IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 15, NO. 1, MARCH 2000
97
A Single Phase Parallely Connected Uninterruptible
Power Supply/Demand Side Management System
Mochamad Ashari, Student Member, IEEE, W. W. L. Keerthipala, Member, IEEE, and Chernmangot V. Nayar
Abstract—This paper presents the application of a single-phase
parallel converter as an uninterruptible power supply and demand
side management system. The proposed system consists of a bi-directional inverter that is connected in parallel to the utility system.
When the grid system fails, the converter will convert the power
from the battery to the ac side (utility side) at the mains voltage and
frequency. On the other hand, when the utility is normal the converter will act as a demand side management system. It charges the
battery during low load and shaves the transient load at the peak
period system. This improves the pattern of the demand variation
in the utility side. This paper presents the operational principle of
the Uninterruptible Power Supply and Demand Side Management
System, laboratory, and simulation results.
Index Terms—Bi-directional inverter, demand side management
system, grid supply system, uninterruptible power supply.
I. INTRODUCTION
HE DEMAND of reliable and high quality power electricity is widely required by industries and residential sites
due to the information industry’s increased use of computers
and related equipment. However, the mains power is usually
fluctuating and contains harmonics, disturbances, sags, and
outage. Power line conditioners, e.g., automatic voltage regulators, filters, and VAR compensators, can be used to improve the
quality of the power, but they cannot solve the problem of black
out [1]–[3]. Hence, an Uninterruptible Power Supply (UPS)
is still chosen for that. However, some typical UPS should
be combined with a power line conditioner, because the UPS
cannot eliminate the disturbances and the voltage variation [4].
A new UPS scheme that can function as a power conditioner
has been proposed by recent research work [5]–[7].
In another case, the load demand variation, e.g., high during
the day (peak load period) and low at night, give to rise another problem. It requires a higher contract capacity, although
the peak load occurs only for few hours a day. It may trip the circuit breaker during the peak period due to a transient load such
as starting of an induction motor, microwave oven, etc. The peak
load also causes extra cost due to high tariff during that time. Demand side management (DSM) is a technique to eliminate the
load variation. The peak shaving, valley filling, and load shifting
T
Manuscript received August 17, 1998; revised June 4, 1999. This work was
supported by CRESTA, Advance Energy Systems Ltd., the School of Electrical and Computer Engineering, Curtin University of Technology, and ACRE.
M. Ashari’s work was funded by a scholorship from the “Engineering Education Development Project (ADB Loan 1432 INO),” Unisearch-UNSW and ITS
Surabaya-Indonesia.
The authors are with the Centre for Renewable Energy Systems Technology
Australia (CRESTA), Curtin University of Technology, Perth, Western Australia
6845 (e-mail: eashari@cc.curtin.edu.au).
Publisher Item Identifier S 0885-8969(00)02219-1.
Fig. 1. UPS/DSM operation modes.
are the techniques used in appropriate combinations to achieve
the load management [8]. In the implementation, a load control
device that selects an appropriate load to switch-on or switch-off
during a period time is commonly used [9]. Thus, the variation
between the high and low of the load pattern can be reduced.
With the recent power electronic developments, it is possible to
shift system loads from daily peaks to daily lows.
This paper presents the application of a single-phase parallel converter for an uninterruptible power supply and demand
side management system. The proposed system consists of a
bi-directional inverter that is connected in parallel to the utility
system. When the grid system fails, the converter will convert
the power from the battery at the desired voltage and frequency.
On the other hand, when the utility is normal the converter will
act as a demand side management. Some laboratory and simulation results are also included in this paper. The simulation uses
a software package EMTDC/PSCAD [10].
II. BASIC OPERATION
The operation of the proposed system can be separated into
two modes: the DSM and UPS as illustrated in Fig. 1. In a
normal utility supply, the system will act as a DSM. When the
grid fails, the system will take over the duty for supplying the
power to the load without any interruption of power. In the
common DSM topology, the system will shave the transient load
during the peak demand period and charge the battery during
low load period. In our case, the shaving technique is controlled
depending on the renewable energy source (photovoltaic array)
which is connected to the DC bus or the battery. When the photovoltaic generates DC power, the converter supplies apart of
the load reducing the power flow from the grid.
The actual operation of the proposed system is discussed
using the simplified equivalent circuit as shown in Fig. 2.
0885–8969/00$10.00 © 2000 IEEE
98
IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 15, NO. 1, MARCH 2000
The current flow from the mains through the coupling inductor
can be obtained as
Z
1 t f (t) 0 (t)g dt
iM (t) =
M
C
pLM 0
= 2 IM sin(!t + )
1
p(VM sin ) + (VC 0 VM cos )
= sin01
p(VM sin(VC) 0+V(MVCcos0 V)M cos ) :
where
IM =
Fig. 2. Simplified circuit for the proposed UPS.
(4)
!LM
2
2
2
2
(5)
(6)
From (4)–(6), it can be found that the mains current, IM , is always a pure sinewave, although the load current is a non-sinusoidal. This is due to the sinewave voltage of the sources across
the inductor. Using (2) and (4), the power flow from the mains
is presented as
SM (t) = M (t)iM (t):
(7)
This can be separated into the active and reactive power voltage
components as follows
V V
PM = M C sin LM
Fig. 3. The power circuit of the converter.
The system comprises two sources, e.g., the mains (VM ) and
the converter (VC ). The mains (grid) voltage is uncontrolled,
but the converter voltage is controllable. The converter is a
bi-directional type, which can convert the power from the
battery to AC at mains voltage and frequency, and can also act
as a battery charger (rectifier). The load as shown in Fig. 2
can be a linear or nonlinear load. Controlling the phase and
amplitude of the converter voltage can control the power flow
from the mains to the load.
The power circuit of the converter that involves full bridge
insulated Gate Bipolar Transistor (IGBT) switches is shown in
Fig. 3. This also involves a transformer and LC filter in the AC
side. The converter voltage is used as the reference.
During supplying the load, the output voltage of the converter
is assumed as
p
C (t) = 2VC sin(!t):
(1)
The mains (grid) voltage is assumed as a pure sinewave, so the
instantaneous mains voltage is given by
p
M (t) = 2VM sin(!t + ):
X
(2)
The load current can be expressed as
p
iL(t) = ~ 2 In sin(n!t + n )
n=1
X
p
p
= 2 Il sin(!t + 1 ) + ~ 2 In sin(n!t + n ):
n=2
(3)
QM =
VM
(V 0 V cos ):
!LM M C
(8)
(9)
Similar to (7), the instantaneous power consumed by the load is
SL (t) = C (t)iL (t):
(10)
If the load is assumed as a linear type, so the high order harmonic
currents can be ignored. The active power required by the load
is
PL = VC I1 cos 1 :
(11)
Fig. 4 shows the phasor diagram of the proposed system.
When the magnitude of the grid voltage is higher than the load
voltage, the converter supplies the reactive power. The inverter
current is 90 leading to the load voltage. XM indicates the
impedance of the inductor LM in . A higher magnitude of current will be drawn from the inverter when the grid voltage drops
below the load voltage as shown in Fig. 4(b).
III. VOLTAGE AND POWER CONTROL CONCEPTS
The output voltage of the proposed UPS/DSM is maintained
constant at a certain value. To keep constant the load voltage is
more important than to keep the power factor at unity, particularly for a weak grid system. A highly fluctuated voltage will affect the load performance and may damage the equipment. The
block diagram of the voltage stabilization control is shown in
Fig. 5. The reference voltage, Vr , is obtained from an oscillator
that is synchronized to the grid voltage. The converter output
voltage, VC , is used as the feedback signal. The magnitude of
the reference voltage and VC is fed to a summing point, resulting
ASHARI et al.: A SINGLE PHASE PARALLELY CONNECTED UPS/DSM SYSTEM
99
Fig. 5. The block diagram of the voltage stabilization control.
Fig. 6. Variation of angle against the mains voltage.
Fig. 4. Phasor diagram of the proposed system.
in an error signal, Verr . Using a PID controller, the Verr is fed
to an integrator, which is periodically reset by the carrier signal.
The integrator time constant, i , should be equal to the time period of the carrier signal. Therefore the integrator output will be
one per unit for one per unit input. The integrator output is a saw
tooth waveform where the amplitude is proportional to the error
signal. Finally, the comparator will produce a pulse width modulated signal for gating the full bridge IGBT’s. When the converter magnitude is lower or higher than the reference, the width
of the PWM pulse will be changed accordingly until the error
signal is minimized. It means the output voltage of the converter
is maintained constant with respect to the reference voltage.
The power flow of the proposed system is arranged as
follows:
• The active power demand should be supplied directly from
the mains.
• The converter should supply the reactive power required
by the load.
To achieve this arrangement, another controller that can change
the phase angle of the reference signal should be used. The
output of this controller is the reference signal, Vr , so it should
be put in series before the voltage stabilization controller.
A general formula of the active power flow for this system
can be drawn as
PM
= PL + PB
(12)
where PB represents the charging power to the battery. When
the battery is fully charged, PB is zero. Thus
PM = PL:
(13)
Fig. 7. Power from the grid as function of angle .
If the battery is assumed fully charged, so the active power will
deliver from the mains directly to the load, the angle can be
calculated from (8), (11), and (13).
VM VC
!LM
sin( ) = VC I1 cos(1 )
= sin01
!L
M I1 cos 1
VM
:
(14)
Equation (14) exhibits that the active power from the mains is
maintained by changing the as function of the load current and
the grid voltage. Fig. 6 shows the variation of angle against the
mains voltage for constant 50 A load current. The angle will
increase when the grid voltage drops.
In the DSM operation, the angle should be controlled such
that the power taken from the grid is reduced below the load
demanded. The converter will supply the unmatched power between the grid and the load demanded. Fig. 7 illustrates the
power taken from the grid as function of at 1.0 per unit voltage.
100
Fig. 8.
load.
IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 15, NO. 1, MARCH 2000
Experimental result: grid voltage and inverter voltage for 5 kW resistive
Fig. 11. Experimental result: the voltage and current for inverter stand-alone
with 5 kW load.
Fig. 9. Experimental result: mains voltage and mains current during supplying
5 kW system load (resistive).
Fig. 12. Experimental result: the mains voltage (simulated by a diesel
generator) and the converter voltage.
Fig. 10. Experimental result: load voltage and load current at 5 kW, resistive
load.
In our case, reducing the power taken from the grid to allow
the converter supplies apart of the load is controlled by a separate DC source. This DC reference represents the photovoltaic
array, which is connected to the DC bus (battery). The power
delivered by the converter will be proportional to the power generated by the DC source. The energy stored in the battery is
controlled through the terminal voltage. When the battery terminal voltage is lower than the default value, the phase shift
is increased until the converter charges the battery at the required power. The entire control topology provides greater value
when the peak load demand coincides with availability of maximum sunshine typically experienced by commercial or industrial consumers. This topology also provides a lower life cycle
cost due to reduced the battery size, which is the major cost of
the system [11].
IV. RESULTS
Laboratory test and simulation results are presented in this
section. A prototype UPS/DSM system consisting of 15 kVA
bi-directional inverter is available at the Centre for Renewable
Energy Systems Technology Australia (CRESTA), Curtin
Fig. 13.
EMTDC/PSCAD simulation of the system in the event of grid failure.
University of Technology. Some test results are presented in
the following figures.
In this test, a 5 kW resistive load is connected to the UPS
system. The load voltage is maintained constant at 220 V, 50 Hz,
while the grid is mainly 240 V. Fig. 8 shows the voltage waveform of the grid and the converter.
Fig. 9 shows the waveform of voltage and current supplied
by the grid. The current is a phase shifted 20.71 as described
in (6). The waveform of the load voltage and current is given in
Fig. 10. This shows that the load current is in phase with the load
voltage. Fig. 11 depicts an in phase waveform of the voltage and
current during inverter stand-alone (UPS) mode.
The voltage stabilization performance is shown in Fig. 12.
As can be seen, the load voltage is almost constant for a high
fluctuating grid voltage. This experiment has been carried out
using a diesel generator for simulating the grid system, so the
voltage can be varied widely.
During the UPS operation, the transient condition of the load
voltage in the event of grid failure is shown in Fig. 13. This
ASHARI et al.: A SINGLE PHASE PARALLELY CONNECTED UPS/DSM SYSTEM
101
REFERENCES
Fig. 14.
Simulation results: the power in the DSM operation.
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Electronics, vol. 11, no. 5, pp. 731–741, Sept. 1996.
[4] D. C. Griffith, Uninterruptible Power Supplies. New York: Marcel
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[5] C. V. Nayar, “A solar/mains/diesel hybrid uninterrupted power system,”
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[7] W. J. Ho, J. B. Lio, and W. S. Feng, “Economic UPS structure with
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[8] V. A. Rabl and W. G. Clark, “The concept of demand-side management,”
in Demand Side Management and Electricity End-Use Efficiency, A. T.
De Almeida and A. H. Rosenfeld, Eds: Kluwer Publishers, 1988, pp.
99–112.
, “Load management technologies and programs in the US,” in
[9]
Demand Side Management and Electricity End-Use Efficiency, A. T.
De Almeida and A. H. Rosenfeld, Eds: Kluwer Publishers, 1988, pp.
113–125.
[10] EMTDC/PSCAD Software Manual. Winnipeg, Canada: Manitoba
HVDC Research Centre, 1994.
[11] M. Ashari, C. V. Nayar, and W. W. L. Keerthipala, “Economic analysis
of a PV-battery-mains hybrid Uninterruptible Power Supply in Perth,
Western Australia,” World Renewable Energy Congress V, Feb. 9–11,
1999.
Fig. 15. Simulation result: the phase shift between the grid voltage and the
converter voltage in the DSM operation.
EMTDC/PSCAD simulation demonstrates that the proposed
system has a smooth transient voltage.
Simulation results of the DSM operation are presented in
Figs. 14 and 15. The load power is simulated constant while the
DC reference (represents the photovoltaic array) is increased to
1.0 kW. The grid supplies almost the entire active power demanded. The grid power is reduced when the converter output
power is increased to follow the DC reference. When the DC reference is zero, the grid reverts to supply the entire active power
demanded.
The phase shift between the grid voltage and the converter
voltage during the simulation is shown in Fig. 15. It shows a
small error between the calculated and the measured values.
This error is due to the dynamic conditions that may occur in
the real implementations.
V. CONCLUSIONS
Application of a parallel inverter for Uninterruptible Power
Supply and demand-side management involving simulations
and laboratory test results has been presented. The proposed
system has the following features:
• It has two modes: the UPS and DSM system.
• The output voltage is stabilized.
• The active power required by the load is directly supplied
from the grid, improving the power factor in the utility
side.
Mochamad Ashari received the Bachelor degree
in electrical engineering from the Institute of
Technology “Sepuluh Nopember” (ITS) Surabaya,
Indonesia, in 1989. He has been with ITS since
1990 as a Lecturer in the Department of Electrical
Engineering. Before receiving the Master of Engineering (electrical) degree from Curtin University
of Technology, Perth, Australia, in 1997, he was
involved in the feasibility study, designing, and
installing the solar-home-systems for rural areas
east of Java. He has been actively involved in the
electrical consultant group for industrial applications including study of
harmonic distortion, design of harmonic filter/power factor correction, relay
setting, and coordination. Currently, he is a full time Research Scholar working
toward his Ph.D. degree at Curtin University of Technology. His research
interests include power electronics and inverter applications, power system
modeling and simulation, and analysis of hybrid power systems.
W. W. L. Keerthipala received the B.Sc. (Engineering) degree with first class honors in electrical
and electronic engineering from the University of
Peradeniya, Sri Lanka, in 1984 and the Ph.D. degree
in power systems and drives from the University of
Cambridge, Trinity College, England, in 1989.
He has worked at the University of Peradeniya
as an Assistant Lecturer for nearly two years during
1985–1986, and at the University of Manitoba,
Canada, as a postdoctoral Research Fellow and
Sessional Lecturer for nearly three years during 1989–1992. From 1992 to July
1997, he was at Nanyang Technological University, Singapore, as a Lecturer
and conducted research work on the latest technology applications in power
systems. His research interests included power system modeling and real-time
simulation, neural network/fuzzy logic based intelligent system protection,
abatement of acoustic noise of power transformers, microprocessor based
intelligent control of induction motors, control of subsynchronous resonance,
and analysis of hybrid power systems. Currently, he is with Curtin University
of Technology, in Australia, as a Senior Lecturer in power systems and power
electronics. His current research is primarily focused on power converter
applications in renewable energy systems.
102
Chernmangot V. Nayar received the B.Sc degree in
electrical engineering from the University of Kerala,
India, in 1969, the Master of Technology degree in
electronics from the Indian Institute of Technology,
Kanpur, in 1976, and the Ph.D. degree in electrical
engineering specializing in wind electrical power
generation from the University of Western Australia
in 1985.
Prior to joining the School of Electrical and Computer Engineering at the Curtin University of Technology, Perth, Australia, in 1986, he held academic
and industry appointments in India, Singapore, and Australia. Prof. Nayar holds
a personal chair in electrical engineering and is the Director of the Centre for
Renewable Energy Systems Technology Australia (CRESTA) at Curtin University of Technology. He has been actively involved in programs investigating advanced technologies in rural electrification suitable for isolated mining towns,
homesteads, and Aboriginal communities in Australia and the rural areas in developing countries. He is the Program Manager of the Power Conditioning Program for the Australian Cooperative Research Centre for Renewable Energy.
He has worked as the United Nation’s Consultant for small-scale wind energy
conversion systems for Asian and Pacific regions and the transfer of technology
for the SPV/diesel hybrid system market development in India. He has a large
number of publications in international journals and conference proceedings.
Prof. Nayar served as a member of the Policy Committee of the Alternative
Energy Development Board in Western Australia. He is also a Chartered Engineer and Corporate Member of the Institution of Electrical Engineers (UK),
and Chartered Professional Engineer and Corporate Member of the Institution
of Engineers, Australia.
IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 15, NO. 1, MARCH 2000