IMPROVEMENT OF POWER QUALITY USING CASCADED H-BRIDGE

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ISSN: 2379-3686 International Journal of Science Research and Technology V olume 1 Issue 2 p p 6-11 15th December 2015 6 A ZERO HARMONIC DISTORTION IMPROVEMENT OF POWER QUALITY USING CASCADED H-BRIDGE Radhey Shyam Meena Mukesh Kr. Lodha Jeetendra Singh Rathore Ritu Raj Soni Member- The Institution of Engineering and Technology IET 1 Dept. Of Electrical Engineering Sri Balaji College of Engineering Technology Jaipur Rajasthan Rajasthan Technical University Kota Abstract- In reactive power compensation cascaded voltage source converter with separated dc sources seems to be the most feasible topology for many reasons. The cascaded converter is constructed with a number of identical H-Bridge inverters. This modular feature makes the cascaded converter very attractive. The cascaded converter topology not only specifies hardware manufacture ability but also makes the entire system flexible in term of power capability. In Multilevel converter has become attractive in the power industries and it can be applied in many applications especially on improvement of the power quality and distortion of zero harmonic components. This paper also presents the generation of triggering signals used to control the cascaded H-bridge multilevel converter. As the number of levels increases the synthesized output waveform adds more steps producing a staircase which approaches the sinusoidal wave with minimum harmonic distortion. Ultimately a zero harmonic distortion of the output wave can be obtained by an infinite number of levels. One of the major limitations of the multilevel converters is the voltage unbalance between different levels. The techniques to balance the voltage between different levels normally involve voltage clamping or capacitor charge control. This paper presents the cascaded multi nine and eleven level converter to improving the voltage sharing problems. Keywords— Multilevel Converter Multilevel Inverter Cascaded Multi nine and eleven Level Converter Power Converter Matlab. I. INTRODUCTION Recently the ―multilevel converter‖ has drawn tremendous interest in the power industry. The general structure of the multilevel converter is to synthesize a sinusoidal voltage from several levels of voltages typically obtained from dc voltage sources. The multilevel converters start from three levels. A three level converter also known as a ―neutral-clamped‖ converter consists of two capacitor voltages in series and uses the centre tap as the neutral. Each phase leg of the three-level converter has two pairs of switching devices in series. The center of each device pair is clamped to the neutral through clamping diodes. The waveform obtained from a three-level converter is a quasi-square wave output. The diode-clamped method can be applied to higher level converters. As the number of levels increases the synthesized output waveform adds more steps producing a stair-case wave which approaches the sinusoidal wave with minimum harmonic distortion. Ultimately a zero harmonic distortion of the output wave can be obtained by an infinite number of levels. More levels also mean higher voltages can be spanned by series devices without device voltage sharing problems. Therefore we used the cascaded multilevel converter to improving the voltage sharing problems. A three phase CHB multilevel converter circuit is designed and simulated using the MATLAB SimPowerSystems software. II. CONVERTER TOPOLOGY A power converter is an electrical or electro-mechanical device for converting electrical energy. It may be converting AC to or from DC or the voltage or frequency or some combination of these. Amongst the many devices that are used for this purpose are-  Switched-mode power supply  Rectifier  Inverter  Motor generator set  DC-DC converter  Transformer But in this paper we considered a multilevel inverter. So first of all define the single inverter and then also explain the multilevel inverter. A device that converts dc power into ac power at desired output voltage and frequency is called an inverter. Some industrial applications of inverters are for adjustable-speed ac drives induction heating stand by air-craft power supplies UPS uninterruptible power supplies for computers HVDC transmission lines etc. Phase controlled converters when operated in the inverter mode are called line-commutated inverters. But line-commutated inverters require at the output terminals an existing ac supply which is used for their commutation. This means that line-commutated inverters can‘t function as isolated ac voltage sources or as variable frequency generators with dc power at the

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ISSN: 2379-3686 International Journal of Science Research and Technology V olume 1 Issue 2 p p 6-11 15th December 2015 7 input. Therefore voltage level frequency and waveform on the ac side of line-commutated inverters cannot be changed. On the other hand force commutated inverters provide an independent ac output voltage of adjustable voltage and adjustable frequency and have therefore much wider applications. Numerous industrial applications have begun to require higher power apparatus in recent years. Some medium voltage motor drives and utility applications require medium voltage and megawatt power level. For a medium voltage grid it is troublesome to connect only one power semiconductor switch directly. As a result a multilevel power converter structure has been introduced as an alternative in high power and medium voltage situations. A multilevel converter not only achieves high power ratings but also enables the use of renewable energy sources. Renewable energy sources such as photovoltaic wind and fuel cells can be easily interfaced to a multilevel converter system for a high power application. However the elementary concept of a multilevel converter to achieve higher power is to use a series of power semiconductor switches with several lower voltage dc sources to perform the power conversion by synthesizing a staircase voltage waveform. Capacitors batteries and renewable energy voltage sources can be used as the multiple dc voltage sources. The commutation of the power switches aggregate these multiple dc sources in order to achieve high voltage at the output however the rated voltage of the power semiconductor switches depends only upon the rating of the dc voltage sources to which they are connected. A multilevel converter has several advantages over a conventional two-level converter that uses high switching frequency pulse width modulation PWM. The attractive features of a multilevel converter can be briefly summarized as follows. ● Staircase waveform quality: Multilevel converters not only can generate the output voltages with very low distortion but also can reduce the d v/dt stresses therefore electromagnetic compatibility EMC problems can be reduced. ● Common-mode CM voltage: Multilevel converters produce smaller CM voltage therefore the stress in the bearings of a motor connected to a multilevel motor drive can be reduced. Furthermore CM voltage can be eliminated by using advanced modulation strategies. ● Input current: Multilevel converters can draw input current with low distortion. ● Switching frequency: Multilevel converters can operate at both fundamental switching frequency and high switching frequency PWM. It should be noted that lower switching frequency usually means lower switching loss and higher efficiency. Unfortunately multilevel converters do have some disadvantages. One particular disadvantage is the greater number of power semiconductor switches needed. Although lower voltage rated switches can be utilized in a multilevel converter each switch requires a related gate drive circuit. This may cause the overall system to be more expensive and complex. To date the MOSFET GTO/Diode semiconductor switches are used to solve above problems. The cascade converter has drawn more interest lately as research shows its remarkable advantages over its counterparts. The simple repetitive modular structure of the converter allows high modification flexibility and greatly simplifies control designs. The technology also permits easy troubleshooting and packaging. Fig.1 Schematic of a 1-phase cascaded-multilevel converter In this circuit a single-phase structure of an m-level cascaded inverter with SDCSs is illustrated in Figure 1. A relatively new converter structure cascaded-inverters with separate dc sources SDCSs is introduces here. This new converter can avoid extra clamping diodes or voltage balancing capacitors. Each separate dc sources SDCSs is connected to a single-phase full-bridge or H-bridge inverter. The ac terminal voltages of different level inverters are connected in series. Each inverter level can generate three different voltage outputs +V dc 0 and –V dc by connecting the dc source to the ac output by different combinations of the four switches S 1 S 2 S 3 and S 4 . To obtain +V dc switches S 1 and S 4 are turned on whereas –V dc can be obtained by turning on switches S 2 and S 3 . By turning on S 1 and S 2 or S 3 and S 4 the output voltage is 0. The ac outputs of each of the different full-bridge inverter levels are connected in series such that the synthesized voltage waveform is the sum of the inverter outputs. The number of output phase voltage levels ‗m‘ in a cascade inverter is defined by m2s+1 where s is the number of separate dc sources. Each single-phase full bridge inverter can generate three level outputs +V dc 0 and –V dc . This is made possible by connecting the dc sources sequentially to the ac side via the n gate-turn-off devices. Similarly the ac output

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ISSN: 2379-3686 International Journal of Science Research and Technology V olume 1 Issue 2 p p 6-11 15th December 2015 8 voltage at each level can be obtained by controlling the conducting angles at different inverter levels. The phase output voltage is synthesized by the sum of individual inverter outputs i.e. V an V 1 +V 2 +V 3 +………….V n Therefore the phase voltage for 11-level cascaded inverter is V an V 1 +V 2 +V 3 +V 4 +V 5 For a stepped waveform such as the one depicted in Fig.5 with s steps the Fourier Transform for this waveform- Error Reference source not found. The conducting anglesError Reference source not found. Error Reference source not found.…….. Error Reference source not found. can be chosen such that the voltage total harmonic distortion is a minimum. Generally these angles are chosen so that predominant lower frequency harmonics 5 th 7 th 11 th and 13 th harmonics2.For the 11-level case the 5 th 7 th 11 th and 13 th harmonics can be eliminated with the appropriate choice of the conducting angles. One degree of freedom is used so that the magnitude of the output waveform corresponds to the reference amplitude modulation index m a which is defined as V L /V Lmax where V L is the amplitude command of the inverter output voltage and V Lmax is the maximum attainable amplitude of the converter i.e. V Lmax s.V dc . Let the equation from above Hn be as follows: Error Reference source not found. Error Reference source not found. Error Reference source not found. Error Reference source not found. Error Reference source not found. The set of nonlinear transcendental equations 5 to 9 can be solved by iterative method such as the Newton-Raphson method. For example using a conducting angles- Therefore the modulation index m a 0.73. This means that if the inverter output is symmetrically switched during the positive half cycle of the fundamental voltage to +V dc at Error Reference source not found. +2V dc at Error Reference source not found. +3V dc at Error Reference source not found.+4V dc at Error Reference source not found. and +5V dc at Error Reference source not found. and similarly in the negative half cycle to –V dc at Error Reference source not found. -2V dc at Error Reference source not found. -3V dc at Error Reference source not found. -4V dc at Error Reference source not found. and-5V dc at Error Reference source not found. the output voltage of the 11-level inverter will not contain the 5 th 7 th 11 th and 13 th harmonic components. For a three-phase system the output voltages of the three cascaded inverters can be connected in either star- or –delta configuration. Fig.2 illustrates the connection diagram for a star- configuration 11-level converter using cascaded-inverters with five SDC sources. Fig. 2 Three phase 11-level CHB Multilevel converter Single phase and Three phase Eleven Levels Cascaded H-bridge Multilevel Converter This type of CHB cascaded multilevel converter has been designed and simulated using MATLAB SimPowerSystems. The multilevel circuits are illustrated in Fig. a b Fig 3 a Simulation model of 3-phase Cascaded Eleven

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ISSN: 2379-3686 International Journal of Science Research and Technology V olume 1 Issue 2 p p 6-11 15th December 2015 9 level converter b Simulation model of 3-phase Cascaded Thirteen level converter. III. MERITS AND DEMERITS OF MULTILEVEL CONVERTER The multilevel inverter approach allows the use of high power and high voltage electric motor drive systems. Advantages/ Merits: The number of possible output voltage levels is more than twice the number of dc sources m 2s+1.The series of H-bridges makes for modularized layout and packaging. This will enable the manufacturing process to be done more quickly and cheaply. Disadvantages/Demerits: Separate dc sources are required for each of the H-bridges. This will limit its application to products that already have multiple SDCSs readily available. IV. SIMULATION RESULTS The simulation results of the cascaded multilevel converter are taken on eleven level converters. And the nine level cascaded multilevel converter is used for the studied purpose. Fig.4 Out put results of Eleven and Thirteen Level Fig.5 Individual waveform of inverter when delays are o 0 18 0 36 0 54 0 72 0 respectively

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ISSN: 2379-3686 International Journal of Science Research and Technology V olume 1 Issue 2 p p 6-11 15th December 2015 10 a b Fig.6 aSingle-phase cascaded 11-level inverter waveform. b Three-phase cascaded 11-level inverter waveform. c line-to-line voltage a b c d Fig. 7 a Bode Diagram and Response For Converter b Impulse Response for Converter c Response in initial conditions d Step response for converter V. CONCLUSON Among recently developed power converter topologies multilevel converters have become an important technology and have been utilized in higher-power applications especially for FACTS controllers. Several multilevel converter topologies have been developed to demonstrate their superiority in such applications. With converter modules in series and with balanced voltage sharing among them the lower-voltage switches can possibly be used in high-voltage systems. Thus the low-voltage-oriented insulated gate bipolar transistor IGBT devices can be stacked for medium-voltage systems. For higher-voltage applications however efforts were made to use GTO-based devices for multilevel converters.

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ISSN: 2379-3686 International Journal of Science Research and Technology V olume 1 Issue 2 p p 6-11 15th December 2015 11 VI. REFERENCES 1. M. H. J. Bollen ―Understanding Power Quality Problems V oltage Sags and Interruptions‖ IEEE Press New York 2000. 2. J. Schlabbach D. Blume and T. Stephanblome ―V oltage Quality in Electrical Power Systems‖ The Institution of Engineering and Technology Stevenage 2000. 3. R. C. Dugan M. F. McGranaghan and H. W. Beaty ―Electric Power Systems Quality‖ 2nd Edition McGraw Hill New York 2006. 4. A. Baggini ―Handbook on Power Quality‖ John Wiley Sons Hoboken 2008. 5. IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems IEEE Std. 519-1992 Apr. 12 1993. 6. Pedro Roncero –Sanchez Jose Enrique Orgeta- CalderonAurelio Garcia-Cervada ―A Versatile Control scheme for a Dynamic V oltage Restorer For Power Quality Improvement‖ IEEE Transaction On Power Delivery V ol.24No.1 Jan 2009PP.277-284. 7. K. Surin and M.T. Leon ―Multilevel power converters‖ Journal in the University of Tennessee 2000 V ol.31pp 150. 8. Jingsheng Liao Keith Corzine and Mehdi Ferdowsi ―A new control Method for Single-DC-Source Cascaded H-Bridge Multilevel Converters using Phase-Shift Modulation‖.

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