Correcting Voltage Sags with Constant-Voltage Transformers

CVTs can mitigate the effects of voltage sags

A well-known solution for electrical noise in industrial plants is the constant-voltage transformer, or CVT. These devices can filter distortion and notches from input voltages (see Fig. 1). CVTs have been used for years for voltage isolation as well. Other plants install CVTs for voltage regulation. Published curves in manufacturer data sheets show that CVTs can provide a fairly constant output voltage even as input voltages are reduced. However, even in today's age of information, many industrial facilities are not aware of one attractive CVT feature — the ability to mitigate the effects of voltage sags. Let's continue on and look at three types of CVTs.

Traditional CVTs

A CVT maintains two separate magnetic paths with limited coupling between them, as shown in Fig. 2, on page 47. The output contains a parallel resonant tank circuit and draws power from the primary to replace power delivered to the load. The transformer is designed so that the resonant path is in saturation while the other is not — a state known as ferroresonance. As a result, a further change in the primary winding will not translate into changes in the saturated secondary winding, resulting in voltage regulation. Single-phase CVTs can be manufactured for a variety of input and output voltages.

CVTs are attractive because they are relatively maintenance-free, with no batteries to replace or moving parts to maintain. They are particularly applicable to industrial-process control devices such as programmable logic controllers, motor-starter coils, and the electronic control circuits of adjustable-speed drives.

Laboratory tests demonstrate that CVTs must be sized properly to serve as effective conditioners for voltage sags. In fact, a properly sized CVT can regulate its output voltage during a voltage sag to 60% of nominal for a significant duration (see Fig. 3, on page 47). However, CVTs are generally not effective during momentary interruptions or extremely deep voltage sags below 50% of nominal.

To properly size a CVT, you must determine two things. First, you must calculate the amount of steady-state current drawn by all of the connected loads during their normal operation. The lower the ratio between the actual current drawn by the connected loads and the rated current of the CVT (% load), the better a CVT can regulate its output voltage (see Fig. 4, on page 47). For example, a 500VA CVT loaded to 500VA will not mitigate voltage sags nearly as well as the same CVT loaded to 250VA, and performance is even better if the same 500VA CVT is only loaded to 125VA. Typically, you would want a factor of 2.5 times the measured load.

Second, you must determine the inrush current of each connected load. Extremely high inrush currents can collapse CVT voltage outputs. Take the peak inrush current measurement for each connected load and multiply that number by the voltage. Then multiply that result by 0.5. After you've made both calculations, use the greater of the two calculations to properly size the CVT.

Ride-Through Transformers

A ride-through transformer (RTT) uses a 3-phase input voltage, but it has a single-phase output. The output is derived from a combination of the three input phases. The RTT enables constant output regulation during input voltage sags, even when two of the input phases are completely interrupted (see Fig. 5). During a voltage sag or brief interruption, an RTT accesses energy in unsagged phases of the supply voltage, even when fully loaded. This means that loads connected to an RTT rarely experience power disturbances (short of a complete 3-phase power loss) that adversely affect them. Of course, this keeps industrial processes running during otherwise costly power variations. RTTs have a unique advantage — an engineer can easily retrofit one in front of an existing control transformer for most industrial processes without having to change out any other components.

Laboratory tests reveal that the performance of the RTT depends upon the phase configuration of the voltage sag or interruption (single-, 2-, or 3-phase). For example, single-phase voltage sags applied to Phase B or C had no affect on the connected loads, even when the applied voltage dipped to zero volts. Simultaneous sags and interruptions of Phases B and C also had no effect on the loads. However, 2-phase sags involving Phase A would drop the loads when voltages were below 50% of nominal. The RTT performed like typical CVTs during 3-phase voltage sags.

To protect equipment against voltage sags below 50% as well as power interruptions, engineers will employ energy-storage devices, such as uninterruptible power supplies. RTTs provide an alternative to energy-storage devices. Fig. 6 and Fig. 7, on page 47, show the response of an RTT to phase-to-neutral and phase-to-phase sags.

Magnetic Synthesizers

The magnetic synthesizer is another ferroresonant device that consists of inductors and capacitors configured in a parallel resonant circuit with a network of six saturating pulse transformers. The output is synthesized by combining pulses of the saturating transformers into “building blocks” similar to the pseudo sine wave of many electronic inverters (see Fig. 8). This device is typically used for applications of 50kVA and larger, where voltage regulation, sag mitigation, and/or isolation are needed.

Magnetic synthesizers are good for solving a wide variety of power quality problems, including voltage sags, sustained undervoltages, notches, transients, and even waveform distortions. It's also effective against voltage sags at full load; however, performance improves if loads are less than 100%.


Engineers have been using ferroresonant technologies for many years. We frequently come across cases where single-phase CVTs have been installed for isolation or noise. But the simple act of oversizing a CVT for an application can provide ride-through during voltage sags as well. Better yet, there are a number of innovative ferroresonant-based products available today that can overcome the known shortcomings of traditional CVTs.

Doug Dorr is the business development manager at EPRI PEAC Corp. in Knoxville, Tenn. You can reach him at [email protected].

Doni Nastasi is a power quality engineer with EPRI PEAC Corp. in Knoxville. You can reach him at [email protected].

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