A PV CONFIGURED UPQC-L FOR POWER QUALITY
ENHANCEMENT IN DISTORTED DISTRIBUTION SYSTEM WITH
INDUCTION MOTOR LOAD
Poongothai.S1*, Srinath.S2 1,2Velammal Engineering College, Chennai, India
[email protected], [email protected]
Abstract - The most familiar issues in the power system is the distorted supply in
distribution system and power electronic based loads. One of the most popular custom
power devices, the Unified Power Quality Conditioner with left shunt configuration
(UPQC-L) as well with PV integration is suitably mitigate all quality issues. UPQC-L
generating reference signals without using any transformation by unit vector template
method (UVT). Hysteresis current controller (HCC) creating switching pulses for Active
Power Filters (APF) of UPQC to mitigate load side as well source side disturbances. In
variety of applications induction motor used in huge number which creates dip in voltage.
Along with power quality tribulations and non sinusoidal supply condition, the effect of
voltage dip also successfully defeated and the performance of the proposed system
validated by MATLAB/SIMULINK.
Keywords: Power Quality, Left Shunt-Unified Power Quality Conditioner, Series Active
Power Filter, Parallel Active Power Filter, Photo Voltaic
1. Introduction
Now a days the disturbances in the distribution side, distribution Generation integration and
different kinds of power electronics base loads pollute the power system [1-3]. Due to various fault
the power system may subjected to sag, swell, unbalance and interruption. As a result at a point of
common coupling (PCC) both voltage and current deviates from its original waveform shape [4].
Sensitive loads could seriously affected by quality issues. Active power filters are the best choice to
secure the power system.
Many researchers identify the best performance in UPQC and it has the capability of compensating
voltage, current and power related problems. It has two active power filters connected back to back
via Capacitor. This capacitor denoted as DC link capacitor which maintains the power balance
between two active filters of UPQC. To maintain stable function of active filters a DG can be
integrated across the DC link of UPQC [7-11]. One filter which is connected in shunt with distribution
system at the point of common coupling act as current source, balance current related tribulations and
maintain the voltage across the DC link capacitor [15-16]. It is connected in the left of UPQC called
as left shunt UPQC (LS-UPQC) [17-19]. As well it maintains the reactive power at load. Another
filter connected in series with distribution system at the point of common coupling act as a voltage
source, balance voltage related tribulations [12-14]. Several investigations and methodologies have
been made to generate firing pulses for DC-DC converter to extract the maximum energy from solar
PV [20-22].
Control strategies for active filters of UPQC are discussed in many papers [5-6]. To reduce the
complexity in transformation techniques for generating reference signals, a unit vector template
method used in this paper. Under non sinusoidal supply condition, combined mode control
implemented, PV integrated LS-UPQC could effectively mitigate various quality harms.
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Section 2 explores the system design of PV integrated UPQC-L and control strategy of Series
Active Power Filter, Parallel Active Power Filter and boost converter used along with PV array.
Section 3 illustrates quality issues and its compensation retorts and the presentation are assessed in
section 4.
2. Materials and Methods
UPQC has two voltage source inverters suitably for three phase distribution system shown in figure
1. PV integrated to DC link of UPQC via boost converter. At the point of common coupling (PCC)
shunt inverter connected through interconnecting reactive elements and series inverter by transformer.
Self supporting DC link (DG not connected) compensate reactive power, harmonics sag and swell.
But during voltage interruption the voltage across DC link capacitor losses its stability which in turn
the active power filters. But DG integrated DC link withstand its functioning during interruption.
UPQC successfully supports undistorted electrical signals to sensitive loads. Linear RL load
considered as a sensitive load. For distorted supply the inductor with the value of 5mH is connected in
series with supply.
Figure 1. Functional Diagram of PV integrated UPQC-L
2.1. Shunt inverter control
Now a day’s most loads are based on power electronics based loads which pollute the system by
creating harmonics. The harmonics in the system deteriorates the performance of all equipments
which are connected to power system. As well most loads are inductive in nature deviates the system
operating from unity power factor. For the proper function of active power filters of UPQC and to
maintain real power of load the DC link voltage has to be maintaining constant. Above mentioned
problem could effectively overcome by PAPF of UPQC. The control strategy of a Parallel Active
Power Filter (PAPF) shown in figure 2. The supply voltage is sensed and it is multiplied by steady
state desired peak value (Vp) and pass on to Phase locked loop to produce sin term based unit vector
templates with proper delay. The actual DC link voltage (Vdc) is compared with reference voltage
(Vdc*) and it is processed by fundamental component of peak amplitude then it gets multiplied with
unit vector templates to obtain reference current signals.
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Figure 2. Parallel Active Filter Control
The reference current signal compared with actual source current in Hysteresis controller to create
necessary gate signals for PAPF of UPQC. Thus the power quality problems correlate with source
current could effectively overcome by PAPF by reducing current harmonics, balancing reactive
power, maintaining unity power factor and DC link voltage.
2.2. Series inverter control
The power system may subjected to various faults due to which the distorted voltage pattern both
in magnitude and frequency being supplied to all other remaining loads that are connected to the same
power system. The series active power filter (SAPF) of UPQC generates voltage in such a way that
makes the load voltage perfect sinusoidal. Therefore the injected voltage in addition to supply voltage
makes the load terminal voltage ideal.
The control strategy for SAPF has to generate pulses for voltage source inverter of SAPF in such a
way it injects voltage at PCC to formulate the load voltage perfect. Based on Unit vector template
method the SAPF has to generate unit templates to generate reference signal. The supply voltage is
sensed and it is multiplied by steady state desired peak value and pass on to Phase locked loop to
produce sin term based unit vector templates with proper delay as shown in equation.
𝑈𝑎 = sin (𝜔𝑡)
𝑈𝑏 = sin (𝜔𝑡 − 120) (1)
𝑈𝑐 = sin (𝜔𝑡 + 120)
Then it is multiplied with desired load voltage magnitude to obtain the reference load voltage
(𝑉𝐿∗(𝜔𝑡)). Then the generated reference voltage compared with actual load voltage (𝑉𝐿) in Hysteresis
current control to generate gate signal for VSI of SAPF. Thus, the power quality troubles correlate
with supply voltage, for example voltage harmonics, voltage sag/swell/unbalance/interruption will get
compensated indirectly.
Figure 3. Series Active Filter Control
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2.3. Boost converter control
The DC-DC boost converter permits to implement different MPPT control strategies, together with
an interface between Photovoltaic and DC link capacitor of UPQC. The circuit diagram of DC-DC
boost converter is shown in Figure.4. A boost converter consists of an inductor, MOSFET switch,
diode and DC link capacitor (output capacitor).
Figure 4. Schematic block diagram of MPPT control in Boost converter.
The common design equations of the boost converter with PV input is given below
𝑉𝑑𝑐 =𝑉𝑝𝑣
1 + 𝐷 (2)
Where
𝑉𝑑𝑐 = Voltage across DC link capacitor
𝑉𝑝𝑣 = Source Voltage
𝐷 = Duty Cycle
The base inductance L required for continuous conduction mode (CCM) operation. The mathematical
expression of L is
𝐿 = ((1 − 𝐷)2) 𝐷𝑅) 2𝑓𝑠 (3)
Where
𝑓𝑠 = MOSFET switching frequency.
R= Load protection resistance
The Capacitance value has been ready with voltage swell purpose. The capacitor has given the voltage
swelling 𝑉𝑟 as
𝑉𝑟 = 𝐶𝑚𝑖𝑛𝐷
𝑅𝑓𝑠𝑉𝑑𝑐 (4)
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Based on the response of duty cycle the output voltage will changed, which is responsible for a
constant output voltage. The load current Idc also depends on the duty cycle, which is given by
equation (5)
𝐼𝑑𝑐 = (1 − 𝐷)𝐼𝑝𝑣 (5)
Where
𝐼𝑝𝑣 = Source Current in PV
𝐼𝑑𝑐 = Current from Converter
The adjustment in the current ∇𝐼 has the estimation of inductance given as
∇𝐼 =𝑉𝑝𝑣(𝑉𝑑𝑐−𝑉𝑝𝑣)
𝑓𝑠𝑉𝑑𝑐 (6)
To get better the effectiveness of the solar panel MPPT is used. According to maximum power
point theorem, output power of one circuit can be maximize by regulating source impedance equal to
the load impedance, so the MPPT algorithm is the same to the hitch of impedance matching. In
present work, the boost Converter is used as impedance matching device between input and output by
changing the duty cycle of the converter circuit. A major advantage of boost converter is that high or
low voltage obtained from the available voltage according to the application. Output voltage of the
converter is depend on the duty cycle, so MPPT is used to calculate the duty cycle for obtain the
maximum output voltage because if output voltage increases then power also increases. In this work
Perturb and Observe (P&O) and constant duty cycle techniques are used, because these require less
hardware complexity and low-cost implementations.
In this Perturb and Observe method, an inconsequential perturbation of output voltage is generated.
If there is an elevation in output power the similar practice is persistent or else perturbation is
upturned till to gain maximum power. Based on P&O algorithm, the generated pulses applied to
converter switch to extract the maximum energy from PV array.
3. Results and Discussion
In simulation part three phase three wired distorted distribution system is connected to linear as
well induction Motor load. To introduce the distortion a series inductor of 5mH is inserted in series
with supply. When the system is subjected to various faults the voltage and current pattern gets
deviated from ideal pattern. Whereas this kind of distorted pattern should not affect the performance
of linear load and induction motor load. As well the voltage dip due to induction motor start also
should not make any disturbance to sensitive load. All these issues overcome by PV integrated
UPQC-L effectively as follows.
Table 1. Configuration parameters Parameters Values
Frequency 50 Hz
Voltage in RMS 415V
Linear Sensitive Load (per phase) 100+j9.42Ω
Dynamic Load (Induction Motor Load) 5.4HP(4KW),400V,50Hz,1430 rpm
DC-Link capacitor 1000µF
DC-Link Voltage 730V
Inductor in converter L 5mH
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3.1 Disturbance due to Induction Motor load start
Induction motor was favoured as the focus of interest owing to its enormous use in variety of
applications. The distribution system along with DVR provide good voltage profile to induction motor
load when the system experienced with various fault and varying load condition [23,24].During
Induction Motor start a voltage dip occurs which in turn affects the sensitive load. As well the current
also sustain longer in transient state shown in figure 5. Non sinusoidal supply voltage profile is also
depicted in Figure 5.
Figure.5 Voltage dip due to IM start and it mitigation responses in distorted 3 Φ distribution system with PV-UPQC. (a)Voltage at PCC (b) Current at PCC
(c) Load Voltage (d) Load current (e) DC link Voltage.
Figure 5 also depict the load voltage and current. It clearly shows the distortion in supply side voltage
as well the dip in voltage due to Induction Motor start is also clearly eradicated. The load current also
attains the stable state early. It shows the constant DC bus voltage under non sinusoidal supply
condition and voltage dip due to Induction Motor start. Since the supply voltage is 415V the capacitor
voltage is maintained by 730V.
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3.2. Disturbance due to Fault
When the system subjected to various faults, the following responses shows the compensation
responses by PV integrated UPQC-L. The simulation responses are taken up to 0.2Sec. Sag created by
23% in the duration of 20ms to 50ms by introducing three phase fault. The SAPF of UPQC
overcomes it by injecting in phase voltage into the system. Swell created by 38% in the duration of
140ms to 170ms by inserting a series resistance with source. The SAPF of UPQC overcomes it by
injecting out of phase voltage into the system. Voltage unbalance created by 18% in the duration of
80ms to 110ms by introducing double line to ground fault shown in figure 6. It depict that undistorted
voltage and current are supplied to sensitive load and induction motor load.
Figure 6. Power Quality issues and its mitigation responses in distorted 3 Φ distribution system with PV-UPQC. (a) Voltage at PCC (b) Current at PCC
(c) Load Voltage (d) Load current (e) DC link Voltage.
From figure 7 the source voltage interruption occurs at the interval of 0.3S to 0.6S. In this proposed system
UPQC come across the capability of injecting power using PV to sensitive load for the period of source voltage
interruption and DC bus voltage also maintained constant.
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Figure 7. Voltage Interruption and it mitigation responses in distorted 3Φ distribution system with PV-UPQC. (a)Voltage at PCC (b) Current at PCC (c)
Load Voltage (d) Load current (e) DC link Voltage.
4. Performance Evaluation
When UPQC not in operation, the system (non sinusoidal supply) subject to more THD in current
at PCC (28.58%) and load voltage (11.12%) with sensitive RL and Induction Motor load. It is clearly
depict that the system need support to mitigate quality issues. One of the types of UPQC that is
UPQC-L is used with PV integration to mitigate power quality issues (sag/swell/unbalance/THD)
including voltage interruption. Due to PV integration the value of DC link capacitor and its settling
time reduced shown in Table 2. This proposed structure supporting Induction motor load as a dynamic
load as well provides regulated voltage and current profile to sensitive loads. Effect of voltage dip due
to Induction motor start along with power quality tribulations could effectively defeated by UPQC-L.
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The transient state of current profile is also reduced. The shunt active filter of UPQC-L effectively
balances the DC link voltage, reactive power requirement of load and power factor.
An attempt has been made with self supporting UPQC-L to show the effectiveness of PV integrated
UPQC-L. From Table 2 it is clearly depicted that the PV supporting UPQC-L effectively supports for
mitigating voltage interruption, the total harmonic distortion reduction, DC link capacitor value
reduced and the settling time of voltage profile of DC link capacitor is also reduced when compared to
self supporting DC link capacitor. Table 2 clearly shows the benefit of the proposed system and THD
also within the limit of IEEE standard.
Table 2. Compensation Effect of PV integrated UPQC-L
No Power
Quality
Issues
Compensati
on Effect System Configuration
Source
Current
THD %
Load
Voltag
e THD
%
1 Voltage Sag Compensated
Self supporting UPQC-L
configured with Non sinusoidal
source and sensitive RL load
plus Induction Motor Load.
*DC Link Capacitor =
10000µF
*Settling Time (tS) = 2.3mS
4.12 1.9
2 Voltage
Swell Compensated 4.31 1.57
3 Voltage
Unbalance Compensated 4.13 1.49
4 Voltage
Interruption
Partial
Compensatio
n
7.12 4.7
5 Voltage Dip
due IM start Compensated 3.7 1.12
6 Voltage Sag Compensated
PV integrated UPQC-L
configured with Non sinusoidal
source and sensitive RL load
plus Induction Motor Load.
*DC Link Capacitor = 1700µF
*Settling Time (tS) = 1.02mS
2.39 1.6
7 Voltage
Swell Compensated 3.41 1.14
8 Voltage
Unbalance Compensated 2.75 1.7
9 Voltage
Interruption Compensated 4.6 2.2
10 Voltage Dip
due IM start Compensated 2.9 1.02
5. Conclusion
The capability of PV integrated UPQC-L animatedly react to a choice of quality issues has been
analyzed. The proposed system regains the voltage and current pattern both in magnitude and
frequency for linear sensitive load under non sinusoidal supply conditions and Induction Motor load
conditions. Beside the influence of induction motor load, different faults are initiated and the quality
issues like voltage sag, voltage swell, voltage unbalance and voltage interruption owing to the fault
have been mitigated. The simulation results shows that the performance of PV integrated UPQC-L is
satisfactory in elimination of source current harmonics, load voltage harmonics and improving the
input power factor. It is found to be effective to meet IEEE 519 standard recommendation on
harmonic levels. Taking into consideration of rating, cost, voltage pattern and settling time of voltage
of DC link capacitor, the PV integrated UPQC-L is superior option to care for sensitive loads
contrasted with self supporting DC link capacitor left shunt UPQC.
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