IMPROVEMENT OF DVR WITH SUPER CAPACITOR AND Z SOURCE INVERTER

AbstractThis paper uses super capacitor as the energy storage device for the DVR to compensate voltage sag, voltage swell and harmonics due to addition of nonlinear load in the distribution line. Based on this topology, DVR consist of super capacitor, z source inverter and injection transformer. Super capacitor produces the necessary dc voltage which is given as the input voltage to the z-source inverter; provide the necessary injecting voltage, which has to be restored. In addition, it also consists of PI controller which provides the necessary control signals for the z source inverter. The simulation results and hardware results of the proposed DVR system are demonstrated.

Index Terms Dynamic Voltage Restorer (DVR), Super Capacitor, Z-source inverter, Sag, Swell, Harmonics, PI controller.INTRODUCTIONVoltage sag, voltage swell, harmonics notching etc are few major power quality problems that has to be taken in to account when maintaing power quality. To overcome these problems some analytical methods have been proposed [1]. Because of this power quality problems both electrical utility and the end users are concerned about the quality of power delivered. Economic value is got down if these problems are not taken into consideration. Few steps involved in evaluating power quality are identifying the problem, problem characterization, identify range of solutions, evaluate solutions and optimum solutions [2]. In order to compensate distortions various custom power devices are used. There are various types in which the power quality problems can be compensated which are load side compensation and compensation using energy storage [3]. Major problems which affect the load side is voltage sag and voltage swell, these voltages have to be compensated in order maintain power factor and to maintain voltage level [4]. One of the custom power devices which uses series compensation is dynamic voltage restorer [5]. Control algorithms are used to provide control signals for the dynamic voltage restorer [6]. Load side compensation of DVR is done by using the load voltage which has a back to back connected voltage source converter with a capacitor in between them which is used to smoothen the voltage from one converter [9]. Limitations of the conventional inverters are replaced by z source inverter for example both buck and boost operation can be done in z source inverter where as it cannot be done in conventional inverter and mis firing of switches are possible in z source inverter so Z source inverter are used here [11]. Comparison of z source inverter and conventional inverter it shows that z source inverter is advantageous over voltage source inverter [12]. So a dynamic voltage restorer uses z source inverter in place conventional inverter has various advantages [14]. Another method used for compensating voltage distortions is using energy storage element. There are various energy storage elements like batteries, flywheel, normal capacitor and super capacitor [15]. Dynamic voltage restorers can be implemented with super capacitor as the energy storage because of various advantages when compared with other energy source device like fast charging and slow dis charging, long life cycle and very less maintenance [17]. In order to overcome all the problems in the previous methods DVR is implemented using Z-Source inverter and Supercapacitor. The most often used controllers to produce control signals are hysteresis controller and PI controller. PI controller compares the reference signal with the actual signal which is taken form the load side and they produce the necessary error signals which are responsible for the pulse generator to produce pulse required for the z source converter, they produce the injecting voltage to be injected in the distribution line.Proposed ModelThe proposed DVR model consists of a energy storage, Z source inverter, harmonic filter and injection transformer. DVR injects voltage into the system which is in synchronous with the supply voltage. DVR can control both the magnitude of the voltage injected and also the phase angle of it. The proposed DVR consist of super capacitor as the energy storage. Which supply the dc voltage required for the z source inverter. Controller provides the reference signal for the pulse generator for pulse production which will turn on the switches in the z source inverter, which helps to produce 3 phase voltage as the output from the inverter. This voltage contains harmonics; they are removed with the help of the harmonic filter which will be a combination of inductor and capacitor. This removes the harmonics in the output from the z source inverter. PI controller produce the necessary reference signal for the pulse generator which produce the pulses required for z source inverter for the production on three phase wave form. Fig 5 shows the basic block diagram of the proposed DVR.

Fig 5 Block diagram of the proposed systemVoltage sag/swell detectionIn this paper dq transformation is used to generate the reference signals in first step the line voltage is converted to component which is given by

Where Q is given by

Now with the help of this values Vd and Vq values are found which is given by

And with the help of source voltage and values we can generate the reference voltage depending upon whether the supply voltage is sag or swell which is given by

And source voltage is given as

Where Va, Vb, Vc are supply voltage.PI ControllerIn this paper PI controller is used to produce the reference signal to the pulse generator which produce the pulses for the z source inverter. PI controller consists of proportional and integral component they compare the reference signal with the actual signal and produce error signals. Reference signals are obtained with the help of dq transformation and actual signal are compared to produce error signal. PI controller is less responsive to noise and very fast in its operation. Fig 6 shows the basic block diagram for PI controller.

Fig 6 Basic block diagram for PI controllerHarmonic FilterIf there is no harmonic filter before injecting transformer voltage stress are directly applied across injection transformer and cause damage to it. Harmonic filter consist of a combination of inductor and capacitor to compensate the harmonics from the output of the Z source inverter. This filter can also be used along the high voltage side but if we use so we need higher rating of transformers because harmonics will enter into the high voltage side.Injection TransformerThe output from the z source inverter is fed to the harmonic filter and given as the input to the injection transformer, which injects the voltage into the supply line to compensate voltage sag, voltage swell and harmonics.Energy StorageThere are various energy storage devices like flywheel, batteries, normal capacitors and super capacitor. In this paper super capacitor is used. Super capacitors has many advantages over other energy storage like fast charging and very slow discharging, can be used over a wide range of temperature. Super capacitor is also called as ultra-capacitor or double layer electrolytic capacitor. Super capacitor has high capacitance value than normal capacitor due to large surface area, which is obtained by inserting a thin paper in between two carbon electrodes. Super capacitor have carbon electrodes separated by a dielectric as like that of the normal capacitor and a thin layer of paper in between them, this thin paper doesnt allow the two carbon electrodes to touch each other thus maintaining high value of capacitance. Ultra capacitor are high power and energy dense and they store more energy when compared to normal capacitor. And another difference between battery and super capacitor is super capacitor require less maintenance. Z Source InverterZ source inverter has two inductor and two capacitors before the voltage source inverter, which is in the shape of X. Due to the presence of this impedance circuit it has the capability of shoot through so that mis firing of switches are acceptable, and both buck and boost operation can be done. The input is give through the energy storage and after passing through the impedance network it passes through the voltage source inverter, it consist of six switches where pulses are given by Pi controller. Conventional converter have eight operation modes six active states and two zero states in z source inverter it have an additional state called shoot through state. In active state, the voltage source is directly applied across the load. In Zero state, load is shorted through either the upper switches or the lower switches. In shoot through state load is shorted to either lower and upper switch of one leg or all the three legs.Simulation resultsA 3 phase, 415V, 50HZ supply line can be connected to more than one load, due to the addition of load or due to fault occurring on the distribution side voltage gets distorted this can be cleared with the help of DVR. In this paper fault occurs at the interval of 0.2Sec to 0.4Sec. During that time DVR is activated and they are deactivated during the rest of the time. In this paper DVR is constructed using a energy storage device which is a super capacitor. By using PI controller when fault occurs they provide the error signal for the pulse generator which produces the pulses for the z source inverter which injects the three phase output voltage in to the distribution line via injection transformer.

Fig 7 Simulation result for three phase voltage sag compensation using DVRDue to the addition of load in the distribution line voltage sag is produced in this paper due to the addition of load voltage gets dipped to 50% (200V) at 0.2Sec and it last till 0.4Sec during this period DVR gets activated and injects 215V so that the voltage is compensated. Before the addition of DVR the total harmonic distortion value was 8.12% which is reduce to 3.78%. Fig 8 shows the total harmonic distortion value for voltage sag.

Fig 8 THD value without DVR

Voltage swell can be produced due to the addition of energizing load or removal of load. In this paper addition of energizing load is added and voltage is swelled. The voltage is raised to 20% (500V) during the interval 0.2Sec to 0.4Sec, during this period DVR gets activated and absorbs this voltage and the load voltage is compensated. Before the addition of DVR total harmonic distortion value was 7.72% which is compensated to 3.12%. Fig 10 shows the THD value for swell creation circuit when we use DVR.

Fig 9 Simulation result for three phase Voltage swell compensation using DVR

Fig 10 THD value for swell

Fig 11 Simulation result for compensating harmonics due to the presence of nonlinear load

Due to the addition of nonlinear load (Diode bridge rectifier) in the distribution side harmonics will be caused and load voltage will be distorted. In this paper diode bridge load is added so harmonics are caused. Due to the addition of load the load voltage gets dipped to 50% (200V) during the time period of 0.2Sec to 0.4Sec during this time period DVR gets activated and inject voltage to compensate the distortion. Fig 12 shows the THD value due to the presence of non linear load which is compensated using DVR.

Fig 12 THD value due to presence of non linear load

CasesBefore CompensationAfter Compensation

Voltage Sag8.12%3.78%

Voltage Swell7.72%3.19%

Nonlinear load27.21%4.50%

Table 1 Comparison of THD values for Voltage sag, Voltage swell and Presence of nonlinear load

Table 1 show the total harmonic distortion values when there is no DVR installed in the system for voltage sag, voltage swell and harmonics due to the presence of nonlinear load, which is compensated with the addition of DVR ant the values of THD are analyzed.HARDWARE SETUPThe schematic block diagram of the entire proposed DVR is shown in the figure 13. Here it consist of three single phase transformer to step down 240V to 100V. Which feds the RL load and an addition of nonlinear load which is a diode bridge rectifier is connected for a short period of time. DVR consist of three single phase transformer connected in series to inject voltage into the system. In DVR, Z- Source inverter is connected with super capacitor which provide the DC voltage which is needed as input to the inverter. Super capacitor is used as the energy storage device which when compared with normal capacitor it has fast charging and slow discharging rate also it has less toxic substance so it can be used in any environmental condition.

Fig 13 Overall block diagram of proposed DVRThe control circuit of the proposed system which produce the necessary pulses to the Z-Source inverter. Reference voltage is taken from step down transformer [50/6V] to the PI controller. Where op-amp [LM324] is act as PI controller. Receiving end voltage is taken from load side to PI controller, which can generate the error value for further process. Next stage consist of upper op-amp and lower op-amp. Upper op-amp is used to check the positive voltage error and lower op-amp is used to check negative voltage error. The error values are in analog, which are converted to digital signals by using a diode rectifier. From the receiving end voltage is taken to the amplifier, which amplifies the analog voltage. The high frequency pulses are gate pulses which are given to opto coupler.Supply Transformer(240/100V) is step down transformer feeding 50V non linear load through bridge rectifier. There are many step down transformer for different circuit like control circuit and isolation circuit. In isolation circuit, the transistor requires 12V AC into 12V DC. Supply for the control circuit is given by the step down transformer voltage of 18V for the op-amp (LM324) used. A series injection transformer 950/100V) is used for injecting the voltage when any distortion occurs. The output voltage is taken across the series injecting transformer and non linear load, load voltage will appear.by varying the rheostat to maximum the supply voltage gets dipped, when the change in toggle switch position DVR compensation takes place. For Swellcreation add special type inductor load with RL load keep the load for some moment, when suddenly remove the inductor swell get created with harmonics. Super capacitor is used as the energy storage device each super capacitor is 5F and 2.5V so 10 capacitors are connected in series.

Fig 14 Entire Hardware SetupB. HARDWARE RESULTSThe following graphs shows the experimental results of implementation of DVR system which consist of super capacitor as energy storage device. Fig 15 shows output from the super capacitor.

Fig 15 Output from Super Capacitor

Sag is created by addition of load. DVR is functioned by a switch. Fig 16 Shows the Voltage sag waveform due to the addition of load. And voltage got reduced to 80V

Fig 16 Voltage Sag waveform

Fig 17 Shows the compensated voltage waveform when DVR is activated

Fig 17 Compensated voltage waveform

Fig 18Shows the voltage swell waveform due to the addition of energizing load. Here voltage is increased from 100V to 110V.

Fig 18 Voltage swell waveform

Fig 19Shows the compensated voltage waveform when DVR is activated

Fig 19 Compensated Voltage waveformconclusionThis paper has proposed a DVR that compensate voltage sag, voltage swell and harmonics due to the presence of nonlinear load by using ultra capacitor as the energy storage, instead of conventional energy storage device. A PI controller is designed for DVR to compensate voltage sag, voltage swell and harmonics due to the presence of nonlinear load. PI controller produces the necessary reference voltage to the Space vector modulation block to generate pulses to the z-source converter. PI controller compares the reference voltage with the actual voltage and produce error signal. These signals are compared in the SVM block to produce pulses. The design of the components of the DVR has been presented. Based on the design procedure, DVR is designed and implemented by simulation and experimental results.In this paper, addition of load during the interval 0.2Sec to 0.4Sec. During this fault time voltage distortion takes place, which recovered by DVR. This DVR get activated at 0.2Sec and inject the distorted voltage up to 0.4Sec. During voltage sag condition, the supply voltage 415V dipped to 50% (200V) at 0.2Sec. With the help of DVR compensator, the dipped voltage gets increase to supply voltage (415V and voltage harmonic distortion caused during sag is 8.12%, using DVR it is compensated to 3.78%. Similarly, in voltage swell condition the supply voltage increased to 20% (500V) at 0.2Sec. With the help of DVR compensator, the voltages above the normal supply voltage are absorbed from 0.2Sec to 0.4Sec and Voltage harmonic distortion caused during swell is 7.72%, using DVR it is compensated to 3.19%. Similarly addition of nonlinear load causes the waveform to get distorted and dipped to 200V, with the help of DVR this voltage is compensated to normal supply voltage 415V and Voltage harmonic distortion due to presence of nonlinear load is 27.21%, using DVR it is compensated to 4.50%. The proposed DVR is simulated under different conditions which includes voltage sag, swell and harmonics. The simulation results are validated the operation of the proposed DVR. And also hardware setup is also showed with creating voltage sag by addition of load from 100V to 80V and creating swell from 100V to 110V by capacitive load. And also it shows the compensated voltage waveform of 100V..References1. Bollen MHJ. Understanding power quality problems. Piscataway, N: IEEE Press; 2006.1. Dugan RC, McGranaghan MF, Beaty HW. Electric power systems quality. 2nd ed. New York: McGraw Hill; 2006.1. Arindam Ghosh, Gerard Ledwich, Power quality enhancement using custom power device, Kluwer academic publishers, 2002.1. Lim PK, Dor DS.: Understanding and resolving voltage sag related problems for sensitive industrial customers. IEEE Power Eng Soc Winter Meet 2000;4:288690.1. Rajasekaran, D.; Dash, Subhransu Sekhar; Vignesh, P. Mitigation of voltage sags and voltage swells by Dynamic Voltage Restorer.Proceedings of the 3rd International Conference on Advances in Recent Technologies in Communication 2011;36- 40.1. Bajpai, R.S.; Gupta, R. Series compensation to mitigate harmonics and voltage sags/swells in distributed generation based on symmetrical components estimation. IEEE International symposium on Industrial Electronics (ISIE), 2011, 1639 1644.1. Krischonme Bhumkittipich, Nadarajah Mithulananthan, Performance Enhancement of DVR for Mitigating Voltage Sag/Swell using Vector Control Strategy. Energy Procedia, Volume 9, 2011, Pages 366-379.1. C. Gopinath ,Dr. R. Ramesh, A Novel Method For Voltage Sag / Swell Compensation Using Dynamic Voltage Restorer, Proceedings of the IEEE - International Conference on Advances in Engineering, Science and Management (ICAESM 2012) 30 & 31 March 2012, E. G. S. Pillay Engineering College, Nagapattinam, pp:333-338. 1. Takushi J., Hideaki F., Hirofumi A.: Design and experimentation of a dynamic voltage restorer capable of significantly reducing an energy-storage element, IEEE Trans. on Ind. Appl., 2008, 44, (3), pp. 817825.1. C. Gopinath, A Series Power Quality Conditioning Device for Voltage Harmonics, International Journal of Power Systems and Power Electronics, Vol:4, No:1, pp:14 21, Oct 2011.1. Fang Zheng Peng. Z-Source Inverter IEEE Transactions on Industry Applications, Vol. 39, No. 2, March/April 2003.1. B. Justus Rabi and R. Arumugam, Harmonics Study and Comparison of Z-source Inverter with Traditional Inverters American Journal of Applied Sciences 2 (10): pp:1418-1426, 2005.1. Dong Cao; Shuai Jiang; Xianhao Yu; Fang Zheng Peng, Low-Cost Semi-Z-source Inverter for Single-Phase Photovoltaic Systems, IEEE Transactions on Power Electronics, 2011, Volume:2, Issue: 12, Pp: 3514- 3523. 1. C. Gopinath, Dr. R. Ramesh, Design and Implementation of Z-Source Inverter Based DVR European Journal of Scientific Research Volume 57, Issue 3 (2011), pp: 514 523.1. Ravi Gupta, N K Sharma, P Tiwari*, Astha Gupta, Nitisha Nigam, Anubha Gupta, Application of energy storage devices in power systems International Journal of Engineering, Science and Technology Vol. 3, No. 1, 2011, pp. 289-297.1. Sachin V. Rajani, Dr. R.C. Jha , Prem Prakash,Deeper and extended voltage sag mitigation by dynamic voltage restorer interfaced with ultracapacitor International Journal of Advanced Engineering Technology E-ISSN 0976-3945, march 2012.1. Reza Teymourfar, Behzad Asaei, Hossein Iman-Eini, Razieh Nejati fard,Stationary super-capacitor energy storage system to save regenerative braking energy in a metro line International Journal on Energy Conversion and Management, Volume 56, April 2012, Pages 206214.1. C.Gopinath, Dr. R. Ramesh, Dynamic Voltage Restorer using Ultra Storage Capacitor, Proceedings of the IET UK sponsored International Conference on Sustainable Energy and Intelligent System Conference (SEISCON 2011) Dr. M.G.R University, Chennai, July 20-22, 2011, pp: 69 - 74. 1. J. Ajay Daniel, C. Gopinath, Dr. R. Ramesh, Z Source Inverter Based Dynamic Voltage Restorer to Mitigate Voltage Sag and Voltage Swell. International Conference on Circuits, Power and Computing Technologies, march 21-22, pp: 37-42.1. Choi B, Li D, Vilathaguwa DM. Design and analysis of the inverter side filter used in the dynamic voltage restorer (DVR). IEEE Trans Power Deliv 2002;17(6):105866.1. C. Gopinath, Dr. R. Ramesh, Design an Interline Dynamic Voltage Restorer for Voltage Sag Compensation Using Z- Source Inverter, International Journal of Electrical Engineering, ISSN 0974-2158 Volume 4, Number 5 (2011), pp. 541-5541. C. Gopinath ,Dr. R. Ramesh, Design and Implementation of Interline Dynamic Voltage Restorer, Proceedings of the First International Conference on Enhancements in Power Electronics and Controls (ICEPEC - 2011) PSG College of Technology, Peelamedu, Coimbatore-641 004, Nov 24 25, 2011, pp: 120 126.1. Alejandro G.Yepes, Jano Malvar, Mitigation of voltage sags, imbalances and harmonics in sensitive industrial loads by means of a series power line conditioner, Electric power system research 84(2012) pp:20-30.

ABOUT THE AUTHORS J. AJAY DANIEL has obtained B.E degree in Electrical and Electronics Engineering from Karunya University, Coimbatore and pursuing M.E Power Electronics and Drives from Easwari Engineering College, Chennai.

C.GOPINATH has obtained his B.E degree in Madurai Kamaraj University, Madurai and completed his M.E degree in Power Systems Engineering from Annamalai University, Chidambaram. He has 12 years of teaching experience. He is presently pursuing part time research at Anna University Chennai. He is currently working as Assistant Professor (Selection Grade), Dept of EEE, Easwari Engineering College, Chennai. He has published over 20 papers in International Journals, International & National Conferences. His research area is power quality improvements. He is a life member of ISTE and Associate member of Institution of Engineers.Dr.R.RAMESH pursued his B.E degree in Electrical and Electronics Engineering at University of Madras, Chennai, and completed his M.E degree in Power systems Engineering in Annamalai University, Chidambaram. He received his PhD degree from Anna University, Chennai and has been a faculty of Electrical and Electronics Engineering Department of College of Engineering, Guindy, Anna University Chennai since 2003. His areas of interest are Real-time Distributed Embedded Control, On-line power system analysis and Web services. He has published several papers in International Journals, International and National Conferences. He is a life member of ISTE and Member of Institution of Engineers.