AC VOLTAGE REGULATOR WITH
AN
INTERM
E
DIATE FREQUENCY
INCREASING
Sergey Yu.Dyaglev
National technical university of Ukraine “Kyiv Polytechnic Institute”
Chair of Industrial Electronics, NTUU “KPI”, Yangelacademician street
,
16/9, Kyiv, 03057,
Ukraine
Tel. +3
8063

830

47

71, E

mail:
sergey5777@rambler.ru
Abstract

In this paper
a
new
high

frequency regulator of AC voltage
is considered,
its
operating principal
is
explained
and a comparison
with an analog
ic
regulator
is
provided.
The
considered r
egulator of AC voltage reduce
s
static power losses
,
has
simpl
e
passive
circuits for
solving
of
‘
commutation
problem’
and
simple
control
requirements
. Regulator provides
smooth
voltage
regulation that could be higher o
r lower th
an
an
input voltage
that
is suitable
for
line conditioners
.
Key words
–
AC voltage regulator, high frequency voltage regulator, PWM voltage regulator.
INTRODUCTION
Nowadays
low

voltage AC

AC
converters(about 400V)
,
which
perform
output
voltage
of
higher or
lower
value
than theinput
voltage
, are of great demand
. Such
converters
are useful in
line conditioners
and
devices of
voltage quality enhancement
. The most
perspective AC

AC converters for this purpose
are converters with int
ermediate frequency
increasin
g. The main reasons of that are good
w
eight and size characteristics, fast
response
,
high quality of output voltage. The most
popular high frequency AC

AC converters
have
buck

boost type
[1]
and buck type with
voltage

adding transformer [2

3]
. S
implicity
a
nd
high
efficiency
of buck

boost converters
make them attractive f
or us
age
, but buck

boost
topology has
drawbacks
,
which limit
their
usage
in practice
for high

power applications
.
Some
of
the buck

boost converters
disadvantages
are
high peak current in
pow
er
switches
that
could be higher than a t
wice
of
amplitude of load current
and
high pulsation of
the
output voltage.
So
, the
usage of buck

boost
AC/AC converters
in practice
is limited byload
power
of
less than few kVAs.
MODULATING AC

AC CONVERTERS
Another
topology of AC

AC converters
that could be
used fo
r high

power regulators
is
called “modulating converters”
[
4
]. The
structure scheme of these converters is showed
on Fig. 1, where M
–
modulator, D

demodulator, L1

C1
–
input rejection filter,
L2

C2
–
outp
ut smoothing filter, TV1
–
power
transformer. When a sinusoidal voltage is
applied to
the converter
input (Fig. 1)
,
modulator
M
c
onverts it into a high

frequency
voltage, that is applied to a primary wi
nding
W1
of power transformer TV1. A voltage from
a sec
ondary winding
W2
is applied to a
demodulator
D
, which is working
synchronous
lywith
modulator
and with
the
same frequency
. Demodulatorprovides high
frequency voltage, which average value could
be regulated. This high frequency voltage is
summing with
the
in
put voltage and filtered by
the
output smoothing filter L2

C2. So
,the
output
voltage
of AC/AC converter c
an
have
both higher and
lower
value
than the input
voltage.
Fig. 1.
Structure of
modulating
AC/AC regulator
D
ifferent topologies could be used
as
modul
ators and demodulators
but as
mentioned earlier
the
goal
of the work
is
a
design of
high

power AC/AC converter, so it’s
necessary
to use
double

ended
topologies.
Oneof possible realizations of modulating AC

AC converters
Fig. 2. Modulating AC/AC converte
r with full

bridge modulator and demodulator
with
double

ended
modulator and demodulator
will be
considered
in the next section.
FULL

BRIDGE MODULATING AC

AC
CONVERTER
A schematic of the full

bridge modulating
AC

AC converter is show
ed
on a Fig. 2.
Mod
ulator of the converter is built on the
transistors VT1

VT8 and diodes VD1

VD8.
Instead of pairs of transistors and diodes,
which form bidirectional power switches it’s
possible to use reverse

blocking IGBT power
modules
[
5
]
. Demodulator of converter consis
ts
of transistors VT9

VT16 and diodes VD9

VD16. In the simplest mode of operation
modulator and demodulator work as voltage
inverters, so that switches VT1, VT2, VT7,
VT8 are switching simultaneously and in
opposite phase to the transistors VT3, VT4,
VT5,
VT6, which are switching
simultaneously
one by one
.
In order t
o avoid a
short circuit current flow through transistors
some dead

time should be added to
the
control
pulses. The transistors of demodulator are
working in the same manner
and
with the same
fre
quency as the modulator’s ones, but w
ith
the some phase shift over the modulator
(Fig.
3).
Changing the phase

shift it is possible to
regulate time interval
when voltage of
secondary winding of transformer TV1 is
added or subtracted from the converter’
s i
nput
voltage
. Therefore it allows regulating of
the
converter’s output voltage.
The
output voltage
in this
operation
mode
can
be calculated
as following:
,
(1)
where V
in
–
input converter voltage, n
–
transformation ra
tio, γ
–
duty cycle.
Fig. 3. Timing diagrams of
full bridge
AC/AC converter
Fig. 4.
S
chematic of proposed AC/AC converter
PROPOSED AC/AC CONVERTOR
The main drawback of
the
full

bridge
modulating AC

AC converter (
Fig.
2
)
is
high
static loses
i
n the power switches
. In this
convertor current flows through four diodes
and four transistors simultaneously. I
ncase
of
reverse

blocked IGBTs
usage the
current
flows
just
through four transistors
simultaneousl
y
, but the static loses are still
high.
In orde
r to
reduce static loses we should
try to use other double

ended topologies such
as push

pull.
Principle schematic of the
proposed
modulating push

pull AC

AC
converter is showed on Fig. 4.
The proposed
c
onverter consists of two back

to

parallel
ly
joined vol
tage inverters with hard switching,
which are
designed from
transistors VT1

VT
4
and
diodes VD1

VD4
,
and
of
two
back

to

parallel
ly
connected
synchronous rectifiers,
which are
designed from
transistors VT5

VT8
and diodes VD5

VD8. Passive circuits VD9

VD24 an
d C2

C9 are intended for solving
of
‘
commutation
problem’. AC

AC converter
also
has
input rejection filter L1

C1 and output
smoothing filter L2

C10.
Convertor
is bi

direct, so it could work with
a
reactive load
and provide energy recuperation back to
the
p
ower
grid
.
As we can see
,
in the
proposed
converter
current
flow
s just
through two
transi
stors
and diodes simultaneously, but
a
voltage on the switched

off transistorsis at
least
twice higher than
a
voltage on
the
transistors
in full

bridge converter (Fig. 2)
that
is a drawback of
the
prop
osed converter.
Timing diagrams of voltages on the elements
of
the
converter are shown
on Fig.
5
, where
V
c
–
input converter voltage waveform, V
w2
–
voltage of secondary transformer windings,
V
out
–
output converter voltage, V
gs1,4
, V
gs2,3
,
V
gs5,8
, V
gs6,7
–
control voltages of transistors
VT1 and VT4, VT2 and
VT3, VT5 and VT8,
VT6 and VT7 respectively.
The
duty cycle is
calculated
by
the
following expression:
(2)
wheret
i
is a ‘cross

over time’ of transistors
control pulses, T
–
a half of
the
convertor’s
operation period.
A value of
an
output voltage
could be calculated by the (1).
As we can see
on Fig.
5
transistors VT1 and VT4, VT2 and
VT3, VT5 an
d VT8, VT6 and VT7 are
switching simultaneously
,
which allow
s
the
converter
to be bi

direct and
recuperat
ing
theload reactive energyto the power grid.
Fig. 5. Timing diagrams of proposed AC/AC converter
SWITCHING PROBLEM SOLVING
As mentioned
in [1] a
switching problem
is a main trouble of AC

AC converters. When
transistors VT5 and VT8 are switching off
,
a
little pause should be hold before switching on
the transistors VT6

VT7
in order
to prevent a
short circuit. When all of the transistors VT5

VT8 are
in off condition a current of
the
inductor
L2 is flowing through the snubber
circuits
VD17

VD20, C6

C7 or VD21

VD24,
C8

C9
. So, the current in the inductor isn’t
discontinued and voltage spikes don’t occur,
but a power is dissipating
on
the supressors
VD1
8, VD19, VD22 and VD23. This power is
as lower as a dead

time
of
the transistors
control pulses lower.
A dissipating power
could be approximately calculated by the
following expression:
(3
)
where f
–
converter operating frequency,
t
dt
–
dead

time, P
L
–
load power, V
out
–
load
voltage, V
BR
–
supressors breakdown voltage.
In most practical cases this power is much less
than the static loses in the converter and don’t
sufficiently affect an efficiency of
the
converter.
AC

AC CONVERT
ER MODELING
On
a
Fig.
6a
SIMULINK model
of the
converter’s schem
atic
is presented.
Simulation
of AC

ACc
onverter was provided for input
voltage of
V
in
=
220V
, duty cycle γ=0.4, serial
active

inductive load R
L
=25Ohms, L
L
=30mH.
Fig. 6. Model schematic of proposed AC/AC converter
Fig. 7. Simulation results, input voltage, output voltage,
output current, input current
Fig. 8. Structure of line

conditioner
with a digital control system
CLOSED LOOP S
Y
STEM
As mentioned
above
, AC

AC converters
of modulating type are useful for line
conditioners and have several advantages over
the other types of AC

AC converters such as
fast response, possibility of construc
ting high

power line

conditioners. To have a complete
system such as line

conditioner
,
a controller
should be added to the AC

AC converter. A
controller may be analogous or digital.
Nowadays digital controllers became more and
more popular what is caused b
y their
flexibility and great possibilities to design a
different service functions such as soft

start
algorithms, protect algorithms, indicate
,
measuring
, interface
functions and others[
6
].
In
this
section a
modeling
of
thelineconditioner prototype is con
sidered.
On
a Fig.8
a structure scheme of line

conditioner is
showed.
Digital control system is based on the
microcontroller
ATMega 8
which has
necessary parts of digital control system such
as ADC, DPWM, program

based PID

regulator and EEPROM that saves ref
erence
table
of the
sinus function.
Synchronization
block is required for synchronizationmoments
of
the
input voltage zero

crossing with a
nulling of thereference table
index
, a
voltage
protect block is required for
giving the
controller know that the inpu
t voltage is in the
proper diapason.
The
results of mode
lingthis
system are showed
on a Fig. 9
.
On a Fig. 10 a prototype of 4kVA line

conditioner under construction is showed.
Fig. 9. Modeling a closed

loop system
Fig. 10. Photo of 4kVA line
condit
ioner
prototype
under construction
CONCLUSIONS
In this paper new modulating AC

AC
converter is proposed which has reduced static
power loses and simpler control requirements
in comparison to the known one.
The proposed AC

AC converter is bi

direct which
makes possible to recuperate
energy from
a
load back to the power grid.
The proposed converter has simple
passive circuits for solving commutation
problem.
Modeling of AC

AC converter with an
open loop
control
and with a digital
microprocessor control sys
tem was provided.
Simple requirements for driver and
control circuits, possibility of constructing
high

power lineconditioners, excellent weight
and size characteristics
and
fast response make
this AC

AC converter
attractive for
commercial using.
REFERENCES
[1]
Vazquez, N. Velazquez, A. Hernandez. “AC
Voltage Regulator Based on the AC

AC Buck

Boost
Converter”. IEEE International Symposium on
Industrial Electronics ISIE’07, pp. 533

537.
[2]
C.A. Petry
, J. C. Fagundes, I. Barbi, “Study of
an AC

AC Indirect Converter for Application as Line
Conditioner”. IEEE International Symposium on
Industrial Electronics ISIE’05, pp.757

762.
[3]
C.A. Petry, J. C. Fagundes, I. Barbi, “New
Direct AC

AC Converters Using
Switching Modules
Solving the Commutation Problem”. IEEE International
Symposium on Industrial Electronics ISIE’06, pp.
864

869.
[4
]
Taltronics. “Application technologies of
reverse

blocking IGBT”. Energize, August 2007, pp.56

60.
[
5
] Kobzev A.V. “Modulati
ng power supplies of
radio

electronic equipment”, Tomsk, “Radio and
broadcast”, p. 14

22, 1990.
[6
] Geof Potter. “An Introduction to Digital
Control of Switching Power Converters”.
DCDC
TechnicalWhitePaperfromAstecPower
, April 2004.
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