S. Arulkumar International Journal of Advanced Engineering

S. Arulkumar et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945

Int J Adv Engg Tech/Vol. VII/Issue II/April-June,2016/1293-1300

Research Paper PERFORMANCE OF UNIFIED POWER QUALITY

CONDITIONER WITH DIFFERENT CONTROLLER S.Arulkumar1 and P.Madhavasarma2

Address for Correspondence

1Research Scholar, Anna University, Chennai, India. 2Principal, Saraswathy College of Engineering and Technology, India.

ABSTRACT The quality of the power is affected 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 of the loads etc. Recent times due to increased usage of loads, maintaining the quality of power is major challenging for control engineer. Unified Power Quality Conditioner (UPQC) can be used for effectively improving the power quality of an electrical power system. It is having shunt APF (Active Power Filter) and series APF (Active Power Filter). The shunt part for compensate the source current and series parts for compensate the load voltage. In this paper presents the reduction of Total Harmonic Distortion (THD) with the help of UPQC using PI controller, improved UPQC controller (IUPQC) and Fuzzy logic controller. The output results are carried out and compare by using MATLAB software. KEY WORDS: UPQC, Power quality, IUPQC, Fuzzy Logic Controller.

1. INTRODUCTION Nowadays Power quality is the major issue in the power system area. E.W.Gunther, et al., developed a new power quality monitoring instrument. This can be used to monitor the steady state quantities and disturbances [1]. In UPQC the active power filters to compensate current and reactive power for voltage compensation [2]. The power electronics based equipment’s are used to improve the power quality issues and power enhancement [3]. In [4, 5] UPQC SCR controlled capacitor banks achieving load compensation by current control method. A new method proposed [6] for power quality improvement and it needs actual sinusoidal power supply. In this paper [7] the universal power quality conditioning system with various compensation are discussed with recent development. The ANN with hysteresis control give assurance for power quality issues like swell, sags etc. [8, 9].Artificial intelligence based 3ɸ UPQC implemented for control purpose [10]. The improvement of power quality issues are discussed in [11, 12]. The authors presented PI based power quality improvement system [13] to improve the THD level. Various control schemes are discussed and applied for synchronous rotating frame to maintain the constant voltage [14]. Thus, the DC Link voltage control unit is intended to keep the average dc bus power absorbed by the shunt inverter .By varying the amplitude of the fundamental component of the reference current, small amount of real power is regulated. The ac source offers some active current to recharge the dc capacitor [15].In this proposes improvement of power quality issues and to compensate the sag, imbalance condition, support the real and reactive power and to eliminating the harmonics in the distribution network with different controller (PI, IUPQC and FUZZY controller). 2. UNIFIED POWER QUALITY CONDITIONER The UPQC is a custom power device that combines the series-and shunt active filters, connected back-to-back on the dc side and sharing a common DC capacitor [15] as shown in Fig.1. It’s having two VSIs that are connected to a common DC energy storage capacitor. Among these two VSIs one is connected in series with the feeder and another one is connected in parallel to the same feeder. The series component of the UPQC is responsible for mitigation of the supply side disturbances viz. voltage sags/swells, flicker, voltage unbalance and harmonics. It inserts voltages for to maintain the load

voltages at a constant level, balanced condition and harmonic distortion free. The shunt component is accountable for reducing the power quality problems caused by the consumer. They are poor power factor, load harmonic currents, load unbalance and DC offset. It injects currents to the ac system such that the source currents become balanced sinusoidal waveform and in phase with the source voltages. The major advantage of UPQC is that it does not require any energy storage. It can be designed to lessen any sag above certain magnitude, independent of its duration. The main difficulty of UPQC is the huge current rating required to mitigate deep sags. For low power, low-voltage equipment this will not be a severe concern, but it might limit the number of large power and medium-voltage applications.

Fig.1 System configuration of UPQC

3. CONTROL STRATEGIES 3.1. COMPENSATING CURRENT GENERATION The magnitude of three phase reference current is considered from the output, isp. The unit current vectors (usa,usb,usc) are in phase with the three phase supply voltages(vsa,vsb,vsc).These current vectors are in phase with the three phase reference current. On multiplying the isp magnitude with the current vectors it results in three phase reference supply currents(isa*,isb*,isc*).On subtracting the load currents(ila,ilb,ilc) from the reference supply current(isa*,isb*,isc*) it gives three phase reference currents(isha*,ishb*.ishc*) for shunt inverter. 3.2. VOLTAGE GENERATION AND COMPENSATION The series inverter operates in current control mode, introduces a voltage source which isolates the load from the supply. This voltage source gives for the deviation of voltages. In closed loop control of series inverter, the 3ɸ load voltages are subtracted from the 3ɸ supply voltages and it is compared with the reference supply voltage.



These reference supply voltages are injected to the load. By suitable transformation of the resource, the reference currents of the series inverter are obtained from the 3ɸ reference voltages. The 3ɸ reference currents along with their sensed counterparts are given to a current (PWM) controller. The current (PWM) controller gating signals, assure that the voltage deviations are meet by the series inverter and hence giving sinusoidal voltage to load. 3.3. THE EQUATIONS OF UPQC 3.3.1. Shunt Inverter Control Quantities From the three phases sensed value the amplitude of the supply voltage is enumerated as: vsm = [2/3(vsa

2+vsb2+vsc

2)] 1/2 (1) The 3ɸ per unit current vectors are given as: usa = vsa/vsm; usb = vsb/vsm; usc = vsc/vsm (2) The three phase supply current is obtained by the product of three phase unit current vectors (usa, usb and usc) and the amplitude of three phase supply current (isp). Multiplication of 3ɸ per unit current vectors with the supply current gives the 3ɸ reference supply currents as follows: isa* = isp.usa; isb* = isp.usb; isc* = isp.usc (3) The reference current is obtained by subtracting the three phase load currents from the three phase supply currents: The reference current obtained from the difference between 3ɸ load currents 3ɸ supply currents: isha* = isa* -ila; ishb* = isb* - ilb ishc* = isc* -ilc (4) In Direct current control technique of shunt inverter these are the iref. Shunt inverter gives the switching signals, the irefare compared with iactin PWM current controller. 3.3.2. Series Inverter Control Quantities The injected voltage as follows: vinj = vs – v1 (5) The magnitude of the injected voltage is expressed as: vinj = |vinj| (6) While, the phase of injected voltage is given as: δinj = tan (Re[vpq]/Im[vpq]) (7) For the compensating the harmonics in load voltage, the given inequalities are followed: a) vinj<vinjmax magnitude control; b) 0 <δinj< 360° phase control; The reference values of the injected voltages are given as: vla* = √2vinjsin(wt+δinj) vlb* = √2vinjsin(wt+2π/3+δinj) vlc* = √2vinjsin(wt-2π/3+δinj) (8) The series inverter’s three phase reference currents (iref) are computed as follows: isea* = vla*/zse ; iseb* = vlb*/zse ; isec* = vlc*/zse ; (9) Secondary windingsare maintaining the currents in ideal, the insertion transformer in order to inject

voltages and hence compensating the voltage sag. The current iref are compared with iact in PWM current controller, six switching signals are obtained for series inverter’s IGBTs. 3.4. CONTROL OF DC VOLTAGE For maintain constant dc current in the energy storage capacitor. Usually a PI Controller is used for determining the magnitude of this compensating current from the error between the average voltage across the dc capacitor and the reference voltage. In a practical approach (Ziegler-Nichols tuning rules) for tuning the PI Controller is proposed. As an alternative to PI Controller, IUPQC Controller and a simple linear control technique of Fuzzy Controller is proposed. The active power which is necessary to keep dc voltage constant in steady state or transient condition. The voltage fluctuations of the dc capacitor are affected by three principal factors; to compensate the alternating power, imbalance during transient and active power absorbed. The FLC contains three parts: fuzzification, interference engine and defuzzification. The FLC is characterized as: i. for each input and output 7 fuzzy set ii. Triangular membership functions for simplicity iii. Continuous universe of discourse for fuzzification. iv. Implication using mamdani’s‘min’ operator v. defuzzification using the ‘height’ method. The knowledge bases are designed in order to get a good dynamic response under uncertainty in process parameters and external disturbances. Dc voltage control using fuzzy logic is shown in figure2. In our application, the fuzzy controller is based on processing the voltage error and its derivation. The numerical values of the variables are converted into linguistic variables in the fuzzification stage. For every input and output variable seven linguistic variables are assigned they are NB(negative big), NM(negative medium), NS(negative small), ZE(zero), PS(positive small), PM(positive medium), and PB(positive big). For fuzzy implementation normalized values are used. As there are seven variables for input and output, 49 possibilities can be obtained (i.e., 7*7=49) as tabulated in table1.

Fig.2. dc voltage control using fuzzy logic

A membership function value between zero and one will be assigned to each of the numerical values in the membership function graph. In this chapter, we applied max-min inference method to get implied fuzzy set of the turning Rules.

Table.1. Fuzzy set rules of inference for the dc voltage CE

E NL NM NS EZ PM PS PL

NL NL NL NL NL NM NS EZ NM NL NL NL NM NS EZ PS NS NL NL NM NS EZ PS PM

EZ NL NM NS EZ PS PM PL PM NM NS EZ PS PM PL PL PS NS EZ PS PM PL PL PL PL EZ PS PM PL PL PL PL



4. SIMULATION RESULT 4.1. PI CONTROLLER

Fig. 3.Simulation diagram for PI controller

The figure 4, 5 and 6 shows the source side and load side voltage, current, real and reactive power performance by using PI controller with Non-linear load connect the time period of 0.1sec to 0.15 sec.

Fig. 4.Load side voltage and current

Fig. 5.Source side voltage and current

Fig. 6.Real and Reactive power



4.2. IUPQC CONTROLLER The figure 8, 9 and 10 shows the source side and load side voltage, current, real and reactive power performance by using IUPQC controller. Non-linear load connect the time period of 0.15sec to 0.25 sec. In that period some amount of voltage is reduce, that

voltage is compensated by using IUPQC controller to regulate the back to back DC capacitor. After 0.25 sec the normal voltage is maintain at constant level and also current, real and reactive power improved. Compare to PI controller it gives better results.

Fig. 7.Simulation diagram for IUPQC controller



Fig. 10.Real and Reactive power



4.3. FUZZY CONTROLLER

Fig. 11.Simulation diagram for FUZZY controller

The figure 12, 13 and 14 shows the source side and load side voltage, current, real and reactive power performance by using Fuzzy controller. Non-linear load connect the time period of 0.1sec to 0.2 sec. In that period some amount of voltage is reduce, that voltage is to compensate by using Fuzzy controller to

regulate the back to back connected DC capacitor. After 0.2 sec the normal voltage is maintain at constant level and also current, real and reactive power improved. Compare to other two controllers it gives better results.





Fig. 14.Real and Reactive power for source side and load side

Fig. 15.Control Surface of the Fuzzy controller

(a) (b)

(c)

Fig.16. Membership functions for input and output variables (a), (b) and (c) Figure16 shows the membership functions of input and output and triangle shaped membership function has the advantages of simplicity and easier implementation and is chosen in this application. 5. RESULT COMPARISON

Fig. 17.Total Harmonic Distortion level of 1.47%



Fig. 18.Total Harmonic Distortion level of 0.88%

Fig.19.Total Harmonic Distortion level of 0.16%

Table.2. RESULT COMPARISON (FFT ANALYSIS THD LEVEL)

CONTROLLER DEVICE

PI CONTROLLER IUPQC CONTROLLER

FUZZY LOGIC CONTROLLER

UPQC 1.47% 0.88% 0.16%

CONCLUSION The simulation results show that UPQC can be used for effectively improving the power quality of an electrical power system. The shunt APF has been used for compensating the source current harmonics and it reduces the source current. The series APF has been used for compensating the load voltage harmonics and it reduces the load Voltage. The performance of UPQC with PI controller (1.47%) , IUPQC controller (0.88%) and Fuzzy controller (0.16%) by using the FFT analysis to get the reduced Total Harmonic Distortion (THD) level, voltage sag compensation, real and reactive power of the system. The fuzzy logic controller provide quickand better results than the PI controller and IUPQC controller. Hence it is proved that fuzzy logic controller is superior when compare to other controllers.

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S. Arulkumar International Journal of Advanced Engineering