AC VOLTAGE REGULATOR WITH AN INTERMEDIATE FREQUENCY INCREASING

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2 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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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.