IEEE Transactions on Power Delivery, 2015
This paper presents a new multilevel cascaded-type dynamic voltage restorer (MCDVR) with fault current limiting function. This topology can operate in two operational modes: 1) compensation mode for voltage fluctuations and unbalances, and 2) short-circuit current limiting mode. The current limiting function of the MCDVR is performed by activating anti-parallel thyristors during the short-circuit fault, and deactivating them during normal operation. The mathematical model of the MCDVR system is also established in this paper. The control scheme design and optimal parameter selection are outlined based on the detailed theoretical analysis of the converter. The transient states of the MCDVR in both the compensation mode and current-limiting mode are also analyzed. Simulation results based on the PSCAD/EMTDC software and experimental results on a laboratory setup help to validate the proposed topology and the theoretical analysis.
- Dynamic voltage restorer (DVR)
- Multilevel inverters
- Fault current limiter
- Voltage restoration
Fig. 1. Schematic diagram of the proposed MCDVR.
EXPECTED SIMULATION RESULTS:
Fig. 2. Simulation results of the MCDVR system. Top to bottom: (a) the supply voltage Us, (b) the load voltage UL, (c) the secondary voltage Udvr, (d) the load current IL, and (e) the dc-link voltage.
Fig. 3. Forward switching simulations of the MCDVR system, top to bottom: (a) the load current IL, (b) the current Iscr in the thyristor’s path, (c) the output current Idvr of the VSI, (d) the dc-link voltage Udc, and (e) the timing sequence
Fig. 4. Backward switching simulations of the MCDVR system, top to bottom: (a) the load current IL, (b) the current Iscr in the thyristor’s path, (c) the output current Idvr of the VSI, (d) the dc-link voltage Udc, and (e) the timing sequence.
Cascaded multilevel inverters have been applied in the industry as a cost-effective means of series sag compensation. However, a large current will be induced into the VSI through a series transformer during faults, and this is harmful to the VSI and the other equipment in the grid. In this paper, the MCDVR was proposed to deal with voltage sags and short-circuit current faults. The MCDVR has not only the advantages of the H-bridge cascade inverter, but also reduces the secondary side current in the preliminary period of the fault. A mathematical model of this system was also established in this paper. A careful analysis of the transient state verified the feasibility of the proposed MCDVR. Based on the theoretical analysis, PSCAD/EMTDC simulations and the experimental results, we can conclude the following:
1) The H-bridge cascade inverter can be adopted to reduce the series transformation ratio and the secondary current during the preliminary period of the fault.
2) The transient state of the MCDVR system was introduced in great detail.
3) The proposed control method can limit fault current with two cycle. The consistencies between the simulation results and experimental results help to verify the proposed topology and theoretical analysis.
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