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ISSN 2319-8885
Vol.03,Issue.04,
April-2014,
Pages:0638-0646
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Power Quality Improvement by Versatile Control Scheme for Unified
Power Quality Conditioner (UPQC) ALAA HAMZAH ABDULLAH
1, SURYA PRAKASH
2, JYOTI SHRIVASTAVA
3
1Dept of Electrical Engineering, SSET, Sam Higginbottom Institute of Agri. Tech. & Sciences Deemed University, Allahabad,
India, E-mail: [email protected]. 2Dept of Electrical Engineering, SSET, Sam Higginbottom Institute of Agri. Tech. & Sciences Deemed University, Allahabad,
India, E-mail: [email protected]. 3Dept of Electrical Engineering, SSET, Sam Higginbottom Institute of Agri. Tech. & Sciences Deemed University, Allahabad,
India, E-mail: [email protected].
Abstract: Conventional power quality mitigation equipment is proving to be inadequate for an increasing number of
applications, and this fact has attracted the attention of power engineers to develop dynamic and adjustable solutions to power
quality problems. This has led to development of Custom Power Devices (CPD).One modern and very promising CPD that
deals with both load current and supply voltage imperfections is the Unified Power Quality Conditioner (UPQC). One modern
solution that deals with both load current and supply voltage imperfections is the Unified Power Quality Conditioner (UPQC)
such a solution can compensate for different power quality phenomena, such as: sags, swells, voltage imbalance, flicker,
harmonics and reactive currents. UPQC is a combination of series and shunt active filters connected in cascade via a common
dc link capacitor. The series active filter inserts a voltage, which is added at the point of the common coupling (PCC) such that
the loads end voltage remains unaffected by any voltage disturbance. The main objectives of the shunt active filter are: to
compensate for the load reactive power demand and unbalance, to eliminate the harmonics from the supply current, and to
regulate the common dc link voltage. This paper deals with the investigation and development of UPQC control schemes and
algorithms for power quality improvement and implementation of a versatile control strategy to enhance the performance of
UPQC. The proposed control scheme gives better steady-state and dynamic response. The validity of the proposed control
method is verified by means of MATLAB/SIMULINK.
Keywords: Edge Unified Power Quality Conditioner (UPQC), Point of the Common Coupling (PCC), MATLAB/Simulink.
I. INTRODUCTION
Reliability of supply and power quality (PQ) are two
most important facets of any power delivery system today.
Not so long ago, the main concern of consumers of
electricity was the continuity of supply. However nowadays,
consumers want not only continuity of supply, but the
quality of power is very important to them too. The power
quality problems in distribution power systems are not new,
but customer awareness of these problems has recently
increased. The power quality at the point of common
coupling (PCC) with the utility grid is governed by the
various standards and the IEEE-519 standard is widely
accepted. Utilities and researchers all over the world have
for decades worked on the improvement of power quality
[1]-[5]. There are sets of conventional solutions to the
power quality problems, which have existed for a long time.
However these conventional solutions use passive elements
and do not always respond correctly as the nature of the
power system conditions change. The increased power
capabilities, ease of control, and reduced costs of modern
semiconductor devices have made power electronic
converters affordable in a large number of applications.
New flexible solutions to many power quality problems
have become possible with the aid of these power electronic
converters [6]-[8].
Nowadays equipment made with semiconductor devices
appears to be as sensitive and polluting as ever. Non-linear
devices, such as power electronics converters, increase
overall reactive power demanded by the equivalent load,
and injects harmonic currents into the distribution grid. It is
well known that the reactive power demand causes a drop in
the feeder voltage and increases the losses. The presence of
harmonic currents can cause additional losses and voltage
waveform distortions, and so cause poor power quality.
Also, the number of sensitive loads that require ideal
sinusoidal supply voltages for their proper operation has
increased. The increasing use of electronic equipment
sensitive to power variations drives the interest in power
conditioning technologies. So, in order to keep the power
quality within limits proposed by standards, it is necessary
to include some sort of compensation [9],-[12].
ALAA HAMZAH ABDULLAH, SURYA PRAKASH, JYOTI SHRIVASTAVA
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.04, April-2014, Pages: 0638-0646
The power electronic based power conditioning devices
can be effectively utilized to improve the quality of power
supplied to customers. One modern solution that deals with
both load current and supply voltage imperfections is the
Unified Power Quality Conditioner (UPQC), which was
first presented in 1995 by Hirofumi Akagi. Such a solution
can compensate for different power quality phenomena,
such as: sags, swells, voltage imbalance, flicker, harmonics
and reactive currents[13]-[17]. UPQC is a combination of
series and shunt active filters connected in cascade via a
common dc link capacitor. The series active filter inserts a
voltage, which is added at the point of the common coupling
(PCC) such that the load end- voltage remains unaffected by
any voltage disturbance. The main objectives of the shunt
active filter are: to compensate for the load reactive power
demand and unbalance, to eliminate the harmonics from the
supply current, and to regulate the common dc link
voltage[15].
II. POWER QUALITY
The contemporary container crane industry, like many
other industry segments, is often enamored by the bells and
whistles, colorful diagnostic displays, high speed
performance, and levels of automation that can be achieved.
Although these features and their indirectly related
computer based enhancements are key issues to an efficient
terminal operation, we must not forget the foundation upon
which we are building. Power quality is the mortar which
bonds the foundation blocks. Power quality also affects
terminal operating economics, crane reliability, our
environment, and initial investment in power distribution
systems to support new crane installations[16].
A. Power Quality Problems
For the purpose of this article, we shall define power
quality problems as: ‘Any power problem that results in
failure or disoperation of customer equipment, manifests
itself as an economic burden to the user, or produces
negative impacts on the environment.’ When applied to the
container crane industry, the power issues which degrade
power quality include: (i) Power Factor, (ii) Harmonic
Distortion, (iii) Voltage Transients, (iv) Voltage Sags or
Dips, (v) Voltage Swells. The AC and DC variable speed
drives utilized on board container cranes are significant
contributors to total harmonic current and voltage distortion.
Whereas SCR phase control creates the desirable average
power factor, DC SCR drives operate at less than this. In
addition, line notching occurs when SCR’s commutate,
creating transient peak recovery voltages that can be 3 to 4
times the nominal line voltage depending upon the system
impedance and the size of the drives. The frequency and
severity of these power system disturbances varies with the
speed of the drive. Harmonic current injection by AC and
DC drives will be highest when the drives are operating at
slow speeds. Power factor will be lowest when DC drives
are operating at slow speeds or during initial acceleration
and deceleration periods, increasing to its maximum value
when the SCR’s are phased on to produce rated or base
speed. Above base speed, the power factor essentially
remains constant. Unfortunately, container cranes can spend
considerable time at low speeds as the operator attempts to
spot and land containers. Poor power factor places a greater
kVA demand burden on the utility or engine-alternator
power source. Low power factor loads can also affect the
voltage stability which can ultimately result in detrimental
effects on the life of sensitive electronic equipment or even
intermittent malfunction. Voltage transients created by DC
drive SCR line notching, AC drive voltage chopping, and
high frequency harmonic voltages and currents are all
significant sources of noise and disturbance to sensitive
electronic equipment[17].
Power quality can be improved through: (i) Power factor
correction, (ii) Harmonic filtering, (iii) Special line notch
filtering, (iv) Transient voltage surge suppression,(v) Proper
earthing systems. In most cases, the person specifying
and/or buying a container crane may not be fully aware of
the potential power quality issues. If this article
accomplishes nothing else, we would hope to provide that
awareness. In many cases, those involved with specification
and procurement of container cranes may not be cognizant
of such issues, do not pay the utility billings, or consider it
someone else’s concern. As a result, container crane
specifications may not include definitive power quality
criteria such as power factor correction and/or harmonic
filtering. Also, many of those specifications which do
require power quality equipment do not properly define the
criteria. Early in the process of preparing the crane
specification:
Consult with the utility company to determine
regulatory or contract requirements that must be
satisfied, if any.
Consult with the electrical drive suppliers and
determine the power quality profiles that can be
expected based on the drive sizes and technologies
proposed for the specific project.
Evaluate the economics of power quality correction
not only on the present situation, but consider the
impact of future utility deregulation and the future
development plans for the terminal.
III. FACTS
Flexible AC Transmission Systems, called FACTS, got
in the recent years a well known term for higher
controllability in power systems by means of power
electronic devices. Several FACTS-devices have been
introduced for various applications worldwide. A number of
new types of devices are in the stage of being introduced in
practice. In most of the applications the controllability is
used to avoid cost intensive or landscape requiring
extensions of power systems, for instance like upgrades or
additions of substations and power lines. FACTS-devices
Power Quality Improvement by Versatile Control Scheme for Unified Power Quality Conditioner (UPQC)
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.04, April-2014, Pages: 0638-0646
provide a better adaptation to varying operational conditions
and improve the usage of existing installations. The basic
applications of FACTS-devices are (i) Power flow control,
(ii) Increase of transmission capability, (iii) Voltage control,
(iv) Reactive power compensation, (v) Stability
improvement, (vi) Power quality improvement, (vii) Power
conditioning, (viii) Flicker mitigation, (ix) Interconnection
of renewable and distributed generation and storages
A. Dynamic Power Flow Controller
A new device in the area of power flow control is the
Dynamic Power Flow Controller (DFC). The DFC is a
hybrid device between a Phase Shifting Transformer (PST)
and switched series compensation. A functional single line
diagram of the Dynamic Flow Controller is shown in Figure
1. The Dynamic Flow Controller consists of the following
components:
A standard phase shifting transformer with tap-
changer (PST).
Series-connected Thyristor Switched Capacitors
and Reactors (TSC / TSR).
A mechanically switched shunt capacitor (MSC).
(This is optional depending on the system reactive
power requirements).
Figure 1: Configuration of DFC.
Based on the system requirements, a DFC might consist
of a number of series TSC or TSR. The mechanically
switched shunt capacitor (MSC) will provide voltage
support in case of overload and other conditions. Normally
the reactance of reactors and the capacitors are selected
based on a binary basis to result in a desired stepped
reactance variation. If a higher power flow resolution is
needed, a reactance equivalent to the half of the smallest one
can be added. The switching of series reactors occurs at zero
current to avoid any harmonics. However, in general, the
principle of phase-angle control used in TCSC can be
applied for a continuous control as well. The operation of a
DFC is based on the following rules:
TSC / TSR are switched when a fast response is
required.
The relieve of overload and work in stressed situations
is handled by the TSC / TSR.
The switching of the PST tap-changer should be
minimized particularly for the currents higher than
normal loading.
The total reactive power consumption of the device
can be optimized by the operation of the MSC, tap
changer and the switched capacities and reactors.
In order to visualize the steady state operating range of
the DFC, we assume an inductance in parallel representing
parallel transmission paths. The overall control objective in
steady state would be to control the distribution of power
flow between the branch with the DFC and the parallel path.
This control is accomplished by control of the injected
series voltage. The PST (assuming a quadrature booster)
will inject a voltage in quadrature with the node voltage.
The controllable reactance will inject a voltage in
quadrature with the throughput current. Assuming that the
power flow has a load factor close to one, the two parts of
the series voltage will be close to collinear. However, in
terms of speed of control, influence on reactive power
balance and effectiveness at high/low loading the two parts
of the series voltage has quite different characteristics. The
steady state control range for loadings up to rated current is
illustrated in Figure 2, where the x-axis corresponds to the
throughput current and the y-axis corresponds to the
injected series voltage.
Figure 2: Operational diagram of a DFC.
Operation in the first and third quadrants corresponds to
reduction of power through the DFC, whereas operation in
the second and fourth quadrants corresponds to increasing
the power flow through the DFC. The slope of the line
passing through the origin (at which the tap is at zero and
TSC / TSR are bypassed) depends on the short circuit
reactance of the PST. Starting at rated current (2 kA) the
short circuit reactance by itself provides an injected voltage
(approximately 20 kV in this case). If more inductance is
switched in and/or the tap is increased, the series voltage
increases and the current through the DFC decreases (and
the flow on parallel branches increases). The operating point
moves along lines parallel to the arrows in the figure. The
ALAA HAMZAH ABDULLAH, SURYA PRAKASH, JYOTI SHRIVASTAVA
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.04, April-2014, Pages: 0638-0646
slope of these arrows depends on the size of the parallel
reactance. The maximum series voltage in the first quadrant
is obtained when all inductive steps are switched in and the
tap is at its maximum.
Now, assuming maximum tap and inductance, if the
throughput current decreases (due e.g. to changing loading
of the system) the series voltage will decrease. At zero
current, it will not matter whether the TSC / TSR steps are
in or out, they will not contribute to the series voltage.
Consequently, the series voltage at zero current corresponds
to rated PST series voltage. Next, moving into the second
quadrant, the operating range will be limited by the line
corresponding to maximum tap and the capacitive step
being switched in (and the inductive steps by-passed). In
this case, the capacitive step is approximately as large as the
short circuit reactance of the PST, giving an almost constant
maximum voltage in the second quadrant.
IV. UNIFIED POWER FLOW CONTROLLER
The UPFC is a combination of a static compensator and
static series compensation. It acts as a shunt compensating
and a phase shifting device simultaneously.
Figure 3: Principle configuration of an UPFC.
The UPFC consists of a shunt and a series transformer,
which are connected via two voltage source converters with
a common DC-capacitor. The DC-circuit allows the active
power exchange between shunt and series transformer to
control the phase shift of the series voltage shown in figure
3. The series converter needs to be protected with a
Thyristor bridge. Due to the high efforts for the Voltage
Source Converters and the protection, an UPFC is getting
quite expensive, which limits the practical applications
where the voltage and power flow control is required
simultaneously.
A. Unified Power Quality Conditioner
The provision of both DSTATCOM and DVR can
control the power quality of the source current and the load
bus voltage. In addition, if the DVR and STATCOM are
connected on the DC side, the DC bus voltage can be
regulated by the shunt connected DSTATCOM while the
DVR supplies the required energy to the load in case of the
transient disturbances in source voltage. The configuration
of such a device (termed as Unified Power Quality
Conditioner (UPQC)) is shown in Figure 4. This is a
versatile device similar to a UPFC. However, the control
objectives of a UPQC are quite different from that of a
UPFC.
Figure 4: Unified Power Quality Conditioner (UPQC).
B. Control Objectives Of UPQC
The shunt connected converter has the following control
objectives
1. To balance the source currents by injecting negative
and zero sequence components required by the load.
2. The compensate for the harmonics in the load
current by injecting the required harmonic currents.
3. To control the power factor by injecting the required
reactive current (at fundamental frequency).
4. To regulate the DC bus voltage.
The series connected converter has the following control
objectives
1. To balance the voltages at the load bus by injecting
negative and zero sequence voltages to compensate
for those present in the source.
2. To isolate the load bus from harmonics present in
the source voltages, by injecting the harmonic
voltages.
3. To regulate the magnitude of the load bus voltage by
injecting the required active and reactive
components (at fundamental frequency) depending
on the power factor on the source side.
4. To control the power factor at the input port of the
UPQC (where the source is connected. Note that the
power factor at the output port of the UPQC
(connected to the load) is controlled by the shunt
converter.
C. Operation Of UPQC
The operation of a UPQC can be explained from the
analysis of the idealized equivalent circuit shown in Figure
5. Here, the series converter is represented by a voltage
source VC and the shunt converter is represented by a
current source IC.
Power Quality Improvement by Versatile Control Scheme for Unified Power Quality Conditioner (UPQC)
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.04, April-2014, Pages: 0638-0646
Figure 5: Operation of UPQC.
Note that all the currents and voltages are 3 dimensional
vectors with phase coordinates. Unlike in the case of a
UPFC, the voltages and currents may contain negative and
zero sequence components in addition to harmonics.
Neglecting losses in the converters, we get the relation
(1)
Where X,Y denote the inner product of two vectors, defined
by
(2)
Let the load current IL and the source voltage VS be
decomposed into two Components given by
(3)
Where I1p, L contains only positive sequence, fundamental
frequency components. Similar comments apply to V 1pS .
IrL and V rS contain rest of the load current and the source
voltage including harmonics. I1pL is not unique and
depends on the power factor at the load bus. However, the
following relation applies for I1p L.
(4) This implies that hIrL; VLi = 0. Thus, the fundamental
frequency, positive sequence component in IrL does not
contribute to the active power in the load. To meet the
control objectives, the desired load voltages and source
currents must contain only positive sequence, fundamental
frequency components and
(5)
Where V ¤ L and I¤S are the reference quantities for the
load bus voltage and the source current respectively. Ál is
the power factor angle at the load bus while Ás is the power
factor angle at the source bus (input port of UPQC). Note
that V ¤ L(t) and I¤S (t) are sinusoidal and balanced. If the
reference current (I¤C ) of the shunt converter and the
reference voltage (V ¤ C) of the series converter are chosen
as
(6)
With the constraint
(7)
We have,
(8) Note that the constraint (14.30) implies that V 1p C is the
reactive voltage in quadrature with the desired source
current, I¤S. It is easy to derive that
(9) The above equation shows that for the operating
conditions assumed, a UPQC can be viewed as a inaction of
a DVR and a STATCOM with no active power flows
through the DC link. However, if the magnitude of V ¤ L is
to be controlled, it may not be feasible to achieve this by
injecting only reactive voltage. The situation gets
complicated if V 1p S is not constant, but changes due to
system disturbances or fault. To ensure the regulation of the
load bus voltage it may be necessary to inject variable active
voltage (in phase with the source current). If we express
(10)
(11)
(12)
In deriving the above, we assume that
(13)
This implies that both ¢VC and ¢IC are perturbations
involving positive sequence, fundamental requency
quantities (say, resulting from symmetric voltage sags). The
power balance on the DC side of the shunt and series
converter. The perturbation in VC is initiated to ensure that
Thus, the objective of the voltage regulation at the load
bus may require exchange of power between the shunt and
series converters.
V. SIMULATION RESULTS
A UPQC simulation model (Figures 6,7 & 8) has been
created in MATLAB/Simulink so as to investigate UPQC
circuit waveforms, the dynamic and steady-state
performance, and voltage and current ratings. The following
typical case studies have been simulated and the results are
presented.
1. Short duration three phase fault conditions.
2. Long duration three phase fault conditions.
3. Dynamic load and three phase fault conditions.
4. Harmonic compensation.
ALAA HAMZAH ABDULLAH, SURYA PRAKASH, JYOTI SHRIVASTAVA
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.04, April-2014, Pages: 0638-0646
5. DC link voltage regulation for the above conditions is
also verified.
Figure 6: Transmission line diagram without UPQC in
MATLAB.
Figure 7: Transmission line diagram with UPQC
without fault.
TABLE 1
DATA USED TO SIMULATE THE MODEL IN
MATLAB
Figure 8: Transmission line diagram with UPQC with
fault condition.
By simulting the above model made in MATLAB and
puttin the data place in table No. 1 the folloewing responses
are obtained(Nikita hari et al. 2011).
Figure 9: Steady state source voltage and load current
waves without UPQC(THD is more).
Figure 9 shows the simulation results for the case where
the system is in steady state. Due to power electronic load
the current waveform is distorted and is unbalanced.
Voltage waveform is also distorted.
Figure 10: Steady state source voltage and load current
waves with UPQC(THD is very less).
Power Quality Improvement by Versatile Control Scheme for Unified Power Quality Conditioner (UPQC)
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.04, April-2014, Pages: 0638-0646
Figure 10 shows the results after UPQC is connected to
the system. The waveforms are balanced and THD is greatly
reduced .This confirms that UPQC compensates harmonics
to a great extent.
Figure 11: steady state DC link voltage (500v).
Fig 11 shows that the DC link voltage is maintained
constant by the shunt compensator.
Figure 12: Response of Long duration three phase fault
conditions Source Voltage without UPQC, from 0.3 to
0.4 seconds (THD is more).
Figure 13: Source voltage when 3 phase fault is
introduced with UPQC.
Figure 13 shows the simulation results when a three
phase fault is introduced, the series active filter(DVR)
injects the compensating voltage so that the source voltage
is maintained constant. This shows that voltage
imperfections are compensated by the series part of UPQC.
Figure 14: Source voltage when a three phase fault is
introduced from 0.2 to 0.25 seconds without UPQC.
Figure 15: Source voltage when a three phase fault is
introduced from 0.1 to 0.2 seconds with UPQC.
D-STATCOM injects current waveforms of opposite
polarity and mitigates the swell in current. Figure 15
validates that UPQC can mitigate long duration sags
effectively. Simulation results show that UPQC mitigates
deeper sags, harmonic compensation is better. It also does
better load regulation and balancing for dynamic loads and
can tolerate long duration fault conditions effectively. Thus
it gives enhanced performance when compared to
DSTATCOM and DVR. Results show that it gives good
steady state and transient performance. The proposed
control scheme is feasible and simple to implement although
further work is needed to optimize the parameters of the
UPQC.
ALAA HAMZAH ABDULLAH, SURYA PRAKASH, JYOTI SHRIVASTAVA
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.04, April-2014, Pages: 0638-0646
VI. CONCLUSIONS
The performance of the UPQC is compared with DVR
and DSTATCOM. The objectives laid down have been
successfully realized through software implementation in
MATLAB/SIMULINK. Simulation results show that, when
the UPQC applying such control strategy is used for the
compensation of the nonlinear/unbalance load conditions in
three-phase three-wire system, the harmonic reduction is
better; unbalance/distortion of load current and source
voltage are compensated well and dc voltage gets regulated
all of which verifies the effectiveness of applying such a
flexible control strategy in UPQC.
VII. REFERENCES
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Press LCC, 2002.
[2] Math H.J. Bollen, Understanding power quality
problems: Voltage sags and Interruptions, New York: IEEE
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Author’s Profile:
Er. Alaa Hamzah Abdullah : Born
in Babel in 1978. He received his
B.Sc from Technical College AL-
musaib(Electrical power Engineering
Techniques) in 2002,He obtained his
Higher Diploma of Techniques in
Electrical power Engineering from
College of Electrical & Electronic Techniques. Presently he
is perusing M. Tech in Power system from SHIATS-
Deemed University, Allahabad, India.
Dr. Surya Prakash belongs to
Allahabad, DOB is 01.05.1971,
Received his Bachelor of Engineering
degree from The Institution of
Engineers (India) in 2003, He
obtained his M.Tech. In Electrical
Engg.(Power System) from KNIT,
Power Quality Improvement by Versatile Control Scheme for Unified Power Quality Conditioner (UPQC)
International Journal of Scientific Engineering and Technology Research
Volume.03, IssueNo.04, April-2014, Pages: 0638-0646
Sultanpur. UP-India in 2009, and he has obtained Ph. D in
Electrical Engg.(Power System) from, SHIATS (Formerly
Allahabad Agriculture Institute, Allahabad-India). His field
of interest includes power system operation & control,
Artificial Intelligent control.
Dr. Jyoti Shrivastava has done her
graduation in Electrical Engineering
and her post graduation in Design of
Heavy Electrical Equipments. At
present she is serving as a Senior
Assistant Professor in Electrical
Engineering department at college of
Engineering and Technology, SHIATS,
Allahabad, India. She has several international and National
papers to her credit.