ÇUKUROVA UNIVERSITY INSTITUTE OF NATURAL AND APPLIED ...

3. FUNDAMENTALS OF DVR Mustafa İNCİ
converters becomes difficult to justify owing to the high voltages that the switches
must block (Roncero-Sánchez et al., 2009).
One solution is to use multilevel voltage-source converters which allow high
power-handling capability with lower harmonic distortion and lower switching
power losses than the two-level converter (Roncero-Sánchez et al., 2009).
Among the different topologies of multilevel converters, the most popular
are: neutral-point-clamped converters (NPC), flying-capacitor converters (FC), and
cascaded-multimodular H-bridge converters. NPC converters require clamping
diodes and are prone to voltage imbalances in their dc capacitors. The H-bridge
converter limitations are the large number of individual inverters and the number of
isolated dc voltage sources required. The main drawback of FC converters is that the
number of capacitors increases with the number of levels in the output voltage.
However, they offer more flexibility in the choice of switching combinations,
allowing more control of the voltage balance in the dc capacitors. Furthermore, the
extension of a converter to a higher level one, beyond three levels, is easier in FC
converters than in NPC converters, which makes the FC topology more attractive
(Roncero-Sánchez et al., 2009).
3.2.2.2. Rectifier
AC to DC converter is used to convert AC into DC form. The topology used
in dynamic voltage restorer is diode rectifier. Installation of a diode-bridge rectifier
circuitry to the DVR provides an economical means for the dc-link to negotiate
active power. Unfortunately, this DVR configuration only allows unidirectional
power flow from the diode-bridge rectifier circuitry to the inverter. When swell or
overvoltage occurs in the power distribution lines, the conventional in-phase and
phase-invariant voltage injection schemes cause inverters to absorb active power
from the lines, which charges up the dc storage capacitors and increases their voltage
levels. Excess dc-link voltage rise will damage the dc storage capacitors and
switching devices. Moreover, the rise in dc-link voltage will nonlinearly increase
switching loss and lowers the DVR’s system efficiency (Lam et al., 2008).
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