[] Topologies for Uninterruptible Power Supplies[1993]{Krishnan}

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

122

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mailto:[email protected]:[email protected]


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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

123


3/6

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

24


4/6

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

125


5/6

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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6/6

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

Documents

[] Topologies for Uninterruptible Power Supplies[1993]{Krishnan}