Constant Voltage Transformer (CVT) or Ferroresonant Transformer (Ferro) is a non-linear transformer, which passively provides a regulated voltage output through an electromagnetic phenomenon called ferroresonance (where the term ferroresonant transformer was derived). Ferroresonance is associated with the behavior of iron cores while operating near a point of magnetic saturation, wherein the core is so strongly magnetized that further variation in the input voltage results in little or no increase in magnetic flux. Basically, the ferroresonant action is a flux limiter rather than a voltage regulator. Nonetheless, with a fairly constant supply frequency, the CVT can maintain an almost constant output voltage even as the input voltage varies widely. In fact, the output voltage of the ferro varies about 1.2% for every 1% change in supply frequency, such that even with a considerable generator frequency change (2-Hertz), results in an output voltage change of only 4%.
In short, CVTs are basically 1:1 transformers that are excited high on their saturation curves, thereby providing an output voltage which is not significantly affected by input voltage variations. This special characteristic is the foundation of the application of ferroresonant transformers as power conditioners.
|Ferroresonant Saturation Curve|
The Constant Voltage Transformer was invented in 1938 by Joseph Sola and continues to sell thousands of units per year. The modern units has undergone minor improvements but remains fundamentally unchanged from its original design. Also, CVTs are single phase devices, but can be used in combination of two or three to provide all of the ferro’s advantages in three phase applications.
Furthermore, constant voltage transformers have an auxiliary secondary (neutralizing) winding paralleled with one or more capacitors, forming a resonant circuit tuned to the power supply frequency. The purpose of the circuit (usually called tank circuit) is to mitigate the negative effect of core saturation that results in distortion of the voltage sine wave shape. In other words, the tank circuit serves as a filter to reject harmonics produced by the core saturation and provides the added benefit of storing energy in the form of AC oscillations, which is available for sustaining output winding voltage for up to ½ cycle of input voltage loss.
The ferroresonant transformer provides several benefits such as:
1. Superior Voltage Regulation - Even with considerable variations in input voltage of up to ±40% of nominal, the output voltage regulation will still be about 1% to 4%.
2. Sag Mitigation
If properly sized, a CVT can regulate its output voltage during a voltage sag to 60% for any duration. However, CVTs are not effective during momentary voltage interruptions or extremely deep voltage sags (generally below fifty percent of nominal). CVTs are often favored over other sag-mitigation devices because they are relatively maintenance-free, with no batteries to replace or moving parts to maintain.
3. Ride-through capability
As mentioned, CVTs also have the special characteristic of being able to store energy for up to 1/2 cycle because of its "tank circuit" design. Combining CVT with an inverter and static transfer switch in a UPS application, this characteristic provides a ride-through capability allowing a completely uninterrupted transfer to an alternate source. This application is very useful in the event of an overload, fault or even in the event of a total loss of the inverter, thereby maintaining power to the load.
4. Current Limiter
In some specialized applications, these devices can be designed to supply a constant current output, rather than constant voltage. Current limiting is an inherent benefit of ferroresonant transformers. In cases where the load current exceeds 150% of rated current of the ferro or CVT, the unit is pulled out of the saturation region, the output voltage collapses to a very small value and the current is thereby limited. However, in applications with high inrush current this characteristic becomes a disadvantage.
5. Isolation from Surges, Harmonics and Noise due to the CVT’s tank circuit design.
6. CVTs are durable and robust that some manufacturers offer warranties in excess of 10 years.
However, amidst all these benefits, the constant voltage transformer is not meant to serve as the only power conditioner to protect industries and facilities from power quality problems. Other power conditioners such as the UPS and TVSS should be use in conjunction for a more effective mitigation, if necessary.
Ironically, the principle of magnetism that provides the CVT its superior voltage regulation capability is also its main weakness. Ferros have low efficiencies compared to regular power transformers. At full load and ideal conditions, its efficiency can reach up to only 93% and substantially drops at light load. Meaning, as the ferroresonant transformer's loading decreases, its efficiency also reduces significantly. Typically, at half load, CVT efficiency ranges from 75% to 85% only as compared to a regular transformer that has an efficiency of more than 95% for the same loading percentage.
In applications with high overload or inrush currents, it is common that the constant voltage transformer be oversized by 2 to 3 times the rated load current. This is to permit the CVT to pass enough current before the output voltage collapses as described above. Consequently, oversizing it considerably reduces its efficiency.
Other disadvantages of CVTs are their large size, humming audible sound and relatively high cost.
Clark, J. (1990). AC Power Conditioners Design and Application
Dugan, R., McGranaghan, M., Santoso, S. and Beaty, H.W. (2004). Electrical Power Systems Quality (2nd ed.). New York: McGraw-Hill.
Utility Systems Technologies, Inc. (2009). AC Automatic Voltage Regulators