chapter  9
Constant-Voltage Transformers
Pages 24

Constant-voltage transformers (CVT) have been used for many years, primarily as a noise-isolation

device. Recently, they have found value when applied as a voltage-sag protection device for industrial

and commercial facilities. The purpose of this section in the handbook is to give power-system engineers

and facility engineers who are unfamiliar with the CVT technology (also known as ferroresonant

transformers) the insights and information necessary to determine the types of electric-service-supply

events that CVTs can mitigate. Items covered in this chapter include operation, characteristics, appli-

cations, specifications, and sizing guidelines of CVTs. The goal here is not to duplicate information

currently available but, rather, to collect information into a single location and then supplement it to

provide:

. Adequate information and procedures to applications personnel in effectively selecting CVTs for

voltage-sag ride-through protection . Application notes to demonstrate how CVTs can improve process voltage-sag ride-through

The industrial use of constant-voltage transformers (also called CVTs and ferroresonant transformers)

goes back to the early 1940s. Living in the U.S. during the 1930s, Joseph G. Sola, a German-born

engineer, discovered the CVT technology [1,2] based on a single transformer rather than an arrangement

of transformers, separate filters, and capacitors. This innovation provides several important advantages:

its inherent robustness (CVT consists of just three or four windings and a high-reliability capacitor), its

imperviousness to continuous short circuits (whether it is turned on into a short circuit or from full

load), and its capability to maintain output-voltage stabilization on a cycle-to-cycle basis for signifi-

cantly large overvoltages and undervoltages.