Download Power Quality Improvement through Unified power quality

Power Quality Improvement through Unified power quality
conditioner (UPQC)
Mohammed Nasser TANDJAOUI, A. KECHICH, C. BENOUDJAFER, M. HABAB∗
Abstract
The quality of the power electric is characterized by the measurement and the analysis of the electric
disturbances which make it possible to understand the origin of the disturbances, to evaluate their impact on the
sensitive equipment, and thus to find and choose the most suitable solution either economically or technically.
This paper deals with Unified Power Quality Conditioners (UPQC’s), which aim at the integration of series and
shunt inverters which can compensate the current and voltage distortion correctly. The results of simulation in
MATLAB/SIMULINK software show appropriate operation and the capability of the proposed system to improve
the power quality at the point of installation on power distribution systems or industrial power systems.
Keywords: power quality, voltage sags, UPQC, harmonic, active power filter
1. Introduction
The term “power quality” (PQ) has gained
significant attention in the past few years [1].
The quality of electric power concerns all the
stakeholders of the energy field is they
managers of networks, suppliers, or
electricity consumers. It has become a
subject of great interest these last years,
primarily for reasons such as the urgent
economic requirements, the generalization of
the equipment sensitive to the disturbances,
and the opening of the electricity market.
Though electricity production is rather
reliable, the quality of this product is not so in
the distribution systems because of the
many nonlinear loads that significantly
affects the purity of the waveform of the
voltages and the cause the loss of the power
supply. This produces many problems of
energy quality due to the significant loads.
Such
a
situation
has
encouraged
researchers to find means of increasing the
quality of energy supply [2].
The most problems known the quality of
∗
Mohammed Nasser TANDJAOUI: University of Bechar,
Department of Technology, Route de Kenadsa, BP.417,
Bechar 08000, Algeria, e-mail: [email protected]
A. KECHICH: University of Bechar, Department of
Technology, Route de Kenadsa, BP.417, Bechar 08000,
Algeri, e-mail: [email protected],
C. BENOUDJAFER: University of Bechar, Department of
Technology, Route de Kenadsa, BP.417, Bechar 08000,
Algeria,
M. HABAB: University of Bechar, Department of
Technology, Route de Kenadsa, BP.417, Bechar 08000,
Algeria
electric are in voltage and current
disturbances.
There are many different methods to
improve the power quality, but the use of a
custom Power device is considered to be the
most efficient method.
The concept of custom Power was
introduced by N.G. Hingorani in 1995. Like
Flexible AC Transmission Systems (FACTS)
for transmission systems, the term custom
power pertains to the use of power
electronics controllers in a distribution
system, especially, to deal with various
power quality problems [3, 4]. Each of
Custom Power devices has its own benefits
and limitations.
The advancement in the semiconductor
device technology has made it possible to
realize most of the power electronics based
devices/prototypes at commercial platform.
The development of power electronic
technology makes it possible to realize many
kinds of Flexible Alternating Current
Transmission Systems devices to obtain
high quality electric energy and enhance the
control over power system [1].
This paper deals with the most effective
type of these devices is considered to be the
Unified Power Quality Conditioners (UPQC)
which aim at the integration of series active
and shunt active filters.
The main purpose of a UPQC is to
compensate
for
supply
voltage
flicker/imbalance, reactive power, negativesequence current, and harmonics.
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ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2
In other words, the UPQC has the
capability of improving power quality at the
point of installation on power distribution
systems or industrial power systems.
2. General UPQC
Unified power quality conditioner (UPQC)
is the powerful tool to settle the power quality
problem. The general configuration of the
UPQC is shown in Figure 1.
Figure 1. General configuration of the UPQC
With ideal compensation, the voltage at
PCC is the fundamental positive sequence
sinusoidal voltage of the power source side
and the load is equal to a resistance. The
currents of the source are sinusoidal current
and the phase angles of them are the same
as the fundamental voltage in phase
respectively.
The UPQC is installed in order to protect
a sensitive load from all disturbances. It is a
combination of series and shunt active filters,
two active filters have different functions and
they are consists of two voltage source
inverters implemented with Insulated gate
Bipolar Transistors (IGBTs) and connected
back to back, sharing a common DC link [5].
The series active filter suppresses and
isolates voltage-based distortions but the
shunt active filter cancels current-based
distortions. The series inverter is connected
through transformers in series between the
source and the common connection point,
acts as a controlled voltage source
maintaining the load voltage sinusoidal and
at desired constant voltage level.
The other inverter is connected in parallel
in load side with the common connection
point through transformers, It acts of helps in
compensating load harmonic current,
reactive current and maintain the dc link
voltage at constant level [5, 6]. The single
phase equivalent circuit for a UPQC is
shown in Figure 2.
Figure 2. Equivalent Circuit of a UPQC
The source voltage, terminal voltage at
PCC and load voltage are denoted by Vs, Vt
and VL respectively. The source and load
currents are denoted by is and iL,
respectively [5]. The voltage injected by
series APF is denoted by Vsr, where as the
current injected by shunt APF is denoted
by ish.
The series PWM converter is modeled
as voltage source, while the shunt PWM
converter as current source. Harmonic
generating load is modeled as generic
current source [6].
The high-pass filter impedance of the
shunt PWM converter referred to the
primary is
Z1− Sh = RSh + jωLSh
(1)
ZSh in series with the network reactor
impedance referred to the secondary of
the series single-phase transformer is
given by
Z 2 − Sh = Z Sh + Z S
(2)
Zp-sr parallel with the high-pass filter
impedance of the series PWM converter
is given by
Z p − Sr =
Z 2 Sh Z Sr
Z ( R + jωLSr )
= 2 Sh Sr
Z 2 Sh + Z Sr Z 2 Sh + ( RSr + jωLSr )
(3)
The impedance of the series PWM
converter referred to the primary is
Z Sr = R Sr + j ω L Sr
(4)
The total impedance ZT connected to
the power supply, when there is no load
is
Z T = Z S + Z Sr + Z p − Sr
(5)
3. The UPQC control strategy
The proposed control strategy is aimed to
generate reference signals for both shunt
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2
and series APFs of UPQC. In the following
section, an approach based on SRF
(Synchronous reference frame) theory
combined of extended p-q theory is used to
get reference signals for the series and
shunt APFs. One of advantages of a d-q
domain derivation of reference signals lies in
easier signal filtration, since the 50Hz
components are transferred into DC.
Reference voltages for the series active filter
can be determined based almost on the
same procedure [7].
The shunt active power filter rating mainly
depends on the compensating provided the
current generated by nonlinear load and the
reactive power of the system need. It acts as
a
controlled
current
generator
that
compensated the load current to force the
source currents drained from the network to
be sinusoidal, balanced and in phase with
the positive-sequence system voltages.
⎤
⎥ =
⎥⎦
2
3
The instantaneous reactive power
(p-q) theory is used to control of shunt
APF in real time [8]. In this theory, the
instantaneous three-phase currents are
transformed to d-q coordinates as shown
in equation (6).
⎡1 / 2 1 / 2
⎤ ⎡V a
1/ 2
⎢
⎥
cos( θ − 2 π / 3 ) cos( θ − 4 π / 3 ) ⎥ ⎢⎢ V b
⎢ cos θ
⎢ − sin θ − sin( θ − 2 π / 3 ) − sin( θ − 4 π / 3 ) ⎥ ⎢ V
⎣
⎦⎣ c
Where θ is the instantaneous supply
voltage angle given by
t
θ = θ0 + ∫ ωtdt
(7)
0
Currents in rotating frame can be
decomposed in DC (50 Hz) and AC
(harmonic, sub harmonic or inter-harmonic)
component:
−
The shunt APF reference current signal
generation block diagram is shown in
Figure 3.
Figure 3. The control system of the PAPF
3.1. Shunt control strategy
⎡ id
⎢
⎢⎣ i q
~
−
41
⎤
⎥
⎥
⎥⎦
(6)
compensation voltage that synthesized by
the PWM converter and inserted in series
with the supply voltage, to force the voltage
of PCC to become sinusoidal and balanced.
The proposed series APF reference voltage
signal generation algorithm is shown in
figure 4.
~
id = id + id , iq = iq + iq
(8)
where id corresponds to the reactive and
iq to the active power component.
3.2. Series control strategy
The series active power filter is to
compensate
the
voltage
disturbance
provided in the source side. It handles no
active or reactive power, it generates the
⎡Vd
⎢
⎢⎣ V q
⎤
⎥=
⎥⎦
Figure 4. The control system of the SAPF
In equation (1), supply voltages vSabc are
transformed to d-q-0 coordinates.
⎡1 / 2 1 / 2
⎤ ⎡ia ⎤
1/ 2
⎥
2⎢
cos θ cos( θ − 2 π / 3 ) cos( θ − 4 π / 3 ) ⎥ ⎢⎢ ib ⎥⎥
⎢
3⎢
⎥⎢ ⎥
⎣ − sin θ − sin( θ − 2 π / 3 ) − sin( θ − 4 π / 3 ) ⎦ ⎣ ic ⎦
(9)
The voltage in d axes (ud) given in (2)
consists of average and oscillating
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ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2
components of source voltages (vSd and
~vSd). The average voltage vSd is
calculated by using second order LPF
(low pass filter).
−
~
Vd = Vd + Vd
(10)
These produced three-phase load
reference voltages are compared with
load line voltages and errors are then
processed by sinusoidal PWM controller
to generate the required switching
signals for series APF IGBT switches [8].
3.3. The DC voltage regulator:
In compensation process, the DC side
voltage will change because UPQC
compensates
the
voltage
/current
disturbance, the active /reactive power and
the losses of switches, etc.
Also, shunt inverter control undertakes
the duty of (stabilizing) DC link voltage
during
series
inverter
operation
to
compensate voltage distortions. DC link
capacitor voltage controlling loop is used
here by applying PI (proportional integrator)
controller [9].
The DC voltage regulator shown in Figure
5 is used to generate a control signal to keep
the voltage be a constant.
The three phase terminal voltages, three
phase load voltages, three phase source
currents and the DC link voltage are sensed
and used to generate the switching patterns
for shunt and series APFs, whereas, an R-L
load with uncontrolled diode rectifier is
considered as a sensitive load to be
protected.
The simulation results are shown in the
figure 6, figure 7 and figure 8. Circuit
parameters used in simulation are located in
table 1.
Table 1. Parameters for the power circuit of UPQC
Source Phase Voltage (rms)
Frequency
DC Link voltage
Shunt inverter Inductance(Lf)
Switching Frequency
Series inverter Inductance (Ls)
Series inverter Capacitance (Cs)
Series inverter Resistance (Rs)
Load Resistance (RL)
Load Inductance (LL)
220 v
50 Hz
850 v
0.001 H
12 kHz
0.6e-3 H
220e-6 F
30e-3 Ω
3.34 Ω
60e-3 H
The maximum simulation time is
regulated on 500 msec. Shunt inverter starts
to operate at time started and series inverter
starts at 150 msec.
Figure 6 shows the performance of series
APF of the UPQC in the system which
supply a linear load R-L.
Figure 5. The DC voltage regulator
It forces the shunt active filter to draw
additional active current from the network.
The study of the regulation of the
continuous voltage at the boundaries of the
storage capacity showed that a compromise
must be done between filtering and the
speed in the control of this voltage. For that,
the studied regulator, proportional integrator
(PI) is more suited to assure an optimal
filtering characteristic and an optimal cost.
4. Simulation results:
The performance of each topology of
UPQC under the steady state conditions
have been verified by computer simulation in
the MATLAB/ SIMULINK environment, when
it is evaluated in terms of voltage sags and
voltage / current harmonics mitigation.
Figure 6. Waveforms of voltage source (Vs), voltage
load (VL) and injected voltage by a series
APF (Vinj)
During the voltage sag condition in an
interval of the time from 0.15 s to 0.35 s, the
series APF is providing the required power of
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2
43
the load to correct this fault by injecting in
phase compensating voltage (50 %) equals
to the difference between the reference load
voltage and source voltage through a series
coupling transformer in order to obtain a
stable, proper and effective load voltage to
protect.
In the second case, the proposed system
developed by supplies a non-linear load with
a source voltage purely sinusoidal. At start
time of simulation, the shunt APF is put into
the operation when the load current is
disturbed by the current harmonics produced
by the diode rectifier.
Simulation results shown in Figure 7
demonstrate the effectiveness of the
developed system for the control of shunt
APF.
Figure 8. Waveforms of voltage/current of source,
load and compensating of UPQC
Figure 7. Waveforms of current source (Is), current
load (IL) and injected current by a shunt
APF (Iinj)
In figure 8, one will analyze the
robustness in terms of speed and precision
of the UPQC compensating for the voltage
sags applied in an interval of the time from
0.05 s to 0.25 s and the voltage harmonics
presented in an interval of the time from
0.30 s to 0.75 s.
The UPQC through the transformer of the
series active filter injects the compensating
voltage necessary to satisfy the request
voltage of the load. It is noted that at the
moment t=0.3s, the UPQC through the
active filter series starts to correct the
voltage
harmonics,
by
injecting
compensating voltage have forms of well
synchronized waves and in opposition of
phase with the voltage of the source.
The shunt APF injects a leading
compensating current and supplies the load,
this quantity of current injecting is a high
current to grid, a part of which is consumed
to feed the load and else is injected to grid to
mitigate the current disturbance and making
the input power close to unity. The shunt
APF controller acts immediately forcing DC
link voltage to settle down at new steady
state value i.e. at 850 V.
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ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2
While series APF is providing the required
real power to the load, the shunt APF is
maintaining the DC link voltage at constant
level such that the series APF can provide
the needed real power to the load. To
maintain the DC link voltage at constant level
the source delivered more current.
5. Conclusion
The work presented the capability of
UPQC lies within the scope of the search for
new solutions for the improvement of the
power quality in the electric supply network.
As the UPQC is a combination of series and
shunt active filters, two active filters have
different functions. It takes advantages of
these filters (series APF and shunt APF) to
compensate the distortions of both source
voltages and load currents. UPQC has a
complicated structure that uses several
elements working together, that’s why we
need rigorous choice of its parameters.
The parallel active filter aimed to
compensate for the harmonic, reactive and
unbalanced interference currents. The active
filter series its objective was the
compensation of the harmonic disturbing
voltage, and of the voltage sags. Finally, the
UPQC was proposed as a general solution
of the compensation of all the disturbances
due to voltage or/and current.
The obtained results of the simulations
show that the UPQC is one of FACTS
equipment
able
to
compensate
all
disturbances of voltage and/or current with a
great efficiency.
References
[1] Benachaiba C., Abdelkhalek O., Dib S., Allali
M. and Dib D., “The Unified Power Quality
Conditioner (UPQC): The Principle, Control
and Application”, in CIGE’10, 03-04 Nov.
2010, Bechar, Algeria.
[2] Tandjaoui M.N., Benachaiba C., Abdelkhalek
O., Doumbia M.L., Mouloudi Y., “Sensitive
Loads Voltage Improvement Using Dynamic
Voltage Restorer”, in Explore IEEE, 17-19
July 2011, Bandung, Indonesia.
[3] Tandjaoui M.N., Benachaiba C., Abdelkhalek
O. and Doumbia M.L., “Mitigation of voltage
sags/swells unbalanced in low voltage
distribution systems”, in IJSAT, (ISSN 22218386), pp.46-51, Vol. 1 No 6, Aug. 2011.
[4] Tandjaoui M.N., Benachaiba C. and
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November 22-24, 2011, Oran, Algeria.
[5] Khadkikar V., Chandra A., Barry A.O. and
Nguyen T.D., “Steady State Power Flow
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(UPQC)”, in Explore IEEE, 2005.
[6] Benachaiba C., Abdelkhalek O., Dib S.,
Haidas M., “Optimization of Parameters of
the Unified Power Quality Conditioner using
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1392 – 124X, Vol.36, No.2, 2007.
[7] Benachaiba C., Ferdi B., Dib S., Rahli M.,
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[8] Kesler M., Ozdemir E., “A Novel Control
Method for Unified Power Quality Conditioner
(UPQC) Under Non-Ideal Mains Voltage and
Unbalanced Load Conditions”, in Explore
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Biography
Mohammed Nasser TANDJAOUI
received the state engineer
degree in Electric Engineering in
2005 from the University of
Sciences and Technology of Oran
(USTO).
He was Magister in electric engineering in 2009
from university of Bechar, Algeria. He currently
was holding the post of Assistant maitre in
university of bechar. He was preparing a
Doctorate of improvement of the quality of
energy electric in a wind network by the
integration of FACTS systems. His research
area interests are power electronics, FACTS,
HVDC, power quality issues, renewable energy
and energy storage