[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Many of the present voltage-regulation technologies can also provide mitigation of other power-quality
problems (e.g., voltage-sag ride-through improvement or isolation for transient overvoltages). There are
seven basic devices in use today. These include:
• Motor-actuated voltage regulator — Generally inexpensive, these devices can handle high kVA
loads, but they are slow to respond to changes in the electric-service supply and can only correct
for gradual load changes. See Section 2.7, Step-Voltage Regulators.
• Saturable reactor regulator — Relatively inexpensive and with a wide load-kVA range, these devices
have a sluggish (five to ten cycles) response time, high output impedance, and are sensitive to a
lagging-load power factor.
• Electronic tap-changing transformers — These devices use triacs or silicon-controlled rectifiers
to change taps quickly on an autotransformer. They respond in 0.5 cycle and are insensitive to
load power factor and voltage unbalances.
• Automatic voltage regulator — These devices function as an uninterruptible power supply with
no energy storage. They have a fast response time (1 to 2 ms), but the need for a fully rated 60-
Hz transformer can make their cost unacceptably high.
• Hybrid electronic voltage regulator — These devices use a series transformer and a power converter
to accomplish the voltage-regulation function.
• Soft-switching automatic voltage regulator — These devices combine the high performance of
active line filters with the lower cost of the more-conventional solutions. The electromagnetic
interference generated by these units is low in spite of the high-frequency switching employed.
• Constant-voltage transformer (ferroresonant transformers) — Appropriate application of a CVT
can handle most low-frequency disturbances, except for deep sags or outages. Detailed descriptions
of these devices are provided in the subsequent sections.
2.8.1.5 What Constant-Voltage Transformers Can and Cannot Do
CVTs are attractive power conditioners because they are relatively maintenance-free; they have no batteries
to replace or moving parts to maintain. They are particularly applicable to providing voltage-sag
protection to industrial process-control devices such as programmable logic controllers (PLC), motorstarter
coils, and the electronic control circuits of adjustable-speed drives.
Ongoing research [3] has demonstrated power-quality attributes of CVTs that include filtering voltage
distortion and notched waveforms. Figure 2.8.7 depicts typical distorted and notched input voltages
versus the filtered CVT output. Also, a CVT can practically eliminate oscillating transients caused by
capacitor switching and can significantly dampen impulsive transients caused by lightning (see
Figure 2.8.7).
To ensure full protection of sensitive electronic loads, CVTs may need to be coupled with other devices
designed to mitigate dynamic disturbances. In addition, CVTs have been used for years for voltage
isolation as well. Many plants install CVTs for voltage regulation. CVTs also offer protection for voltage
swells. If properly sized, a CVT can regulate its output voltage during input voltage sags to 60% of nominal
voltage for virtually any duration (see Figure 2.8.8).
However, many commercial and industrial facilities are not aware of most of the CVT’s attractive
features. At the same time, the CVT technology also has some negative characteristics, which in some
applications may possibly outweigh its benefits. Some of these include:
• CVTs are not effective during momentary voltage interruptions or extremely deep voltage sags
(generally below 50% of nominal).
• Because CVTs have relatively high output impedance, they produce large output drops during
high current demands. As a result, conventionally sized CVTs cannot handle significant changes
in current and are more attractive for constant, low-power loads.
• Because CVTs are physically large devices, it is not always practical to install this type of device
in either a small-office or home environment.
• CVTs produce heat and noticeable operating hum and are sensitive to line-frequency variations.
• CVTs can have relatively poor efficiency and high losses for light loading conditions.
2.8.2 Applications
Distorted Input
Notched Input
Input with Oscillating
Transient
Input with Impulsive
Transient
FIGURE 2.8.7 Filtering ability of a constant-voltage transformer.
Ferro Output
Ferro Output
Ferro Output
Ferro Output
FIGURE 2.8.8 Voltage regulations with a constant-voltage transformer during a voltage sag to 60%.
Constant-voltage transformers have proven to be a reliable means of enhancing voltage-sag tolerance of
industrial process-control elements such as relays, contactors, solenoids, dc power supplies,
programmable logic controllers (PLCs), and motor starters. As mentioned earlier, while the CVT tends
to work well for voltage sags, they are not a good solution for momentary interruptions. Additionally,
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC