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IEEE TRANSACTIONSON INDUSTRIAL ELECTRONICS VOL. 36, NO. 3, AUGUST 1989

44 1

Off-Line Uninterruptible Power Supply with Zero

Transfer Time Using Integrated Magnetics

SALVADOR MARTiNEZ, MANUEL CASTRO, MEMBER,EEE,RAFAEL ANTORANZ, AND

FERNAN DO ,&DANA, SENIOR MEMBER, IEEE

Abstract-An off-line uninterruptible power supply UPS) r emer-

gency power system with zero transfer time is presented. The power

transformer, a tnport-like transformer, acts as an inverter and as a

voltage stabiliz er with no external load ing coil. It has been mad e with

commercial

E l

scrapless laminations. The battery charging circuit is also

integrated into the transformer and improves the dynamic output

response during line-mode operation. The result is a robust, short-circuit-

proof equipment with a harmonic distortion

of

lower than

3

percent, a

static output stab ility better than

1.5

percent, and

a

very high reliability.

1. INTRODUCTION

GREAT effort is being put forth nowadays to implement

A mall and economic uninterruptible power supplies

(UPSS ), mainly to feed personal compu ters and systems. The

most commercialized economic solution is the off-line power

supply in which the line is normally connected to the critical

load,

and when the line fails, an inverter is started and

connected to the load. The transfer o peration is made with fast

electromechanical relays resulting in a typical transfer tim e of

5 ms.

When no transfer tim e is allowed by the critical load, other

solutions are available such as the classical rectifier-inverter-

bypass UPS and the triport

[

11, [2].

The operation of the triport demands an output voltage

stabilizing device when operating in line m ode, and this d evice

is made of a nonintegrated coil and a triac in phase-controlled

working mode.

This paper shows an off-line UPS with zero transfer time

built over a triport transformer without an external stabilizing

coil. In the line-mod e operation, the inverter coil of the triport

is

used to stabilize the output voltage, whereas in the battery-

mode, that coil acts as the inverter primary, which stabilizes

the output operating in one pulse width control. The change of

the electrical topology is made of fast electromechanical relays

with a typical response of 4-6 ms. T he energy stored in the

output capacitor typically keeps the output wave transient

between 15 percent of the nominal voltage. This careful

transfer operation demands keeping the inverter control

synchronized with the line at all times. In a general sen se, the

main electromagnetic device can be seen as an integrated

Manuscript received January 25, 1988; revised December 13, 1988.

S .

Martinez and M. Castro are with the Electronic and Control Department,

R. Antoranz is with Coalba Engergia, Madrid, Spain.

F. Aldana is with the Electronic Engineering Department, Universidad

IEEE

Log

Number 8928461.

Universidad Nacional de educaci6n a Distancia, Madrid, Spain.

Politknica de Madrid, Madrid, Spain.

magnetics compon ent (concept proposed as defined by

S .

Cuk

(8)).

A range of three equipm ent powers has been develop ed (500

VA, lo00 VA, and 1500 VA) intended mainly for operation in

low-quality utility areas.

11 POWER IRCUIT

The power circuit is showed in Fig. 1. With line voltage

between 15 and 20 percent of the nominal value, relay RE1

is closed, and relay RE2 is in position 1 , connecting the triac to

the 8-9 coil of the transformer. The circuit acts as a line

voltage stabilizer keeping the output voltage between

k

1.5

percent in any static condition. The m agnetic shunts SH1 and

SH2 are adjusted to obtain an impedan ce of 0 .333

Zn

and 0.4

Zn (Zn = nominal impedance), respectively. Other lower

values are possible for SHl in case of smaller line voltage

tolerance, but it results in too high an output short circuit

current (31 nominal for 0 .333 Zn in SH1 at nominal input

voltage). Besides, a line failure (short circuit equivalent)

affects the output voltage to a higher degree, while RE1 is still

closed.

The 0.4 Zn value for S H2 has been selected because of the

results of previous work on inverters. Therefore, less than 3

percent of total harmonic distortion in the output voltage in

battery-mode can be obtained. This value also helps keep the

first peak current in the transistor bridge, in case of output

short circuit, under 200 percent of the repetitive peak at 100-

percent load. The inverter control circuit reduces the static

output short circuit current to 1.6

In

In

=

nominal output

current) with the aid of the current transformer (CT).

The LC output filter resonates to the third harmonic. The

equivalent circuit at the fundamental frequency is a capacitor

whose reactive power is 1.66 Pn Pn = nominal power),

which was also selected because of previous work on

inverters. The total harmonic distortion in line-mode is also

less than 3 percent in any condition.

It is worth while to ensure that, with the elected values for

the leakage inductances and the o utput capacitor, the circuit is

able to maintain the output voltage to within 1.5 percent of the

nominal value in the limit situations that follow:

Limit Case

Line voltage 1.15 U ominal

Output voltage