Under what conditions is a dynamic voltage restorer (DVR) preferred over a static VAR compensator (SVC) for voltage sag mitigation?

A voltage sag is a short-duration reduction in voltage. Both Dynamic Voltage Restorers (DVRs) and Static VAR Compensators (SVCs) are used for voltage support, but they address voltage sags differently. An SVC is a shunt-connected device that injects or absorbs reactive power (VARs) to maintain voltage stability. It primarily compensates for voltage fluctuations caused by load changes or reactive power imbalances. A DVR, on the other hand, is a series-connected device that injects voltage into the system to compensate for voltage sags or swells. A DVR is preferred over an SVC when the primary concern is to protect sensitive loads from voltage sags caused by upstream faults or sudden load changes. Unlike an SVC, a DVR actively injects voltage, restoring both the magnitude and phase angle of the voltage, providing more comprehensive voltage support during a sag event. DVRs are particularly advantageous when the load is sensitive to even small voltage variations or when the sag is deep and an SVC alone cannot provide sufficient voltage support. Also, DVRs can correct for voltage unbalance and harmonic distortion to some extent, offering more functionality than a typical SVC. However, DVRs are generally more expensive and have a limited energy storage capacity, making them suitable for mitigating short-duration sags, whereas SVCs are better suited for continuous voltage regulation and longer duration disturbances.