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1 834 VOLTAGE SAG AND SWELL MITIGATION USING CUSTOM POWER DEVICE JYOTHILAL NAYAK BHAROTHU 1 Asst.professor & Head, Department of Electrical & Electronics Engineering, Columbia Institute of Engineering & Technology, Raipur, C.G; India nayakeee@gmail.com ABSTRACT A Power quality problem is an occurrence manifested as a nonstandard voltage, current or frequency that results in a failure or a mis-operation of end user equipments. Utility distribution networks, sensitive industrial loads and critical commercial operations suffer from various types of outages and service interruptions which can cost significant financial losses. With the restructuring of power systems and with shifting trend towards distributed and dispersed generation, the issue of power quality is going to take newer dimensions. In developing countries like India, where the variation of power frequency and many such other determinants of power quality are themselves a serious question, it is very vital to take positive steps in this direction. The present work is to identify the prominent concerns in this area and hence the measures that can enhance the quality of the power are recommended. This work describes the techniques of correcting the supply voltage sag, swell in a distributed system. Reactive power compensation is an important aspect in the control of distribution systems. Reactive current in addition to increasing the distribution system losses, introduces various power quality problems like, harmonics, voltage sag, swell and poor load power factor. These power quality issues result in malfunction of sensitive equipment. And rectifying the same using custom power device Keywords: power quality, voltage, current, harmonics, mat lab. 1. INTRODUCTION The presentation starts with the necessity of quality in electrical power by pointing the causes and consequences that isolates the electrical power from its quality problems like VOLTAGE SAGS AND SWELLS, HARMONICS. Various solutions from the innovative technology, those packs the power with its at most quality are presented. Not only are various guidelines to maintain the power quality but also the analysis and monitoring of power quality vividly highlighted. 1.1POWER QUALITY: In its broadest sense, power quality is a set of boundaries that allows electrical systems to function in their intended manner without significant loss of performance or life. The term is used to describe electric power that drives an electrical load and the load's ability to function properly with that electric power. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor quality power. To improve the power quality some devices need to be installed at a suitable locations. These devices are called custom power devices. Which make sure that customers get pre specified quality and reliability of supply.the compensating devices compensate a load, i.e. its power factor, unbalance conditions or improve the power quality of supplied voltage, etc Power quality is simply the interaction of electric power with electrical equipment. So we have to maintain suitability of electrical power which is helpful to run the electrical equipment is more reliable& without any stress on it. In this presentation we can mitigate power transients which will causes the quality problems in electric power like voltage sag and voltage swell. Modern power systems are complex networks, where hundreds of generating stations and thousands of load centres are interconnected through long power transmission and distribution networks. The main concern of consumers is the quality and reliability of power supplies at various load centres where they are located at. Even though the power generation is fairly reliable, the quality of the supply is not so reliable. But the consumer should provide with good quality of power with proper magnitude of voltage and frequency. Power quality of a supply voltage should be maintained near a pure sinusoidal. But the voltage waveform may not be sinusoidal because presence of non linear loads or occurrences of fault like L-G fault or L-L-G fault or L-L-L-G fault or sudden change in load which will results in increase or decrease in voltage magnitude. Then the electrical equipment will be affected by the power quality issues. To avoid this we will use compensating devices. In this presentation we are applying DSTATCOM (Distribution Static Synchronous Compensator) for voltage sag and swell compensation. 2.1 Power Quality Problems 1. Short duration voltage variations, 2.Long duration voltage variations, 3.Voltage imbalance,4.wave form distortion,5.voltage fluctuation,6. Power frequency variations

2 835 Power quality is most common concern for power utilities as well as for consumers. Today, the world needs increased amount of quality power for its growing population and industrial growth. Voltage sag is a frequently occurring power quality problem. Voltage sag has been defined as reduction in the root mean square (RMS) voltage in the range of 0.1 to 0.9 per unit (p.u.) for duration greater than half a cycle and less than one minute [It may be caused by faults, increased load demand and transitional events such as large motor switching. Voltage sags (also known as voltage dips) can cause loss of production in automated processes, since a voltage sag can trip a motor or cause its controller to malfunction. Normally, the short duration voltage variations are occurring in power system. So we have to mitigate these variations. 2.2 Short Duration Voltage Variations a) Voltage sag: is a fundamental frequency decrease in the supply voltage for a short duration (5 cycles to 1 minute). Fig 2.1 Balanced Voltage Sag Fig 2.2 Unbalanced Voltage Sag b) Voltage Swell: is defined as increasing of fundamental frequency voltage for short duration. Fig 2.3 Balanced Voltages Swell Fig 2.4 Unbalanced Voltages Swell An interruption occurs when the supply voltage (or load current) decrease to less than.1 per unit for period of time not exceeding one minute. 2.3 Reactive Power in Voltage Regulation Voltage Disturbances Voltage sag or dip represents a voltage fall to 0.1 to 0.9 p.u and existing for less than one minute and voltage swell is the rise in voltage of greater than 1.1p.u and exists for less than one minute Voltage Control by Reactive Power Compensation Fig 2.5 Uncompensated Line with Single Load First we consider an uncompensated line. The current drawn by the load depends on the load itself and the line voltage. The current engenders the voltage drops in the transformer and the line reactance. It results in decrease in transmission voltage VT and the distribution voltage VD. The above figure shows the vector diagram for a single load centre connected to the uncompensated line. The voltage drop in the line mainly depends on the current taken by the load as well as the resistance and inductance in the line. It can also be seen that the angle between voltage and current is also playing a major role in maintaining the voltage. Let us consider that the supply voltage is E. Now due to the voltage drops IR and IX the load voltage is V

3 836 Fig 2.6 Compensated Line with Single Load It is possible to bring V=E, just by making the current to lead so that vector diagram will get modified as by the use of shunt compensation. The same principle can be used in case of capacitive load also. If the load is capacitive a lagging current will help in regulation of voltage Voltage Sag/Swell Detection Technique In order to compensate the voltage sag or swell which is obtained due to presence of non linear loads or occurrence of any type of fault we have to identify whether sag occurs or swell occurs. Depending on that we have to apply the compensation technique. To identify sag or swell we have so many techniques.among them one of the most important technique is monitoring the peak value of the supply. Fig 2.7 under Normal Condition If we consider the above waveform the magnitude of voltage is to be maintained at 350v.Let us consider our analysis in the time interval from 0 sec to.025 sec. If we maintain the voltage at constant magnitude then the equipment will not be affected if a fault occurs in the interval in between.015 sec to.025 sec. Then we can t maintain the voltage at 350v.Depending on the fault effect the magnitude of voltage will be affected as shown in below figure. Fig 2.8 under Fault Condition Due to the fault the voltage magnitude is reduced to 200v from 350v.To compensate this we can use compensating device to avoid the problem. In the interval from sec to sec we will apply the compensating device. So the required magnitude of voltage is injected as shown in below figure. Fig 2.9 So we can maintain the voltage magnitude as constant. In this way by monitoring the peak values we can identify the sag/swell and we can rectify it. 3. INTRODUCTION TO FACTS: Flexible AC Transmission Systems (FACTS) is defined as alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability. The FACT controller is defined as a power electronic based system and other static equipment that provide control of one or more Ac transmission system parameters. Depending on the power electronic devices used in the control, the FACTS controllers can be classified as a. Variable Impedance Type., b.voltage Source Converter (VSC) based. The FACTS controllers based on VSC have several advantages over the variable impedance type. For example, a STATCOM is much more compact than a SVC for similar rating and is technically superior. It can supply required reactive current even at low values of the bus voltage and can be designed to have in built short term

4 837 overload capability. Also, a STATCOM can supply active power if it has an energy source or large energy storage at its DC terminals. D-STATCOM is one of the FACTS device so by implementation of D-STATCOM in distribution system we can mitigate short duration voltage variations like voltage sag and voltage swell of dynamic and non linear loads. In case of sensitive and critical load we use DVR & SSTS respectively. The following figure depicts the custom power distribution system. Fig.3.1: Custom power Distribution System 3.1 ABOUT DSTATCOM VSC (Voltage Source Converter i.e., it acts like an inverter or a rectifier depending on the condition) is the backbone of STATCOM and it is a combination of self-commutating solid-state turn-off devices (viz. GTO, IGBT, IGCT and so on) with a reverse diode connected in parallel to them. The solid-state switches are operated either in square-wave mode with switching once per cycle or in PWM mode employing high switching frequencies in a cycle of operation or selective harmonic elimination modulation employing low switching frequencies. A DC voltage source on the input side of VSC, which is generally achieved by a DC capacitor and output, is a multi-stepped Ac voltage waveform, almost a sinusoidal waveform. The turn-off devices make the converter action, where as diode handles rectifier action. STATCOM is essentially consisting of six-pulse VSC units, DC side of which is connected to a DC capacitor to be used as an energy storage device, interfacing magnetic (main coupling transformer and/or intermediate/or inter-phase transformers) that form the electrical coupling between converter AC output voltage waveforms at the point of common coupling (PCC) to regulate reactive current flow by generation and absorption of controllable reactive power by the solid-state switching algorithm. As STATCOM has inherent characteristics for real power exchange with a support of proper energy storage system, operation of such controller is possible in all possible in all four quadrants of P-Q plane and it is governed by the following power flow relation Where S is the apparent power flow, P the active power flow, Q is the reactive power flow, Vs is the main AC phase voltage to neutral (rms), Vc the STATCOM fundamental output AC phase voltage (rms), X (=wl, where, w=2πf), the leakage reactance, L the leakage inductance, f the system frequency and α the phase angle between Vs and Vc. Active power flow is influenced by the variation of α and reactive power flow is greatly varied with the magnitude of the phase voltage between Vc and Vs. For lagging α, the power (P) flows from Vs to Vc and for α=0, the power P is zero and Q is delivered from equation as follows `...3.2

5 838 The AC voltage output (Vc) of STATCOM is governed by DC capacitor voltage (Vdc) and it can be controlles by varying phase difference (α) between Vc and Vs (and also my m, modulation index for PWM control). Functionally, STATCOM injects an almost sinusoidal current (I) in quadrature (lagging or leading) with the line voltage (Vs), and emulates as an inductive or capacitive reactance at the point of connection with the electrical syatem for reactive power control, and it ideally the situation when amplitude of Vs is controlled from full leading (capacitive) to full lagging (inductive) for equals to zero (i.e., both Vs and Vc are in the same phase). The magnitude and phase difference (α) between Vc and Vs across the leakage inductance (L), which in turn controls reactive power flow and DC voltage, Vdc across the capacitor. When, Vc>Vs the STATCOM is considered to be operating in a capacitive mode. When Vc<Vs it is operating in an inductive mode and for Vc=Vs, no reactive power exchange takes place. In the hign rating STATCOM operating under fundamental frequency switching, the principle of phase angle control (α) is generally adopted in control algorithm to compensate converter losses by active power drawn from AC system and also for power flows in or out of the VSC to on directly control the magnitude of DC voltage with charging or discharging of DC bus capacitor enabling control of reactive power flow into the system. 3.2 Modelling of DSTATCOM It is assumed that the source is a balance, sinusoidal three-phase voltage supply with the frequency ω. Since reactive power compensation is desired, it is convenient for this analysis to take the angle of the input the reference angle. However the system is designed based on following assumptions. 1. The three AC mains voltages are balanced, 2.The three - phase load is balanced and linear, 3.The inverter switches are ideal, 4.DC link output is ripple free, and 5.The filter components are reactive and linear A DSTATCOM consists of a two-level Voltage Source Converter (VSC), a DC energy storage device, a coupling transformer connected in shunt to the distribution network through a coupling transformer. The VSC converts the DC voltage across the storage device into a set of three-phase AC output voltages. These voltages are in phase and coupled with the AC system through the reactance of the coupling transformer. Suitable adjustment of the phase and magnitude of the DSTATCOM output voltages allows effective control of active and reactive power exchanges between the DSTATCOM and the AC system. Such configuration allows the device to absorb or generate controllable active and reactive power. A single-phase equivalent circuit of DSTATCOM is shown in Fig Where, R c is included to represent small losses in the switching devices of VSC. R s and L represent the equivalent circuit of the tie-transformer between system voltages U s and the output voltage U I of DSTATCOM. Fig.3.2 Single-phase equivalent circuit of DSTATCOM The U sa, U sb, U sc are defined as instantaneous values of system phase voltage and can be given by: Where, U s is rms value of system phase voltage. The output voltages of DSTATCOM, u ja, u ib and u ic can be given by: Fig 3.3.Vector diagram of D-STATCOM (a) Capacitive mode, (b) Inductive mode, (c) Active power release and (d) Active power absorption

6 839 where, KT is turn s ratio of the tie-transformer, m is the amplitude modulation ratio of VSC output voltage; its value depends on the type of VSC. U DC is the DC-link capacitor`s voltage of VSC and is the phase angle difference between voltage u s and u i. 3.3 VOLTAGE SAG CORRECTION BY A D-STATCOM The schematic diagram of a D-STATCOM is shown in following fig3.2. In this diagram, the shunt injected current Ish corrects the voltage sag by adjusting the voltage drop across the system impedance &h. The value of Ish can be controlled by adjusting the output voltage of the converter. The shunt injected current Ish can be written as The complex power injection of the D-STATCOM can be expressed as It may be mentioned here that the effectiveness of the DSTATCOM in correcting voltage sag depends on the value of Z th or fault level of the load bus. When the shunt injected current I sh is kept in quadrature with V L, the desired voltage correction can again' be achieved without injecting any active power into the system. On the other hand, when the Value of I sh is minimized; the same voltage correction can be achieved with minimum apparent power injection into the system. The voltage sag correction by a D-STATCOM using the above two techniques is discussed in the following. 3.4 A Zero Active Power Injection (ZAPI) In this case, the D-STATCOM is not injecting any active power into the system. Thus the entire load active power (P L ) must be provided by the Thevenin s equivalent of the system. The active power flow through the Thevenin s impedance of above Fig.3.2 (At load side) can be written as From the above eqn. the angle δ can be expressed as For a feasible value of δ, the condition must be satisfied. The above constraint can be rewritten as Thus, when the system voltage magnitude satisfies the above eqn. the D-STATCOM can correct the voltage sag without injecting any active power into the system. For such a case, the injected complex current and apparent power of the DSTATCOM can easily be found from eqns. (3.3) and (3.4), respectively. Note that the injected apparent power will have only the reactive component. 3.5 Minimum Apparent Power Injection (MAPI) As mentioned earlier that, when the magnitude of the injected current is minimized, the D-STATCOM can correct the voltage sag with minimum apparent power injection into the system. Thus the condition of minimum apparent power injection is An analytical expression of Ish can readily be obtained from eqn. (3.3), and the solution of eqn. (3.9) provides the following

7 840 Thus for a given load, the value of δ can easily be found from eqn. (4). Once the value of δ is known, the complex current and apparent power injection of the D STATCOM can again be obtained from eqns. (3.3) and (3.4), respectively WITHOUT DSTATCOM: 3.7. WITH DSTATCOM: 3.8 WITHOUT DSTATCOM: 3.9 WITH DSTATCOM

8 CONCLUSIONS With the help of DSTATCOM two power quality problems namely Voltage sag and Voltage swell are mitigated and the simulation results are verified. The use of computer programs in the simulation of Custom Power (CP) controllers (DSTATCOM), including their controls, is extremely important for the development and understanding of this power electronics based technology. The results achieved through the digital simulations clearly show the capability of the DSTATCOM to mitigate voltage sags providing a continuously variable level of shunt compensation of voltage sags and swells. ACKNOWLEDGEMENTS I Thank to our college principal Dr. TVV Sudhakar for his kind permission and encouragement to write research paper. Also I am very much thankful to our college Chairman Mr. Kishore Jadwani, vice chairman Mr. Vijay Jadwani, Secretary Mr. Harjeet Singh Hura for providing creative environment for this work. I would like to extend my heartfelt thanks to my colleagues. And finally I am very much obliged to my respected parents who inspiring around the clock. REFERENCES [1] G. Yaleinkaya, M.H.J. Bollen, P.A. Crossley, Characterization of voltage sags in industrial distribution systems, IEEE transactions on industry applications, vol.34, no. 4, July/August, pp , [2] Haque, M.H., Compensation of distribution system voltage sag by DVR and D-STATCOM, Power Tech Proceedings, 2001 IEEE Porto, vol.1, pp.10-13, Sept [3] Anaya-Lara O, Acha E., Modeling and analysis of custom power systems by PSCAD/EMTDC, IEEE Transactions on Power Delivery, Vol.17, Issue: 1, Jan. 2002, Pages: [4] Bollen, M.H.J., Voltage sags in three-phase systems Power Engineering Review, IEEE, Vol. 21, Issue: 9, Sept. 2001, pp: 8-11, 15. [5] M.Madrigal, E.Acha., Modelling of Custom Power Equipment Using Harmonic Domain Techniques, IEEE [6] R.Mienski,R.Pawelek and I.Wasiak., Shunt Compensation for Power quality Improvement Using a STATCOM controller: Modelling and Simulation, IEEE Proce., Vol.151, No.2, March [7] C. Benachaiba, B. Ferdi, Power Quality Improvement Using DVR, American Journal of Applied Sciences 6 (3): , [8] S R. Omar, N.A Rahim, Compensation of Different Types of Voltage Sags in Low Voltage Distribution System Using Dynamic Voltage Restorer, Australian Journal of Basic and Applied Sciences, 4(8): , [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, [10] 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): , [11] J O. Viktorin, J.Driesen, Member, IEEE, R.Belmans, Senior Member, IEEE, The Prototype of a Singlephase 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, [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 AUTHOR BIOGRAPHY Jyothilal Nayak Bharothu received the B.E (Electrical &Electronics Engg.) from S.R.K.R Engg. College Bhimavaram (Andhra University) in 2006, and M.Tech;(Power systems- High Voltage Engg) from JNTU KAKINADA University in He has five years of teaching experiences in the field of Electrical Engg. In India, his field of interest is power systems operation and control. Presently, he is H.O.D. of EEE Dept at Columbia Institute of Engineering and Technology RAIPUR (C.G.), INDIA