UNIFIED POWER QUALITY CONDITIONER IN DISTRIBUTION SYSTEM FOR ENHANCING POWER QUALITY

International Journal of Electrical Engineering & Technology (IJEET) Volume 7, Issue 6, Nov Dec, 2016, pp.55 63, Article ID: IJEET_07_06_005 Available online at http://www.iaeme.com/ijeet/issues.asp?jtype=ijeet&vtype=7&itype=6 ISSN Print: 0976-6545 and ISSN Online: 0976-6553 Journal Impact Factor (2016): 8.1891 (Calculated by GISI) www.jifactor.com IAEME Publication 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 http://www.iaeme.com/ijeet/index.asp 55 editor@iaeme.com

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 http://www.iaeme.com/ijeet/index.asp 56 editor@iaeme.com

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 harmonics, unbalances and power factor correction. It maintains the constant voltage across the dc link. The shunt compensator consists of a voltage source inverter. The Dc side of this voltage source inverter is connected to 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 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 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 the Dc voltage to Ac voltage. These inverters will generate the harmonics. The harmonics generated by these inverters are filtered by using passive filters. 3. DC-DC CONVERTER A 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 converters are of three types Buck converter, Boost converter, 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 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 of an inductor to resist changes in current by creating and destroying the magnetic field. Figure 2 DC-DC Converter The output voltage of this 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 with shunt inverters. In 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 parallel with a diode as shown in figure.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 converter is given to the DC-AC voltage source inverter which converts this DC voltage to AC and fed the AC distribution system. http://www.iaeme.com/ijeet/index.asp 57 editor@iaeme.com

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 '(= ( (*) ) '+=,+,) (-) '= ) (.) 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. are http://www.iaeme.com/ijeet/index.asp 58 editor@iaeme.com

Unified Power Quality Conditioner In Distribution System For Enhancing Power Quality 6. SINGLE LINE DIAGRAM OF PROPOSED TEST SYSTEM Figure 4 Single line Diagram of System The proposed test model consists of three buses. The three ends 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 with DC link. The series compensator is used to compensate the voltage disturbances 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 the constant active power flow between series and shunt compensator. The 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. Figure 5 Block Diagram of Proposed Test Model http://www.iaeme.com/ijeet/index.asp 59 editor@iaeme.com

A.S. N. Anjali Devi and G. Madhavi 7. SYSTEM PARAMETERS 8. SIMULATION RESULTS a) Without UPQC PARAMETERS VALUES Three Phase Source RATED VOLTAGE (V) 66KV RATED FREQUENCY (F) 50Hz RESISTANCE (R) 0.18ohm INDUCTANCE (L) Asynchronous Machine POWER VOLTAGE SPEED ACTIVE POWER REACTIVE POWER TRANSFORMER-1 TRANSFORMER-2 TRASFORMER-3 TRANSFORMER-4 PROPORTIONAL GAIN INTEGRAL GAIN Load 1e-3H 10KW 220V 1500RPM 10e3 1e3 Y-G/Y-G POWER-1MVA VOLTAGE-66/33KV Y-G/Y-G POWER-1MVA VOLTAGE-66/3.3KV Y-G/Y-G POWER-1MVA VOLTAGE- 33KV/800V Kp=0.5 Ki=0.5 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. http://www.iaeme.com/ijeet/index.asp 60 editor@iaeme.com

Unified Power Quality Conditioner In Distribution System For Enhancing Power Quality 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. http://www.iaeme.com/ijeet/index.asp 61 editor@iaeme.com

A.S. N. Anjali Devi and G. Madhavi 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%. REFERNCES [1] Raphael j. Millnitz dos santos, A Simplified Control Technique for a Dual Unified Power Quality Conditioner, IEEE Transactions on Industrial Electronics, 61(11), November - 2014. [2] Sanjib Ganguly, Impact of Unified Power-Quality Conditioner Allocation on Line Loading, Losses, and Voltage Stability of Radial Distribution System, IEEE Transactions On Power Delivery, 29(4), August - 2014. [3] Khadkikar, Enhancing electric power quality using UPQC: A comprehensive overview, IEEE Trans. Power Electron, 27(5), pp.2284 2297, May -2012. [4] Ahmed Teke, Mehmet Tumey, A Novel reference signal generation for power-quality conditioner, IEEE Trans on power delivery, 26(4), October -2011 [5] Zhilei Yao and Lanxiao, Control of single phase grid connected inverters with non linear load, IEEE Trans on INDU Electronic, 60(4), April 2013. http://www.iaeme.com/ijeet/index.asp 62 editor@iaeme.com

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