Mitigation of voltage sags/swells unbalanced in low voltage distribution systems

Transcription

1 International Journal of Science and Advanced Technology (ISSN ) Volume 1 No 6 August 211 Mitigation of voltage sags/swells unbalanced in low voltage distribution systems M. N. Tandjaoui, C. Benachaiba, O. Abdelkhalek Department of Technology, University of Bechar University of Bechar Algeria Bechar Algeria Tanjaoui_8@yahoo.fr, chellali99@yahoo.fr M. L. Doumbia Department of Electrical and Computer Engineering, University of Québec (Trois-Rivières) CANADA doumbiam@uqtr.ca Abstract The main problems of the power quality like voltage sags/swells in low voltage distribution systems and on the transmission side due to sensitive loads, the terminology used for the compensation devices is different. Dynamic Voltage Restorer (DVR) is one of the equipments for voltage disturbance mitigation in power systems. It is installed between the incoming supply and the sensitive loads to maintain the voltage at the sensitive load from balance. This paper proposes a mitigation of voltage sags/swells in low voltage distribution system using an effective series compensator (DVR). The compensator should protect sensitive loads against of voltage sags/swells. Performance of the proposed method is investigated under different types of fault in both single phase and three phases for various sensitive load conditions. The simulation was carried out with the help of SIMULINK & MATLAB and the results show appropriate operation of the proposed control system. I. INTRODUCTION Power quality problems like voltage sag, voltage swell and harmonic are major concern of the industrial and commercial electrical consumers due to enormous loss in terms of time and money. This is due to the advent of a large numbers of sophisticated electrical and electronic equipment, such as computers, programmable logic controllers, variable speed drives, and so forth. The use of this equipment often requires very high quality power supplies [1, 2]. Voltage sag/swell that occurs more frequently than any other power quality phenomenon is known as the most important power quality problems in the power distribution systems. IEEE and IEEE describe the voltage sags /swells as shown in Figure1. Voltage sag is defined as a sudden reduction of supply voltage down from 9% to 1% of nominal. According to the standard, a typical duration of sag is l ms to 1 minute. On the other hand, voltage swell, is defined as a sudden increasing of supply voltage up1l% to 18% in rms voltage at the network fundamental frequency with duration from 1 ms to 1 minute. Voltage sag/swell often caused by faults such as single line-toground fault, double line-to-ground fault on the power distribution system or due to starting of large induction motors or energizing a large capacitor bank. Voltage sag/swell can interrupt or lead to malfunction of any electric equipment that is sensitive to voltage variations [5]. Keyword: Power quality, voltage quality, DVR, voltage disturbance, FACTS. Figure 1. Voltage Reduction Standard of IEEE Std There are many different methods to mitigate voltage sags and swells, but the use of a custom Power device is considered to be the most efficient method. The concept of custom Power was introduced by N.G. Hingorani in Like Flexible AC Transmission Systems (FACTS) for transmission systems, the term custom power pertains to the use of power electronics controllers in a distribution system, especially, to deal with various power quality problems [4, 7]. Each of Custom Power devices has its own benefits and limitations. The most effective type of these devices is considered to be the Dynamic Voltage Restorer (DVR). It is a series custom power device, which has excellent dynamic capabilities. It is well suited to protect sensitive loads from duration voltage sag or swell. A DVR is basically a controlled voltage source installed between the supply and a sensitive load. It injects a voltage on the system in order to compensate any disturbance affecting the load voltage. In August 1996, Westinghouse Electric Corporation installed world s first dynamic voltage restorer in Duke Power Company s kv substation in Anderson, South Carolina. This was installed to provide protection to an automated rug 46

2 International Journal of Science and Advanced Technology (ISSN ) Volume 1 No 6 August 211 manufacturing plant. Prior to this connection, the restorer was first installed at the Waltz Mill test facility near Pittsburgh for full power tests. Another was installed to provide service to a large dairy food processing plant in Australia [6, 2]. This paper Introduces Dynamic Voltage Restorer (DVR) and its operating principle. Then, analyses of the voltage compensation methods are presented. At the end, simulation results using MATLAB are illustrated and discussed. II. DYNAMIC VOLTAGE RESTORER A Dynamic Voltage Restorer (DVR) is a recently proposed series connected solid state device. It is normally installed in a distribution system between the supply and the critical load feeder. Its primary function is to rapidly boost up the load-side voltage in the event of a disturbance in order to avoid any power disruption to that load [7]. It injects a voltage into the system in series with the distribution feeder and in synchronicity with the voltages of the distribution system to regulate the load side voltage [8]. The injecting voltages of controllable amplitude, phase angle, and frequency (harmonic) in to the distribution feeder in instantaneous real time via a series-injection transformer (Figure 2). The missing voltage is the difference between the nominal voltage and the actual, it s calculated by a control systems and it send to switchers of converter source converter (VSC) to inject in series in low voltage distribution systems. voltage controller. The VSC is supplied by a dc source of energy storage systems. With this, the DVR can regulate the load voltage at any given magnitude and phase angle. This can be accomplished through real power exchange between the dc source and the ac system through the inverter [13]. There are various circuit topologies and control schemes that can be used to implement a DVR. In addition to voltage sags and swells compensation, DVR can also perform other tasks such as: line voltage harmonics compensation, reduction of transients in voltage and fault current limitations [7]. III. BASIC PRINCIPLE The basic idea of a DVR is to inject the missing voltage cycles vinj(t) into the system through series injection transformer whenever voltage sags/swells are present in the system supply voltage. As a consequence, sag and swell are unseen by the loads. During normal operation, the capacitor receives energy from the energy storage systems. When voltage sags or swells are detected (figure 3), the capacitor delivers DC supply to the converter (VSC). The VSC ensures that only the missing voltage is injected to the transformer to improvement the voltage of sensitive load [8]. Figure 3. Typical schematic of a power system compensated by DVR A relatively small capacitor is present on dc side of the PWM solid state inverter and the voltage over this capacitor is kept constant by exchanging energy with the energy storage reservoir. Figure 2. Typical DVR circuit topology The VSC is a device power electronics employs IGBTs that are operated in a pulse-width modulated (PWM) fashion. The switching pulses of the IGBT are the output from the PI 47

3 International Journal of Science and Advanced Technology (ISSN ) Volume 1 No 6 August 211 Figure 4. Compensation strategy of DVR for voltage sags. A relatively small capacitor is present on dc side of the PWM solid state inverter and the voltage over this capacitor is kept constant by exchanging energy with the energy storage reservoir. The required output voltage is obtained by using pulsewidth modulation switching pattern. As the controller will have to supply active as well as reactive power, some kind of energy storage is needed. In the DVRs that are commercially available now, large capacitors are used as a source of energy. Other potential sources are being considered are: battery banks, superconducting coils, and flywheels.[8] Figs. 4 show a simplified single-line diagram of compensation strategy of medium voltage DVR for voltage sag. During the fault condition, medium voltage DVR will inject both active and reactive power and make appropriate voltage and current compensation [12]. This will thus restore both magnitude and phase of the load voltage. Assuming that load voltage and current in pre-fault conditions are both equal to 1 p.u., the power injected by DVR during voltage sag/swell mitigation is equal to: IV. SIMULATION RESULTS A detailed system as shown in Figure 5 has been modeled by MATLAB/SIMULINK to study the efficiency of suggested control strategy. The system parameters and constant value are listed in Table I. It is assumed that the voltage magnitude of the load bus is maintained at 1 pu during the voltage sags/swells condition. TABLE I PARAMETERS VALUES OF THE SIMULATIONS MODEL Symbol Quantity Parameter Input voltage 22V (3) (4) where is phase shift angle and θ is power factor angle. Observe that power absorbed by the load is given by (1) (2) f Frequency 5Hz P load Active power load 8 W Q load Reactive power load 7 VAR Vdc DC voltage V Rs Resistance of series filter.2 Ω Ls Inductance of series filter mH Cs series filter capacity µf Ki Integrator gain.462 Kp Proportional gain where V load is 1 and I* load is e jθ, therefore, the active and reactive power injected by DVR is given by (3) and (4), respectively. The results of the most important simulations are represented in Figures A DVR is connected to the system through a series transformer with a rapport transformation equal to 1:1. Figure 5. Simulink model for the proposed DVR coupling in power system using Matlab/Simulink 48

4 International Journal of Science and Advanced Technology (ISSN ) Volume 1 No 6 August 211 The DVR is based on three phase voltage PWM inverter with LC output filter to remove high frequency voltage components. An R-L load (R=1Ω, L=1-6H) is considered. A case of single and two phase voltage sag are simulated and the results are shown in Figure 6and 7. Figure 6 shows a 6% voltage sag in one phase and figure 7 shown a voltage dips of 4% in two phases initiated at 4 ms and it is kept until 8 ms, with total voltage sag duration of 4 ms. Figure 6/7 and show the voltage injected by the DVR and the compensated load voltage, respectively Figure 6. Single phase voltage sag;.source voltages,.injected voltages;. Load voltages Figure 7. Two phase voltage sag;.source voltages,.injected voltages;. Load voltages As a result of DVR, the load voltage is kept at 1 p.u. throughout the simulation, including the voltage sag period. Observe that during normal operation, the DVR is doing nothing. It quickly injects necessary voltage components to smooth the load voltage upon detecting voltage sag. As can be seen from the results, the DVR is able to produce the required voltage components for different phases rapidly and help to maintain a balanced and constant load voltage at 1.p.u. The performance of DVR for a voltage swell condition is investigated. Here, the supply voltage swell is generated as shown in Figure 8. The supply voltage amplitude is increased about 12% of nominal voltage. The injected voltage that is produced by DVR in order to correct the load voltage and the load voltage are shown in Figure 8 and, respectively Figure 8. Single phase voltage swell;.source voltages;.injected voltages;.load voltages Figure 9. two phase voltage swell;.source voltages;.injected voltages;.load voltages As can be seen from the results, the load voltage is kept at the nominal value with the help of the DVR. Similar to the case of voltage sag, the DVR reacts quickly to inject the appropriate 49

5 International Journal of Science and Advanced Technology (ISSN ) Volume 1 No 6 August 211 voltage component (negative voltage magnitude) to correct the supply voltage. Notice the constant and balanced voltage at the load throughout the simulation, including during the unbalanced voltage swell event. In this case, during the simulations, the voltage supply is unbalanced in two electrical parameters, first in amplitude of the voltage (Figure 1), order in frequency of supply (figure 11). The effectiveness of the DVR from this disturbance is shown in Figure 1 and 11. Finally, figure 1 and 11, shows once again the same waveforms of the voltage load. 1 Figure 1. Unbalanced of the voltage amplitude;.source voltages;.injected voltages;.load voltages Figure 11. Unbalanced in frequency supply;.source voltages;.injected voltages;.load voltages V. CONCLUSION In this paper, performance of a DVR in power quality is demonstrated with the help of MATLAB. The impact of voltage sags/swells on sensitive equipment is severe. Therefore, DVR is considered to be an efficient solution due to its low cost, small size, and fast dynamic response. DVR is an effective recent custom power device for voltage sags and swells mitigation. A forced commutated voltage sources converter is considered in the DVR along with energy storage to maintain the capacitor voltage. The simulation results show clearly the performance of a DVR in mitigating voltage sags and swells. In the case of voltage sag, which is a condition of a temporary reduction in supply voltage, the DVR injects an equal positive voltage component in all three phases, which are in phase with the supply voltage to correct it. On the other hand, for a voltage swell case, which is a condition of a temporary increase in supply voltage, the DVR injects an equal negative voltage in all three phases which are anti-phase with the supply voltage. For unbalanced conditions, the DVR injects an appropriate unbalanced three-phase voltage component positive or negative depending on whether the condition is an unbalanced voltage sag or unbalanced voltage swell. The DVR handles both balanced and unbalanced situations without any difficulties and injects the appropriate voltage component to correct rapidly any anomaly in the supply voltage to keep the load voltage balanced and constant at the nominal value. REFERENCES [1] H. Ezoji, A. Sheikholeslami, M. Tabasi, M.M. Saeednia, "Simulation of Dynamic Voltage Restorer Using Hysteresis Voltage Control", EJSR, Vol.27 No.1, pp , 29. [2] S.V Ravi Kumar, S. Siva Nagaraju, Simulation of D-STATCOM and DVR in power systems, ARPN Journal of Engineering and Applied Sciences, VOL. 2, NO. 3, JUNE 27. [3] M.R. Hosseini, S.H. Khanmohamadi, S. Gharehpetian, Verification of a new energy control strategy for dynamic voltage restorer by simulation. Simulation Modelling Practice and Theory (ELSEVIER), 25, pp: [4] C. BENACHAIBA, B. FERDI, Voltage Quality Improvement Using DVR, Electrical Power Quality and Utilisation, Journal, Vol. XIV, No. 1, 28. [5] P. Boonchiaml, P. Apiratikull, N. Mithulananthan, Detailed Analysis of Load Voltage Compensation for Dynamic Voltage Restorers. TENCON IEEE Nov. pp: 1-4. [6] Ghosh,A. Ledwich,G. Structures and Control of A Dynamic Voltage Regulator (DVR), IEEE Confrance, pp: [7] C. Benachaiba, B. Ferdi, Power Quality Improvement Using DVR, American Journal of Applied Sciences 6 (3): 396-4, 29. [8] S R. Omar, N.A Rahim, Compensation of Different Types of Voltage Sags in Low Voltage Distribution System Using Dynamic Voltage 5

6 International Journal of Science and Advanced Technology (ISSN ) Volume 1 No 6 August 211 Restorer, Australian Journal of Basic and Applied Sciences, 4(8): , 21. [9] R. Omar, N. A Rahim, M. Sulaiman, New Control Technique Applied in Dynamic Voltage Restorer for Voltage Sag Mitigation, American J. of Engineering and Applied Sciences 3 (1): 42-48, 21. [1] T. I. El-Shennawy, A. Moussa, M. A. El-Gammal, A. Y. Abou-Ghazala, A Dynamic Voltage Restorer for Voltage Sag Mitigation in a Refinery with Induction Motors Loads, American J. of Engineering and Applied Sciences 3 (1): , 21. [11] J O. Viktorin, J.Driesen, Member, IEEE, R.Belmans, Senior Member, IEEE, The Prototype of a Single-phase Dynamic Voltage Restorer. [12] S P. Boonchiam, N. Mithulananthan, Diode-clamped Multilevel Voltage Source Converter Based on Medium Voltage DVR, International Journal of Electrical Power and Energy Systems Engineering, 28. [13] A. Ghosh, Senior Member, IEEE, A. K. Jindal, Student Member, IEEE, A. Joshi, Design of a Capacitor-Supported Dynamic Voltage Restorer (DVR) for Unbalanced and Distorted Loads, IEEE Transactions on Power Delivery, Vol. 19, No. 1, January 24. Mohammed Nasser Tandjaoui received the state engineer degree in Electrical Engineering in 25 from the University of Sciences and Technology of Oran (USTO). He was born here Magister in electrical engineering in 29 from university of Bechar, Algeria. He currently was holding the post of Assistant maitre in university of bechar. He was preparing a Doctorate of improvement of the quality of energy electric in a wind network by the integration of FACTS systems. His research area interests are power electronics, FACTS, HVDC, power quality issues, renewable energy and energy storage. Address: Bechar University Center BP 417 Bechar 8, Algeria. address: tanjaoui_8@yahoo.fr Benachaiba Chellali received the state engineer degree in Electrical Engineering in 1987 from the University of Boumerdes (INH) and the M.S. degree in 1996 from Bechar University Center, Algeria. In 25 he received the doctorate degree from the University of Sciences and Technology of Oran (USTO), Algeria and currently Professor. His current research and teaching interests are in the areas of power quality improvement, active power filters and renewable energy. Presently he is supervising five doctoral students working in the field of power quality and renewable energy. Address: Bechar University Center BP 417 Bechar 8, Algeria. address: chellali@netscape.net Othmane Abdelkhalek was born in Taghit, Bechar (Algeria) in He received the Eng. degree from Bechar University Center in 21, the Magister degree from Sidibel-Abbes University in 24 and the doctorate from the University of Bechar in 21. He is a member in the Laboratory of Physics and Semiconductor Devices. His research area interests are Power electronic, Power quality, Active filtering, DVR, UPQC, UPFC, Control, Digital control, Load flow Optimization. 51