Solar Assisted Relift Luo Converter with UPQC for Voltage Unbalance
Mitigation Using DDSRF Theory
B. PAKKIRAIAH
Dept. of EEE, Gokaraju Rangaraju
Institute of Engineering and
Technology-Autonomous,
Hyderabad-500090, INDIA
M. VENKATESWARLU
Department of EEE, Koneru
Lakshmaiah Education
Foundation, Vaddeswaram,
Guntur, Andhra Pradesh
522502, INDIA
B LOVESWARA RAO
Dept. of EEE, Koneru Lakshmaiah
Education Foundation,
Vaddeswaram, Guntur, Andhra
Pradesh 522502, INDIA
Abstract: The upsurge seen in integration of sensitive non-linear loads and Photovoltaic (PV) system
based power generation has strengthened the demand for enhanced Power Quality (PQ) in distributed
power system. Thereby, a Unified Power Quality Conditioner (UPQC), which is a custom power
device is proposed in this work for improving the PQ of the overall network. The load side PQ issues
is mitigated using the shunt compensator, while the source side PQ issues is mitigated using the
series compensator of the UPQC. The active power to the load is provided from the PV system
through a shunt compensator. The voltage level of the output obtained from the PV is improved with
the aid of Re-lift Luo converter, which is a DC-DC converter of high efficiency and voltage gain.
Moreover, in order to provide a stabilized voltage supply to the UPQC, an Adaptive Proportional
Integral (PI) controller is selected and its gains are adjusted with the application of Fuzzy Logic
Controller (FLC). The Decoupled Double Synchronous Reference Frame (DDSRF) theory is applied
to derive the reference voltage and reference current for balancing source voltage variations and load
current harmonics, respectively. The series and shunt compensators are controlled using Cascaded
Type-2 FLC (CT2FLC). The effectiveness of the proposed PV-UPQC configuration in eliminating
load current harmonics and source voltage fluctuations is evaluated on the basis of MATLAB
simulations.
Keywords: PV-UPQC, Re-lift Luo converter, DDSRF Theory, Adaptive PI controller, CT2FLC
Received: October 25, 2022. Revised: April 29, 2023. Accepted: June 13, 2023. Published: July 18, 2023.
1. Introduction
The Power Quality (PQ) is
instrumental in guaranteeing the proper
working of several electronic and electrical
devices used in industrial, commercial and
domestic applications. Moreover, poor PQ
augments the energy costs and negatively
impacts the operation of the advanced
sensitive equipment coupled to the network.
Consequently, in order to avert the failure of
these devices, the electrical grid must provide
power within the ranges established by
manufacturers [1, 2]. The property of drawing
non-linear current by the power electronic
loads is the major cause of PQ issues in the
form of voltage fluctuations in the distribution
system [3]. Furthermore, the focus on
producing clean energy has increased the
prevalence of PV in distributed power
systems, which in turn causes voltage
instability issues because of its intermittent
nature. These voltage instability issues
effectuates persistent false triggering, false
tripping and malfunctioning of the electronic
systems in addition to capacitor bank heating
[4, 5]. Thereby, the development of a
multifunctional system, which satisfies the
requirement of both clean energy generation
and PQ enhancement is the major objective of
this work. In [6], a multifunctional three phase
solar energy conversion system is proposed,
which has the capability to compensate the PQ
issues in load side. A Distribution Static
Compensator (DSTATCOM) based shunt
International Journal of Electrical Engineering and Computer Science
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E-ISSN: 2769-2507
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active filtering in addition to clean energy
generation is proposed in [7], [8]. The benefit
of excellent load voltage regulation by
DSTATCOM comes at the cost of reactive
power injection. As a result, DSTATCOM is
unable to simultaneously maintain grid current
unity power factor and PCC voltage
regulation. Recently, the use of Dynamic
Voltage Resonator (DVR) [9], which is a
series active filtering device is recommended
to adhere to the strict PQ requirements of
sensitive electronic loads.
For obtaining the additional advantage of
decarbonized energy generation, PV based
DVR configurations are developed in [10],
[11]. In comparison to DVR and
DSTATCOM, an UPQC [12, 13], which
performs both shunt active filtering and series
active filtering is preferred. Utilization of a
suitable controller approach in cooperation
with a DC-DC converter allows for the
elimination of the intermittency-related
limitation of the PV system. The former
stabilizes the high voltage output of the latter
by regulating its duty cycle. A Re-lift Luo
converter is selected for improving the voltage
level of the PV system. The widely used linear
controller approach for controlling the
working of a power electronic converter is PI
controller. However, the nature of being a
fixed gain controller, limits its adaption
capability to deviations in environmental
factors and system parameters.
Therefor an FLC is employed for tuning
the gain values of the PI controller for
enhancing its dynamic response and
expanding its application in wide span of
operating conditions [14-16]. The technique of
cascaded control [17] entailing two T2FLC
[18, 19] is proposed for controlling the UPQC.
The major task of reference signal generation
is accomplished using DDSRF Theory.
In this work, a PV-UPQC that supports
enhanced PQ as well as carbon negative power
generation is presented. A stabilized,
controlled and enhanced voltage level is
obtained from PV system with the application
of Re-lift Luo converter and Fuzzy tuned
Adaptive PI controller. A CT2FLC in addition
to DDSRF theory is used for establishing
control over the working of series and shunt
compensators of the UPQC.
2. Proposed System Description
The foremost responsibility of a utility
system is to provide electric power in the form
of pure sinusoidal current and voltage of
suitable frequency and magnitude at PCC to
the customers. So, a PV-UPQC for enhancing
the PQ of the distributed power system is
presented with the added benefit of carbon
free power generation.
Figure 1: Presented configuration of PV-UPQC
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.10
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E-ISSN: 2769-2507
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The PV is interfaced to the UPQC using the
Re-lift Luo converter and its output is
stabilized with the aid of fuzzy tuned Adaptive
PI controller. The value of error obtained after
comparing the set reference voltage with the
actual voltage of the Re-lift Luo converter is
provided as input to the Adaptive PI
controller. The reference control signal
generated by the controller aids the PWM
generator to produce pulses for governing the
switches of the Re-lift Luo converter. In case
of UPQC, the series converter mitigates the
voltage quality issues in the source side, while
the shunt converter mitigates the current
harmonics issues in load side. The reference
and actual DC-link voltage
() are compared and the
evaluated error in voltage is fed to the
CT2FLC for processing. The reference voltage
signal for stabilizing source voltage
fluctuations and the reference current signal
for minimizing the load side current harmonics
is generated based on DDSRF theory. Finally
using the proposed configuration of PV-UPQC
the PQ of both source side and load side is
enhanced.
3. Proposed System Modelling
3.1 Pv Fed Re-lift Luo Converter
The PV module that encompasses
numerous PV cells is designed on the basis of
single diode model as seen in Figure 2.
Moreover, the output current derived from the
PV cell is given as,
(1)
Here, specifies the reverse saturation
current of the diode, specifies the diode
Ideality constant, specifies the electron
charge, specifies the Boltzmann constant,
and specifies the PV generated current
and the photo generated current respectively
and moreover, the terms and represents
the shunt and series resistances respectively.
Figure 2: PV cell
Re-Lift Luo converter as seen in Figure
3 comprises of three diodes , , ; three
capacitors , ,; three inductors ,
,; two power switches , and an
output capacitor as shown in Figure 2.
Capacitors and possess voltage boosting
characteristics which makes the capacitor
voltage higher than the source voltage.
The inductor serves as a ladder joint for
connecting the capacitors and in order to
raise the capacitor voltage. The operating
modes and operational waveform of the Re-
Lift Luo converter is given in Figure 4 and
Figure 5 respectively.
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Figure 3: Configuration of Re-lift Luo converter
(a)
(b)
Figure 4: Re-lift Luo converter operating modes
Mode 1:
Throughout this mode, both the
switches are in ON condition and the source
energy is absorbed by the inductors and.
The energy for the inductoris obtained from
both the input source and capacitor . A
linear increase in currents is
seen in this mode.
Mode 2:
Unlike mode 1, both the switches are
in OFF condition and the source current is
equivalent to zero. The capacitor is charged
by the current, which means that the stored
energy from inductor is transferred to the
capacitor . Both the currents and
decreases throughout this mode.
During mode 1, the peak to peak variation of
current is given as,
(2)
The variation is equivalent to the current
reduction during mode 2,
(3)
The drop in voltage across inductor is
during mode 2 is given as,
(4)
The current increases during the time
period [mode 1] and a decreases during the
time period [mode 2]
(5)
The voltage across capacitor is give as,
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(6)
Moreover, the current also increases
throughout mode 1 and decreases in mode 2.
Hence,
(7)
The output voltage is given as,
(8)
The output current is given as,
(9)
Figure 5: Operational waveform of Re-Lift Luo converter
The value of the inductor is,
(10)
The value of the inductor is,
(11)
The following equations are used to calculate
the values of the capacitors and :
(12)
(13)
(14)
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(15)
In order to enhance the transient
response of the Re-lift Luo converter, a fuzzy
tuned PI controller is adopted in this research.
3.2 Fuzzy Tuned Adaptive Pi Controller
The preferable characteristics of
conventional PI controller such as quick
response and ease of implementation accredits
to its widespread usage in variety of industrial
applications. The property of being a fixed
gain controller, however, hampers its adaption
capability to deviations in environmental
factors and system parameters. Consequently,
an Adaptable PI controller, which incorporates
the independent and adaptive qualities of FLC
with the aspect of rapid response of a PI
controller, is employed in this work. In
contrast to conventional PI controller, the
gains are adjustable rather than
being fixed and these gains are estimated by
using the FLC. Figure 6 gives the
configuration of the Re-lift Luo converter with
Adaptive PI controller.
Figure 6: Adaptive PI controller for Re-lift Luo converter
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Table 1: Rule base for PI gains
The adaptive PI controller is selected with the
aim of controlling the output voltage of the
Re-lift Luo converter, therefore the input
variables for the FLC are given as the voltage
error () and change in voltage error ().
Each of these two input variables are assigned
with five triangular membership functions as
illustrated in Figure 7 (a). The fuzzy variables
for the inputs are expressed using the
linguistic variables Negative Large (NL),
Negative Small (NS), Zero (Z), Positive Small
(PS) and Positive Large (PL). The process of
fuzzy inference and defuzzification is
accomplished using Min-Max technique and
centre of gravity respectively. The FLC has
two rule bases as seen in Table 1 for
estimating the values of the PI
gains. The membership functions
used for estimating the output variables
are expressed using the linguistic
variables Large (L), Medium (M) and Small
(S) as seen in Figure 7(b).
(a)
(b)
Figure 7: Membership functions of (a) Input Variables and (b) Output Variables
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The rules are developed based on knowledge
of functioning of the converter, error
variations and change in error inputs. For each
rule, the firing strength is given as,
(16)
Where, refers to the membership function.
The singleton values are represented using the
terms. The outs obtained from the
FLC is given as,
(17)
(18)
Where, the number of rules is given as r. Thus
output signal obtained from the PI controller is
given as,
(19)
Thus a stable output voltage is
obtained from the Re-lift Luo converter, under
all operating conditions with the assistance of
FLC tuned adaptive PI controller.
3.3 Modelling of UPQC
The UPQC ensures that a power of
appropriate standard and specification is
supplied constantly without deviation to the
loads coupled to the distributed power system.
The load side PQ issues including reactive
power and harmonics are scaled down with the
aid of shunt compensator, whereas the grid
side PQ issues such as sag/swell are scaled
down with the aid of series compensator.
Moreover, the former enhances the load side
PQ with the injection of current and the latter
enhances the grid side PQ through the
injection of voltage. The parameters
considered for designing the UPQC are,
DC-link Voltage Magnitude
On the basis of the system’s phase
voltage, the minimal value of the DC-link
voltage is determined and is expressed as,
(20)
Where, the grid’s phase-voltage is specified
as and the modulation depth is
specified as.
Shunt Compensator DC-link Capacitor
Value
The capacitance value of the DC-link
capacitor is expressed using the following
equation,
(21)
Where, the shunt compensator’s phase-current
and phase-voltage is specified using the terms
respectively. The energy variation
under dynamic condition is specified as,
overloading factor is specified as and the
reference voltage is specified as.
Inductor Ripple Filter
The shunt compensator is interfaced to
the network through an inductor, which is
mathematically expressed as,
(22)
Where, inductor ripple current is specified
as and the switching frequency is specified
as.
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Series Injection Transformer
In case of a series transformer, the
injection transformer’s maximum value turns
ratio is given as,
(23)
The transformer’s VA rating is given as,
(24)
The grid current is equivalent to current across
series compensator.
Series Compensator Inductor Ripple Filter
The inductor ripple filter is given as,
(25)
Where, inductor ripple current is specified
as. The control of both the series and shunt
compensator is enabled using DDSRF theory
and CT2FLC.
3.4 Modelling of CT2FLC
The control of the operation of UPQC
is entrusted with CT2FLC, which comprises of
two Type 2-FLC (T2FLC). In contrast to its
counterpart, the T1-FLC, the T2-FLC is used
in this work because it is better suited to
handle challenges involving non-linearity and
uncertainty. The control signal originating
from the first T2FLC is used as input for the
second T2FLC.
Figure 8: Configuration of T2FLC
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Figure 9: T2FLC FOU (a)
Figure 8 gives the configuration of
T2FLC, which is a natural extension of
T1FLC but gives additional information in the
secondary membership function. Additionally,
the former varies from the later in terms of
structure by having an output processing block
in place of the defuzzifier block. The
defuzzification block is present within the
output processing module along with a type
reducer. As seen in Figure 9, it uses the
concept of footprint of uncertainty (FOU). The
role of the fuzzification block is to generate T2
fuzzy sets
by converting the input numeric
vector
. The mapping of the input is given as,
(26)
(27)
The rule structure of T2FLC is expressed as,
(28)
Where, and
represents T1 consequent and T2 antecedent
fuzzy sets respectively. The output is specified
as. The fuzzy sets are used for producing
mappings by the inference engine. These
mappings are realized by computing union and
intersection operations. On the basis of the
type reduced set () obtained from the
type reducer, the evaluation of the output of
the defuzzifier is carried out, which is given
as,
(29)
Thus the CT2FLC is used for controlling the
working of the UPQC.
3.5 DDSRF Theory
The equation that represents the
negative, positive and zero component of a
three phase power system is given as,
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=
+
+
(30)
The unsymmetrical source voltage is represented as,
=
=
+
+
(31)
The source current component is given as,
=
=
+
+
(32)
Using Clark’s transformation, the three-phase voltage is transformed as,
=
(33)
Using Clark’s transformation, the three-phase current is transformed as,
After eliminating the zero-sequence component,
=
(35)
=
(36)
The real and imaginary power is given as,
(37)
The resultant voltage is given as,
(38)
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In stationary reference frame the voltage vector has the same frequency as the rotating coordinate
system’s angular frequency. By separating the positive and negative sequence components, a Park's
transformation is used to determine the state of DDSRF.
(39)
(40)
On the basis of equation (39) and (40), it is confirmed that
(41)
(42)
(43)
(44)
The negative and positive sequences are represented using the equations (44) and (43). The
instantaneous PQ theory equations attained from park’s transformation by substitution of decoupled
current and voltage values is given as,
(45)
(46)
(47)
On the basis of decoupled current and voltage, the reference signal produced is given as,
(48)
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(49)
Thus the reference voltage and reference
current for compensating the source voltage
fluctuations and load current harmonics is
attained by using DDSRF theory.
4. Results and Discussions
Due to the increased incidence of
sensitive loads in the distributed power
system, the need for enhanced PQ is drawing
significant amount of attention recently. As a
result, this work presents the implementation
of a PV-UPQC with appropriate control
measures for improving the PQ. An adaptive
PI controller is used for stabilizing the
enhanced voltage output derived from the Re-
lift Luo converter, which interfaces the PV
with the UPQC. Additionally, the CT2FLC in
addition to DDSRF theory used for the control
of UPQC.
Table 1: Parameter Specifications
Parameters
Specifications
PV panel
Power
No. of PV panels
12 V
,
Re-lift Luo converter
Inductors
Capacitors
Capacitors
Power switch
IGBT
Switching frequency
(a)
(b)
Figure 10: Waveforms of (a) PV panel voltage (b) PV panel current
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In order to obtain a higher DC output
voltage, the re-lift Luo converter is supplied
with an input of 125 V from the PV panel, as
shown in Figure 10(a). The Figure 10(b)
shows that the PV panel output
current experiences abrupt changes as a result
of the various operating conditions before
reaching a stable output current of about 32 A.
(a)
(b)
Figure 11: Waveforms of (a) Converter output voltage and (b) Converter output current
The re-lift Luo converter gives out a
stable output voltage and output current of 600
V and 7 A, respectively, with the aid of an
adaptive PI controller as illustrated in Figure
11.
(a)
(b)
Figure 12: Waveforms of (a) Three phase source voltage and (b) Three phase source current
From Figure 12 (a), it is clear that about 400 V
AC voltage is constantly maintained up to 0.1
s and due to the influence of PQ issues, the
voltage drops to 250 V after 0.1s. As
illustrated in Figure 12 (b), the three phase
source current exhibits distortion and is highly
unstable due to the presence of PQ issues.
(a)
(b)
Figure 13: Waveforms of (a) Source voltage and current (b) Power factor
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It is clear from Figure 13 (b) that CT2FLC-
based UPQC effectively improves power
quality and aids for the achievement of unity
power factor. The waveforms of source
voltage and current are illustrated in Figure 13
(a).
(a)
(b)
Figure 14: Waveforms showing (a) real power and (b) reactive power
Real power and reactive power are depicted in
Figures 14 (a) and 14 (b), respectively. Power
fluctuations are visible in the waveform at
first, but after applying the proposed control
technique, the real and reactive power are
distortion-free.
(a)
(b)
Figure 15: Waveforms showing (a) load voltage and (b) load current
With the proposed PQ enhancement
technique, a stable load voltage of 400 V and a
load current of about 35 A are constantly
maintained with no distortions. Hence, the
proposed PV-UPQC with CT2FLC
configuration effectively and successfully
improves the PQ on the load side as shown in
Figure 15.
(a)
(b)
Figure 16: Waveforms of (a) Reference current and (b) actual current for shunt converter
With the injection of desired
magnitude of reactive current in to the line,
effective load current harmonics compensation
is achieved using a shunt converter, which
functions as a controllable current source. A
reference current of 45 A is generated between
0.15s and 0.2s in order to reduce the current
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harmonics that are present on the load side as shown in Figure 16 (a).
(a)
(b)
Figure 17: Waveforms of (a) Reference voltage and (b) actual voltage for series converter
With the injection of desired
magnitude of voltage in to the line, effective
voltage sag compensation is achieved using a
series converter. As shown in the Figure 17
(b), a constant voltage of 180 V is obtained
without any distortions from 0.1s to 0.2s. A
THD of 3.25% is estimated as seen in Figure
18.
Figure 18: THD waveform
(a)
(b)
Figure 19: Comparison of: (a) Efficiency and (b) Voltage gain
As shown in Figure 19, the operational
performance of the Re-lift Luo converter is
compared to a number of other existing
converters on the basis of voltage gain and
efficiency. The Re-lift Luo converter performs
exceptionally well with an excellent voltage
gain ratio of 1:12 and efficiency of 95%.
5. Conclusion
Due to the increasing application of
sensitive power electronic devices, various PQ
issues have arisen in power systems. These
problems could eventually result in significant
economic loss due to entire system failure if
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they are not effectively resolved. A PV based
power generation combined with UPQC
provide a clean energy with improved PQ. The
PV system, as an intermittent low power
source, causes voltage instability. The re-lift
Luo converter along with the adaptive PI
controller improves and stabilizes the voltage
generated from the PV. Moreover, the control
of the UPQC is ensured using CT2FLC and
DDSRF theory. The designed UPQC
configuration is effective in enhancing both
the load side and source side PQ issues on the
basis of the output obtained from MATLAB
simulations. The Re-lift Luo converter
possesses a high voltage gain of ratio 1:12 and
impressive efficiency of 95%.
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_US
International Journal of Electrical Engineering and Computer Science
DOI: 10.37394/232027.2023.5.10
B. Pakkiraiah, M. Venkateswarlu,
B. Loveswara Rao
E-ISSN: 2769-2507
104
Volume 5, 2023
