Research On Key Technologies Of Dynaimc Voltage Restorer

Dynamic voltage restorer (DVR) is designed to protect sensitive loads from the effectsof voltage sags on the distribution feeder. DVR is a very cost effective solution for sensitiveloads protection. However the price of DVR still remains high because of the expensiveenergy storage element and the series transformers. This thesis is aimed at providing newcontrol methods to reduce the size of energy storage element and series transformers bymaking effective use of the energy and eliminating the DC flux of the series transformers.So the DVR system can be designed more cost effective under the new control methods.The capability of DVR in compensating long-duration voltage sags is largelydepended on the amount of stored energy within the restorer which is very expensive. Inorder to reduce the price of DVR, many control strategies have been implemented to makebetter use of the stored energy. Unfortunately, the traditional voltage compensationstrategies are merely designed to minimize the needed compensation voltage to extractmore energy from the DC-link or to decrease the output active power of DVR or to reducethe distortions at the load. To combine the advantages and avoid the drawbacks of thetraditional strategies, an optimized control strategy has been designed and tested byexperiment with a10kW DVR. The new strategy can dramatically increase thecompensation time with low distortion at the load and without enlarging the energy storageof the DC-link. Also, the proposed strategy can perform well in unbalance voltage sagswhile other strategies mainly deal with the balanced voltage sags. In the following thesis,an introduction and comparison of the traditional control strategies will be given. Then, anoptimized control strategy will be developed and theoretically analyzed. Finally,experimental results will be shown to verify and prove the functionality of the proposedstrategy in both balanced and unbalanced voltage sags.The DVR based on a transformer-coupled series-connected inverter is among the mostcost-effective solution to protect sensitive loads. One problem with this system is thetransformer saturation, which could cause serious damage to the DVR system and interruptcompensation. This thesis proposed an attractive method to predict and eliminate the dc magnetic flux, so the problem of transformer saturation can be solved. The proposedmethod takes only3.3ms to elimination the dc magnetic flux, and brings very low distortionto the load voltage. The proposed method had been theoretically analyzed and proved byexperimental results with a10kW DVR system.In this thesis, the proposed control methods can make effective use of the energy formthe DC-link and successfully eliminate the DC flux in the series transformers. As a result,the size of the energy storage element and the series transformers can be reduced. What’smore, the proposed control methods bring very low distortion to the load voltage, so thecontrol methods are suitable for sensitive loads.