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\title{A Review on UPQC for Power Quality Improvement in Distribution System}
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             \author[1]{  B.GOPAL}

             \author[2]{  B.GOPAL}

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\date{\small \em Received: 10 December 2012 Accepted: 31 December 2012 Published: 15 January 2013}

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\begin{abstract}
        


In recent years, Power engineers are increasingly concerned over the quality of the electrical power. In modern power system consists of wide range of electrical, electronic and power electronic equipment in commercial and industrial applications. Since most of the electronic equipments are nonlinear in nature these will induce harmonics in the system, which affect the sensitive loads to be fed from the system. One among the many compensating devices is Unified Power Quality Conditioner (UPQC) which specifically aims at the integration of series-active and shunt-active power filters to mitigate any type of voltage and current fluctuations and power factor correction in a power distribution network, such that improved power quality can be made available at the point of common coupling. In This paper presents a comprehensive review on the unified power quality conditioner (UPQC) to enhance the electric power quality at distribution levels. This is intended to present a broad overview on the different possible UPQC system configurations

\end{abstract}


\keywords{power quality (pq), harmonics, voltage sag, voltage swell, active power filter (apf), unified power quality conditioner (upqc), }

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\let\tabcellsep& 	 	 		 
\section[{Introduction}]{Introduction}\par
he quality of the power is effected by many factors like harmonic contamination, due to the increment of non-linear loads, such as large thyristor power converters, rectifiers, voltage and current flickering due to arc in arc furnaces, sag and swell due to the switching (on and off) of the loads etc. These problems are partially solved with the help of LC passive filters. However, this kind of filter cannot solve random variations in the load current waveform and voltage waveform. Active filters can resolve this problem, however the cost of active filters is high, and they are difficult to implement in large scale. Additionally, they also present lower efficiency than shunt passive filters \hyperref[b0]{[1]}.\par
This paper focuses on a unified power quality condition (UPQC). The UPQC is one of the APF family members where shunt and series APF functionalities are integrated together to achieve superior control over several power quality problems simultaneously. It is noticed that more than half of the papers on UPQC have been reported in the last five years, which indeed suggest the rapid interest in utilizing UPQC to improve the quality of power at the distribution level \hyperref[b1]{[2]}, \hyperref[b2]{[3]}.\par
The UPQC is a combination of series and shunt active filters connected through a common DC link capacitor. The main purpose of a UPQC is to compensate for supply voltage power quality issues such as, sags, swells, unbalance, flicker, harmonics, and for load current power quality problems such as, harmonics, unbalance, reactive current and neutral current \hyperref[b3]{[4]}. 
\section[{II.}]{II.} 
\section[{Power Quality Problems}]{Power Quality Problems}\par
Power quality is very important term that embraces all aspects associated with amplitude, phase and frequency of the voltage and current waveform existing in a power circuit. Any problem manifested in voltage, current or frequency deviation that results in failure of the customer equipment is known as power quality problem.\par
The increasing number of power electronics based equipment has produced a significant impact on the quality of electric power supply. The lack of quality power can cause loss of production, damage of equipment or appliances, increased power losses, interference with communication lines and so forth. Therefore, it is obvious to maintain high standards of power quality \hyperref[b2]{[3]}.\par
The major types of power quality problems are: Interruption, Voltage-sag, Voltage-swell, Distortion, and Harmonics.     Harmonics are sinusoidal voltages or current having frequency that are integer multiples of the fundamental frequency. Here, 3 rd harmonics is seen in the Fig. \hyperref[fig_9]{6}. 
\section[{a) Interruption}]{a) Interruption}\par
In order to meet PQ standard limits, it may be necessary to include some sort of compensation. Modern solutions can be found in the form of active rectification or active filtering. A shunt active power filter is suitable for the suppression of negative load influence on the supply network, but if there are supply voltage imperfections, a series active power filter may be needed to provide full compensation 
\section[{III. Basic Configuration of upqc}]{III. Basic Configuration of upqc}\par
In recent years, solutions based on flexible ac ransmission systems (FACTS) have appeared. The application of FACTS concepts in distribution systems has resulted in a new generation of compensating devices. A unified power-quality conditioner (UPQC) is the extension of the unified power-flow controller (UPFC) concept at the distribution level. It consists of combined series and shunt converters for simultaneous compensation of voltage and current imperfections in a supply feeder. However, a UPFC only needs to provide balance shunt and/or series compensation, since a power transmission system generally operates under a balanced and distortion free environment. On the other hand, a power distribution system may contain dc components, distortion, and unbalance both in voltages and currents. Therefore, a UPQC should operate under this environment while performing shunt and/or series compensation \hyperref[b32]{[5]}. The main purpose of a UPQC is to compensate for supply voltage power quality issues, such as, sags, swells, unbalance, flicker, harmonics, and for load current power quality problems, such as, harmonics, unbalance, reactive current, and neutral current. Fig. \hyperref[fig_0]{1} shows a single-line representation of the UPQC system configuration. The key components of this system are as follows.\par
1. Two inverters one connected across the load which acts as a shunt APF and other connected in series with the line as that of series APF.\par
2. Shunt coupling inductor L Sh is used to Interface the shunt inverter to the network. It also helps in smoothing the current wave shape. Sometimes an isolation transformer is utilized to electrically isolate the inverter from the network. 
\section[{A common dc link that can be formed by using a}]{A common dc link that can be formed by using a}\par
Capacitor or an inductor. In Fig. \hyperref[fig_0]{1}, the dc link is realized using a capacitor which interconnects the two inverters and also maintains a constant self supporting dc bus voltage across it.\par
4. An LC filter that serves as a passive low-pass filter (LPF) and helps to eliminate high-frequency switching ripples on generated inverter output voltage. 5. Series injection transformer that is used to connect the series inverter in the network. A uitable turn ratio is often considered to reduce the current or and voltage rating of the series inverter.\par
The integrated controller of the series and shunt APF of the UPQC to provide the compensating voltage reference V C * and compensating current reference I C * to be synthesized by PWM converters \hyperref[b4]{[6]}, \hyperref[b5]{[7]}.\par
The shunt active power filter of the UPQC can compensate all undesirable current components, including harmonics, imbalances due to negative and zero sequence components at the fundamental frequency. In order to cancel the harmonics generated by a nonlinear load, the shunt inverter should inject a current as governed by the following equation:I C (?t)=I* L (?t)-I S (?t)\textbf{(1)}\par
Where I C (?t), I* L (?t), and I S (?t) represent the shunt inverter current, reference load current, and actual source current, respectively.\par
The series active power filter of the UPQC can compensate the supply voltage related problems by injecting voltage in series with line to achieve distortion free voltage at the load terminal. The series inverter of the UPQC can be represented by following equation:V C (?t)=V* L (?t)-V S (?t)\textbf{(2)}\par
Where V C (?t),V* L (?t),and V S (?t) represent the series inverter voltage, reference load voltage, and actual source voltage, respectively \hyperref[b6]{[8]}  \hyperref[b7]{[9]}. 
\section[{IV. Classification of upqc}]{IV. Classification of upqc}\par
The Unified Power Quality Conditioner are classified on various bases like converter used, topology, supply type and compensation method. The UPQC is classified in two main groups which is based on, Physical structure and Voltage sag compensation \hyperref[b3]{[4]}. 
\section[{a) Physical Structure}]{a) Physical Structure}\par
The key parameters that attribute to these classifications are: Type of energy storage device used, Number of phases, and Physical location of shunt and series inverter. The voltage sag on a system is considered as one of the important power quality problems. There are mainly four methods to compensate the voltage sag in UPQC-based applications.     To mitigate power quality problems in the distribution system and UPQC's different configurations are classified based on the type of supply system. There are mainly two types of supply a) singlephase and b) three-phase.\par
Single-phase two-wire two-Hbridge UPQC configuration is a s shown in Fig. \hyperref[fig_12]{9}. Another two topologies first is 3-leg topology (total 6 switches). Apart from total 6 switches, 4 switches are used in series inverter and 2 switches are used in shunt inverter. Second alf-bridge topology, 2 switches are used in series inverter and 2 switches used in shunt inverter \hyperref[b8]{[10]}.  For neutral current compensation in threephase four-wire (3P4W) system, various shunt inverter configurations are given, namely, four-leg (4L), two split-capacitor (2C) and three-H bridge (3HB) \hyperref[b11]{[13]}  \hyperref[b13]{[15]}.\par
The 3HB topology use three single-phase Hbridge inverter connected to same dc bus of the UPQC. The 2C topology use two split-capacitor on dc side and the midpoint of two capacitor is at zero potential which is used as connection point for the fourth wire. Among all three topologies four-leg (4L) is give better control over neutral current due to four-leg. In this paper three-phase four-wire based on four-leg (4L) shunt inverter topology \hyperref[b24]{[24]}, is shown in Fig. \hyperref[fig_0]{11}.  1. Three-phase UPQC is possible in three-phase three-wire or three-phase four-wire (3P3W or 3P4W) 2. Single-phase UPQC is further classified on:\par
(i) Two H-bridge (ii) 3-Leg topology (iii) Half Bridge 
\section[{2.}]{2.}\par
Three-phase four-wire UPQC is further classified on:\par
(i) Four-Leg (ii) Split Capacitor (iii) Three-H Bridge 3. In single-phase system load reactive current, current harmonics are major problems 3.-In three-phase three wire system apart from reactive current, current harmonics additional problem is current Unbalance. In three phase four-wire system additional neutral current problem 4. Voltage related power quality problems are similar for both single and three phase system except voltage unbalance compensation is not required in single-phase system 4. Voltage related power quality problems are similar for both single and three phase system except voltage unbalance com-pensation is required in three-phase system There are various types of configurations of UPQC is given in above classification. Fig. \hyperref[fig_10]{7} to 11 all are represents right shunt UPQC (UPQC-R) and when in Fig. \hyperref[fig_10]{7} to 11 shunt inverter is located in left at that time it is called left shunt UPQC (UPQC-L). Among this two configurations UPQC-R is commonly used because current flow through series transformer is mostly sinusoidal. The UPQC-L is rarely used due to interference between shunt inverter and passive filters.\par
First, the comparison between Interline UPQC (UPQC-I) and Multi-converter UPQC (UPQC-MC) \hyperref[b23]{[23]} is given in TABLE-3. Second, the comparison between Modular UPQC (UPQC-MD) and Multi-level UPQC (UPQC-ML) \hyperref[b18]{[20]}, \hyperref[b26]{[26]}, \hyperref[b27]{[27]} is given in TABLE-4. 2. The H-bridge modules for shunt inverter is connected in series through multi-winding transformer, while, series inverter is connected in series with using series transformer.\par
2. In UPQC-ML three-level topology require double semiconductor switches.\par
3. UPQC-MD can be useful to achieve higher power levels.\par
3. UPQC-ML can also be useful to achieve higher power levels.\par
Third, the comparison between Distributed UPQC (UPQC-D) and Distributed Generator Integrated UPQC (UPQC-DG) \hyperref[b12]{[14]} [16] [19] \hyperref[b20]{[21]}, is given in TABLE-5. 
\section[{Table 5 : Comparison between UPQC-D \& UPQC-DG Distributed UPQC (UPQC-D)}]{Table 5 : Comparison between UPQC-D \& UPQC-DG Distributed UPQC (UPQC-D)}\par
Distributed Generator Integrated UPQC (UPQC-DG) 1. UPQC-D topology is also known as 3P3W to 3P4W Distributed UPQC because 3P-4W system is realized by using 3P3W system. 
\section[{1.}]{1.}\par
The UPQC can be integrated with one or several DG systems which are nown as UPQC-DG.\par
2. In UPQC-D system the neutral of series transformer is used as neutral of 3P4W system.\par
2. The output of DG system is connected to dc bus of UPQC to compensate voltage and current related problems. 3. Fourth leg is added to 3P 4W UPQC to compensate neutral current flowing towards transformer neutral point.\par
3. In UPQC-DG battery can be added at dc bus which is used as stored power and used as backup which give benefit for removing voltage interruption.\par
Finally, the classification is based on voltage sag compensation is given in this section. There are mainly four methods to compensate voltage sag in UPQC based applications, The comparison between Active Power Control (UPQC-P) and Reactive Power Control (UPQC-Q) is given in   ? The instantaneous active and reactive power or pq theory, and ? Synchronous reference frame method or d-q theory.\par
In p-q theory instantaneous active and reactive powers are computed, while, the d-q theory eals with the current independent of the supply voltage. Both methods transforms voltages and currents from abc frame to stationary reference frame (p-q theory) or synchronously rotating frame (d-q theory) to separate the fundamental and harmonic quantities.\par
In third stage the gating signals for semiconductor switches of UPQC based on derive compensating commands in terms of voltage or current. Then, these compensating commands are given to PWM, hysteresis or fuzzy logic based control techniques  {\ref [25] [28]}.\par
VI. 
\section[{Technical and Economical Consideration}]{Technical and Economical Consideration}\par
Technical literature on the APFs can be found since early 1970s \hyperref[b0]{[1]}. However, the use of UPQC to enhance electric power system quality is reported since mid 1990s \hyperref[b2]{[3]}. Among the various power quality enhancement devices, STATCOM and few others are commercially available \hyperref[b1]{[2]}. The technology to develop commercial UPQC system is available today; however, the overall cost and complexity of such a system still imposes some limitations. A 250-kVA prototype developed at C-DAC, Thiruvananthapuram, India \hyperref[b15]{[17]}, is the most viable reported prototype.\par
The capacity of small and large-scale renewable energy systems based on wind energy, solar energy, etc., installed at distribution as well as transmission levels is increasing significantly. These newly emerging DG systems are imposing new challenges to electrical power industry to accommodate them without violating standard requirements (such as, IEEE 1547, IEEE 519). In terms of power quality, the excessive feeder voltage rise due to reverse power flow from DG system and power system stability is of significant importance. Moreover, most of the DG systems utilize power electronic converters as interfacing device to deliver the generated power to the grid. The switching operation of these systems is contributing as increased harmonic levels both in the grid voltages and currents.\par
In this paper, several UPQC configurations and topologies have been discussed. Among these configurations, UPQC-DG could be the most interesting topology for a renewable energy based power system. This configuration can offer multifunctional options, namely, active power delivery from DG system to grid (normal DG operation), voltage and current related power quality compensation (UPQC operation), and uninterruptible power supply operation. Commercial products have started to appear in the market to increase the renewable energy system connectivity by compensating some of these problems. As the penetration levels of DG system on the existing power system continue to increase, the utilization of active compensating technologies (such as, flexible ac transmission system devices and APFs) is expected to increase gradually. 
\section[{VII.}]{VII.} 
\section[{Conclusion}]{Conclusion}\par
The power quality problems in distribution systems are not new but customer awareness of these problems increased recently. It is very difficult to maintain electric power quality at acceptable limits. One modern and very promising solution that deals with both load current and supply voltage imperfections is the Unified Power Quality Conditioner (UPQC). This paper presented a review on the UPQC as a tool to enhance the electric power quality at distribution level. The UPQC is able to compensate supply voltage power quality issues such as, sags, swells, unbalance, flicker, harmonics, and for load current power quality problems such as, harmonics, unbalance, reactive current and neutral current. Among all these configurations, UPQC-DG and UPQC-ML are the most vital topologies to achieve better reliability and power quality at higher power rating of the system. Therefore with the help of these topologies can meet required load demand in future, increase the production in industries and increase the economy of the country.\begin{figure}[htbp]
\noindent\textbf{1}\includegraphics[]{image-2.png}
\caption{\label{fig_0}Figure 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{4}\includegraphics[]{image-3.png}
\caption{\label{fig_1}Figure 4 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{3}\includegraphics[]{image-4.png}
\caption{\label{fig_3}Figure 3 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{4}\includegraphics[]{image-5.png}
\caption{\label{fig_4}Figure 4 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{5}\includegraphics[]{image-6.png}
\caption{\label{fig_5}Figure 5 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{13}\includegraphics[]{image-7.png}
\caption{\label{fig_6}1 . 3 -}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{}\includegraphics[]{image-8.png}
\caption{\label{fig_7}}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{}\includegraphics[]{image-9.png}
\caption{\label{fig_8}}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{6}\includegraphics[]{image-10.png}
\caption{\label{fig_9}Figure 6 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{7}\includegraphics[]{image-11.png}
\caption{\label{fig_10}Figure 7 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{8}\includegraphics[]{image-12.png}
\caption{\label{fig_11}Figure 8 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{9}\includegraphics[]{image-13.png}
\caption{\label{fig_12}Figure 9 :}\end{figure}
   \begin{figure}[htbp]
\noindent\textbf{1} \par 
\begin{longtable}{P{0.34189944134078215\textwidth}P{0.09022346368715084\textwidth}P{0.29916201117318436\textwidth}P{0.05223463687150837\textwidth}P{0.023743016759776536\textwidth}P{0.04273743016759777\textwidth}}
\multicolumn{2}{l}{Voltage Source Inverter}\tabcellsep \multicolumn{4}{l}{Current Source Inverter}\\
\multicolumn{2}{l}{(VSI) based}\tabcellsep \multicolumn{3}{l}{(CSI) based}\\
\multicolumn{2}{l}{1. The UPQC may be}\tabcellsep 1. The\tabcellsep \multicolumn{2}{l}{UPQC}\tabcellsep may\\
developed\tabcellsep using\tabcellsep \multicolumn{2}{l}{be developed}\tabcellsep \multicolumn{2}{l}{using}\\
PWM\tabcellsep voltage source\tabcellsep PWM\tabcellsep current\tabcellsep \tabcellsep source\\
inverter\tabcellsep \tabcellsep inverter\tabcellsep \tabcellsep \end{longtable} \par
 
\caption{\label{tab_0}Table 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{3} \par 
\begin{longtable}{P{0.2972906403940887\textwidth}P{0.025123152709359605\textwidth}P{0.17167487684729063\textwidth}P{0.35591133004926107\textwidth}}
\tabcellsep \tabcellsep \multicolumn{2}{l}{Multi-converter UPQC}\\
\multicolumn{3}{l}{Interline UPQC (UPQC-I)}\tabcellsep Multi-converter UPQC (UPQC-MC)\\
\multicolumn{3}{l}{1. In Interline UPQC two}\tabcellsep 1. In UPQC-MC third\\
inverters\tabcellsep are\tabcellsep connected\tabcellsep converter is added to\\
between\tabcellsep two\tabcellsep distribution\tabcellsep support dc bus.\\
feeders.\tabcellsep \tabcellsep \tabcellsep \end{longtable} \par
  {\small\itshape [Note: 2. One inverter is connected in series with one feeder while other inverter is connected in shunt with other feeder.2. The third converter is connected either series or parallel with feeder.3. UPQC-I can control and 3. It can control and]} 
\caption{\label{tab_1}Table 3 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{4} \par 
\begin{longtable}{P{0.48874999999999996\textwidth}P{0.36124999999999996\textwidth}}
\multicolumn{2}{l}{Multi-level UPQC}\\
Modular UPQC\tabcellsep Multi-level UPQC\\
(UPQC-MD)\tabcellsep (UPQC-ML)\\
1. In UPQC-MD several H-\tabcellsep 1. UPQC-ML is based on\\
bridge modules are conne-\tabcellsep 3-level neutral point\\
cted in cascade in each\tabcellsep clamped topology.\\
phase.\tabcellsep \end{longtable} \par
 
\caption{\label{tab_2}Table 4 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{-} \par 
\begin{longtable}{P{0.5146788990825688\textwidth}P{0.29243119266055045\textwidth}P{0.010397553516819572\textwidth}P{0.005198776758409786\textwidth}P{0.027293577981651378\textwidth}}
1. The voltage sag is\tabcellsep \multicolumn{4}{l}{1. The voltage sag is}\\
mitigated by injecting active\tabcellsep \multicolumn{2}{l}{mitigated}\tabcellsep by\tabcellsep injecting\\
power through series inverter\tabcellsep \multicolumn{4}{l}{reactive power through}\\
of UPQC.\tabcellsep \multicolumn{4}{l}{series inverter of UPQC.}\\
2. In Active Power Control\tabcellsep \multicolumn{4}{l}{2. In Reactive Power}\\
P is referred as active power.\tabcellsep \multicolumn{4}{l}{Control Q is referred as}\\
\tabcellsep \multicolumn{3}{l}{reactive power.}\\
3. To compensate equal\tabcellsep \multicolumn{4}{l}{3.To compensate equal}\\
percentage of sag UPQC-P\tabcellsep \multicolumn{4}{l}{percentage of sag UPQC-}\\
requires smaller magnitude\tabcellsep Q\tabcellsep \multicolumn{2}{l}{requires}\tabcellsep larger\\
of series injection voltage\tabcellsep \multicolumn{2}{l}{magnitude}\tabcellsep of\tabcellsep series\\
compared to UPQC-Q.\tabcellsep \multicolumn{4}{l}{injection voltage compared}\\
\tabcellsep \multicolumn{2}{l}{to UPQC-P.}\tabcellsep \\
\multicolumn{5}{l}{The comparison between Minimum VA Loading}\\
\multicolumn{5}{l}{(UPQC-VAmin) and Active-Reactive Power Control}\\
\multicolumn{4}{l}{(UPQC-S) [29],[30], [31], is given in TABLE-7.}\\
6.\tabcellsep \tabcellsep \tabcellsep \end{longtable} \par
 
\caption{\label{tab_3}TABLE -}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{7} \par 
\begin{longtable}{P{0.2789986091794158\textwidth}P{0.3180111265646731\textwidth}P{0.07152294853963838\textwidth}P{0.010639777468706537\textwidth}P{0.13477051460361614\textwidth}P{0.028963838664812242\textwidth}P{0.0029554937413073714\textwidth}P{0.0041376912378303194\textwidth}}
\tabcellsep \tabcellsep \multicolumn{4}{l}{Active \& Reactive Power Control}\\
\tabcellsep \multicolumn{3}{l}{Minimum VA loading}\tabcellsep \multicolumn{2}{l}{Active \& Reactive Power}\\
\tabcellsep \multicolumn{3}{l}{(UPQC-VAmin)}\tabcellsep \tabcellsep Control (UPQC-S)\\
\tabcellsep \multicolumn{3}{l}{1. This method is used which}\tabcellsep \multicolumn{2}{l}{1. In UPQC-S the series}\\
\tabcellsep \multicolumn{3}{l}{is injected certain optimal}\tabcellsep \multicolumn{2}{l}{inverter is delivered both}\\
\tabcellsep \multicolumn{3}{l}{angle with respect to source}\tabcellsep \multicolumn{2}{l}{active and reactive power.}\\
\tabcellsep current.\tabcellsep \tabcellsep \tabcellsep \\
\tabcellsep \multicolumn{3}{l}{2. The series voltage}\tabcellsep \multicolumn{2}{l}{2. The series inverter of}\\
\tabcellsep \multicolumn{3}{l}{injection and the current}\tabcellsep \multicolumn{2}{l}{UPQC-S perform voltage}\\
\tabcellsep \multicolumn{3}{l}{drawn by shunt inverter must}\tabcellsep \multicolumn{2}{l}{sag and swell com-}\\
\tabcellsep need\tabcellsep for\tabcellsep determining\tabcellsep \multicolumn{2}{l}{pensation and sharing}\\
\tabcellsep \multicolumn{3}{l}{Minimum VA loading of}\tabcellsep \multicolumn{2}{l}{reactive power with shunt}\\
\tabcellsep UPQC.\tabcellsep \tabcellsep \tabcellsep \multicolumn{2}{l}{inverter.}\\
\tabcellsep \multicolumn{5}{l}{V. Control Strategies of upqc}\\
\tabcellsep \tabcellsep \multicolumn{4}{l}{Control strategy play very important role in}\\
\multicolumn{6}{l}{system's performance. The control strategy of UPQC}\\
\multicolumn{6}{l}{may be implemented in three stages:}\\
i.\tabcellsep \multicolumn{5}{l}{Voltage and current signals are sensed Com}\\
\tabcellsep \multicolumn{4}{l}{pensating commands in}\tabcellsep terms of voltage and\\
\tabcellsep \multicolumn{5}{l}{current levels are derived.}\\
ii.\tabcellsep \multicolumn{2}{l}{The gating}\tabcellsep signals\tabcellsep for\tabcellsep semiconductor\\
\tabcellsep \multicolumn{5}{l}{Switches of UPQC are generatedusing PWM,}\\
\tabcellsep \multicolumn{5}{l}{hysteresis or fuzzy logic based control techniques}\\
\multicolumn{6}{l}{iii. In the first stage voltage signals are sensed}\\
\tabcellsep \multicolumn{5}{l}{using power transformer or voltage sensor and}\\
\tabcellsep \multicolumn{3}{l}{current signals are}\tabcellsep \multicolumn{2}{l}{sensed}\tabcellsep using\tabcellsep current\\
\tabcellsep \multicolumn{5}{l}{transformer or current sensor.}\\
\tabcellsep \tabcellsep \multicolumn{4}{l}{In second stage derivation of compensating}\\
\multicolumn{6}{l}{commands are mainly based on two types of domain}\\
\multicolumn{6}{l}{methods: (1) Frequency domain methods, and (2) Time}\\
\multicolumn{6}{l}{domain method. Frequency domain methods, which,}\\
\multicolumn{6}{l}{is based on the Fast Fourier Transform (FFT) of}\\
\multicolumn{6}{l}{distorted voltage or current signals to extract}\\
\multicolumn{6}{l}{compensating commands. This FFT are not popular}\\
\multicolumn{6}{l}{because of large computation, time and delay.}\end{longtable} \par
 
\caption{\label{tab_4}Table 7 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{6} \par 
\begin{longtable}{P{0.5464285714285715\textwidth}P{0.30357142857142855\textwidth}}
\multicolumn{2}{l}{and Reactive Power Control}\\
Active Power Control\tabcellsep Reactive Power Control\\
(UPQC-P)\tabcellsep (UPQC-Q)\end{longtable} \par
 
\caption{\label{tab_5}Table 6 :}\end{figure}
 			\footnote{© 2013 Global Journals Inc. (US) © 2013 Global Journals Inc. (US)} 			\footnote{© 2013 Global Journals Inc. (US)} 			\footnote{F obal Journal of Researches in Engineering} 			\footnote{, , © 2013 Global Journals Inc. (US)} 		 		\backmatter   			 
\subsection[{Acknowledgements}]{Acknowledgements}\par
The author B. Gopal express sincere thanks to the Management, the Principal and the Staff members of the Department of EEE of Khammam Institute of Technology \& Sciences, Khammam for the continued support in executing this work. 			  			 			 			  				\begin{bibitemlist}{1}
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