVR system, employing the proposed cascaded multilevel direct PWM ac-ac converter, was presented. Compared with the conventional DVR topologies using the VSI, the proposed scheme has the advantages of fewer power stages, higher efficiency, and the elimination of bulky dc-link capacitor. In addition, unlike the existing DVR with the direct PWM ac-ac converter, the proposed DVR ensures stable operation because the proposed cascaded multilevel ac-ac converter has the following unique advantages over the conventional ac-ac converters.

·         It is immune to EMI noise because the switching devices are not damaged by the EMI noise’s misgating on- or off.

·         The commutation problem found in the conventional ac-ac converters can be effectively eliminated without using either dedicated soft commutation strategy or lossy RC snubber circuits.

·         It operates properly even with highly distorted input voltage, which is impossible with the conventional approach using soft commutation strategy.

Furthermore, the proposed multilevel ac-ac converter can obtain high ac output voltage with low-voltage-rating switching devices by cascading unit cells. The equivalent output frequency of the multilevel converter is increased by using a phase-shifted PWM technique, which reduces the size of the output LC filter. The performance of the proposed DVR is successfully verified by using a 1-kW prototype. Finally, a three-phase DVR system using the proposed scheme is verified through simulation.

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