International Journal of Advances in Engineering & Technology, March 2012.
©IJAET
ISSN: 2231-1963
ANALYSIS OF UNIFIED POWER QUALITY CONDITIONER
DURING POWER QUALITY IMPROVEMENT
B. Sridhar1 , Rajkumar Jhapte2
1
Asstt. Prof., Electrical Engg., Christian college of Engg. & Technology, Bhilai, India
2
Sr. Asstt. Prof., EEE., Shri Shankaracharya Technical Campus, Bhilai, India
A BSTRACT
The non linear loads in industry cause an increasing deterioration of the power system voltage and current
waveforms. As a result, harmonics are generated from power converters or nonlinear loads. This causes the
power system to operate at low power factor, low efficiency, increased losses in transmission and distribution
lines, failure of electrical equipments, and interference problem with communication system. So, there is a great
need to mitigate these harmonic and reactive current components. Active Power filters are a viable solution to
these problems. In this manuscript a three-phase unified power quality conditioner (UPQC), with a combination
of shunt active power filter and series active power filter is used to eliminate supply current harmonics,
compensate reactive power, voltage sag and voltage swell compensation on distribution network. The
performance of the active power filter mainly depends on control strategy used to generate reference current for
shunt active power filter (APF) and generate reference voltage for series active power filter. Simulation results
of these two active power filters are carried out separately and as a combination also.
INDEX T ERMS: Unified power quality conditioner (UPQC), shunt active power filter (APF), series active
power filter (APF), point of common coupling (PCC), voltage source converters (VSC)
I.
INTRODUCTION
The non linear equipments during switching ON/OFF of high rated load connected at the point of
common coupling (PCC) highly distorted may result into voltage sags or swells. There are several
sensitive loads, such as computer or microprocessor based AC/DC drive controller, with good voltage
profile requirement; can function improperly can lose valuable data or in certain cases get damaged
due to these voltage sag and swell conditions. One of the effective approaches is to use a unified
power quality conditioner (UPQC) at point of common coupling (PCC) to protect the sensitive loads.
A unified power quality conditioner is a combination of shunt and series active power filters, sharing
a common dc link shown in Fig.1. [1]. It is a versatile device that can compensate almost all power
quality problems such as voltage harmonics, voltage unbalance, voltage flickers, voltage sags &
swells, current harmonics, current unbalance, reactive current, etc. The continuous usage of nonlinear loads injects current and voltage harmonic components into the power system and increases
reactive power demands and power system voltage fluctuations. This analysis will be extended as well
to the optimum UPQC with minimum VA requirement. [15]
Harmonic current components create several problems like,
• Increase in power system losses.
• Overheating and insulator failures in transformers, rotating machinery, Conductor and cables.
• Reactive power burden,
• Low system efficiency,
• Poor power factor,
• System unbalances and causes excessive neutral currents.
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International Journal of Advances in Engineering & Technology, March 2012.
©IJAET
ISSN: 2231-1963
• Malfunctioning of the protective relays and untimely tripping.
The solution is to install line conditioning systems that suppress the power system disturbances.[3]
Fig.1. System configuration of UPQC
II.
OPERATION PRINCIPLE
The principle of operation of active filters is based on the injection of the harmonics required by the
load. Thus the basic principle of Active Power filter is that it generates a current equal and opposite in
polarity to the harmonic current drawn by the load and injects it to the point of coupling thereby
forcing the source current to be pure sinusoidal.
Fig 2.Simulink Model for UPQC
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Vol. 3, Issue 1, pp. 558-565
International Journal of Advances in Engineering & Technology, March 2012.
©IJAET
ISSN: 2231-1963
Active power filter consists of two blocks, first one reference harmonic component calculation block,
which calculates harmonic content on the load side and the second one is inverter block .Calculated
harmonic currents are given to inverter block. Active power filter requires inverter to inject the
harmonics in opposite polarity at point of coupling .The characteristics of the harmonic compensation
are strongly dependent on the filtering algorithm employed for the calculation of load current
harmonics. Many filtering algorithms are developed to extract the harmonic components. Control
method also plays an important role in active power filter control. Reduced Switching losses,
compensation of higher harmonics, less current ripple etc needs a control method to meet all these
requirements.[2]
III.
CONTROL ALGORITHM
Unified power quality conditioner is a combination of shunt APF and series APF. Control algorithm is
been used to generate gating pulses to shunt APF and series APF. Generating gating pulses to shunt
APF for that reference current generator and hysteresis current controller is required. Similarly, for
generating gating pulses to series APF, reference voltage generator and hysteresis voltage controller is
required and complete control strategy.[5]
3.1 Reference Current Generator
The control strategy is based on the extraction of Unit Vector Templates from the distorted input
supply. These templates will be then equivalent to pure sinusoidal signal with unity (p.u) amplitude.
[8]
In order to have distortion less load current, the load current must be equal to these reference signals.
The measured load currents are compared with reference load current signals. The error generated is
then taken to a hysteresis current controller to generate the required gate signals for shunt APF. The
unit vector template can be applied for shunt APF to compensate the harmonic current generated by
nonlinear load. The shunt APF is used to compensate for current harmonics as well as to maintain the
dc link voltage at constant level. To achieve this task the dc link voltage is sensed and compared with
the reference dc link voltage. The error is then processed by a PI controller. The output signal from PI
controller is multiplied with unit vector templates. The source current must be equal to this reference
signal. In order to follow this reference current signal, the 3phase source currents are sensed and
compared with reference current signals. The error generated is then processed by a hysteresis current
controller with suitable band, generating gating signals for shunt APF.
3.2 Hysteresis Current Controller
APF eliminates system harmonics through injecting a current to the system that is equal to the load
harmonic current; therefore the system side will almost have no harmonic current remaining. Since
the load harmonics to be compensated may be very complex and changing rapidly and randomly, APF
has to respond quickly and work with high control accuracy in current tracking.
3.3 Reference Voltage Generator
The control strategy is based on the extraction of Unit Vector Templates from the distorted input
supply. These templates will be then equivalent to pure sinusoidal signal with unity (p.u) amplitude.
The 3 phase distorted input source voltage at PCC contains fundamental component and distorted
component. To get unit input voltage vectors, the input voltage is sensed and multiplied by gain equal
to I/Vm where Vm is equal to peak amplitude of fundamental input voltage.
3.4 Hysteresis Voltage Controller
APF eliminates system voltage sag and voltage swell through injecting a voltage to the system that is
equal to the decrease load voltage magnitude; therefore the system side voltage will almost maintains
constant. Since the load voltage to be compensated may be very complex and changing rapidly and
randomly, APF has to respond quickly and work with high control accuracy in voltage tracking.
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International Journal of Advances in Engineering & Technology, March 2012.
©IJAET
ISSN: 2231-1963
IV.
POWER INVERTERS & CONTROL TECHNIQUES
Inverters are used to inject compensation current in to the system to cancel the harmonics in source
current. The current waveform for canceling harmonics is achieved with the voltage source inverter
(VSI) in the current controlled mode and an interfacing filter. The desired current waveform is
obtained by accurately controlling the switching of the insulated gate bipolar transistors (IGBTs) in
the inverter. Control of the current wave shape is limited by the switching frequency of the inverter
and by the available driving voltage across the interfacing inductance.[9]
In voltage source converters (VSC), variables to be controlled are the amplitude and frequency of the
converter output voltage. PWM techniques enable the modulation of the controllable switches of the
converter. By proper modulation of the controllable switches, a high frequency converter output
voltage can be generated, whose low frequency or localized average during each switching cycle is
same as that of the desired output voltage of the converter at that cycle.
V.
SYSTEM DATA
Supply Voltage, Vs
Supply frequency
DC Bus voltage, Vdc,
Injection Inductor, L
DC side capacitor, C
Sensitive load active power
Sensitive load reactive power
Series filter transformer
VI.
415 V
50 Hz
750 V
10 mH
5000 uF
2460 watts
1000 Vars
10 kVA
RESULTS & DISCUSSIONS
This result shows Shunt APF controller effectively compensates harmonics, reactive power
compensation and maintains D.C link voltage constant. The current waveform for canceling
harmonics is achieved with the voltage source inverter in the current controlled mode and an
interfacing filter. The filter provides smoothing and isolation for high frequency components. The
desired current waveform is obtained by accurately controlling the switching of the insulated gate
bipolar transistors (IGBTs) in the inverter. Control of the current wave shape is limited by the
switching frequency of the inverter and by the available driving voltage across the interfacing
inductance.
Fig.3. Load current from 0.04 sec to 0.2 sec
Fig.4. Unit vector Templates Generation 0.4 sec to 0.5sec
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International Journal of Advances in Engineering & Technology, March 2012.
©IJAET
ISSN: 2231-1963
Fig.5. Reference current from 0.4 sec to 0.5 sec
Fig.6. Shunt APF compensating currents from 0.4 sec to 0.5 sec
Fig.7. DC link capacitor voltage from 0.3 sec to 1 sec
Fig.8. Source current from 0.4 sec to 0.6 sec
Fig.9. Source voltage & current from 0.4 sec to 0.6 sec
6.1 Results For Series APF during voltage sag
This result shows Series APF controller effectively compensates voltage sag and maintains load
voltage constant.
Fig.10. Load Voltage from 0.04sec to 0.2sec during Sag
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Vol. 3, Issue 1, pp. 558-565
International Journal of Advances in Engineering & Technology, March 2012.
©IJAET
ISSN: 2231-1963
Fig.11. Load Voltage from 0.04sec to 0.2sec during Sag
6.2 Results For Series APF during voltage swell
This result shows Series APF controller effectively compensates voltage swell and maintains load
voltage constant.
Fig.12. Load Voltage from 0.04sec to 0.2sec during Swell
Fig.13. Load Voltages from 0.04sec to 0.2sec during Swell
6.3 Results during voltage sag and voltage swell
The powers due to harmonics quantities are negligible as compared to the power at fundamental
component, therefore, the harmonic power is neglected and the steady state operating analysis is done
on the basis of fundamental frequency component only. The unified power quality conditioner is
controlled in such a way that the voltage at load bus is always sinusoidal and at desired magnitude.
Therefore the voltage injected by series active power filter must be equal to the difference between the
supply voltage and the ideal load voltage. Thus the series active power filter acts as controlled voltage
source. The function of shunt active power filter is to maintain the dc link voltage at constant level. In
addition to this the shunt active power filter provides the VAR required by the load, such that the
input power factor will be unity and only fundamental active power will be supplied by the
source[1].This result shows Series APF controller effectively compensates both voltage sag and
swells and maintains load voltage constant.
Fig.14. Load from 0.04sec to 0.24sec during Sag and Swell
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Vol. 3, Issue 1, pp. 558-565
International Journal of Advances in Engineering & Technology, March 2012.
©IJAET
ISSN: 2231-1963
Fig.15. Load Voltages from 0.04sec to 0.24sec during Sag & Swell
VII.
CONCLUSION
From the MATLAB / Simulink based simulation responses, it is evident that the Shunt APF, reference
current generator, hysteresis current controller and also for Series APF, reference voltage generator,
hysteresis voltage controller are performing satisfactorily. The source current waveform is in phase
with the utility voltage and free from harmonic components. The load voltage waveform is
maintaining constant during voltage sag and swell conditions. 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. The shunt active power filter helps series
active power filter during voltage sag and swell condition by maintaining the dc link voltage at set
constant level, such that series active power filter could effectively supply or absorb the active power.
In addition to this shunt active power filter also provides the required load VARs and thus making the
input power factor close to unity.
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ISSN: 2231-1963
[15] Jayanti N.G., Basu Malabika, Conlon Micheal F., et al, “ Rating Requirements of a Unified Power Quality
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Authors Biographies
B. Sridhar was born in Bilaspur, India on November 27, 1979. He graduated from RCET,
Bhilai with First division degree in B.E. Electrical Engineering in 2006 and pursuing
Masters of Engineering (Power System), SSCET, Bhilai. He is currently Assistant Professor
at CCE&T, Bhilai, India. His research interests include Power Quality.
Rajkumar Jhapte was born in Bhilai, India on February 02, 1982. He graduated from
MPCCET, Bhilai with First division degree in B.E. Electrical Engineering in 2004 and
Masters of Engineering (Power System), SSCET, Bhilai in 2009. He is currently Assistant
Professor at SSCET, Bhilai, India. His research interests include Power Quality, Power
Electronics, Power system protection etc.
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