DVRs, or dynamic voltage restorers, are a relatively new static var device that has seen applications in a variety of distribution and subtransmission applications. DVRs are series compensation devices that protect electric load against voltage sags, swells, unbalance and distortion. Though these devices may provide good solutions for customers subject to poor power quality, we caution regarding their application in systems that are subject to prolonged reactive power deficiencies (resulting in low voltage conditions) and in systems that are susceptible to voltage collapse.
The reason for the caution is that in many instances the main protection of networks against voltage collapse is the natural response of load to decrease demand when voltage drop. The implementation of DVRs would act to maintain demand even when incipient voltage conditions are present thus reducing inherent ability to halt a collapse and increasing the risk of cascading outage and blackout.