A three-phase line Interactive UPS system with active power lineAconditioning
rtigo Original
using
/ Original
SRF method
Article
A Three-Phase Line Interactive UPS system with active power line conditioning using SRF method
Um sistema UPS Line-Interactive trifásico com condicionamento ativo de potência usando o
método SRF
Sérgio Augusto Oliveira da Silva* Rodrigo Augusto Modesto**
Lúcio dos Reis Barbosa***
* Universidade Norte do Paraná (UNOPAR).
Universidade Tecnológica Federal do
Paraná (UTFPR).
** Universidade Estadual de Londrina (UEL).
***Universidade Norte do Paraná (UNOPAR).
Universidade Estadual de Londrina (UEL).
Abstract
This paper presents an application of an active compensation method, applied to a
three-phase line-interactive uninterruptible power supply (UPS) system with series
and parallel active power line conditioning capabilities. The control strategy, employed
to achieve the reference currents for the UPS system, is based on the synchronous
reference frame (SRF) method. The reference currents, obtained from the SRF
algorithm, are used to compensate the reactive power and to eliminate harmonic
currents generated from non-linear loads. The reference currents of the SRF-based
controller does not have the task of compensating fundamental zero sequence
component of the neutral current, due to unbalanced loads or unbalance source
voltages conditions. Thus, the efficiency of the three-phase UPS system is increased
when it is feeding unbalanced single-phase non-linear loads. Besides, a mode to
control the active power flow through the series and parallel active power filters of the
UPS system is presented.
Keywords: UPS. Harmonics. Active power flow. Active power filter.
Resumo
Este trabalho apresenta a aplicação de um método de compensação ativa, aplicado
a um sistema de energia ininterrupta (SEI) line-interactive trifásico, com capacidade
de condicionamento ativo de potência série e paralelo. A estratégia de controle,
empregada para a obtenção das referências de corrente do SEI, é baseada no
sistema de referência síncrona (SRF). As referências de corrente obtidas a partir do
algoritmo SRF, são usadas para compensar a potência reativa e eliminar harmônicos
gerados por cargas não lineares. As correntes de referência do controlador SRF, não
possuem a tarefa de compensar a componente fundamental de seqüência zero da
corrente de neutro, presentes no sistema devido a cargas ou tensão da rede elétrica
desequilibradas. Assim, a eficiência do SEI trifásico é aumentada quando a mesma
alimenta cargas não lineares monofásicas desequilibradas. Além disso, é apresentado
um modo para controlar o fluxo de potência ativa através dos filtros ativos de potência
série e paralelo do SEI.
Palavras-chave: SEI. Harmônicos. Fluxo ativo de potência. Filtro ativo de potência.
1 Introduction
The use of non-linear loads by residential, commercial
and industrial customers has contributed to reduce the
power factor and to increase the THD (Total Harmonic
Distortion) of the input line voltages. The problem
increases when single-phase non-linear loads are
connected in three-phase, four-wire systems. In this case,
even perfectly balanced single-phase loads, a very large
third component and its multiples can flow through the
neutral wire. Besides, the neutral current amplitude can
exceed the amplitude of the line currents (GRUZS, 1990).
The excessive neutral current can cause damage both in
the neutral conductor and in the transformer to which the
non-linear loads are connected (QUINN; MOHAN, 1992).
Now, if non-linear loads are unbalanced, the neutral
current will contain both the fundamental and harmonic
components.
Active Power Filters (APF) have been used to
compensate the currents drawn from utility Quinn and
Mohan (1992); Quinn; Mohan and Mehta (1993); Thomas,
et al. (1996). Uninterruptible Power Supply (UPS) systems
have enabled the improvement of power source quality,
providing clean and uninterruptible power to critical loads
such as industrial process controls, computers, medical
equipment, data communication systems, and protection
against power supply disturbances or interruptions (SILVA
et al., 2002; 2003, 2004).
In Silva et al., (2002), the UPS system has been used
for three-phase, three-wire system and in Silva et al.
(2004), the UPS system has been used for three-phase,
four-wire system. In both Silva et al. (2002; 2004), the
source currents have been controlled to be sinusoidal
and balanced providing negative and zero sequence
components compensation.
In this paper a three-phase line-interactive UPS
system is used to feed single-phase non-linear loads.
The implemented algorithm generates the reference
currents to compensate the reactive power and to
eliminate harmonic currents generated from non-linear
loads, without the function of compensating fundamental
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006
31
A three-phase line Interactive UPS system with active power line conditioning using SRF method
zero sequence component of the neutral current, due
to unbalanced loads or unbalance source voltages
conditions. Thus, the efficiency of the three-phase UPS
system is increased when it is feeding unbalanced singlephase non-linear loads. Besides, a mode to control the
active power flow through the UPS system is presented.
2 Operation of the Line-Interactive UPS Topology
The topology of the line-interactive UPS system is
shown in Figure 1. Two PWM converters, coupled with
a common dc-bus, are used to perform the series active
filter and parallel active filter functions. The series active
filter is connected in series with the line and the load
through three single-phase linking transformers. A battery
bank is placed in the dc-bus and a static switch ‘sw’ is
incorporated to the circuit to provide a fast disconnection
between the UPS system and the power supply when an
occasional interruption of the incoming power occurs.
The series active power filter acts as a sinusoidal
current source. It has high impedance, which is enough
to isolate the line from the load with respect to harmonic
currents. An SRF-based controller is used to control the
series active power filter making the line currents (isa, isb,
and isc) sinusoidal with low THD.
The parallel active power filter acts as a sinusoidal
voltage source with constant rms output voltages (vfa,
vfb, and vfc) and low THD. It has low impedance, which
is enough to absorb the harmonic currents of the load.
The output UPS voltages and the line currents are
individually controlled to be in phase with respect to the
line voltages (vsa, vsb, and vsc) respectively. Both the pa-rallel
and the series filter use three independent controllers
acting on half-bridge inverters. In this line-interactive UPS
system, an effective power factor correction is carried out.
Figure 1. Line-interactive UPS system topology.
transformation (1). Then, these quantities are transformed
from a two-phase stationary reference frame (dq)s into a
two-phase synchronous rotating (dq)e reference frame,
based on the transformation (2), where θ = ωt, is the
angular
position
of the reference frame. The coordinates
*
*
*
of unit vector, sinθ and cosθ, are obtained from PLL
system. The currents at the fundamental frequency ω
(ide, iqe) are now dc quantities and all the harmonics,
transformed into non-dc quantities, can be filtered using
a low pass filter (LPF) as shown in Figure 2. Thus, iddc
and iqdc represents the active and reactive fundamental
components of the load current in dq axis, respectively.
32
A. SRF-Based Controller for Current Compensation
(Standby Mode)
An SRF-based controller is used to provide and to
control the sinusoidal compensating reference currents
(ica, icb, and icc) for the series PWM converter shown in
Figure 1. The block diagram of the control scheme for
current compensation is shown in Figure 2.
The three-phase load currents (iLa, iLb, and iLc) are
measured and transformed into a two-phase stationary
reference frame (dq)s quantities (ids, iqs) based on the
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006
A three-phase line Interactive UPS system with active power line conditioning using SRF method
Figure 2. Block Diagram of the Current SRF Based Controller (Conventional strategy).
B. SRF-Based Controller Applied to Unbalanced
Loads
Now, only the dc component current of the synchronous rotating reference frame
must be transformed
into the stationary reference frame (dq)s to obtain the
fundamental references currents
Thus, the reactive and harmonic components
of the load currents were eliminated. As the currents
are balanced, the compensation of the
negative and zero sequence components has been
performed.
The inverse transformation matrix from two-phase
synchronous reference frame to two-phase stationary
reference frame is given by (3). The matrix that provides
the linear transformation from two-phase system to threephase stationary reference frame system is given by (4).
The previous presented synchronous reference
frame method generates balanced reference currents
because it is based on the consideration of balanced
three-phase loads. Thus, additional power rate must be
handled through the converters to perform the current
compensation, decreasing the efficiency of the series
and parallel converters.
Applying a single-phase strategy, in which each phase
is treated separately, is possible to get three-phase load
currents by the acquisition of only one of them, as shown
in Figure 3. By software implementation, the acquired
load current is phase delayed by 120 and 240 degrees,
producing the other two load currents.
As can be waited, the phase delays introduced by
the algorithm produce an increasing in the transient time
and the dynamic response is slower than the response
obtained from the conventional method. However, the
dynamic response can be improved by using the strategy presented in (OLIVEIRA et al., 2001), in which the
conventional filtering of the synchronous reference frame
method can be suppressed.
Based on SRF method applied to a single-phase
strategy shown in Figure 3, the new reference currents
can be achieved by (5). Now the currents
are unbalanced, thus the compensation of the negative
and zero sequence components has not been performed.
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006
33
A three-phase line Interactive UPS system with active power line conditioning using SRF method
Figure 3. SRF-Based Controller applied to a single-phase strategy.
3 Active Power Flow of the Three-Phase UPS System
Depending on the difference between the utility voltage
and the output voltage, the active power flow of the UPS
system can change its direction. When the rms utility
voltage Vs is greater than the rms output voltage Vf , the
active power flows from the power supply to the dc-bus
via series converter and from the dc-bus to the load via
parallel converter, as shown in Figure 4 (a). When Vs is
less than Vf , the active power flows from the power supply
to the dc-bus via the output of the parallel converter and
from the dc-bus to the line via series converter, as shown
in Figure 4 (b). Therefore, controlling the amplitudes of
the series reference currents
the dc-bus furnishes and absorbs power to
make the “balance” of the active power. If the line voltage
Vs is equal to the output voltage Vf and the UPS is feeding
a linear resistive load, there isn’t any active power flow
through both the series and the parallel converters, as
shown in Figure 4 (c).
The direction of the power flow of the UPS system
must be taking into account when the reference currents
are been calculated. Therefore, the amplitude of them
will be ever changing.
Although power compensation can be performed using a dc bus controller, an alternative algorithm can be
implemented applying a constant “k” in the SRF algorithm
of Figure 3. Before the compensation, the constant “k”,
given by (8) can be calculated by division of the dc component of the instantaneous output power
(6) and
the dc component of the input power
(7). Thus,
the real input power
calculated from the reference
currents is given by (9). Now, for an ideal condition, the dc
component of the new input instantaneous power
will be equal to
.
34
4 Simulation Results
The line-interactive UPS system, operating at 20 KHz
switching frequency, has been verified by simulation
working as a series-parallel active power line filter. The
UPS system feeds non-linear loads represented by three
single-phase diode bridge rectifier as shown in Figure 5.
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006
A three-phase line Interactive UPS system with active power line conditioning using SRF method
Fundamental unbalanced voltages of ±15% and harmonic contents have been included in the input voltages
as shown in Figure 6 (a). Figure 6 (b) shows the controlled
output voltages of the line interactive UPS system. They
are balanced and have low harmonic contents.
Figure 5. Line interactive UPS system feeding unbalanced non-linear loads.
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006
35
A three-phase line Interactive UPS system with active power line conditioning using SRF method
The uncompensated non-linear load currents
,
, and
are shown in Figure 7 (a). The compensated line currents
,
, and
that follow the
reference currents
,
, and
obtained from the
current SRF-based controller of Figure 3, are shown in
Figure 7 (b). The implemented algorithm generates the reference currents to compensate the reactive power and to
eliminate the harmonic currents generated by non-linear
loads. As can be noted the fundamental zero sequence
components of the neutral current were not compensated.
In Figure 8 the transition modes from the standby
mode to backup mode (40ms) and from backup to standby
mode (65ms) are shown. The phase “a” output voltage
(
) is shown in Figure 8 (a) and the compensated
input current (
) is shown in Figure 8 (b). In Figure
8 (c), phase “a” parallel inverter current (
) is shown.
It can be noted that the parallel active filter is providing
compensation harmonic of the load currents when the
UPS is operating in the standby mode until 40ms. After
40ms the parallel converter assumes the full power to
feed the load.
Seamless voltages transitions from the standby mode
to backup mode (40ms) and vice-versa (65ms) are obtained as can be observed in the Figure 8 (a).
Figure 9 (b) shows the compensation constant k calculated from equation (8). This constant is used in the
36
current SRF controller of Figure 3 to control the amplitude
of the reference currents so that the dc value of the input
power
and the dc value of the output power
become equals, as shown in Figure 9 (a). Therefore,
controlling the amplitudes of the currents (
), ( ),
and ( ), the dc-bus can furnishes and absorbs power
to make the “balance” of the active power.
Additional simulations are realized considering three
single-phase unbalanced resistive loads as can be shown
by Figures. 10 (a) and 11 (a). In Figure 10 (b) the conventional strategy is used to calculate the balanced reference
currents using the algorithm of Figure 2. As can be noted
the compensation of the fundamental zero sequence
component is realized by the active parallel converter,
shown by Figure 10 (c), decreasing its efficiency. Figure
11 shows the single-phase strategy using the algorithm of
Figure 3. As can be seen the reference currents of Figure
11 (b) are unbalanced, but the compensation currents of
Fig. 11 (c), are null in steady state. Thus, the fundamental
zero sequence components of the load currents are not
compensated by the parallel converter and its efficiency
is increased. Figure 12 shows the instantaneous active
powers of the load (p), parallel converter with conventional
strategy (
) and parallel converter with single-phase
strategy (
), respectively.
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006
A three-phase line Interactive UPS system with active power line conditioning using SRF method
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006
37
A three-phase line Interactive UPS system with active power line conditioning using SRF method
38
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006
A three-phase line Interactive UPS system with active power line conditioning using SRF method
5 Conclusions
An application of an SRF-based method, applied
to a three-phase line-interactive uninterruptible power
supply system with series and parallel active power line
conditioning capabilities has been presented.
The reference currents, obtained from the SRF-based
controller, can be used to compensate only the reactive
power and to eliminate harmonic currents generated from
non-linear loads.
Applying a single-phase strategy, in which each phase
is treated separately, was possible to get three-phase load
currents from the acquisition of only one single-phase
load current.
Thereby, negative and zero sequence components at
fundamental frequency of the three-phase system were
despised. Thus, the power rate of the series and parallel converters decreases and the efficiency of the UPS
system increases when it is feeding unbalanced singlephase non-linear loads. Besides, a mode to control the
active power flow through the UPS system was presented.
References
GRUZS, T. M. A Survey of neutral currents in three-phase
computer power systems. IEEE Transactions on Industry
Applications, v. 26, n. 4, p. 719-725, Jul./Aug. 1990.
OLIVEIRA, L. et al. Improving theddynamic response of
active power filters based on the synchronous reference
frame method. In: PROCEEDINGS OF THE 6TH BRAZILIAN POWER ELECTRONICS CONFERENCE, 2001
ELECTRONICS CONFERENCE AND EXPOSITION,
1992, Boston. APEC'92. Conference Proceedings 1992.
Boston, 1992. p. 829-836.
QUINN, C. A.; MOHAN, N.; MEHTA, H. A four-wire, currentcontrolled converter provides harmonic neutralization in
three-phase, four-wire systems. In: APPLIED POWER
ELECTRONICS CONFERENCE AND EXPOSITION,
1993, San Diego. APEC'93. Conference Proceedings
1993. San Diego, CA, 1993. p. 841-6.
SILVA, S. A. O. et al. A three-phase line-interactive UPS
system implementation with series-parallel active powerline conditioning capabilities. IEEE Transactions on Industry Applications, v. 38, n. 6, p. 1581-1590, Nov./Dec. 2002.
______. et al. Performance analysis of three-phase lineinteractive UPS system with active power-line conditioning. In: ANNUAL CONFERENCE OF THE IEEE INDUSTRIAL ELECTRONICS SOCIETY, 29., 2003. IECON'03.
Roanoke, Virgínia: IEEE Industrial Electronics Society,
2003. v. 1, p. 353-60.
______. et al. A line-Interactive UPS system implementation with series-parallel active power-line conditioning
for three-phase, four-wire systems. Electrical Power and
Energy Systems, v. 26, n. 6, p. 399-411, 2004.
THOMAS, T. et al. Performance evaluation of three-phase
three and four wire active filters. In: CONFERENCE
RECORD OF THE IEEE INDUSTRY APPLICATION
SOCIETY, 31., 1996, San Diego. IAS'96, 1996 Annual
Meeting. San Diego, 1996. p. 1016-1023.
QUINN, C. A.; MOHAN, N. Active filtering of harmonic currents in three-phase, four-wire systems with three-phase
and single-phase non-linear loads. In: APPLIED POWER
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006
39
A three-phase line Interactive UPS system with active power line conditioning using SRF method
Sérgio Augusto Oliveira da Silva*
Doutor em Engenharia Elétrica pela Universidade Federal de
Minas Gerais (UFMG). Docente do Curso de Engenharia da
Computação da Universidade Norte do Paraná (UNOPAR) e do
Curso de Engenharia Elétrica da Universidade Tecnológica Federal
do Paraná (UTFPR).
e-mail: <sergio.computacao@unopar.br>
Rodrigo Augusto Modesto
Tecnólogo em Automação Industrial pela Universidade Tecnológica
Federal do Paraná (UTFPR). Mestrando em Engenharia Elétrica na
Universidade Estadual de Londrina (UEL).
e-mail: <rodrigoaugustodj@hotmail.com >
Lúcio dos Reis Barbosa
Doutor em Engenharia Elétrica pela Universidade Federal de
Uberlândia (UFU). Docente do Curso de Engenharia da Computação
e Engenharia Elétrica da Universidade Norte do Paraná (UNOPAR).
Docente do Curso de Engenharia Elétrica da Universidade Estadual
de Londrina (UEL).
e-mail: <lbarbosa@uel.br>
* Endereço para correspondência:
Rua Francisco Marcelino da Silva, 455 – CEP 86047-160 – Londrina, Paraná, Brasil.
40
SILVA, S. A. O. da.; et al. / UNOPAR Cient., Ciênc. Exatas. Tecnol., Londrina, v. 5, p. 31-40, nov. 2006