Download A Reconfigurable Uninterruptible Power Supply System for Multiple Power Quality Applications

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007
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A Reconfigurable Uninterruptible Power Supply
System for Multiple Power Quality Applications
Chia-Chou Yeh, Student Member, IEEE, and Madhav D. Manjrekar, Member, IEEE
Abstract—A novel topology of a modular per-phase uninterruptible power supply (UPS) system based on reduced-switch-count
configuration is proposed in this paper. The proposed power
conversion methodology offers active front-end filtering capability which ensures unity input power factor (reactive power
compensation) and low input total harmonic distortion (harmonic
power compensation). In addition, this UPS system provides
dynamic voltage sag compensation capability, which consequently
eliminates any series transformer or dc-dc boost converter that
is typically used in conjunction with traditional stand-alone UPS
systems. Furthermore, it has the desirable characteristics of
making seamless transition from normal to backup mode during
power failures and vice versa, as well as providing controlled
voltage charging at the dc bus link. The proposed UPS is also
impervious to load variations, which enables it to operate under
nonlinear load condition. Lastly, the circuit structure is conceived
from a commonly used three-leg six-switch building block, thereby
making it simple and cost-effective, and offering improved power
conversion efficiency as compared to a conventional line-interactive or on-line UPS schemes. A complete set of simulation and
sample experimental results based on a 1-kW test prototype of the
proposed UPS system are presented in this paper to demonstrate
its viability and efficacy.
Index Terms—Active filter, power factor correction, power
quality, reduced-switch-count, uninterruptible power supply
(UPS), voltage sag.
I. INTRODUCTION
N RECENT years, extensive research has been dedicated towards the design of uninterruptible power supply (UPS) systems to provide clean, conditioned, and uninterruptible power
to equipment in critical applications such as servers and storage
systems, personal computers, medical equipment, telecommunication systems, industrial and commercial controls, etc. under
essentially any normal or abnormal utility power conditions. In
order to supply continuous power to the load in the absence of
utility power, energy storage systems such as batteries or flywheels are incorporated in such UPS systems. Typically, power
conversion is accomplished using static power electronic devices such as fast-switching high-current insulated gate bipolar
transistors (IGBTs).
Conventional UPS topologies can mainly be categorized
into three different types: 1) off-line; 2) line-interactive; and 3)
I
Manuscript received April 4, 2006; revised October 7, 2006. Recommended
for publication by Associate Editor P. Mattavelli.
C.-C. Yeh is with the Department of Electrical and Computer Engineering,
Marquette University, Milwaukee, WI 53233 USA (e-mail: chiachou.yeh@marquette.edu; cyeh@ieee.org).
M. D. Manjrekar is with Danaher Power Solutions, Richmond, VA 23231
USA (e-mail: mmanjrekar@danaher-dps.com).
Digital Object Identifier 10.1109/TPEL.2007.900486
Fig. 1. Simplified diagram of off-line UPS topology.
on-line [1]. Off-line UPS, which is also sometimes referred to
as line-preferred or passive-standby UPS, is usually used in low
power applications with power ratings less than 2 kVA [1]–[3].
A typical off-line UPS, as depicted in Fig. 1, consists of a static
bypass switch that connects the critical load directly to the
unconditioned utility mains under normal condition while the
battery is charged through the rectifier/charger. In the event of
power failures, the static switch disconnects the mains, and the
critical load gets fed from the backup battery through the inverter. The transfer switching time is specified within a quarter
of line cycle and hence an interruption such as a voltage sag
or voltage loss might occur at the load side before the backup
power is delivered to the critical load. While the off-line UPS
offers the advantages of simple design, low cost, small size,
and high efficiency, the power supplied to the critical load is
neither regulated nor conditioned. Therefore, the critical load is
not protected from any voltage or frequency fluctuations along
the utility power line. Accordingly, some power line filtering
can be employed to remove large surges, spikes, sags, and other
irregularities that are inherent in the utility power. However, no
output voltage regulation and active power line conditioning, in
terms of voltage correction, disturbance rejection, and reactive
and harmonic compensations, is available at the utility side.
In addition, other factors include performance issues with
nonlinear loads and lack of isolation of the load from the utility
mains.
A line-interactive UPS offers an improved performance
as compared to the off-line UPS and it is normally used in
low to medium power applications [1]. There are two types
of topologies offered for line-interactive UPS. The first and
early topology [4]–[9], as shown in Fig. 2, consists of a series
inductor between the utility mains and the critical load, and a
bilateral converter in parallel with the critical load, acting as a
battery charger under normal condition and an inverter supply
backup power from the battery to the load in the event of power
0885-8993/$25.00 © 2007 IEEE
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Fig. 2. Simplified diagram of line-interactive UPS topology with single converter structure.
Fig. 3. Simplified diagram of line-interactive UPS topology with two converter
structures.
outage. This type of UPS is also sometimes referred to as parallel-processing UPS which involves only one power stage. The
early development of the line-interactive UPS given in [4] and
[5] offers the capability of input current harmonic suppression,
but very little reactive power compensation. Furthermore, the
topology does not provide voltage sag compensation (output
voltage regulation) capability and a tri-port high leakage inductance transformer is incorporated between the mains and the
load for isolation purpose. An improved version of such line-interactive UPS without any tri-port high leakage transformer
has been proposed in [6]–[9] where the UPS has the ability to
suppress input current harmonics. However, the performance
attributes in terms of input power factor correction and output
voltage regulation in these schemes reported in [6]–[9] appear
to be limited.
An alternative type of line-interactive UPS, which has
received considerable attention in recent years, is known as
the series-parallel compensated line-interactive UPS or the
so-called “delta-conversion” UPS [10]–[16]. As illustrated
in Fig. 3, this type of UPS consists of two power conversion
stages, one converter in series with the utility mains through
a series transformer and the other in parallel with the load.
During the normal operation, the utility mains supplies power
directly to the load while the battery is charged through the
parallel converter. When the mains fail, the static switch opens
to disconnect the load from the mains, and the battery maintains the continuity of power to the load through the parallel
converter. Besides functioning as a battery charger, the parallel
power converter also facilitates the input current harmonic
suppression and power factor correction. The complementary
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007
Fig. 4. Simplified diagram of on-line UPS topology.
series power converter acts as a voltage regulator to regulate
the output voltage in the event of utility voltage sag or swell.
This UPS configuration allows an independent control of
the output voltage, input power factor correction, and input
harmonic power compensation, thus providing series-parallel
active power line conditioning capabilities [10]–[16]. Even
though this line-interactive UPS scheme consists of two power
conversion stages, its efficiency is relatively high due to the
fact that the rating of the series power converter is typically
10%–20% of the overall UPS rating, since its purpose is to
compensate for the voltage difference between the mains and
the load [11]. Some of the drawbacks associated with such UPS
are lack of effective isolation between the load and the utility
mains, complex control algorithm, as well as the need for the
series transformer which could be bulky, heavy, and expensive.
The on-line UPS, which is also sometimes referred to as inverter-preferred or double-conversion UPS, has evolved into a
dominant candidate for high power and high voltage applications in industrial and manufacturing plants [1]. This is due
to its ability to supply conditioned and regulated power to the
critical load, as well as its seamless transition from normal to
backup mode and vice versa, and its decoupling capability of
the utility and the load under power outage. A typical on-line
UPS functional block topology is shown in Fig. 4. It mainly
consists of a rectifier/charger that converts the ac input supply
voltage into unregulated dc voltage for the inverter and the battery charging, a battery that supplies backup energy in the event
of an utility power outage, an inverter that converts the unregulated dc voltage from the rectifier (or the battery) into regulated
and filtered ac voltage for the load, and a static bypass switch
that transfers the load to the ac input supply without any interruption in the supply of power in the event of power conditioner
failure.
A number of new and improved on-line UPS systems have
been reported in the past to provide active power line conditioning capability in order to suppress input current harmonics
and to realize close to unity input power factor [17]–[23]. However, when the utility is not functioning in its full operating condition or if there is utility voltage sag, the UPS is unable to
supply full power to the load. Hence, a dc-dc boost converter in
series with the inverter is required to stabilize the output voltage
[17]–[20]. Accordingly, the addition of dc-dc boost converter to
the UPS results in increased cost and footprint of the overall
system. Furthermore, a typical on-line UPS requires two power
conversion stages as depicted in Fig. 4. Both power converters
YEH AND MANJREKAR: RECONFIGURABLE UPS SYSTEM FOR MULTIPLE POWER QUALITY APPLICATIONS
Fig. 5. Single-phase half-bridge inverter.
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Fig. 7. Single-phase half-bridge active rectifier/inverter.
to offer the characteristics of either an “on-line” or “line-interactive” UPS. In other words, the UPS can either act as a
direct voltage feeder (“line-interactive” feature) or an output
voltage regulator (“on-line” feature) during normal operation.
This can be realized through modifying the control algorithm
of the PWM switching scheme. In this paper, the simulation and
sample experimental results based on a 1-kW test prototype are
presented to demonstrate the viability and efficacy of the proposed UPS system. It may be noted that the results presented
here are only pertained to the operation as a direct voltage feeder
(“line-interactive” feature) during normal condition. Nevertheless, the control algorithm for the operation as an output voltage
regulator (“on-line” feature) will be addressed here in this paper.
Fig. 6. Single-phase half-bridge diode rectifier/inverter.
are required to operate at full power rating of the UPS. This also
results in a lower operation efficiency and higher system cost as
compared to the line-interactive UPS.
In this paper, a new and low-cost modular per-phase UPS
based on reduced-switch-count topology is proposed. The proposed topology offers improved power conversion efficiency,
low cost, light weight, and it has a simple circuit structure owing
to an employment of commonly used building block. In addition, this novel UPS system provides active front-end filtering
capability which results in unity input power factor (reactive
power compensation) and low input total harmonic distortion
(harmonic power compensation). Moreover, this UPS system
offers desirable characteristics of making seamless transition
from normal to backup mode during power failures and vice
versa, as well as the ability to control the dc bus link voltage. The
proposed UPS can also function as a dynamic voltage sag compensator (voltage restorer) which ensures output voltage stabilization when the utility is experiencing voltage sag or dip.
Hence, this unique feature eliminates any dc-dc boost converter
used in conjunction with the traditional stand-alone on-line UPS
system. Also, this UPS system is impervious to load variations,
which enables it to operate under nonlinear load condition. Due
to its unique configuration, the proposed UPS has the ability
II. THE PROPOSED UPS SYSTEM TOPOLOGY
To illustrate the development of the proposed UPS system,
let us begin with a simple single-phase half-bridge inverter, the
circuit topology of which is shown in Fig. 5. It can be seen
that this topology operates by causing the load current to flow
directly through the dc capacitors. As the switches are turned
ON and OFF to control the level of the output voltage, the current changes from one capacitor to the other. Depending on the
direction of the current flow, energy is either stored in or extracted from the capacitors. To further improve on the dc bus
charging, a half-bridge diode rectifier is included in the same
circuit topology, as given in Fig. 6. As may be observed in Fig. 6,
the circuit topology offers only limited dc bus charging with a
diode rectifier. Besides that, one would need an alternative path
for current flow from the dc bus capacitors during the backup
mode, that is when the input mains are isolated from the system.
To mitigate this issue, a half-bridge active rectifier is used in
place of the diode rectifier, as shown in Fig. 7. Such an active
rectifier provides active input current shaping as well as input
power factor control. Meanwhile, the dc bus charging can also
be controlled to provide the necessary energy required for the
specified boost. More importantly, this scheme now provides alternative path for current flow when the input mains are isolated
from the system. However, as shown in Fig. 7, the dc bus capacitors are always in the load current path, and hence they experience significant charge fluctuations. In order to reduce the dc
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Fig. 10. Normal mode of operation.
Fig. 8. Proposed UPS topology.
III. MODES OF OPERATIONS
The proposed UPS system can function in three different
modes of operations, namely the normal mode, the backup
mode, and the voltage sag compensation mode.
A. Normal Mode
Fig. 9. Complete topology of proposed UPS system.
bus voltage variations, an additional leg, namely the “source”
leg, is included in the model, as shown in Fig. 8.
In this proposed UPS topology, there are three different
modes of operations, namely: 1) normal mode; 2) backup
(battery) mode; and (3) voltage sag compensation (voltage
restoration) mode. The details of these modes of operations are
described in the following section. A complete UPS topology is
depicted in Fig. 9. The battery is introduced to provide backup
energy storage in the event of power outage, as well as to provide additional voltage when the utility is experiencing voltage
sag. The battery switch is made up of a contactor and a SCR.
During normal mode, the SCR is opened and the contactor
is closed which allows the battery to be charged up by the
active rectifier comprising of the “source” and “neutral” legs.
During the backup mode, the “source leg” transistor switches
are turned-OFF, while the contactor is opened and the SCR is
closed, which enables the battery to act as a dc supply to the
inverter comprising of the “load” and “neutral” legs. During
the voltage sag compensation mode, the “neutral” leg transistor
switches are turned-OFF, while the contactor is opened and the
SCR is closed, which allows the “source” and “load” legs to act
as a voltage restorer to supply the additional voltage from the
battery to the load in the event of utility sag. The bypass static
switch is included in case of power conditioner faults, which
will switch the load directly to the utility.
During the normal mode, that is under the condition of which
there is no power failure or the utility is at least 90% of its
rated operating condition, the “source” leg and the “neutral”
leg operate as an active rectifier (and battery charger), while the
“source” leg and the “load” leg synchronize to operate as a direct voltage feeder (a “line-interactive” feature) from the utility
to the load, as shown in Fig. 10.
In order to obtain sinusoidal input current waveform and
unity input power factor, current-regulated pulse-width modulation (CRPWM) control scheme is employed here [24], [25],
the block diagram of which is given in Fig. 11. As shown in the
figure, there are two feedback control loops, namely the outer dc
bus voltage control loop and the inner current control loop. The
, is sensed and compared with a reference
dc bus voltage,
dc voltage,
, and the error is fed to a proportional-integral
(PI) controller. The output of the PI controller is the amplitude
, which is then multiplied by
of the reference utility current,
a unity waveform, having the same phase angle as the utility
voltage, , to ensure unity input power factor. In other words,
the unity waveform is the normalized utility voltage, , which
can be realized using the normalization method as delineated
in Fig. 12, [26]. Next, this reference utility current, , is
compared with the measured utility current, , and the error
is again conditioned by a PI controller to produce a control
. This control voltage,
, is then compared with
voltage,
, to synthesize the
a fixed frequency triangular waveform,
desired PWM switching patterns for the transistor switches of
Fig. 10. During the normal mode, the CRPWM control scheme
is applied to the “source” and “neutral” legs, while the PWM
switching patterns of the “load” leg is synchronized with that
of the “source” leg.
B. Backup Mode
In the backup (battery) mode, that is when there is a power
outage or when the utility is less than 50% of its rated condition,
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Fig. 11. Control schemes for various modes of operations.
Fig. 12. Normalization method.
the “source” leg transistor switches are turned-OFF, thus preventing backward power feed into the utility, while the “load”
leg and the “neutral” leg operate as an inverter which supplies
power from the battery to the load, as depicted in Fig. 13. During
this mode, the CRPWM scheme cannot be used since the utility
is isolated, which results in a zero utility current. Hence, sinusoidal PWM with unipolar voltage switching scheme is utilized
to synthesize the switching sequences of the “load” and “neutral” legs, see Fig. 11. The desired reference load voltage, , is
, to produce
compared with the same triangular waveform,
the PWM switching patterns. The unipolar voltage switching
is preferred over bipolar voltage switching due to its three-level
PWM output voltages as well as lower harmonic contents, hence
lesser ripples, in the output voltage waveform. With this control
operation for the backup mode, a regulated sinusoidal output
voltage with low total harmonic distortion is delivered.
C. Voltage Sag Compensation Mode
During the voltage sag compensation (voltage restoration)
mode, that is when the utility is operating between 50% and 90%
of its rated condition, the “source” leg and the “load” leg operate
Fig. 13. Backup (battery) mode of operation.
as a voltage restorer which supplies additional power from the
battery to compensate for the utility voltage sag, as shown in
Fig. 14. Meanwhile, the “neutral” leg transistor switches are
turned-OFF. In this voltage sag compensation mode, a refer, in phase synchronization with the utility
ence voltage,
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Fig. 14. Voltage sag compensation mode of operation.
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007
(a)
Fig. 15. Simplified equivalent circuit diagram under voltage sag compensation
mode.
(b)
Fig. 17. (a) Utility input voltage and current waveforms under linear load condition. (b) Load voltage and current waveforms under linear load condition.
Fig. 16. Control strategy to demonstrate output voltage regulation (“on-line”
feature).
voltage, is realized by multiplying the amount of sag voltage,
, with the normalized utility voltage waveform obtained
using the normalization method given in Fig. 12. Accordingly,
, is used to synthesize the
this resulting reference voltage,
desired PWM switching patterns, see Fig. 11. The sinusoidal
PWM control scheme is applied to the “source” and “load”
legs, while no switching is performed on the “neutral” leg. The
voltage supplied from the battery, in this case 0.2 , in addition
to the voltage from the sag utility, in this case 0.8 , ensures desired full voltage of 1.0 to be supplied to the load in order to
stabilize the output voltage at one per unit rated utility voltage,
as illustrated in Fig. 15. It will be seen from the results that the
load voltage and current waveforms regulate seamlessly when
the utility sag occurs, as well as when the utility restores to its
healthy 100% condition.
It is evident that the control methods of Fig. 11 for various
modes of operations are simple and can be easily implemented using a digital microprocessor. With this proposed UPS
topology, one does not need any series transformer used in
conjunction with line-interactive UPS [10]–[16] or any dc-dc
boost converter used in conjunction with on-line UPS [17]–[20]
when compensating for the voltage sag. Furthermore, the
active rectifier (or charger) not only controls the dc bus voltage
charging, but also provides active front-end filtering and power
factor correction capabilities.
In order to control the proposed UPS as an output voltage
regulator (“on-line” feature) during normal mode, the control
strategy employs a modified sinusoidal PWM scheme, as
demonstrated in Fig. 16. The “load” and “neutral” legs operate
as an inverter capable of generating three-level waveform at
the output. Therefore, a conventional unipolar PWM strategy
is adopted which employs positive and negative polarity sine
waves (thin dotted sinusoids) to the “load” and “neutral” legs as
reference commands. If the same reference as in the “neutral”
leg is fed to the “source” leg (thin dotted sinusoids), then
there is no net voltage between the “source” and “neutral”
legs, thereby producing no contribution at the utility front
end because of this inverter modulation. On the other hand,
a conventional current regulated PWM scheme is adopted to
YEH AND MANJREKAR: RECONFIGURABLE UPS SYSTEM FOR MULTIPLE POWER QUALITY APPLICATIONS
Fig. 18. UPS switches from normal to backup mode at t = 0:5 s under linear
load condition. (a) Utility input current. (b) Load voltage and current.
synthesize a three-level rectifier input voltage as a set of two
opposite polarity waveforms at the “source” and “neutral” legs
(thick solid sinusoids). Again, if the same reference command
as the “neutral” leg is applied to the “load” leg (thick solid
sinusoids), then there is no interference in the inverter output
because of modulation of the “source” and “neutral” legs. This
implies that the rectifier and inverter operation are decoupled
from each other. Accordingly, adding together the rectifier and
inverter reference commands for each leg will produce the
resultant reference command that is required to synthesize the
PWM switching patterns for each leg, as shown in Fig. 16. As
mentioned earlier, the results presented here are only pertained
to the operation as a direct voltage feeder depicted in Fig. 10.
IV. SIMULATION RESULTS
The proposed modular per-phase UPS system is simulated in
Matlab-Simulink under linear and non-linear load conditions for
various modes of operations. The specifications of the system
are as follows:
kW;
• Output power,
;
• Utility voltage,
• DC link voltage,
V;
kHz.
• Switching frequency,
A. Normal Mode Under Linear Load Condition
Here, the utility input voltage and current waveforms in
normal mode under resistive (linear) load condition are shown
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Fig. 19. UPS switches from backup to normal mode at t = 0:5 s under linear
load condition. (a) Utility input current. (b) Load voltage and current.
in Fig. 17(a). It can be seen that the utility input current waveform is close to sinusoidal with a total harmonic distortion
(THD) of 3.54% and has a unity input power factor. The
load voltage and current waveforms under the same condition
are shown in Fig. 17(b). Again, both the voltage and current
waveforms are close to sinusoidal with each having a THD of
0.22%. Hence, the system performance demonstrates the active
input current shaping and input power factor control of the
proposed UPS system.
B. Backup Mode Under Linear Load Condition
In the event of power outage or utility voltage sag of more
than 50%, the UPS switches immediately from normal to
backup (battery) mode. Fig. 18(a) shows the utility input
current waveform and Fig. 18(b) shows the load voltage and
current waveforms, in the case of a power outage at
s.
It can be observed that the load voltage and current continue
to regulate with only slight interruptions at the point where the
power failure occurs. The same seamless transition can be seen
in Fig. 19(a) and (b) where the UPS switches from backup to
s.
normal mode at
C. Normal Mode under Non-Linear Load Condition
Here, the UPS is operating in the normal mode under
diode-rectifier (non-linear) load condition. Fig. 20(a) depicts
the utility input voltage and current waveforms. As may be
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(a)
Fig. 20. (a) Utility input voltage and current waveforms under non-linear load
condition. (b) Load voltage and current waveforms under non-linear load condition.
observed therein, the utility input current is close to sinusoidal
with a THD of 6.23% and has a unity input power factor.
Meanwhile, the load voltage and current waveforms under
the same condition are shown in Fig. 20(b). Accordingly, the
proposed UPS is impervious to load variations due to almost
sinusoidal input current and unity input power factor.
D. Backup Mode Under Non-Linear Load Condition
The proposed UPS is operating under non-linear load condition in the backup mode of operation. The utility input current
waveform, and the load voltage and current waveforms are illustrated in Fig. 21(a) and (b), respectively, in the event of power
failure at
s. It can be seen from the figure that it takes
approximately 0.1 s for the load waveforms to stabilize when
s.
the UPS switches from normal to backup mode at
The same scenario can also be observed from Fig. 22(a) and
(b), where the UPS switches from backup to normal mode at
s.
E. Voltage Sag Compensation Mode under Linear Load
Condition
When the UPS experiences a utility voltage sag of 10% to
50% of its full condition, it immediately switches to the voltage
sag compensation (voltage restoration) mode. The utility input
voltage waveform under linear load condition in the event of
Fig. 21. UPS switches from normal to backup mode at t = 0:5 s under nonlinear load condition. (a) Utility input current. (b) Load voltage and current.
s is depicted in Fig. 23(a).
30% utility voltage sag at
Meanwhile, the load voltage and current waveforms are shown
in Fig. 23(b). Notice from the figure that the load waveforms are
restored immediately without any discrepancy when the voltage
sag occurs. When the utility is recovered to its healthy 100%
s, the load waveforms regulate seamcondition at
lessly which demonstrates the dynamic performance of the UPS
system.
V. EXPERIMENTAL SETUP AND RESULTS
The performance of the proposed modular per-phase UPS is
verified experimentally from a 1-kW laboratory prototype rated
, as illustrated in Figs. 24
at a nominal input voltage of 120
and 25. The control method of Fig. 11 was implemented using
Analog Devices ADMC401 DSP board. The PWM switching
frequency was set at 10 kHz. The software was programmed in
assembly language to achieve the maximum speed and to reduce the program overhead. The backup energy storage system
of Fig. 25 consists of several sets of dc capacitor banks capable
of producing dc voltage up to 700 V. The utility input voltage
and current waveforms in normal mode under resistive (linear)
load condition are shown in Fig. 26(a). It can be observed from
the figure that the input current waveform is close to sinusoidal
and has a unity input power factor. The same conclusion can also
be stated for the load voltage and current waveforms under the
YEH AND MANJREKAR: RECONFIGURABLE UPS SYSTEM FOR MULTIPLE POWER QUALITY APPLICATIONS
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(a)
(b)
Fig. 22. UPS switches from backup to normal mode at t = 0:5 s under nonlinear load condition. (a) Utility input current. (b) Load voltage and current.
same operating condition, as given in Fig. 26(b). This demonstrates that the proposed UPS can suppress the input current harmonics as well as the reactive power.
The input voltage and current waveforms, as well as the load
voltage and current waveforms in normal mode under diode-rectifier (non-linear) load condition are given in Fig. 27(a) and (b),
respectively. Again, the input current waveform is close to sinusoidal and has a near unity input power factor. This shows that
the proposed UPS is immune to non-linear load while still maintaining a near sinusoidal input current waveform and unity input
power factor. Note that these experimental results in Figs. 26
and 27 are in good agreement with the simulation results presented earlier in Figs. 17 and 20, respectively.
Meanwhile, the utility input current as well as the load voltage
and current waveforms when the UPS switches from normal to
backup mode are depicted in Fig. 28. Notice therein that the load
voltage and current continue to regulate even when the utility
current is zero. When the utility current is finally restored, the
load voltage and current again regulate seamlessly with only
slight interruptions, as shown in Fig. 29.
Evidently from both the simulation and experimental results,
the proposed UPS clearly demonstrates its superior performance
for multiple power quality applications. In light of its simple
circuit configuration and reduced active switching devices, the
proposed UPS offers a low cost design and higher power conversion efficiency as compared to the line-interactive UPS systems
Fig. 23. (a) Utility input voltage waveform (linear load). 30% utility voltage
sag at t = 0:4s. (b) Load voltage and current waveforms (linear load).
Fig. 24. UPS laboratory prototype.
presented in [10]–[16]. It also eliminates the need of a low frequency series transformer which is bulky, heavy, and expensive.
Comparing the proposed UPS with the on-line UPS systems
presented in [17]–[20], the distinct difference is the elimination
of a dc-dc boost converter used for voltage sag compensation
purpose. In addition, comparing the proposed per-phase system
with a typical single phase on-line UPS [21] as shown in Fig. 30,
the proposed system only requires six active switches as compared to eight active switches shown in Fig. 30. Reducing the
number of active switches not only reduces the system cost, it
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Fig. 25. Backup energy storage system.
(a)
(a)
(b)
Fig. 27. (a) Normal mode—Utility input voltage and current waveforms (nonlinear load). (voltage-50 V/div., current-10 A/div.). (b) Normal mode—Load
voltage and current waveforms (non-linear load). (voltage-50 V/div., current-10
A/div.).
(b)
Fig. 26. (a) Normal mode—Utility input voltage and current waveforms
(linear load). (voltage-50 V/div., current-10 A/div.). (b) Normal mode—Load
voltage and current waveforms (linear load). (voltage-50 V/div., current-10
A/div.).
Fig. 28. Transition from normal to backup mode under linear load condition.
Utility current, load current, and load voltage. (voltage: 100 V/div., current: 10
A/div.).
also enhances the reliability and lowers the size and weight of
the overall system. Another desirable feature of the proposed
UPS is the use of a common neutral connection between the
input and output, thus eliminating the need of an isolation transformer as is used in the system of Fig. 30. The UPS systems presented in [22] and [23] are also based on the reduced-switchcount configuration for single-phase UPS. Although the UPS
systems presented in [22] and [23] only require five and four
active switches, respectively, in the topologies, these structures
are not capable of providing utility voltage sag compensation
capability. For three-phase operation, a three-phase topology
comprising of three per-phase proposed UPS is essential, as
YEH AND MANJREKAR: RECONFIGURABLE UPS SYSTEM FOR MULTIPLE POWER QUALITY APPLICATIONS
Fig. 29. Transition from backup to normal mode under linear load condition.
Utility current, load current, and load voltage. (voltage: 100 V/div., current: 10
A/div.).
1371
can function as a voltage sag compensator when a utility voltage
sag occurs. Meanwhile, the proposed UPS has the ability of
active front-end filtering, as well as seamless transition from
normal to backup mode and vice versa. It also provides grid
isolation without any backward power feed into the utility
in the event of power failure, hence eliminates the needs of
circuit breaker or static switch. More importantly, this UPS is
simple in design at a lower cost as compared to a conventional
line-interactive or on-line UPS owing to the reduced number
of power electronic switching devices. Accordingly, it can be
implemented using a standard commercially available three-leg
six-switch building block.
Finally, a 1-kW UPS prototype was built and tested, and the
experimental results are in good agreement with the simulation
results which demonstrate the effectiveness and soundness of
the proposed UPS system.
ACKNOWLEDGMENT
The authors would like to thank Dr. J. Kikuchi and E. F. Buck
at Eaton Corporation for their support during the experimental
work.
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Fig. 30. A typical single-phase on-line UPS system.
Fig. 31. A three-phase schematic of proposed UPS system based on TRINIT
topology.
illustrated in Fig. 31. This is referred to as
Rectifier Inverter Neutral Inter- ) topology.
(Triple
VI. CONCLUSION
A novel topology of a modular per-phase UPS system based
on reduced-switch-count configuration has been proposed in
this paper. Due to its unique configuration, the proposed UPS
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007
Chia-Chou Yeh (S’99) was born in Taipei, Taiwan,
in 1978. He received the B.S. and M.S. degrees in
electrical engineering from Marquette University,
Milwaukee, WI, in 2000 and 2003, respectively,
where he is currently pursuing the Ph.D. degree in
electrical engineering.
From 2003 to 2007, he was with the Innovation
Center, Eaton Corporation, Milwaukee, WI, as a
Power Electronics Intern, where he was involved
in the research, development, and testing of uninterruptible power supply (UPS) systems and motor
soft-starters. His current research interests include electric machines and
adjustable speed drives, power electronics, modeling and control of electric
machines, as well as condition monitoring and fault diagnosis of electric
machinery drive systems, and their corresponding fault tolerant and mitigation
strategies.
Mr. Yeh is a member of Sigma Xi, Eta Kappa Nu, Tau Beta Pi, and Pi Mu
Epsilon.
Madhav D. Manjrekar (S’96–M’99) received
the B.E. degree from the Government College of
Engineering, University of Pune, India, the M.
Tech. Degree from the Center of Electronic Design
and Technology, Indian Institute of Science, India,
the M.S. degree from Montana State University,
Bozeman, and the Ph.D. degree at the University of
Wisconsin, Madison, in 1993, 1995, 1997, and 1999,
respectively.
From 1999 to 2002, he was with the Technology
and Innovation Center of ABB as a Principal Engineer, where he focused on the design, development, and testing of high power
electronics for variable speed drives, sag correctors, and other power quality
systems. From 2002 to 2005, he was with Eaton Corporation, as a Power Electronics Specialist and then as a Manager, where he was a Lead Technical Consultant for power quality portfolio development. Since January 2006, he has been
with Danaher Power Solutions, Richmond, VA, as a Director of Engineering.
His research interests are modeling, design, and control of power conversion
systems, utility applications of power electronics, adjustable-speed and torque
drives, and nonlinear dynamics and controls.