Among incidents on grids, the sag/swell voltage is considered as the most frequent incident. To solve this problem, custom power devices are used. In particular, the dynamic voltage restorer (DVR) is a modern and efficient customer device. DVRs are used to mitigate voltage sag/swells and harmonics on the load bus, thus protecting the sensitive loads. The DVR is a serial compensator that applies a voltage to the point of common coupling to maintain the voltage of sensitive load at the nominal value. To improve the performance of DVRs, in this study, the control strategy of the two-stage loop circuit is implemented. The external voltage control loop uses a sequence-decoupled resonant (SDR) controller, and the inner current-control loop uses the proportional resonant (PR) controller implemented in the stationary frame αβ.

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Although there are many studies on application of FACTS unit, especially application of reactive power generating unit for power system to enhance voltage stability amplitude and transient stability as considered, the evaluation and selection of suitable power generating unit (STATCOM or SVC) as well as optimal compensation size in the analysis of steady and transient state is not paid attention to. This article studies the above described issues to give a solution to install reasonable reactive power generating source (kind of Var source, location, size) for the power system. The analysis will be based on the technical factors rather than economic ones. 500kV grid system in Vietnam will be used in the calculation until 2015. To deeply evaluate, the implementation plan of reactive power compensation was only studied for 500kV grid in the South, where the load demand is large and can be dramatically increased. The results of power flow calculation, the analysis of characteristics of PV, OPF, and transient stability were studied through PSS/E-30 software.

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