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Active Rectifier for Uninterruptable Power Supply
#1

Active Rectifier for Uninterruptable Power Supply



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Abstract.

The paper contains analysis of power factor
correction circuits that is made in the context of elaboration of
uninterruptable power supply system. Various correctors have
been estimated analytically, by means of simulation and
experimentally. The most significant attention has been paid to
the efficiency of the analyzed converters and their compatibility
with modular approach to development of power converters.
Keywords: uninterruptible power supply systems, power
factor, circuit simulation

I. INTRODUCTION

The proposed uninterruptable power supply system (UPSS)
provides a stable electrical supply for its load not only from
the mains or traditional energy storage - battery, but also from
alternative/extra energy resources like solar panels, fuel cells
or diesel-generator. In case of higher power demand (that is
usually temporary) UPSS may supply the load also from a
supercapacitor.
In order to get the maximum power from diesel-generator it
is very important to load it with sinusoidal current with no
phase shift. The same can be applied to the mains – its current
must be sinusoidal and purely active. To resolve the problem
of non-sinusoidal current an active rectifier with power factor
correction (PFC) must be used. This paper contains the
analysis of efficiency of various PFC circuits.
There are several commonly used topologies of PFC
circuits. Although they are quite different they contain similar
blocks of the transistors and/or diodes connected in series.
This makes a modular approach described, for example, in [1]
possible. The described modules include transistors
(connected as a half-bridge), their transistor drivers, circuits
for voltage/current measurements and interface for control
equipment. The utilized system developed in Riga Technical
University is similar, but has some significant differences [2]:
- modules include 2 DC-link capacitors connected in series
(Fig. 1-a) that allow testing half-bridge PFC circuits with
DC-link voltage up to 800V; One of capacitors can be
shunted if not necessary in the particular configuration;
- they provide voltage measurements on both DC-link
capacitors, helping to avoid voltage unbalance;
- they have optical control interface, to maximize control
device safety and control signal noise immunity;
- they have liquid cooling system that allow to minimize
volume of primary heat sinks.
For the given research the modules have been reconfigured
so that (Fig. 1-b) the third voltage is measured between the
midpoint of capacitors and the input connector. Thus
measurement of the input voltage of PFC becomes possible.

CONCLUSIONS

Six topologies of active PFC converter have been shortly
described, simulated and tested experimentally.
Simulation shows that the bridgeless and half-bridge
topologies have slightly higher efficiency and are good as PFC
circuits that correspond to the initial analytical estimation.
Both circuits at high power ensure input current with cos(f)
near 1, THD near 3-5%. At low power this circuits ensure
input current with cos(f) in range of 0.95-0.99 and THD in
range of 20-40%.
The half-bridge and full-bridge topologies allow ensures not
only power factor correction but also consuming of reactive
power of both types (capacitive/inductive). This phenomenon
can be used to compensate the reactive power of other devices
connected to the same grid.
Experimental result shows also that bridgeless topologies
have very high common-mode noise and need to be improved.
The reason of increased common-mode noise has to be
identified and removed. Fixing of this problem may lead to
significant improvement of the results of the bridgeless
topologies of PFC.
Interleaved boost converter has a big potential of
improvement because of the compensation of the current
ripple in parallel boost branches as it has been shown through
simulation. This topology, however, requires some additional
experimental investigation.
Reply
#2
An uninterruptible power supply, also uninterruptible power source, UPS or battery / steering wheel backrest, is an electrical device that provides emergency power to a load when the input power source or mains fails. A UPS differs from an auxiliary or emergency power system or standby generator because it will provide near-instant protection of incoming power interruptions by supplying stored energy in batteries, supercapacitors or flywheels. The operating time on the battery of most uninterruptible power supplies is relatively short (only a few minutes), but sufficient to start a standby power supply or properly shut down the protected equipment.

A UPS is typically used to protect hardware, such as computers, data centers, telecommunications equipment, or other electrical equipment where an unexpected power outage could cause injury, fatalities, serious business interruption, or data loss. UPS units vary in size from units designed to protect a single computer without a video monitor (about 200 volts amps) to large units that power entire data centers or buildings. The 46-megawatt Battery Electric Storage System (BESS) in Fairbanks, Alaska is the 46-megawatt power supply system that feeds the entire city and surrounding rural communities during outages.
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