Multiple Feedback Loop Control Strategy for
Single-phase Voltage-Source
UPS
Inverter
Naser Abdel-Ftahim,
Student
Member,
IEEE
and
John
E.
QU~~CO~,
Senior
Mamba,
I&=
Faculty
of
Engineering and Applied Science, Memorial University
of
Newfoundland
St. John’s,
NFLD,
Canada
A1B
3x5
Abdroct-
This paper investigates the performance
of
mul-
tiple feedback loop control strategy
for
single-phase voltage-
source
UPS
inverter with an
L-C
fllter.
In
order to select
appropriate feedback variables and
amsew
the stability
of
the
cld
loop
operation
of
the overall system, the power
circuit (inverter and fllter plus load) incremental dynamics
is investigated using the
state-space
averaging technique
and root locus method. The results
of
the stability anal-
ysis
show that
a
control scheme which employs the filter
capacitor current in an inner feedback
loop
and the load
voltage in an outer voltage control loop results in success-
ful operation
of
the
UPS
system.
Computer simulation
results of
a
single-phase voltage-source half-bridge
UPS
in-
verter with
a
second order filter and
ILL
load
is
presented
to demonstrate the performance
of
the proposed control
scheme. Experimental verification
of
a
laboratory model
of
the
UPS
system
is
also
provided for both linear
and
non-
linear loads
so
(UI
to verify the predicted performance
of
the system.
It
is
shown that the control scheme offers im-
proved performance measures over existing schemes.
It
is
simple to implement and capable of producing nearly
per-
fect sinusoidal load voltage waveform
at
moderate switch-
ing frequency and reasonable size
of
Alter parameters. Fur-
thermore, the scheme
has
fast dynamic response and high
voltage utilisation of the DC source.
I.
INTRODUCTION
Uninterruptible Power Supplies
(UPS)
are used to in-
terface critical
loads
such
as
computers and communica-
tion systems to the utility system. The output voltage of
the
UPS
inverter
is
required
to
be siiiusoidal with mini-
mum total harmonic distortion. This
is
usually achieved
by employing
a
combination of pulse width modulation
scheme and a second order filter at the output of the in-
ver ter.
One way of achieving
a
“clean” sinusoidal load volt-
age is by using a sine pulse width modulation (SPWM)
scheme
[l].
In this technique, the load voltage
is
com-
pared with a reference sinusoidal voltage waveform and
the difference in amplitude
is
used to control the modu-
lating signal in the control circuit of the power inverter.
A
more advanced technique employs
a
programmed opti-
mum PWM scheme which
is
based on the harmonic elim-
ination technique
[l].
These schemes have been shown
to
perform well with linear loads. However, with non-linear
loads the
PWM
scheme does not guarantee low distortion
0-7803-1859-5/94/$4.00
’
1994
IEEE
at the load voltage.
To
overcome thia drawback,
a
real-time feedback con-
trol scheme using dead-beat control was propoeed. This
technique employs the capacitor voltage and
its
deriva-
tive in
a
control algorithm to calculate the duration of
the
ON/OFF
states of the inverter switching devices such
that the capacitor voltage
is
exactly equal
to
the reference
voltage at the next sampling instance. Although this tech-
nique has been successfully implemented for single- and
three-phase applications, it has the following drawbacks:
1)
it is complex to implement;
2)
it
is sensitive
to
parame-
ter variations;
3)
its control algorithm requires estimation
of the load parameters
[2].
In order to achieve
a
control scheme which overcomes
the above disadvantages,
a
current regulated control
scheme for DC/AC applications waa proposed in
[3].
In
this technique, the current in the filter capacitor
is
used
as
the feedback variable in
a
two-switch inverter circuit
topology to achieve
a
sinusoidal capacitor current.
An
outer voltage control loop
is
also
incorporated for
load
voltage regulation and compensation for imperfections in
the implementation of the current control loop.
Although the technique results in
a
sinusoidal capacitor
current, the power circuit configuration and the switch-
ing scheme used to implement the technique produce a
load voltage which is sinusoidal with
DC
offset.
For
UPS
applications, the presence
of
the DC voltage offset
is
un-
acceptable.
This paper investigates the .suitability
of
a
current-regulated voltage-controlled strategy for a single-
phase voltage-source half-bridge inverter with
a
second
order filter. The scheme incorporates an inner capaci-
tor current loop, an outer capacitor voltage loop, afixed
switching frequency and variable duty cycle approach to
produce sinusoidal output voltage with minimum
har-
monic distortion. The fixed switching frequency approach
produces
a
defined frequency spectrum at the inverter
output which makes it easier
to
design
an
electromagnetic
interference filter to prevent interference with communica-
958
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