Page 165 - Power Quality in Electrical Systems

Chapter
10

Dynamic Voltage Compensators

Dynamic voltage compensators for voltage sags and surges
represent a simpler and less costly means for achieving
acceptable power quality than battery-powered UPS.

Introduction
Conventional utility power-system equipment has long utilized trans-
formers with automatic under-load tap changers to compensate for devi-
ations in line voltage above and below a desired reference level. The
circuit for a transformer with a tap changer is shown in Figure 10.1. The
controller selects a voltage from the secondary winding and adds or
subtracts it from the secondary voltage through a series transformer to
produce the desired load voltage at terminal X1 [10.1]. The operation of
the mechanical tap changer is too slow to compensate for rapid devia-
tions in line voltage that affect voltage-sensitive equipment. By com-
parison, the dynamic voltage compensator utilizes power-electronic
devices to switch and compensate for line-voltage sags within a half-cycle
time to meet the power-quality requirements of computers and other
voltage-sensitive equipment.
The operation of the dynamic voltage compensator requires two steps.
In the first step, when the source voltage is within an acceptable band,
the compensator utilizes a by-pass switch to connect the source directly
to the load. In the second step, when the source voltage sags outside of
prescribed limits, the compensator injects a correction voltage utilizing
power from the source or internal capacitors. The compensator is less
costly and more efficient than a battery-powered UPS that requires stored
energy in batteries and is usually online continuously supplying the load.
The size, rating, and cost of the components of the compensator
depend upon the maximum voltage and time duration of the deviation
to be compensated. For sags down to zero voltage lasting up to 0.2 s,

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