8/12/2019 [] Topologies for Uninterruptible Power Supplies[1993]{Krishnan}
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Topologies for Uninterruptible Power Supplies
R
Krishnan
and
S.Sriniwan
A h t r a c t
The Bradley Department of ElectricalEngineering
Va.Tech, Blacksburg,VA 24061, USA.
e-
mail [email protected]
Ph:
703 231 4311
)
Fax 703 231 3362)
Tbip paper reviews the development
of
Uninterruptible Power Supply (hereafter referred
to P UP8 )
over the years from one with
E R
front end charger with isolation at h e requency
to one with sinuaoidd current input c-r
with isolation at
high
frequenay (h.f.).
Thip
paper
is
concerned
with the developments
in
the single
phase low power
(less
than 1 kVA UPS topologies.
Each scheme hi
analyzed
briefly and its principal
meritsand demeritsare dentified.
1. ntroduction
UPS systems
are intended to improvethe quality of
ac
power in order to provide continuos operation of
ac powered equipments[l]. To accomplish these
functions a UPS takes in utility ac input and
improves the power quality through power
processing. It also provides a redundant (Back-up)
power source so
that
the load wi l l be interfaced to
the utility
directly
in
the
case of failure of a n y
subsystem. Power quality defects which may be
improved by the UPS include surges, noise, sags and
harmonics. A block diagram of the general UPS is
shown in
Figure
1.
t
Figure
1 Block Diagram of a general
UPS
The most recent developments having a profound
effect
on he
configuration of
UPS
topologies include
High frequency switching
to
minimize
filter
size.
New semiconductor power devices resulting
in a dramatic increase in the
UPS
power
output.
Digital techniques with microprocessors and
DSPs to implement complex control
algorithms.
Some of the desirable features of anUPS are :
Sinusoidal input current drawn fi he utility
mrrine.
Sinusoidal output voltage regardless of
the oad.
Isolationat low weight and cost
High efficiency and hence minimum power
conversionstages.
High reliability.
Acceptance of wide variation of the input voltage
with novariationofoutput voltage.
No single topology satisfies al l these requirements
simultaneously. Every topology tr ies to optimize one
or
many of
the desirable features giving rise
to
a
wide variety of UPS configurations.
The
choice of a
particular topology depends on
the
application and
selective optimization of the desired features. This
paper discusses the variousUPS topologies from th s
view-point.
The UPS
is
classified depending on the power flow
path chosen. If the primary power flow
is
through
the
surge
suppressor and filter to the load
when
the
ac input
is
healthy, and through the battery and
inverter
to
the oad when
the
ac input fails,
then
i t
is
a standby UPS system. If the primary power flow is
through the charger, battery, inverter and load
when the ac input is healthy then
it
is an on-line
UPS system.
The
path through surge suppressor,
filter and transfer switch is resorted to, when the
other path
is
out-of-operation due to failure or for
maintenance.The classification
of UPS
topologies
is
shown in
Figure
2.
The paper is organized
as
ollows. Section 2
contains
standby off-line
opologies.
Section 3 contains
on-
line
UPS
opologies. Line-interactiveUPS topologies
are
described
in Section
4.
Conclusions are
summarized inSection5.
2.
Standby
UPS
characteristics
A
block diagram of this configuration
is
shown in
Figure1. In
the
standby
mode the
charger size is
@7803-1227-9/93/ 3.M)@993 I
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small since
it
has
to
meet the power requirement of
charging the battery alone. Standby mode has a
transfer time associated with
it
during power
failure.
Ups
Topologies
t tandbyoffline
Offline
Online
r
nlinewithbackup
~
Online without backup
Online withLow Frequency
Isolation
Online
with
High Frequency
Isolation
t
Standby Online Hybrid Topology
Line Interactive
Figure
2
:
Classification of
UPS
Topologies
2.1
Triport UPS
A
block diagram of
this
configuration
is
shown in
Figure
3
with
a
special transformer arrangement
with three windings
known
as triport
UPS. Triport
UPS is an example of standby ferro topology, which
furnishes power directly to the load from the
commercial
ac
line through the triport transformer
in the n o d ondition. When the ac input fails
power
is
furnished by the battery through the
inverter. Triport topology uses the ferro resonant
technique, where the transformer
is
used
as
a
voltage
regulator.
Figure 3
Block Diagram of
a
Standby F e m
UPS
Advantages
Line conditioning is passive and the technique
is
very robust.
During normal
power conditions the converter
can be used in the rectifier mode to recharge the
battery.
This essentially
gives rise to a
chargerless
topology.
This
technique has high efficiency and
reliability, as well
as
moderate cost.
. The transformer has a special capability @ e m
Resonant capability) which provides limited
regulation and output wave-form shaping.
Isolation
is
provided
from
the
ac power
transients.
Disadvantages
The ferroresonant transformer tends to be fairly
heavy and
of
low efficiency.
A
conventional triport
topology
tends
to be off
line and transition from ac to inverter can be a
problem under certain low line or high line
conditions.
Tbe quality of output wave form under non-
linear loads
is
generally poor.
2.2
Improvements in Triport
Technology
The instability problem of
almost
periodic oscillation
in triport systems using the constant voltage
transformer (CVT) [2]
has
been addressed.
A
stability problem exista a t light load condition
requiring the
use
of
a
dummy load. Such
a
solution
tends to deteriorate the efficiency and result
in
a
temperature
rise
of the system.
A
stability
improvement technique by utilizing the active filter
as a
feedback element
is
proposed. An improved
version of
the
triport
is
presented[3].
A
tetra port
topology
has
been derived
to
correct ac output
distortion when the inverter
fails. It uses
two
parallel reversible inverters.
This
has the advantage
of redundancy in supplying energy to the load
in
case of a network's failure. Another advantage
is
the
better
waveform given by the inverters, which
allows
no
saturation
in
the
transformer
magnetic
core, and obtaining a high stability in the output
voltage.
3.
On-Line
UPS characteristics
A block diagram of this configuration is shown in
Figure
4.
The power flow in
an
Ups of this ype
has
been explained in the introduction. The other
features of
t h i s
configuration are
In case of power failure, there is no transfer
time associated
with ths
configuration.
=
The charger
is
much larger and
of
much higher
rating
th n
a standby UPS, since
it
has
to
be
designed to meet the load requirements during
normal operation. This w i l l ensure that the
battery
will
not
be
discharged when
the
input
power
is
normal.
. The
flow
of power through the charger and
inverter during normal operation causes
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additional power loss and poorer efficiency
compamdtostandby UPS.
Power
conditioning
c n
be provided during
normal operation.
Schemes 1
to
10 diecussed below
u
a
diode
rectifier bridge, most of themwithout idation at
line
hquency. The
manner
in which the rectified
voltage is pmceecred to charge the battery and
maintain
the input
to the
nverter leads
to
diffemnt
topologies.
3.3.1
Scheme
1
Scheme
1,
own in Figure 6, has the step down
chopper reducing the mctiied ac voltage
to the
evel
of the batfay. Note that larger
the
difPerence
between
them
voltages,
d e r
i l l
be
the
duty
cycle of the chopper which
impoms
a
large
output
filter requirement. The
scheme
has minimum
number
of switches, lende itself to compact
packaging and has a
3
etage power proceasing with
the consequent advantage
of
high
efficiency.
1mdOnWr
Figure6:Schemel
3.3.2 Scheme 2
Figure4 Block
Diagram of
a On-Line
UPS
Scheme
2, shown
in
Figure
7,
has
an
SCR
hopper.
To educe the filter
size,
the chopper
is
preceded by
a
step down lransformer
at
the
ac
input. But this
ad& to the weight, vol- and coat of the s y h
input
transformer
fi amenhanced harmonic 1-8
3.1On-line up with Bypass
This is
an on-line
U PS
haw a transfer switch or
me~haniemmwciated with it
8 that
the l a d Will
be fed by the
a~
input diredly in the event of the
and redb
n
a low p ow er -h d ty packaging.
is poor due to the additional 18 in the
inverter/ charger failure.
3.2 On-Line UPS Without Bypass
A block diagram of this configuration
is
shown
in
Figure 6
In this topology, the general UPS
is
set
to
operate in the on-linemode, but the entire
back-
up path
is
removed.
Since
there
is
no
back-up power
source
or by-
paee, the UPS
does
not provide Back-up power
in
the case of failure of any subsystem.
Redundancy, one of the most important
charactmistics of UPS is not achieved in
this
typeof UPS.
.
nf RT
a4l lERYH C Kt rI
W R IMRlfR
Figure 6
On-Line
UPS witbut b y p a
3.3 On-line Ups with
Low
F'requency
Transformer Isolation
Tlk input power factor is also poor.
Figum7:Scheme2
3.3.3
Scheme
3
A block diagram
of
this configuration is given in
Figure 8. The
use of multi-phaae chopper reduces
the
ipple and
hence the
capacitor
rating.The
nput
p.f. is poor due
to
the use of a buck chopper
to
scale
down
the
rectified
dc
voltage.
Figure8:Scheme3
The topologies under thi s configuration have a
3.3.4sckme4
transformer isolation
at
the
output which
ia
at
low
and large in Size,
--
he Weight and
volume of theUPS.
frequency*This
haS
the baWback Of
m e cheme, shown
in
F ure 9,
a
boost
chopper
in
the hn t-e nd which
is
modulated to proGds a
cine
input ac current and
unity power
factor. S i
he
boost output voltage
ie
greater than the peak input
1
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voltage
,the
inverter operates with this high voltage
with high efficiency and thus removes the drawback
of al l previous schemes. The low voltage battery is
charged through a buck converter.
The
boost
converter charges the dc
link
when the ac input
f d s .
The
buck converter requires a very small
rating
whereas the boost requires a
1
p.u. rating.
Due to hf operation, the front-end converters lend
themselves to compact packaging. Isolation is
provided only
at
the output.
Figure
9
Scheme
4
3.3.5 On-line UPS With
a
Reduced Number
of
Switches
A new
UPS
topology is proposed
[5]
with single
stage power conversion and a reduction
in
the
number of active switches. A block diagram of the
topology is shown in Figure 10. When the input
line
current
is
positive diode
D5
conducts. During this
period turning on switches S1,S2 results in V =O
and turning on
S3,M
c a m s Vx=Vb The inductor
current thus keeps increasing and decreasing for
this switching sequence. Current feed-back helps to
keep the nput current profile sinusoidal.
The
diodes
D5
and
D6
cause
the
power flow from the supply
to
be unidirectional.
The
circuit thus
functions
Like
a
semi-controlled converter. The inverter is of a single
phase
full
bridge type. Turning on S1,S2 or
S 3 , a
causes Vy=O. When
S2 3 are on
V -Vb and when
SI,= are on vy'-vb. Switches d& thus form
single phase inverter to supply the load.
A
current
regulator with load feed-forward is used to obtain
output voltage regulation in the presence of
nonlinear
loads. The transformer tap is selected
such tha t under low-line conditions the inverter can
fully supply the load. The static switches bypass the
inverter and connect the load directly to the supply
in the bypass mode. The topology has many
desirable features like a common neutral between
input and output, sinusoidal input current
irrespective of the load current and batte ry
chargddischarge control without
any
additional
devices. However the
main
draw-back of t h i s
approach is that
the
battery
has
to be rated for the
peak input voltage V . Any effort to decrease the
battery voltage resurts in the necessity for an
additional device negating
the
advantage
of
a
reduced number of switches.
I
1 ... I 1 I
Figure 10 Circuit Schematic of
the
new converter
3.4 On-line Ups with High Frequency
Transformer Isolation
The topologies under
this
category have
a
high
frequency transformer
link.
The use of high
frequency transformer reduces the weight
and
volume of the
Ups
significantly. The use of high
frequency
FWM
techniques reduces the size of
the
filter required in
the UPS
additionally.
Most of the
modern static
UPS
are
in this
category. The
following topologies use the above technology.
3.4.1 Scheme
The acheme
5, shown in
Figure
11,has a high
frequency link for isolation, replacing the line
frequency isolation in
Schemes
1
o4.
he battery
ia
of low voltage type. The differential voltage between
the battery and boost output voltage is handled
through
a
buck converter
stage
the
rating
of which
needs
to
be very4.
Figure
11
Scheme 5
The boost chopper provides sine input current
and
uni ty power factor as in Scheme
4.
The s a m e
boost
chopper is used to charge the dc link from the
battery when the input
fails.
The efficiency of
the
systam may not be high during this mode.
But
during normal operation,
it
wi l l
be
high
since
only
four power processing stages
are
involved and
that
too a t high voltages. The hf transformer educes
the packaging size and cost
3.4.2 Scheme 6
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A UPS wing a
variation
dt eabove topohgy, is
shown m
12 [4].
he
charger cont da of a
isolated dc-dc awitch mode converter using a full
bridge transislm circuit and a
hf
transformer. By
SUitaMe ccmtrol
o f t h e
gata signals to the bridge
circuit,
the
input
mnmnt
is
forced
to
maintain
a
This
mpmystha
input p o w factor of the circuit
to
near
e he output dc
link
voltage
is
mainhind constant by voltage feedback thereby
charsingtbe
bathry.
E i n d d a l protite m
phee with the input voltage.
Figure12: schsme6
The inverter is of
high
voltage
full
bridge type with a
Lx:output
tilter.
Output
voltage
feedback keeps the
w a d - d i d d l a l
b e
ven for
non-linear
oads.
The UPS baamany deejrable
aracbWcs
such
as
high input
p -factor,
high
efPiciency,
small
size
and good r l l r rr i rw reep~nfre.The draw-back of this
lacheme isthat
ths
battery I6 ofhighvoltage type.
3.4.3 Scheme
7
The Scheme 7 hown
n Figure
13
has
a
chopper at
the input to match the required output voltage to
tbe battery volttaget. The battery voltage is then
stepped up to highvoltage through a high frequency
inverterand--.
FA
F igure 13 Scheme 7
The
hf link
ia the
stage
of bolation. The output
inverter
operaters
with a high input voltage and high
efficiency.The disadvantageof
this
scheme
is
that it
requires
5
stages
of
power processing.
3.4.4
Scheme8
T6e Scheme 8 shown
in
Figure
14
has solation at
both the bathery input and output,
at
high and
line
kquency, respectively. Thie is achieved by
removing
the chopper and
comb-
the buck and
hf
link
function
in the
hf
i n d r
stage
with
the
hf
t r e r .
The
dieadvantageof thi s scheme s that
the output inverter has a low voltage input and
hence the aystem efficiency is not high.
Figure14:Scheme8
3.4.6 Scheme9
The
Scheme9 hown in igure
16
has aboost and
buck converter with hf isolation stage
The
booet
stage provides
a eine
input current at unity
power
factor. The buck stagematches
the
syetam voltage to
that of the battery. This
Still has
the drawback of
the
output inverter
operating at
low batte ry voltage.
Figure
15
scheme
9
3.4.6
Scheme10
The scheme 10
shown in
Figure 16,d i s he
desirable features of Scheme 8 nd Scheme
4.
Here
sine input current,
unity
power factar and operation
of output inverter a t high voltage are
obtained.
Figure 16 Scheme 10
The battery is interfaced
w i t h
a buck and booet
converter
for charging and discharging eqmctively.
The
isolation is a t
hf
lea- to
compact.
packaging.
3.5. Standby
On-LineHybrid Topology
A block diagram of this configuration is shown in
Figure
17.
The
load is fed by the filte#iiverhr
combination when the power is normal.
Consequently the size
and
rating of the battery
charger is
small
It thus
has
the advantage of
standby
UPS
topologies. In case of ac power
failure,
the
load is fed by the battery.
The
standby
Wdc converter is switched
on
during the power
failure. It is used to d e
he
batte ry voltage. This
topology
does
not exhibit
a
transfer time
during
power failure. It thus
has
the advantage of on-line
UPS
topoloees also.
There is
no backup power
provided
in
the cane of the failure of any subsystem.
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Figure 17 Hybrid Standby On-line Topology
4 . Line Interactive UPS
A block diagram of
this
configuration is shown in
Figure 18a. Configuration of the cycloconverter is
shown
in
Figure 18b [SI. n
this
hybrid design the
battery and bidirectional cycloconverter are
always
connected to the
output
of the UPS.
tine
hlerrupkr
npllc
B i D i e c t i i o l
Cyrkmverln
V b
Figure 18a Block Diagram of
a
Line
Interactive
UPS
Figure 18b Configuration
of
Bidirectional
cycloconverter
Battery charging is provided by operating the
converter
in the
rectification mode during times of
normal ac input power. In
this
mode no power
conditioning
is
provided since the load is connected
to the
ac input directly. When power fails, the
transfer switch opens and power flows
from
the
battery to the load. The fact
that
the inverter
is
always operational provides better control of
switching transients compared to a stand-by UPS.
Battery isolation is provided with
a h
transformer
in the
bidirectional cycloconverter.
impact on the perfo"e,size,cost, isolation, and
other aspects. Further research and emerging power
devices wi l l
result n
better topologies
thus
enriching
the UPS system practice.
6 .
References
American Power Conversion Technical notes,
1991American Power Conversion Corp,Jan 20
1992.
Harada.K.; Ji, Y.; Katayama.Y.; Chen.C. J.;
Nakamizo.T.,"Stability improvement of
constant voltage transformer for Triport UPS
Murata.K., Harada.K., An improved AC
Triport
without transients in
the
output
voltage",INTELEC'83, p 558-562, Oct.,1983.
Hirachi.K.; Ya"0to.H.; Sakane.M.;
T0mokuni.Y. ;Nagai.Y.,"
A
novel 3-kVA
Ups
using a switch mode rectiier",
Divan,D.M.,"New topology for single phase
Ups systems", IEEE Industry Applicatiod
24th IAS Annual Meeting, part-1,
p931-936,
Oct., 1989.
Tadahito Aoki, Katauichi Yotaunoto, Seichi
Mmy am a, Yoshitaka Kemnochi,"A new UPS
with a bi-directional cycloconverter",
systems", INTELEC'88, p536- 341., Oct.,
1988.
INTELEC'90,p392-399,Oct.,990.
INTELEC'W,&24-429,Oct.,
1990.
7
Acknowledgment
Prof.RKrishnan thanks Mr.RChellapan, Chief
Executive Officer, Numeric Engineers, Mylapore,
Madras-4, India, for introducing to
hi m
the basics,
practice and challenges of UPS systems, both small
and
large.
8 . Intellectual Property
Some schemes, particularly
4, 5
and
10
with
modifications
are
under disclosure.
5 . Conclusions
The UPS technology trend is tracked in
this
paper
with respect
to
topological
developments and their
127