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International Journal of Electrical Engineering & Technology (IJEET)
Volume 7, Issue 6, Nov–Dec, 2016, pp.55–63, Article ID: IJEET_07_06_005
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UNIFIED POWER QUALITY CONDITIONER IN
DISTRIBUTION SYSTEM FOR ENHANCING POWER
QUALITY
A.S. N. Anjali Devi
Student, EEE Department, Prasad V Potluri Siddhartha Institute of Technology, A.P, India
G. Madhavi
Assistant professor, Department of EEE,
Prasad V Potluri Siddhartha Institute of Technology, A.P, India
ABSTRACT
Power quality is one of the major problems in the today’s scenario. The distribution system,
have numerous nonlinear loads, which severely affect the quality of power supplies. The Unified
Power Quality Conditioner (UPQC) is a facts device that is used in the distribution system to
mitigate the disturbances that affect the performance of sensitive and/or critical loads. It is the only
versatile device which can mitigate several power quality problems related with voltage and
current both. Therefore UPQC is a multi functioning device that compensates various voltage
disturbances of the power supply, reduce the voltage fluctuations and prevent the harmonic load
current from entering into the power system.
To reduce the power quality problems in distribution system a dynamic model of UPQC is
developed in MATLAB/SIMULINK with a combination of linear and non-linear loads.
Key words: UPQC, Power Quality, Non-linear Load, Voltage sag, Harmonics, PV, Facts
controllers, series controller, shunt controller.
Cite this Article: A.S. N. Anjali Devi and G. Madhavi, Unified Power Quality Conditioner In
Distribution System For Enhancing Power Quality. International Journal of Electrical Engineering
& Technology, 7(6), 2016, pp. 55–63.
http://www.iaeme.com/IJEET/issues.asp?JType=IJEET&VType=7&IType=6
1. INTRODUCTION
Power quality refers to “maintaining the waveforms of voltages and currents as sinusoidal at rated
frequency and magnitude”. Now-a-days in power systems maintaining Power quality became most
important issue due to the introduction of equipments with power electronic devices which are more
sensitive to power quality problems. The main causes for poor power quality are faults on transmission and
distribution lines, non linear loads, start or stop of heavy loads. The faults on distribution and transmission
network will cause voltage sag or swell. This voltage sag and swell are short duration voltage variations.
The decrease in rms voltage between 0.1 and 0.9pu is known as voltage sag. The duration of this voltage
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A.S. N. Anjali Devi and G. Madhavi
sag is less than one minute. The increase in rms voltage between 1.1 and 1.8pu for the duration of less than
one minute is known as voltage swell.
The nonlinear loads will inject the harmonics into the system. The harmonics are the waveform
distortions in voltages and currents. The switching on/off of heavy loads will cause the under/over
voltages. These under and over voltages are long term voltage variations the duration of these power
quality problems is greater than one minute.
Voltage sag contributes more than 80% of power quality problems that exists in power systems.
Controlling the voltage is a difficult task because Voltages are influenced by frequent load variations.
Voltage regulation can be improved and losses in power system can be considerably reduced by installing
custom power devices or FACTS Controllers at suitable location. FACTS devices provide the flexible and
dynamic control in power systems. These controllers which are also named Distribution Flexible AC
Transmission System (DFACTS) are power electronics based equipments.
These controllers are of four types series controller, shunt controller, combined series-series
controllers, combined series-shunt controllers. By controlling the driving voltage the series controller will
control the current flow ex: Static Synchronous Series compensator, Thyristor controlled series capacitor,
Thyristor controlled series reactor. The shunt controller will provide effective voltage control and damp
voltage oscillations by injecting the current in the line ex: Static Synchronous compensator, Static VAR
compensator and Thyristor switched capacitor. Combined series-series controller will balance the active
and reactive power flow in the lines via power link ex: Interline power flow controller. The combined
series-shunt controller will exchange the real power between series and shunt controllers through Dc link.
These controllers are also known as hybrid controllers ex: Unified Power Quality Conditioner.
2. OPERATING PRINCIPLE OF UPQC
UPQC is a power electronic device which consists of two voltage source inverters (VSI) connected
through a common dc-link capacitor. It is used to mitigate both load current as well as supply voltage
imperfections. The main components of a UPQC are series converter and shunt converter, DC capacitors,
low-pass and high-pass passive filters, and series and shunt injecting transformers.
The purpose of a UPQC is to compensate for supply voltage power quality issues, such as sags, swells,
unbalance, flicker and harmonics. The model of UPQC is shown in figure .1.The series part of UPQC is
known as dynamic voltage restorer (DVR). It is used to maintain constant and balanced voltage at the load.
The series compensator injects a voltage, in such a way that the voltage at the load end remains unaffected
by any voltage disturbance. Whenever there is sag in the supply voltage then series converter injects
suitable voltage to supply. The series inverter of the UPQC injects a voltage represented by the following
equation.
=
−
( )
Where Vc, Vl and Vs represent the series inverter voltage, reference load voltage, and actual source
voltage respectively.
Figure 1 Basic Configuration Of UPQC
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Unified Power Quality Conditioner In Distribution System For Enhancing Power Quality
The shunt part of the UPQC is known as distribution static compensator it is responsible for
compensation of load current
nt harmonics, unbalances and power factor correction. It maintains the constant
voltage across the dc link. The shunt compensator consists
consists of a voltage source inverter.
inverter The Dc side of this
voltage source inverter is connected to the
the Dc link and the Ac side of this inverter is connected to the load
though the shunt coupling transformer. In order to cancel the harmonics generated by a nonlinear load, the
shunt inverter should inject a current given by the following
fo
equation.
= − (2)
Where Ic, Il, and Is represent the shunt inverter current, reference load current, and actual source
current respectively. Thus UPQC improves the power quality by preventing load current harmonics. The
main purpose of the UPQC is to compensate for supply voltage power quality issues such
suc as voltage sag,
swell, unbalance and flicker. During voltage sag/swell conditions the Dc link supplies required voltage for
compensation. Here Dc link connected with pv cell is used. The inverter circuit in UPQC will convert
conve the
Dc voltage to Ac voltage. These inverters
invert will generate the harmonics.. The harmonics generated by these
inverters are filtered by using passive filters.
3. DC-DC CONVERTER
A DC-DC
DC converter is an electronic circuit that converts a source of direct current (DC) from one voltage
level to another voltage level either step up the voltage or step down the voltage. Dc-DC
Dc
converters are of
three types Buck converter, Boost converter, Buck-Boost
Buck Boost converter. The boost converter is used to step up
the voltage. The buck converter is used to step down the voltage. The Buck-Boost
Boost converter either steps up
the voltage or step down the voltage. Here in this project the Boost converter is used to step up the output
voltage of the PV cell. The key principle that drives the Boost converter is the tendency
t
of an inductor to
resist changes in current by creating and
an destroying the magnetic field.
Figure 2 DC-DC Converter
The output voltage of this DC-DC
DC
converter is controlled by using PI controller. The output of this
PI controller is given to the DC link which maintains the active power balance between series and shunt
compensator.
4. PV MODEL
Recently, more work has done in the field of connecting DGs to grid using power electronic converters. In
most of the cases grids are interfaced
nterfaced with shunt inverters. In
I the proposed topology UPQC with PV cell is
connected to DC link. The electrical equivalent circuit model of PV cell consists of a current source in
i
parallel with a diode as shown in figure
fig
.3. The low voltage outputs of photovoltaic units, i.e.Vdc1, Vdc2
have been connected to the DC bus by series connected boost converters. The output of DC-DC
DC
converter
is given to the DC-AC
AC voltage source inverter which converts this DC voltage to AC and fed the AC
distribution system.
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A.S. N. Anjali Devi and G. Madhavi
Figure 3-Electrical Equivalent Circuit Model of PV Cell
5. CONTROL STRATEGY
In the control system of DC-DC converter, the output voltage of converter is compared with a reference
value with PI controller. The output signal of this PI controller is one of the inputs of PWM switching for
adjusting the duty cycle. Photovoltaic cell is used to maintain DC link voltage.
The main purpose of the shunt compensator is to reduce the current harmonics. The operation of the
shunt compensator can be controlled by using shunt controller. Here vector control method is used to
control the shunt compensator. The control scheme comprises of PI controller and 3 phase sine wave
generator for reference current generation and switching signals. The peak values of reference currents are
studied by regulating the DC link voltage. The definite capacitor voltage will be compared with a set of
reference value. The rms voltage source amplitude is calculated from source phase voltage Va, Vb, Vc for
the three phase balanced system and this can be expressed as Vm.
=
(
)( )
=
(
+
) (!)
+
+
(
=
= "# $ (
−
)( )
)% (&)
The in-phase unit vectors are obtained from AC source phase voltages and the rms value of unit vectors
'( =
'+ =
(
)
(*)
are
,+
(-)
,)
' =
(.)
)
The in phase generated reference currents are derived using in-phase unit voltage. The peak value of
the current I so found will be multiplied by the unit sine vectors in phase with the individual source
voltages to obtain the reference compensating currents. The current controller generates the firing pulses to
the VSI by comparing the reference and actual current hence the Hysteresis current control scheme is used
to generate the switching signals to the shunt compensator. The main purpose of the series compensator is
to maintain the balanced and distortion free voltages. The operation of series compensator can be
controlled by using series controller. Here the hysteresis control is used to control the operation of series
compensator. The hysteresis controller will measure the difference between the supply voltage and load
voltage. Based on these differences the hysteresis controller will generate the gate pulses to the series
compensator.
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Unified Power Quality Conditioner In Distribution System For Enhancing Power Quality
6. SINGLE LINE DIAGRAM
IAGRAM OF PROPOSED TEST SYSTEM
Figure 4 Single line Diagram of System
The proposed test model consists of three buses.
buses The three endss of these buses connected with linear
and non linear loads. The fault on one bus has created the voltage sag in remaining buses. In order to
compensate these disturbances UPQC is installed in the system. It is a combination of series and shunt
compensator
pensator with DC link. The series compensator is used to compensate the voltage disturbances
d
and the
shunt compensator is used for the removal of harmonics in load currents. Here the PV cell is used to supply
voltage to the DC link which maintains
maintain the constant
tant active power flow between series and shunt
compensator. The DC-DC
DC converter is used to step up the output voltage of the PV cell. The block diagram
of the proposed test model is shown in figure-5.
fig
Figure 5 Block Diagram of Proposed Test Model
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7. SYSTEM PARAMETERS
PARAMETERS
VALUES
Three Phase Source
RATED VOLTAGE (V)
66KV
RATED FREQUENCY
50Hz
(F)
RESISTANCE (R)
0.18ohm
INDUCTANCE (L)
1e-3H
Asynchronous Machine
POWER
10KW
VOLTAGE
220V
SPEED
1500RPM
Load
ACTIVE POWER
10e3
REACTIVE POWER
1e3
TRANSFORMER-1
TRANSFORMER-2
Y-G/Y-G
POWER-1MVA
VOLTAGE-66/33KV
TRASFORMER-3
Y-G/Y-G
POWER-1MVA
VOLTAGE-66/3.3KV
Y-G/Y-G
POWER-1MVA
VOLTAGE33KV/800V
TRANSFORMER-4
PROPORTIONAL
GAIN
INTEGRAL GAIN
Kp=0.5
Ki=0.5
8. SIMULATION RESULTS
a) Without UPQC
Figure 6 Bus Voltage during Abnormal Conditions without UPQC
Figure -6 illustrate the sag in the bus voltage during fault period from 0.5 to 1 sec .We can observe that
the three phase to ground fault in one bus effected the voltage in the remaining buses of the same system.
As it is a symmetrical fault the voltage sag in three phase system is symmetrical.
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Figure 7 Asynchronous Machine Operating Parameters without UPQC during abnormal conditions
Figure -7 illustrate the variation of asynchronous machine operating parameters such as Torque, speed,
stator current. When electrical fault occurred in bus-1 we can observe the instability in the motor Torque
and the fluctuations in the machine speed between 0.5 to 1 sec.
Figure 8 Total harmonic distortion of the system without UPQC
Figure-8 illustrates the Total Harmonic Distortion of the system during abnormal conditions. Due
to the presence of nonlinear load we can observe the presence of harmonics in the current and the Total
harmonic distortion as 29.71%.
b) With UPQC
Figure 9.BusVoltage during Abnormal Conditions with UPQC
Figure-9 illustrates the bus voltage in the system with the presence of UPQC. We can observe that the
sag in the three bus voltages has been compensated by using UPQC.
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Figure 10.Asynchronous Machine Operating Parameters with UPQC during abnormal conditions
Figure 10 illustrate the operating parameters of the asynchronous machine with the presence of UPQC.
With the presence of UPQC we can observe that the instability in the motor Torque+ and the fluctuations
in the speed have been mitigated and the sag in the stator current is compensated.
Figure 11 Total harmonic distortion of the system with UPQC
Fig 11illustrate the total harmonic distortion of the system with UPQC. We can observe that due to the
presence of UPQC the total harmonic distortion has been reduced from 29.71% to 2.37%.
9. CONCLUSION
This paper deals with the problems related with faults and harmonics in the Distribution system and
introduce a Fact device Unified Power Quality Conditioner (UPQC). For analyzing UPQC, the distribution
system with three bus bars has been considered and a fault is created in one bus bar. The voltage sag in the
bus voltage, which was created by fault, can be compensated effectively with UPQC. After UPQC
implementation in the system, the total harmonic distortion of the load current is minimized from 29.71%
to 2.37%.
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