evelopment of our technology in recent years, the direction of research has shifted to power electronics from power systems to produce the most efficient energy conversion. The power electronics is giving us the opportunity to shape and control large amounts of power with better efficiency. Low cost, smaller size and high energy efficiency are possible because of power electronics. Within the next 30 years, power electronics will shape and condition the electricity somewhere in the transmission network between its generation and all its users. [1] Diode is called the first solid state electronic device. Now-a-days it becomes a weighty part of power electronic era. Diode rectrifiers are used in several power systems. Diode bridge is specially used in high-power applications. Only diode contain some magnificent characteristics such as availabile, light weight, compact, high efficiency, robust for fault high current, less emission noises and etc. Three phase diode bridge constructed with six diodes. It can operate with or without transformer. Like other non-linear devices diode also affected by harmonics. Harmonics are multiples of the Like other non-linear devices diode also affected by harmonics. Harmonics are multiples of the fundamental frequency. The deviation from perfect sine wave isknown as harmonic distortion. Harmonic is acceptablewithin limit. Increase in core losses due to increased iron losses in transformers occured by harmonic currents at harmonic frequencies. It also increased copper losses and stray flux losses result in additional heating, and winding insulation stresses, especially if high levels of dv/dt (i.e., rate of rise of voltage) are present. Temperature cycling and possible resonance between transformer winding inductance and supply capacitance, line notching problems are produced by harmonics. [2] Several harmonic mitigation procedure are available using diode rectifiers. Some of them provide fine uncontrolled dc voltage without harmonic pollution . Every configurations cannot fullfill the demand like autotransformer based schemes fails due to higher rating magnetic, higher number of bridges, resulting in enhancement of capital cost. [3] This paper work with the three-phase multipulse AC to DC conversion system employing a phaseshifting transformer and a three-phase uncontrolled bridge rectifier between the supply and load side of the system. Every such converter provides 6-pulse ripple components on the output voltage, so in order to produce more sets of 6-pulse systems, a uniform phase-shift is required and hence with proper phaseshifting angle, 12, 18, 24, 30, and higher pulse systems can be produced. [4] Phase shifting transformer based configurations are really cost effective and reliable than others. In this paper we design a ac-dc converter with limited harmonic distortion with the help of phase shifting transformer. This paper represent an unique 24pulse converter with bridge rectifier which is able to control harmonic distortion and provide best ripple factor in the output. This can be achieved by applying pulse-multiplication technique. When phase voltage V ab starts D1D6 conduct for 60 0 and for V ba the negative phase voltage starts D3D4 conduct with the supply. Similarly V bc and V cb phase voltage converted to dc at the supply side by connecting D3D2 and D5D6 that time. Lastly Vca and Vac converted to dc using sequence D1D2 and D5D4. The average output voltage is found: V 0 = 2 2? 6 ? ?3 ? 6 0 V m cos ?t d(?t) = 1.654V m ; ------------------------------(1.1) Where Vm is the peak value of rms voltage. When load is purely resistive the rms value of the diode current is : I rms = ? 4 2? ? I m 2 cos 2 ?t d(?t) ? 6 ? 0 ? 1 2 = 0.5518I m ------------------------------(1. 2) Supply the three phases are symmetrical and are Y-connected. The 3-phase voltage sources are defined as: The transformer has a 3-leg core, with 1 primary winding and 1 secondary winding on each leg. Primary and secondary winding ratio for Y-Y is 1 and for Y-Î?" is 0.577 The nodes at the bottom are the neutral points of the Y connections. So 30 0 phase shift required for the 12-pulse converter. For obtaing a 12-pulse ac-dc conversion the phase shift between the two sets of voltages should be either 0° to 30° or +15° to -15° with respect to supply voltage. In this paper phase shift is used for the 0° to 30° phase shift is use for the transformer along with the pulse multiplication technique. Using above equation we calculate the phase shift angle (?) for 24-pulse rectifier is 15 0 . ?? ?? = ?? ?????? * ? 2 3 ? sin?ð??"ð??"ð??"ð??" + ?? * ?? 180 ? ? ?? ?? = ?? ?????? * ? 2 3 ? sin?ð??"ð??"ð??"ð??" + ?? * ?? 180 ? ? 2?? 3 ? ? ?? ?? = ?? ?????? * ? 2 3 ? sin?ð??"ð??"ð??"ð??" + ?? * ?? 180 ? + 2??3 Transformer secondary current waveform for 12-pulse and 24-pulse rectifier with phase shifting. Similarly, RF calculation for 24-pulse, Here, Vrms=460.07 volt and Vdc= 418.94 volt So, RF=0.453 or 45.3% The capacitor absorbs energy during the pulse and delivers this energy to the load between pulses, the output voltage can never fall to zero. However, if the resistance of the load is small, a heavy current will be drawn by the load and the average output voltage will fall. Also, the filter capacitor acts like a short circuit across the rectifier while the capacitor is being charged. Due to these reasons, a simple capacitor filter is not suitable for rectifiers in higher power applications. # V. SIMULATION RESULTS # VI. CONCLUSIONS This paper present the comparison between 12pulse and 24-pulse rectifier. After finishing necessary calculation we have seen 24-pulse contain less harmonic content in input current. It improves power factor with inductive and capacitive load. Although detail analysis has not been described in this paper, but every inevitable information are given. The desirable features of the modified diode rectifier, such as compact, economical, efficient and reliable are added with the new 24-pulse rectifier. Some new features are joined with this configuration such as lower Ripple component and higher power factor. ![Circuit schemes Fig 3.1 shows a simplified circuit diagram of a three-phase twelve-pulse diode rectifier. The resistor and inductance RLin is the total inductance including the line inductance, transformer reactance, and line reactor between the utility and the rectifier. A single unit 3-phase rectifier is called 6-pulse rectifier. Thus, a 12pulse rectifier have 2x6-pulse rectifier. Phase-shifting not only reduce the harmonic input current but also reduces the ripple on the DC output of the rectifier. Three phase full wave rectification diodes D1, D2, D3, D4, D5, D6 are numbered in order of conduction sequences and each one conduct for 120 0 . The conduction sequence for diodes is D1D2, D3D2, D3D4, D5D4, D5D6 and D1D6.](image-2.png "") ![? ? Here line-to-line rms voltage Vrms of the threephase source in V. The resistances and inductances of the three-phase branches are equal. The initial currents are zero. In 12-pulse converter Y-Y and Y-Î?" transformer presents the resistance and inductance values of the secondary winding are referred to the primary side. The relationship between the referred values and the real secondary winding values is: Rs = Rs_real_value * (Np/Ns) 2 Ls = Ls_real_value * (Np/Ns) 2](image-3.png "") 2![Figure 2 : 24-pulse technique with Diode Bridge III. PARAMETERS A Phase-Shifting Transformer is a device for controlling the power flow through specific lines in a complex power transmission network. The basic function of a Phase-Shifting Transformer is to change the effective phase displacement between the input voltage and the output voltage of a transmission line, thus controlling the amount of active power that can flow in the line.To degrade harmonics from input minimum phase shift calculation is as follows: Phase shift= 60 0 /no. of converters](image-4.png "Figure 2 :") 1![Figure 1 : 12-pulse technique with Diode Bridge](image-5.png "Figure 1 :") 3![Figure 3 : Model of a transmission line with and without a PST The phase shift is controllable within certain limits.24-pulse diode rectifier operate with four 6pulse diode rectifier. For obtaing a 24-pulse ac-dc conversion the phase shift among four sets of voltages](image-6.png "Figure 3 :") 4![Figure 4 : 12-pulse input current waveform](image-7.png "Figure 4 :") 5![Figure 5 : 24-pulse input current waveform](image-8.png "Figure 5 :") 6![Figure 6 : 12-pulse primary current for THD calculation](image-9.png "Figure 6 :") 7![Figure 7 : 24-pulse primary current for THD calculation% THD from 12-pulse rectifier is 20.44% whereas 24-pulse contain only 17.45% harmonic distortion. Increasing pulse number decrease 17 th and 22 th harmonics component are eliminated from 24-pulse converter.](image-10.png "Figure 7 :") ![Harmonics Cancellation and Alleviation of Ripple Content from AC-DC Uncontrolled Rectifier by Pulse-Multiplication Technique using Phase-Shifting TransformerIV. FILTER DEISGN When the instantaneous voltage Vs is higher than the instantaneous capacitor voltage Vc, the diodes conduct; and the capacitor is then charged from the supply. If the instantaneous supply voltage falls below Vs the instantaneous voltage Vc, the diodes are reverse biased and the capacitor Ce discharges through the load resistance R. In practice, the ripple factor can be found from --------------------(1.4)From single phase ripple factor calculation assuming RF as 3% then is ----------------------(1.5) from above calculation we 12.2m capacitance for 3% RF in single phase.Using capacitor filter in 12-pulse rectifier Vdc is shown in figure](image-11.png "F") 89![Figure 8 : 12-pulse output voltage with capacitor filter From equation (1.4) RF calculation for 12-pulse rectifier with filter Here, Vrms=422.04 volt and Vdc= 385.49 volt so, RF= 0.441 or 44.1%](image-12.png "Figure 8 :Figure 9 :") 10![Figure 10 : 12-pulse output current and voltage without filte rRF calculation for 12-pulse without filter, Here, Vrms=556.31 volt and Vdc= 554.04 volt So, RF=0.0906 or 9.06%](image-13.png "Figure 10 :") 12![Figure 12 : 12-pulse with RL Load](image-14.png "Figure 12 :") 13![Figure 13 : 24-pulse with RL load](image-15.png "Figure 13 :") 14![Figure 14 : 12-pulse with RLC load](image-16.png "Figure 14 :") 15![Figure 15 : 24-pulse with RLC load](image-17.png "Figure 15 :") © 2015 Global Journals Inc. (US) © 20 15 Global Journals Inc. (US) * Rashid Power Electronics Circuits, Devices And Applications Third Edition HMuhammad * Harmonics in power system-causes, effects and control usa.siemens.com/lv-drives * Design And Analysis of a 24-pulse AC-DC Power Converter By Employing a Pulse Doubling Technique for Asynchronous Drive MKiran PhaniKumar N SrinivasaRao April 2015 3 * Using Multi-Pulse Converters for Supply Current Harmonic Control' Received JayantDewal Prof. NandKishore 05 January, 2014 Accepted 06 * Table I : Comparison Between 12-Pulse And 24-Pulse AC-DC Conversion * Three Phase Harmonic reducing diode rectifier' conf. EPE-PEME NishidaYasuyuki Hiromichi PredragOhyama JohannWPejovic Kolar 2012 Europe * A Method to Decrease Input Current Harmonics of Three-phase Diode Rectifier by Pulse-Doubler Scheme' conf. Proc. of IECON-06 YNishida KMino' Nov. 2006 Paris * Harmonic Cancellation for Twlve pulse rectifier Using DC BUS Modulation NRRaju ADaneshpooy JSchwartzenberg ' 0-7803-7420-7/02/$17. 00 © 2002 IEEE